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3. Maris, H.J.: Nonequilibrium phonons in nonmetallic crystals. Eisenmenger, W., Kaplyanskii, A.A. (eds.), p. 51. Amsterdam: North-Holland 1986
4. Hensel, J.C., Dynes, R.C.: Phys. Rev. Lett.39, 969 (1977)
5. Greenstein, M., Tamor, M.A., Wolfe, J.P.: Phys. Rev. B26, 5604 (1982)