Dirac Magnons in a Honeycomb Lattice Quantum XY Magnet CoTiO3

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 crystalCrI3, and in a 3D Heisenberg magnetCu3TeO6. Here, we report our inelastic neutron scattering investigation on a large single crystal of a stacked honeycomb lattice magnetCoTiO3, which is part of a broad family of ilmenite materials. The magnetically ordered ground state ofCoTiO3features ferromagnetic layers ofCo2+, stacked antiferromagnetically along thecaxis. The magnon dispersion relation is described very well with a simple magnetic Hamiltonian with strong easy-plane exchange anisotropy. Importantly, a magnon Dirac cone is found along the edge of the 3D Brillouin zone. Our results establishCoTiO3as a model pseudospin-1/2material to study interacting Dirac bosons in a 3D quantumXYmagnet.