  By measuring the spatial distribution of differential conductance near
impurities on Fe sites, we have obtained the quasi-particle interference (QPI)
patterns in the (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Zn$_y$Se superconductor with only
electron Fermi surfaces. By taking the Fourier transform on these patterns, we
investigate the scattering features between the two circles of electron pockets
formed by folding or hybridization. We treat the data by using the recent
theoretical approach [arXiv:1710.09089] which is specially designed for the
impurity induced bound states. It is found that the superconducting gap sign is
reversed on the two electron pockets, which can be directly visualized by the
phase-referenced QPI technique, indicating that the Cooper pairing is induced
by the repulsive interaction. Our results further show that this method is also
applicable for data measured for multiple impurities, which provides an easy
and feasible way for detecting the gap function of unconventional
superconductors.
