Neutron-diffraction study of the solid layers at the liquid-solid boundary inHe4films adsorbed on graphite

Abstract

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.

Phase diagram ofH4eadsorbed on graphite

“…9 For the first adsorbed helium layer, there exist preferred adsorption sites, located above the centers of the hexagons formed by the carbon atoms on the graphite surface. A commensurate phase at filling 1/3, as well as an incommensurate solid phase are clearly observed in neutron diffraction experiments, 10,11,12 heat capacity measurements, 13,14,15 and in numerical simulations.…”

“…10,11,12 At intermediate density between these two phases, Greywall and Busch conjectured a commensurate solid with a √ 7 × √ 7 partial registry with respect to the first layer, based on their heat capacity measurements. 1,15 At the same filling, a commensurate solid phase was observed in path integral Monte Carlo (PIMC) simulations; 20 in that study, however, for computational convenience first layer particles were treated as classical, i.e., held fixed in space at their T =0 equilibrium position.…”

The adsorption of helium atoms on coronene cations

“…Experimental studies have investigated features such as the structures of these layers, their phase transitions and the prospects for two-dimensional superfluidity. [1][2][3][4][5] At low temperatures the helium atoms adhere as a 3 3  commensurate phase, in which one third of the hexagonal sites above the graphitic plane are occupied by helium atoms. These atoms are locked into these locations by the corrugated surface.…”

Coupling of phonons to a helium atom adsorbed on graphite

“…These are low-speed phonons, since the adatom speed at a few degrees Kelvin is only a few hundred meters per second. We assume that the graphite surface is in the basal or cleavage plane, and define an x axis in this plane parallel to the phonon lateral momentum K. Note that polarizations in the y direction decouple completely, as can be seen from (5). Let the phonon displacement field be u(r')=Re [< u,o,u.>expUK·R '+ik.z' +iwkt> …”

“…By the virial theorem, this energy is equipartitioned between kinetic and potential energy. Equation (5) -z' is proportional to e"z', and also ikz is replaced by K.…”