1. For a good introduction see, e.g. S. Andersson, in Surface Science, IAEA, Vienna (1975) Vol. 1, p. 77.
2. F. Jona, J. Phys. C11, 4271 (1978).
3. E. Tosatti, in Physics of Semiconductors XIII, ed. by F.G. Fumi, Veschi — North Holland, Rome (1976), p. 21.
4. T.E. Felter, R.A. Barker and P.J. Estrup, Phys. Rev. Letters 38, 1138 (1977); R.A. Barker, P.J. Estrup, F. Jona and P.M. Marcus, Solid State Comm. 25, 375 (1978).
5. M.K. Debe and D.A. King, J. Phys. C10, L303 (1977);