1,390 research outputs found
Dynamics of Crossover from a Chaotic to a Power Law State in Jerky Flow
We study the dynamics of an intriguing crossover from a chaotic to a power
law state as a function of strain rate within the context of a recently
introduced model which reproduces the crossover. While the chaotic regime has a
small set of positive Lyapunov exponents, interestingly, the scaling regime has
a power law distribution of null exponents which also exhibits a power law. The
slow manifold analysis of the model shows that while a large proportion of
dislocations are pinned in the chaotic regime, most of them are pushed to the
threshold of unpinning in the scaling regime, thus providing insight into the
mechanism of crossover.Comment: 5 pages, 3 figures. In print in Phy. Rev. E Rapid Communication
Optical band edge shift of anatase cobalt-doped titanium dioxide
We report on the optical properties of magnetic cobalt-doped anatase phase
titanium dioxide Ti_{1-x}Co_{x}O_{2-d} films for low doping concentrations, 0
<= x <= 0.02, in the spectral range 0.2 to 5 eV. For well oxygenated films (d
<< 1) the optical conductivity is characterized by an absence of optical
absorption below an onset of interband transitions at 3.6 eV and a blue shift
of the optical band edge with increasing Co concentration. The absence of below
band gap absorption is inconsistent with theoretical models which contain
midgap magnetic impurity bands and suggests that strong on-site Coulomb
interactions shift the O-band to Co-level optical transitions to energies above
the gap.Comment: 5 pages, 4 figures, 1 table; Version 2 - major content revisio
A polarized atomic hydrogen beam
We describe the design and operating characteristics of a simple polarized atomic hydrogen beam particularly suitable for applications to crossed beams experiments. In addition to experimental measurements, we present the results of detailed computer models, using Monte-Carlo ray tracing techniques, optical analogs, and phase-space methods, that not only provide us with a confirmation of our measurement, but also allow us to characterize the density, polarization, and atomic fraction of the beam at all points along its path. As a subsidiary result, we also present measurements of the relative and absolute efficiencies of the V/G Supavac mass analyzer for masses 1 and 2
Distinct modes of promoter recognition by two iron starvation σ factors with overlapping promoter specificities
OrbS and PvdS are extracytoplasmic function (ECF) σ factors that regulate transcription of operons required for the biosynthesis of the siderophores ornibactin and pyoverdine in the B. cepacia complex and Pseudomonas spp., respectively. Here we show that promoter recognition by OrbS requires specific tetrameric -35 and -10 element sequences that are strikingly similar to those of the consensus PvdS-dependent promoter. However, whereas P. aeruginosa PvdS can serve OrbS-dependent promoters, OrbS cannot utilise PvdS-dependent promoters. To identify features present at OrbS-dependent promoters that facilitate recognition by OrbS, we carried out a detailed analysis of the nucleotide sequence requirements for promoter recognition by both OrbS and PvdS. This revealed that DNA sequence features located outside of the sigma binding elements are required for efficient promoter utilisation by OrbS. In particular, the presence of an A-tract extending downstream from the -35 element at OrbS-dependent promoters was shown to be an important contributor to OrbS specificity. Our observations demonstrate that the nature of the spacer sequence can have a major impact on promoter recognition by some ECF sigma factors through modulation of the local DNA architecture
Theory of Chiral Modulations and Fluctuations in Smectic-A Liquid Crystals Under an Electric Field
Chiral liquid crystals often exhibit periodic modulations in the molecular
director; in particular, thin films of the smectic-C* phase show a chiral
striped texture. Here, we investigate whether similar chiral modulations can
occur in the induced molecular tilt of the smectic-A phase under an applied
electric field. Using both continuum elastic theory and lattice simulations, we
find that the state of uniform induced tilt can become unstable when the system
approaches the smectic-A--smectic-C* transition, or when a high electric field
is applied. Beyond that instability point, the system develops chiral stripes
in the tilt, which induce corresponding ripples in the smectic layers. The
modulation persists up to an upper critical electric field and then disappears.
