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

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-21 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 Sr2CuO3 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 L3 edge in the related material SrCuO2. Through detailed modeling, we establish that these excitations appear in both the SC and NSC channels of the Cu L3 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.