1. Thomas, D.G. and Hopfield, J.J., Phys. Rev. 1961, vol. 124, p. 657.
2. Gross, E.F., Zakharchenya, B.P., and Konstantinov, O.V., Fiz. Tverd. Tela, 1961, vol. 3, p. 305.
3. Kochereshko, V.P., Mikhailov, G.V., and Ural’tsev, I.N., Fiz. Tverd. Tela, 1983, vol. 25, p. 769 (Sov. Phys. Solid State, 1983, vol. 25, p. 439)
4. Cho, K., Suga, S., Dreybrodt, W., and Willmann, F., Phys. Rev. B, 1975, vol. 11, p. 1512.
5. Suga, S., Cho, K., and Bettini, M., Phys. Rev. B, 1976, vol. 13, p. 943.