1. School of Physics and Optoelectronics, Hunan Key Laboratory for Micro-Nano Energy Materials and Devices, Xiangtan University 1 , Hunan 411105, China
2. Key Laboratory for Matter Microstructure and Function of Hunan Province, Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of Ministry of Education, School of Physics and Electronics, Hunan Normal University 2 , Changsha 410081, China