1. D. S. Gemmell, Rev. Mod. Phys.,46, 129 (1974).
2. A. S. Borovik and E. T. Shipatov, Dep. VENITI No. 2009-76.
3. G. A. Belikov et al., in: Proc. Sixth All-Union Conference on the Physics of the Interaction of Charged Particles with Single Crystals [in Russian], Mosk. Gos. Univ. (1975), p. 385.
4. K. Ozawa et al., Phys. Status Solidi,9a, 323 (1972).
5. M. J. Hollis, Phys. Rev.,B8, 931 (1973).