1. Key Laboratory of Advanced Materials (MOE) and Beijing National Center for Electron Microscopy School of Materials Science and Engineering Tsinghua University Beijing 100084 China
2. Department of Physics and the Quantum Theory Project University of Florida Gainesville FL 32611 USA
3. Institute of Semiconductors Chinese Academy of Sciences Beijing 100084 China
4. Physics Department The Hong Kong University of Science and Technology Clear Water Bay Hong Kong 999077 China