1. Department of Physics, Stanford University, Stanford, California 94305, United States
2. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
3. Department of Applied Physics, Stanford University, Stanford, California 94305, United States
4. Department of Inorganic Materials Science, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands