HYBRIDIZATION, HOLE LOCALIZATION AND PAIR BREAKING IN THE HIGH-TC SUPERCONDUCTING SYSTEM Y1-XPRXBA2CU3O7-DELTA

The Y1-xPrxBa2Cu3O7-delta (delta almost-equal-to 0.05) system has a rich temperature-praseodymium concentration (T-x) phase diagram consisting of a metal-insulator transition at x(cr) almost-equal-to 0.6, high T(c) superconductivity in the metallic phase (0 less-than-or-equal-to x less-than-or-similar-to x(cr)), and antiferromagnetic ordering of the Cu2+ and praseodymium ions at Neel temperatures T(N)(Cu) > T(N)(Pr) in the insulating phase (x(cr) less-than-or-similar-to x less-than-or-equal-to 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 gamma-T 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 CuO2 valence band states. A phenomenological model that incorporates the annihilation of mobile holes in the CuO2 planes and superconducting electron pair breaking by the praseodymium ions can describe T(c) (x, P) for x less-than-or-similar-to 0.2.