"Cigar" Fermi surface as a possible requisite for superconductivity in iron-based superconductors

TL;DR

This study reveals that a specific electronic feature, the 'Cigar' Fermi surface, is likely essential for superconductivity in iron-based superconductors, emphasizing the importance of electronic structure proximity to a van Hove singularity.

Contribution

It demonstrates that superconductivity in Rb-intercalated FeSe compounds is linked to a particular electronic structure feature, providing new insights into the mechanisms of iron-based superconductivity.

Findings

Superconductivity arises from a metallic phase with a typical electronic structure.

Insulating magnetic phase is unrelated to superconductivity.

Proximity of a van Hove singularity to the Fermi level is crucial for superconductivity.

Abstract

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.