1. Department
of Applied Physics, Stanford University, Stanford, California 94305, United States
2. Department
of Physics, Stanford University, Stanford, California 94305, United States
3. Department
of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States
4. Stanford
Institute for Materials and Energy Sciences, SLAC National Accelerator
Laboratory, 2575 Sand
Hill Road, Menlo Park, California 94025, United States