1. State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China
2. Center for Applied Mathematics and Physics, Halmstad University, P.O. Box 823, S-301 18 Halmstad, Sweden and Solid State Physics/Nanometer Structure Consortium, Lund University, P.O. Box 118, S-221 00 Lund, Sweden
3. Science for Life Laboratory, Department of Applied Physics, Royal Institute of Technology, SE-10691 Stockholm, Sweden