1. Department of Physics and Astronomy, University of California, Irvine, California 92697-4575, United States
2. College of Physics and Electrical Engineering, Henan Normal University, Xinxiang, Henan 453007, China
3. College of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, China
4. Department of Chemistry, University of California, Irvine, California 92697-2025, United States