1. Department of Physics, University of California San Diego, La Jolla, California 92093, United States
2. Department of NanoEngineering, University of California San Diego, La Jolla, California 92093, United States
3. Department of Physics, Harvard University, 17 Oxford Street, Cambridge, Massachusetts 02138, United States
4. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States
5. Institute for Theoretical Physics, University of Leipzig, Brderstr.16, 04103 Leipzig, Germany
6. Kavli Institute of Nanoscience, Delft University of Technology, 2628CJ Delft, The Netherlands