Quantum Critical Enhancement of Nematic-Resistivity Anisotropy in Underdoped Ba(Fe_1-xCo_x)₂As₂

The proportionality constant between the thermodynamic order parameter of a symmetry breaking phase and its associated transport coefficient often contains important information about the underlying electronic structure of a material. For instance, the anomalous Hall effect has been used to probe the Berry curvature of the band structures. Here we show how the proportionality constant between resistivity anisotropy and orthorhombicity in the nematic ordered phase is directly related to the elastoresistivity coefficient measured above the phase transition temperature. Using this relation, we discovered that this proportional constant increases significantly as doping approaches optimal, indicating the enhanced coupling between nematicity and conducting electrons near the quantum critical point.