1. Department of Physics, Nanjing Normal University, Nanjing 210023, China
2. National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China
3. Center for Soft Condensed Matter Physics and Interdisciplinary Research, College of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China