10,140 research outputs found
Theory of quantum paraelectrics and the metaelectric transition
We present a microscopic model of the quantum paraelectric-ferroelectric
phase transition with a focus on the influence of coupled fluctuating phonon
modes. These may drive the continuous phase transition first order through a
metaelectric transition and furthermore stimulate the emergence of a textured
phase that preempts the transition. We discuss two further consequences of
fluctuations, firstly for the heat capacity, and secondly we show that the
inverse paraelectric susceptibility displays T^2 quantum critical behavior, and
can also adopt a characteristic minimum with temperature. Finally, we discuss
the observable consequences of our results.Comment: 5 pages, 2 figure
The Mauna Kea Observatories Near-Infrared Filter Set. I: Defining Optimal 1-5 m Bandpasses
A new MKO-NIR infrared filter set is described, including techniques and
considerations given to designing a new set of bandpasses that are useful at
both mid- and high-altitude sites. These filters offer improved photometric
linearity and in many cases reduced background, as well as preserve good
throughput within the JHKLM atmospheric windows. MKO-NIR filters have already
been deployed with a number of instruments around the world as part of a filter
consortium purchase to reduce the unit cost of filters. Through this effort we
hope to establish, for the first time, a single standard set of infrared
fitlers at as many observatories as possible.Comment: PASP, in press; 32 pages, 11 figures, 3 Table
A repulsive atomic gas in a harmonic trap on the border of itinerant ferromagnetism
Alongside superfluidity, itinerant (Stoner) ferromagnetism remains one of the
most well-characterized phases of correlated Fermi systems. A recent experiment
has reported the first evidence for novel phase behavior on the repulsive side
of the Feshbach resonance in a two-component ultracold Fermi gas. By adapting
recent theoretical studies to the atomic trap geometry, we show that an
adiabatic ferromagnetic transition would take place at a weaker interaction
strength than is observed in experiment. This discrepancy motivates a simple
non-equilibrium theory that takes account of the dynamics of magnetic defects
and three-body losses. The formalism developed displays good quantitative
agreement with experiment.Comment: 4 pages, 2 figure
Kinetics of Exciton Emission Patterns and Carrier Transport
We report on the measurements of the kinetics of expanding and collapsing
rings in the exciton emission pattern. The rings are found to preserve their
integrity during expansion and collapse, indicating that the observed kinetics
is controlled by charge carrier transport rather than by a much faster process
of exciton production and decay. The relation between ring kinetics and carrier
transport, revealed by our experiment and confirmed by comparison with a
theoretical model, is used to determine electron and hole transport
characteristics in a contactless fashion.Comment: 6 pages, 4 figure
PSI-ECRIT(S): a hybrid magnetic system with a mirror ratio of 10 for H-like heavy ion production and trapping
At the Paul Scherrer Institut ( PSI, Switzerland) an experimental program is started to measure the ground state shift and width of pionic hydrogen. To calibrate the crystal spectrometer X-ray transitions in hydrogen-like heavy ions (e.g. Ar17+) produced by ECR ion sources, are necessary. In PSI a superconducting cyclotron trap magnet originally developed for high energy experiments will be transformed into an ECR Ion Trap (ECRIT). The SC-magnet can deliver more than 4 Tesla magnetic fields with a mirror ratio of 2. A careful calculation showed this mirror ratio can be increased upto 10 and the trap can operate with frequencies between 5 and 20 GHz. To form a closed resonance zone a relatively large open structure (LBL-AECRU-type) NdFeB hexapole will be applied. The first tests will be performed with 6.4 GHz. Later higher frequencies (10 or 14.5 GHz) and the 2-frequency heating (6.4+10, 6.4+14.5 or 10+14.5) are planned to be applied to get enough quantity of H-like heavy ions. Since the main goal of this machine is to be a trap no extraction is necessary. However, for the fine-tuning of the plasma for very high charge states might require ion charge state spectrums to be analyzed. If this is the case a simple beamline at negative potential will be built. The present paper shows the results of the magnetic system calculations in details and summarises the present state of the ECRIT(S) overall design
Tail States in Disordered Superconductors with Magnetic Impurities: the Unitarity Limit
When subject to a weak magnetic impurity distribution, the order parameter
and quasi-particle energy gap of a weakly disordered bulk s-wave superconductor
are suppressed. In the Born scattering limit, recent investigations have shown
that `optimal fluctuations' of the random impurity potential can lead to the
nucleation of `domains' of localised states within the gap region predicted by
the conventional Abrikosov-Gor'kov mean-field theory, rendering the
superconducting system gapless at any finite impurity concentration. By
implementing a field theoretic scheme tailored to the weakly disordered system,
the aim of the present paper is to extend this analysis to the consideration of
magnetic impurities in the unitarity scattering limit. This investigation
reveals that the qualitative behaviour is maintained while the density of
states exhibits a rich structure.Comment: 18 pages AMSLaTeX (with LaTeX2e), 6 eps figure
Charge transport and phase transition in exciton rings
The macroscopic exciton rings observed in the photoluminescence (PL) patterns
of excitons in coupled quantum wells (CQWs) are explained by a series of
experiments and a theory based on the idea of carrier imbalance, transport and
recombination. The rings are found to be a source of cold excitons with
temperature close to that of the lattice. We explored states of excitons in the
ring over a range of temperatures down to 380 mK. These studies reveal a sharp,
albeit continuous, second order phase transition to a low-temperature ordered
exciton state, characterized by ring fragmentation into a periodic array of
aggregates. An instability at the onset of degeneracy in the cold exciton
system, due to stimulated exciton formation, is proposed as the transition
mechanism.Comment: 8 pages including 4 figure
Face masks to prevent community transmission of viral respiratory infections: A rapid evidence review using Bayesian analysis [Preprint, version 1]
Background: Face masks have been proposed as an important way of reducing transmission of viral respiratory infections, including SARS-CoV-2. /
Objective: To assess the likelihood that wearing face masks in community settings reduces transmission of viral respiratory infections. /
Methods: We conducted a rapid evidence review and used a Bayesian statistical approach to analysing experimental and observational studies conducted in community-dwelling children and adults that assessed the effectiveness of face mask wearing (vs. no face masks) on self-reported, laboratory-confirmed, or clinically diagnosed viral respiratory infections. /
Results: Eleven RCTs and 10 observational studies met the inclusion criteria. The calculation of Bayes factors and cumulative posterior odds from the RCTs showed a moderate likelihood of a small effect of wearing surgical face masks in community settings in reducing self-reported influenza-like illness (ILI) (cumulative posterior odds = 3.61). However, the risk of reporting bias was high and evidence of reduction of clinically- or laboratory-confirmed infection was equivocal (cumulative posterior odds = 1.07 and 1.22, respectively). Observational studies yielded evidence of a negative association between face mask wearing and ILI but with high risk of confounding and reporting bias. /
Conclusions: Available evidence from RCTs is equivocal as to whether or not wearing face masks in community settings results in a reduction in clinically- or laboratory-confirmed viral respiratory infections. No relevant studies concerned SARS-CoV-2 or were undertaken in community settings in the UK
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