Recent Progress in Ultrafast Dynamics of Transition-Metal Compounds Studied by Time-Resolved X-ray Techniques

Abstract: X-ray absorption spectroscopy and X-ray magnetic circular dichroism have long served as indispensable tools for probing the electronic and magnetic properties of transition-metal compounds with elemental selectivity. In recent years, the emergence of femtosecond lasers has opened a new avenue for studying nonequilibrium dynamics in condensed matter. However, conventional optical techniques lack elemental and orbital specificity, making it difficult to disentangle the coupled charge, spin, and lattice responses in complex materials. The development of X-ray free-electron lasers (XFEL) and laboratory high-harmonic generation (HHG) sources has enabled the extension of X-ray absorption and scattering techniques into the femtosecond time domain. Time-resolved X-ray absorption spectroscopy, X-ray magnetic circular dichroism, and resonant soft X-ray scattering now provide direct, complementary access to element- and momentum-resolved ultrafast dynamics. This review summarizes recent progress in these techniques, focusing on pump-probe measurements of laser-induced demagnetization, spin-state transitions, and valence and structural changes in transition-metal compounds. We also discuss advances in tabletop HHG-based X-ray spectroscopy and its integration with large-scale XFEL facilities. These developments provide powerful routes for visualizing the nonequilibrium evolution of charge, spin, orbital, and lattice degrees of freedom, offering new insights into the ultrafast control of quantum materials.