Fractional Spin Excitations in the Infinite-Layer Cuprate ${\mathrm{CaCuO}}_{2}$

Open Access

Fractional Spin Excitations in the Infinite-Layer Cuprate ${CaCuO}_{2}$

Leonardo Martinelli, Davide Betto, Kurt Kummer, Riccardo Arpaia, Lucio Braicovich, Daniele Di Castro, Nicholas B. Brookes, Marco Moretti Sala, and Giacomo Ghiringhelli

Phys. Rev. X 12, 021041 – Published 19 May 2022

Abstract

We use resonant inelastic x-ray scattering (RIXS) to investigate the magnetic dynamics of the infinite-layer 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. Polarization-resolved RIXS spectra reveal the overwhelming dominance of the spin-flip ( $Δ S = 1$ ) character of this continuum with respect to the $Δ S = 0$ 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 $J_{c} \sim J_{1}$ 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 Néel order.

Received 12 October 2021
Accepted 30 March 2022

DOI:https://doi.org/10.1103/PhysRevX.12.021041

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Antiferromagnetism Exchange interaction Magnons Spinon

Antiferromagnets Charge-transfer insulators High-temperature superconductors

Resonant inelastic x-ray scattering

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Leonardo Martinelli ^1,*, Davide Betto², Kurt Kummer ², Riccardo Arpaia ³, Lucio Braicovich ^1,2, Daniele Di Castro^4,5, Nicholas B. Brookes ², Marco Moretti Sala ¹, and Giacomo Ghiringhelli ^1,6,†

¹Dipartimento di Fisica, Politecnico di Milano, piazza Leonardo da Vinci 32, I-20133 Milano, Italy
²ESRF, The European Synchrotron, 71 Avenue des Martyrs, CS 40220, F-38043 Grenoble, France
³Quantum Device Physics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296 Göteborg, Sweden
⁴Dipartimento di Ingegneria Civile e Ingegneria Informatica, Università di Roma Tor Vergata, Via del Politecnico 1, I-00133 Roma, Italy
⁵CNR-SPIN, Università di Roma Tor Vergata, Via del Politecnico 1, I-00133 Roma, Italy
⁶CNR-SPIN, Dipartimento di Fisica, Politecnico di Milano, I-20133 Milano, Italy

^*leonardo.martinelli@polimi.it
^†giacomo.ghiringhelli@polimi.it

Popular Summary

All high-temperature superconducting cuprates share a fundamental building block: ${CuO}_{2}$ layers arranged in a square lattice, each site having a single unpaired spin that interacts with its neighbors through strong antiferromagnetic couplings. The simplicity of the structure, however, veils a deeper complexity: Theoretical studies suggest that this spin- $1 / 2$ square-lattice antiferromagnetism is not stable. Strong couplings to other spins can trigger quantum phase transitions. The fingerprint of such transitions is the splitting of the usual bosonic spin waves into pairs of fermionic excitations (or spinons), though experimental evidence of this fractionalization has so far been scant. Here, we report sound evidence of this phenomenon thanks to unprecedented data quality and the right choice of sample.

We use resonant inelastic x-ray scattering to investigate the magnetic spectrum in the cuprate ${CaCuO}_{2}$ , whose crystal structure enhances the long-range magnetic couplings even with respect to other cuprates. Through an innovative combination of measurements with ultrahigh resolution and full polarization control, we find that, despite the presence of long-range antiferromagnetic order, the spin-wave quasiparticle peak in the short-wavelength region breaks into a continuum of high-energy states, which is related to spinon pairs.

This work demonstrates that the complexity of the spin- $1 / 2$ , antiferromagnetic square lattice has yet to be fully unraveled and paves the way for future studies in the field of quantum magnetism.

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