66 research outputs found
Theoretical aspects of vertical and lateral manipulation of atoms
Using total energy calculations, based on interaction potentials from the
embedded atom method, we show that the presence of the tip not only lowers the
barrier for lateral diffusion of the adatom towards it, but also shifts the
corresponding saddle point. For a Cu adatom at a (100) microfacetted step on
Cu(111) this shift is 0.6 A. The effect of the tip geometry and shape on the
energetics of lateral manipulation was found to be subtle. In the case of
vertical manipulation of a Cu adatom on flat, stepped, and kinked Cu surfaces
we find an unusual but interesting result. It is found that as the tip
approaches the surface, it becomes easier to extract the adatom from the
stepped and kinked surfaces, as compared to the flat surface. This counter
intuitive result can be explained in terms of tip induced changes in the
bonding of the adatom to its low coordinated surroundings.Comment: 8figures, to appear in Surf. Sci., VAS10 proceeding
Surface diffusion coefficients by thermodynamic integration: Cu on Cu(100)
The rate of diffusion of a Cu adatom on the Cu(100) surface is calculated
using thermodynamic integration within the transition state theory. The results
are found to be in excellent agreement with the essentially exact values from
molecular-dynamics simulations. The activation energy and related entropy are
shown to be effectively independent of temperature, thus establishing the
validity of the Arrhenius law over a wide range of temperatures. Our study
demonstrates the equivalence of diffusion rates calculated using thermodynamic
integration within the transition state theory and direct molecular-dynamics
simulations.Comment: 4 pages (revtex), two figures (postscript
Wave packet propagation study of the charge transfer interaction in the F^- -Cu(111) and -Ag(111) systems
The electron transfer between an ion and and
surfaces is studied by the wave packet propagation method in order to determine
specifics of the charge transfer interaction between the negative ion and the
metal surface due to the projected band gap. A new modeling of the ion
is developed that allows one to take into account the six quasi-equivalent
electrons of which are {\it a priori} active in the charge transfer
process. The new model invokes methods of constrained quantum dynamics. The
six-electron problem is transformed to two one-electron problems linked via a
constraint. The projection method is used to develop a wave packet propagation
subject to the modeling constraint. The characteristics (energy and width) of
the ion ion level interacting with the two surfaces are determined and
discussed in connection with the surface projected band gap.Comment: 34 pages, Revtex, 9 figures (postscript
Strain Induced Adatom Correlations
A Born-Green-Yvon type model for adatom density correlations is combined with
a model for adatom interactions mediated by the strain in elastic anisotropic
substrates. The resulting nonlinear integral equation is solved numerically for
coverages from zero to a limit given by stability constraints. W, Nb, Ta and Au
surfaces are taken as examples to show the effects of different elastic
anisotropy regions. Results of the calculation are shown by appropriate plots
and discussed. A mapping to superstructures is tried. Corresponding adatom
configurations from Monte Carlo simulations are shown.Comment: 12 pages, 33 figure
Defect-induced perturbations of atomic monolayers on solid surfaces
We study long-range morphological changes in atomic monolayers on solid
substrates induced by different types of defects; e.g., by monoatomic steps in
the surface, or by the tip of an atomic force microscope (AFM), placed at some
distance above the substrate. Representing the monolayer in terms of a suitably
extended Frenkel-Kontorova-type model, we calculate the defect-induced density
profiles for several possible geometries. In case of an AFM tip, we also
determine the extra force exerted on the tip due to the tip-induced
de-homogenization of the monolayer.Comment: 4 pages, 2 figure
Importance of electronic self-consistency in the TDDFT based treatment of nonadiabatic molecular dynamics
A mixed quantum-classical approach to simulate the coupled dynamics of
electrons and nuclei in nanoscale molecular systems is presented. The method
relies on a second order expansion of the Lagrangian in time-dependent density
functional theory (TDDFT) around a suitable reference density. We show that the
inclusion of the second order term renders the method a self-consistent scheme
and improves the calculated optical spectra of molecules by a proper treatment
of the coupled response. In the application to ion-fullerene collisions, the
inclusion of self-consistency is found to be crucial for a correct description
of the charge transfer between projectile and target. For a model of the
photoreceptor in retinal proteins, nonadiabatic molecular dynamics simulations
are performed and reveal problems of TDDFT in the prediction of intra-molecular
charge transfer excitations.Comment: 9 pages, 8 figures. Minor changes in content wrt older versio
Extended Classical Over-Barrier Model for Collisions of Highly Charged Ions with Conducting and Insulating Surfaces
We have extended the classical over-barrier model to simulate the
neutralization dynamics of highly charged ions interacting under grazing
incidence with conducting and insulating surfaces. Our calculations are based
on simple model rates for resonant and Auger transitions. We include effects
caused by the dielectric response of the target and, for insulators, localized
surface charges. Characteristic deviations regarding the charge transfer
processes from conducting and insulating targets to the ion are discussed. We
find good agreement with previously published experimental data for the image
energy gain of a variety of highly charged ions impinging on Au, Al, LiF and KI
crystals.Comment: 32 pages http://pikp28.uni-muenster.de/~ducree
Surface Modifications by Field Induced Diffusion
By applying a voltage pulse to a scanning tunneling microscope tip the surface under the tip will be modified. We have in this paper taken a closer look at the model of electric field induced surface diffusion of adatoms including the van der Waals force as a contribution in formations of a mound on a surface. The dipole moment of an adatom is the sum of the surface induced dipole moment (which is constant) and the dipole moment due to electric field polarisation which depends on the strength and polarity of the electric field. The electric field is analytically modelled by a point charge over an infinite conducting flat surface. From this we calculate the force that cause adatoms to migrate. The calculated force is small for voltage used, typical 1 pN, but due to thermal vibration adatoms are hopping on the surface and even a small net force can be significant in the drift of adatoms. In this way we obtain a novel formula for a polarity dependent threshold voltage for mound formation on the surface for positive tip. Knowing the voltage of the pulse we then can calculate the radius of the formed mound. A threshold electric field for mound formation of about 2 V/nm is calculated. In addition, we found that van der Waals force is of importance for shorter distances and its contribution to the radial force on the adatoms has to be considered for distances smaller than 1.5 nm for commonly used voltages
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