In-Plane Anisotropic Response to Uniaxial Pressure in the Hidden Order State of URu2Si2

We investigate the uniaxial-pressure dependence of resistivity for URu_2-x Fe _x Si₂ samples with x = 0 and 0.2, which host a hidden order (HO) and a large-moment antiferromagnetic (LMAFM) phase, respectively. For both samples, the elastoresistivity ζ shows a seemingly divergent behavior above the transition temperature T ₀ and a quick decrease below it. We find that the temperature dependence of ζ for both samples can be well described by assuming the uniaxial pressure effect on the gap or certain energy scale except for ζ ₍₁₁₀₎ of the x = 0 sample, which exhibits a nonzero residual value at 0 K. We show that this provides a qualitative difference between the HO and LMAFM phases. Our results suggest that there is an in-plane anisotropic response to the uniaxial pressure that only exists in the hidden order state without necessarily breaking the rotational lattice symmetry.