Am Doping in Copper-Oxide Superconducting Systems
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Am Doping in Copper-Oxide Superconducting Systems
S. Skanthakumar and L. Soderholm Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 ABSTRACT The effect of Am doping on the physical properties of three superconducting systems (RBa2Cu3O7, Pb2Sr2R1-xCaxCu3O8 and R2-xMxCuO4) have been studied using x-ray diffraction, xray absorption spectroscopy (XAS) and magnetic susceptibility experiments. Am is incorporated into all three systems as the tetravalent ion and superconductivity is not observed in any of the resultant compounds. The absence of superconductivity in Pb2Sr2Am0.5Ca0.5Cu3O8 is attributed to the transfer of charge from Am to the Cu-O planes. Systems that cannot incorporate a tetravalent ion, notably AmBa2Cu3O7 and Am2CuO4, do not form. Although Pr1.85Am0.15CuO4 forms with similar structure to Pr1.85Ce0.15CuO4, and Am like Ce is tetravalent, it is not superconducting. We argue that the absence of superconductivity in the Am doped Pr compound it is due to the hybridization of radially-extended f orbitals on the magnetic Am4+ ion with the Cu-O band states. INTRODUCTION Rare earth (R) substitution effects on the physical properties of RBa2Cu3O7, Pb2Sr2R1Ca Cu x 3O8 and R2-xMxCuO4 (M= Ce, Th) have been studied in great detail using various x experimental techniques [1]. Although R3+ carries a magnetic moment as high as 10.6µB, R doping does not affect superconductivity in these systems, with the exception of R= Pr. If Pr has a well separated, nonmagnetic, singlet ground state then Pr doping does not affect the superconductivity. Otherwise Pr f-states, which are more radially extended than those of the heavier rare earths, hybridize with Cu-O bands and suppress superconductivity, a result of which is that the Pr local moments order antiferromagnetically with unusually high ordering temperature (TN) [2]. The actinide 5f orbitals are even more radially extended than the lighterlanthanide 4f-orbitals, and therefore hybridization effects are larger in actinide-containing compounds. In our earlier work [3-6] we have studied Cm doped high-Tc related phases, which do not superconduct but have very high TN. Cm3+ has a 7f configuration, which has a spherically symmetric ground state with a large free-ion effective-magnetic moment (7.94µB) It is argued [6] that superconductivity is always suppressed in these Cm phases due to hybridization. Ce, has the lowest potential for reduction from tetravalent to trivalent of all the rare earths, [7, 8] and is found to be incorporated as a tetravalent ion in these systems [9]. The stability of Ce4+ prevents the formation of CeBa2Cu3O7 and Ce2CuO4. For the samples that do form with Ce4+, notably Pb2Sr2Ce0.5Ca0.5Cu3O8, superconductivity is suppressed by a mechanism other than hybridization, although Ce4+ is nonmagnetic. In this compound, the extra charge introduce by the Ce4+ transfers to CuO2 planes, which are responsible for superconductivity, and reduce the charge carriers on the Cu-O lattice. Since Am ion has a reduction potential that is between those of Ce
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