Symmetric and antisymmetric strain as continuous tuning parameters for electronic nematic order

Abstract

We report the separate response of the critical temperature of the nematic phase transition ${T}_{\mathrm{S}}$ to symmetric and antisymmetric strains for the prototypical underdoped iron pnictide $\mathrm{Ba}{({\mathrm{Fe}}_{0.975}{\mathrm{Co}}_{0.025})}_{2}{\mathrm{As}}_{2}$. This decomposition is achieved by comparing the response of ${T}_{\mathrm{S}}$ to in-plane uniaxial stress and hydrostatic pressure. In addition to quantifying the two distinct linear responses to symmetric strains, we find a quadratic variation of ${T}_{\mathrm{S}}$ as a response to antisymmetric strains ${\ensuremath{\varepsilon}}_{{\mathrm{B}}_{1\mathrm{g}}}=\frac{1}{2}({\ensuremath{\varepsilon}}_{\mathrm{xx}}\ensuremath{-}{\ensuremath{\varepsilon}}_{\mathrm{yy}})$, exceeding the nonlinear response to symmetric strains by at least two orders of magnitude. These observations establish orthogonal antisymmetric strain as a powerful tuning parameter for nematic order.