1. R. M. Martin and L. M. Falicov, in: Light Scattering in Solids, Ed. by M. Cardona, 1979: S. Permogorov, Phys. Stat. Sol. (b),68, 9, 1975
2. S. Permogorov, E. Gross, V. Travnikov and J. Selkin, J. Phys. Chem. Solids,31, 2595, 1970
3. R. Planel, A. Bonnot and C. Benoit à la Guillaume, Phys. Stat. Sol. (b)58, 251, 1973
4. E. L. Ivchenko, J. G. Lang and S. T. Pavlov, Fiz. Tver. Tela19, 1227, 1751, 1977
5. G. C. Trallero and O. Sotolongo Costa, Phys. Stat. Sol. (b),127, 121, 1985.