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TT: Tiefe Temperaturen

TT 19: Metall-Isolator-Überg
änge, Phasenüberg
änge in Quantensystemen II

TT 19.3: Vortrag

Donnerstag, 14. März 2002, 10:15–10:30, H20

Orbitons vs. multi-phonons in LaMnO₃ — •R. Rückamp¹, M. Grüninger¹, M. Windt¹, A. Freimuth¹, P. Reutler^2,3, and A. Revcolevschi² — ¹II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln — ²Laboratoire de Physico-Chimie de l’Etat Solide, Université Paris-Sud, France — ³II. Physikalisches Institut, RWTH-Aachen

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 by measuring both transmission and reflectance of single crystals. 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. We demonstrate that the Raman peaks presented in [1] have to be explained as multi-phonons. Orbital waves yet await their experimental confirmation.

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