1. Department
of Physics, Beihang University, Beijing 100191, P. R. China
2. Beijing
Key Laboratory for Magneto-Photoelectrical Composite and Interface
Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, P. R. China
3. School of Physics & CRANN, Trinity College Dublin, Dublin, Ireland
4. Department
of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China