Unraveling higher-order contributions to spin excitations probed using resonant inelastic x-ray scattering

Abstract

Resonant inelastic x-ray scattering (RIXS) is an evolving tool for investigating the spin dynamics of strongly correlated materials, which complements inelastic neutron scattering. In isotropic spin-$\frac{1}{2}$ Heisenberg antiferromagnetic (HAFM) spin chains, both techniques have observed non-spin-conserving (NSC) excitations confined to the two-spinon phase space. However, a recent O $K$-edge RIXS study of the one-dimensional HAFM ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ observed spin-conserving (SC) four-spinon excitations outside the two-spinon phase space. Here, we demonstrate that analogous four-spinon excitations can also be accessed at the Cu ${L}_{3}$ edge in the related material ${\mathrm{SrCuO}}_{2}$. Through detailed modeling, we establish that these excitations appear in both the SC and NSC channels of the Cu ${L}_{3}$ edge, and are only captured by higher-order terms in the ultrashort core-hole lifetime expansion. Since these terms encode information about spin-spin correlations extending beyond nearest neighbors, our results offer different possibilities for studying nonlocal spin correlations in quantum magnets.