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