1. State Key Laboratory of Precision Measurement and Technology Department of Precision Instrument Tsinghua University Beijing 100084 China
2. Department of Electrical and Computer Engineering Duke University Durham NC 27708 USA
3. Department of Physics Duke University Durham NC 27708 USA
4. Beijing National Laboratory for Condensed Matter Physics Institute of Physics, Chinese Academy of Sciences Beijing 100190 China
5. School of Physics and Technology Wuhan University Wuhan 430072 China