1. A. E. Usachev, Yu. V. Yablokov, L. A. Pozdnyakova, and K. S. Aleksandrov, Fiz. Tverd. Tela (Leningrad) 19, 2156 (1977) [Sov. Phys. Solid State 19, 1263 (1977)].
2. A. E. Usachev, Yu. V. Yablokov, and S. G. L’vov, Fiz. Tverd. Tela (Leningrad) 23, 1439 (1981) [Sov. Phys. Solid State 23, 838 (1981)].
3. M. V. Chernitskii, V. A. Vazhenin, A. E. Nikiforov, A. E. Usachev, A. I. Krotkii, and M. Yu. Artemov, Fiz. Tverd. Tela (Leningrad) 33, 3577 (1991) [Sov. Phys. Solid State 33, 2011 (1991)].
4. V. A. Vazhenin, K. M. Starichenko, M. Yu. Artemov, and M. V. Chernitskii, Fiz. Tverd. Tela (Leningrad) 34, 1633 (1992) [Sov. Phys. Solid State 34, 866 (1992)].
5. V. A. Vazhenin, K. M. Starichenko, M. Yu. Artemov, and A. E. Nikiforov, Fiz. Tverd. Tela (St. Petersburg) 36, 2695 (1994) [Phys. Solid State 36, 1470 (1994)].