Thermoelectric powerand band spectrum transformation in Y1-xCaxBa2-xLaxCu3Oy

Abstract

The superconducting and normal-state transportproperties (resistivity ρ and thermoelectric power S) oftwo sets of ceramic samples with compositionY1-xCaxBa2-xLaxCu3Oy (x = 0-0.4) have been investigated.The co-doping effect was studied at different conditions of the oxygen subsystem, namely, at anear-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 forheavily-doped samples and a rise in the S(T) curve slope as x grows.The results obtained are analysed in terms of aphenomenological narrow-band model that makes it possible to determinethe band spectrum parameters in the normal state and to tracetheir changes with varying composition.In co-doped Y1-xCaxBa2-xLaxCu3Oy, impuritieswith valencies different from those of the native cations cancel out the influence of one another on thecharge balance in the lattice.All the results obtained indicate that the normal-state and superconducting properties of theinvestigated system are mainly determined by this compensation effect.Comparison of the variation of the critical temperatureTc with the changing conduction bandwidth WD shows thatthere is a correlation between these parameters, according towhich the superconducting properties of doped YBa2Cu3Oydepend strongly on the parameters of the band spectrum structurein the normal state.Analysis of how the conduction band is modified and S(T) transforms suggests that calcium isresponsible for additional states in the conduction band.