1. 1. For a brief review, see for example M. Schluter, Thin Solid Films 93, 3 (1982)
2. 2. or W. Monch, in Festkorperprobleme: Advances in Solid State. Physics, ed. P. Grosse (Vieweg, Braunschweig, 1986), Vol. 26, p67.
3. W. Schottky, Phys. Z. 41, 570 (1940).
4. J. Bardeen, Phys. Rev. 71, 717 (1947).
5. V. Heine, Phys. Rev. A138, 1689 (1965).