1. Allen, P. B., Phys. Rev. B
3, 305 (1971).
2. Argyres, P. N., and J. L. Sigel, Phys. Rev. B
9, 3197 (1974).
3. Bass, J., et al., Phys. Rev. Lett.
56, 957 (1986).
4. Bass, J., Landolt-Bornstein, Neue Serie, Band 15a ( Springer Verlag, New York, 1982 ).
5. Beni, G., and C. F. Coll, Phys. Rev. B
11, 573 (1974).