1. Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, United States
2. Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan
3. Department of Physics, Fudan University, Shanghai 200433, China
4. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
5. ICQD/HFNL, University of Science and Technology of China, Hefei, Anhui, 230026, China