1. Department of Physics, Columbia University, New York, NY 10027, USA.
2. Department of Physics, Harvard University, Cambridge, MA 02138, USA.
3. Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
4. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
5. School of Natural Sciences, Institute for Advanced Study, Princeton, NJ 08540, USA.
6. Institute for Theoretical Physics, University of Innsbruck, A-6020 Innsbruck, Austria.
7. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
8. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA.