1. Department of Physics, Queens College of the City University of New York, Flushing, New York 11367, USA
2. Department of Electrical and Computer Engineering, University of Wisconsin - Madison, Madison, Wisconsin 53706, USA
3. Walter-Schottky-Institute, Technical University Munich, Am Coulombwall 4, DE-85748 Garching, Germany
4. Laboratory for Solid State Physics, ETH Zurich, Otto-Stern-Weg 1, 8093 Zurich, Switzerland
5. Center for Hybrid Nanostructures (CHyN) and Institute of Nanostructure and Solid State Physics (INF), University of Hamburg, Jungiusstrasse 9-11, 20355 Hamburg, Germany