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

Yonghao Yuan,Wei Li,Xuemin Fan,Ke He,Qi-Kun Xue,Yan Zhang,Xintong Wang

doi:10.1038/s41467-021-22516-2

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.

Highlights

Superconductivity is significantly enhanced in monolayer FeSe grown on SrTiO3, but not for multilayer films, in which large strength of nematicity develops
Recent studies show that the optimal doping levels of the high-transition temperature superconductors usually correspond to the nematic quantum critical points (QCP), at which the nematic fluctuations are optimized[13,29]
The domains are separated by smectic domain walls, which manifest themselves as white wrinkles with higher differential conductance (Fig. 1b)

Summary

Introduction

Superconductivity is significantly enhanced in monolayer FeSe grown on SrTiO3, but not for multilayer films, in which large strength of nematicity develops. The nematic transition temperature increases from 120 to 170 K when the thickness of the film decreases from 30 to 2 UC21,24,39, much higher than its bulk value 90 K In these multilayer FeSe/STO, besides the enhanced strength of nematicity, short-range stripes, which further breaks translational symmetry, are observed in the vicinity of defects[21], indicating rather strong smectic fluctuation as well as electronic correlation. The long-range ordering degenerates to short-range smectic fluctuations in 3 UC and thicker FeSe films, but vanishes in 1 UC FeSe, indicating the occurrence of an abrupt phase transition Such smectic instability may provide additional superconductivity enhancement in 1 UC FeSe films

Methods

Results

Conclusion