1. G. R. Asatryan and A. A. Mirzakhanyan, Sov. Phys. Solid State 33 (6), 1069 (1991).
2. G. R. Asatryan, G. S. Shakurov, and A. G. Petrosyan, in Proceedings of the 19th All-Russia Conference on Optics and Spectroscopy of Condensed Matter, Krasnodar, 2013, p. 54.
3. G. I. Vetrogon, V. I. Danilenko, V. Ya. Kabanchenko, V. V. Osiko, A. M. Prokhorov, A. N. Terent’evskii, and M. I. Timoshechkin, Sov. Phys. Solid State 22 (11), 1881 (1980).
4. Kh. S. Bagdasarov, Yu. N. Dubrov, I. N. Marov, V. O. Martirosyan, and M. L. Meilman, Phys. Status Solidi B 56, K65 (1973).
5. R. W. Kedzie, D. H. Lyons, and M. Kestigian, Phys. Rev. [Sect.] A 138, A918 (1965).