Superconductivity of the Pr-doped (Y,Eu)Ba<SUB>2</SUB>Cu<SUB>4</SUB>O<SUB>8</SUB> system

A systematic study of the Tc dependence of the disordered R1-xPrxBa2Cu4O8 (RPr-124) system (R=Y,Eu) on the Pr concentration x is reported here. Like (Y,Eu)1-xPrxBa2Cu3O7 (RPr-123), Tc of these systems decreases monotonically with Pr concentration. dTc/dx, however, is found to be smaller in RPr-124 than in RPr-123. Moreover, unlike RPr-123, dTc/dx does not vary monotonically with x at least in the YPr-124 system, although the relative variation of Tc 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 Tc 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 Tc variation in the RPr-124 systems. The different Tc 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 dTc/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