THERMAL-CONDUCTIVITY OF SUPERCONDUCTING UPT3

The thermal conductivity-kappa of the heavy-fermion superconductor UPt3 has been measured on two single crystals of different quality as a function of temperature and applied magnetic field for different orientations of the heat current relative to both the field and the crystalline axes. The temperature dependence of kappa is far from exponential and nearly the same for both crystals, in which the heat current is, respectively, parallel and perpendicular to the hexagonal c axis, suggesting a gap structure with nodes in the basal plane and normal to it. The field dependence of kappa is strongly anisotropic. In the best sample at low fields, where the scattering of heat carriers by vortices is thought to be important, kappa(H) depends on the relative orientation of field and current. On the other hand, at high fields near H(c2) (in both samples), kappa(H) depends on the relative orientation of field and crystalline axes, reflecting an anisotropy in the gap structure and in the Fermi velocities.