1. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
2. Department of Physics, Stanford University, Stanford, California 94305, USA
3. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
4. Department of Applied Physics, Stanford University, Stanford, California 94305, USA
5. Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
6. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA