Origin of the insulating state in the Kitaev candidate Cu$_2$IrO$_3$

TL;DR

This paper investigates the origin of the insulating state in Cu$_2$IrO$_3$, revealing charge ordering and a complex magnetic lattice structure involving Ir and Cu ions that explain its insulating behavior.

Contribution

It proposes a microscopic mechanism involving charge ordering and lattice reorganization that accounts for the insulating state in Cu$_2$IrO$_3$, a Kitaev candidate material.

Findings

Charge ordering of Ir ions with Ir$^{4+}$ and Ir$^{3+}$ states.

Formation of interpenetrated triangular lattices of Ir$^{4+}$ and Cu$^{2+}$ ions.

A novel honeycomb Kitaev lattice with two magnetic ion types.

Abstract

Through a combination of crystal symmetry analysis and density functional theory calculations we unveil a possible microscopic origin of the unexpected insulating behavior reported in the honeycomb Kitaev material Cu$_2$IrO$_3$. Our study suggests that this material hosts an instability towards charge ordering of the Ir ions, with alternating magnetic Ir$^{4+}$ and non-magnetic Ir$^{3+}$ ions arranged on the honeycomb lattice. In this case, the next-nearest-neighbor interactions that couple magnetic Ir$^{4+}$ ions form an enlarged triangular lattice, instead of the expected honeycomb lattice. The magnetic Cu$^{2+}$ ions located at the centre of the iridium honeycomb voids also form a triangular lattice, and additionally contribute to the magnetization of the system. Together, the interpenetrated Ir$^{4+}$ and Cu$^{2+}$ triangular lattices present a novel type of honeycomb Kitaev lattice composed of two types of magnetic ions.