1. a Max-Planck-Institut fur Festkörperforschung , Hochfeld-Magnetlabor 166X, 38042 , Grenoble Cedex , France
2. b International Centre for Theoretical Physics , Trieste , Italy
3. e Institute of Physics , Polish Academy of Sciences , Al. Lotnikow 32/46, 02-668 , Warsaw , Poland
4. c Brooklyn College , City University of New York , Brooklyn , New York , 11210 , U.S.A.
5. d Physics Department , Brown University , Providence , Rhode Island , 02912 , U.S.A.
6. f Dupont Experimental Station , Wilmington , Delaware , 19898 , U.S.A.