1. School of Physics, Southeast University, Nanjing 211189, China
2. Department of Physics, Nanjing University, Nanjing 210093, China
3. Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Land Strasse 400, D-01328 Dresden, Germany
4. National Laboratory of Solid-State Microstructures, Nanjing 210008, China