1. Frankevich, E. L., and Balabanov, E. I., Soviet Phys. Solid State, 8, 682 (1966)
2. Delacote, G., and Schott, M., Solid State Commun., 4, 177 (1966).
3. Smith, R. A., Semiconductors, 124 (Cambridge Univ. Press, 1964).
4. Friedman, L., Phys. Rev., 133, A1668 (1964).
5. Knox, R. S., Solid State Phys., suppl. 5, 29 (1963).