The double life of electrons in magnetic iron pnictides, as revealed by NMR

TL;DR

This paper introduces a two-fluid phenomenological model to explain magnetic excitations in Fe pnictides, successfully fitting NMR data and providing insights into the limitations of low-energy spin fluctuations in accounting for magnetic moments.

Contribution

It proposes a novel two-fluid approach to describe magnetic excitations in Fe pnictides, linking phenomenology with first-principles calculations and experimental data.

Findings

The two-fluid model fits NMR data well.

Low-energy spin fluctuations cannot fully explain the magnetic moment discrepancy.

The phenomenology aligns with LSDA and spin density wave calculations.

Abstract

We present a phenomenological, two-fluid approach to understanding the magnetic excitations in Fe pnictides, in which a paramagnetic fluid with gapless, incoherent particle-hole excitations coexists with an antiferromagnetic fluid with gapped, coherent spin wave excitations. We show that this two-fluid phenomenology provides an excellent quantitative description of NMR data for magnetic "122" pnictides, and argue that it finds a natural justification in LSDA and spin density wave calculations. We further use this phenomenology to estimate the maximum renormalisation of the ordered moment that can follow from low-energy spin fluctuations in Fe pnictides. We find that this is too small to account for the discrepancy between ab intio calculations and neutron scattering measurements.