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.
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