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

摘要

Measurements of the thermal conductivity (k_(xx)) and the thermal Hall effect (k_(xy)) in high magnetic fields 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 (K_(xx)~(el)) of the CuO_2-planes from the phononic thermal conductivity (k_(xx)~(ph)). In YBa_2Cu_3O_(7-δ) k_(xx)~(el) shows a pronounced maximum in the superconducting state. This maximum is much weaker in Bi_2Sr_2CaCu_2O_(8+δ), due to stronger impurity scattering. The maximum of k_(xx)~(el) 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 YBa_2Cu_3O_(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.