Plain s-wave superconductivity in single-layer FeSe on SrTiO3 probed by scanning tunneling microscopy

TL;DR

This study uses low-temperature STM to demonstrate that single-layer FeSe on SrTiO3 exhibits plain s-wave superconductivity with a fully gapped spectrum and impurity response consistent with conventional s-wave pairing.

Contribution

The paper provides direct experimental evidence of plain s-wave pairing symmetry in single-layer FeSe/SrTiO3 using STM and impurity analysis.

Findings

Fully gapped tunneling spectrum observed

Magnetic impurities suppress superconductivity

Non-magnetic impurities do not affect superconductivity

Abstract

Single-layer FeSe film on SrTiO3(001) was recently found to be the champion of interfacial superconducting systems, with a much enhanced superconductivity than the bulk iron-based superconductors. Its superconducting mechanism is of great interest. Although the film has a simple Fermi surface topology, its pairing symmetry is unsettled. Here by using low-temperature scanning tunneling microscopy (STM), we systematically investigated the superconductivity of single-layer FeSe/SrTiO3(001) films. We observed fully gapped tunneling spectrum and magnetic vortex lattice in the film. Quasi-particle interference (QPI) patterns reveal scatterings between and within the electron pockets, and put constraints on possible pairing symmetries. By introducing impurity atoms onto the sample, we show that the magnetic impurities (Cr, Mn) can locally suppress the superconductivity but the non-magnetic impurities (Zn, Ag and K) cannot. Our results indicate that single-layer FeSe/SrTiO3 has a plain s-wave paring symmetry whose order parameter has the same phase on all Fermi surface sections.