Dirac Magnons in a Honeycomb Lattice Quantum XY Magnet CoTiO_{3}

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 our inelastic neutron scattering investigation on a large single crystal 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. 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 establish CoTiO_{3} as a model pseudospin-1/2 material to study interacting Dirac bosons in a 3D quantum XY magnet