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TT: Fachverband Tiefe Temperaturen

TT 30: Superconductivity: Cuprate High-Temperature Superconductors 2

TT 30.5: Vortrag

Mittwoch, 25. März 2009, 15:00–15:15, HSZ 304

Doping by metal-oxygen complexes and phase separation in the superconducting solid solution Bi_2+ySr_2−xLa_xCuO_6+δ — •Jürgen Röhler¹, Christoph Trabant¹, Sebastian Standop¹, Helmut Dwelk², and Alica Krapf² — ¹Universität zu Köln, 50937 Köln — ²Humboldt Universität zu Berlin, 12489 Berlin

Hole doping, n_h (x), of the single-layer superconductor Bi_2+ySr_2−xLa_x
CuO_6+δ (BLSCO) is shown from systematic x-ray absorption measurements at the Cu-, La- and Bi-sites to be controlled by formation of the metal-oxygen complexes [La³⁺–O²⁻], or/and [Bi³⁺–O²⁻]. It is not, as generally assumed, controlled simply by cationic La³⁺/Sr²⁺ mixed valency. Interstitial excess oxygen atoms, taken up together with the substitution of Sr²⁺ by La³⁺ and partially also by excess Bi³⁺, reside at two different sites in two different complex ion formations : [La–O] and/or [Bi–O] close to the faces of the CuO₆ octahedra, and [La₂–O] in between the Sr–O and Bi–O layers. The former dope holes into the CuO₂ planes. The latter do not, but break the van der Waals bonds in the Bi-O double layers allowing for the stabilization of co-existing monoclinic and orthorhombic superstructures. The intricate competition between these three different complex formations is connected with the electronic inhomogeneities in the underdoped, and the macroscopic decomposition in the overdoped regimes. It may be also at the origin of the singular stability of BLSC around its optimum doped composition x_La≃ 0.35, and of the hole saturation in the overdoped regime at n_h≃ 0.2.

Supported by the ESRF through projects HE2644 and HE2955.