Probing the helium-graphite interaction

Abstract

Two separate lines of investigation have recently converged to produce a highly detailed picture of the behavior of helium atoms physisorbed on graphite basal plane surfaces. Atomic beam scattering experiments on single crystals have yielded accurate values for the binding energies of several· states for both 4 He and 'He, as well as matrix elements of the largest Fourier component of the periodic part of the interaction potential. From these data, a complete three-dimensional description of the potential has been constructed, and the energy band structure of a helium atom moving in this potential calculated. At the same time, accurate thermodynamic measurements were made on submonolayer helium films adsorbed on Grafoil .. The binding energy and low-coverage specific heat deduced from these measurements are in excellent agreement with those calculated from the band structures.

Evolutions of Competition of Multiple-Spin-Exchange Interactions and Structure in Submonolayer Solid3HeFilm Adsorbed on Grafoil

“…One of the reasons for the strong competition is the strong adsorption potential, which restricts the exchange tunnel paths in 2D space more strictly and suppresses the MSEs, especially the fewer body MSEs. The strong corrugation of the adsorption potential, whose amplitude is about 40 K in the first layer [8], increases the exchange barrier height and suppresses the MSEs. That must also affect the MSE competition.…”

Orientational ordering in incommensurate solid helium monolayers on graphite

“…(5) and (6) for the sound speeds, and proceeding with the rest of the calculation as described below to obtain a new value of y, which is then used to generate a new solution of Eqs. (5) and (6). The procedure eventually converges to a value of y which does not change from iteration to iteration.…”

“…Other calculations involving this potential have produced remarkable agreement with experiment. 6 One important result of the study of the He-graphite potential is that it cannot be analyzed as a sum of isotropic He-C pair potentials, but must include contributions from the anisotropic electron distribution in the graphite layers, thus enhancing the corrugations of the potential and the degree of orientational ordering.…”

Quantum states and specific heat of low-density He gas adsorbed within carbon nanotube interstitial channels: Band-structure effects and potential dependence

“…Various different forms of effective He-C potential, based on the analysis of Hegraphite interaction, have been proposed in the literature. 17,18 The most important deficiency of the LJ form is the assumption that the potential depends only on the magnitude of r. 17 However, in the view of a lack of knowledge on the precise geometry of a bundle, the LJ form should serve well in obtaining the main features of the interaction potential. The LJ parameters we adopt are ⑀ϭ1.34 meV, ϭ2.75 Å.…”