1. Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, USA.
2. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
3. Department of Physics, Stanford University, Stanford, CA 94305, USA.
4. National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan.
5. Department of Physics, Royal Institute of Technology, SE-106 91 Stockholm, Sweden.
6. Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 201210, China.
7. Department of Applied Physics, Stanford University, Stanford, CA 94305, USA.