1. Institute of Super Microstructure and Ultrafast Process in Advanced Materials, College of Physics and Electronics, Central South University, 605 South Lushan Road, Changsha 410012, People's Republic of China
2. Hunan Key Laboratory for Super Microstructure and Ultrafast Process, Central South University, 932 South Lushan Road, Changsha 410012, People's Republic of China
3. Department of Physics and Astronomy, University of Rochester, Rochester, New York 14534, USA