ORDER-PARAMETER-FLUCTUATION EFFECTS ON THE THERMOELECTRIC-POWER ABOVE THE SUPERCONDUCTING TRANSITION IN POLYCRYSTALLINE YBA2CU3O7-DELTA

This paper reports on detailed measurements of the thermoelectric power, S(T), and of the electrical conductivity, sigma(T), in three polycrystalline YBa2Cu3O7-delta Samples, all with almost the same composition (8 less than or similar 0. 10) and single phase to within 4%, but with different granularity characteristics. The measurements were done in the temperature interval ranging from the superconducting transition ( approximately 91 K) to 300 K, and the temperature resolution was 50 mK for S(T) and 10 mK for sigma(T). Near the transition, the S(T) data are corrected from the effects associated with the nonzero temperature gradient (del T less than or similar 1 K cm-1) needed to perform the measurements. We also propose an empirical procedure to take into account the influence on S(T) of the structural inhomogeneities (grains, crystallites, twinnings) at long length scales, i.e., at length scales much larger than the superconducting correlation length in all directions. From these S(T) and sigma(T) data we extract, following consistent procedures, the corresponding excesses, DELTAS(epsilon) and DELTAsigma(epsilon), as a function of the reduced temperature, epsilon. Our experimental data indicate that in the so-called mean-field-like region (MFR), i.e., between epsilon almost-equal-to 5 X 10(-3) to epsilon almost-equal-to 1.5 X 10(-1), the reduced-temperature behavior of DELTAS(epsilon) is not only very similar for the three different samples studied here, but also to that of DELTAsigma(epsilon). In contrast, the amplitudes of DELTAS(epsilon) and of DELTAsigma(epsilon) appreciably differ from sample to sample. When analyzed in terms of the empirical picture proposed here, that takes into account the influence on DELTAS(epsilon) and on DELTAsigma(epsilon) of the structural inhomogeneities at long length scales, the above results strongly suggest that, at least within the MFR, all the critical behavior of S(epsilon) near but above the transition in YBa2Cu3O7-delta samples is driven by that of sigma(epsilon). These last results confirm, to a quantitative level, our earlier proposal that in single-crystal (in the ab plane) or in polycrystalline YBa2Cu3O7-delta compounds the thermoelectric power coefficient, L(epsilon), relating S(epsilon) to sigma(epsilon), is not affected, to within the measurements resolution, by the presence of superconducting-order-parameter fluctuations. The latter finding seems to be confirmed, at least at a qualitative level, by a recent theoretical calculation of DELTAL(epsilon) in the MFR in layered superconductors.