1. School of Physics and Technology, Wuhan University, Wuhan 430072, China
2. Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China
3. Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China
4. The Key Laboratory of Artificial Micro- and Nano-Structures, Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, China
5. State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China