1. J. M. Ziman:Electrons and Phonons, Chap.5 (Oxford, 1963).
2. R. C. Fivaz:Nuovo Cimento,63 B, 10 (1969).
3. G. Ottaviani, C. Canali, F. Nava, Ph. Schmid, E. Mooser, R. Minder andI. Zschokke: to be published inSolid State Commun.
4. J. Bardeen andW. Shockley:Phys. Rev.,80, 72 (1950).
5. F. Seitz:Phys. Rev.,73, 549 (1948).