Nernst response, viscosity and mobile entropy in vortex liquids

TL;DR

This paper investigates the Nernst effect in vortex liquids of superconductors, revealing a peak related to vortex entropy and viscosity, and discusses entropy exchange mechanisms affecting vortex dynamics.

Contribution

It introduces a detailed analysis of the Nernst response, viscosity, and entropy in vortex liquids, highlighting the entropy leakage from vortex cores and its implications.

Findings

Nernst signal peaks at intermediate temperature and magnetic field.

Minimum in viscosity to entropy density ratio correlates with Nernst peak.

Entropy can leak from vortex cores due to normal quasi-particle exchange.

Abstract

In a liquid of superconducting vortices, a longitudinal thermal gradient generates a transverse electric field. This Nernst signal peaks at an intermediate temperature and magnetic field, presumably where the entropy difference between the vortex core and the superfluid environment is largest. There is a puzzling similarity of the amplitude of this peak across many different superconductors. This peak can be assimilated to a minimum in the viscosity to entropy density ratio of the vortex liquid. Expressed in units of $\frac{\hbar}{k_B}$, this minimum is one order of magnitude larger than what is seen in common liquids. Moreover, the entropy stocked in the vortex core is \textit{not} identical to the entropy bound to a moving magnetic flux line. Due to a steady exchange of normal quasi-particles, entropy can leak from the vortex core. A slowly moving vortex will be peeled off its entropy within a distance of the order of a superconducting coherence length, provided that the $\frac{\Delta}{E_F}$ ratio is sufficiently large.