Figure 3.

(a) Magnetic-field dependent Raman of FePS₃ at $T=5\text{K}$, showing the splitting of ${\text{ψ}}_4$ into two components, ${\text{ψ}}_4^{(1)}$ and ${\text{ψ}}_4^{(2)}$, where the frequency of ${\text{ψ}}_4^{(1)}$ (${\text{ψ}}_4^{(2)}$) decreases (increases) with increasing magnetic field. (b) Frequency vs. magnetic field of ${\text{ψ}}_4^{(1)}$ and ${\text{ψ}}_4^{(2)}$ with the slopes of the linear fits for the two branches. (c) If we consider two magnetization sublattices $M_1$ (pink) and $M_2$ (green) in FePS₃, in the ground state $M_1$ and $M_2$ point in the $+z$ and $-z$ directions. (d, e) The magnon spectrum includes two normal modes with net moments Mnet in the $x,y$ plane that precess in opposite directions. The degeneracy of these modes is broken in an applied field $H_0\widehat{z}$ with the energies of the modes in (d) and (e) respectively decreasing and increasing with $H_0$. In terms of this simple model, the modes in (d) and (e) would represent ${\text{ψ}}_4^{(1)}$ and ${\text{ψ}}_4^{(2)}$ in (b), respectively.