1. Department of Physics and Materials Research Laboratory, University of Illinois, Urbana, IL 61801, U.S.A.
2. Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, U.S.A.
3. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A.
4. Department of Materials Science and Institute of Optoelectronic, Fudan University, Shanghai 200433, China
5. Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, U.S.A.
6. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, U.S.A.
7. Department of Physics, Harvard University, Cambridge, MA 02138, U.S.A.