Strong and selective magnon-phonon coupling in van der Waals antiferromagnet CoPS${}_3$

The Raman scattering response of the biaxial antiferromagnet CoPS${}_3$ has been investigated as a function of both magnetic field and temperature. The peaks observed in the low-frequency spectral range (90–200~cm${}^{-1}$) have been identified as hybrid magnon–phonon excitations. The energies of the bare magnon and phonon modes, as well as the effective coupling strengths between different excitation pairs, have been determined. The strong and selective magnon–phonon interaction largely accounts for the pronounced splitting of two phonon-like modes observed at 152~cm${}^{-1}$ and 158~cm${}^{-1}$ in the antiferromagnetic phase of CoPS${}_3$. Based on the identification of bare magnon excitations and their magnetic-field dependence, we propose an updated set of parameters for the effective exchange ($J_{\mathrm{e}\mathrm{f}\mathrm{f}}=9.9$~meV) and biaxial magnetic anisotropy ($D=4.3$~meV and $E=-0.7$~meV) and advocate for an apparent anisotropic $g$-factor ($g_x=g_y=2$, $g_z=4$) in the CoPS${}_3$ antiferromagnet.