SUPPRESSION OF SUPERCONDUCTIVITY BY Ni AND HOLE-DOPING EFFECT OF Ca IN (Y1-yCay)Ba2Cu3-xNixO7-δ

Abstract

The superconducting and normal behavior of ( Y 1-y Ca y) Ba 2( Cu 3-x Ni x) O 7-δ compounds were investigated by using electrical resistivity and ac susceptibility measurements. For the samples with y = 0, Tc decrease from 91 K to 68 K as the level of Ni doping x increases from 0.0 to 0.25. The conduction mechanism of electrons appears to follow a crossover from a purely metallic regime to a semiconducting regime as the Ni content increases, i.e. for x ≥ 0.15. Ca incorporation in the YBa 2 Cu 2.85 Ni 0.15 O 7-δ samples, is manifested in the metallic behavior (of these samples), and the Tc decreases further to 72 K (for y = 0.05). The transition temperature of the Ni-Ca-doped samples, passes through a broadened maximum at 76 K (for y = 0.2) and then it decreases again. The co-substitution of calcium tends to counteract some of the effect induced by Ni, restoring the linear ρ - T regime and reducing the transition width, but it cannot recover the decrease of the critical temperature.