Strongly anisotropic roughness in surfaces driven by an oblique particle flux

Using field theoretic renormalization, an MBE-type growth process with an obliquely incident influx of atoms is examined. The projection of the beam on the substrate plane selects a "parallel" direction, with rotational invariance restricted to the transverse directions. Depending on the behavior of an effective anisotropic surface tension, a line of second order transitions is identified, as well as a line of potentially first order transitions, joined by a multicritical point. Near the second order transitions and the multicritical point, the surface roughness is strongly anisotropic. Four different roughness exponents are introduced and computed, describing the surface in different directions, in real or momentum space. The results presented challenge an earlier study of the multicritical point.Comment: 11 pages, 2 figures, REVTeX

Wetting layer thickness and early evolution of epitaxially strained thin films

We propose a physical model which explains the existence of finite thickness wetting layers in epitaxially strained films. The finite wetting layer is shown to be stable due to the variation of the non-linear elastic free energy with film thickness. We show that anisotropic surface tension gives rise to a metastable enlarged wetting layer. The perturbation amplitude needed to destabilize this wetting layer decreases with increasing lattice mismatch. We observe the development of faceted islands in unstable films.Comment: 4 pages, 3 eps figure

A molecular-beam study of the dissociative chemisorption of O2 on Ir(110)-(1×2)

The zero-coverage probability of dissociative chemisorption of O2 on Ir(110)-(1×2) has been measured using molecular-beam techniques for a wide range of incident kinetic energies, incident angles, and surface temperatures. The data indicate that a trapping-mediated mechanism is responsible for dissociative chemisorption at low energies, whereas at high energies a direct mechanism accounts for dissociative adsorption. Total energy scaling approximately describes the dissociative dynamics on the very corrugated Ir(110)-(1×2) surface

Changing shapes in the nanoworld

What are the mechanisms leading to the shape relaxation of three dimensional crystallites ? Kinetic Monte Carlo simulations of fcc clusters show that the usual theories of equilibration, via atomic surface diffusion driven by curvature, are verified only at high temperatures. Below the roughening temperature, the relaxation is much slower, kinetics being governed by the nucleation of a critical germ on a facet. We show that the energy barrier for this step linearly increases with the size of the crystallite, leading to an exponential dependence of the relaxation time.Comment: 4 pages, 5 figures. Accepted by Phys Rev Let

Current practice in the diagnosis and management of fetal growth restriction: An international survey

Introduction The aim of this survey was to evaluate the current practice in respect of diagnosis and management of fetal growth restriction among obstetricians in different countries. Material and methods An e-questionnaire was sent via REDCap with "click thru" links in emails and newsletters to obstetric practitioners in different countries and settings with different levels of expertise. Clinical scenarios in early and late fetal growth restriction were given, followed by structured questions/response pairings. Results A total of 275 participants replied to the survey with 87% of responses complete. Participants were obstetrician/gynecologists (54%; 148/275) and fetal medicine specialists (43%; 117/275), and the majority practiced in a tertiary teaching hospital (56%; 153/275). Delphi consensus criteria for fetal growth restriction diagnosis were used by 81% of participants (223/275) and 82% (225/274) included a drop in fetal growth velocity in their diagnostic criteria for late fetal growth restriction. For early fetal growth restriction, TRUFFLE criteria were used for fetal monitoring and delivery timing by 81% (223/275). For late fetal growth restriction, indices of cerebral blood flow redistribution were used by 99% (250/252), most commonly cerebroplacental ratio (54%, 134/250). Delivery timing was informed by cerebral blood flow redistribution in 72% (176/244), used from >= 32 weeks of gestation. Maternal biomarkers and hemodynamics, as additional tools in the context of early-onset fetal growth restriction (<= 32 weeks of gestation), were used by 22% (51/232) and 46% (106/230), respectively. Conclusions The diagnosis and management of fetal growth restriction are fairly homogeneous among different countries and levels of practice, particularly for early fetal growth restriction. Indices of cerebral flow distribution are widely used in the diagnosis and management of late fetal growth restriction, whereas maternal biomarkers and hemodynamics are less frequently assessed but more so in early rather than late fetal growth restriction. Further standardization is needed for the definition of cerebral blood flow redistribution

Area-preserving dynamics of a long slender finger by curvature: a test case for the globally conserved phase ordering

A long and slender finger can serve as a simple ``test bed'' for different phase ordering models. In this work, the globally-conserved, interface-controlled dynamics of a long finger is investigated, analytically and numerically, in two dimensions. An important limit is considered when the finger dynamics are reducible to the area-preserving motion by curvature. A free boundary problem for the finger shape is formulated. An asymptotic perturbation theory is developed that uses the finger aspect ratio as a small parameter. The leading-order approximation is a modification of ``the Mullins finger" (a well-known analytic solution) which width is allowed to slowly vary with time. This time dependence is described, in the leading order, by an exponential law with the characteristic time proportional to the (constant) finger area. The subleading terms of the asymptotic theory are also calculated. Finally, the finger dynamics is investigated numerically, employing the Ginzburg-Landau equation with a global conservation law. The theory is in a very good agreement with the numerical solution.Comment: 8 pages, 4 figures, Latex; corrected typo