1. Hefei National Laboratory for Physical Sciences at the Microscale; University of Science and Technology of China; Hefei Anhui 230026 P. R. China
2. School of Physics; Nankai University; Tianjin 300071 P. R. China
3. Department of Physics and Astronomy; University of Utah; Salt Lake City Utah 84112 USA
4. Department of Modern Mechanics; University of Science and Technology of China; Hefei Anhui 230026 P. R. China