1. Center for Carbon-based Electronics and Key Laboratory for the Physics and Chemistry of Nanodevices, School of Electronics, Peking University , Beijing 100871 , China
2. Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel , Kiel 24098 , Germany
3. Central Laboratory, Tianjin Key Laboratory of Epigenetics for Organ Development in Preterm Infants, the Fifth Central Hospital of Tianjin , Tianjin 300450 , China
4. Institute of Spin Science and Technology, South China University of Technology , Guangzhou 511442 , China