1. College of Physics and Electronic Engineering; Northwest Normal University; Lanzhou 730070 P. R. China
2. Wuhan National High Magnetic Field Center and School of Physics; Huazhong University of Science and Technology; Wuhan 430074 P. R. China
3. Centre for Applied Chemical Research; Frontier Institute of Science and Technology; Xi'an Jiaotong University; Xi'an 710054 P. R. China