Spin waves and Dirac magnons in a honeycomb-lattice zigzag antiferromagnet BaNi2(AsO4)2

The topological properties of massive and massless fermionic quasiparticles have been intensively inves-tigated over the past decade in topological materials without magnetism. Recently, the bosonic analogs of such quasiparticles arising from spin waves have been reported in a two-dimensional (2D) honeycomb-lattice ferromagnet/antiferromagnet and a 3D antiferromagnet. Here, we use time-of-flight inelastic neutron scattering to study spin waves of the S = 1 honeycomb-lattice antiferromagnet BaNi2(AsO4)(2), which has a zigzag antiferromagnetic (AFM) ground state identical to that of the Kitaev quantum spin liquid candidate alpha-RuCl3. We determine the magnetic exchange interactions in the zigzag AFM ordered phase, and show that spin waves in BaNi2(AsO4)(2) have symmetry-protected Dirac points inside the Brillouin zone boundary. These results provide a microscopic understanding of the zigzag AFM order and associated Dirac magnons in honeycomb-lattice magnets, and are also important for establishing the magnetic interactions in Kitaev quantum spin liquid candidates.