Fractional spin excitations in the infinite-layer cuprate CaCuO$_2$

TL;DR

This study uses RIXS to reveal that in CaCuO2, magnons decay into a broad continuum of spinon pairs, highlighting strong quantum magnetic effects due to high ring exchange interactions.

Contribution

It provides evidence for magnon decay into spinon pairs in CaCuO2 and links this to high ring exchange, advancing understanding of quantum magnetism in infinite-layer cuprates.

Findings

80% of magnetic spectral weight is in the continuum near (1/2,0)

Continuum has dominant spin-flip character, similar energy dependence to magnons

High ring exchange Jc ~ J1 influences quantum magnetic effects

Abstract

We use resonant inelastic x-ray scattering (RIXS) to investigate the magnetic dynamics of the infinite-layer cuprate CaCuO2. We find that close to the (1/2,0) point the single magnon decays into a broad continuum of excitations accounting for 80% of the total magnetic spectral weight. Polarization resolved RIXS spectra reveal the overwhelming dominance of spin-flip ($\Delta S = 1$) characterof this continuum with respect to the $\Delta S= 0$ multimagnon contributions. Moreover, its incident energy dependence is identical to that of the magnon, supporting a common physical origin. We propose that the continuum originates from the decay of the magnon into spinon pairs, and we relate it to the exceptionally high ring exchange $J_\mathrm{c} \sim J_1$ of CaCuO2. In the infinite layer cuprates long-range and multi-site hopping integrals are pivotal and amplify the 2D quantum magnetism effects in spite the 3D antiferromagnetic N\'eel order.