1. H. Y. Fan, “Infrared absorption by semiconductors,” Usp. Fiz. Nauk 64:316 (1958).
2. H. Rank, Phys. Rev. 90:202 (1953).
3. H. Briggs, Phys. Rev. 78:287 (1958).
4. N. B. Hannay, Semiconductors [Russian translation], IL, 1962, Chapt. X. 209
5. N. F. Mott and R. W. Gurney, Electronic Processes in Ionic Crystals [Russian translation], IL, 1950, p. 185.