1. Hunan Key Laboratory of Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, P.R. China
2. Shanghai Synchrotron Radiation Facility, Shanghai
Institute of Applied Physics, Chinese Academy of Sciences, 239 Zhangheng Road, Pudong New
Area, Shanghai 201204, P.R. China
3. Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, United States