<i>Colloquium</i>: The unexpected properties of alkali metal iron selenide superconductors

TLDR

Iron‑based superconductors with FeAs layers have been explained by weak‑U Fermi‑surface nesting of hole and electron pockets, supported by small magnetic moments and photoemission evidence. This review surveys the current experimental and theoretical status of alkali‑metal iron selenides, aiming to reassess our understanding of iron‑based superconductivity. Recent related work, including studies of selenide two‑leg ladders and the discovery of superconductivity in monolayer FeSe, is also discussed. Alkali‑metal iron selenides exhibit insulating antiferromagnetic ground states with large moments and lack hole pockets, challenging the nesting paradigm and revealing unresolved conceptual questions.

Abstract

The iron-based superconductors that contain FeAs layers as the fundamental building block in the crystal structures have been rationalized in the past using ideas based on the Fermi surface nesting of hole and electron pockets when in the presence of weak Hubbard $U$ interactions. This approach seemed appropriate considering the small values of the magnetic moments in the parent compounds and the clear evidence based on photoemission experiments of the required electron and hole pockets. However, recent results in the context of alkali metal iron selenides, with generic chemical composition ${A}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2}$ ($A=\mathrm{\text{alkali}}$ metal element), have challenged those previous ideas since at particular compositions $y$ the low-temperature ground states are insulating and display antiferromagnetic order with large iron magnetic moments. Moreover, angle-resolved photoemission studies have revealed the absence of hole pockets at the Fermi level in these materials. The present status of this exciting area of research, with the potential to alter conceptually our understanding of the iron-based superconductors, is here reviewed, covering both experimental and theoretical investigations. Other recent related developments are also briefly reviewed, such as the study of selenide two-leg ladders and the discovery of superconductivity in a single layer of FeSe. The conceptual issues considered established for the alkali metal iron selenides, as well as several issues that still require further work, are discussed.