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. 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.

We have investigated anisotropic optical conductivity spectra over a wide temperature range, 10 KрT р800 K, for a detwinned single crystal of LaMnO 3 , which undergoes the orbital ordering at T OO Ϸ780 K and the A-type antiferromagnetic ordering at T N Ϸ140 K. The charge-gap transition peaking around 2 eV shows a clear anisotropy for the polarizations parallel and perpendicular to the c axis ( Pbnm setting͒. With the increase of temperature, the anisotropy and the gap magnitude are gradually suppressed toward T OO , while least affected by spin disordering at T N , indicating the strong charge-transfer character of gap transition.

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