Neutron inelastic scattering has been used to measure the magnetic excitations in powdered NiPS3, a quasi-two dimensional antiferromagnet with spin S = 1 on a honeycomb lattice. The spectra show clear, dispersive magnons with a ∼ 7 meV gap at the Brillouin zone center. The data were fitted using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb planes. Magnetic exchange interactions up to the third intraplanar nearest-neighbour were required. The fits show robustly that NiPS3 has an easy axis anisotropy with ∆ = 0.3 meV and that the third nearest-neighbour has a strong antiferromagnetic exchange of J3 = −6.90 meV. The data can be fitted reasonably well with either J1 < 0 or J1 > 0, however the best quantitative agreement with high-resolution data indicate that the nearestneighbour interaction is ferromagnetic with J1 = 1.9 meV and that the second nearest-neighbour exchange is small and antiferromagnetic with J2 = −0.1 meV. The dispersion has a minimum in the Brillouin zone corner that is slightly larger than that at the Brillouin zone center, indicating that the magnetic structure of NiPS3 is close to being unstable.

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The ion dynamics of liquid mercury was investigated by inelastic neutron scattering. By exploiting an optimized high-resolution ( approximately 1 meV) experimental configuration, the dynamic response function was accurately measured. Collective excitations extending up to 0.6 A(-1) were observed with an associated velocity of 2100+/-80 m/s. This value is notably greater than the sound velocity, but it is provided by a simple Bohm-Staver calculation. The latter finding emphasizes those electron-related features in the ion dynamics, which are common to systems as different as polyvalent and alkali metals.

The microscopic ion dynamics of liquid gallium was investigated at 320 K-that is, just above the melting point-and 970 K by inelastic neutron scattering experiments and molecular dynamics simulations. The high quality of the experimental data allowed the observation of density fluctuation modes extending up to 1.0 Å −1 and existing at both temperatures. At melting, an acousticlike mode propagating with a velocity definitely exceeding the sound velocity was observed, in agreement with the results of a recent inelastic x-ray scattering experiment. The mode velocity and damping were found to be almost temperature independent. The experimental response function was compared with the results of a molecular dynamics simulation, based on a simple model for the effective ion-ion potential which, however, did not contain any temperature-dependent parameter. The result worth noting is that, despite the simple potential, the simulation was capable to reproduce all the observed features of the measured dynamicstructure factor quantitatively and at both the temperatures.