  The topological properties of massive and massless fermionic quasiparticles
have been intensively investigated over the past decade in topological
materials without magnetism. Recently, the bosonic analogs of such
quasiparticles arising from spin waves have been reported in the
two-dimensional (2D) honeycomb lattice ferromagnet/antiferromagnet and the 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 zig-zag antiferromagnetic (AF) ground state identical to that of
the Kitaev quantum spin liquid candidate alpha-RuCl3. We determine the magnetic
exchange interactions in the zig-zag AF 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 zig-zag AF
order and associated Dirac magnons in honeycomb lattice magnets, and are also
important for establishing the magnetic interactions in Kitaev quantum spin
liquid candidates.
