1. 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. Songshan Lake Materials Laboratory Dongguan Guangdong 523808 China
4. Institute of Nanotechnology Karlsruhe Institute of Technology (KIT) D‐76344 Eggenstein‐Leopoldshafen Germany
5. IPCMS‐CNRS Université de Strasbourg Strasbourg F‐67034 France
6. Department of Physics, Chemistry and Biology (IFM) Linköping University Linköping SE‐58183 Sweden
7. Physics Department E20 Technical University of Munich D‐85748 Garching Germany