34,665 research outputs found
Specific heat of BaKFeAs, and a new method for identifying the electron contribution: two electron bands with different energy gaps in the superconducting state
We report measurements of the specific heat of
BaKFeAs, an Fe-pnictide superconductor with
= 36.9 K, for which there are suggestions of an unusual electron pairing
mechanism. We use a new method of analysis of the data to derive the parameters
characteristic of the electron contribution. It is based on comparisons of
-model expressions for the electron contribution with the total
measured specific heat, which give the electron contribution directly. It
obviates the need in the conventional analyses for an independent, necessarily
approximate, determination of the lattice contribution, which is subtracted
from the total specific heat to obtain the electron contribution. It eliminates
the uncertainties and errors in the electron contribution that follow from the
approximations in the determination of the lattice contribution. Our values of
the parameters characteristic of the electron contribution differ significantly
from those obtained in conventional analyses of specific-heat data for five
similar hole-doped BaFeAs superconductors, which also differ
significantly among themselves. They show that the electron density of states
is comprised of contributions from two electron bands with
superconducting-state energy gaps that differ by a factor 3.8, with 77
coming from the band with the larger gap. The variation of the specific heat
with magnetic field is consistent with extended -wave pairing, one of the
theoretical predictions. The relation between the densities of states and the
energy gaps in the two bands is not consistent with a theoretical model based
on interband interactions alone. Comparison of the normal-state density of
states with band-structure calculations shows an extraordinarily large
effective mass enhancement, for which there is no precedent in similar
materials and no theoretical explanation.Comment: 30 pages, 7 figures, submitte
Experimental investigation of some aspects of insect-like flapping flight aerodynamics for application to micro air vehicles
Insect-like flapping flight offers a power-efficient and highly manoeuvrable basis for micro air vehicles for indoor applications. Some aspects of the aerodynamics associated with the sweeping phase of insect wing kinematics are examined by making particle image velocimetry measurements on a rotating wing immersed in a tank of seeded water. The work is motivated by the paucity of data with quantified error on insect-like flapping flight, and aims to fill this gap by providing a detailed description of the experimental setup, quantifying the uncertainties in the measurements and explaining the results. The experiments are carried out at two Reynolds numbers-500 and 15,000-accounting for scales pertaining to many insects and future flapping-wing micro air vehicles, respectively. The results from the experiments are used to describe prominent flow features, and Reynolds number-related differences are highlighted. In particular, the behaviour of the leading-edge vortex at these Reynolds numbers is studied and the presence of Kelvin-Helmholtz instability observed at the higher Reynolds number in computational fluid dynamics calculations is also verified
Range Corrections to Three-Body Observables near a Feshbach Resonance
A non-relativistic system of three identical particles will display a rich
set of universal features known as Efimov physics if the scattering length a is
much larger than the range l of the underlying two-body interaction. An
appropriate effective theory facilitates the derivation of both results in the
|a| goes to infinity limit and finite-l/a corrections to observables of
interest. Here we use such an effective-theory treatment to consider the impact
of corrections linear in the two-body effective range, r_s on the three-boson
bound-state spectrum and recombination rate for |a| much greater than |r_s|. We
do this by first deriving results appropriate to the strict limit |a| goes to
infinity in coordinate space. We then extend these results to finite a using
once-subtracted momentum-space integral equations. We also discuss the
implications of our results for experiments that probe three-body recombination
in Bose-Einstein condensates near a Feshbach resonance.Comment: 28 pages, 3 figure
Fibrillar templates and soft phases in systems with short-range dipolar and long-range interactions
We analyze the thermal fluctuations of particles that have a short-range
dipolar attraction and a long-range repulsion. In an inhomogeneous particle
density region, or "soft phase," filamentary patterns appear which are
destroyed only at very high temperatures. The filaments act as a fluctuating
template for correlated percolation in which low-energy excitations can move
through the stable pattern by local rearrangements. At intermediate
temperatures, dynamically averaged checkerboard states appear. We discuss
possible implications for cuprate superconducting and related materials.Comment: 4 pages, 4 postscript figures. Discussion of implications for
experiment and theory has been expande
Drivers of intrapopulation variation in resource use in a generalist predator, the macaroni penguin
Intrapopulation variation in resource use occurs in many populations of generalist predators with important community and evolutionary implications. One of the hypothesised mechanisms for such widespread variation is ecological opportunity, i.e. resource availability determined by intrinsic constraints and extrinsic conditions. We combined tracking data and stable isotope analysis to examine how breeding constraints and prey conditions influenced intrapopulation variation in resource use among macaroni penguins Eudyptes chrysolophus. Isotopic variation was also examined as a function of breeding success, individual traits and individual specialisation. Variation in isotope ratios was greatest across multiple tissue types when birds were able to undertake mid-range foraging trips (i.e. during incubation and pre-moult). This variation was highly consistent between years that spanned a 3-fold difference in local krill Euphausia superba density and was also highly consistent at the individual level between 2 years that had similar krill densities. However, by comparing our results with previous work on the same population, it appeared that a decrease in local prey availability can increase intrapopulation variation in resource use during periods with more restricted foraging ranges (i.e. during brood-guard and crèche). This study highlights the importance of considering ecological interactions that operate on multiple spatio-temporal scales when examining the drivers of resource use in populations of generalist predators
An atomic clock with instability
Atomic clocks have been transformational in science and technology, leading
to innovations such as global positioning, advanced communications, and tests
of fundamental constant variation. Next-generation optical atomic clocks can
extend the capability of these timekeepers, where researchers have long aspired
toward measurement precision at 1 part in . This milestone will
enable a second revolution of new timing applications such as relativistic
geodesy, enhanced Earth- and space-based navigation and telescopy, and new
tests on physics beyond the Standard Model. Here, we describe the development
and operation of two optical lattice clocks, both utilizing spin-polarized,
ultracold atomic ytterbium. A measurement comparing these systems demonstrates
an unprecedented atomic clock instability of after
only hours of averaging
How well do CMIP5 climate simulations replicate historical trends and patterns of meteorological droughts?
