1. Key laboratory of Micro‐nano MeasurementManipulation and Physics (Ministry of Education)School of PhysicsBeihang University Beijing 100191 China
2. Beijing Institute of Nanoenergy and NanosystemsChinese Academy of Sciences Beijing 100190 China
3. School of Materials Science and EngineeringTsinghua University Beijing 100084 China
4. Beijing National Laboratory for Condensed Matter PhysicsInstitute of PhysicsChinese Academy of Sciences Beijing 100190 China