1. Department of Physics, Liaoning University, Shenyang 110036, P. R. China
2. Beijing National Laboratory
for Condensed Matter Physics, Institute of Physics, Chinese Academy of Science, P.O. Box 603-146, Beijing, 100190,
P. R. China
3. College of Science, Northeast Forestry University, Harbin 150040, P. R.
China
4. Department of Chemical
Physics, Lund University, Box 124, Lund
22100, Sweden