Superconductivity in the Ag-substituted La_1.8Sr_0.2CuO₄ oxide system

The presence of mixed-valent copper and the nearly two-dimensional character of the K₂NiF₄ structure are considered important for the occurrence of superconductivity in La_1.8Sr_0.2CuO₄ (T_c onset 36 K). The authors have investigated the effect of partial replacement of mixed-valent Cu by monovalent Ag on the superconductivity of this system by studying the compositions La_1.8Sr_0.2Cu_1-xAg_xO₄ (0.0<or=x<or=0.15). All these have the tetragonal K₂NiF₄-type structure. In spite of the charge imbalance caused by Ag substitution, the T_c is found to decrease rather gradually from 36 K for x=0.0 to 20 K for x=0.15. A small increase in the Pauli susceptibility is also observed on partial replacement of Cu by Ag in the above system.