Nanoscale decoupling of electronic nematicity and structural anisotropy in FeSe thin films

Shrinkhala Sharma,Zheng Ren,Ilija Zeljkovic,He Zhao,Hong Li,Ziqiang Wang

doi:10.1038/s41467-020-20150-y

Abstract

In a material prone to a nematic instability, anisotropic strain in principle provides a preferred symmetry-breaking direction for the electronic nematic state to follow. This is consistent with experimental observations, where electronic nematicity and structural anisotropy typically appear hand-in-hand. In this work, we discover that electronic nematicity can be locally decoupled from the underlying structural anisotropy in strain-engineered iron-selenide (FeSe) thin films. We use heteroepitaxial molecular beam epitaxy to grow FeSe with a nanoscale network of modulations that give rise to spatially varying strain. We map local anisotropic strain by analyzing scanning tunneling microscopy topographs, and visualize electronic nematic domains from concomitant spectroscopic maps. While the domains form so that the energy of nemato-elastic coupling is minimized, we observe distinct regions where electronic nematic ordering fails to flip direction, even though the underlying structural anisotropy is locally reversed. The findings point towards a nanometer-scale stiffness of the nematic order parameter.

Highlights

In a material prone to a nematic instability, anisotropic strain in principle provides a preferred symmetry-breaking direction for the electronic nematic state to follow
FeSe presents an excellent playground to explore the interplay of electronic nematicity and symmetry breaking strain due to its structural simplicity and the absence of magnetic ordering that is present in many other Fe-based superconductors[29,30]
In our FeSe films, ranging from 3 to 6 monolayers in thickness, this distance between neighboring modulation lines is approximately 15–20 nm, consistent with the spacing determined from cross-sectional transmission electron microscopy[38] and roughly consistent with the expected value based on the lattice constant mismatch between FeSe and SrTiO3(001) (Supplementary Note 1)

Summary

Introduction

In a material prone to a nematic instability, anisotropic strain in principle provides a preferred symmetry-breaking direction for the electronic nematic state to follow. Out of the array of these experimental handles, anisotropic strain presents a unique tuning knob that can controllably break the symmetry of the lattice This can directly impact the overlap between inequivalent neighboring Fe–Fe orbitals, lifting the dxz and dyz orbital degeneracy, and in principle providing a preferred direction for the electronic nematicity to follow. While the domain size in FeSe single crystals is several micrometers[14,23], it is reduced to ~10 nm length scales in thin films[21,28] This possibly suggests that the substrate, which inevitably has a somewhat different lattice constant compared to the film, may play a role in the formation of smaller electronic nematic domains. We visualize the formation of electronic nematic domains around an underlying network of structural modulations in strained multilayer films of FeSe, and discover a de-coupling of the local antisymmetric strain and electronic nematic order

Methods

Results

Conclusion