1,975 research outputs found
Escherichia coli helicase II (UvrD) protein initiates DNA unwinding at nicks and blunt ends.
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Cyclical population dynamics of automatic versus controlled processing : an evolutionary pendulum
Psychologists, neuroscientists, and economists often conceptualize decisions as arising from processes that lie along a continuum from automatic (i.e., “hardwired” or over-learned, but relatively inflexible) to controlled (less efficient and effortful, but more flexible). Control is central to human cognition, and plays a key role in our ability to modify the world to suit our needs. Given its advantages, reliance on controlled processing may seem predestined to increase within the population over time. Here, we examine whether this is so by introducing an evolutionary game theoretic model of agents that vary in their use of automatic versus controlled processes, and in which cognitive processing modifies the environment in which the agents interact. We find that, under a wide range of parameters and model assumptions, cycles emerge in which the prevalence of each type of processing in the population oscillates between two extremes. Rather than inexorably increasing, the emergence of control often creates conditions that lead to its own demise by allowing automaticity to also flourish, thereby undermining the progress made by the initial emergence of controlled processing. We speculate that this observation may have relevance for understanding similar cycles across human history, and may lend insight into some of the circumstances and challenges currently faced by our species
Stability of Relativistic Matter with Magnetic Fields for Nuclear Charges up to the Critical Value
We give a proof of stability of relativistic matter with magnetic fields all
the way up to the critical value of the nuclear charge .Comment: LaTeX2e, 12 page
The in situ synthesis of PbS nanocrystals from lead(II) n -octylxanthate within a 1,3-diisopropenylbenzene–bisphenol A dimethacrylate sulfur copolymer
The synthesis of lead sulfide nanocrystals within a solution processable sulfur ‘inverse vulcanization’ polymer thin film matrix was achieved from the in situ thermal decomposition of lead(II) n-octylxanthate, [Pb(S2COOct)2]. The growth of nanocrystals within polymer thin films from single-source precursors offers a faster route to networks of nanocrystals within polymers when compared with ex situ routes. The ‘inverse vulcanization’ sulfur polymer described herein contains a hybrid linker system which demonstrates high solubility in organic solvents, allowing solution processing of the sulfur-based polymer, ideal for the formation of thin films. The process of nanocrystal synthesis within sulfur films was optimized by observing nanocrystal formation by X-ray photoelectron spectroscopy and X-ray diffraction. Examination of the film morphology by scanning electron microscopy showed that beyond a certain precursor concentration the nanocrystals formed were not only within the film but also on the surface suggesting a loading limit within the polymer. We envisage this material could be used as the basis of a new generation of materials where solution processed sulfur polymers act as an alternative to traditional polymers
Dispersion of imbibition fronts
We have studied the dispersive behaviour of imbibition fronts in a porous
medium by X-ray tomography. Injection velocities were varied and the porous
medium was initially prewetted or not. At low velocity in the prewetted medium,
the imbibition profiles are found to be distinctly hyperdispersive. The
profiles are anomalously extended when compared to tracer fronts exhibiting
conventional (Gaussian) dispersion. We observe a strong velocity dependence of
the exponent characterizing the divergence of the dispersion coefficient for
low wetting-fluid saturation. Hyperdispersion is absent at high imbibition
velocities or when the medium is not prewetted.Comment: 8 pages, 5 figures; submitted to Europhysics Letter
Spontaneous symmetry breaking in gauge theories via Bose-Einstein condensation
We propose a mechanism naturally leading to the spontaneous symmetry breaking
in a gauge theory. The Higgs field is assumed to have global and gauged
internal symmetries. We associate a non zero chemical potential to one of the
globally conserved charges commuting with all of the gauge transformations.
This induces a negative mass squared for the Higgs field triggering the
spontaneous symmetry breaking of the global and local symmetries. The mechanism
is general and we test the idea for the electroweak theory in which the Higgs
sector is extended to possess an extra global Abelian symmetry. To this
symmetry we associate a non zero chemical potential. The Bose-Einstein
condensation of the Higgs leads, at tree level, to modified dispersion
relations for the Higgs field while the dispersion relations of the gauge
bosons and fermions remain undisturbed. The latter are modified through higher
order corrections. We have computed some corrections to the vacuum
polarizations of the gauge bosons and fermions. To quantify the corrections to
the gauge boson vacuum polarizations with respect to the Standard Model we
considered the effects on the T parameter. We finally derive the one loop
modified fermion dispersion relations.Comment: RevTeX 4, 13 pages. Added references and corrected typo
Reconstruction of three-dimensional porous media using generative adversarial neural networks
To evaluate the variability of multi-phase flow properties of porous media at
the pore scale, it is necessary to acquire a number of representative samples
of the void-solid structure. While modern x-ray computer tomography has made it
possible to extract three-dimensional images of the pore space, assessment of
the variability in the inherent material properties is often experimentally not
feasible. We present a novel method to reconstruct the solid-void structure of
porous media by applying a generative neural network that allows an implicit
description of the probability distribution represented by three-dimensional
image datasets. We show, by using an adversarial learning approach for neural
networks, that this method of unsupervised learning is able to generate
representative samples of porous media that honor their statistics. We
successfully compare measures of pore morphology, such as the Euler
characteristic, two-point statistics and directional single-phase permeability
of synthetic realizations with the calculated properties of a bead pack, Berea
sandstone, and Ketton limestone. Results show that GANs can be used to
reconstruct high-resolution three-dimensional images of porous media at
different scales that are representative of the morphology of the images used
to train the neural network. The fully convolutional nature of the trained
neural network allows the generation of large samples while maintaining
computational efficiency. Compared to classical stochastic methods of image
reconstruction, the implicit representation of the learned data distribution
can be stored and reused to generate multiple realizations of the pore
structure very rapidly.Comment: 21 pages, 20 figure
Lorentz violating kinematics: Threshold theorems
Recent tentative experimental indications, and the subsequent theoretical
speculations, regarding possible violations of Lorentz invariance have
attracted a vast amount of attention. An important technical issue that
considerably complicates detailed calculations in any such scenario, is that
once one violates Lorentz invariance the analysis of thresholds in both
scattering and decay processes becomes extremely subtle, with many new and
naively unexpected effects. In the current article we develop several extremely
general threshold theorems that depend only on the existence of some energy
momentum relation E(p), eschewing even assumptions of isotropy or monotonicity.
We shall argue that there are physically interesting situations where such a
level of generality is called for, and that existing (partial) results in the
literature make unnecessary technical assumptions. Even in this most general of
settings, we show that at threshold all final state particles move with the
same 3-velocity, while initial state particles must have 3-velocities
parallel/anti-parallel to the final state particles. In contrast the various
3-momenta can behave in a complicated and counter-intuitive manner.Comment: V1: 32 pages, 6 figures, 3 tables. V2: 5 references adde
Copper-doped CdSe/ZnS quantum dots : controllable photoactivated copper(I) cation storage and release vectors for catalysis
The first photoactivated doped quantum dot vector for metal-ion release has been developed. A facile method for doping copper(I) cations within ZnS quantum dot shells was achieved through the use of metal-dithiocarbamates, with Cu(+) ions elucidated by X-ray photoelectron spectroscopy. Photoexcitation of the quantum dots has been shown to release Cu(+) ions, which was employed as an effective catalyst for the Huisgen [3+2] cycloaddition reaction. The relationship between the extent of doping, catalytic activity, and the fluorescence quenching was also explored
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