Angle Dependence of the Transverse Thermal Conductivity in YBa$_2$Cu$_3$O$_7$ single crystals: Doppler Effect vs. Andreev scattering

Abstract

We have measured the transverse thermal conductivity $κ_{xy}$ of twinned and untwinned YBa$_2$Cu$_3$O$_7$ single crystals as a function of angle $θ$ between the magnetic field applied parallel to the CuO$_2$ planes and the heat current direction, at different magnetic fields and temperatures. For both crystals we observed a clear twofold variation in the field-angle dependence of $κ_{xy}(θ) = - κ^0_{xy}(T,B) \sin(2θ)$. We have found that the oscillation amplitude $κ^0_{xy}$ depends on temperature and magnetic field. Our results show that $κ^0_{xy} = a B \ln(1/(bB))$ 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.