1. Institute of Super-Microstructure and Ultrafast Process in Advanced Materials, College of Physics and Electronics, Central South University, Changsha 410083, People's Republic of China
2. Department of Mechanical and Materials Engineering and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0656, USA
3. Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA