NEW ASPECTS OF THE THERMOPOWER PEAK ANOMALY NEAR THE SUPERCONDUCTING TRANSITION IN Y(1)BA(2)CU(3)O7-(DELTA) COMPOUNDS - THERMAL FLUCTUATIONS VERSUS OXYGEN-CONTENT INHOMOGENEITIES

The anomalous sharp peak of the thermoelectric power, S(T), that we ha ve recently observed just above the superconducting transition in some non-fully oxygenated Y1Ba2Cu3O7-delta polycrystalline samples, with T -c < 91 K, and also the rounded S(T) behavior around T-c of the fully oxygenated samples (with negative S(T) values), are now analyzed in th is paper on the grounds of different existing approaches for the influ ence of thermal fluctuations on S(T) near the superconducting transiti on. This analysis confirms our previous proposal that these sharp S(T) peaks near T-c, that were also observed by other groups, are not intr insic and that these anomalies are probably due to the presence in the samples olf small oxygen content inhomogeneities (less than 4% of the average sample oxygenation) at long length scales, which lead to inho mogeneities in the critical temperature and in the S(T) sign. Although the uncertainties on the S(T) background may introduce some ambiguity , our present analysis also confirms our earlier proposal that the int rinsic critical behavior of S(T) near T-c in Y1Ba2Cu3O7-delta compound s is mainly driven by that of the electrical conductivity, i.e., we fo und that the thermoelectric coefficient, L(T), of copper-oxide superco nductors does not present a strong critical behavior above the superco nducting transition, in agreement with earlier theoretical predictions of Maki. Also, our previous analysis of the influence on S(T) of unif ormly and non-uniformly distributed oxygen content inhomogeneities is now extended to new possible experimental situations. These results sh ow, in particular, that the strong decrease of the anomalous S(T peak amplitude due to the presence of a magnetic field, an effect observed by various groups and that has remained unexplained until now, may be easily understood in terms of oxygen content inhomogeneities. By takin g into account the dependence of the effective thermopower on the rela tive strength of these inhomogeneities, it is possible to explain the change of sign of S(T) observed by various authors in the normal regio n in some polycrystalline samples. Such a dependence also pro ilides a simple and plausible explanation of the existing controversial experi mental results concerning the possible opposite sign of S in the a and b directions of untwinned crystals. The S(T) peak will also be briefl y compared with the electrical resistivity peak that we have observed previously in other Y1Ba2Cu3O7-delta crystals.