UC San Diego

The diversity of phases and functional properties in quantum materials emerges from competing energy scales, strong couplings between internal degrees of freedom, geometric frustration, topology, and reduced dimensionality. The complexity of these materials consistently challenges our understanding of the underlying physics governing their properties. To this end, ultrafast experimental techniques are invaluable tools for disentangling these aspects by selectively driving degrees of freedom or excitations and observing the cascade of dynamics in the time domain. In this dissertation, we investigate the ultrafast dynamics in various quantum materials, focusing on how the near-equilibrium responses correspond to their magnetic properties. We study beta-Li2IrO3, a Mott insulator with strong spin-orbit coupling, and MnBi2Te4, a topological insulator with intrinsic antiferromagnetic ordering. In the former, we study how photoexcitation leads to the formation of quasiparticles whose dynamics are directly linked to equilibrium order parameters. We demonstrate how these dynamics evolve across the phase diagram and can be associated with short-ranged spin correlations above the conventional magnetic ordering temperatures. In the latter, we observe strong spin-phonon coupling likely originating from the modulation of the interlayer spin exchange due to lattice motion of the phonons. Our findings demonstrate that ultrafast techniques provide critical insights into the complex interplay of spin, charge, and lattice degrees of freedom, offering a deeper understanding of the fundamental properties and potential applications of quantum materials.