| Abstrakt: |
The superconducting and normal behavior of (Y[sub 1-y]Ca[sub y])Ba[sub 2] (Cu[sub 3-x]Ni[sub x])O[sub 7-δ] compounds were investigated by using electrical resistivity and ac susceptibility measurements. For the samples with y = 0, T[sub c] 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[sub 2]Cu[sub 2.85]Ni[sub 0.15]O[sub 7-δ] samples, is manifested in the metallic behavior (of these samples), and the T[sub c] 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. [ABSTRACT FROM AUTHOR] |
