1. Institute of Modern Physics, Northwest University, Xi'an, Shaanxi 710069, China
2. College of Physics, Northwest University, Xi'an, Shaanxi 710069, China
3. Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Northwest University, Xi'an, Shaanxi 710069, People's Republic of China