THERMAL-CONDUCTIVITY AS A PROBE OF UNCONVENTIONAL SUPERCONDUCTING GAP

We have carried out a comparative study of the thermal conductivity of two rectangular single crystals: a niobium sample and a detwinned YBC O. Thermal conductivity was measured as a function of the relative ori entation of the crystaline axes and a magnetic field applied parallel to the sample plane. Different angular variations, two-fold for Nb and four-fold for YBCO, were observed in the two case. We discuss the mic roscopic effects related to the gap structure which are at the origin of this qualitative difference in the behavior of thermal conductivity in these two superconductors.