1,248 research outputs found
Correction-to-scaling exponents for two-dimensional self-avoiding walks
We study the correction-to-scaling exponents for the two-dimensional
self-avoiding walk, using a combination of series-extrapolation and Monte Carlo
methods. We enumerate all self-avoiding walks up to 59 steps on the square
lattice, and up to 40 steps on the triangular lattice, measuring the
mean-square end-to-end distance, the mean-square radius of gyration and the
mean-square distance of a monomer from the endpoints. The complete endpoint
distribution is also calculated for self-avoiding walks up to 32 steps (square)
and up to 22 steps (triangular). We also generate self-avoiding walks on the
square lattice by Monte Carlo, using the pivot algorithm, obtaining the
mean-square radii to ~0.01% accuracy up to N = 4000. We give compelling
evidence that the first non-analytic correction term for two-dimensional
self-avoiding walks is Delta_1 = 3/2. We compute several moments of the
endpoint distribution function, finding good agreement with the field-theoretic
predictions. Finally, we study a particular invariant ratio that can be shown,
by conformal-field-theory arguments, to vanish asymptotically, and we find the
cancellation of the leading analytic correction.Comment: LaTeX 2.09, 56 pages. Version 2 adds a renormalization-group
discussion near the end of Section 2.2, and makes many small improvements in
the exposition. To be published in the Journal of Statistical Physic
Diffraction-Based Interaction-Free Measurements
We introduce diffraction-based interaction-free measurements. In contrast
with previous work where a set of discrete paths is engaged, good quality
interaction-free measurements can be realized with a continuous set of paths,
as is typical of optical propagation. If a bomb is present in a given spatial
region -- so sensitive that a single photon will set it off -- its presence can
still be detected without exploding it. This is possible because, by not
absorbing the photon, the bomb causes the single photon to diffract around it.
The resulting diffraction pattern can then be statistically distinguished from
the bomb-free case. We work out the case of single- versus double- slit in
detail, where the double-slit arises because of a bomb excluding the middle
region.Comment: 8 pages, 4 figure
Diffraction-based Interaction-free Measurements
We introduce diffraction-based interaction-free measurements. In contrast with previous work where a set of discrete paths is engaged, good-quality interaction-free measurements can be realized with a continuous set of paths, as is typical of optical propagation. If a bomb is present in a given spatial region—so sensitive that a single photon will set it off—its presence can still be detected without exploding it. This is possible because, by not absorbing the photon, the bomb causes the single photon to diffract around it. The resulting diffraction pattern can then be statistically distinguished from the bomb-free case. We work out the case of single- versus double-slit in detail, where the double-slit arises because of a bomb excluding the middle region
Fast and Accurate Coarsening Simulation with an Unconditionally Stable Time Step
We present Cahn-Hilliard and Allen-Cahn numerical integration algorithms that
are unconditionally stable and so provide significantly faster
accuracy-controlled simulation. Our stability analysis is based on Eyre's
theorem and unconditional von Neumann stability analysis, both of which we
present. Numerical tests confirm the accuracy of the von Neumann approach,
which is straightforward and should be widely applicable in phase-field
modeling. We show that accuracy can be controlled with an unbounded time step
Delta-t that grows with time t as Delta-t ~ t^alpha. We develop a
classification scheme for the step exponent alpha and demonstrate that a class
of simple linear algorithms gives alpha=1/3. For this class the speed up
relative to a fixed time step grows with the linear size of the system as N/log
N, and we estimate conservatively that an 8192^2 lattice can be integrated 300
times faster than with the Euler method.Comment: 14 pages, 6 figure
Medical student wellbeing - a consensus statement from Australia and New Zealand
Abstract Background Medical student wellbeing – a consensus statement from Australia and New Zealand outlines recommendations for optimising medical student wellbeing within medical schools in our region. Worldwide, medical schools have responsibilities to respond to concerns about student psychological, social and physical wellbeing, but guidance for medical schools is limited. To address this gap, this statement clarifies key concepts and issues related to wellbeing and provides recommendations for educational program design to promote both learning and student wellbeing. The recommendations focus on student selection; learning, teaching and assessment; learning environment; and staff development. Examples of educational initiatives from the evidence-base are provided, emphasising proactive and preventive approaches to student wellbeing. Main recommendations The consensus statement provides specific recommendations for medical schools to consider at all stages of program design and implementation. These are:Design curricula that promote peer support and progressive levels of challenge to students.Employ strategies to promote positive outcomes from stress and to help others in need.Design assessment tasks to foster wellbeing as well as learning.Provide mental health promotion and suicide prevention initiatives.Provide physical health promotion initiatives.Ensure safe and health-promoting cultures for learning in on-campus and clinical settings.Train staff on student wellbeing and how to manage wellbeing concerns. Conclusion A broad integrated approach to improving student wellbeing within medical school programs is recommended. Medical schools should work cooperatively with student and trainee groups, and partner with clinical services and other training bodies to foster safe practices and cultures. Initiatives should aim to assist students to develop adaptive responses to stressful situations so that graduates are prepared for the realities of the workplace. Multi-institutional, longitudinal collaborative research in Australia and New Zealand is needed to close critical gaps in the evidence needed by medical schools in our region
