1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
2. School of Physical Sciences, CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, China
3. Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, DK-2100 Copenhagen, Denmark
4. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China
5. School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, U.K.