Comparative analysis of the effect of La and Co on the superconductivity and energy-band spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba1 -xLax)(2)Cu3Oy and YBa2Cu3-xCoxOy samples (x=0-0.25) have been measured und er maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T-c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists aft er annealing for Y(Ba1-xLax)(2)Cu3Oy and disappears for YBa2Cu3-xCoxOy. A p henomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well a s with annealing. Despite the differences observed in the T-c (x) dependenc e, the critical temperatures for all the sample series studied were found t o correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for t he different influence of annealing on the properties of Y(Ba1-xLax)(2)Cu3O y and YBa2Cu3-xCoxOy are discussed. (C) 1999 American Institute of Physics. [S1063-7834(99)00603-6].