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

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 $L={\kappa }_N{\rho }_0/T=1.3\pm 0.2L_0$. The departure from the value expected by the Wiedemann-Franz law is thus slightly larger than our estimated experimental resolution.

Final published version