Hybridization, Hole Localization and Pair Breaking in the High Tc Superconducting System Y1-xPrxBa2Cu3O7-δ

Abstract The Y 1- x Pr x Ba 2 Cu 3 O 7- δ ( δ ≈ 0.05) system has a rich temperature—praseodymium concentration ( T - x ) phase diagram consisting of a metal—insulator transition at x cr ≈0.6, high T c superconductivity in the metallic phase (0⩽ x ≤ x cr ), and antiferromagnetic ordering of the Cu 2+ and praseodymium ions at Neel temperatures T N (Cu) > T N (Cu)> T N (Pr) in the insulating phase ( x cr ≤ x ⩽1). In the metallic phase, T c decreases monotonically with x and vanishes near x cr , the dependence of T c on pressure P and the upper critical field on temperature are both anomalous, and the low temperature specific heat can be resolved into a large γΓ contribution, reminiscent of heavy fermion behavior, and a spin 1 2 Kondo anomaly; in the insulating phase, the values of T N (Pr) are anomalously large. These phenomena are briefly reviewed within the framework of a picture that is based on appreciable hybridization of the praseodymium localized 4f states and the CuO 2 valence band states. A phenomenological model that incorporates the annihilation of mobile holes in the CuO 2 planes and superconducting electron pair breaking by the praseodymium ions can describe T c ( x , P ) for x ≤ 0.2.