Nematic magnetoelastic effect contrasted betweenBa(Fe1−xCox)2As2and FeSe

To elucidate the origin of nematic order in Fe-based superconductors, we report a Raman scattering study of lattice dynamics, which quantify the extent of ${C}₄$-symmetry breaking, in ${\mathrm{BaFe}}₂{\mathrm{As}}₂$ 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 ${\mathrm{BaFe}}₂{\mathrm{As}}₂$, but unlike ${\mathrm{BaFe}}₂{\mathrm{As}}₂$ it has no long-range magnetic order. We find that the ${E}_g$ phonon-energy split in FeSe becomes substantial only well below ${T}_{\mathrm{s}}$, and its saturated value is much smaller than that in ${\mathrm{BaFe}}₂{\mathrm{As}}₂$. Together with reported results for the $\mathrm{Ba}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_x{)}₂{\mathrm{As}}₂$ family, the data suggest that magnetism exerts a major influence on the lattice.