Bond-dependent anisotropy and magnon decay in cobalt-based Kitaev triangular antiferromagnet

TL;DR

This paper investigates a cobalt-based triangular antiferromagnet CoI2, revealing how bond-dependent anisotropy influences complex spin dynamics, magnon decay, and spiral magnetic order, advancing understanding of frustrated quantum magnetic systems.

Contribution

It demonstrates the role of bond-dependent anisotropy in stabilizing magnetic phases and causing magnon breakdown in a geometrically frustrated triangular lattice.

Findings

Magnon breakdown observed via inelastic neutron scattering.

Bond-dependent anisotropy causes spiral magnetic order.

Complex level repulsion in spin excitations.

Abstract

The Kitaev model, a honeycomb network of spins with bond-dependent anisotropic interactions, is a rare example of having a quantum spin liquid ground state. Although most Kitaev model candidate materials eventually order magnetically due to inevitable non-Kitaev terms, their bond-dependent anisotropy manifests in unusual spin dynamics. It has recently been suggested that bond-dependent anisotropy can stabilise novel magnetic phases and exotic spin dynamics on the geometrically frustrated triangular lattice. However, few materials have been identified with simultaneous geometric frustration and bond-dependent anisotropy. Here, we report a frustrated triangular lattice with bond-dependent anisotropy in the cobalt-based triangular van der Waals antiferromagnet CoI2. Its momentum and energy-resolved spin dynamics exhibit substantial magnon breakdown and complex level repulsion, as measured by inelastic neutron scattering. A thorough examination of excitations in both the paramagnetic and magnetically ordered states reveals that the bond-dependent anisotropy is the origin of the spiral order and the magnon breakdown found in CoI2. Our result paves the way toward a new research direction for the Kitaev model with geometrical frustration.