1. Department of Physics, Clark University, Worcester, Massachusetts 01610, USA
2. Department of Physics, Anhui Normal University, Wuhu, Anhui 241000, China
3. Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
4. Hefei National Laboratory, University of Science and Technology of China, Hefei, Anhui 230088, China
5. Department of Physics, University of Alberta, Edmonton, Alberta, Canada, T6G 2H5