551 research outputs found
Indium-Gallium Segregation in CuInGaSe: An ab initio based Monte Carlo Study
Thin-film solar cells with CuInGaSe (CIGS) absorber are still
far below their efficiency limit, although lab cells reach already 19.9%. One
important aspect is the homogeneity of the alloy. Large-scale simulations
combining Monte Carlo and density functional calculations show that two phases
coexist in thermal equilibrium below room temperature. Only at higher
temperatures, CIGS becomes more and more a homogeneous alloy. A larger degree
of inhomogeneity for Ga-rich CIGS persists over a wide temperature range, which
may contribute to the low observed efficiency of Ga-rich CIGS solar cells
Configuration management for Lyee software
This article presents a configuration management concept for software projects using Lyee methodology. To illustrate this concept, an introduction in configuration management is given. Then, the structure of Lyee programs is defined by sets and their dependencies. From this structure, the actual configuration management concept is deduced and discussed by rendering the structure for an existing configuration
management testbed and describing the involved key players as well as the necessary procedures
Liquid crystal films on curved surfaces: An entropic sampling study
The confining effect of a spherical substrate inducing anchoring (normal to
the surface) of rod-like liquid crystal molecules contained in a thin film
spread over it has been investigated with regard to possible changes in the
nature of the isotropic-to-nematic phase transition as the sample is cooled.
The focus of these Monte Carlo simulations is to study the competing effects of
the homeotropic anchoring due to the surface inducing orientational ordering in
the radial direction and the inherent uniaxial order promoted by the
intermolecular interactions. By adopting entropic sampling procedure, we could
investigate this transition with a high temperature precision, and we studied
the effect of the surface anchoring strength on the phase diagram for a
specifically chosen geometry. We find that there is a threshold anchoring
strength of the surface below which uniaxial nematic phase results, and above
which the isotropic fluid cools to a radially ordered nematic phase, besides of
course expected changes in the phase transition temperature with the anchoring
strength. In the vicinity of the threshold anchoring strength we observe a
bistable region between these two structures, clearly brought out by the
characteristics of the corresponding microstates constituting the entropic
ensemble.Comment: 14 pages, 5 figure
Liquid crystal director fluctuations and surface anchoring by molecular simulation
We propose a simple and reliable method to measure the liquid crystal surface
anchoring strength by molecular simulation. The method is based on the
measurement of the long-range fluctuation modes of the director in confined
geometry. As an example, molecular simulations of a liquid crystal in slab
geometry between parallel walls with homeotropic anchoring have been carried
out using the Monte Carlo technique. By studying different slab thicknesses, we
are able to calculate separately the position of the elastic boundary
condition, and the extrapolation length
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Targeted DamID reveals differential binding of mammalian pluripotency factors.
The precise control of gene expression by transcription factor networks is crucial to organismal development. The predominant approach for mapping transcription factor-chromatin interactions has been chromatin immunoprecipitation (ChIP). However, ChIP requires a large number of homogeneous cells and antisera with high specificity. A second approach, DamID, has the drawback that high levels of Dam methylase are toxic. Here, we modify our targeted DamID approach (TaDa) to enable cell type-specific expression in mammalian systems, generating an inducible system (mammalian TaDa or MaTaDa) to identify genome-wide protein/DNA interactions in 100 to 1000 times fewer cells than ChIP-based approaches. We mapped the binding sites of two key pluripotency factors, OCT4 and PRDM14, in mouse embryonic stem cells, epiblast-like cells and primordial germ cell-like cells (PGCLCs). PGCLCs are an important system for elucidating primordial germ cell development in mice. We monitored PRDM14 binding during the specification of PGCLCs, identifying direct targets of PRDM14 that are key to understanding its crucial role in PGCLC development. We show that MaTaDa is a sensitive and accurate method for assessing cell type-specific transcription factor binding in limited numbers of cells
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Metabolic regulation of pluripotency and germ cell fate through α-ketoglutarate.
An intricate link is becoming apparent between metabolism and cellular identities. Here, we explore the basis for such a link in an in vitro model for early mouse embryonic development: from naïve pluripotency to the specification of primordial germ cells (PGCs). Using single-cell RNA-seq with statistical modelling and modulation of energy metabolism, we demonstrate a functional role for oxidative mitochondrial metabolism in naïve pluripotency. We link mitochondrial tricarboxylic acid cycle activity to IDH2-mediated production of alpha-ketoglutarate and through it, the activity of key epigenetic regulators. Accordingly, this metabolite has a role in the maintenance of naïve pluripotency as well as in PGC differentiation, likely through preserving a particular histone methylation status underlying the transient state of developmental competence for the PGC fate. We reveal a link between energy metabolism and epigenetic control of cell state transitions during a developmental trajectory towards germ cell specification, and establish a paradigm for stabilizing fleeting cellular states through metabolic modulation.J.T. was supported by the Austrian Academy of Sciences, the Wellcome Trust, and the Swiss National Fund for Science, W.H.G. by EMBO and the Wellcome Trust, J.R. and L.W. by the UK Medical Research Council, and E.A. by the Wellcome Trust. M.A.S. is a Wellcome Senior Investigator. Work at the Gurdon Institute is supported by a core grant from the Wellcome Trust and Cancer Research UK
Phase-field-crystal models for condensed matter dynamics on atomic length and diffusive time scales: an overview
Here, we review the basic concepts and applications of the
phase-field-crystal (PFC) method, which is one of the latest simulation
methodologies in materials science for problems, where atomic- and microscales
are tightly coupled. The PFC method operates on atomic length and diffusive
time scales, and thus constitutes a computationally efficient alternative to
molecular simulation methods. Its intense development in materials science
started fairly recently following the work by Elder et al. [Phys. Rev. Lett. 88
(2002), p. 245701]. Since these initial studies, dynamical density functional
theory and thermodynamic concepts have been linked to the PFC approach to serve
as further theoretical fundaments for the latter. In this review, we summarize
these methodological development steps as well as the most important
applications of the PFC method with a special focus on the interaction of
development steps taken in hard and soft matter physics, respectively. Doing
so, we hope to present today's state of the art in PFC modelling as well as the
potential, which might still arise from this method in physics and materials
science in the nearby future.Comment: 95 pages, 48 figure
Topological Defects in Nematic Droplets of Hard Spherocylinders
Using computer simulations we investigate the microscopic structure of the
singular director field within a nematic droplet. As a theoretical model for
nematic liquid crystals we take hard spherocylinders. To induce an overall
topological charge, the particles are either confined to a two-dimensional
circular cavity with homeotropic boundary or to the surface of a
three-dimensional sphere. Both systems exhibit half-integer topological point
defects. The isotropic defect core has a radius of the order of one particle
length and is surrounded by free-standing density oscillations. The effective
interaction between two defects is investigated. All results should be
experimentally observable in thin sheets of colloidal liquid crystals.Comment: 13 pages, 16 figures, Phys. Rev.
Experimental Facilities Development
This research was sponsored by the National Science Foundation Grant NSF PHy 87-1440
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