Identifying $s$-wave pairing symmetry in single-layer FeSe from topologically trivial edge states

TL;DR

This study uses scanning tunneling microscopy and theoretical analysis to provide evidence that single-layer FeSe on SrTiO3 exhibits sign-preserving s-wave pairing symmetry, crucial for understanding its high-temperature superconductivity.

Contribution

The paper offers a topological perspective and experimental evidence supporting sign-preserving s-wave pairing in single-layer FeSe, clarifying a key controversy in its superconducting mechanism.

Findings

Absence of topologically non-trivial edge/corner modes in spectra

Full superconducting gap observed at edges

Spectroscopic features consistent with s-wave pairing

Abstract

Determining the pairing symmetry of single-layer FeSe on SrTiO$_3$ is the key to understanding the enhanced pairing mechanism; furthermore, it guides exploring new superconductors with high transition temperatures ($T_\mathrm{c}$). Despite significant efforts, it remains controversial whether the symmetry is the sign-preserving $s$- or the sign-changed $s_{\pm}$-wave. Here, we investigate the pairing symmetry of single-layer FeSe from a topological point of view. Using low-temperature scanning tunneling microscopy/spectroscopy, we have systematically characterized the superconducting states at isolated edges and corners of single-layer FeSe. The tunneling spectra collected at edges and corners exhibit full gap and substantial dip, respectively, demonstrating the absence of topologically non-trivial edge/corner modes. According to the theoretical calculation, these spectroscopic features are strong evidence for the sign-preserving $s$-wave pairing.