Lifting of gap nodes by disorder in ultranodal superconductor candidate FeSe1-xSx

TL;DR

This study explores how controlled disorder affects the ultranodal superconducting state in FeSe$_{1-x}$S$_x$, revealing that impurity can lift gap nodes and alter low-energy excitations, challenging the topological protection of Bogoliubov Fermi surfaces.

Contribution

It provides experimental evidence that disorder can lift gap nodes in ultranodal FeSe$_{1-x}$S$_x$, showing the fragile nature of BFSs in this superconductor.

Findings

Disorder causes nonmonotonic changes in low-energy excitations.

Impurities can lift gap nodes, turning ultranodal states into nodeless or gapless states.

BFSs in FeSe$_{1-x}$S$_x$ are sensitive to disorder, indicating their accidental nature.

Abstract

The observation of time-reversal symmetry breaking and large residual density of states in tetragonal FeSe$_{1-x}$S$_x$ suggests a novel type of ultranodal superconducting state with Bogoliubov Fermi surfaces (BFSs). Although such BFSs in centrosymmetric superconductors are expected to be topologically protected, the impurity effect of this exotic superconducting state remains elusive experimentally. Here, we investigate the impact of controlled defects introduced by electron irradiation on the superconducting state of tetragonal FeSe$_{1-x}$S$_x$ ($0.18\le x\le 0.25$). The temperature dependence of magnetic penetration depth is initially consistent with a model with BFSs in the pristine sample. After irradiation, we observe a nonmonotonic evolution of low-energy excitations with impurity concentrations. This nonmonotonic change indicates a transition from nodal to nodeless, culminating in gapless with Andreev bound states, reminiscent of the nodal $s_\pm$ case. This points to the accidental nature of the possible BFSs in tetragonal FeSe$_{1-x}$S$_x$, which are susceptible to disruption by the disorder.