1. W. Kohn, in Solid State Physics, edited by F. Seitz and D. Turnbull (Academic Press Inc., N.Y. 1957) Vol. 5, p. 258.
2. G. D. Watkins, in Radiation Effects in Semiconductors, edited by F. L. Vook, ( Plenum Press, N.Y., 1968 ), p. 67.
3. G. F. Köster and J. C. Slater, Phys. Rev. 95, 1167; ibid. 96, 1208 (1954).
4. J.Callaway and A. J. Hughes, Phys. Rev. 156, 860 (1967).
5. N. J. Parada, Phys. Rev. B3, 2042 (1971).