1. M.P. Schulhof, P. Heller, R. Nathans, A. Linz, Phys. Rev. B1, 2403 (1970)
2. E.M. Brody, H.Z. Cummins, Phys. Rev. Lett. 21, 1263 (1968)
3. E.M. Brody, H.Z. Cummins, Phys. Rev. 9, 179 (1974)
4. M. Born, K. Huang, Dynamical Theory of Crystal Lattices. (Oxford Univ. Press, 1968), p. 204
5. J.C. Toledano, E. Errandonea, J.P. Jaguin, Solid State Comm. 20, 905 (1976)