1. S. L. McCall and E. L. Hahn, Phys. Rev. Lett. 18, 908 (1967).
2. N. S. Shiren, Phys. Rev. B 2, 2471 (1970).
3. V. V. Samartsev, B. P. Smolyakov, and R. Z. Sharipov, Pis’ma Zh. Éksp. Teor. Fiz. 20, 644 (1974) [JETP Lett. 20, 296 (1974)].
4. G. A. Denisenko, Zh. Éksp. Teor. Fiz. 60, 2269 (1971) [Sov. Phys. JETP 33, 1220 (1971)].
5. O. Anderson, Physical Acoustics: Principles and Methods, Vol. 3, Part B: Lattice Dynamics, Ed. by W. P. Mason (Academic, New York, 1965; Mir, Moscow, 1968), p. 62.