Abstract
Heat-pulse experiments have been performed in order to test the kinetic theory of phonons in glasses. Based on the coupled Boltzmann equations for phonons and localized tunneling systems field equations for the phonon energy, the tunneling state energy and the phonon momentum have been derived. It is shown that the solution of these symmetric hyperbolic field equations for given boundary and initial values of heat-pulse measurements are in excellent agreement with new experimental data obtained in vitreous silica below 1 K. In the thermodynamic theory presented, the specific heat of glasses is not time dependent explicitly but depends on time via temperature only. Moreover, the theory predicts a new pulse propagation at very short times reflecting a wave-like energy transport due to the absorption and emission of phonons by fast relaxing tunneling states.
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