Sign reversal superconducting gaps revealed by phase referenced quasi-particle interference of impurity induced bound states in (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Zn$_y$Se

TL;DR

This study uses phase-referenced quasi-particle interference to reveal sign-reversal in superconducting gaps of (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Zn$_y$Se, demonstrating a direct method to probe the pairing symmetry in unconventional superconductors.

Contribution

It introduces a phase-referenced QPI technique to directly visualize the sign change of the superconducting gap on electron pockets, confirming the pairing is mediated by repulsive interactions.

Findings

Superconducting gaps are sign-reversed on the two electron pockets.

Phase-referenced QPI effectively visualizes gap sign changes.

Method applicable to data with multiple impurities.

Abstract

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.