1,279 research outputs found
Unified Description of Aging and Rate Effects in Yield of Glassy Solids
The competing effects of slow structural relaxations (aging) and deformation
at constant strain rate on the shear yield stress of simple model
glasses are examined using molecular simulations. At long times, aging leads to
a logarithmic increase in density and . The yield stress also rises
logarithmically with rate, but shows a sharp transition in slope at a rate that
decreases with increasing age. We present a simple phenomenological model that
includes both intrinsic rate dependence and the change in properties with the
total age of the system at yield. As predicted by the model, all data for each
temperature collapse onto a universal curve.Comment: 4 pages, 3 figure
Optimal protocols for Hamiltonian and Schr\"odinger dynamics
For systems in an externally controllable time-dependent potential, the
optimal protocol minimizes the mean work spent in a finite-time transition
between given initial and final values of a control parameter. For an initially
thermalized ensemble, we consider both Hamiltonian evolution for classical
systems and Schr\"odinger evolution for quantum systems. In both cases, we show
that for harmonic potentials, the optimal work is given by the adiabatic work
even in the limit of short transition times. This result is counter-intuitive
because the adiabatic work is substantially smaller than the work for an
instantaneous jump. We also perform numerical calculations of the optimal
protocol for Hamiltonian dynamics in an anharmonic quartic potential. For a
two-level spin system, we give examples where the adiabatic work can be reached
in either a finite or an arbitrarily short transition time depending on the
allowed parameter space.Comment: submitted to J. Stat. Mech.: Theor. Exp
The mechanisms of spatial and temporal earthquake clustering
The number of earthquakes as a function of magnitude decays as a power law.
This trend is usually justified using spring-block models, where slips with the
appropriate global statistics have been numerically observed. However,
prominent spatial and temporal clustering features of earthquakes are not
reproduced by this kind of modeling. We show that when a spring-block model is
complemented with a mechanism allowing for structural relaxation, realistic
earthquake patterns are obtained. The proposed model does not need to include a
phenomenological velocity weakening friction law, as traditional spring-block
models do, since this behavior is effectively induced by the relaxational
mechanism as well. In this way, the model provides also a simple microscopic
basis for the widely used phenomenological rate-and-state equations of rock
friction.Comment: 7 pages, 10 figures, comments welcom
Variety of idempotents in nonassociative algebras
In this paper, we study the variety of all nonassociative (NA) algebras from
the idempotent point of view. We are interested, in particular, in the spectral
properties of idempotents when algebra is generic, i.e. idempotents are in
general position. Our main result states that in this case, there exist at
least nontrivial obstructions (syzygies) on the Peirce spectrum of a
generic NA algebra of dimension . We also discuss the exceptionality of the
eigenvalue which appears in the spectrum of idempotents in
many classical examples of NA algebras and characterize its extremal properties
in metrised algebras.Comment: 27 pages, 1 figure, submitte
Dynamics of Viscoplastic Deformation in Amorphous Solids
We propose a dynamical theory of low-temperature shear deformation in
amorphous solids. Our analysis is based on molecular-dynamics simulations of a
two-dimensional, two-component noncrystalline system. These numerical
simulations reveal behavior typical of metallic glasses and other viscoplastic
materials, specifically, reversible elastic deformation at small applied
stresses, irreversible plastic deformation at larger stresses, a stress
threshold above which unbounded plastic flow occurs, and a strong dependence of
the state of the system on the history of past deformations. Microscopic
observations suggest that a dynamically complete description of the macroscopic
state of this deforming body requires specifying, in addition to stress and
strain, certain average features of a population of two-state shear
transformation zones. Our introduction of these new state variables into the
constitutive equations for this system is an extension of earlier models of
creep in metallic glasses. In the treatment presented here, we specialize to
temperatures far below the glass transition, and postulate that irreversible
motions are governed by local entropic fluctuations in the volumes of the
transformation zones. In most respects, our theory is in good quantitative
agreement with the rich variety of phenomena seen in the simulations.Comment: 16 pages, 9 figure
Boundary lubrication with a glassy interface
Recently introduced constitutive equations for the rheology of dense,
disordered materials are investigated in the context of stick-slip experiments
in boundary lubrication. The model is based on a generalization of the shear
transformation zone (STZ) theory, in which plastic deformation is represented
by a population of mesoscopic regions which may undergo non affine deformations
in response to stress. The generalization we study phenomenologically
incorporates the effects of aging and glassy relaxation. Under experimental
conditions associated with typical transitions from stick-slip to steady
sliding and stop start tests, these effects can be dominant, although the full
STZ description is necessary to account for more complex, chaotic transitions
On the Backbending Mechanism of Cr
The mechanism of backbending in Cr is investigated in terms of the
Projected Shell Model and the Generator Coordinate Method. It is shown that
both methods are reasonable shell model truncation schemes. These two quite
different quantum mechanical approaches lead to a similar conclusion that the
backbending is due to a band crossing involving an excited band which is built
on simultaneously broken neutron and proton pairs in the ``intruder'' subshell
. It is pointed out that this type of band crossing is usually known
to cause the second backbending in rare-earth nuclei.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
VOLT: a novel open-source pipeline for automatic segmentation of endolymphatic space in inner ear MRI
BACKGROUND Objective and volumetric quantification is a necessary step in the assessment and comparison of endolymphatic hydrops (ELH) results. Here, we introduce a novel tool for automatic volumetric segmentation of the endolymphatic space (ELS) for ELH detection in delayed intravenous gadolinium-enhanced magnetic resonance imaging of inner ear (iMRI) data.
METHODS The core component is a novel algorithm based on Volumetric Local Thresholding (VOLT). The study included three different data sets: a real-world data set (D1) to develop the novel ELH detection algorithm and two validating data sets, one artificial (D2) and one entirely unseen prospective real-world data set (D3). D1 included 210 inner ears of 105 patients (50 male; mean age 50.4 ± 17.1 years), and D3 included 20 inner ears of 10 patients (5 male; mean age 46.8 ± 14.4 years) with episodic vertigo attacks of different etiology. D1 and D3 did not differ significantly concerning age, gender, the grade of ELH, or data quality. As an artificial data set, D2 provided a known ground truth and consisted of an 8-bit cuboid volume using the same voxel-size and grid as real-world data with different sized cylindrical and cuboid-shaped cutouts (signal) whose grayscale values matched the real-world data set D1 (mean 68.7 ± 7.8; range 48.9-92.8). The evaluation included segmentation accuracy using the Sørensen-Dice overlap coefficient and segmentation precision by comparing the volume of the ELS.
RESULTS VOLT resulted in a high level of performance and accuracy in comparison with the respective gold standard. In the case of the artificial data set, VOLT outperformed the gold standard in higher noise levels. Data processing steps are fully automated and run without further user input in less than 60 s. ELS volume measured by automatic segmentation correlated significantly with the clinical grading of the ELS (p < 0.01).
CONCLUSION VOLT enables an open-source reproducible, reliable, and automatic volumetric quantification of the inner ears' fluid space using MR volumetric assessment of endolymphatic hydrops. This tool constitutes an important step towards comparable and systematic big data analyses of the ELS in patients with the frequent syndrome of episodic vertigo attacks. A generic version of our three-dimensional thresholding algorithm has been made available to the scientific community via GitHub as an ImageJ-plugin
Phenotypic indicators to identify methionine rich European grain legumes and the correlation of grain methionine contents with the sulphur supply
Home grown legumes are a valuable protein source for pure on-farm diets for livestock in organic farming. Whereas protein of Glycine max naturally has higher contents of methionine nand also lysine typical European grain legumes (Pisum sativum L., Vicia faba L., Lupinus angustifolius L.) used in organic farms as component of animal food are relatively low in those amino acids
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