Test of the Wiedemann-Franz law in an optimally-doped cuprate

TL;DR

This study tests the Wiedemann-Franz law in an optimally-doped cuprate by measuring heat and charge transport, finding a slight deviation from the law's expected Lorenz number.

Contribution

It provides experimental evidence on the validity of the Wiedemann-Franz law in a cuprate superconductor at optimal doping.

Findings

Lorenz number measured as 1.3 ± 0.2 times L0

Slight deviation from Wiedemann-Franz law

Supports d-wave gap symmetry in superconducting state

Abstract

We present a study of heat and charge transport in Bi$_{2+x}$Sr$_{2-x}$CuO$_{6+\delta}$ focused on the size of the low-temperature linear term of the thermal conductivity at optimal doping level. In the superconducting state, the magnitude of this term implies a d-wave gap with an amplitude close to what has been reported. In the normal state, recovered by the application of a magnetic field, measurement of this term and residual resistivity yields a Lorenz number $L = \kappa_{N}\rho_{0}/T = 1.3 \pm 0.2 L_{0}$. The departure from the value expected by the Wiedemann-Franz law is thus slightly larger than our estimated experimental resolution.