1. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
2. State Key Laboratory of Precision Measurement Technology and Instrument, Department of Precision Instrument, Tsinghua University, Beijing 100084, China
3. Laboratoire de Physique du Solide, Namur Institute of Structured Matter, University of Namur, Rue de Bruxelles 61, 5000 Namur, Belgium