THERMOPOWER PEAK ANOMALY NEAR THE SUPERCONDUCTING TRANSITION IN Y1BA2CU3O7-DELTA COMPOUNDS AND CRITICAL-TEMPERATURE INHOMOGENEITIES

We report here the observation, just above the superconducting transit ion, of a sharp peak in the thermopower versus temperature curve, S(T) , in two polycrystalline Y1Ba2Cu3O7-delta compounds. These samples are single phase to within 4%, the resolution of our X-ray diffraction me asurements, but they are not fully oxygenated, their transition temper atures being around 90 K. The corresponding longitudinal resistivity c urves, rho(T), do not present any anomalous peak. The anomalous S(T) p eak 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 theo ry, we show here that this type of sharp S(T) peak above T(c) may be e xplained at a quantitative level by the presence in the samples of sma ll T(c) inhomogeneities, at long length scales and uniformly distribut ed, associated with small (less than 4%) oxygen content inhomogeneitie s. These results confirm our previous proposals that the intrinsic cri tical behaviour of S(T) is driven by that of the electrical conductivi ty, i.e., that the thermoelectric coefficient, L, of copper-oxide supe rconductors does not present any sharp critical divergence above the s uperconducting transition. An example of the influence on S(T) of a no n-uniformly distributed T(c) inhomogeneity is also analyzed here. When combined with our previous results on rho(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 superc onducting transition before the introduction of other more sophisticat ed, and sometimes artificial, explanations.