[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