Low Energy Electrodynamics in Solids 2012

22 Jul 2012, 16:00 → 27 Jul 2012, 13:00 US/Pacific

Chardonnay Ballroom (Embassy Suites Napa Valley)

Dimitri Basov, Michael Martin (LBNL)

LEES 12, co-organized by the University of California, San Diego and Lawrence Berkeley National Laboratory, will be a forum for the interdisciplinary discussion of the low-energy electrodynamics of solids, at both the theoretical and experimental level, with specific emphasis on the electronic and magnetic properties of quantum materials.

Sunday, 22 July

16:00 Registration

19:00 Welcome Reception

Monday, 23 July

Graphene I

Convener: Zhiqiang Li (National High Magnetic Field Lab)

1 Welcome to LEES 2012

Speakers: Dimitri Basov (UC San Diego), Michael Martin (LBNL)

2 Tunable Optical Properties of Graphene

Feng Wang Department of Physics, U.C. Berkeley Graphene, a single layer of carbon atoms, exhibits novel two-dimensional electronic behavior. Optical spectroscopy provides a powerful toolkit study graphene physics. In this talk, I will show how we can use infrared spectroscopy to probe gate-dependent interband transitions as well as intraband transitions. I will also discuss how we can use electrical gating to control inelastic light scattering processes in graphene.

Speaker: Feng Wang (UC Berkeley)

3 Probing Interactions and Dynamics in Single- and Few-Layer Graphene by Optical Spectroscopy

Tony F. Heinz Departments of Physics and Electrical Engineering Columbia University, New York, NY 10027 The optical conductivity of graphene provides a sensitive probe not only of the basic band structure, but also of various interactions in the system. In this talk, we will discuss three recent directions that highlight these possibilities. (1) Probing electron-electron interactions. While the optical conductivity of graphene at low photon energies (~1 eV) conforms well to band structure calculations, at higher photon energies, we observe strong deviations. The existence of a pronounced peak around 4.5 eV is predicted within a single-particle picture. The strongly asymmetrical shape of the peak, however, can be understood only within the picture excitonic corrections, notably in the form of a saddle point exciton. The optical conductivity thus provides a direct indication of the strong many-body electronic interactions in graphene. The effect of carrier screening, as probed by changing the carrier density through electrostatic gating, on these interactions will be discussed. (2) Probing interlayer interactions. The interactions between different layers define the low-energy band structure of few-layer graphene. We have examined the band structure through measurements of the infrared conductivity, emphasizing particularly the role of stacking order - Bernal (ABA) and rhombohedral (ABC) – in few-layer graphene samples. The influence of perpendicular static electric fields on the band structure is shown to depend critically on stacking order. (3) Probing electron-phonon interactions. The infrared conductivity also permits probing of C-C stretching (G-mode) vibrations of appropriate symmetry in few-layer graphene. We show the existence of strong coupling between these discrete modes and the electronic absorption continuum. The line shape of the modes depends strongly on the electronic structure and varies with layer thickness and stacking order.

Speaker: Tony Heinz (Columbia University)

4 Beyond Graphene: Two-Dimensional Crystals

Antonio Castro Neto National University of Singapore Graphene is possibly one of the largest and fastest growing fields in condensed matter research. However, graphene is only one example in a large class of two-dimensional crystals with unusual properties. In this talk I will briefly review the properties of graphene and look at the exciting possibilities that lie ahead.

Speaker: Antonio Castro Neto (National University of Singapore)

10:15 Break

Metal-Insulator Systems

Convener: David Tanner (University of Florida)

5 Common Fermi-liquid origin of T2 resistivity and superconductivity in n-type SrTiO3

Dirk van der Marel, Université de Genève Dook van Mechelen, Université de Genève Igor Mazin, Naval Research Laboratories SrTiO3 is a semiconductor which, when doped with a low density of electrons, becomes a good conductor with relatively high mobility and strong temperature dependence of the electrical resistivity and the infrared optical conductivity. At low temperatures the material becomes superconducting with Tc below 1 K having a dome-shaped doping dependence, both in the 3D bulk material and at the 2D LaAlO3/SrTiO3 interface. The DC resistivity below 100 K has a T^2 temperature dependence. The quasiparticles are in the anti-adiabatic limit with respect to electron-phonon interaction, which renders the interaction mediated through phonons effectively non-retarded. We apply Fermi-liquid theory for the T^2 term in the resistivity, and combine this with expressions for Tc and with the Brinkman-Platzman-Rice (BPR) sum-rule to obtain Landau parameters of n-type SrTiO3. These parameters are comparable to those of liquid 3He, indicating interesting parallels between these Fermi-liquids despite the differences between the composite fermions from which they are formed. The physics of the doped semiconductor SrTiO3 stands in stark contrast with the doped cuprates where Tc’s are two orders of magnitude higher and correlate with the T^1 term of the resistivity.

Speaker: Dirk van der Marel (Université de Genève)

6 Determination of the electromagnon origin in the multiferroic TbMnO3 by Raman scattering

P. Rovillain, M. Cazayous, Y. Gallais, M-A. Measson, A. Sacuto Laboratoire Materiaux et Phenomenes Quantiques Universite Paris 7 France, H. Sakata, Department of physics University of Science Tokyo Japan M. Mochizuki Department of Applied Physics, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Magnetoelectric multiferroics possess coexisting magnetic and ferroelectric phases, with cross-correlation effects between magnetic and electrical degrees of freedom. As such, they can potentially be used to control spin-based properties by electric fields, with very low associated power dissipation [1]. Indeed, just as coupling between magnetic and ferroelectric order parameters exists in multiferroics, coupled spin and lattice excitations termed electromagnons have also been demonstrated. Such mixed excitations have been evidenced at low temperature in multiferroic manganites (TbMnO3, ..) and suspected at room temperature in BiFeO3. Such novel excitations are directly related to electromagnetic coupling and reflect the intimate relationship between magnetic and ferroelectric magnetic orders. However, the exact magnetic and the polar components of the electromagnons has not been yet identified. Electromagnons probably belong to the most challenging open questions in the field and are currently under intense investigation We show using Raman scattering that in multiferroic TbMnO3 a c-axis magnetic field strongly impact the electromagnons excitations [2]. The electrical polarization of the electromagnons is strongly altered under magnetic field whereas their magnetic part is preserved. Entering the paraelectric phase the spectral weight of the electromagnons is transferred to the magnon excitations. The effect of the phase transition on the phonon modes shows that the Mn-O distance is the key that controls the polar character of the electromagnons. The magnetic excitation and the polar activity at the origin of the electromagnons are discussed under the light of Heisenberg simulations. [1] P. Rovillain et al. Nature Materials 9, 975 (2010) [2] P. Rovillain et al., Phys. Rev. Lett. 107, 027202 (2011)

Speaker: Maximilien Cazayous (Laboratoire Matériaux et Phénomènes Quantiques (UMR 7162) Université Paris Diderot Paris 7)

7 Spectroscopic investigation of materials uner extreme conditions

J. L. Musfeldt, T. V. Brinzari, O. Gunaydin-Sen, Q. - C. Sun, J. S. Miller, J. L. Manson, J. A. Schlueter, C. Landee, M. Turnbull, D. Smirnov, Y. J. Wang, M. -H. Whangbo, Z. Liu, G. L. Carr, P. Jena, A. Christianson, S. Baker The interplay between structure and magnetism is of great current interest because it underpins different types of functionality in complex materials. Contrary to previous expectation, we are discovering that many magnetic transitions take place with important elastic effects. Examples include simple low temperature magnetic ordering transitions, magnetic quantum critical transitions, pressure-driven magnetic dimensionality crossovers, and systems in which finite length scale effects can be explored. This talk will focus on the use of vibrational spectroscopy to reveal local lattice distortions and coupling constants associated with magnetoelastic transitions in a variety of materials including Mn(dca)2, Co(dca)2, Cr(Ru2)3, Cu(pyz)(NO3)2, Cu(pyz)F2(H2O)2, and MnO.

Speaker: Janice Musfeldt (University of Tennessee)

8 Existence of Heavy Fermions in the Antiferromagnetic Phase in CeIn3

Takuya Iizuka, Takafumi Mizuno, Shin-ichi Kimura UVSOR Facility, Institute for Molecular Science, and School of Physical Sciences, The Graduate University for Advanced Studies, Okazaki 444-8585, Japan Byeong Hun Min, Yong-Seung Kwon Department of Physics, Sungkyunkwan University, Suwon 440-746, South Korea We report the pressure-dependent optical conductivity spectra of a heavy fermion (HF) compound CeIn3 below the Neel temperature of 10 K to investigate the existence of the HF state in the antiferromagnetic (AFM) phase. The peak due to the interband transition in the hybridization gap between the conduction band and nearly localized 4f states (c-f hybridization) appears at the photon energy of about 20 meV not only in the HF regime but also in the AFM regime. Both the energy and intensity of the c-f hybridization peak continuously increase with the application of pressure from the AFM to the HF regime. This result suggests that the c-f hybridization, as well as the heavy fermions, exists even in the AFM phase of CeIn3.

Speaker: Shin-ichi Kimura (UVSOR Facility, Institute for Molecular Science)

12:30 Lunch

Correlated Physics

Convener: Lance Cooper (University of Illinois, Urbana-Champaign)

9 Far infrared study of magnetic field-induced normal state of the high temperature superconductor LSCO.

Thomas Timusk, Jungseek Hwang McMaster University, Canada T. Rõõm, U. Nagel, D. Hüvonen, Nat. Inst. Chemical Physics and Biophysics, Estonia D. Hawthorn, S. Wakimoto, H. Zhang, University of Toronto, Canada We report on the ab-plane optical properties of the magnetic field induced normal state of La1.94Sr0.06CuO4 (Tc=5.5 K), the first such study. We apply strong magnetic fields up to 16 T along the c-axis . We find that the magnetic fields, which are strong enough to destroy superconductivity, induce a gap-like depression in the optical conductivity at low frequency along with parallel growth of a broad absorption peak at higher frequency, just above the gap. The loss of low frequency conductivity in the gap region is in good agreement with dc magneto resistance measurements on samples from the same batch. The spectral weight loss in the depression at low frequency is recovered by the spectral weight in the broad peak. Significantly, this spectral weight equals the spectral weight of the superconducting condensate. We interpret our data in terms of a field-induced SDW state that coexists with superconductivity in low fields and grows in amplitude with field in agreement with neutron scattering results of Lake et al. and the phase diagram proposed by Demler et al. B. Lake et al. Nature, 415, 299, (2002). E. Demler, S, Sachdev and Y. Zhang, PRL, 87, 067202 (2001).

Speaker: Thomas Timusk (McMaster University)

10 RAMAN SPECTROSCOPY OF IRON BASED SUPERCONDUCTORS

G. Blumberg, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854 I will review results of recent Raman studies of electronic and magnetic excitations in several families of Fe-based superconductors including iron selenide compounds with ordered vacancies which reach 32K superconducting transition temperature while exhibiting antiferromagnetic ordering with Néel temperature as high as 500K. The data reveal the evolution of magnetically ordered state with cooling and the structure of superconducting order. The work is done in collaboration with A. Ignatov and P. Lubik (Rutgers), N.-L. Wang (IOP, Beijing), J. Karpinski (ETH), J. Paglione (U Mariland), E. Giannini (U Geneva). Research at Rutgers is supported by the U.S. Department of Energy under Award DE-SC0005463 and by the National Science Foundation under Award DMR-1104884.

Speaker: Girsh Blumberg (Rutgers University)

11 α-(BEDT-TTF)2I3: Complex electrodynamic response of the charge-ordered phase

Tomislav Ivek [1], Ivan Kovačević [1], Marko Pinterić [1,2], Bojana Korin-Hamzić [1], Silvia Tomić [1], Conrad Clauss [3], Martin Dressel [3], Dieter Schweitzer [4] [1] Institut za fiziku, P.O.Box 304, HR-10001 Zagreb, Croatia [2] Faculty of Civil Engineering, Smetanova 17, 2000 Maribor, Slovenia [3] 1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany [4] 3. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany Much experimental and theoretical attention has been attracted by organic systems with reduced dimensionality and strong Coulomb interactions, and deservedly so due to their novel broken-symmetry phases and corresponding excitations. Here we take a detailed look at the electrodynamics of one of the most prominent charge-ordered systems, the quasi-2D conductor α-(BEDT-TTF)2I3. A semimetal at high temperatures, at 136 K this particular system transitions into an insulating, diamagnetic ground state. Within the insulating phase a long-range commensurate ordering appears in the BEDT-TTF molecular planes, the so-called "horizontal stripe" charge order. [1,2,3] We characterize the charge response of the low-temperature phase using dc resistivity, dielectric and optical spectroscopy in different crystallographic directions within the BEDT-TTF layer. [4,5] Interestingly, two dielectric relaxation modes appear in the kHz-MHz range. The large mode features an anisotropic phason-like behavior, while the small mode presents a soliton-like characteristic. The observed type of excitations agrees with the most relevant physical picture of this charge order as a cooperative bond-charge density wave with ferroelectric-like features. [6] On the other hand, puzzling phenomena including negative differential resistance and voltage oscillations have been reported under application of high electric fields. [7] Our carefully designed electric-field-dependent measurements of conductivity anisotropy within the molecular plane qualitatively confirm these findings; additionally, they reveal novel intriguing behaviors. [8]. [1] Y. Takano et al., J. Phys. Chem. Solids 62, 393 (2001). [2] Organic Conductors, Special Topics Section, J. Phys. Soc. Jpn. 75, No. 5, (2006). [3] T. Kakiuchi et al., J. Phys. Soc. Jpn. 76, 113702 (2007). [4] T. Ivek et al., Phys. Rev. Lett. 104, 206406 (2010) [5] T. Ivek et al., Phys. Rev. B 83, 165128, (2011). [6] R. T. Clay et al., J. Phys. Soc. Jpn. 71, 1816 (2002). [7] K.Tamura et al., J. Appl.Phys.107, 103716(1-5), (2010). [8] T. Ivek et al., to be submitted (2012).

Speaker: Tomislav Ivek (Institut za fiziku)

12 Hidden Order Pseudogap and Hybridization Modulation in URu2Si2

Alexander Balatsky1, Jason Haraldsen1, Yoni Dubi2, Jian Xin Zhu3, Peter Wolfle4 and Matthias Graf3 1 Center for Integrated Nanotechnologies, LANL, USA 2 Center for Integrated Nanotechnologies, LANL, Israel 3 Theory division, LANL, USA 4 Institute for Theory of Condensed Matter and Center for Functional Nanostructures, Karlsruhe Germany, Germany URu2Si2 proved to be a compound that has exhibited similarities with other correlated materials like HighTc oxides in that it has strong correlations, competing phases and intense nanoscale inhomogeneity in Kondo lattice. We point out that Hidden order transition is not a mean field transition, in contrast to prevailing discussion to date. Iit has significant precursor effects, so called pseudogap. Through an analysis and modeling of data from various experimental techniques, we present evidence for the presence of a hidden order pseudogap in URu2Si2 in the temperature range between 25 K and 17.5 K. We evaluate the effects that gap fluctuations would produce on observables like tunneling conductance, neutron scattering and nuclear resonance, and relate them to the experimental findings. We show that the transition into hidden order phase is preceded by the onset of non-coherent hidden order fluctuations. We also discuss nanoscale inhomogeneity seen in URu2Si2 with STM as an evidence for hybridization modulation due to local defects and discuss the role of hybridization modulations in Hidden Order. Hidden order pseudogap in URu$_2$Si$_2$, J. Haraldsen, 10.1103/PhysRevB.84.214410 (2011) Electronic inhomogeneity in a Kondo lattice, E. Bauer et al, PNAS 108, 6857 (2011) How Kondo Holes Create Intense Nanoscale Heavy-Fermion Hybridization Disorder,PNAS 2011 108 (45) 18233-18237; published ahead of print October 17, 2011, doi:10.1073/pnas.1115027108 Anomalous femtosecond quasiparticle dynamics of hidden order state in URu2Si2 Georgi L. Dakovski et al, Phys. Rev. B 84, 161103 (2011) Hybridization Wave as the “Hidden Order” in URu2Si2, Y. Dubi et al, Phys. Rev. Lett. 106, 086401 (2011) Incommensurate spin resonance in URu2Si2, A. V. Balatsky, et al, Phys. Rev. B 79, 214413

Speaker: Alexander Balatsky (Los Alamos National Laboratory)

13 Pressure suppression of unconventional CDW state in PrRu4P12 studied by optical conductivity

H. Okamura(1), M. Matsunami(2), H. Sugawara(1), H. Sato(3), C. Sekine(4) (1) Physics Department, Kobe University, Kobe 657-8501, Japan. (2) Institute for Molecular Science, Okazaki 444-8585, Japan. (3) Tokyo Metropolitan University, Hachioji 192-0397, Japan. (4) Muroran Institute of Technology, Muroran, 050-8585, Japan. Application of external pressure is a powerful tool to explore novel electronic states in strongly correlated materials. Optical conductivity [s(w)] technique has been an important method to probe them, since other spectroscopic methods such as photoemission and tunneling cannot be performed with a pressure cell. We have recently made high pressure s(w) studies of correlated materials including YbS [1], CeRu4Sb12 [2], SrFe2As2 [3] and PrRu4P12 [4]. Here we present results on the filled skutterudite compounds PrRu4P12 [4] and CeRu4Sb12 [2] at high pressures to 14 GPa and at low temperatures to 8 K. PrRu4P12 at ambient pressure and low temperature is insulating with a clear energy gap in s(w) [5]. This insulating state results from an unconventional CDW involving Pr sublattices with different f electron levels, unlike the usual CDW involving lattice deformation. With increasing pressure, the energy gap in s(w) is progressively filled in, and it is completely suppressed at 14 GPa and below 30 K. The pressure evolution of the unique f electron state will be discussed based on the s(w) data. CeRu4Sb12 at ambient pressure, in contrast, is a heavy fermion metal, but large increases of resistivity with pressure had been previously reported. In the measured s(w), a pronounced mid-IR peak, which is due to hybridization gap [6], shifts to higher energy with pressure [2]. Our result suggests that CeRu4Sb12 is tuned by pressure from a heavy fermion metal into a Kondo insulator. [1] M. Matsunami, H. Okamura et al., Phys. Rev. Lett. 103, 237202 (2009). [2] H. Okamura et al., J. Phys. Soc. Jpn. 80, 084718 (2011). [3] H. Okamura et al., Proc. SCES 2011, to appear. [4] H. Okamura et al., submitted (2012). (arXiv:1202.3007) [5] M. Matsunami, H. Okamura et al., Phys. Rev. B 72, 073105 (2005). [6] S. V. Dordevic et al., Phys. Rev. Lett. 86, 684 (2001).

Speaker: Hidekazu Okamura (Kobe University)

15:45 Break

Topological Insulators I

Convener: Alexander Balatsky (Los Alamos National Laboratory)

14 Topological magneto-electric effect

Shoucheng Zhang Stanford University In this talk, I will discuss the quantized topological magneto-electric effect in topological insulators. In particular, I will present a number of proposals to detect this effect in low energy magneto-optical spectroscopy.

Speaker: Shoucheng Zhang (Stanford University)

15 Ultrafast Probing of Dynamical Spin-Charge Coupling in Topological Insulators

Gedik, Nuh MIT The three-dimensional topological insulator (TI) is a new quantum phase of matter that exhibits quantum-Hall-like properties, even in the absence of an external magnetic field. Charge carriers on the surface of a TI behave like a two-dimensional gas of massless helical Dirac fermions for which the spin is ideally locked perpendicular to the momentum. In this talk, I will discuss recent experiments in which we used the angular momentum of circularly polarized ultrafast laser pulses to directly visualize and manipulate the spin-charge coupling in TIs. By using laser pulses in the UV region, we performed novel time of flight based angle-resolved photoemission spectroscopy that enabled simultaneously mapping all three components of spin over the entire Dirac cone of a TI. We find that an idealized description of helical Dirac fermions only applies within a small energy window about the Dirac point, beyond which strong textural deformations occur. Utilizing the pump-probe technique, we selectively obtained time-resolved dynamics of surface and bulk excitations. By using circularly polarized laser pulses in the optical region, we achieved preferential excitation of spin species on one side of the surface Dirac cone, resulting in a charge imbalance in momentum space and thus causing a current flow with a direction dependent on photon helicity.

Speaker: Nuh Gedik (MIT)

16 Optical conductivity of Bismuth-based topological insulators

Stefano Lupi Department of Physics University of Rome La Sapienza Topological Insulators (TI) are new quantum materials with an insulating-gap in the bulk of spin-orbit origin and metallic states at the surface. These states are chiral and show dissipation-less transport properties protected from disorder by the time-reversal symmetry. In addition to their fundamental properties, TI have potential applications in quantum computing, photonics, and spintronic devices. Materials belonging to the V2VI3 (V= Bi, Sb, S; VI = Se, Te, S) family recently emerged, due to their large bulk insulating gap (~300 meV), as the first candidates for the study of topological surface states. However the as-grown crystals usually display, due to impurities/unstoichiometry related to the growing process, an extrinsic degenerate semiconducting behavior. Those impurities affect the low energy transport properties, rendering difficult the separation between the intrinsic 2D metallic behavior due to topological surface states and the 3D metallic conduction induced by extrinsic charges. In this talk I will discuss the optical conductivity σ1(ω) and the optical spectral weight SW of four topological insulators with an increasing chemical compensation (Bi2Se3, Bi2-xCaxSe3 x=0.005, Bi2Se2Te, Bi2Te2Se). The effect of compensation is clearly visible in the infrared spectra, through the suppression of the extrinsic Drude term and the appearance of strong peaks in the terahertz range assigned to electronic transition among impurity states. The SW of the most compensated sample (Bi2Te2Se) is still nearly two orders of magnitude higher than that expected from the surface states.

Speaker: Stefano Lupi (Department of Physics, University of Rome La Sapienza)

18:00 Dinner

Poster Session 1

Graphene, Topological Insulators, Metal-Insulator Systems, Transition Metal Oxides, Correlated Physics, Spin Phenomena

17 AB-Stacked Multilayer Graphene Synthesized via Chemical Vapor Deposition: A Characterization by Hot Carrier Transport

Carlos Diaz-Pinto, Debtanu De, Viktor G. Hadjiev, Haibing Peng, Department of Physics and the Texas Center for Superconductivity, University of Houston, Houston, Texas 77204 We report the synthesis of AB-stacked multilayer graphene via ambient pressure chemical vapor deposition on Cu foils, and demonstrate a method to construct suspended multilayer graphene devices. In four-terminal geometry, such devices were characterized by hot carrier transport at temperatures down to 240 mK and in magnetic fields up to 14 T. The differential conductance (dI/dV) shows a characteristic dip at longitudinal voltage bias V=0 at low temperatures, indicating the presence of hot electron effect due to a weak electron-phonon coupling. Under magnetic fields, the magnitude of the dI/dV dip diminishes through the enhanced intra-Landau level cyclotron phonon scattering. Our results provide new perspectives in obtaining and understanding AB-stacked multilayer graphene, important for future graphene-based applications. (Ref: ACS Nano, Vol 6, p. 1142, 2012 )

Speaker: Haibing Peng (University of Houston, Physics Department)

18 AC Conductivity of AA-stacked and Twisted Bilayer Graphene

C.J. Tabert and E.J. Nicol, University of Guelph, Canada Alternative stacking arrangement for bilayer graphene, differing from the standard Bernal AB-stacking, have been recently been of interest due to the observation of Moire patterns in STM pointing to misorientated or twisted multilayer graphene. Moreover, AA-stacked graphene has also been produced. We have calculated the frequency-dependent AC conductivity of bilayer graphene for the case of AA-stacking and for a simple model of a twisted bilayer. We contrast the conductivity for these cases with that for AB-stacking[1] noting characteristic signatures as a function of twist angle (for the latter case) and finite chemical potential due to charging. We will discuss the physics of our results and show how the important energy scales can be seen in the optics. [1] E.J. Nicol and J.P. Carbotte, Phys. Rev. B 77, 155409 (2008).

Speaker: Calvin Tabert (University of Guelph)

19 An infra-red study of thin film Bi$_2$Se$_3$ electronic structure across multiple thicknesses

K. Post (UCSD), Liang He (UCLA), Xufeng Kou (UCLA), B.C. Chapler (UCSD), Kang L. Wang (UCLA), D.N. Basov (UCSD) The issue of self doping via selenium vacancies in the topological insulator (TI) Bi$_2$Se$_3$ has hindered efforts to produce samples with surface dominated conductivity. In fact, previous transport measurements of Bi$_2$Se$_3$ samples grown by molecular beam epitaxy (MBE) have demonstrated that the charge carrier density varies as a function of the thickness. To explore this issue in Bi$_2$Se$_3$ we have used various methods of infrared spectroscopy, including transmission and ellipsometry, to probe the electronic structure of Bi$_2$Se$_3$ thin films grown using MBE. From these measurements, we have developed an enhanced understanding of how the sample thickness affects the material properties.

Speaker: Kirk Post (University of California, San Diego, Department of Physics)

20 Angle-resolved photoemission on metallic and insulating phase of VO2

Luca Moreschini (1),Young Jun Chang (1,2), Davide Innocenti (3), Andrew L. Walter (1), Jonathan Denlinger (1), Aaron Bostwick (1), Jan Minar (4), Silke Biermann (5), Eli Rotenberg (1) (1) Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, USA (2) Department of Molecular Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany (3) CNR-SPIN, University of Roma "Tor Vergata", Roma, Italy (4) Department Chemie und Biochemie, Ludwig-Maximilians-Universität München, München, Germany (5) Centre de Physique Théorique, École Polytechnique, CNRS, Palaiseau, France Although vanadium dioxide (VO2) has captured the attention of physicists for several years due to its characteristic metal-insulator transition (MIT) above room temperature, there are at present no data published from angle-resolved photoemission (ARPES) experiments. Indeed, ARPES on VO2 has traditionally been hindered by the quality of cleaved single crystals, and the lack of a clear MIT in low photon energy measurements has even lead to the assumption of a surface region with a different electronic structure. We have grown VO2(001) epitaxial films on TiO2 with the new in situ pulsed-laser-deposition (PLD) system available on the MAESTRO beamline at the Advanced Light Source, and we show that the MIT is clearly visible for photon energies within the UV range. We have been able to measure the band dispersion above and below the transition temperature, and discuss our results in comparison with LDA and cluster-DMFT calculations. In addition, we present the evolution of the electronic structure upon W substitutional doping of the VO2 films, which leads to a peculiar phase diagram (Appl. Phys. Lett. 96, 022102 - 2010) where the transition is quenched and then reappears for increasing W content.

