Unraveling higher-order corrections in the spin dynamics of RIXS spectra

TL;DR

This paper reveals higher-order four-spinon excitations in RIXS spectra of SrCuO$_2$, arising from both spin-conserving and non-spin-conserving processes, expanding understanding of spin dynamics in quantum magnets.

Contribution

It demonstrates that four-spinon excitations outside the two-spinon phase space originate from second-order processes involving long-range spin fluctuations in RIXS spectra.

Findings

Four-spinon excitations observed outside the two-spinon phase space.

These excitations arise from second-order terms in the core-hole lifetime expansion.

Both spin-conserving and non-spin-conserving processes contribute to these excitations.

Abstract

Resonant inelastic x-ray scattering (RIXS) is an evolving tool for investigating spin dynamics of strongly correlated materials, which complements inelastic neutron scattering. Both techniques have found that non-spin-conserving (NSC) excitations in quasi-1D isotropic quantum antiferromagnets are confined to the two-spinon phase space. Outside this phase space, only spin-conserving (SC) four-spinon excitations have been detected using O $K$-edge RIXS. Here, we investigate SrCuO$_2$ and find four-spinon excitations outside the two-spinon phase space at both O $K$- and Cu $L_3$-edges. Using the Kramers-Heisenberg formalism, we demonstrate that the four-spinon excitations arise from both SC and NSC processes at Cu $L_3$-edge. We show that these new excitations only appear in the second-order terms of the ultra-fast core-hole lifetime expansion and arise from long-range spin fluctuations. These results thus open a new window to the spin dynamics of quantum magnets.