1. College
of Physics, Qingdao University, Qingdao 266071, P. R. China
2. Functional
Materials Research Laboratory, Tongji University, Shanghai 201804, P. R. China
3. Department
of Physics and Materials Science, City University of Hong Kong, 83 Tat
Chee Avenue, Hong Kong SAR, P. R. China
4. City University of Hong Kong Shenzhen Research Institute, Shenzhen 518057, P. R. China