1. G. Bauer, Springer Tracts in Modem Physics, Vol. 74 (Springer-Verlag, New York, 1974); E. M. Conwell, in Solid State Physics, Suppl. 9, F. Seitz, D. Turnbull, and H. Ehrenreich, eds. (Academic Press, New York, 1967).
2. A. Matulionis, J. Pozela, and A. Reklaitis, Phys. Stat. Sol. (a) 31, 83 (1975); 35, 43 (1976).
3. M. Brauer, Phys. Stat. Sol. (b) 81, 147 (1977).
4. J. T. Devreese, ed., Polarons in Ionic Crystals and Polar Semiconductors (North-Holland, Amsterdam, 1972).
5. W. Shockley, Bell. Syst. Tech. J. 30, 990 (1951).