1. Shanghai
Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China
2. University of Chinese Academy of Sciences, Beijing 100049, P. R. China
3. Department
of Electronic Engineering, Tsinghua University, Beijing 100084, P. R. China
4. School
of Physics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
5. Key
Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Jialuo Road 2019, Jiading District, Shanghai 201800, P. R. China