1. Beijing National Laboratory for Condensed Matter Physics and CAS Key Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People's Republic of China
2. University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
3. Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China
4. Department of Physics and Astronomy, University of California, Irvine, California 92697, USA