1. QUP, Department of Physics, Technical University of Denmark, Building 309, DK-2800 Lyngby, Denmark, Department of Natural Sciences, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK 1871 Frederiksberg C, Denmark, and Nano-Science Center, University of Copenhagen, Universitetsparken 5, DK-2100 København Ø, Denmark