1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-146, Beijing 100190, PR China
2. School of Science, Dalian Nationalities University, Dalian 116600, PR China
3. Departamento de Ciencia de los Materiales, Ingeniería Metalúrgica y Química Inorgánica, Facultadde Ciencias, Universidad de Cádiz (Campus Río San Pedro), E-11510 Puerto Real, Cádiz, Spain
4. Division of Solid State Physics, Lund University, Lund 22100, Sweden