1. Department of Physics, Shaoyang University, Shaoyang 422001, China
2. Department of Physics and Key Laboratory for Low-Dimensional Structures and Quantum Manipulation (Ministry of Education), Hunan Normal University, Changsha 410081, China
3. Hunan Key Laboratory for Micro-Nano Energy Materials and Device and Department of Physics, Xiangtan University, Xiangtan 411105, China