1. College of Physics/State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, P. R. China
2. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3. Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), D-76128 Karlsruhe, Germany
4. College of Physics, Beihua University, Jilin, P. R. China