1. Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics, University of Science and Technology of China, Hefei 230026, People's Republic of China
2. Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, USA
3. Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Science, Hefei 230026, People's Republic of China
4. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China