Dirac magnons in a honeycomb lattice quantum XY magnet CoTiO3

Abstract

The discovery of massless Dirac electrons in graphene and topological Dirac-Weyl materials has prompted a broad search for bosonic analogues of such Dirac particles. Recent experiments have found evidence for Dirac magnons above an Ising-like ferromagnetic ground state in a two-dimensional (2D) kagome lattice magnet and in the van der Waals layered honeycomb crystal CrI${}_3$, and in a 3D Heisenberg magnet Cu${}_3$TeO${}_6$. Here we report on our inelastic neutron scattering investigation on large single crystals of a stacked honeycomb lattice magnet CoTiO${}_3$, which is part of a broad family of ilmenite materials. The magnetically ordered ground state of CoTiO${}_3$ features ferromagnetic layers of Co${}^{2+}$, stacked antiferromagnetically along the $c$-axis. We discover that the magnon dispersion relation exhibits strong easy-plane exchange anisotropy and hosts a clear gapless Dirac cone along the edge of the 3D Brillouin zone. Our results establish CoTiO${}_3$ as a model pseudospin-$1/2$ material to study interacting Dirac bosons in a 3D quantum XY magnet.

7 pages, 5 figures; Supplemental Materials Available upon request