Hump Structure below Tc in the thermal conductivity of MgB2 superconductor

TL;DR

This study investigates the absence of a hump in MgB2's thermal conductivity below Tc, showing that defect-induced suppression of multigap features can reveal a hump, with analysis indicating a specific gap ratio and defect scattering effects.

Contribution

It demonstrates that defect-induced suppression of multigap features in MgB2 reveals a hump in thermal conductivity, providing insights into gap structure and scattering effects.

Findings

Hump structure appears in MgB2 with higher residual resistivity.

Electronic thermal conductivity is significantly lower than Wiedemann-Franz law predictions.

Large defect scattering reduces electronic thermal conductivity, revealing the hump.

Abstract

A reasonable cause of absence of hump structure in thermal conductivity of MgB2 below the superconducting transition temperature (Tc) lies in the appearance of multigap structure. The gaps of lower magnitude can be suppressed by defects so that this system becomes effectively a single gap superconductor. When such a situation is created, it is hoped that thermal conductivity will show hump below Tc. Proceeding along these lines, a sample of MgB2 with a relatively higher residual resistivity (33.3 mili-Ohm-cm)has been found to show a hump structure below Tc. The actual electronic thermal conductivity kel of this sample is less than that expected from the Wiedeman- Franz law by more than a factor of 2.6 in the considered temperature range. Modifying the Wiedeman- Franz law for the electronic contribution by replacing the Lorenz number by an effective Lorenz number Leff (L0) we have obtained two sets of kel, namely those with Leff = 0.1L0 and 0.2L0. Corresponding to these two sets of kel, two sets of the phonon thermal conductivity kph are obtained. kph has been analyzed in terms of an extended Bardeen- Rickayzen- Tewordt theory. The main result of this analysis is that the hump structure corresponds to a gap ratio of 3.5, and that large electron-point defect scattering is the main source of drastic reduction of the electronic thermal conductivity from that given by the usual Wiedeman- Franz law.