SUBSTITUTION OF GA FOR CU IN RBA2CU3-XGAXO7-Y SYSTEMS (R=YB, ER, Y, DY, GD, EU, AND ND)

We have investigated the effect of Ga doping on the structure, oxygen content, and superconducting transition temperature T(c) of RBa2Cu3-xGaxO7-y (R = Yb, Er, Y, Dy, Gd, Eu, and Nd, and x = 0, 0.05, 0.1, 0.15, 0.2, and 0.3). We observed that in these systems, the superconducting T(c), the melting point, and the critical Ga concentration x(O-T) at which the samples undergo an orthorhombic-tetragonal transition, are all rare-earth-ion size dependent. The results suggest that the decrease of density of states N(E(F)) and/or the localization of the carriers, which is likely related to a Mott transition due to Ga substitution, are the possible origins for the suppression of superconductivity and the observed metal-semiconductor transition.