Incommensurate smectic phase in close proximity to the high-Tc superconductor FeSe/SrTiO3

TL;DR

This study reveals an incommensurate smectic phase in bilayer FeSe films on SrTiO3, which diminishes with increasing thickness, and suggests a link between this phase and high-temperature superconductivity in monolayer FeSe.

Contribution

It uncovers the presence of a smectic phase in bilayer FeSe and its abrupt disappearance in monolayer, linking electronic phase transitions to superconductivity enhancement.

Findings

Smectic phase appears only in bilayer FeSe.

Smectic order diminishes with increasing layer thickness.

Doping suppresses smectic phase and promotes superconductivity.

Abstract

Superconductivity is significantly enhanced in monolayer FeSe grown on SrTiO3, but not for multilayer films, in which large strength of nematicity develops. However, the link between the high-transition temperature superconductivity in monolayer and the correlation related nematicity in multilayer FeSe films is not well understood. Here, we use low-temperature scanning tunneling microscopy to study few-layer FeSe thin films grown by molecular beam epitaxy. We observe an incommensurate long-range smectic phase, which solely appears in bilayer FeSe films. The smectic order still locally exists and gradually fades away with increasing film thickness, while it suddenly vanishes in monolayer FeSe, indicative of an abrupt smectic phase transition. Surface alkali-metal doping can suppress the smectic phase and induce high-Tc superconductivity in bilayer FeSe. Our observations provide evidence that the monolayer FeSe is in close proximity to the smectic phase, and its superconductivity is likely enhanced by this electronic instability as well.