1,662 research outputs found
A multi-phase field model of planar dislocation networks
In this paper we extend the phase-field model of crystallographic slip of Ortiz (1999 J. Appl. Mech. ASME 66 289â98) and Koslowski et al (2001 J. Mech. Phys. Solids 50 2957â635) to slip processes that require the activation of multiple slip systems, and we apply the resulting model to the investigation of finite twist boundary arrays. The distribution of slip over a slip plane is described by means of multiple integer-valued phase fields. We show how all the terms in the total energy of the crystal, including the long-range elastic energy and the Peierls interplanar energy, can be written explicitly in terms of the multi-phase field. The model is used to ascertain stable dislocation structures arising in an array of finite twist boundaries. These structures are found to consist of regular square or hexagonal dislocation networks separated by complex dislocation pile-ups over the intervening transition layers
The electronic structure of amorphous silica: A numerical study
We present a computational study of the electronic properties of amorphous
SiO2. The ionic configurations used are the ones generated by an earlier
molecular dynamics simulations in which the system was cooled with different
cooling rates from the liquid state to a glass, thus giving access to
glass-like configurations with different degrees of disorder [Phys. Rev. B 54,
15808 (1996)]. The electronic structure is described by a tight-binding
Hamiltonian. We study the influence of the degree of disorder on the density of
states, the localization properties, the optical absorption, the nature of
defects within the mobility gap, and on the fluctuations of the Madelung
potential, where the disorder manifests itself most prominently. The
experimentally observed mismatch between a photoconductivity threshold of 9 eV
and the onset of the optical absorption around 7 eV is interpreted by the
picture of eigenstates localized by potential energy fluctuations in a mobility
gap of approximately 9 eV and a density of states that exhibits valence and
conduction band tails which are, even in the absence of defects, deeply located
within the former band gap.Comment: 21 pages of Latex, 5 eps figure
Singular kernels, multiscale decomposition of microstructure, and dislocation models
We consider a model for dislocations in crystals introduced by Koslowski,
Cuiti\~no and Ortiz, which includes elastic interactions via a singular kernel
behaving as the norm of the slip. We obtain a sharp-interface limit
of the model within the framework of -convergence. From an analytical
point of view, our functional is a vector-valued generalization of the one
studied by Alberti, Bouchitt\'e and Seppecher to which their rearrangement
argument no longer applies. Instead we show that the microstructure must be
approximately one-dimensional on most length scales and exploit this property
to derive a sharp lower bound
Cloud computing in nanoHUB powering education and research
We present a tool that uses a phase field approach to simulate plastic deformation in nanocrystalline materials. It captures the competing grain-boundary and dislocation-mediated deformation mechanisms that govern plastic deformation in these materials. The model is based on a multiphase field approach in which dislocations and grain boundary sliding are represented by means of scalar phase fields described in âThe role of grain boundary energetics on the maximum strength of nanocrystalline Niâ, Koslowski, Lee and Lei, Journal of the Mechanics and Physics of Solids, 59 1427â1436, 2011. The tool enables users to quantify how uncertainties in the input parameters (materials properties such as elastic constants, Peierls energy barrier for dislocation glide, and activation barrier for grain boundary sliding) affect the prediction of the yield stress. In addition, it provides a sensitivity analysis that quantifies the relative importance of each input variable. In order to achieve this, the phase field simulation code is orchestrated by the PRISM Uncertainty Quantification tool that enables users to select various state-of-the-art methods for uncertainty propagation
The Link between General Relativity and Shape Dynamics
We show that one can construct two equivalent gauge theories from a linking
theory and give a general construction principle for linking theories which we
use to construct a linking theory that proves the equivalence of General
Relativity and Shape Dynamics, a theory with fixed foliation but spatial
conformal invariance. This streamlines the rather complicated construction of
this equivalence performed previously. We use this streamlined argument to
extend the result to General Relativity with asymptotically flat boundary
conditions. The improved understanding of linking theories naturally leads to
the Lagrangian formulation of Shape Dynamics, which allows us to partially
relate the degrees of freedom.Comment: 19 pages, LaTeX, no figure
Preferred foliation effects in Quantum General Relativity
We investigate the infrared (IR) effects of Lorentz violating terms in the
gravitational sector using functional renormalization group methods similar to
Reuter and collaborators. The model we consider consists of pure quantum
gravity coupled to a preferred foliation, described effectively via a scalar
field with non-standard dynamics. We find that vanishing Lorentz violation is a
UV attractive fixed-point of this model in the local potential approximation.
Since larger truncations may lead to differing results, we study as a first
example effects of additional matter fields on the RG running of the Lorentz
violating term and provide a general argument why they are small.Comment: 12 pages, no figures, compatible with published versio
Cold Reactions of Alkali and Water Clusters inside Helium Nanodroplets
The reaction of alkali (Na, Cs) clusters with water clusters embedded in
helium nanodroplets is studied using femtosecond photo-ionization as well as
electron impact ionization. Unlike Na clusters, Cs clusters are found to
completely react with water in spite of the ultracold helium droplet
environment. Mass spectra of the Cs+(HO) reaction products are
interpreted in terms of stability with respect to fragmentation using
high-level molecular structure calculations
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