1,494 research outputs found
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 “The role of grain boundary energetics on the maximum strength of nanocrystalline Ni”, Koslowski, Lee and Lei, Journal of the Mechanics and Physics of Solids, 59 1427–1436, 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
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
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
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
Finite size effects and localization properties of disordered quantum wires with chiral symmetry
Finite size effects in the localization properties of disordered quantum
wires are analyzed through conductance calculations. Disorder is induced by
introducing vacancies at random positions in the wire and thus preserving the
chiral symmetry. For quasi one-dimensional geometries and low concentration of
vacancies, an exponential decay of the mean conductance with the wire length is
obtained even at the center of the energy band. For wide wires, finite size
effects cause the conductance to decay following a non-pure exponential law. We
propose an analytical formula for the mean conductance that reproduces
accurately the numerical data for both geometries. However, when the
concentration of vacancies increases above a critical value, a transition
towards the suppression of the conductance occurs.
This is a signature of the presence of ultra-localized states trapped in
finite regions of the sample.Comment: 5 figures, revtex
Mixed Atomistic–Continuum Models of Material Behavior: The Art of Transcending Atomistics and Informing Continua
The recent development of microscopes that allow for the examination of defects at the atomic scale has made possible a more direct connection between the defects and the macroscopic response they engender (see, e.g., the December 1999 issue of MRS Bulletin)
Semiempirical Quantum-Chemical Orthogonalization-Corrected Methods: Theory, Implementation, and Parameters
Semiempirical orthogonalization-corrected methods (OM1, OM2, and OM3) go beyond the standard MNDO model by explicitly including additional interactions into the Fock matrix in an approximate manner (Pauli repulsion, penetration effects, and core–valence interactions), which yields systematic improvements both for ground-state and excited-state properties. In this Article, we describe the underlying theoretical formalism of the OMx methods and their implementation in full detail, and we report all relevant OMx parameters for hydrogen, carbon, nitrogen, oxygen, and fluorine. For a standard set of mostly organic molecules commonly used in semiempirical method development, the OMx results are found to be superior to those from standard MNDO-type methods. Parametrized Grimme-type dispersion corrections can be added to OM2 and OM3 energies to provide a realistic treatment of noncovalent interaction energies, as demonstrated for the complexes in the S22 and S66×8 test sets
Produção, composição bromatológica e extração de potássio pela planta de milho para silagem colhida em duas alturas de corte.
O presente experimento teve como objetivo avaliar as características agronômicas, composição químico-bromatológica e extração de potássio de cinco genótipos de milho para silagem. O delineamento foi realizado em parcelas subdivididas no delineamento em blocos ao acaso, com 3 híbridos (DKB 390, AGX 8517, A-2560) e 2 variedaes (AL-Bianco, Piratininga), em 2 alturas de corte (20 e 40 cm acima do solo) e 4 repetições..
Three-Dimensional Quantum Percolation Studied by Level Statistics
Three-dimensional quantum percolation problems are studied by analyzing
energy level statistics of electrons on maximally connected percolating
clusters. The quantum percolation threshold \pq, which is larger than the
classical percolation threshold \pc, becomes smaller when magnetic fields are
applied, i.e., \pq(B=0)>\pq(B\ne 0)>\pc. The critical exponents are found to
be consistent with the recently obtained values of the Anderson model,
supporting the conjecture that the quantum percolation is classified onto the
same universality classes of the Anderson transition. Novel critical level
statistics at the percolation threshold is also reported.Comment: to appear in the May issue of J. Phys. Soc. Jp
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