1. V. P. Zhuze and I. A. Smirnova, Rare-Earth Semiconductors (Nauka, Leningrad, 1977).
2. A. Rogalev, J. Goulon, and Ch. Brouder, J. Phys.: Condens. Matter 11, 1115 (1999).
3. A. N. Gusatinskii, G. I. Alperovich, and A. V. Soldatov, Phys. Status Solidi B 112, 599 (1982).
4. A. V. Soldatov and A. N. Gusatinskii, Phys. Status Solidi B 125, K129 (1984).
5. A. V. Soldatov, G. I. Al’perovich, and A. N. Gusatinskii, Fiz. Tverd. Tela (Leningrad) 27, 1237 (1985) [Sov. Phys. Solid State 27, 747 (1985)].