1. E. V. Korovkin and T. A. Lebedkina, Sov. Phys. Solid State 33 (10), 1595 (1991).
2. E. V. Korovkin and T. A. Lebedkina, Phys. Solid State 35 (3), 329 (1993).
3. E. V. Korovkin and T. A. Lebedkina, Phys. Solid State 37 (11), 1945 (1995).
4. E. V. Korovkin and T. A. Lebedkina, Phys. Solid State 42 (8), 1451 (2000).
5. S. I. Pekar, Zh. Eksp. Teor. Fiz. 16, 341 (1946)