1. Beijing Key Laboratory of Nano-Photonics and Nano-Structure (NPNS), Department of Physics, Capital Normal University, Beijing 100048, PR China
2. Center for Nanoscience and Nanotechnology and School of Physics and Technology, Wuhan University, Wuhan 430072, PR China
3. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Box 603-146, Beijing 100190, PR China
4. School of Science, Dalian Nationalities University, Dalian 116600, PR China
5. Division of Solid State Physics/The Nanometer Consortium, Lund University, Box 118, S-22100 Lund, Sweden