1. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA.
2. Department of Physics, University of Texas at Dallas, Richardson, TX, USA.
3. Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
4. Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan.