1. Anastassakis, E.: In: Dynamical properties of solids. Horton, G.K., Maradudin, A.A. (eds.), Vol. 4, p. 157. Amsterdam: North-Holland 1980
2. Thalmeier, P., Fulde, P.: Z. Phys. B-Condensed Matter and Quanta26, 323 (1977)
3. L�thi, B.: In: Dynamical properties of solids. Horton, G.K., Maradudin, A.A. (eds.), Vol. 3, p. 245, Amsterdam: North-Holland 1980
4. Schaack, G.: Solid State Common.17, 505 (1975)
5. Ahrens, K., Schaack, G.: Phys. Rev. Lett.42, 1488 (1979)