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

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.

5 pages, 4 figures; small changes through the text, references updated