617 research outputs found
New Physics of the Partial Dislocation in Silicon Revealed through {\em Ab Initio} Calculation
Based on {\em ab initio} calculation, we propose a new structure for the
fundamental excitation of the reconstructed 30 partial dislocation in
silicon. This soliton has a rare structure involving a five-fold coordinated
atom near the dislocation core. The unique electronic structure of this defect
is consistent with the electron spin resonance signature of the hitherto
enigmatic thermally stable R center of plastically deformed silicon. We present
the first {\em ab initio} determination of the free energy of the soliton,
which is also in agreement with the experimental observation. This
identification suggests the possibility of an experimental determination of the
density of solitons, a key defect in understanding the plastic flow of the
material.Comment: 6 pages, 5 postscript figure
Wenn die Akzeptanz der Supportangebote sinkt – Fehlentwicklung oder strukturelle Notwendigkeit
Das Supportangebot des E-Learning Zentrums der TU Wien wurde im Rahmen des Projekts Delta 3 entwickelt und war damit an der Nominierung als Finalist des Medida Prix 2007 nominiert. In den zwölf Monaten seit der Einreichung ging die Nachfrage – speziell für die Weiterbildungsworkshops – deutlich zurück. Die möglichen Gründe dafür werden selbstkritisch und auch aus strategischer Sicht analysiert, um daraus potenzielle Verbesserungsmaßnahmen ableiten zu können. (DIPF/ Orig.
Recommended from our members
Machine learning based interatomic potential for amorphous carbon
We introduce a Gaussian approximation potential (GAP) for atomistic simulations of liquid and amorphous elemental carbon. Based on a machine learning representation of the density-functional theory (DFT) potential-energy surface, such interatomic potentials enable materials simulations with close-to DFT accuracy but at much lower computational cost. We first determine the maximum accuracy that any finite-range potential can achieve in carbon structures; then, using a hierarchical set of two-, three-, and many-body structural descriptors, we construct a GAP model that can indeed reach the target accuracy. The potential yields accurate energetic and structural properties over a wide range of densities; it also correctly captures the structure of the liquid phases, at variance with a state-of-the-art empirical potential. Exemplary applications of the GAP model to surfaces of “diamondlike” tetrahedral amorphous carbon (-C) are presented, including an estimate of the amorphous material’s surface energy and simulations of high-temperature surface reconstructions (“graphitization”). The presented interatomic potential appears to be promising for realistic and accurate simulations of nanoscale amorphous carbon structures.V.L.D. gratefully acknowledges a postdoctoral fellowship from the Alexander von Humboldt Foundation and support from the Isaac Newton Trust (Trinity College Cambridge). This work used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk) via EPSRC Grant No. EP/K014560/1
A general-purpose machine-learning force field for bulk and nanostructured phosphorus
Elemental phosphorus is attracting growing interest across fundamental and applied fields of research. However, atomistic simulations of phosphorus have remained an out- standing challenge. Here we show that a universally applicable force field for phosphorus can be created by machine learning (ML) from a suitably chosen ensemble of quantum- mechanical results. Our model is fitted to density-functional theory plus many-body dis- persion (DFT+MBD) data; its accuracy is demonstrated for the exfoliation of black and violet phosphorus (yielding monolayers of “phosphorene” and “hittorfene”); its transfer- ability is shown for the transition between the molecular and network liquid phases. An application to a phosphorene nanoribbon on an experimentally relevant length scale ex- emplifies the power of accurate and flexible ML-driven force fields for next-generation materials modelling. The methodology promises new insights into phosphorus as well as other structurally complex, e.g., layered solids that are relevant in diverse areas of chem- istry, physics, and materials science
Deadlocks and waiting times in traffic jam
In a city of right moving and upmoving cars with hardcore constraint, traffic
jam occurs in the form of bands. We show how the bands are destroyed by a small
number of strictly left moving cars yielding a deadlock phase with a rough edge
of left cars. We also show that the probability of waiting time at a signal for
a particular tagged car has a power law dependence on time, indicating the
absence of any characteristic time scale for an emergent traffic jam. The
exponent is same for both the band and the deadlock cases. The significances of
these results are discussed.Comment: 8 pages including 4 eps figures, one in colour, uses revtex to appear
in Physica
Tailoring Graphene with Metals on Top
We study the effects of metallic doping on the electronic properties of
graphene using density functional theory in the local density approximation in
the presence of a local charging energy (LDA+U). The electronic properties are
sensitive to whether graphene is doped with alkali or transition metals. We
estimate the the charge transfer from a single layer of Potassium on top of
graphene in terms of the local charging energy of the graphene sheet. The
coating of graphene with a non-magnetic layer of Palladium, on the other hand,
can lead to a magnetic instability in coated graphene due to the hybridization
between the transition-metal and the carbon orbitals.Comment: 5 pages, 4 figure
Blackbox Lernprozess und informelle Lernszenarien
Im Kontrast zu weit verbreiteten Auffassungen ist es aus der Sicht von Lernpsychologie und Hirnforschung nicht möglich, individuelle Lernprozesse exakt zu steuern. Im Gegenteil: Der individuelle Lernprozess stellt sich als Blackbox dar, deren Output immer wieder nur erstaunt zur Kenntnis genommen werden kann. Alle Versuche, dieses Problem zu lösen, erweisen sich regelmäßig als Ressourcenverschwendung. Als deutlich effizienter könnte es sich hingegen offenbaren, informelle Lernformen als Methode der Wahl massiv einzusetzen und somit den - ohnehin unrealistischen - Kontrollanspruch als Lehrende endgültig aufzugeben. (DIPF/Orig.
Evaluation of the MACE Force Field Architecture: from Medicinal Chemistry to Materials Science
The MACE architecture represents the state of the art in the field of machine
learning force fields for a variety of in-domain, extrapolation and low-data
regime tasks. In this paper, we further evaluate MACE by fitting models for
published benchmark datasets. We show that MACE generally outperforms
alternatives for a wide range of systems from amorphous carbon, universal
materials modelling, and general small molecule organic chemistry to large
molecules and liquid water. We demonstrate the capabilities of the model on
tasks ranging from constrained geometry optimisation to molecular dynamics
simulations and find excellent performance across all tested domains. We show
that MACE is very data efficient, and can reproduce experimental molecular
vibrational spectra when trained on as few as 50 randomly selected reference
configurations. We further demonstrate that the strictly local atom-centered
model is sufficient for such tasks even in the case of large molecules and
weakly interacting molecular assemblies
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