Superconductivity of the Pr-doped (Y,Eu)Ba₂Cu₄O₈ system

Abstract

A systematic study of the T_c dependence of the disordered R_1-xPr_xBa₂Cu₄O₈ (RPr-124) system (R=Y,Eu) on the Pr concentration x is reported here. Like (Y,Eu)_1-xPr_xBa₂Cu₃O₇ (RPr-123), T_c of these systems decreases 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 system, 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 discussed 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 attributed the failure of the AG theory in the RPr-124 systems to the linear x dependence of the carrier-impurity scattering rate. A modified AG 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-124 systems are qualitatively understood on the basis of an indirect effect of the Pr impurity on the coupling of the Y,Eu state with the Cu₃d–O 2p state. The relatively smaller value of dT_c/dx in Y,Eu-124 than in Y,Eu-123 is understood on the basis of the larger separation of the CuO₂-(Y,Eu/Pr)-CuO₂ regions in the former system.