  We studied the uniaxial-pressure dependence of the resistivity for
URu$_{2-x}$Fe$_x$Si$_2$ 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 found
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
non-zero 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.

Strongly Correlated Electrons