4,782 research outputs found
Casimir interaction among heavy fermions in the BCS-BEC crossover
We investigate a two-species Fermi gas with a large mass ratio interacting by
an interspecies short-range interaction. Using the Born-Oppenheimer
approximation, we determine the interaction energy of two heavy fermions
immersed in the Fermi sea of light fermions as a function of the s-wave
scattering length. In the BCS limit, we recover the perturbative calculation of
the effective interaction between heavy fermions. The p-wave projection of the
effective interaction is attractive in the BCS limit while it turns out to be
repulsive near the unitarity limit. We find that the p-wave attraction reaches
its maximum between the BCS and unitarity limits, where the maximal p-wave
pairing of heavy minority fermions is expected. We also investigate the case
where the heavy fermions are confined in two dimensions and the p-wave
attraction between them is found to be stronger than that in three dimensions.Comment: 11 pages, 6 figure
Probing strongly interacting atomic gases with energetic atoms
We investigate properties of an energetic atom propagating through strongly
interacting atomic gases. The operator product expansion is used to
systematically compute a quasiparticle energy and its scattering rate both in a
spin-1/2 Fermi gas and in a spinless Bose gas. Reasonable agreement with recent
quantum Monte Carlo simulations even at a relatively small momentum k/kF>1.5
indicates that our large-momentum expansions are valid in a wide range of
momentum. We also study a differential scattering rate when a probe atom is
shot into atomic gases. Because the number density and current density of the
target atomic gas contribute to the forward scattering only, its contact
density (measure of short-range pair correlation) gives the leading
contribution to the backward scattering. Therefore, such an experiment can be
used to measure the contact density and thus provides a new local probe of
strongly interacting atomic gases.Comment: 35 pages, 11 figures; (v4) published with the new titl
Toshisada Nishida (1941â2011): Chimpanzee Rapport
Frans de Waal pays tribute to pioneering primatologist Toshisada Nishida, who transformed our understanding of chimpanzee behavior and culture and galvanized efforts to ensure their conservation
Visual perception of liquids: Insights from deep neural networks
Visually inferring material properties is crucial for many tasks, yet poses significant computational challenges for biological vision. Liquids and gels are particularly challenging due to their extreme variability and complex behaviour. We reasoned that measuring and modelling viscosity perception is a useful case study for identifying general principles of complex visual inferences. In recent years, artificial Deep Neural Networks (DNNs) have yielded breakthroughs in challenging real-world vision tasks. However, to model human vision, the emphasis lies not on best possible performance, but on mimicking the specific pattern of successes and errors humans make. We trained a DNN to estimate the viscosity of liquids using 100.000 simulations depicting liquids with sixteen different viscosities interacting in ten different scenes (stirring, pouring, splashing, etc). We find that a shallow feedforward network trained for only 30 epochs predicts mean observer performance better than most individual observers. This is the first successful image-computable model of human viscosity perception. Further training improved accuracy, but predicted human perception less well. We analysed the networkâs features using representational similarity analysis (RSA) and a range of image descriptors (e.g. optic flow, colour saturation, GIST). This revealed clusters of units sensitive to specific classes of feature. We also find a distinct population of units that are poorly explained by hand-engineered features, but which are particularly important both for physical viscosity estimation, and for the specific pattern of human responses. The final layers represent many distinct stimulus characteristicsânot just viscosity, which the network was trained on. Retraining the fully-connected layer with a reduced number of units achieves practically identical performance, but results in representations focused on viscosity, suggesting that network capacity is a crucial parameter determining whether artificial or biological neural networks use distributed vs. localized representations
An Operational Remote Sensing Algorithm of Land Surface Evaporation
Partitioning of solar energy at the Earth surface has significant implications in climate dynamics, hydrology, and ecology. Consequently, spatial mapping of energy partitioning from satellite remote sensing data has been an active research area for over two decades. We developed an algorithm for estimating evaporation fraction (EF), expressed as a ratio of actual evapotranspiration (ET) to the available energy (sum of ET and sensible heat flux), from satellite data. The algorithm is a simple two-source model of ET. We characterize a landscape as a mixture of bare soil and vegetation and thus we estimate EF as a mixture of EF of bare soil and EF of vegetation. In the estimation of EF of vegetation, we use the complementary relationship of the actual and the potential ET for the formulation of EF. In that, we use the canopy conductance model for describing vegetation physiology. On the other hand, we use âVI-Tsâ (vegetation index-surface temperature) diagram for estimation of EF of bare soil. As operational production of EF globally is our goal, the algorithm is primarily driven by remote sensing data but flexible enough to ingest ancillary data when available. We validated EF from this prototype algorithm using NOAA/AVHRR data with actual observations of EF at AmeriFlux stations (standard error â
0.17 and R2 â
0.71). Global distribution of EF every 8 days will be operationally produced by this algorithm using the data of MODIS on EOS-PM (Aqua) satellite
Search for a stochastic background of 100-MHz gravitational waves with laser interferometers
This letter reports the results of a search for a stochastic background of
gravitational waves (GW) at 100 MHz by laser interferometry. We have developed
a GW detector, which is a pair of 75-cm baseline synchronous recycling
(resonant recycling) interferometers. Each interferometer has a strain
sensitivity of ~ 10^{-16} Hz^{-1/2} at 100 MHz. By cross-correlating the
outputs of the two interferometers within 1000 seconds, we found h_{100}^2
Omega_{gw} < 6 times 10^{25} to be an upper limit on the energy density
spectrum of the GW background in a 2-kHz bandwidth around 100 MHz, where a flat
spectrum is assumed.Comment: Accepted by Phys.Rev.Lett.; 10 pages, 4 figure
Counting Majorana zero modes in superconductors
A counting formula for computing the number of (Majorana) zero modes bound to
topological point defects is evaluated in a gradient expansion for systems with
charge-conjugation symmetry. This semi-classical counting of zero modes is
applied to some examples that include graphene and a chiral p-wave
superconductor in two-dimensional space. In all cases, we explicitly relate the
counting of zero modes to Chern numbers.Comment: 21 pages, 3 figure
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