Atomic-scale imaging of electronic nematicity in ferropnictides

TL;DR

This study visualizes atomic-scale electronic nematicity in a ferropnictide superconductor using scanning tunneling microscopy, revealing its energy dependence, impurity sensitivity, and suppression by doping, which are crucial for understanding its role in superconductivity.

Contribution

First direct real-space visualization of atomic-scale electronic nematicity in a ferropnictide, linking it to energy-dependent orbital splitting and impurity effects.

Findings

Nematic order appears as 4a_Fe-spaced stripes within nano-domains.

Nematic order parameter changes sign around 30 meV energy.

Cobalt doping suppresses nematicity and enhances superconductivity.

Abstract

Electronic nematicity, a correlated state characterized by broken rotational symmetry, has been recognized as a ubiquitous feature intertwined with unconventional electron pairing in various iron-based superconductors. Here we employ spectroscopic-imaging scanning tunneling microscopy to visualize atomic-scale electronic nematicity directly on FeAs planes of a prototypical ferropnictide BaFe$_2$As$_2$. Spatially, the nematic order appears as 4$a_{\textrm{Fe}}$-spaced stripes ($a_{\textrm{Fe}} \sim $ 0.28 nm is the in-plane Fe-Fe distance) within homogeneously and orthogonally oriented nano-domains. The energy-resolved conductance maps reveal a pronounced energy-dependence of the nematic order parameter that experiences a sign change at approximately 30 meV. This characteristic behavior coincides with energy-dependent orbital splitting previously identified in momentum space, but is remarkably visualized in real space for the first time in our study. Moreover, the electronic nematicity exhibits pronounced sensitivity to single impurities and is notably suppressed by cobalt substitution for Fe atoms, promoting optimal superconductivity when nematic fluctuations are strongest. Our results provide pivotal experimental insights for developing a microscopic model of nematic order, thus paving the way to study its complex relationship with unconventional superconductivity.