1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
2. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China
3. Department of Physics, University of Washington, Seattle, Washington 98195, USA
4. Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
5. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China