Thermoelectric power and band spectrum transformation in Y_1-xCa_xBa_2-xLa_xCu₃O_y

The superconducting and normal-state transport properties (resistivity ρ and thermoelectric power S) of two sets of ceramic samples with composition Y_1-xCa_xBa_2-xLa_xCu₃O_y (x = 0-0.4) have been investigated. The co-doping effect 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 increasing doping level, while the S(T) dependence acquires additional features, exhibiting a linear increase with decreasing temperature for heavily-doped samples and a rise in the S(T) curve slope as x grows. The results obtained are analysed in terms of a phenomenological narrow-band model that makes it possible to determine the band spectrum parameters in the normal state and to trace their changes with varying composition. In co-doped Y_1-xCa_xBa_2-xLa_xCu₃O_y, impurities with valencies different from those of the native cations cancel out the influence of one another on the charge balance in the lattice. All the results obtained indicate that the normal-state and superconducting properties of the investigated system are mainly determined by this compensation effect. Comparison of the variation of the critical temperature T_c with the changing conduction bandwidth W_D shows that there is a correlation between these parameters, according to which the superconducting properties of doped YBa₂Cu₃O_y depend strongly on the parameters of the band spectrum structure in the normal state. Analysis of how the conduction band is modified and S(T) transforms suggests that calcium is responsible for additional states in the conduction band.