Electronic Structure of Pyrochlore Iridates: From Topological Dirac Metal to Mott Insulator

TL;DR

This paper explores the electronic phases of pyrochlore iridates, revealing a transition from a Dirac semi-metal to a Mott insulator driven by electron correlation strength, with topological surface states and potential axion insulator phases.

Contribution

It provides a comprehensive theoretical analysis of the phase diagram of pyrochlore iridates, identifying a Dirac semi-metal phase with topological properties as a key ground state.

Findings

Discovery of a Dirac semi-metal phase with Fermi arc surface states.

Identification of a transition from magnetic metal to Mott insulator with increasing U.

Prediction of a narrow axion insulator phase at intermediate U.

Abstract

In 5d transition metal oxides such as the iridates, novel properties arise from the interplay of electron correlations and spin-orbit interactions. We investigate the electronic structure of the pyrochlore iridates, (such as Y$_{2}$Ir$_{2}$O$_{7}$) using density functional theory, LDA+U method, and effective low energy models. A remarkably rich phase diagram emerges on tuning the correlation strength U. The Ir magnetic moment are always found to be non-collinearly ordered. However, the ground state changes from a magnetic metal at weak U, to a Mott insulator at large U. Most interestingly, the intermediate U regime is found to be a Dirac semi-metal, with vanishing density of states at the Fermi energy. It also exhibits topological properties - manifested by special surface states in the form of Fermi arcs, that connect the bulk Dirac points. This Dirac phase, a three dimensional analog of graphene, is proposed as the ground state of Y$_{2}$Ir$_{2}$O$_{7}$ and related compounds. A narrow window of magnetic `axion' insulator, with axion parameter $\theta=\pi$, may also be present at intermediate U. An applied magnetic field induces ferromagnetic order and a metallic ground state.