Ultrafast Bragg-peak kinetics in a spin orbit Mott insulator

Spin-orbit Mott insulators like Sr₂IrO₄ sit at the edge of intertwined electronic, magnetic, and lattice orders. To capture how this system reacts to a direct, resonant spin excitation, we performed an all-x-ray pump–probe experiment at the XCS instrument (LCLS/SLAC) using the split-and-delay system to create two equal femtosecond x-ray pulses from the same shot. One is used to “pump” the system resonantly, while the other to “probe.” By monitoring a chosen Bragg peak, we obtain a direct, momentum-resolved readout of how the ordered state responds from sub-picoseconds to a few picoseconds. Our measurement protocol interleaves “x-ray pump on” shots (pump+probe pair) with simple reference shots (“x-ray pump off,” single-pulse frames) so that background and slow drifts are cleanly removed. With consistent ROIs and shot-averaging at fixed delays, we extract the normalized Bragg-intensity change ΔI/I and track peak-profile metrics. The response shows a prompt intensity change at the earliest delays, followed by a partial recovery over several picoseconds. The initial amplitude and early-time slope increase with incident energy on the sample (fluence), indicating a photo-driven disturbance that quickly hands energy to the lattice before full thermalization. In this talk, I will introduce spin-orbit Mott insulators, like Sr₂IrO₄, and explain the x-ray pump/x-ray probe method in practical terms. I will discuss fluence, momentum, and time dependence of our observations. Together, these results establish a clear picture of ultrafast relaxation pathways in Sr₂IrO₄ and provide motivation for future pump–probe diffraction studies of correlated oxides.