596 research outputs found
Dynamic response of phenolic resin and its carbon-nanotube composites to shock wave loading
We investigate with nonreactive molecular dynamics simulations the dynamic response of phenolic resin and its carbon-nanotube (CNT) composites to shock wave compression. For phenolic resin, our simulations yield shock states in agreement with experiments on similar polymers except the
“phase change” observed in experiments, indicating that such phase change is chemical in nature. The elastic–plastic transition is characterized by shear stress relaxation and atomic-level slip, and phenolic resin shows strong strain hardening. Shock loading of the CNT-resin composites is applied parallel or perpendicular to the CNT axis, and the composites demonstrate anisotropy in wave
propagation, yield and CNT deformation. The CNTs induce stress concentrations in the composites and may increase the yield strength. Our simulations suggest that the bulk shock response of the composites depends on the volume fraction, length ratio, impact cross-section, and geometry of the CNT components; the short CNTs in current simulations have insignificant effect on the bulk
response of resin polymer
Low-density, one-dimensional quantum gases in a split trap
We investigate degenerate quantum gases in one dimension trapped in a
harmonic potential that is split in the centre by a pointlike potential. Since
the single particle eigenfunctions of such a system are known for all strengths
of the central potential, the dynamics for non-interacting fermionic gases and
low-density, strongly interacting bosonic gases can be investigated exactly
using the Fermi-Bose mapping theorem. We calculate the exact many-particle
ground-state wave-functions for both particle species, investigate soliton-like
solutions, and compare the bosonic system to the well-known physics of Bose
gases described by the Gross-Pitaevskii equation. We also address the
experimentally important questions of creation and detection of such states.Comment: 7 pages, 5 figure
Three-dimensional molecular dynamics simulations of void coalescence during dynamic fracture of ductile metals
Void coalescence and interaction in dynamic fracture of ductile metals have
been investigated using three-dimensional strain-controlled multi-million atom
molecular dynamics simulations of copper. The correlated growth of two voids
during the coalescence process leading to fracture is investigated, both in
terms of its onset and the ensuing dynamical interactions. Void interactions
are quantified through the rate of reduction of the distance between the voids,
through the correlated directional growth of the voids, and through correlated
shape evolution of the voids. The critical inter-void ligament distance marking
the onset of coalescence is shown to be approximately one void radius based on
the quantification measurements used, independent of the initial separation
distance between the voids and the strain-rate of the expansion of the system.
The interaction of the voids is not reflected in the volumetric asymptotic
growth rate of the voids, as demonstrated here. Finally, the practice of using
a single void and periodic boundary conditions to study coalescence is examined
critically and shown to produce results markedly different than the coalescence
of a pair of isolated voids.Comment: Accepted for publication in Physical Review
The 1D Bose Gas with Weakly Repulsive Delta Interaction
We consider the asymptotic solutions to the Bethe ansatz equations of the
integrable model of interacting bosons in the weakly interacting limit. In this
limit we establish that the ground state maps to the highest energy state of a
strongly-coupled repulsive bosonic pairing model.Comment: 8 pages, to appear in Proceedings of The International Conference on
the Statistical Physics of Quantum Systems, Sendai, 17-20 July 200
Protein tyrosine phosphatases: the problems of a growing family
Protein tyrosine phosphorylation is now recognized as an important component of the control of many fundamental aspects of cellular function, including growth and differentiation, cell cycle and cytoskeletal integrity. In vivo, the net level of phosphorylation of tyrosyl residues in a target substrate reflects the balance between the competing action of kinases and phosphatases. We are examining physiological roles for protein tyrosine phosphorylation, pursuing the problem from the perspective of the enzymes that catalyze the dephosphorylation reaction, the protein tyrosine phosphatases (PTPases). The PTPases have, until recently, been somewhat neglected relative to the protein tyrosine kinases (PTKs). However, considerable progress has been made in identifying new members of the PTPase family, and it appears that they constitute a novel class of signal transducing molecules that rival the PTKs in their structural diversity and complexity.
One of the principal reasons that the study of PTPases has lagged behind that of the..
Crystal structure of ED-Eya2: insight into dual roles as a protein tyrosine phosphatase and a transcription factor
Eya proteins are transcription factors that play pivotal roles in organ formation during development by mediating interactions between Sine Oculis (SO) and Dachshund (DAC). Remarkably, the transcriptional activity of Eya proteins is regulated by a dephosphorylating activity within its Eya domain (ED). However, the molecular basis for the link between catalytic and transcriptional activities remains unclear. Here we report the first description of the crystal structure of the ED of human Eya2 (ED-Eya2), determined at 2.4-angstrom resolution. In stark contrast to other members of the haloacid dehalogenase (HAD) family to which ED-Eya2 belongs, the helix-bundle motif (HBM) is elongated along the back of the catalytic site. This not only results in a structure that accommodates large protein substrates but also positions the catalytic and the SO-interacting sites on opposite faces, which suggests that SO binding is not directly affected by catalytic function. Based on the observation that the DAC-binding site is located between the catalytic core and SO binding sites within ED-Eya2, we propose that catalytic activity can be translated to SO binding through DAC, which acts as a transcriptional switch. We also captured at two stages of reaction cycles-acyl-phosphate intermediate and transition state of hydrolysis step, which provided a detailed view of reaction mechanism. The ED-Eya2 structure defined here serves as a model for other members of the Eya family and provides a framework for understanding the role of Eya phosphatase mutations in disease.-Jung, S.-K., Jeong, D. G., Chung, S. J., Kim, J. H., Park, B. C., Tonks, N. K., Ryu, S. E., Kim, S. J.. Crystal structure of ED-Eya2: insight into dual roles as a protein tyrosine phosphatase and a transcription factor. FASEB J. 24, 560-569 (2010). www.fasebj.or
Fermionization of a bosonic gas under highly-elongated confinement: A diffusion quantum Monte Carlo study
The diffusion quantum Monte Carlo technique is used to solve the many-body
Schroedinger equation fully quantum mechanically and nonperturbatively for
bosonic atomic gases in cigar-shaped confining potentials. By varying the
aspect ratio of the confining potential from 1 (spherical trap) to 10000
(highly elongated trap), we characterize the transition from the
three-dimensional regime to the (quasi-)one-dimensional regime. Our results
confirm that the bosonic gas undergoes ``fermionization'' for large aspect
ratios. Importantly, many-body correlations are included explicitly in our
approach.Comment: 10 pages, 8 figure
Optical dipole traps and atomic waveguides based on Bessel light beams
We theoretically investigate the use of Bessel light beams generated using
axicons for creating optical dipole traps for cold atoms and atomic
waveguiding. Zeroth-order Bessel beams can be used to produce highly elongated
dipole traps allowing for the study of one-dimensional trapped gases and
realization of a Tonks gas of impentrable bosons. First-order Bessel beams are
shown to be able to produce tight confined atomic waveguides over centimeter
distances.Comment: 20 pages, 5 figures, to appear in Phys. Rev.
Bosons in cigar-shape traps: Thomas-Fermi regime, Tonks-Girardeau regime, and between
We present a quantitative analysis of the experimental accessibility of the
Tonks-Girardeau gas in the current day experiments with cigar-trapped alkalis.
For this purpose we derive, using a Bethe anzats generated local equation of
state, a set of hydrostatic equations describing one-dimensional
delta-interacting Bose gases trapped in a harmonic potential. The resulting
solutions cover the_entire range_ of atomic densities.Comment: 4 pages, 4 figure
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