TC DEPRESSION IN THE YBA2CU4-XMXO8 SYSTEM FOR M=FE, NI, ZN, AND GA

Observations show that T(c) depression of the YBa2Cu4O8 system resulti ng from disorder caused by partial replacement of Cu by metallic dopan ts Fe, Ni, Zn, and Ga, in small concentrations, is significantly faste r than in the YBa2CU3O7 SYStem. Another feature of T(c) suppression in the YBa2CU4O8 system is that the effect of Fe and Ni is much stronger in this system than that of Zn. These features are analyzed in terms of the hole occupancy of the doubly charged impurity atoms in two circ umstances-first when the impurity is isolated, and the other when it i s in the crystal environment. On this basis the observations support a mechanism of T(c) suppression wherein the moments of the impurity acq uire effectively more isolated-atom-like character in the YBa2CU4O8 sy stem and wherein the pairing interaction is also suppressed directly.