Finite Frequency and Nonlinear Elastoresistance Measurements in the Fe-based superconductors

Elastoresistance measurements have previously revealed a divergent electronic nematic susceptibility as a universal feature of underdoped Fe-based superconductors upon cooling through the tetragonal-to-orthorhombic structural transition. Elastoresistivity is described by a high rank tensor (fourth-rank +) as it relates changes in resistivity (second-rank) to strain (second-rank) experienced by a material. Recent technical advances have enabled elastoresistance measurements to both go beyond linear response and to measure the frequency dependence (up to 3 kHz) as well as significantly speeding up the data acquisition process. In this talk, in the context of Fe-based superconductors, I will outline the improvements in the elastoresistance technique, including a new AC strain method based on amplitude demodulation. Furthermore, I will show the large role nematic fluctuations play in the isotropic electronic response of these materials—as evidenced by a diverging nonlinear symmetric (A_1g) elastoresistance response to antisymmetric (B_2g) strain.