Thermopower in Y1-xCaxBa2-xLaxCu3Oy and Y1-xCaxBa2Cu3-xCoxOy

A systematic study of the thermopower behavior in codoped systems with comp ositions of Y1-xCaxBa2-xLaxCu3Oy (0 less than or equal to x less than or eq ual to 0.4) and Y1-xCaxBa2Cu3-xCoxOy (0 less than or equal to x less than o r equal to 0.3) has been carried out. Both of these systems have been inves tigated in cases pf a near-stoichiometric oxygen content and that reduced b y vacuum annealing. For all the sample series increasing doping level was r evealed to initiate a well-marked transformation of the S(T) dependences wh ich is noncharacteristic of thr YBa2Cu3Oy, system in the case of different deviations from the stoichiometry. This transformation is assumed to be a c onsequence of a specific Ca influence on the band spectrum in the normal sl ate. The thermopower at T= 300 K remains almost constant: with x for Y1-xCa xBa2-xLaxCu3Oy while it increases gradually For Y1-xCaxBa2Cu3-xCoxOy in the case of a near-stoichiometric oxygen content and changes nonmonotonically after oxygen-reducing annealing. Comparing the results obtained for two dif ferent kinds of codoping we have concluded that in both of the investigated systems nonisovalent impurities placed at different lattice sites counterb alance the influence of each other on the charge balance in the lattice and properties of the charge-carrier system, but to a different extent. In die case of Coma doping the effect of total charge compensation is realized an d this determines all the sample properties, while for the Ca/Co doping the compensation effect is accompanied by an additional mechanism affecting th e properties of the charge-carrier system. [S0163-1829(99)13505-7].