Speaker
Description
Understanding how magnetic phase transitions in correlated quantum systems respond to external perturbations requires a rigorous many-body description of localized states and ligand coordination. Because these macroscopic states emerge from a subtle interplay between crystal-field effects, spin-orbit coupling, and strong electronic correlations, core-level techniques like X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) take relevance. As element-specific probes, XAS and RIXS directly measure local cluster physics, core-hole multiplets, and crystal-field excitations. This provides a microscopic view of magnetism that complements macroscopic transport measurements; while transport probes the anomalous Hall effect (AHE) from delocalized conduction states, XAS and RIXS target localized orbitals to extract direct information about local magnetic moments.
To capture these dynamics, we developed a theoretical framework that maps first-principles Density Functional Theory (DFT) calculations to construct an Anderson Impurity Model (AIM) for localized subspaces, focusing on the 3$d$ shell for transition metals and the 4$f$ shell for rare earths. We use Dynamic Mode Decomposition (DMD) to discretize the hybridization, allowing us to mimic the host density of states (DOS) with a minimal number of bath sites. By solving the resulting finite-size Hamiltonian through exact diagonalization, we can compute core-level XAS/XMCD and RIXS/RIXS-MCD spectra to serve as direct fingerprints of the local magnetic states.
We applied this framework to resolve different physical phenomena in two different materials. For the magnetic system CrPS$_4$, we analyze spin-flip excitations using a minimal model that couples local spin flips to the crystal-field manifold, capturing how these excitations track the broader magnetic phase transitions. For the magnetic Weyl candidate PrAlGe, we model the Pr 4$f$-4$f$ excitations to explain why local magnetic moments and circular dichroism persist up to $T \sim 35$ K, well above the bulk ferromagnetic transition at $T_c \sim 16$ K.