1. Geballe Laboratory for Advanced Materials, Stanford University, 476 Lomita Mall, Stanford, California 94305, USA
2. Department of Applied Physics, Stanford University, 348 Via Pueblo Mall, Stanford, California 94305, USA
3. Stanford Institute for Materials and Energy Science, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
4. Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, California 94305, USA