1. Department of Mathematics and Physics, Haifa University at Oranim, 36006 Tivon, Israel
2. Department of Chemistry, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel
3. Max-Planck Institut für Festkörperforschung, Hochfeld-Magnetlabor, B.P. 166X, F-38042, Grenoble Cedex 9, France