Physical Review Letters
American Physical Society
Inelastic neutron spectra of the commensurate √3 ×3 herringbone monolayer phase of nitrogen physisorbed on the graphite (002) surface at low temperature have been compared with lattice dynamics calculations of the polycrystalline-averaged one-phonon coherent neutron cross section. The observed zone-center energy gap of ∼0.4 THz in the acoustic-phonon branches is a factor of 2 larger than calculated from central atom-atom potentials. We conclude that current models of the corrugation in the adatom substrate potential greatly underestimate the lateral restoring forces in this relatively simple molecular film.
F. Y. Hansen, L. W. Bruch, H. Taub, V. L. P. Frank, H. J. Lauter, and JR Dennison, "Corrugation in the Nitrogen-graphite Holding Potential Determined from Analysis of Inelastic Neutron Scattering Data," Phys. Rev. Lett. 64, 764-767 (1990).