THERMAL-CONDUCTIVITY OF HIGH-T-C SUPERCONDUCTORS - EFFECT OF VANHOVE SINGULARITIES

The electronic contribution kappa(c) to the thermal conductivity of a two-dimensional superconductor presenting saddle points at the Fermi l evel in the band structure, corresponding to logarithmic Van Hove sing ularities in the 2D density of states, is calculated within a variatio nal method. We compare theoretical results obtained for both isotropic s-wave and anisotropic d(x2-y2)-wave gap parameters. The peak structu re obverved below T-c on the thermal conductivity of a single crystal of Bi2Sr2CaCu2O8 can be reproduced by considering both gap parameter s ymmetries whereas the very low temperature behavior of kappa can be on ly explained by considering a d-wave gap parameter. In this latter cas e, the Van Hove singularities are found to limit the type of power law dependence of kappa at low temperatures. The model also predicts weak coupling values for the in-plane electron-phonon transport coupling c onstant lambda(tr) in this material, in agreement with values obtained from the analysis of electrical resistivity data.