1. Department of Physics, School of Sciences, Wuhan University of Technology, 205 Luoshi Road, Hongshan district, Wuhan, Hubei 430070, People's Republic of China
2. College of Physical Sciences and Technology, Sichuan University, Chengdu 610065, People's Republic of China
3. School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, UK