Speaker
David Broun
(Simon Fraser University)
Description
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.
Primary author
David Broun
(Simon Fraser University)
