Thermal conductivity and thermal Hall effect in Bi- and Y-based high-T_c superconductors

Measurements of the thermal conductivity (k_xx) and the thermal Hall effect (k_xy) in high magnetic Gelds in Y- and Bi-based high-T_c superconductors are presented. We describe the experimental technique and test measurements on a simple metal (niobium). In the high-T_c superconductors k_xx and k_xy increase below T_c and show a maximum in their temperature dependence. k_xx has contributions from phonons and quasiparticle (QP) excitations, whereas k_xy is purely electronic. The strong increase of k_xy below T_c gives direct evidence for a strong enhancement of the QP contribution to the heat current and thus for a strong increase of the QP mean free path. Using k_xy and the magnetic field dependence of k_xx we separate the electronic thermal conductivity (f^el_xx) of the CuCO₂-planes from the phononic thermal conductivity (k^ph_xx). In YBa₂Cu₃O_7-δ k^el_xx shows a pronounced maximum in the superconducting state. This maximum is much weaker in Bi₂Sr₂CaCu₂O_8+δ, due to stronger impurity scattering. The maximum of k^el_xx is strongly suppressed by a magnetic field, which we attribute to the scattering of QPs on vortices. An additional magnetic field independent contribution to the maximum of k_xx occurs in YBa₂Cu₃O_7-δ, reminiscent of the contribution of the CuO-chains, as determined from the anisotropy in untwined single crystals. Our data analysis reveals that below T_c as in the normal state a transport (τ) and a Hall (τ_H) relaxation time must be distinguished: The inelastic (i.e. temperature dependent) contribution to τ is strongly enhanced in the superconducting state, whereas τ_H displays the same temperature dependence as above T_c. We determine also the electronic thermal conductivity in the normal state from k_xy and the electrical Hall angle. It shows an unusual linear increase with temperature.