1. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02138, USA.
2. School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14850, USA.
3. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
4. Research Laboratory of Electronics, MA Institute of Technology, Cambridge, MA, USA.
5. Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba 305-0044, Japan.
6. Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan.
7. Department of Physics, Harvard University, Cambridge, MA 02138, USA.