1. Department of Electrical and Computer Engineering, University of California San Diego, La Jolla, California 92093, United States
2. Hubei Yangtze Memory Laboratories, Wuhan, Hubei 430205, P. R. China
3. School of Physics and Microelectronics, Key Laboratory of Material Physics Ministry of Education Zhengzhou University, Zhengzhou, Henan 450052, P. R. China
4. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom Kowloon, Hong Kong 999077, P. R. China
5. Macao Institute of Materials Science and Engineering (MIMSE), MUST-SUDA Joint Research Center for Advanced Functional Materials, Macau University of Science and Technology, Taipa 999078, Macau, China