1. National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
2. School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, China
3. College of Science, Northeastern University, 110004 Shenyang, China
4. Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA