747 research outputs found
The Polarisation Signatures of Microlensing
It has already been shown that microlensing can give rise to a non-zero
variable polarisation signal. Here we use realistic simulations to demonstrate
the additional information that can be gained from polarimetric observations of
lensing events.Comment: 4 pages, LaTeX, 2 figures from 6 postscript files. Uses elsart.sty,
graphicx.sty and harvard.sty. To appear in New Astronomy Reviews -
proceedings of the Oxford Workshop `Gravitational Lensing: Nature's Own
Weighing Scales
Dynamic characterisation of interlaminar fracture toughness in carbon fibre epoxy composite laminates
In this work, the rate dependence of mode I interlaminar fracture toughness for two different materials systems,
IM7/8552 and IM7/M91, both unidirectional UD carbon-fibre epoxy composite laminates have been examined
over a wide range of loading rates from 0.5 mm/min up to 2000 mm/s at room temperature. Quasi-static fracture
tests were performed using a DCB (double-cantilever beam) method with a screw-driven testing machine, while
the dynamic tests were carried out using a WIF (wedge-insert fracture) specimen loaded dynamically in a hydraulic
system. For performing the tests at high displacement rates, a special setup was designed and manufactured
which allowed the insertion of the wedge within the DCB specimens at different cross-head displacement
rates. The experimental technique used a pair of strain gauges attached to the bending surface of one of the
arms of the cantilever beams and far from the initial crack tip. The peak values of the recorded strain were used
to determine the fracture toughness under dynamic conditions through use of the compliance calibration
method. A finite element model was developed to check the consistency of the measurements and validate the
data reduction method used. The results exhibited rate insensitive behaviour in the case of the IM7/8552 laminates
while IM7/M91 showed the contrary behaviour with maximum peak at 500 mm/s of displacement rate,
with a toughness increase of 95% with respect to the quasi-static conditions.The research leading to these results has received funding from the European Union as Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 722096, DYNACOMP project
Magnetoresistance and electronic structure of asymmetric GaAs/AlGaAs double quantum wells in the in-plane/tilted magnetic field
Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs
buffer of a standard heterostructure were investigated by magnetotransport
measurements. In magnetic fields oriented parallel to the electron layers, the
magnetoresistance exhibits an oscillation associated with the depopulation of
the higher occupied subband and the field-induced transition into a decoupled
bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields
allow to reconstruct the evolution of the electron concentration in the
individual subbands as a function of the in-plane magnetic field. The
characteristics of the system derived experimentally are in quantitative
agreement with numerical self-consistent-field calculations of the electronic
structure.Comment: 6 pages, 5 figure
c-Axis tunneling in YBa2Cu3O7-\delta/PrBa2Cu3O7-\delta superlattices
In this work we report c-axis conductance measurements done on a superlattice
based on a stack of 2 layers YBa2Cu3O{7-\delta} and 7 layers
PrBa2Cu3O{7-\delta} (2:7). We find that these quasi-2D structures show no clear
superconducting coupling along the c-axis. Instead, we observe tunneling with a
gap of \Delta_c=5.0\pm 0.5 meV for the direction perpendicular to the
superconducting planes. The conductance spectrum show well defined
quasi-periodic structures which are attributed to the superlattice structure.
From this data we deduce a low temperature c-axis coherence length of
\xi_c=0.24\pm 0.03 nm.Comment: 15 pages, 5 figures. To appear in Phys.Rev.
Ammonia in a time of COVID-19. A submission of evidence to Defra/AQEG
A submission to the Air Quality Expert Group (AQEG), an expert committee of the Department for Environment, Food and Rural Affairs (Defra) • Ammonia gas (NH3) is a priority pollutant both as a precursor to particulate matter and for ecosystem impacts. • Three scenarios for UK emission reductions during COVID-19 in emission sectors, where activity is likely reduced ,have been assessed. • Total UK emissions of NH3 are likely to have decreased slightly (~2%), which is within the uncertainty and meteorological variability of the UK atmosphere. • Urban background and urban on road and roadside emissions of NH3 are likely to have decreased, by as much as 30% and 90% respectively compared with usual emissions before COVID-19. • Unratified data from three of the five UK automatic NH3 analysers (Auchencorth Moss, Chilbolton Observatory, and Manchester OSCA Observatory) show typical springtime NH3 concentrations across the UK. •
Data from the non-automatic National Ammonia Monitoring Network will enable analysis at UK level in the months ahead. This includes roadside data from London Cromwell Rd. • Evidence gaps & future approaches are outlined. Future analysis of the Defra UKEAP rural networks proposed. • The key measurement gap is urban roadside NH3 (and PM ammonium) as there is only one long-term site in the UK measuring roadside NH3 concentrations. It is suggested that a roadside network of samplers and/or analysers are urgently put in place (perhaps aligned with the UK Urban NO2 Network; UUNN) to monitor NH3 at roadsides during and post COVID-19 lock down where possible
Distance Estimation in Cosmology
In this paper we outline the framework of mathematical statistics with which
one may study the properties of galaxy distance estimators. We describe, within
this framework, how one may formulate the problem of distance estimation as a
Bayesian inference problem, and highlight the crucial question of how one
incorporates prior information in this approach. We contrast the Bayesian
approach with the classical `frequentist' treatment of parameter estimation,
and illustrate -- with the simple example of estimating the distance to a
single galaxy in a redshift survey -- how one can obtain a significantly
different result in the two cases. We also examine some examples of a Bayesian
treatment of distance estimation -- involving the definition of Malmquist
corrections -- which have been applied in recent literature, and discuss the
validity of the assumptions on which such treatments have been based.Comment: Plain Latex version 3.1, 18 pages + 2 figures, `Vistas in Astronomy'
in pres
Scholarly communication in transition: The use of question marks in the titles of scientific articles in medicine, life sciences and physics 1966–2005
The titles of scientific articles have a special significance. We examined nearly 20 million scientific articles and recorded the development of articles with a question mark at the end of their
titles over the last 40 years. Our study was confined to the disciplines of physics, life sciences and medicine, where we found a significant increase from 50% to more than 200% in the number of articles with question-mark titles. We looked at the principle functions and structure of the titles of scientific papers, and we assume that marketing aspects are one of the decisive factors behind the growing usage of question-mark titles in scientific articles
Emergence of quasi-metallic state in disordered 2D electron gas due to strong interactions
The interrelation between disorder and interactions in two dimensional
electron liquid is studied beyond weak coupling perturbation theory. Strong
repulsion significantly reduces the electronic density of states on the Fermi
level. This makes the electron liquid more rigid and strongly suppresses
elastic scattering off impurities. As a result the weak localization, although
ultimately present at zero temperature and infinite sample size, is
unobservable at experimentally accessible temperature at high enough densities.
Therefore practically there exists a well defined metallic state. We study
diffusion of electrons in this state and find that the diffusion pole is
significantly modified due to "mixture" with static photons similar to the
Anderson - Higgs mechanism in superconductivity. As a result several effects
stemming from the long range nature of diffusion like the Aronov - Altshuler
logarithmic corrections to conductivity are less pronounced.Comment: to appear in Phys. Rev.
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