1. The Institute for Advanced Studies, Wuhan University, Wuhan 430072, P.R. China
2. Department of Physics, Wuhan University, Wuhan 430072, P.R. China
3. School of Mathematics and Physics, China University of Geosciences (Wuhan), Wuhan 430074, Hubei P.R. China
4. Department of Physics, Hunan Normal University, Changsha 410081, P.R. China