Multiphonon

Multiphonon refers to processes in solids in which more than one phonon participates in a single interaction event, such as the creation or annihilation of two or more lattice vibrational quanta. Phonons are quantized vibrations of the crystal lattice carrying energy and crystal momentum. Multiphonon phenomena arise from anharmonic terms in the lattice potential and from higher-order couplings between electronic states and lattice vibrations.

In spectroscopic contexts, multiphonon effects appear as two-phonon or higher-order processes that produce overtones and combination bands in Raman or infrared spectra, as well as in neutron scattering. Two-phonon scattering is often observable when single-phonon processes are weak or symmetry-forbidden. The observed frequencies in spectra tend to be sums or combinations of fundamental phonon frequencies, and intensities depend on temperature due to phonon populations.

Theoretical treatment commonly uses perturbation theory applied to anharmonic lattice Hamiltonians, with selection rules determined by crystal symmetry. Multiphonon processes are typically weaker than single-phonon processes but can become significant at elevated temperatures or in materials with strong anharmonicity or resonant electronic structure.

Applications and relevance include material characterization, probing phonon dispersion, and studying thermal properties. In specific materials, multiphonon features are used to infer coupling strengths and to identify crystal structures or disorder. A well-known example is the appearance of two-phonon Raman bands in carbon-based materials such as graphene and graphite, where double-resonant processes give characteristic features.