8,696 research outputs found
Improvements to context based self-supervised learning
We develop a set of methods to improve on the results of self-supervised
learning using context. We start with a baseline of patch based arrangement
context learning and go from there. Our methods address some overt problems
such as chromatic aberration as well as other potential problems such as
spatial skew and mid-level feature neglect. We prevent problems with testing
generalization on common self-supervised benchmark tests by using different
datasets during our development. The results of our methods combined yield top
scores on all standard self-supervised benchmarks, including classification and
detection on PASCAL VOC 2007, segmentation on PASCAL VOC 2012, and "linear
tests" on the ImageNet and CSAIL Places datasets. We obtain an improvement over
our baseline method of between 4.0 to 7.1 percentage points on transfer
learning classification tests. We also show results on different standard
network architectures to demonstrate generalization as well as portability. All
data, models and programs are available at:
https://gdo-datasci.llnl.gov/selfsupervised/.Comment: Accepted paper at CVPR 201
Observation of valley Landau-Zener-Bloch oscillations and pseudospin imbalance in photonic graphene
We demonstrate inter-valley Bloch oscillation (BO) and Landau-Zener tunneling
(LZT) in an optically-induced honeycomb lattice with a refractive index
gradient. Unlike previously observed BO in a gapped square lattice, we show
non-adiabatic beam dynamics that are highly sensitive to the direction of the
index gradient and the choice of the Dirac cones. In particular, a
symmetry-preserving potential leads to nearly perfect LZT and coherent BO
between the inequivalent valleys, whereas a symmetry-breaking potential
generates asymmetric scattering, imperfect LZT, and valley-sensitive generation
of vortices mediated by a pseudospin imbalance. This clearly indicates that,
near the Dirac points, the transverse gradient does not always act as a simple
scalar force as commonly assumed, and the LZT probability is strongly affected
by the sublattice symmetry as analyzed from an effective Landau-Zener
Hamiltonian. Our results illustrate the anisotropic response of an otherwise
isotropic Dirac platform to real-space potentials acting as strong driving
fields, which may be useful for manipulation of pseudospin and valley degrees
of freedom in graphene-like systems
Stochastic Resolution of Identity for Real-Time Second-Order Green's Function: Ionization Potential and Quasi-Particle Spectrum.
We develop a stochastic resolution of identity approach to the real-time second-order Green's function (real-time sRI-GF2) theory, extending our recent work for imaginary-time Matsubara Green's function [ Takeshita et al. J. Chem. Phys. 2019 , 151 , 044114 ]. The approach provides a framework to obtain the quasi-particle spectra across a wide range of frequencies and predicts ionization potentials and electron affinities. To assess the accuracy of the real-time sRI-GF2, we study a series of molecules and compare our results to experiments as well as to a many-body perturbation approach based on the GW approximation, where we find that the real-time sRI-GF2 is as accurate as self-consistent GW. The stochastic formulation reduces the formal computatinal scaling from O(Ne5) down to O(Ne3) where Ne is the number of electrons. This is illustrated for a chain of hydrogen dimers, where we observe a slightly lower than cubic scaling for systems containing up to Ne ≈ 1000 electrons
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Development of Container Free Sample Exposure for Synchrotron X-ray Footprinting.
