Thermopower peak anomaly near the superconducting transition in Y 1Ba 2Cu 3O 7−δ compounds and critical temperature inhomogeneities

Abstract

We report here the observation, just above the superconducting transition, of a sharp peak in the thermopower versus temperature curve, S( T), in two polycrystalline Y 1Ba 2Cu 3O 7−δ compounds. These samples are single phase to within 4%, the resolution of our X-ray diffraction measurements, but they are not fully oxygenated, their transition temperatures being around 90 K. The corresponding longitudinal resistivity curves, p( T), do not present any anomalous peak. The anomalous S( T) peak disappears after a new oxygen annealing of the samples, which also increases somewhat their T c whereas their corresponding S( T) become negative. By using mean field approaches of the effective medium theory, we show here that this type of sharp S( T) peak above T c may be explained at a quantitative level by the presence in the samples of small T c inhomogeneities, at long length scales and uniformly distributed, associated with small (less than 4%) oxygen content inhomogeneities. These results confirm our previous proposals that the intrinsic critical behaviour of S( T) is driven by that of the electrical conductivity, i.e., that the thermoelectric coefficient, L, of copper-oxide superconductors does not present any sharp critical divergence above the superconducting transition. An example of the influence on S( T) of a non-uniformly distributed T c inhomogeneity is also analyzed here. When combined with our previous results on p( T) in inhomogeneous samples, our present results on S( T) confirm the need of a thorough check in terms of T c inhomogeneities of any anomalous behaviour near a superconducting transition before the introduction of other more sophisticated, and sometimes artificial, explanations.