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TT: Tiefe Temperaturen

TT 26: Elektronen und Phononen in HTSL

TT 26.2: Vortrag

Freitag, 30. März 2001, 10:45–11:00, A

Are Ca and oxygen equivalent dopants of Y_1−yCa_yBa₂Cu₃O_x ? — •J. Röhler¹, M. Leufgen¹, A. Gädke¹, E. Kaldis², and G. Böttger² — ¹Universität zu Köln, D-50937 Köln — ²ETH Zürich, CH-8093 Zürich

Substitution of Y³⁺ by Ca²⁺ in Y_1−yCa_yBa₂Cu₃O_x is widely believed to dope the superconducting CuO₂ planes with mobile holes as the uptake of oxygen does. Dual doping with oxygen and Ca²⁺ is frequently used to tune the overdoped state of fully oxygenated YBa₂Cu₃O₇ into a more extended range. The mechanism of exactly how Ca-dopants affect the superconductivity in Y_1−yCa_yBa₂Cu₃O_x is however unclear. Conflicting results have been reported on perfectly deoxygenated but Ca doped Y_1−yCa_yBa₂Cu₃O₆. Dependent on subtle details of the deoxygenation procedure Y_0.8Ca_0.2Ba₂Cu₃O₆ appears to be a superconducting metal, or an insulator. We report on EXAFS-measurements of the local atomic structure at the Y- and the Ca-sites in oxygen overdoped Y_1−yCa_yBa₂Cu₃O_{≃ 6.96}. We confirmed Ca ought to substitute Y as a single impurity, not Ba. Important structural differences between the two sites appear in the nn distances to the CuO₂ planes. The Ca–Cu2 bondlength is found to be significantly shorter than the Y–Cu2 bondlength, pointing to a reduced planar dimpling of s=0.2 Å around the Ca sites compared to s=0.25 Å around the Y sites. The former number corresponds to the reduced planar dimpling in fully deoxygenated and insulating YBa₂Cu₃O₆. Discussed are the crucial role of the lattice degrees of freedom perpendicular to the planes in cuprate superconductors, and their control through the Cu4s hybridization strength.