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TT: Tiefe Temperaturen

TT 11: Postersitzung II (Metall-Isolator-Überg
änge, Phasenüberg
änge in Quantensystemen, Theorie: Systeme korrelierter Elektronen)

TT 11.16: Poster

Dienstag, 25. März 2003, 14:30–19:00, P2c, P2d

Experimental quest for orbitons in LaMnO₃ — •R. Rückamp¹, M. Grüninger¹, C. Hilgers¹, A. Freimuth¹, P. Reutler^2,3, K.-Y. Choi², P. Lemmens², B. Büchner², G. Güntherodt², and A. Revcolevschi³ — ¹II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln — ²II. Physikalisches Institut, RWTH-Aachen — ³Lab. de Chimie des Solides, Université Paris-Sud, France

Orbital physics currently attracts much interest. In an orbitally ordered state one expects new collective elementary excitations, namely orbital waves or orbitons, which involve transitions between different orbital states. Recently, Saitoh et al. [1] reported three peaks in the Raman data of orbitally ordered LaMnO₃ and interpreted this as the first observation of orbital waves as elementary excitations in a solid.

We determined the optical conductivity σ(ω) of LaMnO₃. The peaks observed in σ (ω) are very similar to the Raman features. However, direct orbital excitations are not infrared active due to parity. Therefore the observation of similar peak frequencies in the Raman spectrum and in σ(ω) strongly challenges the interpretation of these peaks as orbitons [2]. We identify these features as multi-phonons and present Raman data which show two-phonon, three-phonon and four-phonon scattering. We discuss the search for orbital excitations in σ(ω) at higher energies and in related materials.

[1] E. Saitoh et al., Nature 410, 180 (2001).

[2] M. Grüninger et al., Nature 418, 39 (2002).