1. Xinjiang Key Laboratory of Solid State Physics and Devices, Xinjiang University, Urumqi Xinjiang 830046, P. R. China
2. School of Physics Science and Technology, Xinjiang University, Urumqi, Xinjiang 830046, P. R. China
3. Hunan Key Laboratory of Super Micro-structure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, P. R. China