Furthermore, even in the uniform state, the system shows chiral fluctuations,
including both incipient chiral stripes and localized chiral vortices. We
compare these predictions with observed chiral modulations and fluctuations in
smectic-A liquid crystals.Comment: 11 pages, including 9 postscript figures, uses REVTeX 3.0 and
epsf.st
A Rotating Disc Electrochemical Reactor to Produce Iron Powder for the Co2-Free Iron Fuel Cycle
Iron (Fe) is a promising candidate for energy carriers due to its high energy density, abundance, and transportability. Energy is released during the combustion of iron powder. Iron oxide particles are the product of combustion, which can be easily collected and reduced back to metallic iron, thus enforcing an iron fuel cycle. Electrochemical reduction of iron oxide in alkaline media is a promising approach for the reduction process as it is CO2-free and requires low temperature/energy. In the context of the iron fuel cycle, we promote electroreduction with dendrite-rich structures rather than compact deposit layers for easy harvesting and conversion of deposits to iron powder. This study presents the design and performance of an electrochemical reactor with a rotating disc system, designed for the continuous production of electrolytic iron powder. The reactor facilitates an integrated and automated process of electroreduction of iron oxide (from electroreduction to cleaning, drying, and dendrite/powder harvesting). Our proof of concept experiments show that iron deposits with dendritic structures can be produced in various conditions (anode configurations and rotating speeds), and are mainly located on the disc edge. The growth of dendrites at the edge of the disc favour harvesting and conversion to iron powder. Current efficiencies of more than 85-90 % are achieved in this study. Insights from the present study open new perspectives for the circularity of the iron fuel cycle. Furthermore, this technique provides a novel contribution to powder production in sustainable iron/steel-making technologies
Ab initio and finite-temperature molecular dynamics studies of lattice resistance in tantalum
This manuscript explores the apparent discrepancy between experimental data
and theoretical calculations of the lattice resistance of bcc tantalum. We
present the first results for the temperature dependence of the Peierls stress
in this system and the first ab initio calculation of the zero-temperature
Peierls stress to employ periodic boundary conditions, which are those best
suited to the study of metallic systems at the electron-structure level. Our ab
initio value for the Peierls stress is over five times larger than current
extrapolations of experimental lattice resistance to zero-temperature. Although
we do find that the common techniques for such extrapolation indeed tend to
underestimate the zero-temperature limit, the amount of the underestimation
which we observe is only 10-20%, leaving open the possibility that mechanisms
other than the simple Peierls stress are important in controlling the process
of low temperature slip.Comment: 12 pages and 9 figure
HIV Linkage to Care and Retention in Care Rate Among MSM in Guangzhou, China
Quantifying HIV service provision along the HIV care continuum is increasingly important for monitoring and evaluating HIV interventions. We examined factors associated with linkage and retention in care longitudinally among MSM (n = 1974, 4933 person-years) diagnosed and living in Guangzhou, China, in 2008–2014. We measured longitudinal change of retention in care (≥2 CD4 tests per year) from linkage and antiretroviral therapy initiation (ART). We examined factors associated with linkage using logistic regression and with retention using generalized estimating equations. The rate of linkage to care was 89% in 2014. ART retention rate dropped from 71% (year 1) to 46% (year 2), suggesting that first-year retention measures likely overestimate retention over longer periods. Lower CD4 levels and older age predicted retention in ART care. These data can inform interventions to improve retention about some subgroups
To wet or not to wet: that is the question
Wetting transitions have been predicted and observed to occur for various
combinations of fluids and surfaces. This paper describes the origin of such
transitions, for liquid films on solid surfaces, in terms of the gas-surface
interaction potentials V(r), which depend on the specific adsorption system.
The transitions of light inert gases and H2 molecules on alkali metal surfaces
have been explored extensively and are relatively well understood in terms of
the least attractive adsorption interactions in nature. Much less thoroughly
investigated are wetting transitions of Hg, water, heavy inert gases and other
molecular films. The basic idea is that nonwetting occurs, for energetic
reasons, if the adsorption potential's well-depth D is smaller than, or
comparable to, the well-depth of the adsorbate-adsorbate mutual interaction. At
the wetting temperature, Tw, the transition to wetting occurs, for entropic
reasons, when the liquid's surface tension is sufficiently small that the free
energy cost in forming a thick film is sufficiently compensated by the fluid-
surface interaction energy. Guidelines useful for exploring wetting transitions
of other systems are analyzed, in terms of generic criteria involving the
"simple model", which yields results in terms of gas-surface interaction
parameters and thermodynamic properties of the bulk adsorbate.Comment: Article accepted for publication in J. Low Temp. Phy
W=0 Pairing in Carbon Nanotubes away from Half Filling
We use the Hubbard Hamiltonian on the honeycomb lattice to represent the
valence bands of carbon single-wall nanotubes. A detailed symmetry
analysis shows that the model allows W=0 pairs which we define as two-body
singlet eigenstates of with vanishing on-site repulsion. By means of a
non-perturbative canonical transformation we calculate the effective
interaction between the electrons of a W=0 pair added to the interacting ground
state. We show that the dressed W=0 pair is a bound state for resonable
parameter values away from half filling. Exact diagonalization results for the
(1,1) nanotube confirm the expectations. For nanotubes of length ,
the binding energy of the pair depends strongly on the filling and decreases
towards a small but nonzero value as . We observe the existence
of an optimal doping when the number of electrons per C atom is in the range
1.21.3, and the binding energy is of the order of 0.1 1 meV.Comment: 16 pages, 6 figure
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