1. V. L. Gilyarov, A. I. Slutsker, V. P. Volodin, and L. A. Laius, Fiz. Tverd. Tela (St. Petersburg) 39, 153 (1997) [Phys. Solid State 39, 132 (1997)].
2. A. I. Slutsker, V. L. Gilyarov, and A. I. Luk’yanenko, Fiz. Tverd. Tela (St. Petersburg) 48, 1832 (2006) [Phys. Solid State 48, 1947 (2006)].
3. G. Alvarez, C. J. Howls, and H. J. Silverstone, J. Phys. A 35, 4017 (2002).
4. P. Amore, A. Aranda, and A. De Pace, J. Phys. A 37, 3515 (2004).
5. S. Dusuel and G. S. Uhrig, J. Phys. A 37, 9275 (2004).