Superconductivity at the Border of Electron Localization and Itinerancy

TL;DR

This paper compares different theories of superconductivity in iron-based materials, showing that strong electron interactions lead to similar pairing regardless of Fermi surface nesting, especially near the localization-itinerancy boundary.

Contribution

It demonstrates that strong-coupling pairing mechanisms are consistent across materials with different Fermi surfaces, highlighting the importance of the localization boundary.

Findings

Similar pairing amplitudes in different iron superconductors

Largest pairing occurs near the boundary of electron localization and itinerancy

Superconductivity may emerge in materials with specific electronic correlation characteristics

Abstract

The superconducting state of iron pnictides and chalcogenides exists at the border of antiferromagnetic order. Consequently, these materials could provide clues about the relationship between magnetism and unconventional superconductivity. One explanation, motivated by the so-called bad-metal behaviour of these materials, proposes that magnetism and superconductivity develop out of quasi-localized magnetic moments which are generated by strong electron-electron correlations. Another suggests that these phenomena are the result of weakly interacting electron states that lie on nested Fermi surfaces. Here we address the issue by comparing the newly discovered alkaline iron selenide superconductors, which exhibit no Fermi-surface nesting, to their iron pnictide counterparts. We show that the strong-coupling approach leads to similar pairing amplitudes in these materials, despite their different Fermi surfaces. We also find that the pairing amplitudes are largest at the boundary between electronic localization and itinerancy, suggesting that new superconductors might be found in materials with similar characteristics.