Evidence for unidirectional nematic bond ordering in FeSe

TL;DR

This study reveals a unidirectional nematic bond order in FeSe, characterized by band separation without orbital splitting, challenging previous orbital degeneracy explanations for nematicity.

Contribution

The paper introduces the concept of unidirectional nematic bond ordering in FeSe, supported by high-resolution ARPES data and tight-binding calculations, offering a new understanding of nematic electronic structure.

Findings

Hole pocket shows fourfold-symmetry-breaking distortion below 90K.

Electron pockets do not require fourfold symmetry-breaking.

Band separation supports unidirectional nematic bond ordering.

Abstract

The lifting of $d_{xz}$-$d_{yz}$ orbital degeneracy is often considered a hallmark of the nematic phase of Fe-based superconductors, including FeSe, but its origin is not yet understood. Here we report a high resolution Angle-Resolved Photoemission Spectroscopy study of single crystals of FeSe, accounting for the photon-energy dependence and making a detailed analysis of the temperature dependence. We find that the hole pocket undergoes a fourfold-symmetry-breaking distortion in the nematic phase below 90~K, but in contrast the changes to the electron pockets do not require fourfold symmetry-breaking. Instead, there is an additional separation of the existing $d_{xy}$ and $d_{xz/yz}$ bands - which themselves are not split within resolution. These observations lead us to propose a new scenario of "unidirectional nematic bond ordering" to describe the low-temperature electronic structure of FeSe, supported by a good agreement with 10-orbital tight binding model calculations.