1. School of Physics, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China
2. State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Integrate Circuit Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
3. Shenzhen Institute for Advanced Study, University of Electronic Science and Technology of China, Shenzhen 518110, P. R. China