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3. Steiner, P., Höchst, H., Hüfner, S.: Z. Physik B30, 129 (1978); see also for further references
4. Kowalczyk, S.P.: Thesis, Berkeley 1976
5. Shirley, D.A.: Top. Appl. Phys.26, 166. Berlin, Heidelberg, NewYork: Springer 1978; see also for further references.