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

TLDR

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 pyrochlore iridates, such as Y₂Ir₂O₇. We use density functional theory, LDA+U, and effective low‑energy models. The calculations reveal a rich phase diagram: a magnetic metal at weak U, a Mott insulator at strong U, and an intermediate Dirac semi‑metal with vanishing density of states, topological Fermi‑arc surface states, and a narrow axion‑insulator window that becomes ferromagnetic and metallic under an applied magnetic field.

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.