1. Department of Materials Science and Engineering, University of Maryland College Park, College Park, MD 20742, USA.
2. Department of Physics, Yale University, New Haven, CT 06511, USA.
3. Department of Physics and Astronomy, University of California, Irvine, Irvine, CA 92697, USA.
4. Departments of Electrical Engineering and Applied Physics, Yale University, New Haven, CT 06520, USA.