Critical divergence of the symmetric (A1g) nonlinear elastoresistance near the nematic transition in an iron-based superconductor

We report the observation of a nonlinear elastoresistivity response for the prototypical underdoped iron pnictide $\mathrm{Ba}{({\mathrm{Fe}}_{0.975}{\mathrm{Co}}_{0.025})}_{2}{\mathrm{As}}_{2}$. Our measurements reveal a large quadratic term in the isotropic (${A}_{1g}$) electronic response that was produced by a purely shear (${B}_{2g}$) strain. The divergence of this quantity upon cooling towards the structural phase transition reflects the temperature dependence of the nematic susceptibility. This observation shows that nematic fluctuations play a significant role in determining even the isotropic properties of this family of compounds.