1. Department of Physics, City University of Hong Kong, Hong Kong 999077, P.R. China
2. College of Physics and Materials Science, Tianjin Normal University, Tianjin 300387, P.R. China
3. Center for Neutron Scattering, City University of Hong Kong, Hong Kong 999077, P.R. China
4. School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, P.R. China
5. Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, P.R. China