Nematic magnetoelastic effect contrasted between $\mathrm{Ba(Fe_{1-x}Co_x)_2As_2}$ and FeSe

Whether the nematic order ubiquitously found in Fe-based superconductors is driven by the spin or the charge or orbital degree of freedom is currently under heated debate. To elucidate its microscopic origin, we report a Raman scattering study of lattice dynamics, which quantify the extent of $C_4$-symmetry breaking, in $\mathrm{BaFe_2As_2}$ and FeSe. FeSe possesses a nematic ordering temperature $T_{\mathrm{s}}$ and orbital-related band-energy split below $T_{\mathrm{s}}$ that are similar to those in $\mathrm{BaFe_2As_2}$, but unlike $\mathrm{BaFe_2As_2}$ it has no long-range magnetic order. We find that the $E_g$ phonon-energy split in FeSe sets in only well below $T_{\mathrm{s}}$, and its saturated value is substantially smaller than that in $\mathrm{BaFe_2As_2}$. Together with reported results for the $\mathrm{Ba(Fe_{1-x}Co_{x})_2As_2}$ family, the data suggest that magnetism exerts a major influence on the lattice.