4,870 research outputs found
Effect of atomic scale plasticity on hydrogen diffusion in iron: Quantum mechanically informed and on-the-fly kinetic Monte Carlo simulations
We present an off-lattice, on-the-fly kinetic Monte Carlo (KMC) model for simulating stress-assisted diffusion and trapping of hydrogen by crystalline defects in iron. Given an embedded atom (EAM) potential as input, energy barriers for diffusion are ascertained on the fly from the local environments of H atoms. To reduce computational cost, on-the-fly calculations are supplemented with precomputed strain-dependent energy barriers in defect-free parts of the crystal. These precomputed barriers, obtained with high-accuracy density functional theory calculations, are used to ascertain the veracity of the EAM barriers and correct them when necessary. Examples of bulk diffusion in crystals containing a screw dipole and vacancies are presented. Effective diffusivities obtained from KMC simulations are found to be in good agreement with theory. Our model provides an avenue for simulating the interaction of hydrogen with cracks, dislocations, grain boundaries, and other lattice defects, over extended time scales, albeit at atomistic length scales
The Dual Description of Long Distance QCD and the Effective Lagrangian for Constituent Quarks
We describe long distance QCD by a dual theory in which the fundamental
variables are dual potentials coupled to monopole fields and use this dual
theory to determine the effective Lagrangian for constituent quarks. We find
the color field distribution surrounding a quark anti-quark pair to first order
in their velocities. Using these distributions we eliminate the dual potentials
and obtain an effective interaction Lagrangian depending only upon the quark and anti-quark
coordinates and velocities, valid to second order in their velocities. We
propose as the Lagrangian describing the long distance interaction of
constituent quarks
Regional muscle features and their association with knee extensors force production at a single joint angle
This study aimed (i) to investigate the role of regional characteristics of the knee extensors muscles (vastus lateralis: VL, vastus intermedius: VI and rectus femoris: RF) in determining maximum-voluntary force (MVF); and (ii) to understand which regional parameter of muscle structure would best predict MVF. Muscle architecture (e.g., pennation angle and fascicle length), muscle volume (Vol), anatomical (ACSA) and physiological cross-sectional-area (PCSA) were measured in the proximal (0-33% of the muscle length), middle (33-66% of the muscle length) and distal (66-100% of the muscle length) portions of each muscle in fifteen healthy males using ultrasound and Magnetic Resonance Imaging (MRI). Knee extensors force was calculated in isometric condition at a single knee joint angle of 90 degrees. Regional ACSA, Vol and PCSA were correlated with MVF production. Regional muscle geometry showed no significant correlations with MVF. Among regions, the middle portion of each muscle was largely correlated with MVF compared to all the other regions (distal and proximal). To understand which regional structural parameter best predicted MVF, a stepwise multiple linear regression was performed. This model showed a significant explanatory power (P < 0.001, R-2 = 0.76, adjusted R-2 = 0.71), including muscle Vol collected in the mid portions of VL and RF. Even if no significant differences were reported between Vol, PCSA and ACSA in determining MVF, our results showed that the RF and VL volume collected in the middle portion of the muscle length are strong determinants of MVF produced by the knee extensors at 90 degrees joint angle
Noise-induced macroscopic bifurcations in globally-coupled chaotic units
Large populations of globally-coupled identical maps subjected to independent
additive noise are shown to undergo qualitative changes as the features of the
stochastic process are varied. We show that for strong coupling, the collective
dynamics can be described in terms of a few effective macroscopic degrees of
freedom, whose deterministic equations of motion are systematically derived
through an order parameter expansion.Comment: Phys. Rev. Lett., accepte
The effect of the displacement damage on the Charge Collection Efficiency in Silicon Drift Detectors for the LOFT satellite
The technology of Silicon Drift Detectors (SDDs) has been selected for the
two instruments aboard the Large Observatory For X-ray Timing (LOFT) space
mission. LOFT underwent a three year long assessment phase as candidate for the
M3 launch opportunity within the "Cosmic Vision 2015 -- 2025" long-term science
plan of the European Space Agency. During the LOFT assessment phase, we studied
the displacement damage produced in the SDDs by the protons trapped in the
Earth's magnetosphere. In a previous paper we discussed the effects of the Non
Ionising Energy Losses from protons on the SDD leakage current. In this paper
we report the measurement of the variation of Charge Collection Efficiency
produced by displacement damage caused by protons and the comparison with the
expected damage in orbit.Comment: 17 pages, 7 figures. Accepted for publication by Journal of
Instrumentatio
On the Nature of the Cosmological Constant Problem
General relativity postulates the Minkowski space-time to be the standard
flat geometry against which we compare all curved space-times and the
gravitational ground state where particles, quantum fields and their vacuum
states are primarily conceived. On the other hand, experimental evidences show
that there exists a non-zero cosmological constant, which implies in a deSitter
space-time, not compatible with the assumed Minkowski structure. Such
inconsistency is shown to be a consequence of the lack of a application
independent curvature standard in Riemann's geometry, leading eventually to the
cosmological constant problem in general relativity.
We show how the curvature standard in Riemann's geometry can be fixed by
Nash's theorem on locally embedded Riemannian geometries, which imply in the
existence of extra dimensions. The resulting gravitational theory is more
general than general relativity, similar to brane-world gravity, but where the
propagation of the gravitational field along the extra dimensions is a
mathematical necessity, rather than being a a postulate. After a brief
introduction to Nash's theorem, we show that the vacuum energy density must
remain confined to four-dimensional space-times, but the cosmological constant
resulting from the contracted Bianchi identity is a gravitational contribution
which propagates in the extra dimensions. Therefore, the comparison between the
vacuum energy and the cosmological constant in general relativity ceases to be.
Instead, the geometrical fix provided by Nash's theorem suggests that the
vacuum energy density contributes to the perturbations of the gravitational
field.Comment: LaTex, 5 pages no figutres. Correction on author lis
Automation in optometry (the implications and impact) and the use of computers in optometry
Automation in optometry (the implications and impact) and the use of computers in optometr
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