95 research outputs found
Molecular Dynamics Simulations of Dynamic Force Microscopy: Applications to the Si(111)-7x7 Surface
Molecular dynamics simulations have been performed to understand true atomic
resolution, which has been observed on the Si(111)-77 surface by
dynamic force microscopy in ultra high vacuum(UHV). Stable atomic-scale
contrast is reproduced in simulations at constant mean height above a critical
tip-sample separation when monitoring the interaction force between tip and
sample. Missing or additional adatoms can be recognized in such scans, although
they are less well resolved than native adatoms. The resonance frequency shift,
as well as arbitrary scans, e.g. at constant force can be computed from a
series of force-distance characteristics. By means of dynamic simulations we
show how energy losses induced by interaction with an oscillating tip can be
monitored and that they occur even in the non-contact range.Comment: 5 pages, 5 figures, accepted publication in Applied Surface Scienc
Site-specific force-distance characteristics on NaCl(001): Measurements versus atomistic simulations
A scanning force microscope was used to measure the frequency shift above various atomic sites on a NaCl(001) surface at 7K. The data was converted to force and compared to the results of atomistic simulations using model NaCl and MgO tips. We find that the NaCl tip demonstrates better agreement in the magnitude of the forces in experiments, supporting the observation that the tip first came into contact with the sample. Using the MgO tip as a model of the originally oxidized silicon tip, we further demonstrate a possible mechanism for tip contamination at low temperatures.Peer reviewe
Exact Results for Tunneling Problems of Bogoliubov Excitations in the Critical Supercurrent State
We show the exact solution of Bogoliubov equations at zero-energy in the
critical supercurrent state for arbitrary shape of potential barrier. With use
of this solution, we prove the absence of perfect transmission of excitations
in the low-energy limit by giving the explicit expression of transmission
coefficient. The origin of disappearance of perfect transmission is the
emergence of zero-energy density fluctuation near the potential barrier.Comment: 6 pages, 3 figures; Proceedings of QFS200
Absence of Anomalous Tunneling of Bogoliubov Excitations for Arbitrary Potential Barrier under the Critical Condensate Current
We derive the exact solution of low energy limit of Bogoliubov equations for
excitations of Bose-Einstein condensate in the presence of arbitrary potential
barrier and maximum current of condensate. Using this solution, we give the
explicit expression for the transmission coefficient against the potential
barrier, which shows partial transmission in the low energy limit. The
wavefunctions of excitations in the low energy limit do not coincide with that
of the condensate. The absence of the perfect transmission in the critical
current state originates from local enhancement of density fluctuations around
the potential barrier.Comment: 4 pages, 1 figur
Theoretical study of short- and long-range forces and atom transfer in scanning force microscopy
We investigate the interaction energy, the short-range force components, and the electron potential between two Al(001) slabs, which mimic a blunt tip close to an atomically corrugated sample in scanning force microscopy. The adhesive energy and perpendicular force calculated using the self-consistent-field pseudopotential method in the local-density approximation are site dependent, but can be accurately represented by a universal function in terms of scaled variables in the attractive range. The lateral force which determines friction variations on an atomic scale is not simply proportional to the perpendicular force and is typically one order of magnitude smaller. At larger separations the effect of the total long-range Van der Waals force and of its gradient are estimated to be small for a sharp conical support tip, but quite appreciable for a rounded support tip with a radius as small as 200. By calculating the interaction energy of an Al atom between two slabs, we also study the possibility of single-atom transfer between tip and sample, and show that the double well in the interaction energy collapses into a single minimum at a slab separation larger than two bulk interlayer spacings. The atom is preferentially located on the side of the deeper minimum, but can hop between the two wells at finite temperatures. Moreover, the position of the deeper minimum relative to the electrodes can vary as the tip is scanned against the sample. Finally we explore possible relations between the short-range perpendicular force and the tunneling conductance through the potential barrier between two semi-infinite jellium slabs as a function of their separation. © 1992 The American Physical Society
Adhesive energy, force and barrier height between simple metal surfaces
Using the self-consistent field pseudopotential method we calculated the adhesive energy, perpendicular and lateral forces and barrier height between two rigid A1(001) slabs modeling the sample and a blunt tip. We found that the adhesive energy and forces are site specific, and can lead to a significant corrugation in the constant force mode with negative force gradient. Lateral forces, which determine friction on the atomic scale are not simply proportional to the perpendicular force, and are typically one order of magnitude smaller. Our results confirm that perpendicular tip force and barrier height are interrelated for separations where the force gradient is positive. © 1992
Distance dependence of force and dissipation in non-contact atomic force microscopy on Cu(100) and Al(111)
The dynamic characteristics of a tip oscillating in the nc-AFM mode in close
vicinity to a Cu(100)-surface are investigated by means of phase variation
experiments in the constant amplitude mode. The change of the quality factor
upon approaching the surface deduced from both frequency shift and excitation
versus phase curves yield to consistent values. The optimum phase is found to
be independent of distance. The dependence of the quality factor on distance is
related to 'true' damping, because artefacts related to phase misadjustment can
be excluded. The experimental results, as well as on-resonance measurements at
different bias voltages on an Al(111) surface, are compared to Joule
dissipation and to a model of dissipation in which long-range forces lead to
viscoelastic deformations
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