Heat transport in Bi_{2+x}Sr_{2-x}CuO_{6+\delta}: departure from the Wiedemann-Franz law in the vicinity of the metal-insulator transition

TL;DR

This study investigates heat transport in Bi_{2+x}Sr_{2-x}CuO_{6+\delta} near the metal-insulator transition, revealing deviations from the Wiedemann-Franz law in less doped or more disordered samples at very low temperatures.

Contribution

It provides experimental evidence of departure from the Wiedemann-Franz law near the metal-insulator transition in cuprates, highlighting doping and disorder effects on heat conduction.

Findings

Wiedemann-Franz law holds in overdoped samples.

Deviations from WF law occur in underdoped or disordered samples.

Heat conduction exceeds WF law predictions near the metal-insulator crossover.

Abstract

We present a study of heat transport in the cuprate superconductor Bi_{2+x}Sr_{2-x}CuO_{6+\delta} at subkelvin temperatures and in magnetic fields as high as 25T. In several samples with different doping levels close to optimal, the linear-temperature term of thermal conductivity was measured both at zero-field and in presence of a magnetic field strong enough to quench superconductivity. The zero-field data yields a superconducting gap of reasonable magnitude displaying a doping dependence similar to the one reported in other families of cuprate. The normal-state data together with the results of the resistivity measurements allows us to test the Wiedemann-Franz(WF) law, the validity of which was confirmed in an overdoped sample in agreement with previous studies. In contrast, a systematic deviation from the WF law was resolved for samples displaying either a lower doping content or a higher disorder. Thus, in the vicinity of the metal-insulator cross-over, heat conduction in the zero-temperature limit appears to become significantly larger than predicted by the WF law. Possible origins of this observation are discussed.