Hump structure below Tc in the thermal conductivity of MgB2 superconductor

A reasonable cause of absence of hump structure in thermal conductivity of MgB2 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 (kappa) Will show hump below T-c. Proceeding along these lines, a sample of MgB2 with a relatively higher residual resistivity rho(o) = 33.8 Q cm has been found to show a hump structure below T-c. The actual electronic thermal conductivity K,1 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 L-0 = 2.45 x 10(-8) W Omega K-2 by an effective Lorenz number L-eff (<L-0) we have obtained two sets of kappa(el), namely those with L-eff = 0.1 L-0 and 0.2 L-0. Corresponding to these two sets of kappa(el), two sets of the phonon thermal conductivity kappa(ph) are obtained. kappa(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. (C) 2009 Elsevier B.V. All rights reserved.