1. Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
2. Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
3. Department of Physics, Stanford University, Stanford, CA 94305-4060, USA.
4. SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.
5. National Institute of Standards and Technology, Boulder, CO 80305, USA.
6. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA.