Condensed Matter Physics, 2012, vol. 15, No. 4, p. 43007:1-10
DOI:10.5488/CMP.15.43007           arXiv:1212.6140

Title: Hydrodynamic states of phonons in insulators
  S.A. Sokolovsky (Prydniprovs'ka State Academy of Civil Engineering and Architecture)

The Chapman-Enskog method is generalized for accounting the effect of kinetic modes on hydrodynamic evolution. Hydrodynamic states of phonon system of insulators have been studied in a small drift velocity approximation. For simplicity, the investigation was carried out for crystals of the cubic class symmetry. It has been found that in phonon hydrodynamics, local equilibrium is violated even in the approximation linear in velocity. This is due to the absence of phonon momentum conservation law that leads to a drift velocity relaxation. Phonon hydrodynamic equations which take dissipative processes into account have been obtained. The results were compared with the standard theory based on the local equilibrium validity. Integral equations have been obtained for calculating the objects of the theory (including viscosity and heat conductivity). It has been shown that in low temperature limit, these equations are solvable by iterations. Steady states of the system have been considered and an expression for steady state heat conductivity has been obtained. It coincides with the famous result by Akhiezer in the leading low temperature approximation. It has been established that temperature distribution in the steady state of insulator satisfies a condition of heat source absence.

Key words: phonons of insulator, Umklapp processes, relaxation degrees of freedom, local equilibrium, the Chapman-Enskog method, phonon hydrodynamics, small drift velocity, low temperatures, steady states
PACS: 05.20.Dd, 51.10.+y, 63.20.-e, 63.20.Kr, 66.90.+r, 78.66.Nk.

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