1. Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA.
2. Walter Schottky Institut and Physik-Department, Technische Universität München, Am Coulombwall 4a, 85748 Garching, Germany.
3. Institute of Physics, University of Münster, Wilhelm-Klemm-Str.10, 48149 Münster, Germany.
4. Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA.
5. Department of Physics and Astronomy, Purdue University, IN 47907, USA.
6. Birck Nanotechnology Center, Purdue University, IN 47907, USA.
7. Microsoft Station Q Purdue, Purdue University, IN 47907, USA.
8. School of Materials Engineering and School of Electrical and Computer Engineering, IN 47907, USA.
9. Department of Physics, Columbia University, New York, NY 10027, USA.