Dichotomy of superconductivity between monolayer FeS and FeSe

TL;DR

This study compares the electronic structures of monolayer FeS and FeSe on SrTiO3, revealing that despite similarities, monolayer FeS does not exhibit high-temperature superconductivity, highlighting the importance of interfacial effects.

Contribution

The paper provides the first detailed band structure analysis of monolayer FeS on SrTiO3, elucidating why high Tc superconductivity is absent in FeS despite similarities to FeSe.

Findings

Monolayer FeS shows similar electronic structure to FeSe, including electron doping and electron-phonon coupling.

High Tc is absent in monolayer FeS, linked to weak superconducting pairing.

Interfacial electron-phonon coupling enhances Tc only when combined with intrinsic pairing interactions.

Abstract

The discovery of high-temperature (Tc) superconductivity in monolayer FeSe on SrTiO3 raised a fundamental question whether high Tc is commonly realized in monolayer iron-based superconductors. Tetragonal FeS is a key material to resolve this issue because bulk FeS is a superconductor with Tc comparable to that of isostructural FeSe. However, difficulty in synthesizing tetragonal monolayer FeS due to its metastable nature has hindered further investigations. Here we report elucidation of band structure of monolayer FeS on SrTiO3, enabled by a unique combination of in-situ topotactic reaction and molecular-beam epitaxy. Our angle-resolved photoemission spectroscopy on FeS and FeSe revealed marked similarities in the electronic structure, such as heavy electron doping and interfacial electron-phonon coupling, both of which have been regarded as possible sources of high Tc in FeSe. However, surprisingly, high-Tc superconductivity is absent in monolayer FeS. This is linked to the weak superconducting pairing in electron-doped multilayer FeS in which the interfacial effects are absent. Our results strongly suggest that the cross-interface electron-phonon coupling enhances Tc only when it cooperates with the pairing interaction inherent to the superconducting layer. This finding provides a key insight to explore new heterointerface high-Tc superconductors.