Regensburg 2002 – wissenschaftliches Programm
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. Reutler2,3, 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.  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  have to be explained as multi-phonons. Orbital waves yet await their experimental confirmation.
 E. Saitoh et al., Nature 410, 180 (2001).