1. Institute for Theoretical Solid State Physics, IFW-Dresden, 01069 Dresden, Germany
2. Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, U.S.A.
3. Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, China
4. Institute of Theoretical Physics, Chinese Academy of Sciences, P. O. Box 2735, Beijing 100190, China