New aspects of the thermopower peak anomaly near the superconducting transition in Y 1Ba 2Cu 3O 7−δ compounds: thermal fluctuations versus oxygen content inhomogeneities

Abstract

The anomalous sharp peak of the thermoelectric power, S( T), that we have recently observed just above the superconducting transition in some non-fully oxygenated Y 1Ba 2Cu 3O 7−δ 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 this paper on the grounds of different existing approaches for the influence of thermal fluctuations on S( T) near the superconducting transition. This analysis confirms our previous proposal that these sharp S( T) peaks near T c, that were also observed by other groups, are not intrinsic and that these anomalies are probably due to the presence in the samples of small oxygen content inhomogeneities (less than 4% of the average sample oxygenation) at long length scales, which lead to inhomogeneities 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 intrinsic critical behavior of S( T) near T c in Y 1Ba 2Cu 3O 7−δ compounds is mainly driven by that of the electrical conductivity, i.e., we found that the thermoelectric coefficient, L( T), of copper-oxide superconductors does not present a strong critical behavior above the superconducting transition, in agreement with earlier theoretical predictions of Maki. Also, our previous analysis of the influence on S( T) of uniformly and non-uniformly distributed oxygen content inhomogeneities is now extended to new possible experimental situations. These results show, 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 taking into account the dependence of the effective thermopower on the relative strength of these inhomogeneities, it is possible to explain the change of sign of S( T) observed by various authors in the normal region in some polycrystalline samples. Such a dependence also provides a simple and plausible explanation of the existing controversial experimental results concerning the possible opposite sign of S in the a and b directions of untwinned crystals. The S( T) peak will also be briefly compared with the electrical resistivity peak that we have observed previously in other Y 1Ba 2Cu 3O 7−δ crystals.