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The energy of each phonon mode is fluctuating around its equilibrium value, and its power spectral density was derived with a quartz specimen which was illuminated by a laser light. Fluctuations of the Brillouin scattered light intensity were measured by means of the photon-counting technique. The power spectral density has been derived to be 8×{}10^-5/f per phonon mode provided that phonon-number fluctuations are independent from mode to mode.
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Toshimitsu et al. (1990) studied this question.
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