A perspective on pnictide superconductors

TL;DR

This paper explores the complex superconducting behavior of pnictides, especially FeSe, highlighting the interplay of RVB and excitonic insulator physics, and compares it to other layered superconductors with similar properties.

Contribution

It introduces a unified perspective on pnictide superconductors, emphasizing the role of excitonic insulator physics and structural sensitivity, linking them to other layered superconductors.

Findings

Superconductivity involves RVB and excitonic insulator physics.

Structural and electronic properties are highly sensitive to Fe-X coordination.

Similar behaviors are observed in other layered, semimetallic superconductors.

Abstract

FeSe is employed as reference material to elucidate the observed high Tc superconducting behaviour of the related layered iron pnictides. The structural and ensuing semimetallic band structural forms are here rather unusual, with the resulting ground state details extremely sensitive to the precise shape of the Fe-X coordination unit. The superconductivity is presented as coming from a combination of Resonant Valence Bond (RVB) and Excitonic Insulator physics, and incorporating Boson-Fermion degeneracy. Although sourced in a very different fashion the latter leads to some similarites with the high temperature superconducting (HTSC) cuprates. The Excitonic Insulator behaviour sees SDW, CDW/PLD and superconductive instabilities all vie for ground state status. The conflict leads to a very sensitive and complex set of properties, frequently mirroring HTSC cuprate behaviour. The delicate balance between ground states is made particularly difficult to unravel by the micro-inhomogeneity of structural form which it can engender. It is pointed out that several other notable superconductors, layered in form, semimetallic with indirect overlap and possessing homopolar bonding, would look to fall into the same general category, beta-ZrNCl and MgB2 and the high-pressure forms of several elements, like sulphur, phosphorus, lithium and calcium, being cases in point.