1. State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China
2. College of Science, Civil Aviation University of China, Tianjin 300300, China
3. Department of Physics, Umeå University, SE-90187 Umeå, Sweden
4. Institute for Solid State Physics and Optics Wigner Research Centre for Physics, P.O. Box 49, H-1525 Budapest, Hungary
5. Faculty of Light Industry and Environmental Engineering, Óbuda University, Doberdóút 6, H-1034 Budapest, Hungary