1. E. L. Nagaev, Pis’ma Zh. Éksp. Teor. Fiz. 6, 484 (1968) [JETP Lett. 6, 18 (1967)].
2. E. L. Nagaev, Zh. Éksp. Teor. Fiz. 54, 228 (1968) [Sov. Phys. JETP 27, 122 (1968)].
3. N. Mott and Z. Zinamon, Rep. Prog. Phys. 33, 81 (1970).
4. N. F. Mott and E. A. Davis, Electronic Processes in Non-Crystalline Materials (Clarendon, Oxford, 1971; Mir, Moscow, 1974).
5. E. L. Nagaev, J. Magn. Magn. Mater. 110, 39 (1992).