1. J. F. Ncye: Physical Properties of Crystals (Clarendon Press, Oxford, 1964), Chapter 13.
2. G. B. Benedek, K. Fpritsch: Phys. Rev. 149, 647 (1966).
3. M. Born, K. Hcuang: Dynamical Theory of Crystal Lattices (Clarendon Press, Oxford, 1954), Section 50.
4. L. D. Landau, E. M. Lifshitz: Electrodynamics of Continuous Media (Pergamon Press, New York, 1958), Chapter 14.
5. D. F. Nelson, P. D. Lazay: Phys. Rev. Letters 25, 1187 (1970).