Elastic interactions between surface adatoms and between surface clusters

One major term is omitted in most conventional treatments of interactions between surface adatoms or groups of adatoms. This is the elastic interaction, in which adatoms interact through mutual distortion of the substrate. The indirect elastic interaction explains a range of observed surface phenomena in a consistent quantitative way. These phenomena include static and dynamic effects ranging from ordered structures to correlated motions of adatom clusters. It is likely also that substrate distortion is important in clean surface reconstruction. The elastic interaction is typically comparable with or larger than the indirect electronic interactions usually presumed. It follows that detailed calculations which ignore the substrate distortions produced will be of limited value only.     

Effects of size and surface anisotropy on thermal magnetization and hysteresis in the magnetic clusters

Based on Monte Carlo simulation, the spin configurations, thermal magnetization and hysteresis loops of the clusters coated by the surface shell with radial anisotropy are studied. Interestingly, a new multidomain containing a few of subdomains whose easy directions are along those of the configurational anisotropy, a magnetization curve in steps and a first order phase transition from the single domain to the multidomain in the thermal and field magnetization processes, are found, which is as a result of the interplay of the configurational anisotropy, the size effect, the surface anisotropy, the applied field and the thermal fluctuation. In this first order transition, we find a critical temperature, a critical surface anisotropy and a critical size. The simulated temperature dependence of the coercivity of the cluster with the surface anisotropy can be fitted by H_c (T)=H_c (0)(1-C_αT^α) with low value of α, which explains well the experimental results of the nanoparticles. Moreover, it is found that the hysteresis loops and coercivity are strongly affected by the cluster size and the thickness of the surface layer.     

Effects of corotron size and parameters on the dielectric substrate surface charge

Effects of variation of parameters of a corona device (corotron) used in electro-photographic machines on the amount of surface charge build-up on the surface of dielectric substrate were studied. Particular attention was given to the effect of corotron dimension including wire–shields and wire–plate distances, substrate thickness, shields insulation and the substrate speed on the amount of substrate surface charge. The computational analyses were performed for a two-dimensional cross-section of the corotron under steady-state condition. The Maxwell equations were solved and the electrical quantities in a rectangular positive single wire corotron were evaluated. The simulation results showed that for a fixed wire voltage, the corotron size, the substrate thickness, insulation of shields and the substrate speed will affect the distributions of electrical quantities in the corotron. The wire–substrate distance and the substrate speed, however, were found to be the main parameters that control the amount of surface charge build-up on the substrate.     

Study of Si and C adatoms and SiC clusters on the silicon surface by the molecular dynamics method

Individual Si and C adatoms, as well as SiC clusters, on a Si surface are simulated by the molecular dynamics method in the course of investigation of the initial stages of formation of a SiC layer on silicon with the help of molecular beam epitaxy. The potential energy surfaces for Si and C adatoms on the (2 × 1) reconstructed Si(001) surface and on the nonreconstructed Si(111) surface, as well as on the Si(111) surface with a SiC cluster, are calculated and analyzed. The values of migration barriers for adatoms on these surfaces are calculated. The effect of the SiC cluster on deformation of the surface region of Si(111) and on the migration of adatoms is investigated. The deep minima observed on the potential energy surfaces immediately above a cluster and at its boundaries can trap diffusing adatoms. The distributions of stresses and strains in the silicon lattice under a cluster on the surface are studied and described.     

Analysis of interactions between Co adatoms on the vicinal Cu(111) surface

The energies of magnetic interactions between Co adatoms at the vicinal Cu(111) surface are calculated in the framework of the density functional theory using the Korringa-Kohn-Rostoker type Green’s functions. It is demonstrated that the interactions between Co adatoms appreciably depend on the distance from a surface step. Our calculations show that the magnitude of the repulsive barrier related to the surface step is larger for Co adatoms located at the upper surface terrace than for those located at the lower surface terrace.     

Electromagnetic effects on a molecule at a metal surface: I. Effects of nonlocality and finite molecular size

The electromagnetic effects on a molecule near a metal surface are considered with the view to understanding the surface-enhanced-Raman-scattering (SERS) effect. The image enhancement effect is calculated including the nonlocal response of the metal and finite molecular size. The effect is much reduced (× 10^?5) from that for a point molecule above a local metal but can still give a gain ≈ 10³. The power emitted by a dipole above a smooth surface is also calculated. For an Ag surface the power emitted in the form of photons, surface plasmons, and electron-hole excitations are found to be in the ratio 1 : 3 : 10⁶. The numerical results are calculated using the semi-classical infinite-barrier model of the metal surface with a Lindhard dielectric function modified to take into account finite electron lifetime and core polarization.     

On the interactions of adatoms on metal surfaces

Pariwise interactions of the adatoms and ions adsorbed on metal surfaces has been investigated with the help of the pole approximation in integral equations of the Lippman-Swinger type. These interactions have been shown to depend mainly both on the type of the metal spectrum and on the electron structure of the adatoms. The theory developed in this paper gives a satisfactory explanation of several facts observed in experiments, especially the longrange oscillating interaction between adatoms.     

Effects of the bulk anisotropy on the surface magnetization

We study in this paper the magnetization singularity at the bulk critical temperature of a semi-infinite anisotropic Heisenberg model. In this analysis the surface is described by a Heisenberg model, while the bulk is given by an anisotropic Heisenberg model. Within the Green's function formalism we find the slope of the surface magnetization at the bulk critical temperature. We show that this slope is discontinuous and the difference between the derivatives of the surface magnetization, just below and above the bulk critical temperature, can exhibit a minimum at a given value of the bulk anisotropy parameter.     

