Tensor magnetothermal resistance in YBa2Cu3O7-x via Andreev scattering of quasiparticles

A magnetic field in the a-b plane of an untwinned single crystal of YBa₂Cu₃O_7-x (T_c = 92 K) is found to induce field-dependent off-diagonal elements in the thermal conductivity tensor at 10 K. The principal axes of the field-dependent part of the tensor lie along equivalent (110) directions; a principal component is maximized when the field is along the corresponding principal axis. A model that considers Andreev reflection of thermally excited quasiparticles by vortex screening currents gives semiquantitative agreement with experiment only if the gap function has nodes in the vicinity of 〈110〉.