Assessing the uncertainties and understanding the deficiencies of climate models are fundamental to developing adaptation strategies. The objective of this study is to understand how well Coupled Model Intercomparison-Phase 5 (CMIP5) climate model simulations replicate ground-based observations of continental drought areas and their trends. The CMIP5 multimodel ensemble encompasses the Climatic Research Unit (CRU) ground-based observations of area under drought at all time steps. However, most model members overestimate the areas under extreme drought, particularly in the Southern Hemisphere (SH). Furthermore, the results show that the time series of observations and CMIP5 simulations of areas under drought exhibit more variability in the SH than in the Northern Hemisphere (NH). The trend analysis of areas under drought reveals that the observational data exhibit a significant positive trend at the significance level of 0.05 over all land areas. The observed trend is reproduced by about three-fourths of the CMIP5 models when considering total land areas in drought. While models are generally consistent with observations at a global (or hemispheric) scale, most models do not agree with observed regional drying and wetting trends. Over many regions, at most 40% of the CMIP5 models are in agreement with the trends of CRU observations. The drying/wetting trends calculated using the 3 months Standardized Precipitation Index (SPI) values show better agreement with the corresponding CRU values than with the observed annual mean precipitation rates. Pixel-scale evaluation of CMIP5 models indicates that no single model demonstrates an overall superior performance relative to the other models
Solving Tree Problems with Category Theory
Artificial Intelligence (AI) has long pursued models, theories, and
techniques to imbue machines with human-like general intelligence. Yet even the
currently predominant data-driven approaches in AI seem to be lacking humans'
unique ability to solve wide ranges of problems. This situation begs the
question of the existence of principles that underlie general problem-solving
capabilities. We approach this question through the mathematical formulation of
analogies across different problems and solutions. We focus in particular on
problems that could be represented as tree-like structures. Most importantly,
we adopt a category-theoretic approach in formalising tree problems as
categories, and in proving the existence of equivalences across apparently
unrelated problem domains. We prove the existence of a functor between the
category of tree problems and the category of solutions. We also provide a
weaker version of the functor by quantifying equivalences of problem categories
using a metric on tree problems.Comment: 10 pages, 4 figures, International Conference on Artificial General
Intelligence (AGI) 201
Opioids Delay Healing of Spinal Fusion: A Rabbit Posterolateral Lumbar Fusion Model
Background Context Opioid use is prevalent in the management of pre- and postoperative pain in patients undergoing spinal fusion. There is evidence that opioids downregulate osteoblasts in vitro, and a previous study found that morphine delays the maturation and remodeling of callus in a rat femur fracture model. However, the effect of opioids on healing of spinal fusion has not been investigated before. Isolating the effect of opioid exposure in humans would be limited by the numerous confounding factors that affect fusion healing. Therefore, we have used a well-established rabbit model to study the process of spinal fusion healing that closely mimics humans. Purpose The objective of this work was to study the effect of systemic opioids on the process of healing of spinal fusion in a rabbit posterolateral spinal fusion model. Study Design/Setting This is a preclinical animal study. Materials and Methods Twenty-four adult New Zealand white rabbits were studied in two groups after approval from the Institutional Animal Care and Use Committee (IACUC). The opioid group (n=12) received 4 weeks\u27 preoperative and 6 weeks\u27 postoperative transdermal fentanyl. Serum fentanyl levels were measured just before surgery and 4 weeks postoperatively to ensure adequate levels. The control group (n=12) received only perioperative pain control as necessary. All animals underwent a bilateral L5–L6 posterolateral spinal fusion using iliac crest autograft. Animals were euthanized at the 6-week postoperative time point, and assessment of fusion was done by manual palpation, plain radiographs, microcomputed tomography (microCT), and histology. Results Twelve animals in the control group and 11 animals in the opioid group were available for analysis at the end of 6 weeks. The fusion scores on manual palpation, radiographs, and microCT were not statistically different. Three-dimensional microCT morphometry found that the fusion mass in the opioid group had a lower bone volume (p=.09), a lower trabecular number (p=.02), and a higher trabecular separation (p=.02) compared with the control group. Histologic analysis found areas of incorporation of autograft and unincorporated graft fragments in both groups. In the control group, there was remodeling of de novo woven bone to lamellar organization with incorporation of osteocytes, formation of mature marrow, and relative paucity of hypertrophied osteoblasts lining new bone. Sections from the opioid group showed formation of de novo woven bone, and hypertrophied osteoblasts were seen lining the new bone. There were no sections showing lamellar organization and development of mature marrow elements in the opioid group. Less dense trabeculae on microCT correlated with histologic findings of relatively immature fusion mass in the opioid group. Conclusions Systemic opioids led to an inferior quality fusion mass with delay in maturation and remodeling at 6 weeks in this rabbit spinal fusion model. These preliminary results lay the foundation for further research to investigate underlying cellular mechanisms, the temporal fusion process, and the dose-duration relationship of opioids responsible for our findings
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