1. M. Born and K. Huang, Dynamical Theory of Crystal Lattices (New York, 1954).
2. R. J. Birgeneau, J. Cordes, G. Dolling, and A. D. B. Woods, Phys. Rev. A 136, 1359 (1964).
3. G. L. Squires, Ark. Fys. 25, 21 (1963).
4. J. de Klerk, Proc. Phys. Soc. 73, 337 (1959).
5. G. A. Alers, R. Neighbours, and H. Sato, Bull. Am. Phys. Soc. 4, 131 (1959).