1. See for instance:Bassani; F., Greenaway, D. L., and Fisher, G., in Physics of Semiconductors:Proceedings of the 7th International Conference, edited by M. Hulin (Academic Press, New York, 1964).
2. Aulich, E.; Brebner, J. L.; Mooser, E.: Phys. Status Solidii, 31 29 (1969).
3. Besson, J.M.; Jain, K.P. and Kuhn, A.: Phys. Rev. Letters, 32, 936 (1974).
4. Balzarotti, A. and Piacentini, M.: Solid State Commun., 10, 421, (1972).
5. Khelladi, F. Z.: Ph. D. thesis, Université de Paris VI, unpublished (1 961).