1. Department of Physics, Boston College, Chestnut Hill, MA 02467, USA.
2. GMZ Energy, Incorporated, 12A Hawthorn Street, Newton, MA 02458, USA.
3. Department of Mechanical Engineering, Massachusetts Institute of Technology (MIT), Cambridge, MA 02139, USA.
4. Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, China.
5. Department of Physics and Department of Electrical Engineering and Computer Science, MIT, Cambridge, MA 02139, USA.