1. Materials and Nano-Physics, Department of Applied Physics, SCI, KTH Royal Institute of Technology, SE-106 91 Stockholm, Sweden
2. Physics Department/Center for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway
3. School of EECS, KTH Royal Institute of Technology, Electrum 229, SE-164 40 Kista, Sweden