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A neutron scattering study of the structure of 4He films adsorbed on graphite is reported. Diffraction from helium monolayers at a temperature of 1.2 K shows the formation of an incommensurate, triangular-lattice solid of high density. As the coverage is increased above two layers, the diffraction pattern changes indicating solidification of a second layer. The observed two-layer patterns can be indexed with either a pair of incommensurate, triangular-lattice solid layers of different densities or a close-packed bilayer; the experimental information available is not sufficient to make a more precise identification. A measurement of the height of the first helium layer above the graphite basal plane was also made. This was done by determining the coverage-dependent shift in the position of the graphite (002) diffraction peak (assumed to arise from interference between film and substrate scattering) and fitting it to a simple structural model. Values for the monolayer height above the graphite plane and for the lattice constants of the possible bilayer structures are given.

The elementary excitations in superfluid helium have been studied in the wave-vector range 0 1.5 &Q & 2. 3 A by inelastic neutron scattering. The single-excitation scattering function g& (Q, her) was measured for pressures between 1 atm and the solidification pressure and for temperatures from 1. 3'K to above the transition temperature T&. The dispersion curves were fitted to parabolas and the appropriate Landau parameters tabulated vs temperature and pressure. By accounting for the effect of the instrumental resolution it was possible to observe a line broadening at low temperatures which sets in when the slope of the dispersion curve equals the sound velocity. There are strong indications that this is due to the rotonphonon interaction proposed by Pitaevskii. The. line broadening observed when the temperature approaches Tz is well accounted for by the roton-roton interaction as calculated by Landau and Khalatnikov.The results have been related through thermodynamics to other properties of the liquid and the spectral form of the excitations compared to those observed in magnetic systems close to the critical temperature.

The wavelength and frequency dependence of neutrons magnetically scattered from iron has been studied with high resolution from low temperatures to 1.057V At low temperatures, the spin waves can be satisfactorily described in terms of a model Hamiltonian containing Heisenberg and dipole-dipole terms. The spin-wave energies vary as 1 -T/T c to the power 0.37±0.03 over a temperature range 0.005< 1 -T/T c <0.2. At a temperature a few degrees below T c , the spin waves become over-critically damped. In the spin-wave region, no peak corresponding to a diffusive mode has been observed in the scattering, in contrast to the antiferromagnet RbMnF 3 , where the existence of such a peak has been clearly demonstrated. Above T c , in the hydrodynamic region, the scattering can be described in terms of a diffusion equation. The diffusion constant and static susceptibility vary as 1 -T c /T to the powers 0.14±0.04 and 1.30±0.06, respectively, over the temperature range 0.008 <1 -T C /T<0.05. The observed power laws indicate that near the Curie temperature the spin-wave energies vary with temperature in the same way as the magnetization, while the diffusion constant varies more slowly with temperature than has been predicted. In agreement with earlier measurements, the data for the static susceptibility indicate that the power law of the divergence is close to 1.30. Values in the range 1.33-1.43 are predicted by high-temperature expansion techniques. There is evidence for the existence of a strongly damped propagating mode in the transition region at and above T c . At the critical temperature, linewidths scale as the wave vector to the power 2.7=b0.3; within error, this is the same as the power 2.5 predicted on the basis of the dynamic scaling laws. * Work performed under the auspices of the U. S. Atomic

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