EFFECT OF MAGNETIC-FIELD ON THERMAL-CONDUCTIVITY OF YBA2CU3O7-DELTA SINGLE-CRYSTALS

We report measurements of the thermal conductivity in a magnetic field on single crystals of YBa2Cu3O7-delta. Unlike in sintered samples, the zero-field thermal conductivity of these crystals above the superconducting transition temperature shows a marked decrease with increasing temperature, demonstrating the importance of phonon-phonon processes and the high quality of the crystals. We find that the thermal conductivity kappa is very sensitive to the presence of fluxoids which are strong scatterers of phonons. The degradation of kappa is particularly large when the magnetic field H is oriented perpendicular to the Cu-O planes. The anisotropic behavior of kappa-(H) is in qualitative agreement with a geometric argument based on the antisotropy of the superconducting coherence length. We detect hysteresis at the level of several percent in the dependence of kappa on the field.