1. Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, P. R. China
2. College of Physics, Jilin Normal University, Siping 136000, China
3. National Demonstration Center for Experimental Physics Education, Jilin Normal University, Siping 136000, China
4. State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China