Angle dependence of the transverse thermal conductivity in YBa(2)Cu(3)O(7) single crystals: Doppler shift and Andreev...

We have measured the transverse thermal conductivity kappa(xy) of twinned and untwinned YBa(2)Cu(3)O(7) single crystals as a function of angle theta between the magnetic field applied parallel to the CuO(2) planes and the heat current direction. For both crystals we observed a clear twofold variation in the field-angle dependence of kappa(xy)(theta) = -kappa(0)(xy)(T,B)sin(2 theta). We have found that the oscillation amplitude kappa(0)(xy) depends on temperature and magnetic field. Our results show that kappa(0)(xy) = aBln(1/bB) with the temperature- and sample-dependent parameters a and b. We discuss our results in terms of Andreev scattering of quasiparticles by vortices and a recently proposed theory based on the Doppler shift in the quasiparticle spectrum.