1. Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA
2. Institut für Kristallzüchtung, D-12489 Berlin, Germany
3. Physikalisch-Technische Bundesanstalt, D-38116 Braunschweig, Germany
4. VITCON Projectconsult GMBH, D-07745 Jena, Germany
5. Department of Physics, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
6. School of Fundamental Science and Technology, Keio University, Yokohama, Kanagawa 2238522, Japan
7. London Centre for Nanotechnology, University College London, London WC1H 0AH, United Kingdom
8. Department of Physics and James Franck Institute, University of Chicago, Chicago, Illinois 60637, USA