868 research outputs found
Emergence of quasiparticle Bloch states in artificial crystals crafted atom-by-atom
The interaction of electrons with a periodic potential of atoms in
crystalline solids gives rise to band structure. The band structure of existing
materials can be measured by photoemission spectroscopy and accurately
understood in terms of the tight-binding model, however not many experimental
approaches exist that allow to tailor artificial crystal lattices using a
bottom-up approach. The ability to engineer and study atomically crafted
designer materials by scanning tunnelling microscopy and spectroscopy (STM/STS)
helps to understand the emergence of material properties. Here, we use atom
manipulation of individual vacancies in a chlorine monolayer on Cu(100) to
construct one- and two-dimensional structures of various densities and sizes.
Local STS measurements reveal the emergence of quasiparticle bands, evidenced
by standing Bloch waves, with tuneable dispersion. The experimental data are
understood in terms of a tight-binding model combined with an additional
broadening term that allows an estimation of the coupling to the underlying
substrate.Comment: 7 figures, 12 pages, main text and supplementary materia
Irreversible nucleation in molecular beam epitaxy: From theory to experiments
Recently, the nucleation rate on top of a terrace during the irreversible
growth of a crystal surface by MBE has been determined exactly. In this paper
we go beyond the standard model usually employed to study the nucleation
process, and we analyze the qualitative and quantitative consequences of two
important additional physical ingredients: the nonuniformity of the
Ehrlich-Schwoebel barrier at the step-edge, because of the existence of kinks,
and the steering effects, due to the interaction between the atoms of the flux
and the substrate. We apply our results to typical experiments of second layer
nucleation.Comment: 11 pages. Table I corrected and one appendix added. To be published
in Phys. Rev. B (scheduled issue: 15 February 2003
Island nucleation in the presence of step edge barriers: Theory and applications
We develop a theory of nucleation on top of two-dimensional islands bordered
by steps with an additional energy barrier for descending atoms.
The theory is based on the concept of the residence time of an adatom on the
island,and yields an expression for the nucleation rate which becomes exact in
the limit of strong step edge barriers. This expression differs qualitatively
and quantitatively from that obtained using the conventional rate equation
approach to nucleation [J. Tersoff et al., Phys. Rev. Lett.72, 266 (1994)]. We
argue that rate equation theory fails because nucleation is dominated by the
rare instances when two atoms are present on the island simultaneously. The
theory is applied to two distinct problems: The onset of second layer
nucleation in submonolayer growth, and the distribution of the sizes of top
terraces of multilayer mounds under conditions of strong step edge barriers.
Application to homoepitaxial growth on Pt(111) yields the estimate eV for the additional energy barrier at CO-decorated steps.Comment: 13 pages, 3 figure
Spatio-temporal distribution of nucleation events during crystal growth
We consider irreversible second-layer nucleation that occurs when two adatoms
on a terrace meet. We solve the problem analytically in one dimension for zero
and infinite step-edge barriers, and numerically for any value of the barriers
in one and two dimensions. For large barriers, the spatial distribution of
nucleation events strongly differs from , where is the
stationary adatom density in the presence of a constant flux. The probability
that nucleation occurs at time after the deposition of the second
adatom, decays for short time as a power law [] in and
logarithmically [] in ; for long time it decays
exponentially. Theories of the nucleation rate based on the assumption
that it is proportional to are shown to overestimate by a
factor proportional to the number of times an adatom diffusing on the terrace
visits an already visited lattice site.Comment: 4 pages, 3 figures; accepted for publication on PR
Linear theory of unstable growth on rough surfaces
Unstable homoepitaxy on rough substrates is treated within a linear continuum
theory. The time dependence of the surface width is governed by three
length scales: The characteristic scale of the substrate roughness, the
terrace size and the Ehrlich-Schwoebel length . If (weak step edge barriers) and ,
then displays a minimum at a coverage , where the initial surface width is reduced by a factor
. The r\^{o}le of deposition and diffusion noise is analyzed. The
results are applied to recent experiments on the growth of InAs buffer layers
[M.F. Gyure {\em et al.}, Phys. Rev. Lett. {\bf 81}, 4931 (1998)]. The overall
features of the observed roughness evolution are captured by the linear theory,
but the detailed time dependence shows distinct deviations which suggest a
significant influence of nonlinearities
Renal impairment after liver transplantation - a pilot trial of calcineurin inhibitor-free vs. calcineurin inhibitor sparing immunosuppression in patients with mildly impaired renal function after liver transplantation
<p>Abstract</p> <p>Objectives</p> <p>Chronic kidney disease is frequent in patients after orthotopic liver transplantation (OLT) and has impact on survival. Patients receiving calcineurin inhibitors (CNI) are at increased risk to develop impaired renal function. Early CNI reduction and concomitant use of mycophenolat mofetil (MMF) has been shown to improve renal function.</p> <p>Methods</p> <p>The aim of this trial was to compare dose-reduced CNI/MMF versus CNI-free MMF/prednisone-based treatment in stable patients after OLT with respect to glomerular filtration rate (GFR). 21 patients [GFR 44.9 ± 9.9 mL/min/1.73 m<sup>2 </sup>measured by 99m-Tc-DTPA-clearance, serum creatinine (SCr) 1.5 ± 0.42 mg/dL] were randomized either to exchange CNI for 10 mg prednisone (group 1; n = 8) or to receive CNI at 25% of the initial dose (group 2; n = 13) each in combination with 1000 mg MMF b.i.d.</p> <p>Results</p> <p>At month 12 mean SCr (-0.3 ± 0.4 mg/dL, p = 0.031) and GFR improved (8.6 ± 13.1 mL/min/1.73 m<sup>2</sup>, p = 0.015) in group 2 but remained unchanged in group 1. Main side effects were gastroinstestinal symptoms (14.3%) and infections (4.8%). Two biopsy proven, steroid-responsive rejections occurred. In group 1 mean diastolic blood pressure (BP) increased by 11 ± 22 mmHg (p = 0.03).</p> <p>Conclusions</p> <p>Reduced dose CNI in combination with MMF but not CNI-free-immunosuppression leads to improvement of GFR in patients with moderately elevated SCr levels after OLT. Addition of steroids resulted in increased diastolic blood pressure presumably counterbalancing the benefits of CNI withdrawal on renal function.</p
The limnology of an African lake: Lake Naivasha, Kenya
Lake Naivasha is a shallow freshwater lake which shares a
common depression with two saline lakes, Elementeita and Namkuru in the Eastern or Gregory Rift Valley of Kenya. The
Nakuru-Naivasha basin is bounded,to the east by the Aberdare Range and the Kinangop Plateau, by the Mau Escarpment to the west (Figure 1). The valley floor, extensively broken by secondary faulting, is still volcanically active. To the south of Lake Naivasha, Longonot and several smaller volcanoes form a barrier which is breached by the Gorge ,a former outlet of the lake. To the the Naivasha basin is partially
separated from the Elementeita-Nakurubasin by the Eburu
mountains
Simulations of energetic beam deposition: from picoseconds to seconds
We present a new method for simulating crystal growth by energetic beam
deposition. The method combines a Kinetic Monte-Carlo simulation for the
thermal surface diffusion with a small scale molecular dynamics simulation of
every single deposition event. We have implemented the method using the
effective medium theory as a model potential for the atomic interactions, and
present simulations for Ag/Ag(111) and Pt/Pt(111) for incoming energies up to
35 eV. The method is capable of following the growth of several monolayers at
realistic growth rates of 1 monolayer per second, correctly accounting for both
energy-induced atomic mobility and thermal surface diffusion. We find that the
energy influences island and step densities and can induce layer-by-layer
growth. We find an optimal energy for layer-by-layer growth (25 eV for Ag),
which correlates with where the net impact-induced downward interlayer
transport is at a maximum. A high step density is needed for energy induced
layer-by-layer growth, hence the effect dies away at increased temperatures,
where thermal surface diffusion reduces the step density. As part of the
development of the method, we present molecular dynamics simulations of single
atom-surface collisions on flat parts of the surface and near straight steps,
we identify microscopic mechanisms by which the energy influences the growth,
and we discuss the nature of the energy-induced atomic mobility
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