Inhaltszusammenfassung

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-? 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 fiel...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-? 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-? 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-? and Bi2Sr2CaCu2O8 x, corresponding to a Ginzburg-Landau coherence length ?ab=14.5 and 14.2 Å, respectively. The Hall resistivity ?xy of both materials showed a sign anomaly in the mixed state and scaled with the longitudinal resistivity ?xx as ?xy(T)=K-1?xx?(T) with ??2 and a magnetic-field-independent coefficient K. » weiterlesen» einklappen

Autoren

Klassifikation

DFG Fachgebiet:
Physik der kondensierten Materie

DDC Sachgruppe:
Physik