Impurity Study of the Lanthanum Copper Metal Oxide System

We have studied the thermodynamic and transport properties of the La_{rm 2-x }Sr_{rm x}Cu _{rm 1-x}M _{rm x}O_4 and the La_2Cu_ {rm 1-x}M_{ rm x}O_4 systems with M = Ni and Zn. We have determined the superconducting transition temperature as a function of impurity concentration in the La_{1.85}Sr _{0.15}Cu_{ rm 1-x}M_{rm x}O_4 and the Neel temperature as a function of impurity concentration in the La_2Cu_{ rm 1-x}M_{rm x}O_4 system. These measurements indicate that the superconducting transition temperature and the Neel temperature is severely depressed by both impurities. This data supports the conjecture that a weak BCS theory is applicable to these new oxide materials if the traditional phonon mediated electron-electron interaction is replaced by an interaction mediated by magnetic interactions. A comparison to similar studies for the YBa_2(Cu_{rm 1-x}M_{rm x} )_3O_{ rm 7-delta} system show some significant differences. In particular, the effects of the impurities, Ni and Zn have similar effects on T_{ rm c} in the La_{1.85 }Sr_{0.15}Cu _{rm 1-x}M _{rm x}O_4 system, they have significantly different effects on the YBa_2(Cu_ {rm 1-x}M_{ rm x})_3O _{7-delta} system. In addition, localization seems to be more prevalent for Zn doping in the YBa_2(Cu_{ rm 1-x}M_{rm x})_3O_ {7-delta} systems than in the La _{1.85}Sr_ {0.15}Cu_{rm 1-x}M_{rm x} O_4 system.