1. Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of Ministry of Education, College of Physics and Information Science, Hunan Normal University, Changsha 410081, People's Republic of China
2. School of Physics and Microelectronics Science, Hunan University, Changsha 410082, People's Republic of China
3. School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081, People's Republic of China