In-Plane Anisotropic Response to Uniaxial Pressure in the Hidden Order State of URu_2Si_2

We investigate the uniaxial-pressure dependence of resistivity for URu_2-xFe_xSi_2 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 \zeta shows a seemingly divergent behavior above the transition temperature T_0 and a quick decrease below it. We find that the temperature dependence of \zeta for both samples can be well described by assuming the uniaxial pressure effect on the gap or certain energy scale except for \zeta_(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.