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, Stanford University, Stanford, California 94305, United States
4. Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, United States
5. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, United States
6. Department of Applied Physics, Stanford University, Stanford, California 94305, United States
7. Graduate School of Science & Technology, Seikei University, Musashino, Tokyo 180-8633, Japan