  Recently discovered A-Fe-Se (A - alkali metal) materials have questioned the
most popular theories of iron-based superconductors because of their unusual
electronic structure [1]. Controversial photoemission data taken in the
superconducting state [2-7] are in conflict with highly magnetic state seen by
neutron-, muSR-spectroscopies and transport/thermodynamic probes [8-10]. These
results lead to suggestions to consider all iron-based materials as originating
from Mott-insulators or semiconductors, thus once again raising the question of
close relation between the cuprates and Fe-based superconductors [e.g. 2]. Here
we study electronic and magnetic properties of Rb0.77Fe1.61Se2 (Tc = 32.6 K) in
normal and superconducting states by means of photoemission and muSR
spectroscopies as well as band structure calculations. We demonstrate that the
puzzling behavior of these novel materials is the result of separation into
metallic (~12%) and insulating (~ 88%) phases. Only the former becomes
superconducting and has a usual electronic structure of electron-doped
FeSe-slabs. Our results thus imply that the antiferromagnetic insulating phase
is just a byproduct of Rb-intercalation and its magnetic properties have hardly
any relation to the superconductivity. Instead, we find that also in this,
already third class of iron-based compounds, the key ingredient for
superconductivity is a certain proximity of a van Hove singularity to the Fermi
level. These findings set the direction for effective search of new
superconducting materials.
