Direct observation of multi-spinon excitations outside of the two-spinon continuum in the antiferromagnetic spin chain cuprate Sr2CuO3

TL;DR

This paper demonstrates that resonant inelastic x-ray scattering (RIXS) can directly observe four-spinon excitations in a 1D antiferromagnetic spin chain, revealing complex quantum dynamics beyond the two-spinon continuum.

Contribution

It provides the first direct measurement of four-spinon excitations using RIXS, expanding the understanding of quasiparticle interactions in quantum spin chains.

Findings

RIXS can access four-spinon excitations outside the two-spinon continuum.

First direct experimental observation of four-spinon states.

RIXS probes different correlation functions than INS, enabling new insights.

Abstract

One-dimensional (1D) magnetic insulators have attracted significant interest as a platform for studying emergent phenomena such as quasiparticle fractionalization and quantum criticality. The antiferromagnetic Heisenberg chain of spins-1/2 is an important reference system; its elementary excitations are spin-1/2 quasiparticles called spinons that are always created in pairs. However, while inelastic neutron scattering (INS) experiments routinely observe the excitation continuum associated with two-spinon states, the presence of more complex dynamics associated with four-spinon states has only been inferred from comparison with theory. Here, we show that resonant inelastic x-ray scattering (RIXS) is capable of accessing the four-spinon excitations directly, in a spectroscopic region separated from the two-spinon continuum. Our results provide the first direct measurement of four-spinon excitations, which is made possible by the fundamentally different correlation functions probed by RIXS compared to INS. This advance holds great promise as a tool in the search for novel quantum states, in particular quantum spin liquids.