1. Key Laboratory for the Physics and Chemistry of NanodevicesDepartment of ElectronicsPeking University Beijing 100871 P. R. China
2. Academy for Advanced Interdisciplinary StudiesPeking University Beijing 100871 P. R. China
3. State Key Laboratory of Low‐Dimensional Quantum PhysicsDepartment of Physics and Tsinghua‐Foxconn Nanotechnology Research CenterTsinghua University Beijing 100084 China