Absence of a metallicity effect for ultra-short-period planets
Ultra-short-period (USP) planets are a newly recognized class of planets with
periods shorter than one day and radii smaller than about 2 Earth radii. It has
been proposed that USP planets are the solid cores of hot Jupiters that lost
their gaseous envelopes due to photo-evaporation or Roche lobe overflow. We
test this hypothesis by asking whether USP planets are associated with
metal-rich stars, as has long been observed for hot Jupiters. We find the
metallicity distributions of USP-planet and hot-Jupiter hosts to be
significantly different (), based on Keck spectroscopy of
Kepler stars. Evidently, the sample of USP planets is not dominated by the
evaporated cores of hot Jupiters. The metallicity distribution of stars with
USP planets is indistinguishable from that of stars with short-period planets
with sizes between 2--4~. Thus it remains possible that the USP
planets are the solid cores of formerly gaseous planets smaller than Neptune.Comment: AJ, in pres
A two-qubit engine fueled by entangling operations and local measurements
We introduce a two-qubit engine that is powered by entangling operations and
projective local quantum measurements. Energy is extracted from the detuned
qubits coherently exchanging a single excitation. This engine, which uses the
information and back-action of the measurement, is generalized to an N-qubit
chain. We show that by gradually increasing the energy splitting along the
chain, the initial low energy of the first qubit can be up-converted
deterministically to an arbitrarily high energy at the last qubit by successive
neighbor swap operations and local measurements. Modeling the local measurement
as the entanglement of a qubit with a meter, we identify the measurement fuel
as the energetic cost to erase correlations between the qubits.Comment: 5 pages, 4 figure
A low stellar obliquity for WASP-47, a compact multiplanet system with a hot Jupiter and an ultra-short period planet
We have detected the Rossiter-Mclaughlin effect during a transit of WASP-47b,
the only known hot Jupiter with close planetary companions. By combining our
spectroscopic observations with Kepler photometry, we show that the projected
stellar obliquity is . We can firmly exclude a
retrograde orbit for WASP-47b, and rule out strongly misaligned prograde
orbits. Low obliquities have also been found for most of the other compact
multiplanet systems that have been investigated. The Kepler-56 system, with two
close-in gas giants transiting their subgiant host star with an obliquity of at
least 45, remains the only clear counterexample.Comment: 5 pages, 2 figures, Accepted for publication on ApJL, comments
welcom
Global data for ecology and epidemiology: a novel algorithm for temporal Fourier processing MODIS data
Background. Remotely-sensed environmental data from earth-orbiting satellites are increasingly used to model the distribution and abundance of both plant and animal species, especially those of economic or conservation importance. Time series of data from the MODerate-resolution Imaging Spectroradiometer (MODIS) sensors on-board NASA's Terra and Aqua satellites offer the potential to capture environmental thermal and vegetation seasonality, through temporal Fourier analysis, more accurately than was previously possible using the NOAA Advanced Very High Resolution Radiometer (AVHRR) sensor data. MODIS data are composited over 8- or 16-day time intervals that pose unique problems for temporal Fourier analysis. Applying standard techniques to MODIS data can introduce errors of up to 30% in the estimation of the amplitudes and phases of the Fourier harmonics. Methodology/Principal Findings. We present a novel spline-based algorithm that overcomes the processing problems of composited MODIS data. The algorithm is tested on artificial data generated using randomly selected values of both amplitudes and phases, and provides an accurate estimate of the input variables under all conditions. The algorithm was then applied to produce layers that capture the seasonality in MODIS data for the period from 2001 to 2005. Conclusions/Significance. Global temporal Fourier processed images of 1 km MODIS data for Middle Infrared Reflectance, day- and night-time Land Surface Temperature (LST), Normalised Difference Vegetation Index (NDVI), and Enhanced Vegetation Index (EVI) are presented for ecological and epidemiological applications. The finer spatial and temporal resolution, combined with the greater geolocational and spectral accuracy of the MODIS instruments, compared with previous multi-temporal data sets, mean that these data may be used with greater confidence in species' distribution modelling
The California-Kepler Survey. II. Precise Physical Properties of 2025 Kepler Planets and Their Host Stars
We present stellar and planetary properties for 1305 Kepler Objects of
Interest (KOIs) hosting 2025 planet candidates observed as part of the
California-Kepler Survey. We combine spectroscopic constraints, presented in
Paper I, with stellar interior modeling to estimate stellar masses, radii, and
ages. Stellar radii are typically constrained to 11%, compared to 40% when only
photometric constraints are used. Stellar masses are constrained to 4%, and
ages are constrained to 30%. We verify the integrity of the stellar parameters
through comparisons with asteroseismic studies and Gaia parallaxes. We also
recompute planetary radii for 2025 planet candidates. Because knowledge of
planetary radii is often limited by uncertainties in stellar size, we improve
the uncertainties in planet radii from typically 42% to 12%. We also leverage
improved knowledge of stellar effective temperature to recompute incident
stellar fluxes for the planets, now precise to 21%, compared to a factor of two
when derived from photometry.Comment: 13 pages, 4 figures, 4 tables, accepted for publication in AJ; full
versions of tables 3 and 4 are include
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