Speaker: Luca Moreschini (Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, USA)

21 ARPES in 1D CDW systems: taking a fresh look at an old problem

M. Grioni1, M. Valbuena1, S. Pons1, C. Tournier-Colletta1, E. Frantzeskakis2, M.C. Asensio2, J. Avila2, L. Moreschini3, S. Moser1,3, A. Bostwick3, E. Rotenberg3, E. Canadell4 1 Institute of Condensed Matter Physics, EPFL, CH-1015 Lausanne (Switzerland) 2 Synchrotron SOLEIL, ANTARES beamline, F-91192 Gif-sur-Yvette, France 3 Advanced Light Source, LBNL, Berkeley, CA 94720 (USA) 4 Institut de Ciència de Materials de Barcelona, UAB, 08193 Bellaterra (Spain) Improved experimental conditions at state-of-the-art synchrotron ARPES facilities enable more accurate measurements that reveal new and interesting aspects of the electronic structure of paradigmatic quasi-1D charge-density wave (CDW) materials. With micro-ARPES (SOLEIL) we could select and measure high-quality hair-like single crystals of NbSe3. By varying the light polarization we observe two sets of linearly dispersive features, previously undetected and not predicted by band structure calculations, suggestive of spin-charge separation in this compound. In (TaSe4)2I an extensive band mapping (at the ALS) clarifies the effect of transverse coupling. The ARPES data indicate sizeable 2D interactions, and an antiphase arrangement of the CDW on neighboring chains. The corresponding wiggled Fermi surface is well nested by a transverse wave vector, supporting the Peierls mechanism of the metal-insulator transition.

Speaker: Marco Grioni (ICMP-EPFL)

22 Broadband microwave study of 2D superconductor-insulator quantum phase transition

Wei Liu, Johns Hopkins University Minsoo Kim, SUNY Buffalo, Dr. G. Sambandamurthy, SUNY Buffalo, Dr. N. Peter Armitage, Johns Hopkins University We incorporated a 8 Tesla magnet into our newly developed Corbino broadband microwave spectrometer, which allows us to perform measurements over a range from 0.21 GHz to 15 GHz at temperatures down to 300 mK. We investigate the complex AC conductance of disordered InO$_x$ films as a function of magnetic field through the 2D superconductor-insulator quantum phase transition. We study the behaviors of the frequency dependent complex response function of a particular InO$_x$ sample near the critical point in the limit of $\hbar \omega < K _{B} T$ and $\hbar \omega > K _{B} T$ and compare our results to theoretical models. We carry out a fully dynamic investigation of how superconductivity is destroyed through the transition. Our data would show evidence for this particular InO$_x$ film whether Bosonic or Fermionic picture of the quantum phase transition could apply.

Speaker: Wei Liu (Johns Hopkins University)

23 Dispersive Kondo resonance and hybridized bands in Yb-based compounds

Masaharu Matsunami, UVSOR Facility Tetsuya Hajiri, UVSOR Facility Hidetoshi Miyazaki, UVSOR Facility Masashi Kosaka, Saitama University Shin-ichi Kimura, UVSOR Facility Heavy-fermion or valence-fluctuation phenomena in strongly-correlated f-electron systems can be derived from the hybridization between conduction band and 4f states (c-f hybridization). Therefore, to directly probe the c-f hybridized electronic structures in the momentum space is important for understanding the heavy-fermion physics. Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for this purpose. In this work, we have performed ARPES for Yb-based compounds such as YbAl2, a prototypical valence fluctuation system, to observe the energy-momentum dispersion curves in the three-dimensional momentum space using a synchrotron radiation. For YbAl2, the c-f hybridized bands and the dispersive Kondo resonance peak have been clearly observed in the APRES spectra. The results are closely associated with the extremely high Kondo temperature (>2000 K). On the basis of these findings, the c-f hybridized electronic structures in Yb compounds will be discussed in comparison with theoretical models such as LDA band calculations and periodic Anderson model.

Speaker: Masaharu Matsunami (UVSOR Facility, Institute for Molecular Science)

24 Dynamic Jahn-Teller effect in the expanded fullerides Cs3C60

Katalin Kamarás, Gyöngyi Klupp and Péter Matus Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box 49, H 1525 Budapest, Hungary Alexey I. Ganin, Alec McLennan and Matthew J. Rosseinsky Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK Yasuhiro Takabayashi, Martin T. McDonald and Kosmas Prassides Department of Chemistry, Durham University, Durham DH1 3LE, UK The mechanism of superconductivity in fulleride superconductors (A3C60, where A is an alkali metal) is being reconsidered from BCS towards another model, similar to cuprates: a phase diagram where an antiferromagnetic Mott insulator is changing into a strongly correlated superconductor. Typical model systems are the so-called expanded fullerides where Mott localization happens because the distance between fulleride ions exceeds a critical value. Two cubic Cs3C60 polymorphs (A15 and fcc)1 are the newest members of this class, with transition temperatures around 35 K at a few kilobars. The normal state of these compounds is low-spin (S=1/2) indicating a spin-pairing mechanism for which the Jahn-Teller-effect was proposed. We will present evidence by infrared spectroscopy for a dynamic Jahn-Teller effect in the insulating state of the expanded fulleride Cs3C60 in the temperature range 28-480 K at ambient pressure. Jahn-Teller distortions of the size ~0.04 Angstroms can be easily detected by vibrational spectroscopy, due to the symmetry lowering of the fullerene balls. The temperature dependence of the spectra can be explained by a molecular effect, the gradual transformation of two temperature-dependent solid-state conformers to a single one, typical and unique for Jahn--Teller systems in solids.2 These results unequivocally establish the relevance of the dynamic Jahn-Teller effect overcoming Hund's rule local exchange interactions, leading to a magnetic Mott-Jahn-Teller insulator.3 1 Y. Takabayashi, A.Y. Ganin, P. Jeglic, D. Arcon, T. Takano, Y. Iwasa, Y. Ohishi, M. Takata, N. Takeshita, K. Prassides, M.J. Rosseinsky: Science 323, 1585 (2009) 2 I.B. Bersuker: “The Jahn-Teller Effect”, Cambridge University Press, 2006 3 M. Capone, M. Fabrizio, C. Castellani, E. Tosatti: Rev. Mod. Phys. 81, 943 (2009)

Speaker: Katalin Kamaras (Wigner Research Centre for Physics, Hungarian Academy of Sciences)

25 Electronic and optical properties of LaAlO3/SrTiO3 superlattices: interface charge density, Fermi surfaces, and optical reflectivity spectra

Se Young Park and Andrew J. Millis, Columbia University We present calculations of the charge density profile, Fermi surface topology, and reflectivity spectra of the electron gas at the LaAlO3/SrTiO3 interface. The calculations are based on a self-consistent Hartree/RPA approximation and a tight binding parametrization of the band structure. The subband occupancy is studied as a function of polar discontinuity and dielectric constants. The number of occupied xy bands changes significantly whereas for all reasonable dielectric constant profiles only one yz and one xz bands are occupied for polar discontinuity larger than a critical value depending on dielectric constant profiles. These yz and xz band give dominant contributions to the long-distance tail of the interface charge. The optical reflectance and ellipsometry angle are calculated including optical phonon of SrTiO3. The results are compared with experimental data. Support: DOE ER-046169

Speaker: Se Young Park (Columbia University)

26 Gate tunable real-space plasmons in graphene revealed by infrared nano-imaging

Z. Fei(1), A. S. Rodin(1), G. O. Andreev(1), W. Bao(2,3), A. S. McLeod(1), M. Wagner(1), L. M. Zhang(4), Zeng Zhao(3), G. Dominguez(6), M. Thiemens(5), M. M. Fogler(1), A. H. Castro-Neto(7), C. N. Lau(3), F. Keilmann(8), D. N. Basov(1) (1) Department of Physics, University of California, San Diego (2) Materials Research Science and Engineering Center, University of Maryland, College Park (3) Department of Physics and Astronomy, University of California, Riverside (4) Department of Physics, Boston University (5) Department of Chemistry and Biochemistry, University of California, San Diego, (6) Department of Physics, California State University, San Marcos (7) Graphene Research Centre and Department of Physics, National University of Singapore (8) Max Planck Institute of Quantum Optics and Center for Nanoscience We report infrared nano-imaging of plasmonic fringe patterns in graphene at mid-infrared (IR) frequency range. By tuning the gate voltage, we were able to systematically change the carrier density of graphene while monitoring the evolution of these fringe patterns. We ascribe these fringe patterns to the interference of plasmon waves launched by the near-field probe with those reflected from the edges, which is further verified by our electrostatic simulation. Plasmon dissipation quantified through our modeling and analysis of the fringe patterns is linked to the exotic electrodynamics of graphene. Standard plasmonic figures of merits of our tunable graphene devices surpass that of common metal-based structures.

Speaker: Zhe Fei (Department of Physics, University of California, San Diego)

27 High THz field induced dynamics of Sr14Cu24O41

Elsa Abreu, Department of Physics, Boston University, USA Ravi Singh, Department of Physics, University of Warwick, UK Verner K. Thorsmolle, Department of Condensed Matter Physics, University of Geneva, Switzerland Geetha Balakrishnan, Department of Physics, University of Warwick, UK Richard D. Averitt, Department of Physics, Boston University, USA Dynamical studies of far infrared excitations in materials have become possible following the recent development of techniques that enable the generation of terahertz electromagnetic radiation with high electric field values [1]. This energy range is extremely relevant for the study of many charge density wave compounds, which exhibit low frequency excitations associated with a collective response due to pinning. Sr14Cu24O41 is a quasi one-dimensional material whose structure consists of alternating layers of Cu2O3 chains and CuO2 ladders [2]. Both substructures become charge ordered below ~200K. Understanding the dynamics of Sr14Cu24O41 excitations in the far infrared has the potential not only to shed light onto the complex nature of charge ordering in this material, but also to help provide a better understanding of the nature of superconducting behavior in two-dimensional high temperature superconducting cuprates. In our work, high field terahertz pulses are used to investigate the static and dynamic properties of single crystal Sr14Cu24O41 samples grown by the traveling solvent floating zone method. We will present preliminary results of a field and time dependent terahertz study of Sr14Cu24O41. [1] P. U. Jepsen et al., Laser Phot Rev 5, 124 (2011) [2] B. Gorshunov et al., Phys Rev B 66, 060508 (2002)

Speaker: Elsa Abreu (Department of Physics, Boston University)

28 Hybridization gap and anisotropic far-infrared optical conductivity of URu2Si2

J. Levallois (1), F. Lévy-Bertrand (1,2), M.K. Tran (1), D. Stricker (1), J.A. Mydosh (3), Y.-K. Huang (4) and D. van der Marel (1) 1) Département de Physique de la Matière Condensée, Université de Genève, CH-1211 Genève 4, Switzerland 2) Institut Néel, CNRS et Université Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France 3) Kamerlingh Onnes Laboratory, Leiden University, 2300RA Leiden, The Netherlands 4) Van der Waals-Zeeman Institute, University of Amsterdam, 1018XE Amsterdam, The Netherlands We present far-infrared reflectivity measurements on URu2Si2 as a function of temperature along the a-axis and the c-axis of the tetragonal structure. Our results demonstrate that in addition to a pronounced anisotropy, a partial gap emerges at ∼ 30 K, well above the hidden order temperature T_HO and whose amplitude seems to be T-independent and not affected by the hidden order transition. As this suppression of spectral weight coincides with the development, at the same energy, of a maximum in the scattering rate, indicating optical interband transitions, we propose that this partial gap is a hybridization gap and that this change in the bandstructure below 30 K is a precursor of the hidden order state. However, since these changes have no noticeable impact on the entropy nor on the DC transport properties, we suggest that this is a crossover phenomenon rather than a thermodynamic phase transition.

Speaker: Julien Levallois (DPMC - University of Geneva)

29 Imaging the Electric Breakdown in V2O3 at the Nanoscale

S. Guénon (1) S. Scharinger (2) S. Wang (1) J. G. Ramirez (1) D. Koelle (2) R. Kleiner (2) Ivan K. Schuller (1) (1) Department of Physics and Center for Advanced Nanoscience, University of California San Diego (2 ) Physikalisches Institut–Center for Collective Quantum Phenomena and their Applications in LISA+, Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen, Germany We have measured the electric properties of a 200 micron wide and 10 micron long bridge in a 100 nm thick V2O3 thin film at the metal-insulator transition. Discontinuous jumps to lower voltages in the current voltage characteristic (IV) followed by a more or less constant voltage progression of the curve for high currents indicate an electric breakdown of the device. In addition, the IV curve shows hysteresis and a training effect, i.e. the succeeding IV loops are different from the first IV loop of the thermal cycled device. Low temperature scanning electron microscopy indicates that the electric breakdown is caused by a combination of several mechanism like self-heating as well as percolative and bolt-like switching. This work has been supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences under Award FG03-87ER-45332.

Speaker: Stefan Guénon (Department of Physics and Center for Advanced Nanoscience, University of California San Diego)

30 In-plane electronic anisotropy in the optical spectra of Ba(Fe1-xCox)2As2

M. Nakajima1,2,3, S. Ishida1,2,3, K. Kihou2,3, C. H. Lee2,3, A. Iyo2,3, H. Eisaki2,3, T. Kakeshita1,3, and S. Uchida1,3 1Department of Physics, University of Tokyo 2National Institute of Advanced Industrial Science and Technology 3JST, Transformative Research-Project on Iron Pnictides The magnetostructural phase in undoped and underdoped iron-arsenide superconductors attracts much interest as the proximate phase to the superconducting phase. In this phase, both the crystal structure and the spin arrangement break the fourfold rotational symmetry, and various experimental techniques have successfully probed the in-plane anisotropy. We measured the optical spectrum of detwinned Ba(Fe1-xCox)2As2 (x=0, 0.02, and 0.04) using the polarized light along the crystallographic a and b axis in the orthorhombic phase. In the present work, the annealed crystals of high quality were used [1]. For x=0, in the low-temperature orthorhombic-antiferromagnetic phase, the isotropic Drude term dominates the low-energy optical conductivity spectrum, consistent with the resistivity data. Intrinsic anisotropy shows up in the higher-energy region [2]. With Co doping, the anisotropy in the high-energy region weakens, whereas the width of the Drude component contributing to the dc conductivity becomes anisotropic. This anisotropy well explains the anisotropy in the resistivity. Our results show that an anisotropic scattering rate gives rise to the dc conductivity anisotropy, suggesting that the dopant Co atom works as an anisotropic scattering center. [1] S. Ishida et al., Phys. Rev. B 84, 184514 (2011). [2] M. Nakajima et al., Proc. Natl. Acad. Sci. U.S.A. 108, 12238 (2011).

Speaker: Masamichi NAKAJIMA (Department of Physics, University of Tokyo)

31 Infrared probe of p-doped GaAs in the ferromagnetic semiconductor GaMnAs and non-magnetic system GaBeAs

B. C. Chapler, University of California San Diego S. Mack, University of California Santa Barbara R. C. Myers, The Ohio State University L. Ju, University of California Berkeley B. W. Boudouris, University of California Berkeley R. A. Segalman, University of California Berkeley K. S. Burch, University of Toronto N. Samarth, The Pennsylvania State University F. Wang, University of California Berkeley D. D. Awschalom, University of California Santa Barbara D. N Basov, University of California San Diego The III-Mn-V ferromagnetic semiconductor GaMnAs has emerged has an ideal test bed for prototype spintronic devices and effects, as it has the most ideal suite of properties for spintronics applications. Substitutional Mn in a GaAs host acts as a single acceptor, and additionally contributes a local magnetic moment, thus understanding the resultant the interplay between the electronic, magnetic, and optical properties of GaMnAs presents an enticing challenge in this unique material. Here we report on extensive studies exploring the electromagnetic response of GaMnAs at energy scales of several meV to the order of the fundamental band gap of the GaAs host (1.5 eV). In these studies, we use several techniques for tuning the hole concentration in our samples. These techniques include, tuning the chemical doping concentration, post-growth annealing to remove compensating Mn-interstitials, designing a spatial gradient of compensating As-antisite defects, as well as charge accumulation and depletion via electric field effect. We contrast these results with similar studies of the non-magnetic p-doped system GaBeAs, which is an ideal system to isolate affects attributable to the presence of magnetism in GaMnAs. Through these detailed studies and comparisons, we highlight the unconventional nature of the insulator-to-metal transition and conducting state of the enigmatic magnetic system, GaMnAs.

Speaker: Brian Chapler (University of California San Diego)

32 Infrared signatures of ambipolar injection in narrow gap Donor-Acceptor polymer transistors

Omar Khatib (UCSD), Jonathan D. Yuen (Univ. of Waterloo), Jim Wilson (UCSD), Rajeev Kumar (Nanoterra), Fred Wudl (UCSB), Max Di Ventra (UCSD), Alan J. Heeger (UCSB), Dimitri N. Basov (UCSD) Donor-Acceptor (D-A) copolymers have recently emerged as versatile materials for use in a large variety of device applications. Specifically, these systems possess extremely narrow bandgaps, enabling ambipolar charge transport when integrated in solution-processed field-effect transistors (OFETs). However, the fundamentals of electronic transport in this class of materials remain unexplored. We present a systematic investigation of ambipolar charge injection in D-A conjugated polymers polybenzobisthiadiazole-dithienopyrrole (PBBTPD) and polybenzobisthiadiazole-dithienocyclopentane (PBBTCD) using infrared spectroscopy. We observed a significant modification of the absorption edge in both PBBTPD- and PBBTCD-based OFETs under the applied electric field. The absorption edge reveals hardening under electron injection and softening under hole injection. Additionally, we registered localized vibrational resonances associated with charge injection. These findings indicate that several physical processes likely contribute to the field-induced changes in IR transmission spectra, including possibly electron- and hole-induced polaron absorption, and/or electric field-induced modulation of the polymer band edge (the linear Stark effect). Requisite for the latter effect is the existence of a built-in electrical dipole moment, as well as quasi-ordering of the polymer chains near the semiconductor/insulator interface. Additionally, we carried out microscopic IR measurements to characterize the ambipolar injection profile between electrodes.

Speaker: Omar Khatib (UCSD)

33 Infrared study of carrier scattering in graphene field effect device 2h0'

Joo Youn Kim (University of Seoul, Seoul 130-743, Republic of Korea) Kwangnam Yu (University of Seoul, Seoul 130-743, Republic of Korea) Chul Lee (University of Seoul, Seoul 130-743, Republic of Korea) Sukang Bae (Sungkyunkwan University, Suwon 440-746, Republic of Korea) Sang Jin Kim (Sungkyunkwan University, Suwon 440-746, Republic of Korea) Keun Soo Kim (Sejong University, Graphene Research Institute, Seoul 143-747, Republic of Korea) Byung Hee Hong (Seoul National University, Seoul 151-742, Republic of Korea) E. J. Choi (University of Seoul, Seoul 130-743, Republic of Korea) We determined carrier scattering rate (Γ) of grapheme from Far-IR transmission measurement on CVD-graphene/SiO2/p-Si field effect device. As carrier density (n) is varied by applying the gate voltage exhibits distinct n-dependent change which is represented by two polynomial scatterings as Γ(n) = A/n + B∙√n. The A/n-scattering and B∙√n-scattering plays dominant role in the low-n and high-n regime respectively, whereas they have equal strength at n = nc = 2×10^12 cm-2 . We calculated dc-conductivity (σ0(n)) from Γ(n) finding that σ0(n) exhibits the linear-to-sublinear crossover at n = nc due to that Γ(n) switches from A/n to B∙√n at this density. It accounts for the sub-linear behavior of I-V curve, long-standing puzzle in graphene physics. We discuss possible origin of the A/n and B∙√n scattering in terms of the charged-impurity, phonon, and short-range adatom scattering.

Speaker: E.J. Choi (Univ. of Seoul, Korea)

34 Infrared study of tunable magnetoplasmons in graphene

Hugen Yan (IBM T. J. Watson Research Center) Zhiqiang Li (National High Magnetic Field Laboratory) Wenjuan Zhu (IBM T. J. Watson Research Center) Phaedon Avouris (IBM T. J. Watson Research Center) Fengnian Xia (IBM T. J. Watson Research Center) The Dirac plasmons in graphene are of great fundamental and practical interest due to their very unique properties. We present infrared spectroscopy of 2D plasmon excitations in graphene in high magnetic fields. The plasmon resonance in patterned graphene disks splits into edge and bulk plasmon modes in magnetic fields. Remarkably, the edge plasmons develop increasingly long lifetimes in high fields due to the suppression of back-scattering. The characteristics of the magnetoplasmons, such as their resonance frequency and splitting rate in field, can be well controlled by the dimensions of the patterned structure and chemical doping. This work demonstrates the intriguing physics of magnetoplasmons in graphene as well as their great potential for tunable terahertz magneto-optical devices.

Speaker: Zhiqiang Li (National High Magnetic Field Laboratory)

35 Infrared study on the electronic structure of metallic pyrochlore Bi2Ir2O7

Y. S. Lee1,2, S. J. Moon1, Scott C. Riggs3, M. C. Shapiro3, I. R. Fisher3, and D. N. Basov 1 University of California at San Diego, La Jolla, CA 92093 2 Soongsil University, Seoul 156-743, Korea 3 Stanford University, Stanford, CA 94305 We investigated the electronic structure of a metallic pyrochlore Bi2Ir2O7 by using infrared spectroscopy. Consistent with a metallic response in transport, the optical conductivity spectra of this compound exhibit a Drude component which is rather narrow. Two interband transitions are identified near 0.2 and 1.0 eV, which are assigned as the d-d transitions related to Jeff,1/2 and Jeff, 3/2 bands, respectively. The temperature dependence in optical spectra is found to be weak. The electronic structure of Bi2Ir2O7 is well-described with the significance of spin-orbit coupling in 5d oxides.

Speaker: Yunsang Lee (UCSD/Soongsil University)

36 Low energy electrodynamics of the Kondo-lattice antiferromagnet CeCu$_2$Ge$_2$

Grace Bosse, The Institute for Quantum Matter, Department of Physics and Astronomy, Johns Hopkins University L. Bilbro, The Institute for Quantum Matter, Department of Physics and Astronomy, Johns Hopkins University R. Valdes Aguilar, The Institute for Quantum Matter, Department of Physics and Astronomy, Johns Hopkins University L. Pan, The Institute for Quantum Matter, Department of Physics and Astronomy, Johns Hopkins University W. Liu, The Institute for Quantum Matter, Department of Physics and Astronomy, Johns Hopkins University A.V. Stier, The Institute for Quantum Matter, Department of Physics and Astronomy, Johns Hopkins University Y. Li, Department of Physics and Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign J. Eckstein, Department of Physics and Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign N.P. Armitage, The Institute for Quantum Matter, Department of Physics and Astronomy, Johns Hopkins University We present time-domain THz spectroscopy data of a thin film of the heavy fermion compound CeCu$_2$Ge$_2$. Measurements to obtain the frequency dependent complex conductivity were taken as a function of temperature down to temperatures below the onset of magnetic order. At low temperatures a narrow Drude-like peak forms, which is likely associated with the heavy fermion or spin density wave state. Using this data in conjunction with DC resistivity measurements and Fourier transformed infrared reflectivity data, we obtain the frequency dependence of the scattering rate and the mass renormalization through an extended Drude model analysis. At lowest temperatures, an effective mass of almost 80 times the band mass is observed, which is consistent with reported specific heat measurements. The persistence and further development of the heavy fermion state well into the antiferromagnetic state, which challenges the conventional understanding of the low temperature competition between Kondo screening and the Ruderman-Kittel-Kasuya-Yosida interaction, will be discussed.

Speaker: Grace Bosse (The Institute for Quantum Matter, Department of Physics and Astronomy, Johns Hopkins University)

37 Low energy electronic excitations and magneto-phonon resonance in graphite and graphene

Y. Kim1, J. M. Poumirol1, Y. Ma2, A.Imambekov2, A. Lombardo3, N. G. Kalugin4, J. Kono5, T. Georgiou6, A. K. Geim6, K. S. Novoselov6, A. C. Ferrari3, and D. Smirnov1 1 National High Magnetic Field Laboratory, Tallahassee, FL 32310, USA 2 Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA 3 Engineering Department, Cambridge University, Cambridge, CB3 0FA, UK 4 Department of Materials and Metallurgical Engineering, New Mexico Tech, Socorro, NM 87801, USA 5 Department of Electrical & Computer Engineering, Rice University, Houston, TX 77005, USA 6 School of Physics & Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK Recently, much attention has been paid to electron-phonon coupling in graphene. The zone-centre, doubly degenerate E2g phonon, strongly interacts with electrons, resulting in renormalization of phonon frequencies and line broadening. These phenomena are predicted to be tunable by electric and magnetic fields, through Fermi-energy shifts and Landau quantization, respectively. In particular, the Raman G peak is predicted to exhibit magneto-phonon resonance manifested as strong anti-crossings when the E2g phonon energy matches the separation of two Landau levels (LLs) [1,2]. Here, we report high-field magneto-Raman measurements of graphene and graphite in magnetic fields up to 45 T. In single-layer graphene, the Raman G peak exhibits clear splitting at approximately 27 T, which we attribute to the fundamental magneto-phonon resonance (MPR) associated with (0,1) inter-LL transitions. The coupled electron-phonon modes demonstrate characteristic anti-crossing behavior allowing for an accurate determination of the electron-phonon coupling strength in graphene [3]. Circularly polarized Raman scattering measurements allows revealing unique polarization- and filling-factor dependence of MPR in graphene, predicted in Ref.[2]. Graphene’s parent compound, graphite is expected to exhibit even richer carrier-phonon coupling phenomena. Graphite, a bulk semimetal containing both electrons and holes even at zero temperature, has a linear (“massless") dispersion for the hole pocket around the H-point of the Brillouin Zone and a parabolic (“massive") dispersion for the electron pocket around the K-point. We demonstrate a complex picture of MPR effects caused by coupling of the E2g phonon to both H-point (SLG-like) and K-point (BLG-like) inter-LL excitations and extract the strengths of electron-phonon coupling [4]. References: 1. T. Ando, J. Phys. Soc. Jpn. 76, 024712 (2007). 2. M. O. Goerbig, J.-N. Fuchs, K. Kechedzhi, and V. I. Falko, Phys. Rev. Lett. 99, 087402 (2007). 3. Y. Kim et al, 2012 APS March Meeting (American Physical Society, Boston, 2012). 4. Y. Kim et al, Phys. Rev B Rapid Comm. (accepted 2012) , arXiv:1112.3884.

Speaker: Dmitry Smirnov (National High Magnetic Field Laboratory)

38 Low-frequency optics on Sr_(1−x)Ca_(x)RuO_(3)

Marc Scheffler, 1. Physikalisches Institut, Universität Stuttgart, 70550 Stuttgart, Germany Diana Geiger, 1. Physikalisches Institut, Universität Stuttgart, 70550 Stuttgart, Germany Martin Dressel, 1. Physikalisches Institut, Universität Stuttgart, 70550 Stuttgart, Germany Melanie Schneider, I. Physikalisches Institut, Georg-August-Universität Göttingen, 37077 Göttingen, Germany Philipp Gegenwart, I. Physikalisches Institut, Georg-August-Universität Göttingen, 37077 Göttingen, Germany The pseudo-cubic perovskite ruthenates SrRuO_(3) and CaRuO_(3) have recently attracted interest due to their unconventional electronic properties. For both materials, non-Fermi liquid behavior has been reported in previous optical studies at infrared frequencies. In addition to these two pure compounds, the doping series Sr_(1−x)Ca_(x)RuO_(3) offers a rich phase diagram: going from the itinerant ferromagnet SrRuO_(3) to the paramagnet CaRuO_(3), there are indications for a quantum phase transition at x approximately 0.8. Using low-frequency optical spectroscopy, we have studied Sr_(1−x)Ca_(x)RuO_(3) thin-film samples of very high quality, which were prepared by metalorganic aerosol deposition. In order to be sensitive to the small energy scales expected close to the quantum phase transition, we address the THz and GHz frequency ranges at temperatures down to 2 K. We present optical data, in particular the frequency-dependent conductivity, and discuss it in the framework of the extended Drude model with frequency-dependent relaxation rates and effective masses. While for pure SrRuO_(3) as well as for doped systems (approaching the quantum phase transition) we find conventional metallic Drude behavior at frequencies below 1 THz, CaRuO_(3) exhibits highly unusual optical properties: the relaxation rate increases strongly with frequency. We compare the optical data to dc measurements and discuss them with respect to the temperature ranges with non-Fermi liquid behavior.

Speaker: Marc Scheffler (Universität Stuttgart, Germany)

39 Magnetic field induced color change in α-Fe2O3 single crystals

P. Chen,1 N. Lee,2 S. McGill,3 S. -W. Cheong,2 and J. L. Musfeldt1 1 Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA 2 Rutgers Center for Emergent Materials and Department of Physics & Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA 3 National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA We investigated the magneto-optical properties of α-Fe2O3 in order to understand the interplay between charge and magnetism in a model transition metal oxide. We discovered that hematite appears more red in applied magnetic field than in zero field conditions, an effect that is amplified by the presence of the spin flop transition. Analysis of the exciton pattern on the edge of the d-d color band reveals C2/c monoclinic symmetry in the high field phase. These findings advance our understanding of magnetoelectric coupling away from the static limit and motivate spectroscopic work on other iron-based materials under extreme conditions.

Speaker: Peng Chen (Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA)

40 Measuring Collective CDW Excitations throughout momentum space using XUV ARPES

S. Kaiser 1, J.C. Petersen 1,2, A. Simoncig 1, H.Y. Liu 1, N. Dean 1,2 ,A.L. Cavalieri 1, C. Cacho 3, I.C.E. Turcu 3, E. Springate 3, F. Frassetto 4, L. Poletto 4, S. S. Dhesi 5, H.Berger 6, T. Cuk 7, and A. Cavalleri 1,2 1 Max Planck Dept. for Structural Dynamics, Centre for Free-Electron Laser Science, University of Hamburg, Hamburg, Germany 2 Clarendon Laboratory, Oxford University, Parks Road, Oxford, UK 3 Central Laser Facility, STFC Rutherford Appleton Laboratory, Harwell, United Kingdom 4 LUXOR, CNR-INFM, Padova, Italy 5 Diamond Light Source Ltd., Harwell, United Kingdom 6 Institute of Physics of Complex Matter, EPFL, Lausanne, Switzerland 7 UC Berkeley, Berkeley, USA We use time- and angle-resolved photoemission spectroscopy with sub-30-fs XUV pulses to map the time- and momentum-dependent electronic structure of photoexcited 1T–TaS2. This compound is a two-dimensional Mott insulator with charge-density wave (CDW) ordering. The ultrafast time-resolution and wide angular acceptance of the experiment allow us to clock the melting dynamics of the CDW state and decompose its charge and lattice order throughout momentum space. Upon charge transfer photo-excitation, or photo-doping, not only the Mott gap at the Fermi level but also the CDW gaps at finite binding energy melt on sub-vibrational timescales: Charge order, evidenced by splitting between occupied sub-bands at the Brillouin zone boundary melt. Surprisingly, this occurs well before the lattice responds. This indicates that the CDW gaps close in response to a redistribution of charge, rather than a repositioning of atom. Subsequently, spectral intensity migrates from shadow bands in the first zone back out to higher momentum, revealing the unfolding of the Brillouin-zone that accompanies incipient lattice relaxation along the coordinate of the Raman active amplitude mode following photo-doping. At longer timescales, charge order and lattice distortions lock again, displaying a modulation of spectral weight at the frequency of the amplitude mode. In blue bronze (K0.3MoO3), a prototypical 1D-CDW system, we investigate the collective response of a melting CDW wave. We find the excitation of both amplitude and phason response are excited coherently, making it possible to reconstruct the coherent dynamics of a complex order parameter and to connect it to the complex band structure. These experiments show the power of time- and angle resolved photoemission experiments performed with XUV radiation, a significant technical improvement compared to the more common use of 6-7 eV radiation.

Speaker: Stefan Kaiser (Max Planck Department for Structural Dynamics, CFEL, U Hamburg)

41 Na2IrO3 by ARPES: a spin-orbit-induced band insulator.

R. Comin,1 G. Levy,1 B. Ludbrook,1 Z.-H. Zhu,1 C. Veenstra,1 J. Rosen,1 Y. Singh,2 P. Gegenwart,3 D. Stricker,4 J. Hancock,4 D. van der Marel,4 I. Elfimov,1, 5 and A. Damascelli1, 5 1 Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada 2 Indian Institute of Science Education and Research (IISER) Mohali, Knowledge city, Sector 81, Mohali 140306, India 3 I. Physikalisches Institut, Georg-August-Universitat Gottingen, D-37077 Gottingen, Germany 4 Departement de Physique de la Matiere Condensee, Universite de Geneve, quai Ernest-Ansermet 24, CH 1211 Geneve 4, Switzerland 5 Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada We study the low-energy electronic structure of the insulating compound Na2IrO3 by angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). Since in DFT calculations within the local density approximation (LDA) this system is metallic, yet with very narrow bands, it has been proposed that Na2IrO3 might be the realization of a Mott insulator in a 5d transition metal oxide. The ARPES experiments were performed both on the pristine cleaved surfaces and as a function of in-situ carrier doping via potassium deposition. From the K-induced chemical potential shift and the emergence of spectral weight across the insulating gap, as well as complementary optical conductivity experiments, we have been able to obtain a precise quantitative estimate of the optical gap Delta=350 meV. By performing detailed LDA calculations, accounting also for spin-orbit interaction (SO) and Coulomb repulsion U, we find that while the correct gap magnitude can only be reproduced in LDA+SO+U calculations, a clear gap is already open everywhere in momentum space simply with the inclusion of spin-orbit (LDA+SO). This establishes Na2IrO3 as a spin-orbit-induced band insulator.

Speaker: Riccardo Comin (University of British Columbia)

42 Near-field spectroscopy of optically doped graphene

M. Wagner(1), Z. Fei(1), A. S. McLeod(1), A. S. Rodin(1), W. Bao(2,3), L. M. Zhang(4), Zeng Zhao(3), M. Thiemens(5), M. M. Fogler(1), A. H. Castro-Neto(6), C. N. Lau(3), F. Keilmann(7), D. N. Basov(1) 1-Department of Physics, University of California San Diego, La Jolla, California 92093, USA 2-Materials Research Science and Engineering Center, University of Maryland, College Park, Maryland 20742, USA 3-Department of Physics and Astronomy, University of California, Riverside, California 92521, USA 4-Department of Physics, Boston University, Boston, Massachusetts 02215, USA 5-Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, USA 6-Graphene Research Centre and Department of Physics, National University of Singapore, 117542, Singapore 7-Max Planck Institute of Quantum Optics and Center for Nanoscience, 85714 Garching, Germany Scattering near-field infrared spectroscopy with its spatial resolution in the 20 nm region is an ideal spectroscopic tool to study graphene. Our previous work has revealed important insight in the SiO2 substrate graphene interaction and showed an enhancement and blueshift of the SiO2 surface phonon resonance in the mid-infrared spectral region due to plasmon phonon coupling [1]. Additionally, pronounced plasmonic effects occurred in the form of standing wave patterns near interfaces and boundaries in single- and bilayer graphene for ungated and gated samples [2]. Here we extend our previous studies and report on optically induced effects for single- and bilayer graphene on SiO2 substrates. 100 fs near-infrared laser pulses are used to change the carrier density of graphene via optical pumping before probing the sample’s mid-infrared scattering near-field response. We analyze the effects on the hybrid plasmon-phonon system with respect to the pump power and time-delay between pump and probe pulses. [1] Z. Fei et al., Nano Lett. 11, 4701 (2011). [2] Z. Fei et al. arXiv:1202.4993

Speaker: Martin Wagner (UCSD)

43 Optical Properties of (SrMnO3)n/(LaMnO3)2n Superlattices: An Insulator-to-Metal Transition Observed in the Absence of Disorder

A. Perucchi1, L. Baldassarre1, A. Nucara2, P. Calvani2, C. Adamo3, D.G. Schlom3, P. Orgiani4, L. Maritato4, S. Lupi5 1) Sincrotrone Trieste, Area Science Park, I-34012 Trieste, Italy 2) CNR-SPIN and Dipartimento di Fisica, Università Sapienza, P.le A. Moro 2, I-00185 Rome, Italy 3) Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, USA 4) CNR-SPIN and Dipartimento di Matematica ed Informatica, Università di Salerno, Baronissi, I-84081 Salerno, Italy 5) CNR-IOM and Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 2, I-00185 Rome, Italy We measure the optical conductivity, σ1(ω), of (SrMnO3)n/(LaMnO3)2n superlattices (SL) for n=1, 3, 5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator to metal transition (IMT) for n=3, driven by the softening of a polaronic mid-infrared band. At n=5 that softening is incomplete, while at the largest-period n=8 compound the MIR band is independent of T and the SL remains insulating. One can thus first observe the IMT in a Manganite system in the absence of the disorder due to chemical doping. Unsuccessful reconstruction of the SL optical properties from those of the original bulk materials suggests that (SrMnO3)n/ (LaMnO3)2n heterostructures give rise to a novel electronic state.

Speaker: Andrea Perucchi (ELETTRA - Sincrotrone Trieste)

44 Orbital fluctuations and orbital order below the Jahn-Teller transition in Sr3Cr2O8

Zhe Wang, Joachim Deisenhofer, Michael Schmidt, Franz Mayr, Hans-Abrecht Krug von Nidda, and Alois Loidl Experimental Physics V, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany Yuan Wan Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA Diana Lucia Quintero-Castro, A. T. M. Nazmul Islam, and Bella Lake Helmholtz-Zentrum Berlin für Materialien und Energie, D-14109 Berlin, Germany We report on the magnetic-, phononic-, and crystal-field-excitation spectrum of a spin-gapped system Sr3Cr2O8 determined by Terahertz and infrared (IR) spectroscopy across the Jahn-Teller (JT) transition at T_JT = 285 K. We identify the spin singlet-triplet excitations in the dimerized ground state and reveal the corresponding selection rules, which rely on an inter-dimer Dzyaloschinskii-Moriya (DM) interaction with DM vector parallel with crystalline a-axis. The temperature-dependent feature of magnetic and phononic excitations supports the existence of strong orbital fluctuation in an extended temperature regime T* < T < T_JT with T* ~120K, in agreement with the results from Raman spectroscopy and electron spin resonance (ESR) measurements.[1,2] The strong fluctuation regime results from the competition between spin-orbital interaction and JT interaction, since, in contrary to the effect of JT distortion, spin-orbital interaction stabilizes the chromium orbital of d_{x^2-y^2} with respect to d_{z^2}. The excitation corresponding to the split of the orbitals due to spin-orbital interaction is observed from T>T_JT down to T*, whose energy is around 3 meV consistent with the estimation based on the results of ESR and crystal-field-excitation spectrum. Below T*, JT interaction is dominant that the ordering of lower-lying d_{z^2} orbital is achieved. [1] Zhe Wang et al., Phys. Rev. B 83 201102 (2011) [2] D. Wulferding et al., Phys. Rev. B 84, 064419 (2011)

Speaker: Zhe Wang (EP 5, EKM, Institute for Physics, Augsburg University, Augsburg, Germany)

45 Plasmarons and phonons in finite momentun optical conductivity of graphene

J.P. Carbotte, McMaster University J.P.F. LeBlanc, University of Guelph E.J. Nicol, University of Guelph Mid infrared nanoscopy has recently been successfully applied to graphene. In principle, this technique could be used to obtain some information on the finite momentum q optical conductivity sigma(q,omega) as a function of energy omega. Consequently, this novel experimental probe could allow one to examine regions of momentum space (k) of the Dirac fermions in graphene which were not previously accessible to q=0 optics. With this in mind, we calculate sigma(q,omega) for graphene in the presence of many-body renormalizations. We also demonstrate how sigma(q,omega) could be used to image parts of the renormalized charge carrier dispersion curves. In particular, we discuss how electron-electron interactions as well as electron-phonon interactions present themselves in sigma(q,omega). For instance, if the region near the Dirac point at k=0 is probed with a q of order the Fermi momentum kF, structure is revealed which is due to plasmarons. These are collective modes of an electron plus a plasmon which have recently been seen in ARPES experiments on graphene. Here, we calculate that plasmarons will provide a visible signature in sigma(q,omega) even for probing q < kF/2.

Speaker: Jules Carbotte (McMaster University)

46 Quantum Hall effect from 2D surface states of the 3D topological insulator HgTe

Andreas V. Stier, Department of Physics and Astronomy, The Johns Hopkins University Liang Wu, Department of Physics and Astronomy, The Johns Hopkins University Christopher Morris, Department of Physics and Astronomy,The Johns Hopkins University Rolando Valdes-Aguilar,Department of Physics and Astronomy,The Johns Hopkins University Tony Almeida, US Army NVESD Alexey Suslov, National High Magnetic Field Laboratory N. Peter Armitage, Department of Physics and Astronomy, The Johns Hopkins University Three dimensional (3D) topological insulators (TI) exhibit two dimensional (2D) topologically protected conducting surface states created by strong spin-orbit coupling. Those states show a dispersion relation of massless Dirac fermions and exhibit spin-momentum locking. We present our results on high field (31T) magneto-transport experiments of the 3D topological insulator HgTe. In-plane tensile (tetragonal) strain exerted on the MBE grown 70nm thin film HgTe samples from the zinc doped CdTe substrate opens a small gap (~20 meV) and therefore lifts the band degeneracy in the center of the Brillouin zone. We study samples grown on two different crystallographic directions of the substrate material and compare the effect of varying crystallographic direction on the transport result. We observe evidence for quantized Hall (QH) resistance in both samples developing at cryogenic temperatures. We confirm the 2D character of the probed states through tilted magnetic field measurements. The observed effect is also confirmed to derive from Dirac fermions of the two TI surfaces as shown through a non-zero Berry's phase by an extrapolation of the filling factors of the QH plateaus to the large magnetic field limit. DC magneto-transport experiments are compared to zero field temperature dependent THz time domain spectroscopy data. Work at JHU was supported by the Gordon and Betty Moore foundation. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-0654118, the State of Florida, and the U.S. Department of Energy.

Speaker: Andreas V. Stier (Johns Hopkins University)

47 Reconfigurable gradient index using VO2 memory metamaterials

M. D. Goldflam (UCSD), T. Driscoll (UCSD, Duke), B. Chapler (UCSD), O. Khatib (UCSD), N. Marie Jokerst (Duke), S. Palit (Duke), D. R. Smith (Duke), B.-J. Kim (ETRI), G. Seo (UST), H.-T. Kim (ETRI, UST), M. D. Ventra (UCSD), and D. N. Basov (UCSD) We have demonstrated tuning of a hybrid metamaterial device that incorporates a form of spatial gradient control. Hybrid metamaterials are constructed through incorporation various non-metallic materials into standard metamaterials structures. Electrical tuning of the metamaterial was achieved through a vanadium dioxide (VO2) layer which interacts with an array of lithographically fabricated gold split ring resonators (SRR). Through design of the device and contact geometry, we achieved a spatial gradient in the magnitude of permittivity, writeable using a single transient electrical pulse which creates dissipative heating in the device. This induced gradient in our device was observed on spatial scales on the order of one wavelength at 1 THz. Additionally, gradients created in such hybrid VO2-SRR devices are persistent, remaining even after the applied voltage has been removed, due to the hysteresis present in the insulator to metal transition of VO2. Thus we have demonstrated the viability of elements for use in future devices with potential applications in beamforming and communications [1]. To continue forward with this research, various contact geometries are currently being investigated in hopes of gaining improved control over gradients and expanding on the possible applications of such devices. Transmission measurements of the full two dimensional spatial extent of the gradients created in these new devices are underway. [1] M. D. Goldflam, T. Driscoll, B. Chapler, O. Khatib, N. Marie Jokerst, S. Palit, D. R. Smith, B.-J. Kim, G. Seo, H.-T. Kim, M. D. Ventra, and D. N. Basov, Applied Physics Letters 99, 044103 (2011).

Speaker: Michael Goldflam (University of California, San Diego)

48 Sliding Phononic Gaps in the Charged Incommensurate Planes of Sr14Cu24O41

Adrian Gozar Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA Christopher Homes Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA Girsh Blumberg Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA Verner Thorsmolle Laboratory for Quantum Magnetism, Ecole Polytechnique Federale de Lausanne (EPFL), CH-101 Switzerland Henrik Ronnow Laboratory for Quantum Magnetism, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Switzerland The terahertz (THz) excitations in the quantum spin-ladder system Sr14Cu24O41 have been determined along the c-axis using infrared, Raman and THz time-domain spectroscopy. Low-frequency infrared and Raman active modes are observed above and below the charge-ordering temperature T_co ~ 200 K over the narrow interval of 1 - 2 meV. A new infrared mode around 1 meV develops below 100 K. The temperature dependence of these modes shows that they are coupled to the charge- and spin-density-wave correlations in this system. These low-energy features are conjectured to originate in the gapped sliding-motion of the chain and ladder sub-systems, which are both incommensurate and charged.

Speaker: Adrian Gozar (Brookhaven National Laboratory)

49 Structure of the hidden order gap in URu$_2$Si$_2$ revealed by optical spectroscopy

Jesse Hall (McMaster University), Toomas R\~o\~om, Taaniel Uleksin, Urmas Nagel (Estonian National Institute for Chemical Physics and Biophysics), Noravee Kanchanavatee, Marc Janoschek, M. Brian Maple (University of California San Diego), Travis Williams, Graeme Luke, and Tom Timusk (McMaster University). Among the heavy fermion compounds URu2Si2 has attracted considerable study due to the second order phase transition it undergoes at 17.5 K for which the order parameter has yet to be identified. We present high quality infrared optical data tracking the onset and evolution of the gap in the conductivity associated with the transition. This allows the detailed evolution of the electronic behaviour to be studied in both the coherent scattering regime above the transition and the hidden order state below the transition, and the evolution of the electronic density of states to be studied. Measurements on both the ab-plane and the c-axis of the tetragonal crystal structure reveal pronounced anisotropies as well as different gap structure. Finally, preliminary results on dependence of the evolution of the gap structure with Re doping will be presented.

Speaker: Jesse Hall (McMaster University)

50 Synthesis and Magnetic Dynamics of Multiferroic Chromates

Anna Pimenov (1), Ch. Kant (1), V. Tsurkan (2), A. Loidl (2), A. Vasiliev (3), L. Svistov (3) , L. Prozorova (3), A. Pimenov (1) (1) Institute of Solid State Physics, Vienna University of Technology, 1040 Vienna, Austria (2) Experimentalphysik V, EKM, University of Augsburg, 86159 Augsburg, Germany (3) Kapitza Institute of Physical Problems, 119334 Moscow, Russia We describe the preparation routes for the series of polycrystalline ACrO2 (A= Cu, Ag, Li, Pd) chromates using a solid-state reaction technique at different temperatures and partly using a two-stage substitution procedure. All samples have been characterized using X-ray and magnetic measurements. In addition, single crystals of CuCrO2 have been grown by flux method. CuCrO2 and AgCrO2 have been investigated using high field Electron-Spin-Resonance spectroscopy and quasioptical transmittance technique in the frequency range between 70 GHz and 600 GHz. Two eigenmodes of the antiferromagnetic resonance can be detected. Clear signatures of the spin-flop transition are observed for specific magnetic domains in CuCrO2.

Speaker: Andrei Pimenov (Vienna University of Technology)

51 Terahertz Response and Colossal Kerr Rotation from the Surface States of the Topological Insulator Bi2Se3

R. Valdés Aguilar1, A. V. Stier1, W. Liu1, L. S. Bilbro1, D. K. George2, N. Bansal3, L. Wu1, J. Cerne2, A. G. Markelz2, S. Oh3, and N. P. Armitage1 1. The Institute for Quantum Matter, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA 2. Department of Physics, University at Buffalo, State University of New York. Buffalo, New York 14260, USA 3. Department of Physics and Astronomy, Rutgers the State University of New Jersey. Piscataway, New Jersey 08854, USA We report the THz response of thin films of the topological insulator Bi2Se3. At low frequencies, transport is essentially thickness independent showing the dominant contribution of the surface electrons. Despite their extended exposure to ambient conditions, these surfaces exhibit robust properties including narrow, almost thickness-independent Drude peaks, and an unprecedentedly large polarization rotation of linearly polarized light reflected in an applied magnetic field. This Kerr rotation can be as large as 65° and can be explained by a cyclotron resonance effect of the surface states. In addition, we will report on the long term effects of atmospheric exposure of thin films of Bi2Se3

Speaker: Rolando Valdes Aguilar (Department of Physics and Astronomy. Johns Hopkins University)

52 Three-dimensional band structure of heavy fermion CeCoIn5 measured by photon-dependent angle resolved photoemission

J. D. Denlinger, Lawrence Berkeley National Laboratory J. W. Allen, University of Michigan L. Dudy, University of Würzburg K. Rossnagel, University of Kiel P. M. Oppeneer, Uppsala University V. S. Zapf, Los Alamos National Laboratory M. B. Maple, U. of California at San Diego Photon-energy dependent angle-resolved photoemission spectroscopy (ARPES) is used to map the 3D band structure and Fermi Surface of the archetypal Kondo lattice material, CeCoIn5. ARPES measurements from two different <001> and <100> cleave surfaces give complementary orthogonal views of the electronic structure with photon-energy dependent kz-broadenings in different crystallographic momentum directions, thereby partially overcoming an inherent ARPES limitation for three-dimensional systems. Polarization dependence of 4d-4f resonantly-excited ARPES is then used to help identify the k-locations of high Ce 4f spectral weight. Detailed comparison is made to local density functional (LDA) calculations in which the Ce 4f-electrons are treated as part of the conduction electrons or as unhybridized localized core states. Clear overall agreement to the localized LDA calculation is observed for all binding energies up to the very low energy scale near EF where signatures of f-d hybridization are observed. However some high binding energy scale disagreement with LDA also exists for CeCoIn5, but not for CeRhIn5 and CeIrIn5, and indicates the need for improved theoretical treatment of Co 3d electron correlations relative to the 4d and 5d systems. The resulting detailed experimental knowledge of the full 3D band structure, k-locations of strong f-d hybridization, and differences between 3d, 4d and 5d Ce115 systems are key steps towards verifying the recent LDA+DMFT predictions of the temperature-dependent Fermi surface evolution in CeIrIn5 [1]. [1] H. C. Choi, B. I. Min, J. H. Shim, K. Haule and G. Kotliar, Phys. Rev. Lett. 108, 016402 (2012).

Speaker: Jonathan Denlinger (Lawrence Berkeley National Lab)

53 Three-Dimensional Electronic structure of Li1+xFeAs

Tetsuya Hajiri ^A, B^ Takahiro Ito ^A,C^ Ryosuke Niwa ^A^ Satoshi Hirate ^A^ Masaharu Matsunami ^B,D^ Byeong Hun Min ^E^, Yong Seung Kwon ^E^ Shin-ichi Kimura ^B,D^ ^A^ Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan ^B^ UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan ^C^ Nagoya University Synchrotron Radiation Research Center, Nagoya University, Nagoya, 464-8603, Japan ^D^ School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan ^E^ Department of Emerging Materials Science, DGIST, Daegu 711-873, Korea LiFeAs is an intriguing iron-based superconductor because it exhibits superconductivity (TC = 18 K) without any structural phase and SDW/AFM transitions [1]. In pristine LiFeAs, we have demonstrated that the electronic structure can be fundamentally explained by LDA band structure [2]. In this system, both of the excess and deficiency of Li-ions (Li1+xFeAs) from the stoichiometry suppresses the superconductivity [3] and enlarges spin fluctuation [4]. Up to date, the reason of the suppression of the superconductivity has not been clarified yet. To elucidate the effect of the Li excess or deficiency, we investigated the electronic structure of non-superconducting Li1+xFeAs by a polarization-dependent three-dimensional angle-resolved photoemission spectroscopy (3D-ARPES). As a result, we observed the difference in the low-energy electronic structure between pristine LiFeAs and Li1+xFeAs. We also found two kink structures in the band dispersions near the center of the Brillouin zone; one is located at about 20 meV, which originates from phonons, and the other at about 100 meV, which cannot be explained by the phonon origin. The higher-energy kink is considered to originate from magnetic excitations. [1] X. C. Wang et al., Solid State Commun. 148, 538-540 (2008). [2] T. Hajiri et al., to be published in Phys. Rev. B. [3] M. Wang et al., Phys. Rev. B 83, 220515(R) (2011). [4] L. Ma et al., Phys. Rev. B 82, 180501(R) (2010).

Speaker: Tetsuya Hajiri (Nagoya University)

54 Time-domain THz spectroscopy of central modes in BaTiO3 and SrTiO3

H. Němec, V. Skoromets, C. Kadlec, T. Ostapchuk, J. Hlinka, J. Petzelt, and P. Kužel Institute of Physics AS CR, Na Slovance 2, 18221 Praha, Czech Republic The dielectric response to terahertz (THz) waves of many materials exhibiting ferroelectric phase transition provides unique information about its origin and driving mechanisms. We will discuss this issue within the frame of recently obtained results in perovskite BaTiO3 and SrTiO3 single crystals and strained thin films and multilayers. Crystals in the vicinity of ferroelectric phase transitions are characterized by a delicate compensation of various microscopic forces. Two excitations, a soft phonon mode and a central mode, are often coexisting in the THz spectral range and represent the driving force of the phase transition. Due to their polar character, they can be easily detected in the THz transmission measurements. The THz spectra can be fitted with a term describing a damped oscillator and a coupled Debye relaxation [1]. Close to the ferroelectric phase transition, anharmonic properties of the crystalline lattice potential often play a crucial role and can be experimentally accessed [2] e.g. by investigating the electric-field dependence of the response. In parallel, molecular dynamics simulations of the soft mode based on an effective Hamiltonian method [1] allow one to attribute microscopic interpretation to the above phenomenological description. By comparing the experimental data to the simulations in BaTiO3 we have shown [1] that the coexistence of two excitations in this compound is an attribute of a single degree of freedom (Ti displacements within the O6 octahedra) with a complex anharmonic potential. [1] J. Hlinka, T. Ostapchuk, D. Nuzhnyy, J. Petzelt, P. Kužel, C. Kadlec, P. Vaněk, I. Ponomareva, and L. Bellaiche, Phys. Rev. Lett. 101, 167402 (2008). [2] V. Skoromets, F. Kadlec, C. Kadlec, H. Němec, I. Rychetský, G. Panaitov, V. Müller, D. Fattakhova-Rohlfing, P. Moch, and P. Kužel, Phys. Rev. B 84, 174121 (2010); C. Kadlec, V. Skoromets, F. Kadlec, H. Němec, J. Schubert, G. Panaitov, and P. Kužel, Phys. Rev. B 80, 174116 (2009).

Speaker: Hynek Nemec (Institute of Physics AS CR, Na Slovance 2, 18221 Praha, Czech Republic)

55 Topological surface state dispersion measured using THz magneto-ellipsometry

Jason N. Hancock, J. L. M. van Mechelen, Alexey Kuzmenko, Dirk van der Marel, University of Geneva Christophe Brune, Elena Novik, Georgy Astakhov, Hartmut Buhmann, Laurens Molenkamp, Wurzburg University We present an ambient field and magneto-optical study of the three-dimensional topological insulator, strained HgTe. The ac conductance of the thin film is in good agreement with the dc transport data which which show a quantum Hall effect of topological surface states at high field and low temperature. Using polarization-sensitive time-domain THz spectroscopy in a magnetic field, reliable information on the Drude weight and cyclotron resonance frequency severely constrain the details of surface state dispersion within 1meV of the Fermi level. Details of the technique and its prospect for future observation of axion electrodynamics using THz spectroscopy will also be discussed. C. Brune, et al, Phys. Rev. Lett. 106, 126803 (2011), Hancock, et al. Phys. Rev. Lett. 107, 136803 (2011)

Speaker: Jason Hancock (University of Geneva/University of Connecticut)

56 Ultrafast carrier dynamics in CVD graphene probed by terahertz spectroscopy

Alex Frenzel, Harvard University / MIT Nityan Nair, Massachusetts Institute of Technology Nathaniel Gabor, Massachusetts Institute of Technology Nuh Gedik, Massachusetts Institute of Technology The relaxation of the electronic system in graphene plays a crucial role in a variety of proposed optoelectronic devices, including bolometers, photodetectors, and solar cells. Time-resolved terahertz spectroscopy affords the ability to probe the low-energy electrodynamics of carriers during relaxation. This is accomplished by exciting the electronic system with a strong 100 fs, 1.5 eV optical pulse and probing at variable time delay with a picosecond far-infrared pulse. Using this method to investigate carrier dynamics in CVD-grown graphene, we observe a positive change in the differential terahertz transmission after optical excitation. This new behavior contrasts with the negative change seen in previous measurements. Our experiments reveal a maximum change in transmission which decreases with increasing temperature. Additionally, the relaxation dynamics slow down with increasing excitation density, a trend which is not expected from typical electron-hole recombination dynamics. This qualitatively different response reveals new opportunities for manipulating optical response in graphene.

Speaker: Alex Frenzel (Harvard / MIT)

Tuesday, 24 July

Transition Metal Oxides

Convener: Pascale Roy (Synchrotron Soleil)

57 Infrared ellipsometry studies of oxide-based heterostructures

C. Bernhard Infrared ellipsometry studies of the electronic and structural properties of oxide-based heterostructures will be presented. As a first example, it is shown that this technique enables one to directly probe the mobility and the depth profile of the concentration of the confined electrons at the LaAlO3/SrTiO3 interface [1]. In addition, preliminary results on the electric field effect on the electronic and structural properties of the interfacial layer will be presented. [1] A. Dubroka et al., Phys. Rev. Lett. 104, 156807 (2010).

Speaker: Christian Bernhard (Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg)

58 Mott-Hubbard excitons in edge-sharing CuO4 chains

Y. Matiks, P. Horsch, B. Keimer, and A. V. Boris Max Planck Institute for Solid State Research, Heisenbergstr. 1, DE-70569 Stuttgart, Germany Compounds composed of chains of edge sharing Cu2+O4 plaquettes have the peculiar property that the magnitude of the nearest-neighbor hopping matrix element along the chains is anomalously small due to the orthogonality of the 2pσ orbitals on oxygen ions adjacent to the Cu ion. By virtue of their exceptionally narrow electronic bandwidths, these compounds provide a highly favorable platform for the investigation of exciton formation and the interplay between spin and charge correlations in cuprates. We have performed a comprehensive ellipsometric study of the charge excitations across the optical gap in high-quality LiCuVO4 [1], NaCu2O2 [2], Li2CuO2, CuGeO3 and α-CuV2O6 single crystals. In all these compounds, the excitonic states associated with the Zhang-Rice singlet state were assigned. For photon polarization along the chains, the data reveal a weak but well-resolved two-peak structure forming the absorption edge whose spectral weight is strongly enhanced upon cooling near the magnetic ordering temperature. These bands were identified as exciton doublets, originating from the long-range Coulomb interaction between the nearest and the next-nearest-neighbour Cu sites along the chains. These results have not only persuasively demonstrated the formation of the Mott-Hubbard excitons, but also quantified characteristic energy scales, such as the local Hubbard U (2.55 − 4.3 eV) and long-range Coulomb V (0.8 − 1.6 eV) interactions. [1] Y. Matiks, P. Horsch, R. K. Kremer, B. Keimer, A. V. Boris, Phys. Rev. Lett. 103, 187401 (2009) [2] Y. Matiks, A. N. Yaresko, K. Myung-Whun, A. Maljuk, P. Horsch, B. Keimer, A. V. Boris, Phys. Rev. B 84, 245116 (2011)

Speaker: Alexander Boris (Max Planck Institute for Solid State Research)

59 How bad metals turn good: spectroscopic signatures.

Antoine Georges College de France, Paris Many materials with strong electronic correlations display metallic-like resistivity up to very high temperature, with values exceeding the Ioffe-Regel-Mott (IRM) criterion. Yet, at low enough temperature, good metallic conductivity obeying Fermi liquid behaviour can be recovered. In this talk, I will explore how this crossover takes place. I will show that the Fermi liquid scale, which is strongly suppressed by strong correlations, should not be confused with the much higher Brinkman-Rice scale, at which incoherent transport sets in. In between these two scales, an extended regime of metallic transport applies, in which the resistivity is smaller than the IRM value but does not follow a T2 Fermi-liquid law. Well-defined quasiparticle excitations do exist in this regime, as manifested in the one-particle spectral function and optical conductivity, with properties distinct from Landau and Drude theories. For a hole-doped Mott insulator, a strong particle-hole asymmetry applies down to low-energy: electron-like excitations are much longer lived, placing these quasiparticle excitations on the `dark side' for ARPES spectroscopy. This also has implications for the temperature dependence of the thermopower. Based on work done in collaboration with: Xiaoyu Deng (Ecole Polytechnique, Palaiseau, France) Jernej Mravlje, (College de France, Paris and Ecole Polytechnique, Palaiseau, France) Rok Zitko (Joszef Stefan Institute, Ljubljana, Slovenia)

Speaker: Antoine Georges (École Polytechnique)

60 Terahertz study of magnetic excitations in a chiral iron-langasite

S. de Brion1, F. Levy-Bertrand1, R. Ballou1, V. Simonet1, P. Lejay1, J-B. Brubach2 and P. Roy2 1 Institut Néel, CNRS et Université Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France 2 SOLEIL Lorme des Merisiers, Saint Aubin 09192 Gif sur Yvette, France For the first time, down to low temperature (from 300 K to 5 K), successful terahertz measurements (from 8 to 60 cm-1) have been performed on the AILES synchrotron line at SOLEIL. Those measurements were realized on the iron-langasite single crystals Ba3(Ta,Nb)Fe3Si2O14. These compounds are unique examples of complete chirality (structural chirality and double magnetic chirality [1,2]). They also present magneto-electric effects, which suggest that they are good candidate for multiferroicity and occurrence of electromagnons.On a structural point of view, the studied iron-langasite single crystals are single domain: they are growth with one structural chirality, opposite for the Ta and Nb compound. On a magnetic point of view, the Fe3+ network consists of layers with triangles arranged on a triangular lattice. The helicoidal magnetic structure, observed below TN=27 K, has a periodicity of about 7 times the crystal structure in the stacked direction while the magnetic moments are arranged at 120° in the triangles [1]. Our terahertz measurements revealed two distinct energy ranges for the magnetic excitations with very different and unexpected temperature dependence. The low energy spectra below 15 cm-1 in Ba3TaFe3Si2O14 disappears above TN, while the high energy part remains up to 150K. We have studied these excitations as a function of the sample structural chirality and electromagnetic field polarization. REFERENCES 1. K. Marty et al PRL 101, 247201 (2008) 2. M. Loire et al PRL 106, 207201 (2011)

Speaker: Sophie de Brion (Institut Néel, CNRS-UJF)

10:15 Break

Graphene II

Convener: Zhi-Xun Shen (Stanford University)

61 Genzel Prize: Terahertz magneto-optics and magneto-plasmonics of graphene and graphite

A.B. Kuzmenko, I. Crassee, J. Levallois, and D. van der Marel DPMC, University of Geneva, Quai Ernest Ansermet 24, 1211 Geneva 4, Switzerland Graphene attracts a lot of attention as a novel optical and plasmonic material. Its optical response is exceptionally sensitive to a magnetic field due to the small cyclotron mass of the Dirac-like charge carriers. This does not only make magneto-optics a useful tool to study this material but also gives rise to giant magneto-optical effects, potentially useful for terahertz applications. In this talk, I’ll overview our magneto-optical studies of large-scale single- and multilayer graphene grown epitaxially on the Si- and C-faces of silicon carbide respectively [1,2,3]. In the highly doped monolayer graphene we observe a strong Drude peak at zero field and a quasiclassical, field-linear, cyclotron resonance at finite fields, which gives rise to a giant Faraday rotation exceeding 0.1 radians at modest fields [1]. In this type of graphene we also found an unexpectedly strong terahertz plasmonic absorption [3] due to natural defects such as terrace steps in SiC. When a field is applied, the plasmon peak splits in two modes, which is a hallmark of the magnetoplasmon physics found earlier in 2D electron gases in semiconductors [4] and on the surface of liquid helium [5]. In quasineutral twisted multilayer graphene, often regarded as a set of decoupled monolayers, we observe quantum LL transitions with the expected square-root like dependence on magnetic field. However, the optical intensity of these transitions is several times smaller than the Kubo formula predicts and its field dependence is not square-root like, as one would expect in ideal and decoupled monolayers [2]. I’ll contrast this serious discrepancy to our recent magneto-optical Kerr rotation spectroscopy experiments in Bernal stacked graphite [6], where a very good agreement between the experiment and the tight-binding Kubo formula is achieved in a broad range of magnetic fields. [1] I. Crassee, J. Levallois, A.L. Walter, M. Ostler, A. Bostwick, E. Rotenberg, Th. Seyller, D. van der Marel, and A.B. Kuzmenko, Nature Physics 7, 48 (2011). [2] I. Crassee, J. Levallois, D. van der Marel, A. L. Walter, Th. Seyller, and A. B. Kuzmenko, Phys. Rev. B 84, 035103 (2011). [3] I. Crassee et al., submitted. [4] S.J. Allen, H.L. Stormer, and J.C.M. Hwang. Phys. Rev. B 28, 4875 (1983). [5] D.B. Mast, A.J. Dahm, and A.L. Fetter, Phys. Rev. Lett. 54, 1706 (1985); D.C. Glatti, E.Y. Andrei, G. Deville, J. Poitrenaud, and F.I.B. Williams, Phys. Rev. Lett. 54, 1710 (1985). [6] J. Levallois, M.K. Tran, and A.B. Kuzmenko, arXiv: 1110.2754; Solid State Communications, Special Issue on Graphene (2012), DOI: 10.1016/j.ssc.2012.04.036.

Speaker: Alexey Kuzmenko (University of Geneva)

62 Tuning the Electronic Structure of Graphene

Eli Rotenberg Advanced Light Source, Lawrence Berkeley National Laboratory

Speaker: Eli Rotenberg (LBNL)

63 Many-body interactions in graphene and graphite via infrared magnetospectroscopy

Li-Chun Tung (Department of Physics and Astrophysics, University of North Dakota) Wenlong Yu (School of Physics, Georgia Institute of Technology) Jean-Mari Poumirol (National High Magnetic Field Laboratory) Dmitry Smirnov (National High Magnetic Field Laboratory) Zhigang Jiang (School of Physics, Georgia Institute of Technology) The electronic band structures of graphene and graphite exhibit unusual low-energy dispersion relation, radically different from the parabolic bands common to conventional two-dimensional semiconductors. Most interestingly, the charge carriers in graphitic systems mimic relativistic, massless Dirac particles, leading to intriguing new phenomena. In this talk, I focus on infrared optical studies of graphene and graphite in high magnetic fields (up to 35 T). In particular, we resolved resonances between hole Landau levels and electron Landau levels (intraband transitions), as well as resonances between hole and electron Landau levels (interband transitions). We argue that many-body correlations of massless Dirac Fermions, considering electron-electron, electron-phonon and electron-plasmon interactions, contribute considerably to our experimental results. Our work suggests that rich interacting physics exists in graphitic materials, which may have profound implications in future optoelectronics.

Speaker: Zhigang Jiang (Georgia Insitute of Technology)

64 Ultrafast Terahertz Spectroscopy of Carbon Nanomaterials

Hyunyong Choi (1,2), Dominik Pfaff (1), Ryan Smith (1), and Robert A. Kaindl (1) (1) Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA (2) School of Electrical and Electronic Engineering, Yonsei University, Seoul, 120-749, Korea (present address) Confined carriers in carbon nanomaterials - fullerenes, nanotubes, and graphene – exhibit a rich variation in electronic and optical properties. Understanding their excitations, correlations and carrier dynamics is of fundamental interest and can yield insight for future high-speed device applications. Here, we present experiments that employ broadband ultrafast THz pulses to investigate single-walled carbon nanotubes and graphene. Photoexcitation of semiconducting nanotubes leads to the emergence of an induced conductivity around 1.7 THz, whose frequency, chirality enhancement, and temperature dependence agree with the observation of intra-excitonic THz transitions. The picosecond decay of this conductivity follows a bi-molecular kinetics, revealing exciton-exciton annihilation dynamics. Moreover, we have carried out first ultrafast mid-IR-pump, THz probe experiments of exfoliated graphene. An ultrabroadband THz probe pulse spanning up to 20 THz was employed to cover both intra- and interband transitions and map the Dirac fermion distribution functions. We have observed transient conductivity changes in single-layer graphene on Si/SiO2 and will discuss its complex spectral response and fast picosecond relaxation. These experiments provide insights into low-energy excitations and dynamics of carbon nanomaterials, motivating studies with intense THz fields for resonant excitation.

Speaker: Robert Kaindl (Materials Sciences Division, Lawrence Berkeley National Laboratory)

12:30 Lunch

Pnictides

Convener: Laszlo Mihaly (Stony Brook University)

65 Anisotropic in-plane optical conductivity in detwinned iron-pnictides

A. Dusza, A. Lucarelli and L. Degiorgi Dep. of Physics, ETH Zurich, Switzerland J.-H. Chu and I.R. Fisher Dep. of Physics, Stanford University, U.S.A. We study the anisotropic in-plane optical conductivity of detwinned Ba(Fe_(1-x)Co_x)_2As_2 single crystals for x=0, 2.5% and 4.5% in a broad energy range (3 meV-5 eV) across their structural and magnetic transitions. For temperatures below the Neel transition, the topology of the reconstructed Fermi surface, combined with the distinct behavior of the scattering rates, determines the anisotropy of the low frequency optical response. For the itinerant charge carriers, we are able to disentangle the evolution of the Drude weights and scattering rates and to observe their enhancement along the orthorhombic antiferromagnetic a-axis with respect to the ferromagnetic b-axis. For temperatures above the structural phase transition, uniaxial stress leads to a finite in-plane anisotropy. The anisotropy of the optical conductivity, leading to a significant dichroism, extends to high frequencies in the mid- and near-infrared regions. The temperature dependence of the dichroism at all dopings scales with the anisotropy ratio of the dc conductivity, suggesting the electronic nature of the structural transition. Our findings bear testimony to a large nematic susceptibility that couples very effectively to the uniaxial lattice strain. In order to clarify the subtle interplay of magnetism and Fermi surface topology we compare our results with theoretical calculations obtained from density functional theory within the full-potential linear augmented plane-wave method.

Speaker: Leonardo Degiorgi (ETH Zurich)

66 Optical spectroscopy study on Fe-pnictides/chalcogenides

N. L. Wang, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China I present our optical spectroscopic measurements on several different Fe-based superconducting systems. We find that, for all investigated FeAs-based compounds, the optical conductivity spectra contain, in addition to the free carrier response at low frequency, a peculiar temperature-dependent gap-like suppression at rather high energy scale near 0.6 eV. This suppression evolves with the As-Fe-As bond angle induced by electron- or hole-doping. Furthermore, the feature weakens in the Fe-chalcogenide compounds. We elaborate that the feature is mainly caused by the strong Hund's coupling effect between the itinerant electrons and localized electron moment arising from the multiple Fe 3d orbitals. I shall also present our recent optical spectroscopy study on the iron-selenide superconductor K$_{0.75}$Fe$_{1.75}$Se$_2$. The measurement revealed the development of a sharp reflectance edge below T$_c$ at frequency much smaller than the superconducting energy gap on a relatively incoherent electronic background, a phenomenon which was not seen in any other Fe-based superconductors so far investigated. Our analysis indicates that this edge structure develops from a Josephson-coupling plasmon in the superconducting condensate due to the presence of nanoscale phase separation between superconductivity and magnetism. Work done with Z. G. Chen, R. H. Yuan, W. Z. Hu, B. Cheng, T. Dong, P. Zheng.

Speaker: Nan-Lin Wang (Institute of Physics, Chinese Academy of Sciences, Beijing, China)

67 Optical conductivity of (Ba,K)Fe2As2: observation of a normal state pseudogap and the effect of impurity scattering in the superconducting order parameter

R.P.S.M. Lobo LPEM, ESPCI-CNRS, Paris, France Y.M. Dai and B. Xu, LPEM, ESPCI-CNRS, Paris, France IOP, CAS, Beijing, China H.H. Wen IOP, CAS, Beijing, China Nanjing University, Nanjing, China X.G. Qiu IOP, CAS, Beijing, China We measured the detailed temperature dependence of the optical conductivity, down to 15 cm-1 (2 meV), of (Ba,K)Fe2As2 iron-pnictide superconductors in the underdoped and optimally doped regimes. In the underdoped samples we observe prominent gaps opening due to the appearance of a spin density wave (SDW) order. Below the SDW temperature but far above the superconducting transition, we observe the opening of a pseudogap. The energy scale at which this pseudogap opens is comparable to the that of the superconducting gap. A spectral weight analysis shows that this pseudogap shares the same electronic states as the superconducting condensate and suggests that it is a precursor to superconductivity. When entering the superconducting state, a decrease in the low energy optical conductivity indicates the formation of a superconducting condensate and the opening of a superconducting gap. In the optimally doped sample (Tc = 39.1 K) the optical conductivity vanishes, within experimental error, below 160 cm-1 (20 meV), indicating a fully open gap. A deeper analysis indicates that 2 superconducting gaps are required to quantitatively describe the data. The gap values agree with STM and ARPES data and qualitatively follow the expected behavior for a strongly coupled two band superconductor. The fully open gaps observed in (Ba,K)Fe2As2 contrasts to the strong residual optical conductivity measured in Ba(Fe,Co)2As2. We can reconcile this discrepancy in the framework of a s+/- gap symmetry where FeAs in-plane non magnetic impurities act as pair-breaking centers.

Speaker: Ricardo Lobo (LPEM, ESPCI-CNRS)

68 Infrared pseudogap in the $ab$-plane and $c$-axis responses of the pnictide high-$T_c$ superconductors

S. J. Moon$^{1,2}$, A. A. Schafgans$^{1}$, M. A. Tanatar$^{3}$, R. Prozorov$^{3}$, A. Thaler$^{3}$, P. C. Canfield$^{3}$, S. Kasahara$^{4}$, T. Shibauchi$^{5}$, T. Terashima$^{4}$, Y. Matsuda$^{5}$, A. S. Sefat$^{6}$, D. Mandrus$^{6,7}$, and D. N. Basov$^{1}$ $^{1}$Department of Physics, University of California, San Diego, La Jolla, California 92093, USA $^{2}$Department of Physics, Hanyang University, Seoul 133-791, South Korea $^{3}$Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA $^{4}$Research Center for Low Temperature and Materials Science, Kyoto University, Kyoto 606-8502, Japan $^{5}$Department of Physics, Kyoto University, Kyoto 606-8502, Japan $^{6}$Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA $^{7}$Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA The nature of the pseudogap phase and its relation to high-$T_c$ cuprate superconductors remains a significant yet unresolved problem in condensed matter physics. The central question concerns whether the pseudogap is related to precursor superconductivity or other possible broken symmetry state. Irrespective of the origin, the pseudogap is universally regarded as an essential piece of the physics of unconventional cuprate superconductors. While the pseudogap in the cuprate family of high-$T_c$ superconductors has been extensively documented, spectroscopic manifestations of the pseudogap in the iron-based materials remained elusive. We report on the infrared studies of the ab-plane and c-axis charge dynamics of a prototypical pnictide system: the BaFe$_2$As$_2$ (Ba122) family. Our experiments have identified hallmarks of the $ab$-plane and $c$-axis pseudogap in the Ba122 system that mirror the spectroscopic manifestations of the pseudogap in the underdoped cuprates. Moreover, the evolution of the charge dynamics across the phase diagram suggests that the pseudogap is not directly related to precursor superconductivity.

Speaker: Soonjae Moon (Department of Physics, Hanyang University)

15:45 Break

Photoinduced Studies I

Convener: Larry Carr (Brookhaven National Laboratory)

69 Exploring Dynamic Phase Transitions in the Vanadates using Terahertz Spectroscopy

Richard D. Averitt, Boston University In this presentation, I will discuss our recent work using transient strain and transient electric fields to drive phase transitions in complex materials. The goal of such studies is to investigate nonequilibrium phenomena in materials with strongly coupled charge, orbital, lattice, and spin interactions to explore myriad pathways in driving phase transitions with the ultimate goal of accessing new metastable states that, in a given material, are not thermally accessible. Specifically, I will discuss our results on the vanadates VO2 and V2O3. V2O3 undergoes a transition from antiferromagnetic insulator at low temperatures to a strongly correlated metal above ~160K and VO2 exhibits an insulator to metal transition at 340K. Optical-pump THz-probe studies on V2O3 thin films reveal metallic-state coherent oscillations in the far-infrared conductivity. The 100 ps conductivity oscillations result from optically induced strain that drives V2O3 from the correlated metallic state towards a paramagnetic insulating phase. In addition, I will discuss experiments using metamaterial enhanced high field terahertz (THz) pulses (up to ~4MV/cm) to induce the insulator-to-metal transition in vanadium dioxide (VO2) thin films. Ultrafast THz field enhancement in the gaps of metamaterial split ring resonators releases free electrons in VO2 via the Poole-Frenkel effect. The accelerated hot electrons transfer energy to lattice via electron phonon coupling inducing the persistent VO2 phase transition. This electric-field induced phase transition occurs on a picosecond time scale.

Speaker: Richard Averitt (Boston University)

70 Fast Dynamics of Exchange Biased Bilayers

Ivan K. Schuller, UCSD, USA; Shimshon Barad, Tel Aviv University, Israel; W. A. A. Macedo, Centro de Desenvolviemeinto da Tecnologia Nuclear, Brazil; Chris Leighton, University of Minnesota, USA; Ultrafast optical excitation of a ferromagnet/antiferromagnet (Ni/FeF2) exchange biased bilayer produces novel magnetization dynamics unlike ever observed before. An unexpected precession of the magnetization, in reverse magnetic fields that exceed the exchange bias, originates from a reorientation of frustrated spins at the interface. As the laser-excited interface approaches the blocking temperature, an exchange bias reversal can also be induced with a single excitation pulse, showing that not only the ferromagnet but also the antiferromagnet is strongly affected by the optical perturbation. This non-trivial response cannot be extrapolated from the known slow dynamics of magnetic bilayers, and provides important information on the physics of the interlayer coupling. We will contrast these measurements with our earlier studies using field pulses. Work supported by the US Department of Energy

Speaker: Ivan K. Schuller (UCSD)

71 Ultrafast Mid-infrared Spectroscopy of the Charge- and Spin-Ordered Nickelate La1.75Sr0.25NiO4

G. Coslovich(1), B. Huber(1), W.-S. Lee(2), Y.-D. Chuang(3), Y. Zhu(1), T. Sasagawa(4), Z. Hussain(3), H. A. Bechtel(3), M. C. Martin(3), R. W. Schoenlein(1), Z.-X. Shen(2), and R. A. Kaindl(1) (1) Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA (2) SIMES, SLAC National Accelerator Laboratory and Stanford University, Menlo Park, CA 94025, USA (3) Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA (4) Materials and Structures Laboratory, Tokyo Institute of Technology, Kanagawa 226-8503, Japan Here we report the first ultrafast mid-infrared study of charge and spin-ordered nickelates. A strong photo-induced modulation of the optical conductivity is observed on sub-picosecond timescales, indicating the filling and subsequent re-establishment of the pseudogap in the time-domain. The fast timescale of this process (about 600 fs) suggests a major role of short-range correlations of polaronic carriers. Spectral analysis at energies resonant to the Ni-O stretching mode allows decoupling the phonon dynamics from the large pseudogap modulation. This approach reveals the interplay between specific lattice modes and the electronic degrees of freedom in nickelates.

Speaker: Giacomo Coslovich (Lawrence Berkeley National Lab)

72 Ultrafast quantum modulation spectroscopy of a solid state Mott insulator

D. Nicoletti (1), S. Kaiser (1), S. R. Clark (2), G. Cotugno (1,2), R. I. Tobey (2), N. Dean (2), S. Lupi (3), H. Okamoto (4), J. Tsutsumi (4), T. Hasegawa (4), D. Jaksch (2), and A. Cavalleri (1,2) (1) Max Planck Department for Structural Dynamics, University of Hamburg, Center for Free-Electron Laser Science, Hamburg, Germany (2) Department of Physics, Oxford University, Clarendon Laboratory, Oxford, United Kingdom (3) Department of Physics, University of Rome “La Sapienza”, Rome, Italy (4) National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan The interplay of interactions that leads to electronic order in Mott insulators is of fundamental importance to explain several remarkable phenomena, including high-Tc superconductivity. In particular, the microscopic physics of the Hamiltonian is encapsulated in effective parameters, which include the role of all degrees of freedom to determine hopping and interaction energies. Yet, these microscopic parameters are not easily extracted from most experimental techniques, making the range of validity of a single Hubbard model often difficult to determine. We introduce here a new type of spectroscopy that investigates microscopic couplings by detecting changes in the excitation spectrum when different low-energy modes are selectively driven. In the organic salt ET-F2TCNQ, a prototypical one-dimensional Mott insulator [1], we excite two specific local vibrational modes of the ET molecule with ultrashort mid-infrared pulses. Such technique is reminiscent of what routinely explored in optical lattices, thus bringing non-equilibrium many body dynamics of model systems to solid state compounds. The conductivity, measured as a function of pump-probe delay over the whole infrared range, displays a pronounced red-shift of the Mott gap after excitation of a 10-µm IR active mode, due to an effective screening of the on-site correlation. Furthermore, a direct frequency modulation of the hopping leads to the formation of an intragap sideband manifold, and no Drude peak, unlike what typically observed for above gap (~eV) excitations [2,3]. A “locally-vibrating” dynamic Hubbard model is able to reproduce and explain these lineshapes, revealing a pronounced asymmetry in the behavior of holons and doublons. This is of general interest, as hole and electron doping are known to affect correlated electron systems in different ways, notably in high-Tc cuprates. The quantum modulation spectroscopy introduced here is applicable to systematic deconstruction of the Hubbard Hamiltonian in a broad range of materials, addressing not only vibrational but also magnetic and electronic degrees of freedom. References: [1] T. Hasegawa et al., Solid State Comm. 103, 489-493 (1997). [2] H. Okamoto et al., Phys. Rev. Lett. 98, 037401 (2007). [3] S. Wall et al., Nature Physics 7, 114-118 (2011).

Speaker: Daniele Nicoletti (Max Planck Department for Structural Dynamics, University of Hamburg, Center for Free-Electron Laser Science, Hamburg, Germany)

18:00 Dinner

Poster Session 2

High-Tc Cuprates, Pnictides, Photo-induced, Nanoscale spectroscopies, Plasmonics, Metamaterials, New techniques

73 Anisotropic electron-phonon coupling in MgB2 by ARPES

B. M. Ludbrook, Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada C. N. Veenstra, Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada G. Levy, Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada N. D. Zhigadlo, Laboratory for Solid State Physics, ETH Zurich, Switzerland A. Damascelli, Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada We present high-quality angle-resolved photoemission measurements on the superconductor MgB2. The renormalization of the sigma bands due to the electron-phonon coupling is clearly visible for the first time. Using a self-consistent fitting procedure we determine the self-energy, extract the bosonic 'glue', and calculate the momentum-dependent coupling parameter. The large values of \lambda~1.6 measured at specific momenta in the Brillouin zone are in agreement with ab-initio theoretical estimates. Comparison of these values with the Fermi surface average value of \lambda~0.7 is a direct demonstration of the strong electron-phonon coupling anisotropy, thought to be the origin of the large Tc in this material.

Speaker: Bart Ludbrook (Department of Physics & Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada)

74 ARPES evidence of translational symmetry breaking in superconducting Fe(Te1-xSex)

L. Moreschini[1], P.-H. Lin[2,3], A. Bostwick[1], E. Rotenberg[1], E. Giannini[4], R.Viennois[4], K.W. Yeh[5], M.K. Wu[5], and M. Grioni[2] 1 Advanced Light Source, LBNL, Berkeley, CA 94720 (USA) 2 Institute of Condensed Matter Physics, EPFL, CH-1015 Lausanne (Switzerland) 3 LPS, Université de Paris-sud, F-91140 Orsay (France) 4 DPMC, Université de Genève, CH-1211 Genève (Switzerland) 5 Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan In systems with coexisting translational periodicities, the momentum distribution of the ARPES spectral weight A(k,ω) encodes the strength of the underlying potentials [1,2]. We have performed an unusually broad survey of k-space in superconducting Fe(Te1-xSex) samples, covering several Brillouin zones. We find that A(k,ω) does not exhibit the overall periodicity of the crystal, with a unit cell (Fe2) containing two formula units. A(k,ω)follows instead the periodicity of the Fe layer, with a smaller and rotated (Fe1) unit cell. This result demonstrates that translational symmetry is broken in the “11” phase, most likely by a modulation of the positions of the chalcogen atoms. [1] J. Voit et al., Science 290, 501 (2000). [2] C.-H. Lin et al., Phys. Rev. Lett. 107, 257001 (2011).

Speakers: Marco Grioni, P.-H. Lin (ICMP-EPFL)

75 Charge density wave formation near band degeneracies

H.-M. Eiter (1), M. Lavagnini(1), A. Baum (1), R. Hackl (1), E.A. Nowadnick (2)(3), A.F. Kemper (2)(4), T.P. Devereaux (2)(4), J.-H. Chu (2)(4), J. G. Analytis (2)(4), I.R. Fisher (2)(4), and L. Degiorgi (5) (1) Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, D-85748 Garching, Germany (2) Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA (3) Geballe Laboratory for Advanced Materials & Physics Department, Stanford University, CA 94305, USA (4) Geballe Laboratory for Advanced Materials & Dept. of Applied Physics, Stanford University, CA 94305, USA (5) Laboratorium f\"ur Festk\"orperphysik, ETH - Z\"urich, CH-8093 Z\"urich, Switzerland We present Raman scattering experiments on rare-earth tri-telluride single crystals to study low dimensional interacting electron gases and the transition into a charge density wave (CDW) phase. In the case of ErTe3 there are two CDW phase transitions at 265K and 155K with orthogonal ordering vectors. We analyze the data and the Raman selection rules and find a strong enhancement of the light scattering intensity near band degeneracies. For symmetry reasons the electron-phonon coupling is also enhanced at these points. This is an additional contribution to the phonon renormalization and therefore influences CDW formation in multiband systems.

Speaker: Hans-Martin Eiter (Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, D-85748 Garching, Germany)

76 Charge transport in semiconductor nanostructures investigated by terahertz spectroscopy

H. Němec, Z. Mics, and P. Kužel Institute of Physics AS CR, Na Slovance 2, 18221 Praha, Czech Republic The optical pump—THz probe spectroscopy provides a sub-picosecond time resolution and a convenient access to the spectral part of the conductivity spectra where the responses of free and localized charge carriers usually fundamentally differ. We developed a microscopic model of far-infrared conductivity, which comprises two contributions: the contribution of local (depolarization) fields (accounted for by Maxwell-Garnett or Bruggeman effective medium model) and that directly related to the localization of charge carriers [1]. We employ a Monte Carlo method to simulate the carrier motion within nanoparticles and, subsequently, we use the Kubo formula to calculate the far-infrared conductivity. The character of the complex conductivity spectrum is essentially determined by the probability of the inter-particle transport of carriers and by the ratio of the nanoparticle size and the bulk carrier mean free path. We demonstrate our approach on the interpretation of experimental results we obtained with CdS nanoparticles with various sizes [2]. We found that a certain confinement of photogenerated electrons exists, but there is a strong coupling and percolation between adjacent nanocrystals which permits an efficient inter-nanocrystal transport. We also found that electron mobility strongly depends on carrier excess energy. Low mobility is observed for carriers with low excess energy (after excess energy relaxes at low excitation densities), since these charges are localized between potential barriers. When the excess energy is high, the mobility is also high as charge carriers can easily pass over potential barriers. This happens either immediately after photoexcitation of charges with short-wavelength radiation, or in the case of high excitation densities when quasi-Fermi energy is high above the conduction band minimum. [1] H. Němec et al., J. Photochem. Photobiol. A 215, 123 (2010). [2] Z. Mics et al., Phys. Rev. B 83, 15 5326 (2011).

Speaker: Hynek Nemec (Institute of Physics AS CR, Na Slovance 2, 18221 Praha, Czech Republic)

77 Coherent broadband THz spectrometer using photomixers for accurate determination of complex dielectric function

Roggenbuck, Axel [1,2] Langenbach, Malte [1] Schmitz, Holger [1] Vidal, Ernesto [1] Deninger, Anselm [2] Cámara Mayorga, Iván [3] Güsten, Rolf [3] Hemberger, Joachim [1] Grüninger, Markus [1] Affiliations: [1] : II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany, [2] : TOPTICA Photonics AG, Lochhamer Schlag 19, D-82166 Gräfelfing, Germany [3] : Max-Planck-Institute for Radio Astronomy, Auf dem Hügel 69, D-53121 Bonn, Germany We discuss the development of a cw THz spectrometer for solid-state spectroscopy at low temperatures as well as high magnetic fields. The spectrometer is based on the principle of THz generation using frequency mixing of two near-infrared distributed feedback diode lasers with frequency stabilization [1]. The laser beat is converted into THz radiation by a photomixer, which efficiently generates THz radiation from 60 GHz to 1.8 THz. The THz radiation is detected by a second photomixer via homodyne mixing of the THz signal and the laser beat. A fast phase modulation technique using fiber stretchers is used to determine the amplitude and the phase at a given frequency with excellent reliability [2]. More recently, we have implemented a third laser which increases the phase accuracy by enabling a correction for phase drifts mainly caused by thermal fluctuations. Thus, the complex dielectric function can be determined very accurately with a very high frequency resolution. Furthermore, the performance of the photomixers at low temperatures down to 5 K and high magnetic fields up to 8 T has been tested extensively to integrate this spectrometer within a commercial magneto-cryostat for spectroscopic investigations. Various aspects of the above-mentioned developments will be outlined. [1] A. Roggenbuck, H. Schmitz, A. Deninger, I. Cámara Mayorga, J. Hemberger, R. Güsten, and M. Grüninger, New J. Phys., 12, 043017 (2010). [2] A. Roggenbuck, K. Thirunavukkuarasu, H. Schmitz, J. Marx, A. Deninger, I. Cámara Mayorga, R. Güsten, J. Hemberger, and M. Grüninger, J. Opt. Soc. Am. B, in print (2012).

Speaker: Komalavalli Thirunavukkuarasu (II. Physikalisches Institut, Universität zu Köln)

78 Coherent intersubband polarization switching in a semiconductor quantum well using terahertz pulses

M. Wagner(1), M. Helm(1), M. S. Sherwin(2), D. Stehr(1) 1-Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, P.O. box 510119, 01314 Dresden, Germany 2-Physics Department, University of California Santa Barbara, California 93106 Intersubband transitions in semiconductor quantum wells with their large dipole moments and their polarization decay within a few hundreds of femtoseconds [1] to a few picoseconds are interesting for optical switching with picosecond switching times. Up to now coherent control of an intersubband polarization has only been demonstrated in multiple quantum wells in the mid-infrared spectral region where the infrared transmission change between a doped sample and a similar undoped one was measured [2]. Here we demonstrate coherent switching of an excited intersubband polarization in a single GaAs/AlGaAs quantum well in the far-infrared spectral region in a more direct way [3]. To this end a bias modulation is applied to fill and empty the doped quantum well via the Stark effect and simultaneously monitor changes in the THz transmission. A first THz pulse centered around the 2 THz intersubband transition energy excites a coherent, macroscopic polarization between the n=1 and n=2 conduction band states. A temporally delayed control pulse is used to either switch the polarization off or to refresh it, depending on the relative time delay between pump and control pulse. The polarization switching is directly monitored in the time domain by measuring the free induction decay of the induced polarization with standard electro-optic sampling. Model calculations based on the optical Bloch equations agree well with the experiment. [1] R. A. Kaindl et al., Phys. Rev. Lett. 80, 3575 (1998). [2] F. Eickemeyer et al., Appl. Phys. Lett. 79, 165 (2001). [3] M. Wagner et al., Appl. Phys. Lett. 99, 131109 (2011).

Speaker: Martin Wagner (Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany)

79 Controlling incandescence using Metamaterials

Xianliang Liu, Willie J. Padilla Boston College Talmage Tyler, Nan Marie Jokerst Duke University Tatiana Starr, Anthony F. Starr SensorMetrix, Inc. A blackbody is an idealized object that absorbs all radiation incidents upon it and reradiates energy solely determined by its temperature, as described by Planck’s law. The phenomenon that material emits light at high temperature is also known as incandescence. The desire to control incandescence has long been a research topic of interest for scientists—one particular theme being the construction of a selective emitter whose thermal radiation is much narrower than that of a blackbody at the same temperature. In this work we demonstrate, for the first time, selective thermal emitters based on metamaterial perfect absorbers. We experimentally realize a narrow band mid-infrared (MIR) thermal emitter. Multiple metamaterial sub-lattices further permit construction of a dual-band MIR emitter. By performing both emissivity and absorptivity measurements, we find that emissivity and absorptivity agree very well as predicted by Kirchhoff’s law of thermal radiation. Our results directly demonstrate the great flexibility of metamaterials for tailoring blackbody emission. Reference: 1, Liu, X. et al. Phys. Rev. Lett. 107, 045901 (2011).

Speaker: Xianliang Liu (Boston College)

80 d-wave quasiparticles and the origin of vortex viscosity in cuprates

David Broun, Simon Fraser University Xiaoqing Zhou, Simon Fraser University Wendell Huttema, Simon Fraser University Patrick Turner, Simon Fraser University Ruixing Liang, University of British Columbia Doug Bonn, University of British Columbia Walter Hardy, University of British Columbia The frictional force experienced by a quantized flux line moving in a conventional superconductor arises primarily from induced vortex electric fields coupling to charge excitations within the vortex core. This was first captured by Bardeen-Stephen theory, which treats the vortex core as a tube of normal metal embedded in a superconducting background. The theory is applicable to conventional superconductors for two reasons: the vortex cores are large and contain a nearly continuous spectrum of single-particle states; and s-wave pairing symmetry results in a low density of extended states surrounding the vortex core. In cuprate superconductors the opposite situation holds: small vortex cores contain at most a few discrete states, with a continuum of low lying states outside the vortex core due to the nodes in the d-wave energy gap. To explore these differences, microwave techniques have been used to probe the frequency dependent vortex viscosity of underdoped, Ortho-II YBCO. The measurements reveal a vortex viscosity with surprisingly strong frequency dependence that bears a striking resemblance to the zero-field quasiparticle conductivity. This implies that the dominant dissipative mechanism for the flux lines is induced electric fields coupling to extended, long-lived d-wave quasiparticle states outside the vortex cores, a remarkable upending of the conventional Bardeen-Stephen picture. Analysis of viscosity spectra reveals the presence of a second, shorter timescale in the relaxation dynamics that grows in importance with increasing field, with a dynamical crossover observed in the vortex-liquid regime above which the viscous dynamics have a single, fast relaxation rate. At low temperatures the flux-flow resistivity has a log(1/T) form that is reminiscent of the DC resistivity of cuprates in the pseudogap regime.

Speaker: David Broun (Simon Fraser University)

81 Dispersive high-energy spin excitations in iron pnictide superconductors investigated with resonant inelastic x-ray scattering

K. J. Zhou1, Y. B. Huang2,1, C. Monney1, N. L. Wang2, P. C. Dai2,3,4, X. Dai2, J. Van den Brink5, H. Ding2, and T. Schmitt1 1Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 2Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 3Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996, USA 4Neutron Scattering Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 5Institute for Theoretical Solid State Physics, IFW Dresden, 01171 Dresden, Germany The discovery of iron-based high temperature superconductivity has triggered tremendous research efforts in searching for novel high-Tc superconductors. Unlike the cuprates whose parent compounds are long-range ordered antiferromagnetic Mott insulators, the iron-based parent compounds are ‘spin-density wave’ metals with delocalized electronic structure and more itinerant magnetism. ARPES studies suggest that superconductivity in iron-based materials may be connected with interband scattering between the quasi-nested electron-hole Fermi surfaces. On the other hand, the observation of spin fluctuations by Inelastic Neutron Scattering (INS) in these materials, similar to those seen in cuprates, suggests that cuprate and iron-based high-Tc superconductors may share a common pairing mechanism. Recent developments of the high-resolution resonant inelastic X-ray scattering (RIXS) technique [1] have enabled investigations of magnetic excitations in cuprates [2,3], which show excellent agreement with results from INS. In this presentation we demonstrate that RIXS can be used to measure collective magnetic excitations in iron-based superconductors and their parent compounds despite their much stronger itinerancy compared to cuprates. The persistence of high-energy spin excitations even in optimally doped pnictide superconductors of the ‘122’ and ‘1111’ families in a wide range of temperatures strongly suggests a spin-mediated Cooper pairing mechanism as proposed in cuprate superconductors [4]. References [1] G. Ghiringhelli et al., Rev. Sci. Instrum. 77, 113108 (2006); V. N. Strocov et al., J. Synch. Radiat. 17, 631 (2010). [2] J. Schlappa et al., Phys. Rev. Lett. 103, 047401 (2009). [3] L. Braicovich et al., Phys. Rev. Lett. 104, 077002 (2010). [4] M. Le Tacon et al., Nature Physics (2011), doi:10.1038/nphys2041

Speaker: Thorsten Schmitt (Paul Scherrer Institut)

82 Evidence for a Peierls phase-transition in a three-dimensionnal multiple charge-density-waves solid.

B. Mansart Laboratory for Ultrafast Microscopy and Electron Scattering, ICMP, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland and Laboratory of Ultrafast Spectroscopy, ISIC, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland M. Cottet Laboratory for Ultrafast Microscopy and Electron Scattering, ICMP, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland T.J. Penfold Laboratory of Ultrafast Spectroscopy, ISIC, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland and Laboratory of Computational Chemistry and Biochemistry, ISIC, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland and SwissFEL, PSI, CH-5232 Villigen, Switzerland S.B. Dugdale H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United Kingdom R. Tediosi Département de Physique de la matière Condensée, Université de Genève, CH-1211 Genève 4, Switzerland M. Chergui Laboratory of Ultrafast Spectroscopy, ISIC, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland F. Carbone Laboratory for Ultrafast Microscopy and Electron Scattering, ICMP, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland We studied the ultrafast photoinduced charge-density-wave (CDW) to metal phase transition in a complex solid, namely Lu5Ir4Si10. After melting the charge ordering using infrared laser pulses, the consequent charge redistribution is probed with fs time resolution through the spectral weight analysis of the transient optical response over a broad energy range. The time-dependent spectral weight reveals a signature of the CDW melting and the time-scale of this photo-induced phase transition. This new kind of analysis allows us to show that the charge order remains preserved until the lattice distorts sufficiently to induce the phase transition. These results are completed by ab-initio modeling of the electronic band structure, identifying the orbitals involved in the CDW and the electronic transitions leading to the photo-induced melting of the charge order. This allows us to reveal the Peierls origin of multiple CDW in this three-dimensional solid.

Speaker: Barbara Mansart (Ecole Polytechnique Fédérale de Lausanne)

83 High speed terahertz modulation from metamaterials with embedded high electron mobility transistors

David Shrekenhamer, Department of Physics, Boston College, USA Saroj Rout, NanoLab, Electrical and Computer Engineering, Tufts University, USA Andrew Strikwerda, Department of Physics, Boston University, USA Chris Bingham, Department of Physics, Boston College, USA Richard Averitt, Department of Physics, Boston University, USA Sameer Sonkusale, NanoLab, Electrical and Computer Engineering, Tufts University, USA Willie Padilla, Department of Physics, Boston College, USA We have designed and demonstrated the performance of a novel terahertz (THz) device resulting from hybridization of metamaterials (MMs) with pseudomorphic high electron mobility transistors (HEMTs), fabricated in a commercial gallium arsenide (GaAs) process. Monolithic integration of transistors into each unit cell permits modulation at the metamaterial resonant frequency of 0.46 THz. Characterization is performed using a THz time-domain spectrometer (THz-TDS) and we demonstrate modulation values over 30%, and THz modulation at frequencies up to 10 megahertz (MHz).

Speaker: David Shrekenhamer (Boston College)

84 Infrared Phonon Fingerprinting of Nanocrystals through Broadband Near-Field Spectroscopy

Alexander S. McLeod - University of California San Diego Gerardo Dominguez - California State University San Marcos Zack Gainsforth - University of California Berkeley Space Sciences Laboratory Priscilla Kelly - University of California San Diego Mark Thiemens - University of California San Diego D. N. Basov - University of California San Diego Near-field infrared spectroscopy has recently been demonstrated with the capability to resolve optical properties of sub-wavelength sample areas across a broad range of infrared frequencies. This method holds promise for the direct identification of sub-wavelength chemical composition in nanostructured and heterogeneous samples. We apply this technique to the study of phonon- resonant silicon carbide nanocrystals tens of nanometers in size using an apertureless scanning near-field optical microscope (SNOM) coupled to a pulsed broadband infrared laser source and FTIR spectrometer. We present measurements of nanocrystal near-field spectra in the range of 700-1200 cm−1 evaluated in comparison with the near-field spectra of bulk silicon carbide, calibrated using ellipsometry. A detailed analytic model of the probe-sample near-field interaction is applied for the identification of nanoscale resonant size effects. These techniques provide a powerful method for identifying and characterizing sub-wavelength nanocrystals in heterogeneous samples via near-field infrared “phonon fingerprinting.”

Speaker: Alex McLeod (University of California San Diego)

85 Intense terahertz pulse-induced breaking of BCS superconducting phase in NbN

Ryusuke Matsunaga^1 and Ryo Shimano^1 1 Department of Physics, School of Science, The University of Tokyo We studied ultrafast dynamics of a nonequilibrium BCS state in superconductor NbN by terahertz pump-terahertz probe spectroscopy. Intense THz pulse excitation induces the suppression of superconductivity within 2 ps due to direct photo-injection of high-density quasiparticles. The optical conductivity in the terahertz pulse-induced nonequilibrium BCS state cannot be explained by the increase of effective temperature. The complex conductivity spectrum shows the essential importance of the spatial condensation of quasiparticles and the extraordinary nonthermal quasiparticle distribution due to the phonon bottleneck effect.

Speaker: Ryusuke Matsunaga (Department of Physics, School of Science, The University of Tokyo)

86 Nanoscale investigation of multilayer, dispersive mirrors damage by spectroscopic near-field microscopy

S. Amarie(1), I. Ivanov(1), V. Pervak(2), F. Keilmann(1) and A. Apolonskiy(1,2) 1) Max Planck Institute of Quantum Optics, Garching, Germany. 2) Ludwig-Maximilians-University Munich, Garching, Germany. Multilayer dielectric mirrors are the limiting factor to scale up and simplify high-power femtosecond laser systems towards shorter pulse durations and higher average or peak powers. Detailed characterization including laser-induced damage is pursued by using nano-FTIR, which features strong material contrast at lateral resolution of only 20 nm. The high sensitivity of chemical recognition is due to phonon resonance of the constituent layers. For the investigated mirror these are TaO5 and SiO2 showing resonances at 680 and 1050 cm-1, respectively. Using 30 fs pulses we observe two different damage mechanisms.

Speaker: Sergiu Amarie (Max Planck Institute of Quantum Optics)

87 Nanoscale layering of antiferromagnetic and superconducting phases in Rb2Fe4Se5

A. Charnukha,1 A. Cvitkovic,2 T. Prokscha,3 D. Pr¨opper,1 N. Ocelic,2 A. Suter,3 Z. Salman,3 E. Morenzoni,3 J. Deisenhofer,4 V. Tsurkan,4, 5 A. Loidl,4 B. Keimer,1 and A. V. Boris1 1Max-Planck-Institut f¨ur Festk¨orperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany 2Neaspec GmbH, D-82152 Martinsried (Munich), Germany 3Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute (PSI), CH-5232 Villigen PSI, Switzerland 4Experimental Physics V, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, D-86159 Augsburg, Germany 5Institute of Applied Physics, Academy of Sciences of Moldova, MD-2028 Chisinau, R. Moldova We studied phase separation in a single-crystalline antiferromagnetic superconductor Rb2Fe4Se5 (RFS) using a combination of scattering-type scanning near-field optical microscopy (s-SNOM) and low-energy muon spin rotation (LE-mSR). We demonstrate that the antiferromagnetic and superconducting phases segregate into nanometer-thick layers perpendicular to the iron-selenide planes, while the characteristic in-plane size of the metallic domains reaches 10 mm. By means of LE-mSR we further show that in a 40-nm thick surface layer the ordered antiferromagnetic moment is drastically reduced, while the volume fraction of the paramagnetic phase is significantly enhanced over its bulk value. Self-organization into a quasiregular heterostructure indicates an intimate connection between the modulated superconducting and antiferromagnetic phases.

