Speaker
Katalin Kamaras
(Wigner Research Centre for Physics, Hungarian Academy of Sciences)
Description
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)
Primary author
Katalin Kamaras
(Wigner Research Centre for Physics, Hungarian Academy of Sciences)
