1. Department of Physics, James Franck Institute, University of Chicago, Chicago, Illinois 60637, USA;
2. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
3. Department of Physics, Stanford University, Stanford, California 94305, USA
4. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory and Stanford University, Menlo Park, California 94025, USA
5. Institute for Theoretical Physics, University of Cologne, Cologne 50937, Germany