Thermoelectric power and band spectrum transformation in Y1-xCaxBa2-xLaxCu3Oy

The superconducting and normal-state transport properties (resistivity rho and thermoelectric power S) of two sets of ceramic samples with composition Y1-xCaxBa2-xLaxCu3Oy (x = 0-0.4) have been investigated. The co-doping eff ect was studied at different conditions of the oxygen subsystem, namely, at a near-stoichiometric oxygen content and at that decreased by annealing in vacuum atmosphere. The thermoelectric power changes only slightly with inc reasing doping level, while the S(T) dependence acquires additional feature s, exhibiting a linear increase with decreasing temperature for heavily-dop ed samples and a rise in the S(T) curve slope as x grows. The results obtai ned are analysed in terms of a phenomenological narrow-band model that make s it possible to determine the band spectrum parameters in the normal state and to trace their changes with varying composition. In co-doped Y1-xCaxBa 2-xLaxCu3Oy, impurities with valencies different from those of the native c ations cancel out the influence of one another on the charge balance in the lattice. All the results obtained indicate that the normal-state and super conducting properties of the investigated system are mainly determined by t his compensation effect. Comparison of the variation of the critical temper ature T, with the changing conduction bandwidth W-D shows that there is a c orrelation between these parameters, according to which the superconducting properties of doped YBa2Cu3Oy depend strongly on the parameters of the ban d spectrum structure in the normal state. Analysis of how the conduction ba nd is modified and S(T) transforms suggests that calcium is responsible for additional states in the conduction band.