Abstract
A basic concept in solid-state physics is that when some kind of symmetry in a solid is spontaneously broken, collective excitations will arise1. For example, phonons are the collective excitations corresponding to lattice vibrations in a crystal, and magnons correspond to spin waves in a magnetically ordered compound. Modulations in the relative shape of the electronic clouds in an orbitally ordered state2,3,4,5,6,7,8,9 could in principle give rise to orbital waves, or ‘orbitons’, but this type of elementary excitation has yet to be observed experimentally. Systems in which the electrons are strongly correlated—such as high-temperature superconductors and manganites exhibiting colossal magnetoresistivity—are promising candidates for supporting orbital waves, because they contain transition-metal ions in which the orbital degree of freedom is important10,11. Orbitally ordered states have been found in several transition-metal compounds12,13, and orbitons have been predicted theoretically for LaMnO3 (refs 4, 5). Here we report experimental evidence for orbitons in LaMnO3, using Raman scattering measurements. We perform a model calculation of orbiton resonances which provides a good fit to the experimental data.
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Acknowledgements
We thank N. Nagaosa, T. Katsufuji, P. Prelovšek and S. E. Barnes for discussions. This work was supported in part by NEDO Japan, CREST Japan, the Science and Technology Special Coordination Fund for Promoting Science and Technology, and Grant-in-Aid for Scientific Research Priority Area from the Ministry of Education, Science and Culture of Japan. S.O. acknowledges the financial support of JSPS. Part of the numerical calculation was performed in the supercomputing facilities in IMR, Tohoku University.
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Saitoh, E., Okamoto, S., Takahashi, K. et al. Observation of orbital waves as elementary excitations in a solid. Nature 410, 180–183 (2001). https://doi.org/10.1038/35065547
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DOI: https://doi.org/10.1038/35065547
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