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 superconductors around the superconducting transition. The Seebeck effect coefficient has been widely used to study the dissipative phenomena of the vortex lattice and its interaction with quasiparticles, in conjunction with the resistivity to which it is correlated in the framework of all existing theories. As is well known, Seebeck effect measurements are far from the accuracy of resistivity measurements because the presence of a thermal gradient across the sample may alter the shape of the curve. We make a quantitative analysis of the errors involved in ac measurement techniques; the error percentage grows considerably in the low-dissipation region where the Seebeck effect vanishes. We propose a method that drastically reduces the experimental artifacts; moreover, we present the Seebeck effect measurements of a Y-Ba-Cu-O thin film, performed with the standard steady-flux ac technique and with the one proposed here, and we discuss the results in light of our calculations. [S0163-1829(98)06842-8].