1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-146, Beijing, 100080, P. R. China, Department of Physics, Liaoning University, Shenyang,110036, China, Department of Physics, Peking University, Beijing,100871, P. R. China, and Division of Solid State Physics, Lund University, Lund 22100, Sweden