The method of X-ray footprinting and mass spectrometry (XFMS) on large protein assemblies and membrane protein samples requires high flux density to overcome the hydroxyl radical scavenging reactions produced by the buffer constituents and the total protein content. Previously, we successfully developed microsecond XFMS using microfluidic capillary flow and a microfocused broadband X-ray source at the Advanced Light Source synchrotron beamlines, but the excessive radiation damage incurred when using capillaries prevented the full usage of a high-flux density beam. Here we present another significant advance for the XFMS method: the instrumentation of a liquid injection jet to deliver container free samples to the X-ray beam. Our preliminary experiments with a liquid jet at a bending magnet X-ray beamline demonstrate the feasibility of the approach and show a significant improvement in the effective dose for both the Alexa fluorescence assay and protein samples compared to conventional capillary flow methods. The combination of precisely controlled high dose delivery, shorter exposure times, and elimination of radiation damage due to capillary effects significantly increases the signal quality of the hydroxyl radical modification products and the dose-response data. This new approach is the first application of container free sample handling for XFMS and opens up the method for even further advances, such as high-quality microsecond time-resolved XFMS studies
Quasiparticle Interference on the Surface of the Topological Insulator BiTe
The quasiparticle interference of the spectroscopic imaging scanning
tunneling microscopy has been investigated for the surface states of the large
gap topological insulator BiTe through the T-matrix formalism. Both the
scalar potential scattering and the spin-orbit scattering on the warped
hexagonal isoenergy contour are considered. While backscatterings are forbidden
by time-reversal symmetry, other scatterings are allowed and exhibit strong
dependence on the spin configurations of the eigenfunctions at k points over
the isoenergy contour. The characteristic scattering wavevectors found in our
analysis agree well with recent experiment results.Comment: 5 pages, 2 figures, Some typos are correcte
A Critical Examination of Hypernova Remnant Candidates in M101. II. NGC 5471B
NGC 5471B has been suggested to contain a hypernova remnant because of its
extraordinarily bright X-ray emission. To assess its true nature, we have
obtained high-resolution images in continuum bands and nebular lines with the
Hubble Space Telescope, and high-dispersion long-slit spectra with the Kitt
Peak National Observatory 4-m echelle spectrograph. The images reveal three
supernova remnant (SNR) candidates in the giant HII region NGC 5471, with the
brightest one being the 77x60 pc shell in NGC 5471B. The Ha velocity profile of
NGC 5471B can be decomposed into a narrow component (FWHM = 41 km/s) from the
background HII region and a broad component (FWHM = 148 km/s) from the SNR
shell. Using the brightness ratio of the broad to narrow components and the Ha
flux measured from the WFPC2 Ha image, we derive an Ha luminosity of
(1.4+-0.1)x10^39 ergs/s for the SNR shell. The [SII]6716,6731 doublet ratio of
the broad velocity component is used to derive an electron density of ~700
cm^-3 in the SNR shell. The mass of the SNR shell is thus 4600+-500 Mo. With a
\~330 km/s expansion velocity implied by the extreme velocity extent of the
broad component, the kinetic energy of the SNR shell is determined to be
5x10^51 ergs. This requires an explosion energy greater than 10^52 ergs, which
can be provided by one hypernova or multiple supernovae. Comparing to SNRs in
nearby active star formation regions, the SNR shell in NGC 5471B appears truly
unique and energetic. We conclude that the optical observations support the
existence of a hypernova remnant in NGC 5471B.Comment: 27 pages, 9 figures, to appear in May 2002 issue of The Astronomical
Journa
AtomSim: web-deployed atomistic dynamics simulator
AtomSim, a collection of interfaces for computational crystallography simulations, has been developed. It uses forcefield-based dynamics through physics engines such as the General Utility Lattice Program, and can be integrated into larger computational frameworks such as the Virtual Neutron Facility for processing its dynamics into scattering functions, dynamical functions etc. It is also available as a Google App Engine-hosted web-deployed interface. Examples of a quartz molecular dynamics run and a hafnium dioxide phonon calculation are presented
Longitudinal effects of metabolic syndrome on Alzheimer and vascular related brain pathology.
Background/aimsThis study examines the longitudinal effect of metabolic syndrome (MetS) on brain-aging indices among cognitively normal (CN) and amnestic mild cognitive impairment (aMCI) groups [single-domain aMCI (saMCI) and multiple-domain aMCI (maMCI)].MethodsThe study population included 739 participants (CN = 226, saMCI = 275, and maMCI = 238) from the Alzheimer's Disease Neuroimaging Initiative, a clinic-based, multi-center prospective cohort. Confirmatory factor analysis was employed to determine a MetS latent composite score using baseline data of vascular risk factors. We examined the changes of two Alzheimer's disease (AD) biomarkers, namely [(18)F]fluorodeoxyglucose (FDG)-positron emission tomography (PET) regions of interest and medial temporal lobe volume over 5 years. A cerebrovascular aging index, cerebral white matter (cWM) volume, was examined as a comparison.ResultsThe vascular risk was similar in all groups. Applying generalized estimating equation modeling, all brain-aging indices declined significantly over time. Higher MetS scores were associated with a faster decline of cWM in the CN and maMCI groups but with a slower decrement of regional glucose metabolism in FDG-PET in the saMCI and maMCI groups.ConclusionAt the very early stage of cognitive decline, the vascular burden such as MetS may be in parallel with or independent of AD pathology in contributing to cognitive impairment in terms of accelerating the disclosure of AD pathology
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