1. State
Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, People’s Republic of China
2. Key
Laboratory for the Physics and Chemistry of Nanodevices and Department
of Electronics, Peking University, Beijing 100871, People’s Republic of China
3. Division
of Solid State Physics, Lund University, S-22100 Lund, Sweden
4. Department
of Physics, Tsinghua University, Beijing 100084, People’s Republic of China