1. Department of Physics, The Ohio State University, Columbus, OH 43210, USA.
2. Department of Physics, Beijing Jiaotong University, Beijing 100044, China.
3. Department of Physics, Wright State University, Dayton, OH 45435, USA.
4. Department of Physics, Tongji University, Shanghai 200092, China.
5. Department of Physics, University of Maryland, Baltimore County, Baltimore, MD 21250, USA.
6. Department of Physics, Beijing Normal University, Beijing 100875, China.
7. Department of Physics, Tsinghua University, Beijing 100084, China.