1. For a review see: Amorphous solids: low temperature properties. In: Springer Series of Topids in Current Physics, Phillips, W.A., (ed.), Vol. 24. Berlin, Heidelberg, New York: Springer 1981
2. Anderson, P.W., Halperin, B.I., Varma, C.M.: Philos. Mag.25, 1 (1972); Phillips, W.A.: J. Low Temp. Phys.7, 351 (1972)
3. For a recent view see: Laermans, C.: In: Structure and bonding in non-crystalline solids, p. 325. Walrafen, G., Revesz, A. (eds.) New York: Plenum Press 1986
4. Laermans, C., Esteves, V., Vanelstraete, A.: Radiat. Eff.97, 175 (1986)
5. Raychaudhuri, A.K., Hunklinger, S.: Z. Phys. B ? Condensed Matter57, 113 (1984)