QUASI-PARTICLE MEAN FREE-PATH IN YBA2CU3O7 MEASURED BY THE THERMAL HALL CONDUCTIVITY

Asymmetric scattering of quasiparticles by pinned vortices in YBa2Cu3O7 produces an unusually large thermal Hall conductivity kappa(xy) in the mixed state. In high fields, the kappa(xy) vs held profile displays strong curvature, which allows the zero-field mean free path l(0) of the quasiparticles to be determined. We find that l(0) undergoes a remarkable increase from 90 Angstrom near 90 K to 2500 Angstrom at 15 K. The scattering rate changes by a factor of 3 between 90 and 75 K. We discuss implications for the scattering mechanism. From kappa(xy) and l(0), we obtain a new estimate of the thermal conductivity of inplane quasiparticles.