Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator $\mathrm{Sr_2IrO_4}$

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understandingof the pseudogap, non-Fermi liquids and high-temperaturesuperconductivity$^{1–4}$. Recently, photo-excitation has been used to induce similarly exotic states transiently$^{5–7}$. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electronlaser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr$_2$IrO$_4$. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D)long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. The marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics