Direct Evidence for Dominant Bond-directional Interactions in a Honeycomb Lattice Iridate Na2IrO3

TL;DR

This paper provides direct experimental evidence of dominant bond-directional interactions in Na2IrO3, revealing their role in magnetic frustration and supporting the realization of Kitaev physics in honeycomb iridates.

Contribution

It offers the first direct evidence of bond-directional interactions in Na2IrO3, linking them to magnetic frustration and Kitaev physics.

Findings

Diffuse magnetic x-ray scattering shows broken spin-rotational symmetry.

Three spin components exhibit nano-scale correlations along different directions.

Bond-directional interactions lead to magnetic frustration in Na2IrO3.

Abstract

Heisenberg interactions are ubiquitous in magnetic materials and have been prevailing in modeling and designing quantum magnets. Bond-directional interactions offer a novel alternative to Heisenberg exchange and provide the building blocks of the Kitaev model, which has a quantum spin liquid (QSL) as its exact ground state. Honeycomb iridates, A2IrO3 (A=Na,Li), offer potential realizations of the Kitaev model, and their reported magnetic behaviors may be interpreted within the Kitaev framework. However, the extent of their relevance to the Kitaev model remains unclear, as evidence for bond-directional interactions remains indirect or conjectural. Here, we present direct evidence for dominant bond-directional interactions in antiferromagnetic Na2IrO3 and show that they lead to strong magnetic frustration. Diffuse magnetic x-ray scattering reveals broken spin-rotational symmetry even above Neel temperature, with the three spin components exhibiting nano-scale correlations along distinct crystallographic directions. This spin-space and real-space entanglement directly manifests the bond-directional interactions, provides the missing link to Kitaev physics in honeycomb iridates, and establishes a new design strategy toward frustrated magnetism.