1,597 research outputs found
Cellular solid behaviour of liquid crystal colloids. 1. Phase separation and morphology
We study the phase ordering colloids suspended in a thermotropic nematic
liquid crystal below the clearing point Tni and the resulting aggregated
structure. Small (150nm) PMMA particles are dispersed in a classical liquid
crystal matrix, 5CB or MBBA. With the help of confocal microscopy we show that
small colloid particles densely aggregate on thin interfaces surrounding large
volumes of clean nematic liquid, thus forming an open cellular structure, with
the characteristic size of 10-100 micron inversely proportional to the colloid
concentration. A simple theoretical model, based on the Landau mean-field
treatment, is developed to describe the continuous phase separation and the
mechanism of cellular structure formation.Comment: Latex 2e (EPJ style) EPS figures included (poor quality to comply
with space limitations
Grain Boundary Loops in Graphene
Topological defects can affect the physical properties of graphene in
unexpected ways. Harnessing their influence may lead to enhanced control of
both material strength and electrical properties. Here we present a new class
of topological defects in graphene composed of a rotating sequence of
dislocations that close on themselves, forming grain boundary loops that either
conserve the number of atoms in the hexagonal lattice or accommodate
vacancy/interstitial reconstruction, while leaving no unsatisfied bonds. One
grain boundary loop is observed as a "flower" pattern in scanning tunneling
microscopy (STM) studies of epitaxial graphene grown on SiC(0001). We show that
the flower defect has the lowest energy per dislocation core of any known
topological defect in graphene, providing a natural explanation for its growth
via the coalescence of mobile dislocations.Comment: 23 pages, 7 figures. Revised title; expanded; updated reference
Orbital and physical parameters of eclipsing binaries from the ASAS catalogue -- III. Two new low-mass systems with rapidly evolving spots
We present the results of our spectroscopic and photometric analysis of two
newly discovered low-mass detached eclipsing binaries found in the All-Sky
Automated Survey (ASAS) catalogue: ASAS J093814-0104.4 and ASAS J212954-5620.1.
Using the GIRAFFE instrument on the 1.9-m Radcliffe telescope at SAAO and the
UCLES spectrograph on the 3.9-m Anglo-Australian Telescope, we obtained
high-resolution spectra of both objects and derived their radial velocities
(RVs) at various orbital phases. The RVs of both objects were measured with the
TODCOR technique using synthetic template spectra as references. We also
obtained V and I band photometry using the 1.0-m Elizabeth telescope at SAAO
and the 0.4-m PROMPT instruments located at the CTIO. The orbital and physical
parameters of the systems were derived with PHOEBE and JKTEBOP codes. We
compared our results with several sets of widely-used isochrones. Our
multi-epoch photometric observations demonstrate that both objects show
significant out-of-eclipse modulations, which vary in time. We believe that
this effect is caused by stellar spots, which evolve on time scales of tens of
days. For this reason, we constructed our models on the basis of photometric
observations spanning short time scales (less than a month). Our modeling
indicates that (1) ASAS-09 is a main sequence active system with nearly-twin
components with masses of M1 = 0.771(33) Msun, M2 = 0.768(21) Msun and radii of
R1 = 0.772(12) Rsun and R2 = 0.769(13) Rsun. (2) ASAS-21 is a main sequence
active binary with component masses of M1 = 0.833(17) Msun, M2 = 0.703(13) Msun
and radii of R1 = 0.845(12) Rsun and R2 = 0.718(17) Rsun. Both systems confirm
the characteristic of active low-mass stars, for which the observed radii are
larger and the temperatures lower than predicted by evolutionary models. Other
parameters agree within errors with the models of main sequence stars.Comment: 15 pages, 7 figures, 7 tables, to appear in A&
The Magnetic Electron Ion Spectrometer (MagEIS) Instruments Aboard the Radiation Belt Storm Probes (RBSP) Spacecraft
