Recent numerical computations of the phonon spectrum of quantum crystals have demonstrated the existence of an anomalous extra phonon branch which lies above the usual acoustic modes. The purpose of the present note is to explain the physical origin of the anomalous branch in terms of two-phonon resonances, and to discuss briefly the possibility of observing these resonances in quantum crystals by neutron-scattering and Raman-scattering experiments.
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J. Ruvalds (1971) studied this question.
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