Non-trivial role of interlayer cation states in iron-based superconductors

TL;DR

This paper investigates why certain iron germanide compounds do not exhibit high-temperature superconductivity, revealing that electronic structure changes and ferromagnetic tendencies suppress superconductivity despite structural similarities to known superconductors.

Contribution

It demonstrates that superconductivity suppression in iron germanides is due to electronic structure modifications and ferromagnetic tendencies, challenging the adequacy of simple $d$ or $dp$ models.

Findings

Superconductivity in iron germanides is suppressed by ferromagnetic tendencies.

Electronic structure changes upon atom substitution influence superconductivity.

Superconductivity cannot be fully explained by simple $d$ or $dp$ models.

Abstract

Unconventional superconductivity in iron pnictides and chalcogenides has been suggested to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology of the Fermi surface in these materials. Based on this premise, one would also expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors. As a matter of fact, they, however, superconduct at very low temperatures or not at all. In this work we establish that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies, which surprisingly do not originate from changes in bond-angles or -distances with respect to iron pnictides and chalcogenides, but are due to changes in the electronic structure in a wide range of energies happening upon substitution of atom species (As by Ge and the corresponding spacer cations). Our results indicate that superconductivity in iron-based materials may not always be fully understood based on $d$ or $dp$ model Hamiltonians only.