Fig. S4.

(A) Comparison between the $B_{1g}$ charge nematic susceptibility of FeSe and Co doped BaFe₂ As₂ (23) plotted as a function of T–$T_0$ where $T_0$ is the Curie–Weiss temperature of each sample. The arrows indicate the structural transition $T_S$ for each sample. (B) Electron–lattice coupling energy $T_S$–$T_0$ as a function of $T_S$ for FeSe and Co-Ba122.