MEASUREMENTS OF THE THERMOPOWER COEFFICIENT ABOVE THE SUPERCONDUCTING TRANSITION IN POLYCRYSTALLINE Y1BA2CU3O7-DELTA - ABSENCE OF APPRECIABLE FLUCTUATION EFFECTS

This paper reports on detailed measurements of the thermoelectric power coefficient, L(T) = S(T)sigma(T), relating the thermoelectric power S(T) to the electrical conductivity sigma(T) in three polycrystalline Y1Ba2Cu3O7-6 samples. All the samples have almost the same composition (delta less than or similar to 0.10) and they are single-phase to within 4%, but they have very different structural inhomogeneities at long length scales, i.e. at scales much larger than the superconducting correlation length in all directions. Our measurements in different single-phase samples indicate that all the critical behaviour of S(T) above the superconducting transition is driven by sigma(T). Therefore, these results strongly suggest, within our experimental resolutions of DELTAL/L less than or similar to 2%, that L(T) in Y1Ba2Cu3O7-6 compounds is not affected, over the entire mean-field-like region (MFR) above the superconducting transition, by thermodynamic fluctuations of the superconducting order parameter amplitude (OPF). The reduced temperature behaviour of the thermopower excess DELTAS(epsilon) is the same for all the single-phase samples studied here, i.e. fractal effects are not observed in the MFR, in agreement with our previous paraconductivity results. Also, the OPF effects arising on S(T) through a(T) are confirmed to be essentially three-dimensional (3D) over the entire MFR.