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 AxFe2ySe2 (A ΒΌ 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 ironbased
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.
