1. Southwestern Institute of Physics, Post Office Box 432, Chengdu 610041, China
2. Department of Engineering physics, Tsinghua University, Beijing 100084, China
3. CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France
4. KTX Laboratory and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China