A new method is presented for measuring terms in the elastoresistivity tensor $m_{ ij}$ of single crystal samples with tetragonal symmetry. The technique is applied to a representative underdoped Fe-arsenide, Ba(Fe$_{ 0.975}$Co$_{ 0.025}$)$_2$As$_2$, revealing an anomalously large and anisotropic elastoresistance in comparison to simple metals. The $m_{ 66}$ coefficient follows a Curie-Weiss temperature dependence, providing direct evidence that the tetragonal-to-orthorhombic structural phase transition that occurs at $T_s$ = 97.5 K in this material is not the result of a true-proper ferro-elastic transition. Rather, the material suffers a pseudo-proper transition for which the lattice strain is not the primary order parameter.
Categories: cond-mat.supr-con cond-mat.str-el