1. Department of Physics, Shanghai University, 99 Shangda Road, 200444 Shanghai, P. R. China
2. State Key Laboratory of Advanced Special Steel & School of Materials Science and Engineering, Shanghai University, 99 Shangda Road, 200444 Shanghai, P. R. China
3. Department of Physics, Humboldt-Universität zu Berlin, 12489 Berlin, Germany
4. Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee 22, 14195 Berlin, Germany