Abstract
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.
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