Observation of orbital waves as elementary excitations in a solid

• Published in: Nature (London) Vol. 410; no. 6825; pp. 180 - 183
• Main Authors: Tokura, Y, Saitoh, E, Okamoto, S, Takahashi, K. T, Tobe, K, Yamamoto, K, Kimura, T, Ishihara, S, Maekawa, S
• Format: Journal Article
• Language: English
• Published: London Nature Publishing 08.03.2001; Nature Publishing Group
• Subjects: Clouds; Collective effects; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Crystals; Electron states; Electronics; Electrons; Elementary excitations; Exact sciences and technology; Excitation; High temperature; Manganites; Mathematical models; Metals; Orbitals; Physics; Spin waves; Strongly correlated electron systems; heavy fermions; Strongly correlated systems; heavy fermions; Waves; Raman spectra; Degrees of freedom; Inorganic compounds; Ternary compounds; Transition element compounds; Electron-phonon interactions; Rare earth compounds; Experimental study; Phonon mode; Electron wave; Strongly correlated electron systems; Elementary excitation; Orbital; Lanthanum Manganites
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/35065547

A basic concept in solid-state physics is that when some kind of symmetry in a solid is spontaneously broken, collective excitations will arise. 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 state 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 important. Orbitally ordered states have been found in several transition-metal compounds, 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.