This paper describes the Magnetic Electron Ion Spectrometer (MagEIS) instruments aboard the RBSP spacecraft from an instrumentation and engineering point of view. There are four magnetic spectrometers aboard each of the two spacecraft, one low-energy unit (20–240 keV), two medium-energy units (80–1200 keV), and a high-energy unit (800–4800 keV). The high unit also contains a proton telescope (55 keV–20 MeV). The magnetic spectrometers focus electrons within a selected energy pass band upon a focal plane of several silicon detectors where pulse-height analysis is used to determine if the energy of the incident electron is appropriate for the electron momentum selected by the magnet. Thus each event is a two-parameter analysis, an approach leading to a greatly reduced background. The physics of these instruments are described in detail followed by the engineering implementation. The data outputs are described, and examples of the calibration results and early flight data presented
Toxic equivalency factors (TEFs) after acute oral exposure of azaspiracid 1,-2 and-3 in mice
Azaspiracids (AZAs) are marine algal toxins that can be accumulated by edible shellfish to cause a foodborne gastrointestinal poisoning in humans. In the European Union, only AZA1, -2 and -3 are currently regulated and their concentration in shellfish is determined through their toxic equivalency factors (TEFs) derived from the intraperitoneal lethal potency in mice. Nevertheless, considering the potential human exposure by oral route, AZAs TEFs should be calculated by comparative oral toxicity data. Thus, the acute oral toxicity of AZA1, -2 and -3 was investigated in female CD-1 mice treated with different doses (AZA1: 135-1100 mu g/kg; AZA2 and AZA3: 300-1100 mu g/kg) and sacrificed after 24 h or 14 days. TEFs derived from the median lethal doses (LD50) were 1.0, 0.7 and 0.5, respectively for AZA1, -2 and -3. In fact, after 24 h from gavage administration, LD(50)s were 443 mu g/kg (AZA1; 95% CL: 350-561 mu g/kg), 626 mu g/kg (AZA2; 95% CL: 430-911 mu g/kg) and 875 mu g/kg (AZA3; 95% CL: 757-1010 mu g/kg). Mice dead more than 5 h after the treatment or those sacrificed after 24 h (doses: = 175 mu g AZA1/kg, >= 500 mu g AZA2/kg and >= 600 mu g AZA3/kg) showed enlarged pale liver, while increased serum markers of liver alteration were recorded even at the lowest doses. Blood chemistry revealed significantly increased serum levels of K+ ions (>= 500 mg/kg), whereas light microscopy showed tissue changes in the gastrointestinal tract, liver and spleen. No lethality, macroscopic, tissue or haematological changes were recorded two weeks post exposure, indicating reversible toxic effects. LC-MS/MS analysis of the main organs showed a dose-dependency in gastrointestinal absorption of these toxins: at 24 h, the highest levels were detected in the stomach and, in descending order, in the intestinal content, liver, small intestine, kidneys, lungs, large intestine, heart as well as detectable traces in the brain. After 14 days, AZA1 and AZA2 were still detectable in almost all the organs and intestinal content
Fourier Transform Scanning Tunneling Spectroscopy: the possibility to obtain constant energy maps and the band dispersion using a local measurement
We present here an overview of the Fourier Transform Scanning Tunneling
spectroscopy technique (FT-STS). This technique allows one to probe the
electronic properties of a two-dimensional system by analyzing the standing
waves formed in the vicinity of defects. We review both the experimental and
theoretical aspects of this approach, basing our analysis on some of our
previous results, as well as on other results described in the literature. We
explain how the topology of the constant energy maps can be deduced from the FT
of dI/dV map images which exhibit standing waves patterns. We show that not
only the position of the features observed in the FT maps, but also their shape
can be explained using different theoretical models of different levels of
approximation. Thus, starting with the classical and well known expression of
the Lindhard susceptibility which describes the screening of electron in a free
electron gas, we show that from the momentum dependence of the susceptibility
we can deduce the topology of the constant energy maps in a joint density of
states approximation (JDOS). We describe how some of the specific features
predicted by the JDOS are (or are not) observed experimentally in the FT maps.
