263 research outputs found
Two Dimensional Ir-Cluster Lattices on Moir\'e of Graphene with Ir(111)
Lattices of Ir clusters have been grown by vapor phase deposition on graphene
moir\'{e}s on Ir(111). The clusters are highly ordered, spatially and thermally
stable below 500K. Their narrow size distribution is tunable from 4 to about
130 atoms. A model for cluster binding to the graphene is presented based on
scanning tunneling microscopy and density functional theory. The proposed
binding mechanism suggests that similar cluster lattices might be grown of
materials other than Ir.Comment: Submitted to PRL on 27Apr0
Information technology as a tool to improve the competitiveness of the organization
In this article we are talking about competitiveness and how competitiveness is enhanced through information technology
Breakdown of step-flow growth in unstable homoepitaxy
Two mechanisms for the breakdown of step flow growth, in the sense of the
appearance of steps of opposite sign to the original vicinality, are studied by
kinetic Monte Carlo simulations and scaling arguments. The first mechanism is
the nucleation of islands on the terraces, which leads to mound formation if
interlayer transport is sufficiently inhibited. The second mechanism is the
formation of vacancy islands due to the self-crossing of strongly meandering
steps. The competing roles of the growth of the meander amplitude and the
synchronization of the meander phase are emphasized. The distance between
vacancy islands along the step direction appears to be proportional to the
square of the meander wavelengthComment: 7 pages, 9 figure
Island size distributions in submonolayer growth: successful prediction by mean field theory with coverage dependent capture numbers
We show that mean-field rate equations for submonolayer growth can
successfully predict island size distributions in the pre-coalescence regime if
the full dependence of capture numbers on both the island size and the coverage
is taken into account. This is demonstrated by extensive Kinetic Monte Carlo
simulations for a growth kinetics with hit and stick aggregation. A detailed
analysis of the capture numbers reveals a nonlinear dependence on the island
size for small islands. This nonlinearity turns out to be crucial for the
successful prediction of the island size distribution and renders an analytical
treatment based on a continuum limit of the mean-field rate equations
difficult.Comment: 4 pages, 4 figue
Spiral Growth and Step Edge Barriers
The growth of spiral mounds containing a screw dislocation is compared to the
growth of wedding cakes by two-dimensional nucleation. Using phase field
simulations and homoepitaxial growth experiments on the Pt(111) surface we show
that both structures attain the same characteristic large scale shape when a
significant step edge barrier suppresses interlayer transport. The higher
vertical growth rate observed for the spiral mounds on Pt(111) reflects the
different incorporation mechanisms for atoms in the top region and can be
formally represented by an enhanced apparent step edge barrier.Comment: 11 pages, 4 figures, partly in colo
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The role of the hippocampus in weighting expectations during inference under uncertainty
Making inference under uncertainty requires an optimal weighting of prior expectations and observations. How this weighting is realized in the brain remains elusive. To investigate this, we recorded functional neuroimaging data while participants estimated a number based on noisy observations. Crucially, the prior expectation about the variability of observations (an expected variability) was manipulated. Consistent with normative models, when novel observations were characterized by higher expected or observed variability, participants’ estimates relied more on expectations than novel observations and were characterized by higher stochasticity. Activity in hippocampus increased when novel evidence was characterized by higher expected or observed variability. Response in superior parietal cortex reflected a precision-weighted prediction error signal (i.e., the distance between observations and expectations) that was modulated by hippocampal activity. Our findings implicate the hippocampus during inference under uncertainty, suggesting a role in weighting prior representations over observations and in modulating responsivity of superior parietal cortex to prediction error
Re-entrant Layer-by-Layer Etching of GaAs(001)
We report the first observation of re-entrant layer-by-layer etching based on
{\it in situ\/} reflection high-energy electron-diffraction measurements. With
AsBr used to etch GaAs(001), sustained specular-beam intensity oscillations
are seen at high substrate temperatures, a decaying intensity with no
oscillations at intermediate temperatures, but oscillations reappearing at
still lower temperatures. Simulations of an atomistic model for the etching
kinetics reproduce the temperature ranges of these three regimes and support an
interpretation of the origin of this phenomenon as the site-selectivity of the
etching process combined with activation barriers to interlayer adatom
migration.Comment: 11 pages, REVTeX 3.0. Physical Review Letters, in press
Kinetic Monte Carlo simulations of oscillatory shape evolution for electromigration-driven islands
The shape evolution of two-dimensional islands under electromigration-driven
periphery diffusion is studied by kinetic Monte Carlo (KMC) simulations and
continuum theory. The energetics of the KMC model is adapted to the Cu(100)
surface, and the continuum model is matched to the KMC model by a suitably
parametrized choice of the orientation-dependent step stiffness and step atom
mobility. At 700 K shape oscillations predicted by continuum theory are
quantitatively verified by the KMC simulations, while at 500 K qualitative
differences between the two modeling approaches are found.Comment: 7 pages, 6 figure
Kinetic modelling of epitaxial film growth with up- and downward step barriers
The formation of three-dimensional structures during the epitaxial growth of
films is associated to the reflection of diffusing particles in descending
terraces due to the presence of the so-called Ehrlich-Schwoebel (ES) barrier.
We generalize this concept in a solid-on-solid growth model, in which a barrier
dependent on the particle coordination (number of lateral bonds) exists
whenever the particle performs an interlayer diffusion. The rules do not
distinguish explicitly if the particle is executing a descending or an
ascending interlayer diffusion. We show that the usual model, with a step
barrier in descending steps, produces spurious, columnar, and highly unstable
morphologies if the growth temperature is varied in a usual range of mound
formation experiments. Our model generates well-behaved mounded morphologies
for the same ES barriers that produce anomalous morphologies in the standard
model. Moreover, mounds are also obtained when the step barrier has an equal
value for all particles independently if they are free or bonded. Kinetic
roughening is observed at long times, when the surface roughness w and the
characteristic length scale as and where
and , independently of the growth
temperature.Comment: 15 pages, 7 figure
Strong anisotropy in surface kinetic roughening: analysis and experiments
We report an experimental assessment of surface kinetic roughening properties
that are anisotropic in space. Working for two specific instances of silicon
surfaces irradiated by ion-beam sputtering under diverse conditions (with and
without concurrent metallic impurity codeposition), we verify the predictions
and consistency of a recently proposed scaling Ansatz for surface observables
like the two-dimensional (2D) height Power Spectral Density (PSD). In contrast
with other formulations, this Ansatz is naturally tailored to the study of
two-dimensional surfaces, and allows to readily explore the implications of
anisotropic scaling for other observables, such as real-space correlation
functions and PSD functions for 1D profiles of the surface. Our results confirm
that there are indeed actual experimental systems whose kinetic roughening is
strongly anisotropic, as consistently described by this scaling analysis. In
the light of our work, some types of experimental measurements are seen to be
more affected by issues like finite space resolution effects, etc. that may
hinder a clear-cut assessment of strongly anisotropic scaling in the present
and other practical contexts
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