1. College of Electrical and Information Engineering, Hunan University, Changsha, China
2. School of Electronics, Key Laboratory for the Physics and Chemistry of Nanodevices, Peking University, Beijing, China
3. Information Technology Institute, Peking University, Tianjin, China
4. Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center, State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing, China