1. Department
of Physics, University of California-San Diego, La Jolla, California 92093-0319, United States
2. Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States
3. Center for Free-Electron Laser Science (CFEL), Deutsches Elektronensynchrotron (DESY), Notkestrasse
85, 22607 Hamburg, Germany
4. Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States
5. London Center for Nanotechnology, University College London, London WC1E 6BT, United Kingdom
6. Research Complex at Harwell, Didcot, Oxfordshire OX11 0DE, United Kingdom