1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-146, Beijing 100190, People's Republic of China
2. Department of Applied Physics, Chongqing University, Chongqing 400044, People's Republic of China
3. College of Chemistry, Liaoning University, Shenyang 110036, People's Republic of China
4. Departamento de Ciencia de los Materiales, Ingeniería Metalúrgicay Química Inorgánica, Facultadde Ciencias, Universidad de Cádiz (Campus Río San Pedro), E-11510 Puerto Real (Cadiz), Spain
5. Division of Solid State Physics, Lund University, Lund 22100, Sweden