Fractional Spin Excitations in the Infinite-Layer Cuprate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>CaCuO</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>
Leonardo Martinelli, Davide Betto, K. Kummer, Riccardo Arpaia, L. Braicovich, D. Di Castro, N. B. Brookes, M. Moretti Sala, G. Ghiringhelli
Abstract
We use resonant inelastic x-ray scattering (RIXS) to investigate the magnetic dynamics of the infinitelayer cuprate CaCuO 2 . We find that close to the 1=2; 0 point, the single magnon decays into a broad continuum of excitations accounting for about 80% of the total magnetic spectral weight. Polarizationresolved RIXS spectra reveal the overwhelming dominance of the spin-flip (S 1) character of this continuum with respect to the 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 c J 1 of CaCuO 2 . In the infinite-layer cuprates, long-range and multisite hopping integrals are very important, and they amplify the 2D quantum magnetism effects in spite of the 3D antiferromagnetic Nel order.