1. Laboratory for Nanodevices, National Center for Nanoscience and Technology, Beijing
100190, China
2. CAS Key Laboratory of Standardization and Measurement for Nanotechnology, Beijing 100190, China
3. Department of Physics, Beijing University of Aeronautics and Astronautics,
Beijing 100191, China
4. Beijing
National Laboratory
of Condensed Matter, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China