1. School of Physics and Electronics, Central South University, 932 South Lushan Road, Changsha 410083, China
2. Advanced Device Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
3. Center for Emergent Matter Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
4. Department of Electrical and Computer Engineering, Institute for Applied Life Sciences, University of Massachusetts, Amherst, Massachusetts 01003, USA