1. Department of Applied Physics, Via Pueblo Mall, Stanford University, Stanford, CA 94305, USA.
2. Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany.
3. Stanford Synchrotron Radiation Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.
4. Fritz-Haber-Institut, Department of Physical Chemistry, Faradayweg 4-6, 14195 Berlin, Germany.
5. Geballe Laboratory for Advanced Materials, 476 Lomita Mall, Stanford University, Stanford, CA 94305, USA.