Nernst, Seebeck, and Hall effects in the mixed state of YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+x thin films: A comparative study

The anisotropy of the high-temperature superconductors has strong impact on their transport properties in the mixed state. We have performed a comparative study of the Nernst, Seebeck, and Hall effects of the anisotropic and extremely anisotropic high-temperature superconductors YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+x, respectively. High-quality, c-axis-oriented epitaxial thin films have been used for our study. The temperature and the magnetic-field dependence of the Nernst and Seebeck electric fields were measured both in the mixed state and in the fluctuation regime above the thermodynamic critical temperature. The Seebeck-effect data can be explained well by an extended two-fluid counterflow picture in analogy to the fountain effect in superfluids. From the Nernst-effect data the temperature and magnetic-field dependence of the transport entropy of magnetic-flux lines has been derived. For the magnetic field applied parallel to the c axis of the films, approximately the same value of the transport entropy was found for YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+x, showing that the different anisotropy of these materials has little influence on this quantity. From our experimental data in the mixed state and the fluctuation regime well above the mean-field critical temperature, the upper-critical-field slope was derived. For both regimes, dHc2/dT=-2.4±0. 2 and -2.5±0.2 T/K for YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+x, corresponding to a Ginzburg-Landau coherence length ξab=14.5 and 14.2 , respectively. The Hall resistivity ρxy of both materials showed a sign anomaly in the mixed state and scaled with the longitudinal resistivity ρxx as ρxy(T)=K-1ρxxβ(T) with β2 and a magnetic-field-independent coefficient K.