# Raman scattering from Higgs mode oscillations in the two-dimensional antiferromagnet Ca$_2$RuO$_4$

## TL;DR

This study uses Raman spectroscopy and theoretical modeling to detect and analyze the Higgs mode in the two-dimensional antiferromagnetic Mott insulator Ca$_2$RuO$_4$, providing evidence for excitonic magnetism.

## Contribution

It presents the first direct observation of the Higgs mode in Ca$_2$RuO$_4$ using Raman scattering, supported by exact diagonalization calculations and neutron scattering data.

## Key findings

- Detection of Higgs mode in $A_g$ polarization
- Observation of single-magnon and two-magnon peaks in $B_{1g}$ geometry
- Quantitative agreement between experiments and model calculations

## Abstract

We present and analyze Raman spectra of the Mott insulator Ca$_2$RuO$_4$, whose quasi-two-dimensional antiferromagnetic order has been described as a condensate of low-lying spin-orbit excitons with angular momentum $J_{eff}=1$. In the $A_g$ polarization geometry, the amplitude (Higgs) mode of the spin-orbit condensate is directly probed in the scalar channel, thus avoiding infrared-singular magnon contributions. In the $B_{1g}$ geometry, we observe a single-magnon peak as well as two-magnon and two-Higgs excitations. Model calculations using exact diagonalization quantitatively agree with the observations. Together with recent neutron scattering data, our study provides strong evidence for excitonic magnetism in Ca$_2$RuO$_4$ and points out new perspectives for research on the Higgs mode in two dimensions.