Tc depression in the YBa2Cu4-xMxO8 system for M=Fe, Ni, Zn, and Ga.

Observations show that T c depression of the YBa 2 Cu 4 O 8 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 YBa 2 Cu 3 O 7 system. Another feature of T c suppression in the YBa 2 Cu 4 O 8 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 YBa 2 Cu 4 O 8 system and wherein the pairing interaction is also suppressed directly.