This talk will explain how X-ray Free Electron Lasers (Xfels) provide a unique opportunity to directly determine the magnetic correlations in transient photo-excited states. To do this we made the first implementation of time-resolved resonant inelastic X-ray scattering (tr-RIXS) at LCLS. We photo-dope a model quantum antiferromagnetic Mott insulator Sr2IrO4 and show how tr-RIXS directly clarifies the nature of ultrafast magnetic and orbital dynamics. We find that the non-equilibrium state 2 ps after photo-excitation has strongly suppressed long range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. The magnetism restores its two-dimensional (2D) in-plane Neel correlations on a timescale of a few ps, while the three dimensional (3D) long-range magnetic order recovers over a far longer timescale of a few 100 ps. We achieved a total energy resolution of 70 meV, comparable to those from regular synchrotron sources, and within a feasible time for data collection. This makes tr-RIXS an attractive, powerful new spectroscopy, especially in magnetic materials without long-range magnetic order. We end by describing how LCLC-II has potential to transform the experimental possibilities for tr-RIXS.

Y. Cao1*, M. P. M. Dean1*, X. Liu2, S. Wall3, D. Zhu4, R. Mankowsky5, 6, V.

Thampy1, X. M. Chen1, J. Vale7, D. Casa8, Jungho Kim8, A. H. Said8, P.

Juhas1, R. Alonso-Mori4, M. Glownia4, A. Robert4, J. Robinson4, M. Sikorski4, S. Song4, M. Kozina4, H. Lemke4, L. Patthey9, S. Owada10, T. Katayama11, M.

Yabashi10, Y. Tanaka10, T. Togashi11, J. Liu12, 13, B. J. Kim14, L. Huber15, C.-L. Chang16, D. F. McMorrow7, M. Först5, 6, and J. P. Hill1

1) Department of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, New York 11973, USA 2)Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 3) ICFO Institut de Ciències Fotòniques, Mediterranean Technology Park, 08860, Castelldefels, Barcelona, Spain 4) Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California, USA 5)Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany 6) Center for Free Electron Laser Science, Hamburg, Germany 7) London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, UK 8) Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA 9) SwissFEL, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

10) RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan 11) Japan Synchrotron Radiation Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan

12) Department of Physics, University of California, Berkeley, California 94720, USA 13) Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 14) Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany 15) Institute for Quantum Electronics, Physics Department, ETH Zurich, CH-8093 Zurich, Switzerland 16) Zernike Institute for Advanced Materials, University of Groningen, Groningen, NL 9747AG

* ycao@bnl.gov, mdean@bnl.gov