1. A. G. Danese, F. G. Curti, and R. A. Bartinski, Phys. Rev. B: Condens. Matter 70, 165420 (2004).
2. D. V. Buturovich, M. V. Kuz’min, M. V. Loginov, and M. A. Mittsev, Fiz. Tverd. Tela (St. Petersburg) 48(11), 2085 (2006) [Phys. Solid State 48 (11), 2205 (2006)].
3. M. V. Kuz’min, M. V. Loginov, and M. A. Mittsev, Fiz. Tverd. Tela (St. Petersburg) 50(2), 354 (2008) [Phys. Solid State 50 (2), 369 (2008)].
4. M. V. Kuz’min, M. V. Loginov, M. A. Mittsev, and T. V. Krachino, Fiz. Tverd. Tela (St. Petersburg) 37(4), 1030 (1995) [Phys. Solid State 37 (4), 559 (1995)].
5. T. V. Krachino, M. V. Kuz’min, M. V. Loginov, and M. A. Mittsev, Fiz. Tverd. Tela (St. Petersburg) 39(2), 256 (1997) [Phys. Solid State 39 (2), 224 (1997)].