In the Blink of an X-ray: Femtosecond X-ray Absorption of Transition Metal Perovskites

This thesis has explored ultrafast element-specific dynamics in correlated transition metal oxides using femtosecond X-ray absorption spectroscopy (fs-XAS), resonant inelastic X-ray scattering (RIXS), and photoelectron spectroscopies, complemented by advanced theoretical modeling. Spanning eight chapters, the work offers a comprehensive investigation into how charge, spin, orbital, and lattice degrees of freedom evolve under nonequilibrium conditions in perovskite materials. Chapter 3 laid the foundation by demonstrating the capability of fs-XAS to resolve photoinduced electronic and structural changes in epitaxial LaFeO3 thin films. Using 400 nm excitation and probing at the iron 2p and oxygen 1s edges, we captured the ultrafast formation of intermediate- and high-spin Fe2+ states via ligand-to-metal charge transfer (LMCT), followed by polaron formation. Density functional theory (DFT) and multiplet charge transfer (CTM) theory were employed to interpret these dynamics, revealing that decreased Fe–O hybridization accounts for both spectral redshifts and changes in crystal field strength. In Chapter 4, we extended fs-XAS to LaNiO3, a correlated metallic nickelate of interest for catalysis. We observed an ultrafast redshift of the nickel–oxygen antibonding states, with stronger shifts at the oxygen 1s edge than at the Ni 2p3/2 edge—an indication of differing core-hole screening. These results revealed a transient reduction in nickel–oxygen covalency and formation of ligand holes, reshaping the band structure on sub-picosecond timescales and providing information on band engineering under light excitation. Chapter 5 returned to LaFeO3, focusing on spin-state dynamics at the Fe 3p edge in nanosheet-templated textured thin films. Here, we resolved the evolution from LMCT to polaronic trapping and subsequent structural relaxation. Compared to Fe2O3, the dynamics in LaFeO3 unfolded significantly faster, reflecting differences in covalency and symmetry. Notably, beyond 3 ps, the system displayed persistent Fe3+-like distortions without full reoxidation—underscoring the role of lattice reorganization. In Chapter 6, we investigated a LaNiO3/LaFeO3 heterostructure to probe interfacial charge transfer dynamics. fs-XAS at both iron and nickel 3p edges revealed longer-lived Fe2+-like states compared to LaFeO3 alone, indicating electron transfer into the nickel layer. At the Ni edge, excited-state features assigned to Ni2+ and Ni3+ emerged and decayed within picoseconds. These results confirmed photoinduced interfacial charge redistribution and showed how layer architecture governs carrier lifetime and localization. Chapter 7 provided a depth-resolved analysis of the electronic structure using soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES). By comparing LaCrO3, LaFeO3, and LaCoO3 in both thin-film and single-crystal forms, we characterized the valence states, charge transfer satellites, and spin–orbit splittings across multiple core levels. Finally, Chapter 8 turned to high-resolution RIXS on strained LaCrO3 films. Dispersive magnetic excitations and d–d transitions were resolved, with clear signatures of crystal field splitting and anisotropic spin alignment. These results validated CTM-based simulations and confirmed the role of epitaxial strain in modifying magnetic properties. XMLD and SQUID measurements further supported an out-of-plane spin axis and a reduced Néel temperature, reflecting strain-tuned exchange interactions. In conclusion, the results attest that ultrafast X-ray techniques are not only diagnostic tools but also powerful probes for discovering and controlling the emergent physics of quantum materials.