Speaker: Aliaksei Charnukha (Max Planck Institute for Solid State research)

88 Non-equilibrium superconductivity in light-stimulated YBa2Cu3Ox

C. R. Hunt (1,2), D. Nicoletti (1), S. Kaiser (1), W. Hu (1), I. Gierz (1), H. Liu (1), M. Le Tacon (3), T. Loew (3), D. Haug (3), B. Keimer (3), A. Cavalleri (1,4) (1) Max Planck Department for Structural Dynamics, University of Hamburg, Center for Free-Electron Laser Science, Hamburg, Germany (2) Department of Physics and the Frederick Seitz Material Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois (3) Max Planck Institute for Solid State Research, Stuttgart, Germany (4) Department of Physics, Oxford University, Clarendon Laboratory, Oxford, United Kingdom Using femtosecond mid-infrared pulses, we demonstrate that a photoinduced non-equilibrium superconducting state can be generated in underdoped YBa2Cu3Ox above Tc. Pumping nearly resonant to the Cu-O phonon mode at 15.8 micron along the c-axis (perpendicular to the Cu-O layers) establishes phase coherence throughout the pseudogap region of the phase diagram. The transient response at three doping levels (x = 6.5, 6.6, 7) was fully characterized using THz time-domain spectroscopy at temperatures ranging from below Tc to room temperature. We measure the non-equilibrium superconducting state in two different ways: First, probing in-plane (along the a-axis) reveals a London-like inverse frequency dependence in the imaginary part of the conductivity which has a lifetime in the 20 ps range. Second, probing perpendicular to the Cu-O planes (along the c-axis) shows evidence of the so-called Josephson plasma resonance, which arises in the superconducting state of cuprates due to a tunneling current of Cooper pairs between the layers. These results offer a new perspective on condensate formation in high temperature superconductors. The photoinduced non-equilibrium state will be discussed in the context of pre-formed pairs in the pseudogap phase.

Speaker: Cassandra Hunt (University of Illinois at Urbana-Champaign;Max Planck Department for Structural Dynamics)

89 Optical and Raman spectrum in the magnetic state of iron pnictides

Noel García, ICMM-CSIC Madrid (Spain) M.J. Calderón, ICMM-CSIC Madrid (Spain) E. Bascones, ICMM-CSIC Madrid (Spain) G. León, ICMM-CSIC Madrid (Spain) E. Cappelluti, ISC, INFM-CNR Rome (Italy) and ICMM-CSIC Madrid (Spain) S. Ciuchi, CNISM Universita dell'Aquila, L'Aquila (Italy) and ISC, INFM-CNR, Rome (Italy) One of the big puzzles in iron-pnictides is to understand the unexpected anisotropy in the magnetic state between x and y direction seen in resistivity and optical conductivity[1] among other techniques. Raman experiments[2] have also seen a symmetry dependence of the arsenide phonon intensity indicating a strong in-plane anisotropy in the magnetic state. There is a strong debate about whether this anisotropy is driven by magnetism or by orbital ordering. Both degrees of freedom are coupled and difficult to disentangle. We consider a 5 orbital tight-binding model[3] at the Hartree-Fock level and calculate the Drude weight, the optical conductivity and the Raman response. Our results point against orbital ordering as the origin of the anisotropy in the Drude weight which may be ascribed to the anisotropy of the Fermi velocity for small magnetic moment[4]. We compare the orbital conductivity anisotropy with the frequency dependent orbital reorganization and find that the anisotropy is mostly magnetic in origin. We study the reorganization of the spectral weight in the optical and Raman spectrum upon varying the Hubbard interaction and the Hund's coupling. We also study the Raman response of the coupling of the arsenide optical phonon with the electronic continuum and compare with Raman experiments.[5] [1] J.-H. Chu et al., Science 329, 824 (2010); Dusza et al, EPL 93 37002 (2011) [2] S. Sugai et al., arXiv:1010. 6151; L. Chauvière et al., Physical Review B 84, 104508 (2011) [3] M.J. Calderon, B. Valenzuela, E. Bascones, Phys. Rev. B 80, 094531 (2009). [4] B. Valenzuela, E. Bascones, M. J. Calderón, Phys. Rev. Lett. 105, 207202 (2010) [5] N. García, B. Valenzuela, M. J. Calderón, E. Bascones, G. León, E. Cappelluti, S. Chiuchi, (in preparation)

Speaker: Belen Valenzuela (Instituto de Ciencia de Materiales de Madrid ICMM-CSIC)

90 Optical properties of mechanically-exfoliated Bi2Sr2CaCu2O8

Luke Sandilands (University of Toronto, Canada) Viktoriya Baydina (University of Toronto, Canada) Alexander Su (University of Toronto, Canada) Anjan Reijnders (University of Toronto, Canada) Tor Pedersen (Canadian Light Source, Canada) Ferenc Borondics (Canadian Light Source, Canada) Genda Gu (Brookhaven National Laboratory, USA) Shimpei Ono (University of Tokyo, Japan) Yoichi Ando (Osaka University, Japan) Kenneth Burch (Institute for Optical Sciences, University of Toronto, Canada) We report on the mid-infrared (0.175 to 0.65 eV) optical conductivity of mechanically exfoliated optimally-doped Bi2Sr2CaCu2O8 thin crystals on SiO2/Si substrates. The conductivity of thicker (greater than 100 nm) samples is comparable to bulk while that of thinner (20 nm) samples is markedly suppressed and suggestive of insulating behaviour. We attribute this change to the presence of a degraded surface layer in our samples. Using an effective medium approximation, we are able to explain the observed trend and to extract the 'intrinsic' optical properties of BSCCO as a function of thickness. Our results indicate no thickness-induced change in the optical response of BSCCO samples as thin as 3 bilayers.

Speaker: Luke Sandilands (University of Toronto)

91 Optical Study of the Pseudogap State in YBa2(Cu1-xZnx)3O7-δ

Ece UYKUR Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, JAPAN Takahiko MASUI Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, JAPAN Kiyohisa TANAKA Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, JAPAN Shigeki MIYASAKA Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, JAPAN Setsuko TAJIMA Department of Physics, Graduate School of Science, Osaka University, Osaka 560-0043, JAPAN It has been shown that the c-axis optical spectra of the high-Tc cuprate superconductors are sensitive to the antinodal region of the Fermi surface that is reflecting the strong effects of the pseudogap. Unlike the other spectroscopic techniques, the optical spectra can in principle distinguish the pseudogap and the superconducting condensation, based on the behavior of the spectral weight transfer. However, in reality, the additional structures such as the transverse Josephson plasma (TJP) mode [1] in the c-axis spectra of the YBa2Cu3O7-δ make the discussion difficult and complicated. In this study, we carried out the temperature dependent c-axis reflectivity measurements on the Zn-substituted YBa2Cu3O7-δ single crystals, where the additional anomalies in the spectra were suppressed with the Zn-doping [2]. In the absence of the TJP resonance mode, we have observed the continuous transfer of the low energy spectral weight to the higher energy region even below Tc as the response of the pseudogap. This is the evidence of the coexistence of the pseudogap and the superconducting gap, suggesting an inhomogeneous state. Furthermore, we found an unexpected Zn-effect, the disappearance of the change in kinetic energy at the superconducting transition which is pronounced in the underdoped regime. This might be due to the weakening of the carrier confinement by Zn-doping [3,4]. [1] C. Bernhard et al., Phys. Rev. B 61, 618 (2000) [2] R. Hauff et al., Phys. Rev. Lett. 77, 4620 (1996) [3] T. Masui et al., Advances in Superconductivity XII (Springer-Verlag) pp. 221 (2000) [4] K. Tomimoto et al., Phys. Rev. B 60, 114 (1999) ; S. Tajima et al., J. Low Temp. Physics 117, 413 (1999)

Speaker: Ece UYKUR (Department of Physics, Graduate School of Science, Osaka University)

92 Persistent high-energy spin excitations in RIXS spectra of optimally doped Bi-2212

M. Guarise, B. Dalla Piazza, H. Ronnow, H. Berger, M. Grioni Institute of Condensed Matter Physics, EPFL, CH-1015 Lausanne (Switzerland) We have exploited high-resolution resonant inelastic x-ray scattering (RIXS) at the Cu L3 edge to measure the full spectrum of spin excitations in Bi-2212. We find that the magnon dispersion and spectral line shape of the insulating Y-substituted parent compound are very similar to those of the AFM insulator Sr2CuO2Cl2 [1]. Namely, we observe a dispersion along the magnetic zone boundary, indicative of interactions beyond nearest-neighbors. We also find that clear signatures of spin waves persist in superconducting underdoped and even optimally doped Bi-2212 samples, but develop a remarkable anisotropy. Along the nodal direction the magnon peaks are well-defined and follow the dispersion of the parent insulator. By contrast, only a broad quasielastic tail is seen in the anti-nodal direction. These results suggest a similar anisotropy of the coherence length of the paramagnons. [1] M. Guarise et al., Phys. Rev. Lett. 105, 157006 (2010)

Speaker: Marco Grioni (ICMP-EPFL)

93 Precursor superconducting phase at temperatures as high as 180 K in superconducting cuprate crystals from infrared spectroscopy

A. Dubroka1,2, M. Rössle1, K.W. Kim1, V. K. Malik1, D. Munzar2, D. N. Basov3, A. A. Schafgans3, S. J. Moon3, C. T. Lin4, D. Haug4, V. Hinkov4, B. Keimer4, Th. Wolf5, J. G. Storey6, J. L. Tallon6, and C. Bernhard1 1.) Physics Department and Fribourg Center for Nanomaterials (FriMat), University of Fribourg, Chemin du Musée 3, CH-1700 Fribourg, Switzerland 2.) Institute of Condensed Matter Physics, Faculty of Science, Masaryk University and Central European Institute for Technology, Kotlářská 2, Brno, Czech Republic 3.) Department of Physics, University of California, San Diego, La Jolla, California 92093, USA 4.) Forschungszentrum Karlsruhe, IFP, D-76021 Karlsruhe, Germany. 5.) Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany 6.) MacDiarmid Institute for Advanced Materials & Nanotechnology,Industrial Research Ltd, Gracefield Road, P.O. Box 31310, Lower Hutt, New Zealand We present results of our detailed study of the infrared c-axis response of underdoped cuprate high-temperature superconductors RBa2Cu3O7 (R=Y; Gd; Eu) [1]. In addition to competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*>>Tc, our analysis enables us to identify the onset of another phase below Tons >Tc. In this phase in contrast to that of related with T*, the low-energy spectral weight increases with decreasing temperature. Below Tc, it transforms into the condensate and the underlying electronic states are susceptible to magnetic fields. All these characteristics are hallmarks of superconducting fluctuations and thus we conclude that the phase corresponds to a precursor superconducting state. Our conclusions are strongly supported by the data of the in-plane infrared conductivity where a gap opens below Tons which is accompanied by the shift of spectral weight towards lower frequencies [2]. We map out the doping phase diagram of Tons which reaches a maximum of 180 K at strong underdoping. A very intriguing property of the precursor superconducting phase is that it involves a very large fraction of the low-energy electronic states that increases with underdoping to that extent that for very strongly underdoped samples, the effects above Tc are much stronger than those below Tc. Our results helps to understand the mysterious phenomenology of the pseudogap showing that this phase involves two different phenomena which is likely a source of the ongoing dispute of the origin of the pseudogap. [1] A. Dubroka et al., Phys. Rev. Lett. 106, 047006 (2011) [2] See supplemental material at http://link.aps.org/supplemental/10.1103/PhysRevLett.106.047006

Speaker: Adam Dubroka (Masaryk University)

94 Raman response in density wave materials

E.A. Nowadnick, A.F. Kemper, B. Moritz (Stanford and SLAC), R. Hackl (Walther Meissner Institut), T.P. Devereaux (Stanford and SLAC) Raman spectroscopy, which uses different incoming and outgoing light polarizations to measure different areas of the Brillouin zone, allows researchers to probe the nature of charge and spin density wave gaps. We present calculations of the Raman response for two density wave materials: rare earth tri-tellurides in the charge density wave state and the iron pnictides in the spin density wave state. Both of these materials have phase diagrams which can be further understood by clarifying the nature of the density wave state. For example, in the tri-tellurides, either one or two charge density wave gaps are present depending on the type of rare earth element in the compound. In the pnictides, superconductivity coexists with or is in close proximity to an antiferromagnetic spin density wave state, and there is debate over whether the magnetism is best described by an itinerant or local moment picture. We discuss what can be learned from our calculations and compare to experimental results.

Speaker: Elizabeth Nowadnick (Stanford University)

95 Single-layer terahertz metamaterials with bulk optical constant

W.-C. Chen1, A. Totachawattana1, K. Fan2, J. L. Ponsetto1, A. C. Strikwerda3, X. Zhang2, R. D. Averitt3, and W. J. Padilla1 1Department of Physics, Boston College, 140 Commonwealth Ave., Chestnut Hill, Massachusetts 02467, USA 2Department of Mechanical Engineering, Boston University, 15 Saint Mary's St., Brookline, Massachusetts 02446, US 3Department of Physics, Boston University, 590 Commonwealth Ave., Boston, Massachusetts 02215, USA Metamaterials have been drawn a great deal of attention, because they offer many exotic properties that might not be readily available for naturally occurring materials - negative refractive index being the prime example.[1] The true power of metamaterials originating from their ability to construct materials with a specific electric and magnetic response - the electric permittivity (epsilon) and the magnetic permeability (μ). Those designer materials are widely used for many real-life applications. For many cases, singly layer metamaterials are sufficient for creating the desired functionality. As for some other cases, one might require to use multiple layered metamaterials to achieve much sophisticated responses. However, it is well known single layer metamaterials might not have true bulk properties, even though the extracted parameters can predict the optical responses very precisely. To this regard, we perform an experimental and computational study on what the conditions under which single layer metamaterials may be described by bulk optical constants. Terahertz time domain spectroscopy is utilized to investigate two types of geometries, both with two different sizes of embedding dielectric — cubic and tetragonal unit cells. The tetragonal metamaterials are shown to yield layer dependent optical constants, whereas the cubic metamaterials yielded layer independent optical constants. We establish guidelines for when epsilon and μ can be used as material parameters for single layer metamaterials. Experimental results at terahertz frequencies are presented and supported by full wave three-dimensional electromagnetic simulations. Reference: [1]R. A. Shelby, D. R. Smith, S. Shultz, "Experimental Verification of a Negative Index of Refraction". Science 292, 77–79. (2001). [2] W.-C. Chen, A. Totachawattana, K. Fan, J. L. Ponsetto, A. C. Strikwerda, X. Zhang, R. D. Averitt, and W. J. Padilla,"Single-layer terahertz metamaterials with bulk optical constants" Physical Review B 85, 035112 (2012).

Speaker: Wenchen Chen (Boston College)

96 Single-layer terahertz metamaterials with bulk optical constant

W.-C. Chen1, A. Totachawattana1, K. Fan2, J. L. Ponsetto1, A. C. Strikwerda3, X. Zhang2, R. D. Averitt3, and W. J. Padilla1 1Department of Physics, Boston College, 140 Commonwealth Ave., Chestnut Hill, Massachusetts 02467, USA 2Department of Mechanical Engineering, Boston University, 15 Saint Mary's St., Brookline, Massachusetts 02446, US 3Department of Physics, Boston University, 590 Commonwealth Ave., Boston, Massachusetts 02215, USA Metamaterials have been drawn a great deal of attention, because they offer many exotic properties that might not be readily available for naturally occurring materials - negative refractive index being the prime example.[1] The true power of metamaterials originating from their ability to construct materials with a specific electric and magnetic response - the electric permittivity () and the magnetic permeability (μ). Those designer materials are widely used for many real-life applications. For many cases, singly layer metamaterials are sufficient for creating the desired functionality. As for some other cases, one might require to use multiple layered metamaterials to achieve much sophisticated responses. However, it is well known single layer metamaterials might not have true bulk properties, even though the extracted parameters can predict the optical responses very precisely. To this regard, we perform an experimental and computational study on what the conditions under which single layer metamaterials may be described by bulk optical constants. Terahertz time domain spectroscopy is utilized to investigate two types of geometries, both with two different sizes of embedding dielectric — cubic and tetragonal unit cells. The tetragonal metamaterials are shown to yield layer dependent optical constants, whereas the cubic metamaterials yielded layer independent optical constants. We establish guidelines for when  and μ can be used as material parameters for single layer metamaterials. Experimental results at terahertz frequencies are presented and supported by full wave three-dimensional electromagnetic simulations. Reference: [1]R. A. Shelby, D. R. Smith, S. Shultz, "Experimental Verification of a Negative Index of Refraction". Science 292, 77–79. (2001). [2] W.-C. Chen, A. Totachawattana, K. Fan, J. L. Ponsetto, A. C. Strikwerda, X. Zhang, R. D. Averitt, and W. J. Padilla,"Single-layer terahertz metamaterials with bulk optical constants" Physical Review B 85, 035112 (2012).

Speaker: Wenchen Chen (Boston College)

97 Spectroscopic near-field microscopy of biominerals in the phonon region

Sergiu Amarie Fritz Keilmann Max Planck Institute of Quantum Optics, Garching, Germany Spectroscopic near-field imaging is enabled by combining a coherent mid-infrared continuum source with 20nm-resolving tip-scattering near-field microscopy (s-SNOM). Broadband amplitude and phase spectra are acquired together with topographical height at every pixel while scanning. As a first application we present studies of biological nanocomposites, namely mollusc shell and human bone specimens which contain mineral nanocrystals in organic matrices. The mineral parts exhibit strongly resonant spectral contrasts due to phonons. Local variations of resonance positions and shapes are observed and correlated with known contrasts from electron microscopy. They give information on chemical identity, crystal structure, and mineral density. Our method should be straightforwardly applicable in many fields of general mineralogy, solid state research, and materials science.

Speaker: Fritz Keilmann (Max Planck Institute of Quantum Optics, Garching, Germany)

98 Superconducting coherence along c-axis in the stripe phase of high-Tc cuprates

K. Tanaka, Y. Sakai, T. Miyake, S. Miyazaki, S. Miyasaka and S. Tajima, Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan M. Tonouchi Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan T. Sasagawa Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama Kanagawa 226-8503, Japan Recently, a two-dimensional superconducting state has been reported from transport studies in the stripe-ordered La2−xBaxCuO4 with x = 1/8 [1]. This is consistent with the results of c-axis (E//c) infrared optical studies for La2−xBaxCuO4 and La2−x−yNdySrxCuO4, that the Josephson plasma edge originating from the Josephson coupling of the CuO2 planes disappears in the stripe phase [2]. These results indicate the disappearance of the superconducting coherence along c-axis and the decoupling of the CuO2 planes. To clarify the universality of this phenomena, we performed terahertz time-domain spectroscopy (THz-TDS) measurement, in which one can obtain lower frequency information than the conventional Fourier transform type spectrometer, on static stripe-ordered La1.84−yEuySr0.16CuO4 (y = 0, 0.1, 0.2) and La2−x−yNdySrxCuO4 (x=0.125, 0.16, y=0.1~0.5), where the stability of the stripe phase can be controlled by y. We found that the Josephson plasma edge shows systematic shift to the lower frequency with increasing y and survives in the extremely low frequency region in the stripe-ordered phase. By comparing the superfluid density along c-axis and in-plane, we conclude that the system is not going toward two-dimensional superconducting state with stabilizing the stripe order. [l] Q. Li, M. Hucker, G.D. Gu, A.M. Tsvelik, and J.M. Tranquada, Phys. Rev. Lett. 99, 067001 (2007). [2] A.A. Schafgans, C.C. Homes, G. D. Gu, Seiki Komiya, Yoichi Ando, and D.N. Basov, Phys. Rev. B 82, 100505(R) (2010).

Speaker: Kiyohisa TANAKA (Osaka University)

99 Terahertz excitations in the 1D Ising chain quantum magnet CoNb2O6

Christopher M. Morris, Rolando Valdés Aguilar, Seyed Koopayeh, Colin L. Broholm, N. Peter Armitage The Institute for Quantum Matter, Department of Physics & Astronomy, The Johns Hopkins University, Baltimore, MD 21218 The one-dimensional magnet CoNb2O6 was recently demonstrated to be an excellent realization of a one-dimensional quantum Ising spin chain. It has been shown to undergo a quantum phase transition in a magnetic field oriented transverse to its ferromagnetically aligned spin chains. Low energy spin-flip excitations in the chains were recently observed via inelastic neutron scattering [1]. The energy spectrum of these excitations was shown to have an interesting energy scaling governed by symmetries of the E8 exceptional Lie group. Here, time-domain terahertz spectroscopy (TDTS) is used to investigate optically active low energy excitations in CoNb2O6. We take advantage of the polarization sensitivity of this technique to characterize both electric and magnetic dipole active excitations in this compound. A connection is made from the q=0 response observed here to the excitations observed by neutron scattering. Additionally, a number of magnetic dipole excitations not evident in the neutron scattering experiments are observed. Finally, we will show preliminary data on the terahertz spectra of this material as it undergoes the magnetic field-tuned quantum phase transition. [1] R. Coldea et al, Science 327, 177 (2010)

Speaker: Christopher M. Morris (Johns Hopkins University)

100 THz Induced Breakdown of Superconductivity in NbN

X. Xi X.-J. Wang Y. Shen J.B. Murphy X. Yang and G.L. Carr Photon Sciences, Brookhaven National Laboratory We have begun an experimental study of superconductivity breakdown behavior in a thin NbN film when exposed to a single, strong-field THz pulse. By limiting the THz spectral content to below-gap frequencies, the breakdown is assumed to take place when the induced current density exceeds the critical current density - but the details for this process are mostly unknown due to the onset of thermal effects in typical transport measurements. A finite-difference time-domain approach has been developed to model the process and compared with experimental results that show significant non-linear THz upconversion as breakdown occurs.

Speaker: Xiaoxiang Xi (Brookhaven National Laboratory)

101 Towards understanding the c-axis infrared response of underdoped cuprate superconductors

Dominik Munzar and Jiri Vasatko, Department of Condensed Matter Physics, Faculty of Science, and Central European Institute of Technology, Masaryk University, Kotlarska 2, 611 37 Brno, Czech Republic The c-axis infrared (IR) conductivity of underdoped high-Tc cuprate superconductors reveals a pronounced pseudogap (PG) and, for materials with two CuO2 planes per unit cell, signatures of coherent electronic coupling within the pair of closely spaced planes, in particular the so-called transverse plasma mode (TPM) located around 400 cm-1. The PG develops below T* much greater than Tc, the TPM below Tons, Tc less than Tons much less than T*. We report on results of our recent studies aiming at understanding these phenomena. (a) The formulas frequently used to describe the c-axis response of the coupled electron-phonon system of bilayer cuprate superconductors, that were originally obtained at the level of the phenomenological multilayer model (MLM), have been derived by using diagrammatic perturbation theory [1,2]. This provides a support for several important findings based thereon, in particular those of [3]. (b) The reported magnetic field (H perpendicular to the planes) induced changes of the TPM [4,3] have been clarified using the MLM [5]. Results of our analysis suggest that the response at H=0 and T=Tc is close to that at H = ca 25 T less than Hc2 and T much less than Tc, in accord with theories attributing the above Tc state to that of a superconductor lacking the long range phase coherence. (c) The qualitative difference between the manifestations of the PG in the c-axis IR response and those in the in-plane one belongs to major unsolved problems in the physics of the cuprates. In the third part of our contribution, implications of the MIR data reported by Yu et al. [6] will be discussed. [1] J. Chaloupka, C. Bernhard, D. Munzar, Phys. Rev. B 79, 184513 (2009). [2] J. Vasatko and D. Munzar, submitted to Phys. Rev. B, cond-mat 1203.6523v1. [3] A. Dubroka et al., Phys. Rev. Lett. 106, 047006 (2011). [4] A. D. LaForge et al., Phys. Rev. B 76, 054524 (2007). [5] J. Marek and D. Munzar, Journ. of Phys.: Condensed Matter 23, 415703 (2011). [6] L. Yu et al., Phys. Rev. Lett. 100, 177004 (2008).

Speaker: Dominik Munzar (Masaryk University)

102 Tuning the plasmonic regime in high temperature superconductor metamaterials

Giliberti, Valeria; Department of Physics, University of Rome La Sapienza, Rome (Italy) Ortolani, Michele; Department of Physics, University of Rome La Sapienza, Rome (Italy) Di Gaspare, Alessandra; CNR-Istituto di Fotonica e Nanotecnologie, Rome (Italy) De Marzi, Gianluca;ENEA:Fusion-Superconductivity, Frascati (Italy) Lupi, Stefano; Department of Physics, University of Rome La Sapienza, Rome (Italy) Superconductors are promising candidates for active and tunable metamaterials in the terahertz (THz) range. In this work, we demonstrate the modulation, through temperature, of the intensity, linewidth and frequency of plasmonic features of high temperature superconductors (cuprates and MgB2) based metamaterials. Through a detailed analysis of the parameters characterizing these plasmonic excitations, we also extract some important properties of superconductor compounds, like the penetration depth and the superfluid density. These parameters are usually measured in microwaves using guide propagation and cavity techniques. These techniques, for physical limitations, are hard to extend in the THz range. Using free-space propagation and virtual resonant cavity induced on samples by litographed sub-wavelength resonators, we obtain the temperature dependence of the superfluid density and penetration depth for frequencies up to 3 THz.

Speaker: Odeta Limaj (Department of Physics, University of Rome La Sapienza)

103 Ultrafast conductivity dynamics in the colossal magnetoresistance La1-xCaxMnO3 thin films

J. Zhang, Boston University R. D. Averitt, Boston University X. Tan, University of Science and Technology of China W. Wu, University of Science and Technology of China La1-xCaxMnO3 is a classic colossal magnetoresistance (CMR) material where the conductivity displays a marked sensitivity to an external magnetic field for reasons that are still not fully understood. The underlying rich physics is a result of the strong coupling of spin, lattice, orbital and charge degrees of freedom. Optical spectroscopy provides experimental access to the underlying interactions in the manganites including, as examples, spin and orbital ordering, and the metal-insulator transition. Further, time-resolved optical spectroscopy can dynamically probe photoinduced changes that drive phase transitions. In this work we report on time resolved terahertz spectroscopic studies of strained La1-xCaxMnO3 thin films, where we observe and control electron-spin-lattice relaxation dynamics with 1.5 eV excitation pulses. We will describe, in detail, the observed differences in the conductivity dynamics as a function of lattice strain.

Speaker: Jingdi Zhang (Boston University)

104 Ultrafast optical study of the electron doped cuprate NCCO

James Hinton, Jake Koralek (LBNL, UC Berkeley) Guichan Yu, Mun Chan (University of Minnesota) Neven Barisic (University of Minnesota, University of Stuttgart) Martin Greven (University of Minnesota) Joe Orenstein (LBNL, UC Berkeley) We employed time resolved reflectivity (TRR) and transieng grating spectroscopy (TGS) to study the electron doped cuprate superconductor Nd(2-x)Ce(x)CuO4 across a broad temperature and doping range. The pseudogap (PG) response above Tc is consistent with excitation of fluctuating antiferromagnetic order, previously observed in two-magnon Raman scattering. Cooling below Tc, we see the onset of a response due to photo-excitation of superconducting quasiparticles. This SC signal coexists with the PG signal, which is partially suppressed below Tc, suggestive of competition between the SC and PG orders.

Speaker: James Hinton (UC Berkeley, LBNL)

105 Unique Automatic Beamsplitter Exchange Unit for a Research Vacuum FT-IR Spectrometer

Michael Joerger Guenter Zachmann Since the introduction of FT-IR spectroscopy, it is very well accepted by the research community that a spectrometer based on a vacuum optics bench provides IR spectra which are free of residual H2O- and CO2- absorption bands. Such residual absorptions of the laboratory room air are typically visible under purge conditions and may mask weak spectral features of the measured sample spectrum. But measurements of a sample in the complete IR or THz spectral range require different types of optical components which are mainly detectors, sources and beamsplitters (BMS). The technology for automatic detector and source switching is well established. In contrary to that automatic BMS exchange under vacuum condition has been not yet realized in modern FT-IR spectrometers because of the high technical demand for precision, its complexity and costs. Therefore it was necessary to vent and re-evacuate the spectrometer optics bench for the manual BMS exchange. In this contribution the functionality of a new and unique automatic beamsplitter exchange unit (BMS-c) for the VERTEX 80v research bench-top vacuum FT-IR spectrometer will be presented. Spectra measured with up to four different types of BMS in connection with a unique Transmittance/Reflectance accessory as well as with a diamond micro ATR unit will be shown. Up to four different types of BMS are mounted on the exchange unit which are e.g. far IR/THz Mylar-BMS with 50µm thickness and the far IR multilayer Mylar BMS as well as the standard mid IR KBr beamsplitter and the wide band UV-VIS-NIR beamsplitter. The used BMS is selected via the instrument user interface and automatically positioned without the need of venting and re-evacuation the spectrometer optics bench. To demonstrate the usability example spectra of different solid and powdered material in the very wide spectral range from 10cm-1 (0.3THz) up to 25,000cm-1 (400nm) have been measured. In particular for measurements using a liquid He cooled cryostat or samples with unstable morphology the automated BMS exchange unit is of advantage. It avoids any change of the thermal conditions of the optics bench by venting and evacuation and provides therefore very stable measurement results and at the same time significant shorter measurement times.

Speaker: Michael Joerger (Bruker Optik GmbH, Rudolf-Plank-Strasse 27, 76275 Ettlingen, Germany)

106 Vacuum FT-IR spectrometer: Research Tool at Synchrotron Infrared Beamlines

Michael Joerger Guenter Zachmann In the 70s of the last century first attempts to make use of the advantage of bright and highly collimated e-synchrotron radiation not only in the short wavelength range (UV, X-Ray) but also in the long wavelength range (FIR, typically 400 to 10cm-1) failed. The theoretically expected advantage of the synchrotron radiation as bright but expensive IR sources could not be shown. Today the system parameters for the e-synchrotron beam generating radiation as well as for the optics design for guiding the synchrotron radiation to an IR beamline are well explored. Successful experiments providing new insight resulted in the adoption of synchrotron radiation as a radiation source for spectrometers and consequently an increase in the number of IR beamlines with spectrometers attached. Outstanding data have been published using these beamline spectrometer combinations in the field of IR microscopy and imaging [1], low temperature far IR ellipsometry [2] and high pressure physics [3]. In recent years the long wavelength IR spectral range (THz, typically <100cm-1), became of high interest due to new technologies and challenging applications [4]. The combination of high resolution and flexible vacuum FT-IR spectrometers installed at IR beamlines proofed to be an ideal combination for the study of the characteristics and new applications of IR and THz radiation. In this presentation an overview of applications utilizing vacuum FT-IR spectrometers at e-synchrotron IR beamlines will be presented. The advantages of synchrotron radiation versus thermal radiation sources with respect to spectral range, resolution and scanning speed will be discussed as well as synchronization of the scanner to the e-synchrotron pulses, the use of automation options and modern detector technology [5]. References: [1] D. Moss et al., Infrared Phys. Technol., 49, 53–56 (2006) [2] C. Bernhard et al., Thin Solid Films, 455 –456 (2004), 143–149 [3] S. Lupi et al., Nature Communications 1, Article number: 105 doi:10.1038/ncomms1109 [4] Karantzoulis et al., Infrared Phys. & Technol., 53 (2010) 300–303 [5] P. Innocenzi et al., J. Phys. Chem. C 2007, 111, 5345-5350

Speaker: Michael Joerger (Bruker Optik GmbH, Rudolf-Plank-Strasse 27, 76275 Ettlingen, Germany)

107 Vortex-State Electrodynamics in Superconducting Thin Films Studied by Far-Infrared Spectroscopy

Xiaoxiang Xi,a,b) G. L. Carr,a) J.-H. Park,c) D. Graf c) and D. B. Tanner b) a) Photon Sciences, Brookhaven National Laboratory, Upton, NY 11973 b) Department of Physics, University of Florida, Gainesville, FL 32611 c) National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310 In a type-II superconductor, a magnetic field above the lower critical field creates vortices and dramatically changes the superconductor's electrodynamic response. Such changes have been observed in thin film samples by our infrared magneto-spectroscopy experiments with field normal to the film surface. The complex optical conductivity was extracted, and was compared to effective medium theories for the electrodynamic response of the vortex state. We found a good agreement between our optical data and the Maxwell Garnett theory, which treats the mixed state as having normal-metal disks (representing the vortex cores) surrounded by superconductor. Our data also show the effect of magnetic-field-induced pair breaking on the superconducting fraction outside of the vortices.

Speaker: Xiaoxiang Xi (Brookhaven National Lab)

Wednesday, 25 July

Spin Phenomena

Convener: George Gruner (UCLA)

108 Quantum information processing with spins in diamond and silicon carbide

W. F. Koehl, B. B. Buckley, F. J. Heremans, G. Calusine, and D. D. Awschalom Center for Spintronics and Quantum Computation, University of California, Santa Barbara, CA 93106 USA A strong motivation behind modern research into quantum physics has been to identify robust quantum states that can be easily controlled, for future use in advanced information technologies. During the past few years, an optically active point defect in diamond known as the nitrogen-vacancy (N-V) centre has attracted a great deal of interest because it possesses an atomic scale electronic spin state that can be used as an individually addressable, solid state quantum bit (qubit) even at room temperature. The N-V center’s optical transitions become coherent, spin-dependent, and may be precisely controlled with gate voltages in micron-scale devices, with promise for future photonic applications [1]. Moreover, engineered coupling of this electron spin with the proximal single nitrogen nuclear spin enables a scalable quantum memory [2]. These exceptional coherent properties have motivated theoretical efforts to predict and identify similar defects in other semiconductors, since they may offer an expanded range of functionality not available to the diamond N-V [3]. We show that several defect spin states in various polytypes of SiC can be optically addressed and coherently controlled in the time domain at temperatures ranging from 20 – 300 K. Using optical and microwave techniques similar to those used with diamond N-V qubits, we study the ground states of the neutral carbon-silicon divacancy. These defects are optically active near telecom wavelengths, and inhabit a host material for which there already exist industrial scale crystal growth and advanced microfabrication techniques. In addition, optical spectroscopy and spin resonance measurements reveal that they possess desirable spin coherence properties comparable to those of the diamond N-V. This makes them promising candidates for various photonic, spintronic, and quantum information applications that merge quantum degrees of freedom with classical electronic and optical technologies [4]. [1] L.C. Bassett*, F.J. Heremans*, C.G. Yale*, B.B. Buckley*, & D.D. Awschalom, Phys. Rev. Lett. 107, 266403 (2011). [2] G. D. Fuchs, G. Burkard, P. V. Klimov, and D. D. Awschalom, Nature Physics 7, 789 (2011). [3] J. R. Weber*, W. F. Koehl*, J. B. Varley*, A. Janotti, B. B. Buckley, C. G. Van de Walle, and D. D. Awschalom, Proc. Natl Acad. Sci. USA 107, 8513 – 8518 (2010). [4] W. F. Koehl, B. B. Buckley, F. J. Heremans, G. Calusine, and D. D. Awschalom, Nature 479, 84 (2011).

Speaker: David Awschalom (Center for Spintronics and Quantum Computation, University of California, Santa Barbara, CA 93106)

109 Ultrafast probes of spin and charge dynamics

J. Orenstein (UC Berkeley and LBNL) I will describe two time-resolved optical studies, one on the electron doped high Tc superconductor NCCO and the other on the two dimensional electron gas in GaAs quantum wells. The former experiments were performed in collaboration with Martin Greven's group U. Minnesota, and the latter with Mike Lilly of Sandia. In both cases, low-energy excitations, on the scale of meV's, are revealed, despite the fact that the optical probe is at the rather high energy of 1.5 eV. In NCCO, time-resolved optics sees critical slowing down of fluctuations with decreasing T that suggest the approach to a quantum critical point. In the 2DEG experments, phase-resolved transient grating techniques reveal low energy propagating modes of helical spin polarization.

Speaker: Joe Orenstein (UC Berkeley and LBNL)

110 Topological Structures of Novel Quantum System – Views from Scanning Microwave Impedance Microscope

Zhi-Xun Shen, Stanford University Understanding and controlling local conductivity have been a corner stone for important scientific breakthroughs and technological inventions, as exemplified by transistor, integrated circuit, Anderson localization, quantum hall effect and fractional quantum hall effect. In these cases, local visualization and control of doping, mobility, gating, dielectric, heterostructure, and inter-diffusion are important. Microwave Impedance microscope provides a new platform to measure local electrical properties. Microwave has several inherent advantages. It is coherent so both amplitude and phase information can be analyzed to gain quantitative insight. Its high frequency naturally leads to efficient capacitive coupling, thus no contact is needed for electrical measurement. It has much higher inherent contrast for electrical properties than optical microscopy, for conductivity diverges for metal but approaches zero for insulator. However, it also has two disadvantages – relatively poor spatial resolution and stray field coupling that compromises quantitative analysis. We will report our progress in developing an AFM based, and scalable (batch processed tip) non-resonance microwave impedance microscope that achieves a resolution ~ 50 nm and reduces stray field coupling. The non-resonance approach and merge with the AFM platform also greatly reduce many of the “practical problems” that severely compromises advances of the earlier resonator based scanning microwave microscope, such as thermal drift, height control, and tip consistency – all critical for quantitative and repeatable measurements. We will show samples images from a range of materials – semiconductors, dielectrics, complex oxides, phase change memory materials, graphene, and topological insulators, also functional properties such as metal-insulator transition, semiconductor metrology, photoconductivity, imaging in water and bio-cells . The highlight of this talk will be the physics insight we gain by applying this technique to investigate topological structures of novel quantum systems, including edge state of topological order in QHE system, the percolation pathways in colossal magnetoresistive manganites and domain walls and vertex of ferroelectric and multi-ferro systems.

Speaker: Zhi-Xun Shen (Stanford University)

111 Observation of Spin-Orbital Separation in the spin chain Sr2CuO3 with Resonant Inelastic X-ray Scattering

T. Schmitt1, J. Schlappa1, K. Wohlfeld2, K. J. Zhou1, M. Mourigal3, M. W. Haverkort4, V. N. Strocov1, L. Hozoi2, C. Monney1, S. Nishimoto2, S. Singh5, A. Revcolevschi5, J.-S. Caux6, L. Patthey1, H. M. Ronnow3, J. van den Brink2 1Paul Scherrer Institut, Swiss Light Source, CH-5232 Villigen PSI, Switzerland 2Leibniz Institute for Solid State and Materials Research IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany 3Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland 4Max-Planck-Institut für Festkörperforschung, D-70506 Stuttgart, Germany 5ICMMO - UMR 8182 - Bât. 410, Université Paris-Sud 11, 91405 Orsay Cedex, France 6Institute for Theoretical Physics, Universiteit van Amsterdam, 1090 GL Amsterdam, The Netherlands Resonant Inelastic X-ray Scattering (RIXS) is a powerful probe of excitations from the electronic ground state in transition-metal oxides. In this talk we present high-resolution RIXS studies of magnetic and electronic excitations in the low dimensional spin chain system Sr2CuO3 performed at the ADvanced RESonant Spectroscopies (ADRESS) beamline of the Swiss Light Source with the SAXES spectrometer [1]. In general, quantum effects become important when the space symmetry is lowered. In the extreme case of one-dimensional-materials the electron can break up into separate quasi-particles, i.e., spinons, holons and orbitons that carry their respective spin, charge and orbital degrees of freedom [2]. Sr2CuO3 is an ideal realization of the one-dimensional Heisenberg spin-1/2 chain. When an electron is removed from this spin-chain one can for instance observe how spin and charge degrees of freedom are splitting in the so called spin-charge separation mechanism [3]. Our Cu L3-RIXS measurements on Sr2CuO3 reveal the fractionalization of magnons into two-spinons and higher order excitations as previously reported from neutron scattering [4]. Furthermore, we observe the splitting of an orbital excitation into the independently propagating spinon and orbiton quasi-particles [5]. This newly observed spin-orbital separation phenomenon gives thereby rise to strongly dispersive orbital excitations (orbitons) [6]. [1] V. N. Strocov et al., J. Synchrotron Rad. 17, 631–643 (2010); G. Ghiringhelli et al., Rev. Sci. Instrum. 77, 113108 (2006). [2] T. Giamarchi, Quantum Physics in One Dimension (Clarendon Press, Oxford, 2004) and references therein. [3] B.J. Kim et al., Nature Physics 2, 397–401 (2006). [4] A.C. Walters et al., Nature Physics 5, 867 (2009). [5] J. Schlappa, K. Wohlfeld, J. van den Brink, T. Schmitt et al., Nature (in press, February 2012). [6] K. Wohlfeld, M. Daghofer, S. Nishimoto, G. Khaliullin and J. van den Brink, Phys. Rev. Lett. 107, 147201 (2011).

Speaker: Thorsten Schmitt (Paul Scherrer Institut)

10:15 Break

Photoinduced Studies II

Convener: Ricardo Lobo (Laboratoire de Physique et d'Etude des Materiaux, CNRS)

112 Light-induced non-equilibrium superconductivity throughout the pseudogap phase of cuprates

A. Cavalleri, S. Kaiser, D. Nicoletti, C. Hunt, M. LeTacon, B. Keimer, T. Takayama, H. Takagi In this talk I will discuss some of our recent work aimed at controlling superconductivity in the High Tc cuprates with light. High-field THz and mid-infrared radiation are used to manipulate low frequency excitations in the superconducting and in the underdoped phases. The first key result is that ordered states that compete with superconductivity, such as for instance the striped phase in LESCO ant 1/8 hole doping, can be melted with light, making superconductivity emerge on the ultrafast timescale. Ultrafast soft x-ray scattering experiments with the Stanford LCLS X-ray Free Electron Laser allow for a hierarchy of events to be established, clocking and quantifying the relaxation of lattice and charge order. In recent experiments, transient superconductivity is been demonstrated throughout the pseudogap phase of YBCO, including at room temperature for the underdoped O6.5 compounds. In this case, the underlying physical picture is less clear than in the LESCO case, but it may be connected to the effect of coherent modulation of the microscopic electronic properties through coherent excitation of the apical oxygen postion. A straightforward conceptual analogy can be found with experiments that that study driven dynamics of strongly correlated atomic gases in optical lattices.

Speaker: Andrea Cavalleri (Max Planck Department for Structural Dynamics Center For Free Electron Laser – Hamburg, Department of Physics – University of Oxford)

113 Electronic matter wave rescattering at a nanoscale metal tip on attosecond time scales

Peter Hommelhoff, Max Planck Institute of Quantum Optics When femtosecond laser pulses are focused on nanometric metal tips, electrons are emitted. For few-cycle laser pulses, the liberated electronic matter wave can be driven back towards to parent tip when the laser field flips sign. At the tip's surface it can scatter elastically, gain more energy in the laser field, and travel towards the detector. We have observed this process, which is the basis of attosecond science and well known from atoms and molecules in the gas phase -- for the first time from a solid-state, nanoscale metal tip. We show electronic matter wave interference in the time-energy domain, and first steps towards a new attosecond low-energy electron diffraction surface imaging technique.

Speaker: Peter Hommelhoff (Max Planck Institute of Quantum Optics)

114 LIGHT INDUCED MELTDOWN OF QUASIPARTICLES IN HIGH TC SUPERCONDUCTORS

Alessandra Lanzara Physics Department - University California, Berkeley Materials Science Division, Lawrence Berkeley National Laboratory We use high resolution time- and angle- resolved photoemission spectroscopy (tr-ARPES) to directly probe collective dynamics after optical excitation and study their influence on quasiparticles dynamics, Cooper pair formation, superconducting gap and other competing orders. In particular, through systematic pump fluence dependence we were first able to induce a meltdown of quasiparticles and measure their recovery dynamics. Interestingly we observed that only quasiparticles beyond a particular boson mode respond to the pump laser excitations, while the others remain untouched and that the entire decay is governed by two different time scale. We observe that quasiparticles recombination is also a fluence and momentum dependent process with enhanced recombination at the antinode. As we move away from the nodal direction and along the Fermi arc, we observe a closing of the superconducting gap, a crossover from a weakly perturbed to a strongly perturbed regime. These results point to a new dichotomy between the ultrafast gap and quasiparticles response within and beyond the Fermi arc and reveal a new window into the nature of the pairing interaction in high Tc superconductors. [1] J. Graf et al. Nature Physics 7, 806 (2011) [2] C. L. Smallwood et al. Science 336, 1137 (2012)

Speaker: Alessandra Lanzara (UC Berkeley)

12:30 Lunch

13:45 Wine Country Excursions or Free Time to explore Napa Valley

Thursday, 26 July

Plasmonics

Convener: Girsh Blumberg (Rutgers University)

115 Towards a q-dependent optics

Martin Dressel The response of matter on a temporally and spatially varying electromagnetic field at a certain point and a certain time depends on the field strength prior to this time at places close to this point. Hence the material parameters become a function of frequency $\omega$ and wavevector q, in general. While the frequency dispersion is common knowledge and widely utilized, spatial dispersion is usually disregarded. However, it is crucial in many metamaterials and inhomogeneous matter. The definition of effective optical parameters by any of the classical effective medium approximations becomes deceptive. We will discuss the theoretical background, the relation of spatial dispersion and magnetoelectric effect (bi-anisotropy), for instance. The novel method of Mueller-matrix spectroscopic ellipsometry allows us to map the complete q-dependent optical response. Experiments of toy models such as a metallic-dielectric nanostructures and split-ring-resonator array demonstrate the power of this method and elucidate the rule of spatial dispersion.

Speaker: Martin Dressel (Universitat Stuttgart)

116 Tapered terahertz plasmonic waveguides

Daniel Mittleman, Rice University Concentrating optical energy into an ultra-small spot beyond the diffraction limit has long been an interesting topic in photonics. One of the most popular methods is to use subwavelength-sized plasmonic waveguides, based on the excitation of surface plasmon polaritons (SPPs) on metallic surfaces. While most of the studies on plasmonic waveguides have been focused in the optical regime, subwavelength plasmonic waveguides in the THz spectral regime have recently attracted a great deal of attention. Here, we discuss two different types of tapered waveguides which can enable deep subwavelength focusing of broadband terahertz signals. A single wire can support a surface plasmon in the Sommerfeld mode, while a pair of parallel plates can support edge plasmons which effectively confine the electromagnetic wave. In both cases, confinement of the wave to below lambda/100, over a broad spectral bandwidth, is demonstrated.

Speaker: Daniel Mittleman (Rice University)

117 Control of light with phase discontinuities: photonics with metasurfaces

Federico Capasso, Francesco Aieta, Patrice Genevet, Nanfang Yu, Mikhail A. Kats, Zeno Gaburro School of Engineering and Applied Sciences, Harvard University Cambrideg, MA02138 Conventional optical components such as lenses and holograms rely on gradual phase shifts accumulated during light propagation to shape light beam. New degrees of freedom in optical design are attained by introducing in the optical path abrupt phase changes over the scale of the wavelength. In this talk, we will discuss the results presented in [1], where a two-dimensional array of plasmonic resonators in form of V-shape antennas with spatially varying phase response and sub-wavelength separation can imprint such phase discontinuities on propagating light. We demonstrated that a linear phase variation on the interface between two media leads to anomalously reflected and refracted beams in accordance with generalized laws of reflection and refraction derived from Fermat’s principle. If we consider an interface with a phase gradient arbitrarily oriented with respect to the plane of incidence rather than parallel to it as in Ref. [1], the reflected and refracted beams are non-coplanar with the incident beam, leading to a three-dimensional generalization of the new laws of reflection and refraction [2] . Out of plane refraction has been experimentally demonstrated [2]. Phase discontinuities enable wavefront engineering with unprecedented flexibility, which is promising for a wide variety of planar optical components [3]. Optical phase array with subwavelength control of light parameters could lead to a surface optics technology capable of large scale applications including novel spatial wave modulators [1] N. Yu, et al. Science 334, 333 (2011) [2] F. Aieta, et al. NanoLetters, Published on line Feb. 15, 2012 DOI: 10.1021/nl300204s [3] P. Genevet, et al. Appl. Phys. Lett. 100, 13101 (2012)

Speaker: Frederico Capasso (Harvard University)

118 The Orientation of Luminescent Excitons in Layered Nanomaterials

J.A. Schuller [1,2], S. Karavelli [2], T. Schiros [1], K. He [3], S. Yang [1], I. Kymissis [1], J. Shan [3], and R. Zia [2] [1] Energy Frontier Research Center, Columbia University [2] School of Engineering, Brown University [3] Department of Physics, Case Western Reserve University Measurements of optical anisotropies can elucidate the morphology and electronic excitations in nanomaterials with inherent structural anisotropies. Here, we exploit anisotropies in layered nanomaterials to resolve the orientation of luminescent excitons and isolate photoluminescence (PL) signatures from distinct intra- and interlayer optical transitions. We combine analytical calculations with energy- and momentum-resolved spectroscopy to distinguish between in-plane (IP) and out-of-plane (OP) oriented excitons in materials with weak or strong interlayer coupling—MoS2 and PTCDA respectively. We prove that PL from MoS2 mono-, bi-, and tri-layers originates solely from IP intralayer excitons whereas PTCDA supports distinct IP and OP exciton species with different spectra, dipole strengths, and temporal dynamics. Our work provides fundamental insight into exciton physics in layered nanomaterials and highlights the importance of designing optical systems that efficiently excite and collect light from exciton species with different orientations.

Speaker: Jon Schuller (Columbia University)

10:15 Break

High Tc Cuprates I

Convener: Harold Weinstock (AFOSR)

119 Gap signatures in the IR and THz properties of the cuprate and iron-based superconductors

Christopher Homes Condensed Matter Physics and Materials Sciences Department Brookhaven National Laboratory Advances in infrared spectroscopy have allowed the complex optical properties of superconductors to be examined with unprecedented accuracy. These studies have been particularly revealing in the cuprates and the recently discovered iron-based superconductors, both of which have high critical temperatures (Tc’s) [1]. The d-wave symmetry of the superconducting energy gap in the hole-doped cuprates results in a rather ambiguous energy gap in the copper-oxygen planes; however, in the electron-doped materials the energy gap appears to be non-monotonic resulting in clear optical gap below Tc. The cuprates are two dimensional and the superconductivity along the poorly-conducting c axis is due to Josephson coupling between the planes, resulting in the formation of a striking Josephson plasma edge in the reflectance below Tc. The iron-based superconductors are more three dimensional, multiband systems, consisting of electron and hole pockets at the Fermi surface. Superconductivity may be found in a number of different structures, and of these BaFe2As2 (122) is one of the most studied, displaying an anisotropic gapping of the Fermi surface below the structural and magnetic transition, as well as a phonon anomaly [2]. Superconductivity may be induced through cobalt substitution, and a clear optical signature of superconductivity is observed with the formation of at least one gap [3]. Superconductivity is also observed in the even simpler iron-chalcogenide FeTe1-xSex (11) materials, which appear to be strongly correlated [4], with evidence of multiple gaps below Tc [5]. In the iron-based superconductors scattering between the electron and hole pockets is thought to be a necessary element of the pairing mechanism. Thus in the purely hole-doped KFe2As2 it is not surprising that the critical temperature is very low (Tc ~ 3 K); however in the electron-doped K0.8Fe2-ySe2 the critical temperature is an order of magnitude higher (Tc ~ 31 K), suggesting that the pairing mechanism may have to be re-evaluated. Interestingly, while no evidence for a Josephson plasma edge has been observed along the c-axis of the iron-bases superconductors, K0.8Fe2-ySe2 appears to be an inhomogeneous material in which the superconductivity is due to Josephson coupling [6]. [1] D. C. Johnston, Adv. Phys. 59, 803 (2010). [2] A. Akrap et al., Phys. Rev. B 80, 180502(R) (2009). [3] J. J. Tu et al., Phys Rev. B 82, 174509 (2010). [4] Z. P. Yin, K. Haule and G. Kotliar, Nat. Mater. 10, 932 (2011). [5] C. C. Homes et al., Phys. Rev. B 81, 180508(R) (2010); J. Phys. Chem. Sol. 72, 505 (2011). [6] C. C. Homes et al., arXiv:1110.5529v1 (2011).

Speaker: Christopher Homes (Brookhaven National Laboratory)

120 Control of correlated electrons in metal-oxide superlattices

Bernhard Keimer Max Plank Institute for Solid State Research Stuttgart, Germany We will outline recent results of an experimental program aimed at controlling the phase behavior of correlated electrons through the synthesis and characterization of metal-oxide superlattices, with particular emphasis on copper and nickel oxides. Control parameters include the occupation of transition-metal d-orbitals [1] and the dimensionality of the electron system [2]. In particular, we will demonstrate control of the electron-phonon interaction in cuprate superlattices [3], and of the spin density wave polarization in nickelate superlattices [4]. These results also highlight the power of resonant x-ray scattering [1,4], spectral ellipsometry [2], and Raman scattering [3] as microscopic probes of the electron system in metal-oxide heterostructures and superlattices. [1] E. Benckiser, M. W. Haverkort, S. Brück, E. Goering, S. Macke, A. Frañó, X. Yang, O. K. Andersen, G. Cristiani, H. U. Habermeier, A. V. Boris, I. Zegkinoglou, P. Wochner, H. J. Kim, V. Hinkov, and B. Keimer, Nature Mater. 10, 189 (2011). [2] A. V. Boris, Y. Matiks, E. Benckiser, A. Frano, P. Popovich, V. Hinkov, P. Wochner, M. Castro-Colin, E. Detemple, V. K. Malik, C. Bernhard, T. Prokscha, A. Suter, Z. Salman, E. Morenzoni, G. Cristiani, H.-U. Habermeier, and B. Keimer, Science 332, 937 (2011). [3] N. Driza, S. Blanco-Canosa, M. Bakr, S. Soltan, M. Khalid, L. Mustafa, K. Kawashima, G. Christiani, H.-U. Habermeier, G. Khaliullin, C. Ulrich, M. Le Tacon, and B. Keimer, submitted. [4] A. Frano, E. Schierle, M. W. Haverkort, Y. Lu, M. Wu, S. Blanco-Canosa,Y. Matiks, A. V. Boris, P. Wochner, G. Cristiani, G. Logvenov, H.U. Habermeier, V. Hinkov, E. Benckiser, E. Weschke, and B. Keimer, in preparation.

