EFFECTS OF A CHANGE IN THE CARRIER CONCENTRATION AND DISORDER ON THE SUPERCONDUCTING TRANSITION-TEMPERATURE OF THE LA2-XSRXCU1-YNIYO4 SYSTEM

Electrical resistivity and magnetization measurements have been carrie d out on the La2-xSrxCu1-yNiyO4 system with 0.05 less-than-or-equal-to x less-than-or-equal-to 0.25 and 0 less-than-or-equal-to y less-than- or-equal-to 0.08. The superconducting transition temperature (T(c)) as well as the Sr composition range at which superconductivity occurs bo th show a progressive decrease with increasing Ni content or disorder (y). Above a critical Ni concentration or disorder (y(c)) a complete s uppression of superconductivity is found in the entire range of Sr (ho le) concentration. The rho(T) and chi(T) data indicate that weak disor der (y < y(c)) modifies the electronic properties of the superconducti ng phase, which undergoes three successive changes as the Sr concentra tion is increased from 0.05 to 0.25: (1) a localized regime which show s Mott variable range hopping type transport at low temperatures; (2) a superconducting regime with a reduced T(c) and Meissner fraction com parable to the parent Ni-free system; and (3) a metallic regime with w eak superconducting fluctuations. When y > y(c), however, the supercon ducting phase is completely suppressed; with increasing carrier concen tration a crossover occurs from a strongly localized to a normal metal lic phase, which exhibits weak localization effects at low temperature s. From these results an electronic phase diagram is derived which des cribes the effect of changes in the carrier concentration as well as t he strength of disorder on the superconducting transition temperature.