Regensburg 1998 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 11: Elektronenstruktur und Phononen in HTSL

TT 11.2: Vortrag

Dienstag, 24. März 1998, 15:45–16:00, H 20

Displacive Structural Transformation in the CuO₂ Planes of YBa₂Cu₃O_x at the Underdoped-Overdoped Phase Separation Line — •J. Röhler¹, E. Liarokapis², and E. Kaldis³ — ¹U Köln, D-50937 Köln — ²NTU, GR-15780 Athens — ³ETH, CH-8093 Zürich

A structural phase transformation in the CuO₂ planes of YBa₂Cu₃O_x has been observed at the onset of the overdoped regime, x=6.95 [1]. We have measured as a function of x the static dimpling in the CuO₂ planes by yttrium EXAFS, and the O2,3 in-phase ”dimpling mode“ by Raman scattering. We find at x≃ 6.95 anomalously large displacements of the Cu2 atoms off the O2,3 layer, and a gap in the distribution of the O2,3 in-phase Raman shifts. Above x_opt the c-axis exhibits a significant upturn correlated with a downturn in a. Thereby the orthorhombicity of the martensitic crystal increases. We conclude a second martensitic form of orthorhombic YBa₂Cu₃O_x to develop in the overdoped regime.

[1] E. Kaldis, J. Röhler, E. Liarokapis, N. Poulakis, K. Conder, P.W. Loeffen, Phys. Rev. Lett. 79 (1997).