Speaker: Keimer Bernhard (Max Plank Institute for Solid State Research)

121 Superconducting gap in the pnictides – theory and ARPES

Andrey V. Chubukov University of Wisconsin, Madison I review recent theory works on the gap structure in Fe-pnictides and compare theory predictions with laser and Synchrotron ARPES measurements. I discuss the arguments for s++, s+-, and d-wave gaps and argue in favor of s+- gap for both moderately and strongly doped materials. I further discuss the evidence for symmetry-allowed angle variation of the s+- gap and for potential gap nodes, and suggest new ARPES experiments to verify recent theory proposals of vertical loop nodes in P-doped pnictides.

Speaker: Andrey Chubukov (University of Wisconsin, Madison)

122 Log(1/T) flux-flow resistivity: a dynamical signature of vortices in cuprate superconductors

David Broun, Simon Fraser University Xiaoqing Zhou, Simon Fraser University Benjamin Morgan, University of Cambridge Wendell Huttema, Simon Fraser University Darren Peets, University of British Columbia & Max-Planck-Institut für Festkörperforschung, Stuttgart Patrick Turner, Simon Fraser University John Waldram, University of Cambridge Ahmad Hosseini, University of British Columbia Ruixing Liang, University of British Columbia Doug Bonn, University of British Columbia Walter Hardy, University of British Columbia High magnetic fields have played a key role in extending our understanding of the normal state of the cuprate superconductors to low temperatures. Early pulsed-field measurements, by Ando and Boebinger, revealed a transition in the underdoped regime to a state in which resistivity increases with decreasing temperature, with a puzzling log(1/T) form. Recent quantum oscillation studies have provided a wealth of additional information on electronic structure. However, the true nature of the nonsuperconducting ground state remains uncertain. A central issue is the role of local superconducting pairing in the pseudogap regime: to resolve this, a distinct physical signature of the presence of vortices is ideally needed. Our measurements of flux-flow resistivity reveal that the transition from metallic low temperature resistivity to log(1/T) behaviour in fact occurs on the overdoped side of the phase diagram. Working deep within the superconducting phase, with the cleanest available samples of YBCO and Tl2201, we take a different approach from previous experiments and measure the response of vortices to microwave-frequency driving currents. This circumvents flux pinning and reveals a log(1/T) flux-flow resistivity that persists throughout the superconducting region of the cuprate phase diagram. This includes a regime at high carrier dopings in which the normal-state transport is Fermi-liquid-like, indicating that the log(1/T) flux-flow resistivity observed in the superconducting state is not simply a reflection of the normal state, but a dynamical property of the vortices themselves. This would imply, in turn, that the log(1/T) resistivity seen in the pseudogap phase is a signature of vortex fluctuations and therefore that local superconducting pairing persists to high magnetic fields in the underdoped cuprates.

Speaker: David Broun (Simon Fraser University)

12:30 Lunch

Metamaterials

Convener: Martin Dressel (University of Stuttgart)

123 Taming the Blackbody

Willie J. Padilla (Boston College), Xianliang Liu (Boston College) Since the first experimental demonstration of negative index, research into metamaterials has grown enormously. The ability of metamaterial to achieve nearly any electromagnetic response in nearly any frequency band suggest many exotic applications including invisible cloaks and perfect lenses. One recent field of research is that of metamaterial perfect absorbers (MPAs) due to their unique ability to achieve unity absorption with high efficiency. Here we experimentally demonstrate terahertz, infrared, and optical metamaterials able to control the absorption and emission of electromagnetic waves over a broad bandwidth. A metamaterial absorber which controls the emissivity spectrum of a body at a particular temperature over a bandwidth of 50% is demonstrated which may be applied as a coating to materials to control their blackbody emission spectra.

Speaker: Prof. Willie Padilla (Boston College)

124 Active Terahertz Metamaterials

Hou-Tong Chen, Abul K. Azad, Ranjan Singh, Matthew T. Reiten, Dibakar Roy Chowdhury, Suchitra Ramani Quanxi Jia, Stuart A. Trugman and Antoinette J. Taylor Los Alamos National Laboratory In recent years terahertz (THz) technology has become an optimistic candidate for numerous sensing, imaging, and diagnostic applications. Yet, THz technology still suffers from a deficiency in sources, detectors, modulators and other functional elements ubiquitous in neighboring microwave and infrared frequency bands. One of the greatest obstacles in this progress is the lack of materials that naturally respond well to THz radiation. The potential of metamaterials for THz applications originates from their resonant electromagnetic response, which significantly enhances their interaction with THz radiation. Thus, metamaterials offer a route towards helping to fill the so-called “THz gap”. Here, we present a series of novel THz metamaterials with designed active functionality, enabling dynamic tuning of the amplitude, frequency and polarization state of a THz wave. In these materials the critical dependence of the resonant response on the supporting substrate and/or the fabricated structure enables the creation of active THz metamaterial devices. We show that the resonant response can be controlled using optical or electrical excitation and thermal tuning, enabling efficient THz devices which will be of importance for advancing numerous real-world THz applications.

Speaker: Hou-Tong Chen (Los Alamos National Lab)

125 Tunable and Nonlinear Metamaterial Composites

David R. Smith, Duke University After spending more than a decade showing the promise of artificially structured metamaterials that are mostly passive and linear, the community is now putting forth significant efforts into active, tunable and nonlinear metamaterials. These functional metamaterials connect the electronic and other fundamental properties of a material with the metamaterial structure to introduce unprecedented control of electromagnetic waves. As the metamaterials field fuses with plasmonics and nanophotonics, more emphasis is being placed on the nano-interfaces between materials, which can surprisingly dominate the electromagnetic behavior of the composite. An example is the enormous enhancement of second harmonic generation in nanostructured metallic metamaterials, in which the hydrodynamic properties of the free electron gas in the metal interacts with the nanosurface to achieve the necessary symmetry. We will describe several examples, showing how the physics of materials can be leveraged to form dynamic, nonlinear or tunable metamaterial composites.

Speaker: David R. Smith (Duke University)

15:45 Break

High Tc Cuprates & Pnictides

Convener: Setsuko Tajima (Osaka University)

126 Overview of ARPES studies of cuprate and pnictide superconductors

Dan Dessau University of Colorado, Boulder I will overview a few of the recent important contributions using ARPES to study correlated electron high Tc superconductors, including cuprates and pnictides. Topics will include superconducting gaps, pseudogaps and Fermi arcs.

Speaker: Dan Dessau (University of Colorado, Boulder)

127 High Temperature Proximity Effect in Topological Insulators

Kenneth Burch1, Parisa Zareapour1, Alex Hayat1,2, Shu Yang F. Zhao1, Anjan Reijnders1, Mikhail Kreshchuk1, Achint Jain1, Daniel C. Kwok3, Nara Lee3, Sang-Wook Cheong3, Zhijun Xu4, Alina Yang4, G. D. Gu4, and R. Cava5 1Department of Physics and Institute for Optical Sciences, University of Toronto, 60 St. George Street, Toronto ON, M5S 1A7, Canada 2Centre for Quantum Information and Quantum Control University of Toronto, 60 St. George Street, Toronto ON, M5S 1A7, Canada 3Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, 136 Frelinghuysen Road, Piscataway, New Jersey 08854, USA 4CMP&MS Department, Brookhaven National Laboratory, Upton, New York 11973, USA 5 Department of Chemistry, Princeton University, New Jersey 08544, USA Producing new effects through the combination of different materials has a long history in science and technology. One of the most intriguing recent ideas is the emergence of Majorana fermions when a topological insulator is placed in proximity with a superconductor. Towards this goal, we produced high-temperature superconductivity in topological insulators via the proximity to a high temperature superconductor. In this talk I will describe the simple mechanism to achieve this. In addition I will present our extensive tunnelling spectroscopy studies that confirm the existence of the proximity effect. Lastly I will discuss some potential future directions this technique offers for novel optical devices.

Speaker: Kenneth Burch (University of Toronto)

128 Disentangling the electronic and phononic glue in a high-Tc superconductor

S. Dal Conte1,2, C. Giannetti3, G. Coslovich4,5, F. Cilento4, D. Bossini3,6, T. Abebaw4, F. Banfi3, G. Ferrini3, H. Eisaki7, M. Greven8, A. Damascelli9,10, D. van der Marel11 & F. Parmigiani4,12 1 Department of Physics A. Volta, Universit`a degli Studi di Pavia, Pavia I-27100, Italy. 2 Present address: Physics Department, Eindhoven University of Technology, the Netherlands. 3 Department of Physics, Universit`a Cattolica del Sacro Cuore, Brescia I-25121, Italy. 4 Department of Physics, Universit`a degli Studi di Trieste, Trieste I-34127, Italy. 5 Present address: Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, CA 94720, USA. 6 Present address: Institute forMolecules andMaterials, Radboud University Nijmegen, the Netherlands. 7 Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan. 8 School of Physics and Astronomy, University of Minnesota, Minneapolis,Minnesota 55455, USA 9 Department of Physics & Astronomy, University of British Columbia, Vancouver, Canada. 10 Quantum Matter Institute, University of British Columbia, Vancouver, Canada. 11 D´epartement de Physique de la Mati`ere Condens´ee, Universit´e de Gen`eve, Switzerland. 12 Sincrotrone Trieste S.C.p.A., Basovizza I-34012, Italy. The mechanism behind the formation of Cooper pairs in high-Tc superconductors is a subject of strong debate. In particular the research narrows down to two mechanisms: phonons and excitations of electronic origin are both considered the main candidates of the pairing mechanism in these compounds. Experiments performed at equilibrium conditions have the capability to reconstruct the glue spectrum. On the contrary they are insensitive to the nature of the glue and failed to separate these two contributions as they could coexist on the same energy scale (<100 meV). Here we tackle this problem by measuring the optical response of a Bi2Sr2Ca0.92Y0.08Cu2O8+d crystal on both time and frequency domains[1]. By fitting the experimental data to the calculated variation of reflectivity at different delay times, we are able to unambiguously disentangle the electronic and phononic contribution of the glue on the basis of their different temporal dynamics[2]. We find that, on the time scale faster than electron-phonon thermalization, the quasiparticles are already thermalized with the excitations of electronic origin participating to the glue. The strength of this interaction (lambda∼1.1) fully accounts for the high critical temperature of the system. These results represent an important step ahead in the understanding of the pairing mechanism in cuprates and pave the way for the investigation of the electron-boson coupling in a variety of complex materials, ranging from transition-metal oxides to iron-based superconductors. [1] Monthoux et al. Superconductivity without phonons. Nature 450, 1177 (2007). [2] Kresin et al. Colloquium: Electron-lattice interaction and its impact on high Tc superconductivity. Rev. Mod. Phys. 81, 481 (2009). [3] C. Giannetti et al. Revealing the high-energy electronic excitations underlying the onset of high-temperature superconductivity in cuprates. Nature Communications 2, 353 (2011). [4] S. Dal Conte et al. Disentangling the electronic and phononic glue in a high-Tc superconductor. Science, in press.

Speaker: Stefano Dal Conte (Eindhoven University of Technology)

129 Non-retarded pairing interaction in a high-Tc cuprate from coherent charge fluctuations spectroscopy

B. Mansart Laboratory for Ultrafast Microscopy and Electron Scattering, ICMP, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland and Laboratory of Ultrafast Spectroscopy, ISIC, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland J. Lorenzana ISC-CNR and Dipartimento di Fisica, Università di Roma La Sapienza, P.le Aldo Moro, I-00185 Roma, Italy M. Scarongella Laboratory of Ultrafast Spectroscopy, ISIC, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland M. Chergui Laboratory of Ultrafast Spectroscopy, ISIC, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland F. Carbone Laboratory for Ultrafast Microscopy and Electron Scattering, ICMP, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland Despite their obvious physical difference, from a mathematical (or purely formal) point of view, magnetism and superconductivity are closely linked phenomena. Coherent charge and pairing fluctuations can be described in terms of precession of pseudospins operators, first introduced by Anderson, and behaving as spin-1/2 operators [1]. In our experiment, a polarized ultrafast laser pulse excites the superconductor through the Impulsive Stimulated Raman Scattering (ISRS) effect [2]. The coherent oscillations of the Cooper pairs condensate are detected via delayed supercontinuum pulses and enable a new technique, Coherent Charge Fluctuation Spectroscopy (CCFS), to distinguish the electronic excitations that couple to the superconducting quasiparticles [3]. This is of pivotal importance for cuprates, as the applicability of conventional pairing theories [4], based on retarded interactions between electrons mediated by low energy glue bosons, has been doubted and a completely different framework has been proposed involving non-retarded interactions associated with high-energy electronic scales [5]. We found that the superconducting condensate oscillations resonate at the typical scale of Mott physics (2.6 eV), implying a substantial contribution of non-retarded interactions to the pairing, as in unconventional (non Migdal-Eliashberg) theories. [1] P.W. Anderson, "Random-Phase Approximation in the theory of superconductivity“, Phys. Rev. 112, 1900 (1958). [2] R. Merlin, "Generating coherent THz phonons with light pulses", Solid State Comm. 102, 207 (1997). [3] B. Mansart, J. Lorenzana, M. Scarongella, M. Chergui and F. Carbone, "Direct observation of real-time oscillations of the Cooper-pairs condensate in a high-Tc superconductor", arXiv:1112.0737 [4] G.M. Eliashberg, "Interactions between electrons and lattice vibrations in a superconductor", Soviet Physics JETP 11, 696 (1960). [5] P.W. Anderson, "The Resonating Valence Bond state in La2CuO4 and Superconductivity", Science 235, 1196 (1987).

Speaker: Barbara Mansart (Ecole Polytechnique Fédérale de Lausanne)

130 Optical and transport properties in pnictides: anomalous effects due to interband interactions

Lara Benfatto ISC-CNR The occurrence of superconductivity in pnictides renewed in the last year the interest in the physics of multiband superconductors. However, what makes the case of pnictides very peculiar is the fact that interactions have mainly an interband character, as due to exchange of spin fluctuations between hole and electron pockets. These two characteristics make the theoretical description of pnictides much more involved than what is usually believed, forcing us to revise our standard paradigms for correlated electron systems. In this talk I will review some of our recent results based on a multiband model with retarded interactions treated within Eliashberg theory. In particualr I will discuss the redistribution of spectral weight between coherent and incoherent optical processes, that leads to an anomalous temperature dependece of the sum rule. I will also show that when the momentum dependence of the interaction is taken into account explicitly the vertex corrections to the quasiparticle current lead to a predominant hole or electron character of the transverse conductivity, explaining the experimental results on the Hall effect.

Speaker: Lara Benfatto (ISC-CNR and Department of Physics, Sapienza University of Rome)

18:00 Transport to Conference Dinner Location

Conference Dinner

19:00 Arrive, Group Photo, Tour Winery Grounds

19:45 Dinner

131 Student Poster Award Winner Presentations

132 Remembering Mike Tinkham

Paul Richards UC Berkeley

Speaker: Paul Richards (UC Berkeley)

22:00 Transport back to Embassy Suites

Friday, 27 July

Topological Insulators II

Convener: Kenneth Burch (University of Toronto)

133 STM Studies of Topological Insulators

Aharon Kapitulnik Stanford Univeristy

Speaker: Aharon Kapitulnik (Stanford University)

134 Low frequency electrodynamics of topological insulator surface states

R. Valdés Aguilar, A. V. Stier, W. Liu, L. S. Bilbro The Johns Hopkins University D. K. George, L. Wu, J. Cerne, A. G. Markelz University of Buffalo N. Bansal, M. Brahlek, S, Oh Rutgers University I will report on our studies of high quality MBE grown Bi2Se3 and strained HgTe topological insulator thin films using time domain terahertz spectroscopy (TDTS). In the Bi2Se3 case, we explicitly demonstrate the 2D character of the response by studying films of different thicknesses. We show that prolonged exposure of these thin films to atmospheric conditions actually suppresses their bulk response. In our measurements we take advantage of a unique feature of TDTS that allows to use the time structure of the THz pulses to measure the Faraday and Kerr rotation angles in a single experiment. We find an unprecedentedly large value of the Kerr rotation that is due to the cyclotron resonance of the 2D Dirac fermions. I will talk about our extension of these measurements to the strained HgTe topological insulator system.

Speaker: N. Peter Armitage (The Johns Hopkins University)

135 Novel electronic states of topological insulators studied by ARPES

Takafumi Sato, Department of Physics, Tohoku University The surface state of a three-dimensional topological insulator is characterized by a Dirac-cone dispersion protected by the time-reversal symmetry. Breaking the TRS by a magnetic order leads to the opening of a gap in the surface state and consequently the Dirac fermions become massive. It has been proposed theoretically that such a mass acquisition is necessary for realizing novel topological phenomena, but achieving a sufficiently large mass is an experimental challenge. We report an unexpected discovery [1] that the surface Dirac fermions in TlBi(S1-xSex)2 acquires a mass without explicitly breaking the TRS. ARPES data around the Brillouin-zone center measured for various sulfur concentrations x revealed a finite energy gap at the Dirac point in 0.6 ≤ x ≤ 0.9, while such an energy gap is absent in x = 1.0. This indicates that the masless Dirac fermions transforms into a massive state by simply replacing Se with S in the crystal. Present result provides a new route to achieving the massive Dirac state required for realizing the exotic topological phenomena. We will also present our recent ARPES results on tunable Dirac carriers in topological insulator Bi2-xSbxTe3-ySey [2]. This study has been performed in collaboration with S. Souma, T. Arakane, K. Nakayama, K. Kosaka, T. Takahashi (Tohoku Univ.), Kouji Segawa, K. Eto, T. Minami, Z. Ren, and Yoichi Ando (Osaka Univ.). [1] T. Sato et al., Nat. Phys. 7, 840 (2011). [2] T. Arakane et al., Nat. Commun. 3:636, ncomms1639 (2012).

Speaker: Takafumi Sato (Department of Physics, Tohoku University)

136 Terahertz Quantum Hall Effect in Topological Insulator HgTe

A. M. Shuvaev(1), G. V. Astakhov (2), G. Tkachov (3), C. Brüne (3) H. Buhmann (3), L. W. Molenkamp (3), A. Pimenov (1) (1) Institute of Solid State Physics, Vienna University of Technology, 1040 Vienna, Austria (2) Physikalisches Institut (EP6), Universität Würzburg, 97074 Würzburg, Germany (3) Physikalisches Institut (EP3), UniversitÄat Würzburg, 97074 Würzburg, Germany Using terahertz spectroscopy in external magnetic felds we investigate the low-temperature charge dynamics of the topological insulator HgTe. Faraday rotation angle and ellipticity could be well described using a classical Drude model in two dimensions (2D). From these data complete characterization of the charge carriers could be done, including 2D density, scattering rate and cyclotron effective mass. In these experiments the quantum Hall oscillations are observed at terahertz frequencies. The two-dimensional density, estimated from the period of the oscillations, agrees well with direct transport experiments. The effective mass of the 2D carriers is found to be close to that in the bulk of the unstrained sample.

Speaker: Andrei Pimenov (Vienna University of Technology)

10:15 Break

Nanoscale Spectroscopies

Convener: Fritz Keilmann (Max Planck Institut für Quantenoptik)

137 Nano-optical imaging and spectroscopy of mesoscopic phase behavior in quantum matter

Markus Raschke University of Colorado The rich phase behavior of correlated matter including colossal magnetoresistance, superconductivity, or multiferroicity has made them candidates for a wide range of technological applications. However, the underlying microscopic electronic, lattice, and spin interactions that give rise to these complex materials properties are yet poorly understood. The investigation of these materials is further complicated by frustration and degenerate ground states that can lead to phase competition and coexistence of multiple phases.This often gives rise to structural and electronic inhomogeneities and mesoscopic spatial phase separation on atomic to microscale dimensions. We will discuss the combination of the new nano-optical scanning probe techniques of scattering scanning near-field optical microscopy (s-SNOM) with different linear, inelastic, nonlinear, and ultrafast spectroscopies. By selective optical near-field coupling to electronic, lattice, and spin degrees of freedom and their symmetries these different spectroscopic implementations allow for probing the associated domain architecture with nanometer spatial resolution. By selecting one or more suitable optical interactions, multiple order parameter can be imaged simultaneously under the influence of sample strain, high magnetic or electric fields, or cryogenic and variable temperature. Specific examples include the investigation of the complex nanoscale phase separation between metallic and different insulating phases in the metal-to-insulator transition in VO_2 microcrystals by combining near-field Raman and IR (Drude response) spectroscopy. The resulting nanoscale strain-temperature phase diagram provides insight into the competition between the intrinsic phase behavior and external strain, temperature, and photo doping. The extension of tip-enhanced Raman spectroscopy allows for phonon Raman nano-crystallography, e.g., for the study of finite-size effects on the ferroelectric order in BaTiO_3 nanocrystals or BiFeO_3. The spatial and time-reversal symmetry selectivity of optical second-harmonic generation can access the coupled ferroelectric and antiferromagnetic nanoscale domain topology and order in intrinsic multiferroics, e.g., YMnO_3, and provide insight into the mechanisms of the magnetoelectric coupling. In conclusion I will provide an outlook based on our recent developments on femtosecond optical control for nanoscale ultrafast spatio-temporal imaging.

Speaker: Markus Raschke (University of Colorado)

138 Graphene as a tunable plasmonic material

Z. Fei, A. S. Rodin, G. O. Andreev, A. S. McLeod, M. Wagner, M. M. Fogler, D. N. Basov, Department of Physics, University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA L. M. Zhang, Department of Physics, Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215, USA W. Bao, Zeng Zhao, C. N. Lau, Department of Physics and Astronomy, University of California, Riverside, California 92521, USA G. Dominguez, M. Thiemens, Department of Chemistry, University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093 A. H. Castro-Neto, Graphene Research Centre and Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore F. Keilmann, Max Planck Institute of Quantum Optics and Center for Nanoscience, 85714 Garching, Germany Graphene is a novel plasmonic medium whose electronic and optical properties can be conveniently controlled by electrostatic gates. Near-field nano-imaging shows that at technologically relevant infrared frequencies common graphene/Si oxide/Si back-gated structures support surface plasmons with wavelength of the order of 200 nm and the propagation length several times this distance. Such plasmons represent concentration of electromagnetic energy on the spatial scale two orders of magnitude smaller than the photon wavelength. Both the amplitude and the wavelength of the plasmons are shown to be tunable by the gate voltage. Plasmon standing waves arise when plasmons launched by a sharp tip of a scanned probe interfere with their reflection off sample edges and inhomogeneities. These interference patterns are shown to depend on the location of the tip and the shape of the sample. Theoretical modeling provides quantitatively accurate description of the plasmonic interference patterns. Plasmonic dispersion and damping, extracted from the spatial decay of the interference fringes sheds light on the exotic electrodynamics of Dirac quasiparticles in graphene.

Speaker: Michael Fogler (UC San Diego)

139 Infrared nanospectroscopy of plasmons in semiconductors, metal nanoantennas and graphene

Rainer Hillenband, CIC nanoGUNE, San Sebastian, Spain Optical spectroscopy has tremendous impact in science and technology, particularly in the infrared (IR) and terahertz (THz) spectral range, where photons can probe molecule vibrations, phonons, as well as plasmons and electrons in non-metallic conductors. However, diffraction limits the spatial resolution to the micrometer scale, thus strongly limiting its application in nano- and biosciences. To overcome this drawback, we developed near-field microscopy based on elastic light scattering from atomic force microscope tips (scattering-type scanning near-field optical microscopy, s-SNOM) [1]. Collection of the tip-scattered light yields nanoscale resolved IR and THz images, beating the diffraction limit in the terahertz spectral range by more than three orders of magnitude. Combined with thermal radiation and a Fourier-transform (FT) spectrometer, s-SNOM can map broadband IR spectra with a spatial resolution below 100 nm (nano-FTIR) [2]. For nanoscale infrared dielectric mapping and vibrational spectroscopy we employ metalized AFM tips acting as infrared antennas. The illuminating light is converted into strongly concentrated near fields at the tip apex (nanofocus), which provides a means for localized excitation of molecule vibrations, plasmons or phonons in the sample surface. Spectroscopic mapping of the scattered light thus allows for nanoscale chemical recognition of (bio)materials, mapping of free-carrier concentration in semiconductor nanodevices and nanowires [1, 2] or nanoimaging of strain [3]. Another application of s-SNOM is the imaging of the vectorial infrared near-field distribution of plasmonic nanostructures. In this application, a dielectric tip scatters the near fields at the sample surface, allowing for mapping the hot spots in plasmonic infrared gap antennas or for verifying IR energy transport and compression in nanoscale transmission lines [4]. With these studies we establish a basis for the development of nanoscale infrared circuits based on antennas and transmission lines, which could have interesting application potential for the development of ultra-compact infrared sensors, spectrometers and novel near-field probes. s-SNOM also enables the launching and detecting of propagating and localized plasmons in graphene nanostructures. Spectroscopic real-space images of the plasmon modes allow for direct measurement of the ultrashort plasmon wavelength and for visualizing plasmon control by gating the graphene structures. [1] A. J. Huber, et al. Nano Lett. 8, 3766 (2008) [2] F. Huth et al., Nature Mater. 10, 352, (2011) [3] J. A. Huber, et al, Nature Nanotech. 4, 157, (2009) [4] M. Schnell, et al. Nature Photon. 5, 283 (2011)

Speaker: Rainer Hillenbrand (CIC nanoGUNE)

Summary / Conclusions / LEES 2014

Conveners: Dimitri Basov, Michael Martin, Ricardo Lobo