Influence of temperature on the nodal properties of the longitudinal thermal conductivity of YBa₂Cu₃O_(7-x)

The angle dependence at different temperatures of the longitudinal thermal conductivity $\kappa_{xx}(\theta)$ in the presence of a planar magnetic field is presented. In order to study the influence of the gap symmetry on the thermal transport angular scans were measured up to a few Kelvin below the critical temperature $T_c$. We found that the four-fold oscillation of $\kappa_{xx}(\theta)$ vanishes at $T > 20 $K and transforms into a one-fold oscillation with maximum conductivity for a field of 8 T applied parallel to the heat current. Nevertheless, the results indicate that the d-wave pairing symmetry is the main pairing symmetry of the order parameter up to $T_c$. Numerical results of the thermal conductivity using an Andreev reflection model for the scattering of quasiparticles by supercurrents under the assumption of d-wave symmetry provide a semiquantitative description of the overall results.