1. T. R. Koehler, inLattice Dynamics, A. A. Maradudin and G. K. Horton, eds. (North-Holland, Amsterdam, 1974; H. Horner, inLattice Dynamics, A. A. Maradudin and G. K. Horton, eds. (North-Holland, Amsterdam, 1974); H. R. Glyde inRare Gas Solids, M. K. Klein and J. A. Venables, eds. (Academic Press, New York, in press); and other papers cited in these reviews.
2. L. H. Nosanow,Phys. Rev. 146, 120 (1966).
3. J. H. Hetherington, W. J. Mullin, and L. H. Nosanow,Phys. Rev. 154, 175 (1967).
4. F. G. Mertens,Z. Physik 250, 1 (1972).
5. J. A. Krumhansl and S. Y. Wu,Phys. Lett. 28A, 263 (1968).