1. Departments of Applied Physics, Physics, and Stanford Synchrotron Laboratory, Stanford University, Stanford, CA 94305, USA.
2. Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277–8561, Japan.
3. Institute for Chemical Research, Kyoto University, Uji, Kyoto 611–0011, Japan.
4. RIKEN, The Institute for Physical and Chemical Research, Wako 351–0198, Japan.