The onset of convection in a finite container due to surface tension and buoyancy

A method for predicting instabilities which combines recent techniques from bifurcation theory with the finite-element method is described. It is applied to the prediction of the onset of convection driven by both surface tension and buoyancy in rectangular containers. For zero buoyancy, the critical values of the Marangoni number for the first two bifurcations from the trivial solution are found for a two-dimensional cavity of aspect ratio 2. The variation of these critical values with aspect ratio is obtained by continuation methods and this reveals an interlacing of modes as the container size increases. It is established that the bifurcation to an even number of cells is transcritical rather than pitchfork and the turning point on the subcritical branch is located as a function of aspect ratio. The hysteresis associated with the transcritical bifurcation is small. As the surface tension forces decrease to zero, so that the convection is driven by buoyancy alone, the amount of hysteresis associated with the transcritical bifurcation becomes vanishingly small. The reason for this is not fully understood.     

Magnetization of nanomagnet assemblies: Effects of anisotropy and dipolar interactions

We investigate the effect of anisotropy and weak dipolar interactions on the magnetization of an assembly of nanoparticles with distributed magnetic moments, i.e., assembly of magnetic nanoparticles in the one-spin approximation, with textured or random anisotropy. The magnetization of a free particle is obtained either by a numerical calculation of the partition function or analytically in the low and high field regimes, using perturbation theory and the steepest-descent approximation, respectively. The magnetization of an interacting assembly is computed analytically in the range of low and high field, and numerically using the Monte Carlo technique. Approximate analytical expressions for the assembly magnetization are provided which take account of the dipolar interactions, temperature, magnetic field, and anisotropy. The effect of anisotropy and dipolar interactions are discussed and the deviations from the Langevin law they entail are investigated, and illustrated for realistic assemblies with the lognormal moment distribution.     

Subleading long-range interactions and violations of finite size scaling

We study the behavior of systems in which the interaction contains a long-range component that does not dominate the critical behavior. Such a component is exemplified by the van der Waals force between molecules in a simple liquid-vapor system. In the context of the mean spherical model with periodic boundary conditions we are able to identify, for temperatures close above T _c, finite-size contributions due to the subleading term in the interaction that are dominant in this region decaying algebraically as a function of L. This mechanism goes beyond the standard formulation of the finite-size scaling but is to be expected in real physical systems. We also discuss other ways in which critical point behavior is modified that are of relevance for analysis of Monte Carlo simulations of such systems. Received 21 November 2000 and Received in final form 28 February 2001     

Tuning surface interactions to control shape and array behavior of diblock copolymer micelles on a silicon substrate

The micellar shape of liquid crystalline diblock copolymers, PEO_m-b-PMA(Az)_n, consisting of high surface energy components was controlled by tuning surface interactions. On a fluorinated surface, the diblock copolymers formed ordered arrays of spherical micelles consisting of PEO cores surrounded by PMA(Az) coronas. Gold ions could be doped into the PEO cores by immersion in a solution of the gold ion. The Au³⁺-doped micelles were subsequently etched and reduced by VUV radiation to form hexagonally ordered gold nanodots.     

Band effects and the dipole interaction between two adatoms on a metal surface

The contribution of band effects to the adatom interaction energy is investigated to second order in perturbation theory. These are found to bring an important correction to the dipole law derived recently by Kohn and Lau.     

Self-organization of Cs adatoms on Ag(111) caused by quantum confinement of surface electrons on Ag clusters

The effect of quantum confinement formed by Ag nanoclusters on the self-organization of Cs adatoms on Ag(111) was studied. The existence of allowed and forbidden zones was found for the diffusion motion of Cs adatoms near and on top of the clusters. The formation of the orbits of an adatom’s motion was demonstrated for self-organization process in a Cs adatom ensemble near and on top of the clusters.     

Statistical study of the radiation losses due to surface roughness for a dilectric slab deposited on a metal substrate

This paper presents an original method of analyzing radiation loss from dielectric slab with random wall imperfections. The method is based on Maxwell’s equations under their covariant form written in a non-orthogonal coordinate system. The solution is found by using a perturbation method applied to the smooth surface problem. The statistical characteristics of the radiation intensity, the average value and the probability density function, are analytically determined.     

Elastic displacement of surface due to laser picosecond pulse heating of gold

Laser picosecond pulse heating initiates nonequilibrium energy transport in the surface vicinity of the metallic substrates. In this case, electron temperature rises rapidly while lattice site temperature is slow during the heating period. Although the rise of lattice site temperature is low, the temperature gradient is high. This results in elastic displacement of the surface. To model the heating process thermomechanical coupling needs to be introduced in the analysis. In the present study, picosecond pulse heating of gold surface is considered. Three-dimensional analysis of electron kinetic theory is introduced when formulating the electron and the lattice site temperatures. The analysis is extended to include the thermomechanical coupling due to mechanical response of the substrate material. The lattice site temperatures obtained from the electron kinetic theory are compared with the predictions of the two-equation model. It is found that both models predict almost identical temperature profiles in the surface vicinity of the substrate material. In addition, the surface displacement on the order of 10⁻¹¹ m is predicted.     

Strong influence of defects on the electronic structure of Pt adatoms and clusters on graphite

We study the specific impact of defects such as step edges at the graphite surface on the electronic configuration of adsorbed Pt atoms and Pt₈ clusters. Surface diffusion is strongly reduced by depositing Pt and Pt₈ into a thin rare gas layer. In this configuration a very narrow adatom Pt 4f spectrum is found at an exceptionally small binding energy, similar to Pt surfaces. Both, adatom and cluster spectra are strongly shifted towards higher binding energy when allowed to diffuse towards defects like step edges. The strong shifts are indicative of a chemical reaction at the step edges and are conjectured to be part of the particle size dependent binding energy shifts typically observed for transition metal clusters grown on the surface of graphite.