The dual nature of As-vacancies in LaFeAsO-derived superconductors: magnetic moment formation while preserving superconductivity

TL;DR

This paper reveals that arsenic vacancies in LaFeAsO superconductors create local magnetic moments without harming superconductivity, and may even enhance it by suppressing competing magnetic orders.

Contribution

It introduces a microscopic theory showing how As-vacancies induce magnetic moments while preserving or improving superconductivity in LaFeAsO-derived materials.

Findings

As-vacancies create ferromagnetic local moments.

V_As do not deteriorate superconductivity and may enhance it.

V_As suppress competing magnetic orders.

Abstract

As-vacancies (V_As) in La-1111-systems, which are nominally non-magnetic defects, are shown to create in their vicinity by symmetry ferromagnetically oriented local magnetic moments due to the strong, covalent bonds with neighboring Fe atoms that they break. From microscopic theory in terms of an appropriately modified Anderson-Wolff model, we find that the moment formation results in a substantially enhanced paramagnetic susceptibility in both the normal and superconducting (SC) state. Despite the V_As act as magnetic scatterers, they do not deteriorate SC properties which can even be improved by V_As by suppressing a competing or coexisting commensurate spin density wave or its remnant fluctuations. Due to the induced local magnetic moments an s_++-scenario in related systems is unlikely.