1. State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, P. R. China
2. Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong, P. R. China
3. Analysis and Testing Center, Soochow University, Suzhou 215123, P. R. China
4. College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, P. R. China
5. Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249, United States
6. Division of Quantum Phases & Devices, Department of Physics, Konkuk University, Seoul 143-701, Korea
7. Department of Materials Design and Innovation, University at Buffalo − The State University of New York, Buffalo, New York 14260, United States