T_c depression in the YBa₂Cu_4-xM_xO₈ system for M=Fe, Ni, Zn, and Ga

Abstract

Observations show that T_c depression of the YBa₂Cu₄O₈ system resulting from disorder caused by partial replacement of Cu by metallic dopants Fe, Ni, Zn, and Ga, in small concentrations, is significantly faster 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 circumstances-first when the impurity is isolated, and the other when it is in the crystal environment. On this basis the observations support a mechanism of T_c suppression wherein the moments of the impurity acquire effectively more isolated-atom-like character in the YBa₂Cu₄O₈ system and wherein the pairing interaction is also suppressed directly.