1. See the discussion in C. Kittel: Introduction to Solid State Physics, Fifth Ed. (Wiley, New York 1976) p. 191
2. See the theoretical discussion given by J.D. Dow, S.Y. Ren, K. Hess: Phys. Rev. B25, 6218 (1982)
3. D.L. Mills, E. Burstein: Rep. Prog. Phys. 37, 817 (1974)
4. V.M. Agranovich, D.L. Mills (eds.): Surface Polaritons, (North-Holland, Amsterdam 1982)
5. R.E. Camley, D.L. Mills: Phys. Rev. B29,1695 (1984)