THERMOPOWER MEASUREMENTS OF HIGH-TEMPERATURE SUPERCONDUCTORS - EXPERIMENTAL ARTIFACTS DUE TO APPLIED THERMAL-GRADIENT AND A TECHNIQUE FOR AVOIDING THEM

We discuss thermoelectric measurements of high-temperature superconduc tors around the superconducting transition. The Seebeck effect coeffic ient has been widely used to study the dissipative phenomena of the vo rtex lattice and its interaction with quasiparticles, in conjunction w ith the resistivity to which it is correlated in the framework of all existing theories. As is well known, Seebeck effect measurements are f ar from the accuracy of resistivity measurements because the presence of a thermal gradient across the sample may alter the shape of the cur ve. We make a quantitative analysis of the errors involved in ac measu rement techniques; the error percentage grows considerably in the low- dissipation region where the Seebeck effect vanishes. We propose a met hod that drastically reduces the experimental artifacts; moreover, we present the Seebeck effect measurements of a Y-Ba-Cu-O thin film, perf ormed with the standard steady-flux ac technique and with the one prop osed here, and we discuss the results in light of our calculations. [S 0163-1829(98)06842-8].