1. Department of Physics and Astronomy, University of Texas, San Antonio, Texas 78249, USA
2. Department of Physics, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212003, China
3. North Carolina State University, Raleigh, North Carolina 27695-8204, USA
4. Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, Dalian University of Technology, Dalian 116024, China