Role of internal electric field on suppression of superconductivity by Pr in copper oxides

Abstract

The influence of Pr on the superconductivity in copper oxides is argued to depend critically on the energy difference between the Fermi energy E F and the energy E f of the Pr 3+: 4f 2 level. With a small f F − E f , hybridization between Pr-4f and O-2p orbitals transforms the itinerant holes of the p-type superconductors into small polarons, which suppresses T c. With a large E F − E f , any such hybridization is too small to change the itinerant character of the mobile holes, and Pr has no more influence on T c than do the other trivalent rare earths. Comparisons of the influence of Pr on the transition temperature T c have been made for several systems. In each case, Pr 3+ ions occupy sites having four oxygen atoms of a CuO 2 sheet as nearest neighbors, and p-type superconductivity is suppressed by Pr only where the sum of the formal positive charge Q per formula unit at the planes adjacent to a CuO 2 sheet exceed a critical value. We argue that changes in the local electric field at a Cu atom of the CuO 2 sheets lowers E F relative to E f with increasing Q.