Superconductivity, Seebeck coefficient, and band structure transformation in Y1-xCaxBa2Cu3-xCoxOy (x=0-0.3)

The temperature dependences of the resistivity and of the Seebeck coefficie nt S is studied in three series of Y1-xCaxBa2Cu3-xCoxOy samples (x = 0-0.3) differing in oxygen content. It was found that the critical temperature de creases for y approximate to 7.0, and S(T = 300 K) increases with doping, w hereas oxygen deficiency results in a nonmonotonic variation of these quant ities with increasing x. The band structure parameters have been determined from an analysis of the S(T) relations using a phenomenological theory of electron transport. It was found that an increase in x results in a gradual increase in band asymmetry, which is caused by calcium-induced creation of additional states in the band responsible for conduction in the normal pha se. An analysis has shown that high impurity concentrations in oxygen-defic ient Y1-xCaxBa2Cu3-xCoxOy samples bring about an additional ordering of the structure, which may be caused by formation of a cobalt superlattice. It h as also been shown that, in the case of Ca and Co codoping, the dependence of critical temperature on effective conduction-band width coincides with t he universal correlation relation observed in the YBa2Cu3Oy system with sin gle substitutions in various lattice sites. (C) 1999 American Institute of Physics. [S1063-7834(99)00408-6].