1. Department of Physics; CAS Key Laboratory of Strongly-coupled Quantum Matter Physics; Hefei National Laboratory for Physical Sciences at the Microscale; University of Science and Technology of China; Hefei 230026 P. R. China
2. Department of Electronic Engineering; Key Laboratory of Polar Materials and Devices; Ministry of Education; East China Normal University; Shanghai 200241 P. R. China
3. Collaborative Innovation Center of Advanced Microstructures; Nanjing 210093 P. R. China