Correlation effects in the iron pnictides

TL;DR

This paper investigates the electron correlation strength in iron pnictides, suggesting they are near the boundary between itinerant and localized electrons, which influences their magnetic properties and quantum critical behavior.

Contribution

It introduces a controlled theoretical framework based on the fraction of coherent spectral weight to analyze correlations and magnetic phenomena in iron pnictides.

Findings

Parent compounds are metallic with strong correlations near Mott physics.

The system is closer to intermediate coupling than simple antiferromagnetic metals.

A multiband t-J1-J2 model is proposed for doped iron pnictides.

Abstract

One of the central questions about the iron pnictides concerns the extent to which their electrons are strongly correlated. Here we address this issue through the phenomenology of the charge transport and dynamics, single-electron excitation spectrum, and magnetic ordering and dynamics. We outline the evidence that the parent compounds, while metallic, have electron interactions that are sufficiently strong to produce incipient Mott physics. In other words, in terms of the strength of electron correlations compared to the kinetic energy, the iron pnictides are closer to intermediately-coupled systems lying at the boundary between itinerancy and localization, such as V2O3 or Se-doped NiS2, rather than to simple antiferromagnetic metals like Cr. This level of electronic correlations produces a new small parameter for controlled theoretical analyses, namely the fraction of the single-electron spectral weight that lies in the coherent part of the excitation spectrum. Using this expansion parameter, we construct the effective low-energy Hamiltonian and discuss its implications for the magnetic order and magnetic quantum criticality. Finally, this approach sharpens the notion of magnetic frustration for such a metallic system, and brings about a multiband matrix t-J1-J2 model for the carrier-doped iron pnictides.