1. Beijing National Laboratory for Condensed Matter, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
2. Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
3. Faculty for Physics, Ludwig-Maximilians University of Munich (LMU), Am Coulombwall 1, D-85748 Garching, Germany
4. Research Center for Exotic Nanocarbons, Shinshu University, Nagano City 380-8553, Japan
5. Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China