1. N. D. Gavrilova, E. G. Maksimov, V. K. Novik, and S. I. Drozhdin, Fiz. Tverd. Tela (Leningrad) 27(9), 2597 (1985) [Sov. Phys. Solid State 27, 1559 (1985)].
2. I. K. Novik and N. D. Gavrilova, Fiz. Tverd. Tela (St. Petersburg) 42(6), 961 (2000) [Phys. Solid State 42, 991 (2000)].
3. G. Heiland and H. Ibach, Solid State Commun. 4(3), 353 (1966).
4. I. Born and E. Huang, Dinamical Theory of Crystal Lattices (Clarendon, Oxford, 1954).
5. J. Mangin and A. Hadni, Phys. Rev. B: Condens. Matter Mater. Phys. 27, 7730 (1983).