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

Abstract

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 ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{x}}$, respectively. High-quality, c-axis-oriented epitaxial thin films have been used for our study. The temperature and the magnetic-field dependence of the Nernst and Seebeck electric fields were measured both in the mixed state and in the fluctuation regime above the thermodynamic critical temperature. The Seebeck-effect data can be explained well by an extended two-fluid counterflow picture in analogy to the fountain effect in superfluids. From the Nernst-effect data the temperature and magnetic-field dependence of the transport entropy of magnetic-flux lines has been derived. For the magnetic field applied parallel to the c axis of the films, approximately the same value of the transport entropy was found for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{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, ${\mathit{dH}}_{\mathit{c}2}$/dT=-2.4\ifmmode\pm\else\textpm\fi{}0.2 and -2.5\ifmmode\pm\else\textpm\fi{}0.2 T/K for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{x}}$, corresponding to a Ginzburg-Landau coherence length ${\ensuremath{\xi}}_{\mathit{a}\mathit{b}}$=14.5 and 14.2 \AA{}, respectively. The Hall resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{x}\mathit{y}}$ of both materials showed a sign anomaly in the mixed state and scaled with the longitudinal resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{x}\mathit{x}}$ as ${\mathrm{\ensuremath{\rho}}}_{\mathit{x}\mathit{y}}$(T)=${\mathit{K}}^{\mathrm{\ensuremath{-}}1}$${\mathrm{\ensuremath{\rho}}}_{\mathit{x}\mathit{x}}^{\mathrm{\ensuremath{\beta}}}$(T) with \ensuremath{\beta}\ensuremath{\simeq}2 and a magnetic-field-independent coefficient K.