The 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 syst ematically the nux density, B, and the temperature, T. The Nernst coefficie nt was observed over the temperature region of a fairly wide superconductin g vs. normal resistive transition under the applied flux density, B. The tr ansport entropy of a fluxoid, S-phi, estimated directly from the present me asurements showed a small tail decreasing exponentially with increasing tem perature, even above the mean-field critical temperature, T-c(B), whereas t he existing theory predicted S-phi to become zero above T-c(B). Then the ex pression for S-phi above Tc(B) was derived. The expression for the resistiv ity around the temperatures of the resistive transition was also derived, i ncluding the resistivity due to the flux flow. With the aid of these expres sions, the observed Nernst coefficient was explained quantitatively by the present theory. (C) 1999 Elsevier Science B.V. All rights reserved.