Nonlinear uniaxial pressure dependence of the resistivity in Sr_1-xBa_xFe_1.97Ni_0.03As₂

Nematic order and its fluctuations have been widely found in iron-based superconductors. Above the nematic order transition temperature, the resistivity shows a linear relationship with the uniaxial pressure or strain along the nematic direction and the normalized slope is thought to be associated with nematic susceptibility. Here we systematically studied the uniaxial pressure dependence of the resistivity in Sr_1-xBa_xFe_1.97Ni_0.03As₂, where nonlinear behaviors are observed near the nematic transition temperature. We show that it can be well explained by the Landau theory for the second-order phase transitions considering that the external field is not zero. The effect of the coupling between the isotropic and nematic channels is shown to be negligible. Moreover, our results suggest that the nature of the magnetic and nematic transitions in Sr_1-xBa_xFe₂As₂ is determined by the strength of the magnetic-elastic coupling.