Spin fluctuations and unconventional pairing in KFe$_2$As$_2$

TL;DR

This study explores how spin fluctuations influence unconventional superconductivity in KFe$_2$As$_2$, revealing a close competition between s-wave and d-wave pairings driven by incommensurate spin fluctuations.

Contribution

The paper develops a multiorbital model and uses RPA to connect spin fluctuations with pairing symmetry, explaining experimental observations in KFe$_2$As$_2$.

Findings

Incommensurate spin fluctuations originate from interband interactions.

s-wave and d-wave pairings are nearly degenerate, with sign-changing s-wave driven by spin fluctuations.

Possible explanations for the superconducting gap nodes in KFe$_2$As$_2$ are proposed.

Abstract

We study the relation between the spin fluctuation and superconductivity in an heavily hole doped end material KFe$_2$As$_2$. We construct a five orbital model by approximately unfolding the Brillouin zone of the three dimensional ten orbital model obtained from first principles calculation. By applying the random phase approximation, we obtain the spin susceptibility and solve the linearized Eliashberg equation. The incommensurate spin fluctuation observed experimentally is understood as originating from interband interactions, where the multiorbital nature of the band structure results in an electron-hole asymmetry of the incommensurability in the whole iron-based superconductor family. As for superconductivity, s-wave and d-wave pairings are found to be in close competition, where the sign change in the gap function in the former is driven by the incommensurate spin fluctuations. We raise several possible explanations for the nodes in the superconducting gap of KFe$_2$As$_2$ observed experimentally.