1. Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
2. Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
3. Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
4. Department of Physics, Nanjing University, Nanjing 210008, China
5. Department of Physics, Saitama University, Saitama 338-8570, Japan
6. Department of Physics, Niigata University, Niigata 950-2181, Japan
7. TRIUMF, 4004 Wesbrook Mal, Vancouver, British Columbia V6T 2A3, Canada
8. Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351-0198, Japan
9. Institute of Physics, University of Tsukuba, Ibaraki 305-8571, Japan