1. School of Physics, Dalian University of Technology, Dalian 116024, People's Republic of China
2. School of Science, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China
3. Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, People's Republic of China
4. Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, People's Republic of China