Diffusion of Na atoms on a Cu(001) surface

Abstract

We study the equilibrium properties and the diffusive motion of Na atoms adsorbed on a Cu͑001͒ surface at finite coverages via a model Hamiltonian. Our model Hamiltonian starts with an adatom-substrate interaction potential based on the previous analysis of the zerocoverage He scattering data. The dipole-dipole interaction between the alkali adatoms at finite coverages is then included. We use Monte Carlo simulations to study the ordered structure of the system at low temperatures and molecular-dynamics simulations for evaluating the dynamical correlation functions relevant for both the tracer and collective diffusive constants.

Onset of 3D Collective Surface Diffusion in the Presence of Lateral Interactions:Na/Cu(001)

“…This change indicates that the Na=Cu001 dynamics are governed by correlated motion; i.e., the Na motion can no longer be described as isolated self-diffusion, even at these relatively low coverages. The characteristic shape of the curve, featuring a new peak at intermediate K values, followed by a de Gennes narrowing dip [17], has been predicted both numerically and analytically for surface diffusion of repulsive particles [11,12,18], but has never been observed previously due to the lack of experimental resolution. The location of the de Gennes dip, corresponds to a peak in the static structure factor [18] and verifies the repulsive nature of the force and the coverage estimation of the adsorbate.…”

“…Limited experimental resolution has meant that all previous QHAS measurements, including the system mentioned above, have required assumptions about the quasielastic line shape, which limited the validity of analysis to ultrafast and relatively simple diffusion processes [10]. In particular, attempts to analyze measurements of Na=Cu001 with higher adsorbate coverages and non-negligible adsorbate interactions, failed to reproduce the data quantitatively [11,12]. The ultrahigh resolution of our 3 He spin-echo spectrometer enables us to observe the line shape directly in the present system.…”

“…The lateral interaction was included using a dipole-dipole interaction, which describes the interaction purely in terms of repulsive electrostatic forces [23]. The dipole moments used in the calculation were extracted from work function measurements [11].…”

Crucial electronic contributions to measures of surface diffusion by He atom scattering

“…We analyze theoretically Na/Cu͑001͒. Recent HSE measurements of this system 10 have addressed the previously unexplained coverage dependence of Na surface diffusion 11,12 and have identified a new component of motion oriented perpendicular to the surface. Such perpendicular motion could be detected from a Na coverage of 0.04 ML, and its effect rapidly increased with deposition of adatoms.…”

Potential energy surface of alkali atoms adsorbed on Cu(001)

“…11 More recently, the much improved resolution of the 3 He spin-echo technique 12 has allowed for a deeper analysis of the diffusion process at larger coverage. 13 This study interpreted in terms of collective phenomena a coverage dependence of Na surface diffusion previously reported and unexplained: 14,15 indeed, the diffusion process involves nontrivial electrondensity rearrangements, 16,17 a phenomenon further showing the complexity of such systems. Additionally to the information on the given studied system, high-precision HAS measurements may act as a reference to other experimental and theoretical methods.…”