1. Department of Physics and Institute of Condensed Matter Theory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A.
2. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, U.S.A.
3. Department of Applied Physics, Stanford University, Stanford, CA 94305, U.S.A.
4. Department of Chemistry, Stanford University, Stanford, CA 94305, U.S.A.
5. Department of Physics, Stanford University, Stanford, CA 94305, U.S.A.
6. Department of Materials Science and Engineering, Stanford University, CA 94305, U.S.A.
7. Geballe Laboratory for Advanced Materials, Stanford University, CA 94305, U.S.A.