1. Department of Microtechnology
and Nanoscience (MC2), Chalmers University of Technology, SE-412 96 Göteborg, Sweden
2. Division of Solid State Physics, Lund University, Box 118, S-221 00 Lund, Sweden
3. Key Laboratory for the Physics
and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China