Fractional spin excitations in the infinite-layer cuprate CaCuO₂

The spin 1/2 square-lattice antiferromagnet is one of the most studied systems in condensed matter theory because of its purely quantum mechanical nature. After the seminal paper by Anderson, much effort has been put in the quest for exotic ground states such as the Resonating Valence Bond (RVB), which have been predicted to arise in many Heisenberg Hamiltonians with frustrating or multi-spin couplings. These states are interesting because they exhibit fractionalized collective excitations, each carrying spin 1/2, that are the 2-dimensional version of spinons in 1D spin chains. Despite many efforts, these fractional magnetic excitations have so far eluded conclusive observation in the AF square lattice. In this context, we have used Cu L₃ Resonant Inelastic X-ray Scattering (RIXS) to study the spin excitations in the infinite-layer CaCuO₂, which is special for the very large nearest neighbour AF coupling J (∼180 meV), and an exceptionally large ring exchange J_c ∼ J. Close to the magnetic zone boundary, we observe a decay of the magnon into a broad asymmetric continuum at high energies, accounting for more than 80% of the total spectral weight, and composed of ΔS=1 excitations. Our observation are in line for what is expected from a continuum of spinon pairs.