Dresden 2003 – scientific programme

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TT: Tiefe Temperaturen

TT 3: Supraleitung: Elektronenstruktur, Phononen, Tunneln, Ordnungsparameter I

TT 3.4: Talk

Monday, March 24, 2003, 10:15–10:30, HSZ301

Electronic structure of Ba_1−xK_xBiO₃ and (Ba,K)₃Bi₂O₇ — •M. Merz¹, M. Meven¹, N. Nücker², S. Schuppler², S. Ustinovich³, A. Soldatov³, S. Shiryaev³, S. Barilo³, D. Arena⁴, J. Dvorak⁴, S. Stadler⁴ und Y. Idzerda⁴ — ¹Institut für Kristallographie, RWTH-Aachen — ²Forschungszentrum Karlsruhe, IFP, Karlsruhe — ³Academy of Sciences, Minsk — ⁴Naval Research Laboratory, Washington

With O 1s near–edge x–ray absorption spectroscopy, the unoccupied electronic structure of Ba_1−xK_xBiO₃ and (Ba,K)₃Bi₂O₇ single crystals as well as of highly–ordered Ba_1−xK_xBiO₃ films has been investigated using the surface–insensitive fluorescence yield. Clear indications for both, a charge–density wave derived band as well as a conduction band are found. The data support a picture, where the intrinsically doped holes in BaBiO₃ predominantly reside on the oxygen orbitals of the compressed Bi^V+–O octahedra, leading to the well–known static charge disproportionation and the alternation of enlarged and compressed octahedra. Upon substitution of Ba by K, the intensity attributed to the unoccupied states resulting from the charge–density wave is effectively reduced. Concomitantly, the spectral weight of the conduction band develops and shows a marked increase with growing K content. For higher potassium concentrations (x ≥ 0.5), on the other hand, a small amount of holes still remains in the O 2p states of the charge–density wave, thereby pointing to a local, and most probably dynamical, breathing mode of the BiO₆ octahedra.