Nonlinear Elastoresistivity Response in the $A_{1g}$ Symmetry Channel of the Iron Superconductor Ba(Fe$_{0.975}$Co$_{0.025}$)$_2$As$_2$

Elastoresistivity relates changes in resistance of a material to strains that it experiences. Previously we have shown how the $B_{2g}$ component of the elastoresistivity tensor is proportional to the nematic susceptibility, and hence can be used to infer a divergence of the nematic susceptibility approaching the tetragonal-to-orthorhombic structural phase transition in the Fe-based superconductors. In this talk I will introduce a new application of elastoresistance measurements for probing the resistivity response in the $A_{1g}$ symmetry channel. This is not a nematic symmetry; rather, it describes the isotropic response to strains that the material experiences. This technique was performed on a stereotypical iron based superconductor, Ba(Fe$_{0.975}$Co$_{0.025}$)$_2$As$_2$. We find that the response in the $A_{1g}$ channel is nonlinear with a quadratic elastoresistance coefficient that diverges close to the tetragonal to orthorhombic structural transition. I will explain the significance of these measurements and how they fit with our understanding from previous measurements of the $B_{2g}$ elastoresistance response.