[1] Northwest Normal Univ, Coll Phys & Elect Engn, Key Lab Atom & Mol Phys & Funct Mat Gansu Prov, Lanzhou 730070, Peoples R China
[2] Northwest Normal Univ, Joint Lab Atom & Mol Phys, Lanzhou 730070, Peoples R China
[3] Chinese Acad Sci, Inst Modern Phys, Lanzhou 730070, Peoples R China
[4] Univ Coll Dublin, Sch Phys, Dublin 4, Ireland