1. Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe 76021, Germany
2. Department of Physics, Freie Universität Berlin, Berlin 14195, Germany
3. Institute of Materials Science, Technische Universität Darmstadt, Darmstadt 64287, Germany
4. Physics Department, University of Antwerp, Antwerp 2020, Belgium
5. Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States