1. See e.g. M. Reed in:Semiconductors and Semimetals eds. R.K. Willardson and A.C. Beer (Academic, New York, 1992) Vol. 35.
2. A.P. Volodin, and V.S. Edel’man, JETP Lett. 37 (1983) 7.
3. D. Marty, J. Phys. C: Solid State Phys. 19 (1986) 6097.
4. Yu. Z. Kovdrya and V. Nikolaenko, Sov. J. Low Temp. Phys 18 (1992) 894.
5. P.K.H. Sommerfeld and R.W. van der Heijden, J. Phys.: Condens. Matter 7 (1995) 9731.