# Magnetic excitations and amplitude fluctuations in insulating cuprates

## TL;DR

This study uses light scattering to analyze magnetic excitations in insulating cuprates, revealing universal spectral features, amplitude fluctuations, and chiral excitations, with results aligning with theoretical predictions.

## Contribution

The paper provides the first detailed symmetry-resolved light scattering spectra of multiple cuprates, confirming theoretical models and identifying new amplitude fluctuation and chiral excitation features.

## Key findings

- Spectral shape in B1g symmetry is nearly universal across samples.
- Asymmetric line shape relates to amplitude fluctuations of magnetization.
- Support for chiral excitations as a continuum below 3J, with an additional excitation in La2CuO4.

## Abstract

We present results from light scattering experiments on three insulating antiferromagnetic cuprates, YBa$_2$Cu$_3$O$_{6.05}$, Bi$_2$Sr$_2$YCu$_2$O$_{8+\delta}$, and La$_2$CuO$_4$ as a function of polarization and excitation energy {using samples of the latest generation. From the raw data we derive symmetry-resolved spectra.} The spectral shape in $B_{1g}$ symmetry is found to be nearly universal and independent of the excitation energy. The spectra agree quantitatively with predictions by field theory [\onlinecite{Weidinger:2015}] facilitating the precise extraction of the Heisenberg coupling $J$. {In addition, the asymmetric line shape on the high-energy side is found to be related to amplitude fluctuations of the magnetization. In La$_2$CuO$_4$ alone minor contributions from resonance effects may be identified.} The spectra in the other symmetries are not universal. The variations may be traced back to weak resonance effects and extrinsic contributions. For all three compounds we find support for the existence of chiral excitations appearing as a continuum in $A_{2g}$ symmetry having an onset slightly below $3J$. In La$_2$CuO$_4$ an additional isolated excitation appears on top of the $A_{2g}$ continuum.