Nernst effect in high-T_c cuprate superconductors

Detailed measurements of both the Nernst coefficient, N_e, and the current vs. voltage characteristics were carried out in a YBCO film by varying systematically the flux density, B, and the temperature, T. The Nernst coefficient was observed over the temperature region of a fairly wide superconducting vs. normal resistive transition under the applied flux density, B. The transport entropy of a fluxoid, S_φ estimated directly from the present measurements showed a small tail decreasing exponentially with increasing temperature, even above the mean-field critical temperature, T_c(B), whereas the existing theory predicted S_φ to become zero above T_c(B). Then the expression for S_φ above T_c(B) was derived. The expression for the resistivity around the temperatures of the resistive transition was also derived, including the resistivity due to the flux flow. With the aid of these expressions, the observed Nernst coefficient was explained quantitatively by the present theory.