SUPERCONDUCTIVITY OF THE PR-DOPED (Y, EU)BA2CU4O8 SYSTEM

A systematic study of the T(c) dependence of the disordered R1-xPrxBa2 Cu4O8 (RPr-124) system (R=Y,Eu) on the Pr concentration x is reported here. Like (Y,Eu)1-xPrxBa2Cu3O7 (RPr-123), T(c) of these systems decre ases monotonically with Pr concentration. dT(c)/dx, however, is found to be smaller in RPr-124 than in RPr-123. Moreover, unlike RPr-123, dT (c)/dx does not vary monotonically with x at least in the YPr-124 syst em, although the relative variation of T(c) with the rare-earth (Y,Eu) ionic radius is similar in both the cases. The observed results are d iscussed in terms of the hole-filling model and the Abrikosov-Gorkov ( AG) theory. It is found that the AG theory is not adequate to explain the observed T(c) variation with x in the RPr-124 systems. We have att ributed the failure of the AG theory in the RPr-124 systems to the lin ear x dependence of the carrier-impurity scattering rate. A modified A G theory considering the observed carrier-impurity scattering rate is found to provide a reasonable explanation of T(c) variation in the RPr -124 systems. The different T(c) variations in the YPr-124 and EuPr-12 4 systems are qualitatively understood on the basis of an indirect eff ect of the Pr impurity on the coupling of the Y,Eu state with the Cu 3 d-O 2p state. The relatively smaller value of dT(c)/dx in Y,Eu-124 tha n in Y,Eu-123 is understood on the basis of the larger separation of t he CuO2-(Y,Eu/Pr)-CuO2 regions in the former system.