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. Institute for Solid State Physics and Optics; Wigner Research Centre for Physics; Hungarian Academy of Sciences; P.O. Box 49 H-1525 Budapest Hungary
4. Faculty of Light Industry and Environmental Engineering; Óbuda University; Doberdó út 6 H-1034 Budapest Hungary
5. Department of Physics; Umeå University; S-901 87 Umeå Sweden