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