Neutron-diffraction study of the solid layers at the liquid-solid boundary in 4He films adsorbed on graphite

URL:http://link.aps.org/doi/10.1103/PhysRevB.24.1170 DOI:10.1103/PhysRevB.24.1170A 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.K. C. wishes to thank NATO for financial assistance. Work at Brookhaven was supported by the Division of Basic Energy Sciences, U. S. Department of Energy, under Contract No. DE-AC02-76CH00016. Work at the University of Missouri-Columbia was supported by the National Science Foundation under Grant No. DMR-7905958