FeSe and the missing electron pocket problem

TL;DR

This paper reviews recent experimental and theoretical efforts to understand the missing electron pocket in FeSe's Fermi surface, a key issue in unraveling nematicity and superconductivity in iron-based superconductors.

Contribution

It synthesizes experimental findings and theoretical models, emphasizing the role of the $d_{xy}$ orbital in explaining the missing electron pocket in FeSe.

Findings

Recent ARPES and STM experiments reveal the nematic Fermi surface structure.

Incorporating the $d_{xy}$ orbital improves theoretical explanations of the missing electron pocket.

The missing electron pocket impacts the understanding of superconductivity in FeSe.

Abstract

The nature and origin of electronic nematicity remains a significant challenge in our understanding of the iron-based superconductors. This is particularly evident in the iron chalcogenide, FeSe, where it is currently unclear how the experimentally determined Fermi surface near the M point evolves from having two electron pockets in the tetragonal state to exhibiting just a single electron pocket in the nematic state. This has posed a major theoretical challenge, which has become known as the missing electron pocket problem of FeSe, and is of central importance if we wish to uncover the secrets behind nematicity and superconductivity in the wider iron-based superconductors. Here, we review the recent experimental work uncovering this nematic Fermi surface of FeSe from both ARPES and STM measurements, as well as current theoretical attempts to explain this missing electron pocket of FeSe, with a particular focus on the emerging importance of incorporating the $d_{xy}$ orbital into theoretical descriptions of the nematic state. Furthermore, we will discuss the consequence this missing electron pocket has on the theoretical understanding of superconductivity in this system and present several remaining open questions and avenues for future research.