Influence of graphene coating on the adsorption and tribology of Xe on Au(1 1 1) substrate.

The adsorption and tribological properties of graphene have received increasing attention for the further development of graphene-based coatings in applications. In this work, we performed first principles calculations with the inclusion of the nonlocal van der Waals correction to study the effect of graphene coating on the adsorption geometries, sliding frictions and electronic properties of Xe monolayer on the Au(1 1 1) substrate. The calculated activation energies indicate that Xe becomes movable on pure Au(1 1 1) surface at a temperature of around 30 K, whereas its motion can be activated only at a high temperature of ~50 K on graphene and on graphene-coated Au(1 1 1) substrates, in good agreement with recent experimental measurements by quartz crystal microbalance technique.

Effects of the commensurability and disorder on friction for the system Xe/Cu.

We present molecular dynamics simulations of static friction for a monolayer of Xe deposited on a thick slab of Cu for two different geometries. The interaction potential between Xe and Cu has been derived from DFT calculations. The first geometry is the commensurate adsorption geometry (√3 × âˆš3 suggested by LEED, corresponding to a coverage 1/3, where all Xe atoms are on top positions. The second one corresponds to a coverage 0.36 and is characterized by a large surface unit cell, containing 9 Xe atoms in different disordered positions. This large unit cell mimics an incommensurate case. Our analysis points out the effect of the order/disorder in tribological properties of a realistic three-dimensional system.

Why sliding friction of Ne and Kr monolayers is so different on the Pb(111) surface.

To understand the tribological properties of Ne and Kr on Pb(111), the potential energy surfaces for sliding motion of Ne, Kr, and Xe monolayers on the Pb(111) surface are examined through density functional calculations, using either local density or self-consistent nonlocal van der Waals functionals. The calculated adsorption energy for Xe/Pb(111) agrees well with experiment, validating the present approach and parameters. Activation energies along a sliding path indicate that Ne motion is much faster than Kr and Xe on Pb(111) at T∼6 K, which explains the puzzling experimental observation.

Sliding friction of N2 on Pb(111).

Molecular dynamics simulations of the sliding friction between two thick solid slabs are performed. The upper body is formed of light N(2) particles and the substrate of heavy Pb atoms. Among the various mechanisms that are responsible for the friction, we consider the phonon-phonon interaction between the two blocks. To provide evidence of the phonon interaction, we compare two different systems. For the first we consider the substrate as formed of atoms fixed in the equilibrium (111) positions. In the second system the Pb atoms can be displaced from the ideal positions, under their mutual interactions. A comparison with recently obtained experimental data will be discussed.

Theoretical investigation of the anticorrugation effects on the tribological properties of the Xe/Cu interface.

We present a molecular dynamics study of the slip time and static friction for a slab of Xe deposited on a slab of Cu. To put in evidence the role played by the phonon field of the two blocks, we compare results obtained with a substrate formed by fixed atoms with one formed by mobile atoms. In the last case the scattering between Xe and Cu mobile atoms is inelastic and there is an exchange of momentum and energy between the two blocks which produces disorder in the interface plane. This disorder favors a decrease of the static friction and a consequent increase of the slip time. We describe the interaction between Xe and Cu with a phenomenological multi-ion potential which gives rise to an anticorrugation of the charge distribution and reproduces very well the ab initio density functional calculations. Our model potential is a linear superposition of a corrugating potential and an anticorrugating one. For this reason we can study the static friction by passing from an anticorrugated to a fully corrugated system. We also investigate the slip time and we compare our results with recent experimental data measured with the quartz crystal microbalance technique.

Thermal effects in static friction: thermolubricity.

We present a molecular dynamics analysis of the static friction between two thick slabs. The upper block is formed by N2 molecules and the lower block by Pb atoms. We study the effects of the temperature as well as the effects produced by the structure of the surface of the lower block on the static friction. To put in evidence the temperature effects we will compare the results obtained with the lower block formed by still atoms with those obtained when the atoms are allowed to vibrate (e.g., with phonons). To investigate the importance of the geometry of the surface of the lower block we apply the external force in different directions, with respect to a chosen crystallographic direction of the substrate. We show that the interaction between the lattice dynamics of the two blocks is responsible for the strong dependence of the static friction on the temperature. The lattice dynamics interaction between the two blocks strongly reduces the static friction, with respect to the case of the rigid substrate. This is due to the large momentum transfer between atoms and the N2 molecules which disorders the molecules of the interface layer. A further disorder is introduced by the temperature. We perform calculations at T = 20K which is a temperature below the melting, which for our slab is at 50K . We found that because of the disorder the static friction becomes independent of the direction of the external applied force. The very low value of the static friction seems to indicate that we are in a regime of thermolubricity similar to that observed in dynamical friction.

Transition from smooth sliding to stick-slip motion in a single frictional contact.

We show that the transition from smooth sliding to stick-slip motion in a single planar frictional junction always takes place at an atomic-scale relative velocity of the substrates.

Corrugating and anticorrugating static interactions in helium-atom scattering from metal surfaces.

We perform a density-functional-theory calculation of the static repulsive potential of He scattering off a noble and a simple metal surface. The classical turning point of He on Cu(111) is found to be closer to the metal when the adatom is at top than at bridge site (anticorrugating effect). The potential of He on Al(111) is instead corrugated. By comparing the results of the two systems, we conclude that the He-metal anticorrugating effect occurs when the kinetic energy difference for He at top and bridge sites is larger than the electrostatic one, and an induced localized dipole on He is formed.

Underdamped commensurate dynamics in a driven Frenkel-Kontorova-type model.

The dynamics of a Frenkel-Kontorova chain subject to a substrate potential with a multiple-well structure and driven by an external dc force is studied in the underdamped regime. Making a rational choice among the three inherent length scales characterizing the system allows us to consider the possible formation of commensurate structures during sliding over the complex on-site potential. We comment both on the nature of the particle dynamics in the vicinity of the pinning-depinning transition point, and on the dynamical states displayed during the chain motion at different strengths of the dc driving. Varying the number of particles in the simulations allows us to consider, on a multiple-well substrate, the role played by the coverage variable on the depinning mechanism. The dependence of the minimal force required to initiate the chain motion (static friction) on the ratio of the model interaction strengths is analyzed and compared to the well-known case of the standard Frenkel-Kontorova model, which has only two inherent lengths.

Driven, underdamped Frenkel-Kontorova model on a quasiperiodic substrate.

We consider the underdamped dynamics of a chain of atoms subject to a dc driving force and a quasiperiodic substrate potential. The system has three inherent length scales which we take to be mutually incommensurate. We find that when the length scales are related by the spiral mean (a cubic irrational) there exists a value of the interparticle interaction strength above which the static friction is zero. When the length scales are related by the golden mean (a quadratic irrational) the static friction is always nonzero. From considerations based on the connection of this problem to standard map theory, we postulate that zero static friction is generally possible for incommensurate ratios of the length scales involved. However, when the length scales are quadratic irrationals, or have some commensurability with each other, the static friction will be nonzero for all choices of interaction parameters. We also comment on the nature of the depinning mechanisms and the steady states achieved by the moving chain.

Spontaneous pattern formation in driven nonlinear lattices

We demonstrate the spontaneous formation of spatial patterns in a damped, ac-driven cubic Klein-Gordon lattice. These patterns are composed of arrays of intrinsic localized modes characteristic for nonlinear lattices. We analyze the modulation instability leading to this spontaneous pattern formation. Our calculation of the modulational instability is applicable in one- and two-dimensional lattices; however, in the analyses of the emerging patterns we concentrate particularly on the two-dimensional case.