Observation of discrete conventional Caroli-de Gennes-Matricon states in the vortex core of single-layer FeSe/SrTiO3

TL;DR

This study used STM to observe discrete Caroli-de Gennes-Matricon states in the vortex cores of single-layer FeSe/SrTiO3, revealing s-wave pairing without sign change in this high-temperature superconductor.

Contribution

First direct observation of discrete CdGM states in single-layer FeSe/SrTiO3, confirming their energy dependence and indicating s-wave pairing symmetry.

Findings

Discrete CdGM states follow E = μΔ^2/E_F formula

Boundary spectra show fully gapped states without zero-bias peaks

Results support s-wave pairing without sign change

Abstract

Using low-temperature scanning tunneling microscopy (STM), we studied the vortex states of single-layer FeSe film on SrTiO3 (100) substrate, and the local behaviors of superconductivity at sample boundaries. We clearly observed multiple discrete Caroli-de Gennes-Matricon (CdGM) states in the vortex core, and quantitative analysis shows their energies well follow the formula: E = {\mu}{\Delta}^2/E_F, where {\mu} is a half integer and {\Delta} is the mean superconducting gap over the Fermi surface. Meanwhile, a fully gapped spectrum without states near zero bias is observed at [110](Fe) oriented boundary of 1 ML and 2 ML FeSe films, and atomic step edge of 1 ML FeSe. Accompanied with theoretical calculations, our results indicate a s-wave pairing without sign-change in the high-TC FeSe_SrTiO3 superconductor.