3,466 research outputs found
A Two-Region Diffusion Model for Current-Induced Instabilities of Step Patterns on Vicinal Si(111) Surfaces
We study current-induced step bunching and wandering instabilities with
subsequent pattern formations on vicinal surfaces. A novel two-region diffusion
model is developed, where we assume that there are different diffusion rates on
terraces and in a small region around a step, generally arising from local
differences in surface reconstruction. We determine the steady state solutions
for a uniform train of straight steps, from which step bunching and in-phase
wandering instabilities are deduced. The physically suggestive parameters of
the two-region model are then mapped to the effective parameters in the usual
sharp step models. Interestingly, a negative kinetic coefficient results when
the diffusion in the step region is faster than on terraces. A consistent
physical picture of current-induced instabilities on Si(111) is suggested based
on the results of linear stability analysis. In this picture the step wandering
instability is driven by step edge diffusion and is not of the Mullins-Sekerka
type. Step bunching and wandering patterns at longer times are determined
numerically by solving a set of coupled equations relating the velocity of a
step to local properties of the step and its neighbors. We use a geometric
representation of the step to derive a nonlinear evolution equation describing
step wandering, which can explain experimental results where the peaks of the
wandering steps align with the direction of the driving field.Comment: 11 pages, 10 figure
The role of entrepreneurial human capital as a driver of endogenous economic growth
We model investment in entrepreneurial human capital (EHC)-the representative enterprise’s share of production capacity allocated to investment in innovative industrial and commercial knowledge-as a distinct channel through which firmspecific human capital drives endogenous growth. Ourmodel suggests that institutional factors supporting free markets for goods and ideas and higher-educational attainments of entrepreneurs and workers enhance endogenous economic growth by augmenting the efficiency of investment inEHCrather than exclusively by themselves. We test these implications, using data from Global Entrepreneurship Monitor’s Adult Population Survey of 63 countries over 2002-10, and find robust support for these hypotheses
Diffusion Boundary Condition at Surface Steps
This Communication reports a geometrical factor that is necessary in the
diffusion boundary condition across surface steps. Specifically, this factor
relates adatom concentration to its spatial gradient at a surface step, and it
describes the fraction of jump attempts that cross the step. In this
Communication, the authors show that the factor is 1/\Pi using theoretical
formulation and further verify the formulation using numerical simulations for
triangular, square, and hexagonal surface lattices
Current-Induced Step Bending Instability on Vicinal Surfaces
We model an apparent instability seen in recent experiments on current
induced step bunching on Si(111) surfaces using a generalized 2D BCF model,
where adatoms have a diffusion bias parallel to the step edges and there is an
attachment barrier at the step edge. We find a new linear instability with
novel step patterns. Monte Carlo simulations on a solid-on-solid model are used
to study the instability beyond the linear regime.Comment: 4 pages, 4 figure
Anisotropy of Growth of the Close-Packed Surfaces of Silver
The growth morphology of clean silver exhibits a profound anisotropy: The
growing surface of Ag(111) is typically very rough while that of Ag(100) is
smooth and flat. This serious and important difference is unexpected, not
understood, and hitherto not observed for any other metal. Using density
functional theory calculations of self-diffusion on flat and stepped Ag(100) we
find, for example, that at flat regions a hopping mechanism is favored, while
across step edges diffusion proceeds by an exchange process. The calculated
microscopic parameters explain the experimentally reported growth properties.Comment: RevTeX, 4 pages, 3 figures in uufiles form, to appear in Phys. Rev.
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Spiral surface growth without desorption
Spiral surface growth is well understood in the limit where the step motion
is controlled by the local supersaturation of adatoms near the spiral ridge. In
epitaxial thin-film growth, however, spirals can form in a step-flow regime
where desorption of adatoms is negligible and the ridge dynamics is governed by
the non-local diffusion field of adatoms on the whole surface. We investigate
this limit numerically using a phase-field formulation of the
Burton-Cabrera-Frank model, as well as analytically. Quantitative predictions,
which differ strikingly from those of the local limit, are made for the
selected step spacing as a function of the deposition flux, as well as for the
dependence of the relaxation time to steady-state growth on the screw
dislocation density.Comment: 9 pages, 3 figures, RevTe
Cardio-metabolic risk factors and cortical thickness in a neurologically healthy male population: results from the psychological, social and biological determinants of ill health (pSoBid) study
<p>Introduction: Cardio-metabolic risk factors have been associated with poor physical and mental health. Epidemiological studies have shown peripheral risk markers to be associated with poor cognitive functioning in normal healthy population and in disease. The aim of the study was to explore the relationship between cardio-metabolic risk factors and cortical thickness in a neurologically healthy middle aged population-based sample.</p>
<p>Methods: T1-weighted MRI was used to create models of the cortex for calculation of regional cortical thickness in 40 adult males (average age = 50.96 years), selected from the PSOBID study. The relationship between cardio-vascular risk markers and cortical thickness across the whole brain, were examined using the general linear models. The relationship with various covariates of interest was explored.</p>
<p>Results: Lipid fractions with greater triglyceride content (TAG, VLDL and LDL) were associated with greater cortical thickness pertaining to a number of regions in the brain. Greater C reactive protein (CRP) and Intercellular adhesion molecule (ICAM-1) levels were associated with cortical thinning pertaining to perisylvian regions in the left hemisphere. Smoking status and education status were significant covariates in the model.</p>
<p>Conclusions: This exploratory study adds to a small body of existing literature increasingly showing a relationship between cardio-metabolic risk markers and regional cortical thickness involving a number of regions in the brain in a neurologically normal middle aged sample. A focused investigation of factors determining the inter-individual variations in regional cortical thickness in the adult brain could provide further clarity in our understanding of the relationship between cardio-metabolic factors and cortical structures.</p>
The drivers of Chinese CO2 emissions from 1980 to 2030
China's energy consumption doubled within the first 25 years of economic reforms initiated at the end of the 1970s, and doubled again in the past 5 years. It has resulted of a threefold CO2 emissions increase since early of 1980s. China's heavy reliance on coal will make it the largest emitter of CO2 in the world. By combining structural decomposition and input–output analysis we seek to assess the driving forces of China's CO2 emissions from 1980 to 2030. In our reference scenario, production-related CO2 emissions will increase another three times by 2030. Household consumption, capital investment and growth in exports will largely drive the increase in CO2 emissions. Efficiency gains will be partially offset the projected increases in consumption, but our scenarios show that this will not be sufficient if China's consumption patterns converge to current US levels. Relying on efficiency improvements alone will not stabilize China's future emissions. Our scenarios show that even extremely optimistic assumptions of widespread installation of carbon dioxide capture and storage will only slow the increase in CO2 emissions
Impurity-induced diffusion bias in epitaxial growth
We introduce two models for the action of impurities in epitaxial growth. In
the first, the interaction between the diffusing adatoms and the impurities is
``barrier''-like and, in the second, it is ``trap''-like. For the barrier
model, we find a symmetry breaking effect that leads to an overall down-hill
current. As expected, such a current produces Edwards-Wilkinson scaling. For
the trap model, no symmetry breaking occurs and the scaling behavior appears to
be of the conserved-KPZ type.Comment: 5 pages(with the 5 figures), latex, revtex3.0, epsf, rotate, multico
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