We report a comprehensive spin wave analysis of the semiconducting honeycomb
van der Waal antiferromagnet NiPS3​. Using single crystal inelastic neutron
scattering, we map out the full Brillouin zone and fit the observed modes to a
spin wave model with rigorously defined uncertainty. We find that the third
neighbor exchange J3​ dominates the Hamiltonian, a feature which we fully
account for by ab-initio density functional theory calculations. We also
quantify the degree to which the three-fold rotation symmetry is broken and
account for the Q=0 excitations observed in other measurements, yielding a
spin exchange model which is consistent across multiple experimental probes. We
also identify a strongly reduced static ordered moment and reduced low-energy
intensity relative to the linear spin wave calculations, signaling unexplained
features in the magnetism which requires going beyond the linear spin wave
approximation.Comment: 7 pages, 8 figures; 5 pages and 6 additional figures of appendice