1. See, for example,a)H. J. Zeiger andG. W. Pratt:Magnetic Interaction in Solids, (Clarendon Press, Oxford, 1973) Chapt. 6;b)F. Bassani andG. P. Parravicini:Electronic States and Optical Transition in Solids (Pergamon Press, Oxford, 1975), Chapt. 5;c)O. Madelung:Introduction to Solid State Theory (Springer-Verlag, Berlin, 1978), Chapt. 6.
2. J. Bardeen, F. J. Blatt andL. H. Hall: inProceeding of the Conference on Photoconductivity (John Wiley and Sons, New York, N. Y., 1956), p. 146.
3. L. M. Roth, B. Lax andS. Zwerdling:Phys. Rev.,114, 90 (1959).
4. T. Ohta, M. Nagae andT. Miyakawa:Progr. Theor. Phy.,23, 229 (1960).
5. J. Halpern andB. Lax:J. Phys. Chem. Solids,26, 911 (1965).