1. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China
2. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P.R. China
3. Physics Department E20, Technical University of Munich, Garching 85748, Germany
4. School of Physics, Beijing Institute of Technology, Beijing 100081, P.R. China
5. Advanced Research Institute for Multidisciplinary Science, Beijing Institute of Technology, Beijing 100081, P.R. China
6. CAS Key Laboratory of Vacuum Physics, Chinese Academy of Sciences, Beijing 100190, P.R. China
7. Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0QX, United Kingdom
8. Department of Physics & IRIS Adlershof, Humboldt-Universität zu Berlin, Newtonstrasse 15, Berlin 12489, Germany