1. Department of Physics; Southern University of Science and Technology; 1088 Xueyuan Blvd. Shenzhen Guangdong 518055 P. R. China
2. Key Laboratory of Materials Physics; Institute of Solid State Physics; Hefei Institutes of Physical Science; Chinese Academy of Sciences; Hefei Anhui 230031 P. R. China
3. Department of Physics; Harbin Institute of Technology; 92 West Da-Zhi St. Harbin Heilongjiang 150001 P. R. China
4. Materials Characterization and Preparation Center; Southern University of Science and Technology; 1088 Xueyuan Blvd. Shenzhen Guangdong 518055 P. R. China