The role of the phase factors which are neglected in the rough JDOS
approximation is described using the stationary phase conditions. We present
also the technique of the T-matrix approximation, which takes into account
accurately these phase factors. This technique has been successfully applied to
normal metals, as well as to systems with more complicated constant energy
contours. We present results recently obtained on graphene systems which
demonstrate the power of this technique, and the usefulness of local
measurements for determining the band structure, the map of the Fermi energy
and the constant-energy maps.Comment: 33 pages, 15 figures; invited review article, to appear in Journal of
Physics D: Applied Physic
Predicting mortality with pneumonia severity scores: importance of model recalibration to local settings
In patients with community-acquired pneumonia (CAP) prediction rules based on individual predicted mortalities are frequently used to support decision-making for in-patient vs. outpatient management. We studied the accuracy and the need for recalibration of three risk prediction scores in a tertiary-care University hospital emergency-department setting in Switzerland. We pooled data from patients with CAP enrolled in two randomized controlled trials. We compared expected mortality from the original pneumonia severity index (PSI), CURB65 and CRB65 scores against observed mortality (calibration) and recalibrated the scores by fitting the intercept α and the calibration slope β from our calibration model. Each of the original models underestimated the observed 30-day mortality of 11%, in 371 patients admitted to the emergency department with CAP (8·4%, 5·5% and 5·0% for the PSI, CURB65 and CRB65 scores, respectively). In particular, we observed a relevant mortality within the low risk classes of the original models (2·6%, 5·3%, and 3·7% for PSI classes I-III, CURB65 classes 0-1, and CRB65 class 0, respectively). Recalibration of the original risk models corrected the miscalibration. After recalibration, however, only PSI class I was sensitive enough to identify patients with a low risk (i.e. <1%) for mortality suitable for outpatient management. In our tertiary-care setting with mostly referred in-patients, CAP risk scores substantially underestimated observed mortalities misclassifying patients with relevant risks of death suitable for outpatient management. Prior to the implementation of CAP risk scores in the clinical setting, the need for recalibration and the accuracy of low-risk re-classification should be studied in order to adhere with discharge guidelines and guarantee patients' safet
Para to Ortho transition of metallic dimers on Si(001)
Extensive electronic structure calculations are performed to obtain the
stable geometries of metals like Al, Ga and In on the Si(001) surface at 0.5 ML
and 1 ML coverages. Our results coupled with previous theoretical findings
explain the recent experimental data in a comprehensive fashion. At low
coverages, as shown by previous works, `Para' dimers give the lowest energy
structure. With increasing coverage beyond 0.5 ML, `Ortho' dimers become part
of low energy configurations leading toward a `Para' to `Ortho' transition at 1
ML coverage. For In mixed staggered dimers (`Ortho' and `Para') give the lowest
energy configuration. For Ga, mixed dimers are non-staggered, while for Al
`Para' to `Ortho' transition of dimers is complete. Thus at intermediate
coverages between 0.5 and 1 ML, the `Ortho' and `Para' dimers may coexist on
the surface. Consequently, this may be an explanation of the fact that the
experimental observations can be successfully interpreted using either
orientation. A supported zigzag structure at 0.5 ML, which resembles , does not undergo a dimerization transition, and hence stays
semi-metallic. Also, unlike the soliton formation is ruled out
for this structure.Comment: 8 pages, 6 figure
Nature of the metal-nonmetal transition in metal-ammonia solutions. I. Solvated electrons at low metal concentrations
Using a theory of polarizable fluids, we extend a variational treatment of an
excess electron to the many-electron case corresponding to finite metal
concentrations in metal-ammonia solutions (MAS). We evaluate dielectric,
optical, and thermodynamical properties of MAS at low metal concentrations. Our
semi-analytical calculations based on a mean-spherical approximation correlate
well with the experimental data on the concentration and the temperature
dependencies of the dielectric constant and the optical absorption spectrum.
The properties are found to be mainly determined by the induced dipolar
interactions between localized solvated electrons, which result in the two main
effects: the dispersion attractions between the electrons and a sharp increase
in the static dielectric constant of the solution. The first effect provides a
classical phase separation for the light alkali metal solutes (Li, Na, K) below
a critical temperature. The second effect leads to a dielectric instability,
i.e., polarization catastrophe, which is the onset of metallization. The locus
of the calculated critical concentrations is in a good agreement with the
experimental phase diagram of Na-NH3 solutions. The proposed mechanism of the
metal-nonmetal transition is quite general and may occur in systems involving
self-trapped quantum quasiparticles.Comment: 13 figures, 42 page
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