1. Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität u Kiel, D-24098 Kiel, Germany
2. School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, P. R. China
3. State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, P. R. China
4. RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan
5. Changzhou University and Jiangnan Graphene Research Institute, Changzhou 213000, P. R. China
6. Institut für Physik, Technische Universität Ilmenau, D-98693 Ilmenau, Germany