Observation of orbital waves as elementary excitations in a solid

A basic concept in solid-state physics is that when some kind of symmetry in a solid is spontaneously broken, collective excitations will arise(1). 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(2-9) 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 important(10,11). Orbitally ordered states have been found in several transition-metal compounds(12,13), 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.