Quantum critical phase of FeO spans conditions of Earth's lower mantle

TL;DR

This study reveals that FeO in Earth's lower mantle exists in a quantum critical state with a gradual insulator-metal transition, affecting Earth's electromagnetic properties and magnetic field behavior.

Contribution

It provides the first systematic theoretical exploration of FeO's electronic structure at extreme conditions, identifying a quantum critical state relevant to Earth's interior.

Findings

FeO undergoes a gradual orbitally selective insulator-metal transition at high P-T conditions.

FeO in the lower mantle exists in a quantum critical state with moderate electrical conductivity.

The electronic state of FeO influences electromagnetic interactions between Earth's mantle and core.

Abstract

Earth's interior consists primarily of an insulating rocky mantle and a metallic iron-dominant core. Recent work has shown that mountain-scale structures at the core-mantle boundary may be highly enriched in FeO reported to exhibit high conductivity and metallic behavior at extreme pressure-temperature (P-T) conditions. However, the underlying electronic processes in FeO remain poorly understood and controversial. Here we systematically explore the electronic structure of B1-FeO at extreme conditions with large-scale theoretical modeling using state-of-the-art embedded dynamical mean field theory (eDMFT). Fine sampling of the phase diagram at more than 350 volume-temperature conditions reveals that, instead of sharp metallization, compression of FeO at high temperatures induces a gradual orbitally selective insulator-metal transition. Specifically, at P-T conditions of the lower mantle, FeO exists in an intermediate "quantum critical" state, characteristic of strongly correlated electronic matter. Transport in this regime, distinct from insulating or metallic behavior, is marked by incoherent diffusion of electrons in the conducting t_{2g} orbital and a band gap in the e_g orbital, resulting in moderate electrical conductivity (~ 10^5 S/m) with modest P -T dependence as observed in experiments. FeO-rich regions in Earth's lowermost mantle could thus influence electromagnetic interactions between the mantle and the core, producing several features observed in Earth's rotation and magnetic field evolution.