Hump structure below Tc in the thermal conductivity of MgB2 superconductor

Abstract

A reasonable cause of absence of hump structure in thermal conductivity of MgB 2 below the superconducting transition temperature (T c ) 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 (κ) will show hump below T c . Proceeding along these lines, a sample of MgB 2 with a relatively higher residual resistivity ρ o = 33.8 µΩ-cm has been found to show a hump structure below T c . The actual electronic thermal conductivity κ el 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 numberby an effective Lorenz number L eff (<L 0 ) we have obtained two sets of κ el , namely those with L eff = 0.1L 0 and 0.2L 0 . Corresponding to these two sets of κ el , two sets of the phonon thermal conductivity κ ph are obtained. κ ph 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.