First-principles study on cubic pyrochlore iridates Y2Ir2O7 and Pr2Ir2O7

TL;DR

This study uses first-principles calculations to explore the electronic and magnetic phases of pyrochlore iridates Y2Ir2O7 and Pr2Ir2O7, revealing how Coulomb interactions and structural parameters influence their topological and magnetic properties.

Contribution

It provides a detailed first-principles analysis of the magnetic and topological phase transitions in Y2Ir2O7 and Pr2Ir2O7, highlighting the effects of Coulomb repulsion and structural distortion.

Findings

Y2Ir2O7 becomes a trivial AF insulator at U=1.3 eV

Pr2Ir2O7 remains a paramagnetic semimetal with low carrier density

Structural changes induce a transition to a Z2 topological insulator in Pr2Ir2O7

Abstract

Fully relativistic first-principles electronic structure calculations based on a noncollinear local spin density approximation (LSDA) are performed for pyrochlore iridates Y$_2$Ir$_2$O$_7$ and Pr$_2$Ir$_2$O$_7$. The all-in, all-out antiferromagnetic (AF) order is stablized by the on-site Coulomb repulsion $U>U_c$ in the LSDA+$U$ scheme, with $U_c\sim1.1$~eV and 1.3~eV for Y$_2$Ir$_2$O$_7$ and Pr$_2$Ir$_2$O$_7$, respectively. AF semimetals with and without Weyl points and then a topologically trivial AF insulator successively appear with further increasing $U$. For $U=1.3$~eV, Y$_2$Ir$_2$O$_7$ is a topologically trivial narrow-gap AF insulator having an ordered local magnetic moment $\sim0.5\mu_B$/Ir, while Pr$_2$Ir$_2$O$_7$ is barely a paramagnetic semimetal with electron and hole concentrations of $0.016$/Ir, in overall agreements with experiments. With decreasing oxygen position parameter $x$ describing the trigonal compression of IrO$_6$ octahedra, Pr$_2$Ir$_2$O$_7$ is driven through a non-Fermi-liquid semimetal having only an isolated Fermi point of $\Gamma_8^+$, showing a quadratic band touching, to a $Z_2$ topological insulator.