1. Beijing National Laboratory for Condensed; Matter Physics and Institute of Physics; Chinese Academy of Sciences; Box 603-146 Beijing 100190 China
2. Center for Nanoscience and Nanotechnology and School of Physics and Technology; Wuhan University; Wuhan 430072 China
3. Division of Solid State Physics/The Nanometer Structure Consortium; Lund University; Box 118 S-22100 Lund Sweden