1. Department of Physics, Stanford University, Stanford, California 94305, USA
2. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
3. Department of Applied Physics, Stanford University, Stanford, California 94305, USA
4. Department of Electrical Engineering, University of California, Los Angeles, Los Angeles, California 90095, USA
5. Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA