1. State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China
2. College of Physics, Jilin University, Changchun 130012, China
3. State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Basic Research Center of Excellence for Functional Molecular Engineering, Nanotechnology Research Center, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China
4. Department of Physics, Umeå University, SE-90187 Umeå, Sweden