[1] Univ Birmingham, Sch Phys & Astron, Birmingham B15 2TT, England
[2] Shaanxi Normal Univ, Sch Phys & Informat Technol, Xian 710119, Peoples R China
[3] Univ Nottingham, Sch Phys & Astron, Nottingham NG7 2RD, England
[4] Xian Univ, Sch Mech & Mat Engn, Xian 710065, Peoples R China