FIELD-DEPENDENT THERMAL-CONDUCTIVITY OF TL2BA2CUO6 - EVIDENCE FOR ANDREEV SCATTERING OF QUASI-PARTICLES

We have studied the thermal conductivity of Tl2Ba2CuO6 (Tl-2201) cryst als as functions of temperature and magnetic field. A thermal conducti vity peak is observed near 0.6 T-c which is greatly reduced on applica tion of a magnetic field along the c axis. We argue that the suppressi on of the peak at relatively low fields (less than or equal to 8T much less than H-c2) permits us to separate electronic and phononic contri butions. Fields applied in plane are less effective, but give the same reduction when applied either parallel or perpendicular to the temper ature gradient. A model based on Andreev reflection by vortex screenin g currents is shown to be consistent with these results.