1. Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States
2. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States
3. Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, United States
4. Department of Physics, Stanford University, Stanford, California 94305, United States
5. Department of Applied Physics, Stanford University, Stanford, California 94305, United States
6. Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
7. Department of Electrical and Computer Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States
8. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States