1. Beijing National Laboratory for Condensed Matter Physics; Institute of Physics; Chinese Academy of Sciences; P. O. Box 603-146 Beijing 100190 China
2. Present address: Leibniz Institute of Photonic Technology; Albert-Einstein-Str. 9, 07745 Jena, Germany and Physical Chemistry, Friedrich-Schiller University Jena; Helmholtzweg 07743 Jena Germany
3. Present address: Department of Electrical and Computer Engineering; University of California; San Diego 9500 Gilman Drive La Jolla, California 92093 USA
4. Department of Physics; Liaoning University; Shenyang 110036 China