1. Department of Physics and Key Laboratory for Low-Dimensional Quantum Structures and Manipulation (Ministry of Education), Hunan Normal University, Changsha 410081, China
2. Department of Science, Nanchang Teachers College, Nanchang 330029, China
3. School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
4. International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110015, China