1. R. J. Elliot and R. Loudon, J. Phys. (France) 15, 196 (1960).
2. G. V. Godiak, Yu. E. Lozovik, and M. S. Obrekht, Fiz. Tverd. Tela (Leningrad) 25, 1063 (1983) [Sov. Phys. Solid State 25, 613 (1983)].
3. Y. Kuramoto and M. Morimoto, J. Phys. Soc. Jpn. 44, 1759 (1978).
4. L. V. Keldysh and T. A. Onishchenko, JETP Lett. 24, 59 (1976).
5. S. T. Chui, Phys. Rev. B 9, 3438 (1974).