394 research outputs found
Advanced Diagnostics for the Study of Linearly Polarized Emission. II: Application to Diffuse Interstellar Radio Synchrotron Emission
Diagnostics of polarized emission provide us with valuable information on the
Galactic magnetic field and the state of turbulence in the interstellar medium,
which cannot be obtained from synchrotron intensity alone. In Paper I (Herron
et al. 2017b), we derived polarization diagnostics that are rotationally and
translationally invariant in the - plane, similar to the polarization
gradient. In this paper, we apply these diagnostics to simulations of ideal
magnetohydrodynamic turbulence that have a range of sonic and Alfv\'enic Mach
numbers. We generate synthetic images of Stokes and for these
simulations, for the cases where the turbulence is illuminated from behind by
uniform polarized emission, and where the polarized emission originates from
within the turbulent volume. From these simulated images we calculate the
polarization diagnostics derived in Paper I, for different lines of sight
relative to the mean magnetic field, and for a range of frequencies. For all of
our simulations, we find that the polarization gradient is very similar to the
generalized polarization gradient, and that both trace spatial variations in
the magnetoionic medium for the case where emission originates within the
turbulent volume, provided that the medium is not supersonic. We propose a
method for distinguishing the cases of emission coming from behind or within a
turbulent, Faraday rotating medium, and a method to partly map the rotation
measure of the observed region. We also speculate on statistics of these
diagnostics that may allow us to constrain the physical properties of an
observed turbulent region.Comment: 34 pages, 25 figures, accepted for publication in Ap
Local Rotation Invariant Patch Descriptors for 3D Vector Fields
Abstract—In this paper, we present two novel methods for the fast computation of local rotation invariant patch descriptors for 3D vectorial data. Patch based algorithms have recently become very popular approach for a wide range of 2D computer vision problems. Our local rotation invariant patch descriptors allow an extension of these methods to 3D vector fields. Our approaches are based on a harmonic representation for local spherical 3D vector field patches, which enables us to derive fast algorithms for the computation of rotation invariant power spectrum and bispectrum feature descriptors of such patches. Keywords-local feature; 3D vector field; invariance; I
Parallel algorithm for determining motion vectors in ice floe images by matching edge features
A parallel algorithm is described to determine motion vectors of ice floes using time sequences of images of the Arctic ocean obtained from the Synthetic Aperture Radar (SAR) instrument flown on-board the SEASAT spacecraft. Researchers describe a parallel algorithm which is implemented on the MPP for locating corresponding objects based on their translationally and rotationally invariant features. The algorithm first approximates the edges in the images by polygons or sets of connected straight-line segments. Each such edge structure is then reduced to a seed point. Associated with each seed point are the descriptions (lengths, orientations and sequence numbers) of the lines constituting the corresponding edge structure. A parallel matching algorithm is used to match packed arrays of such descriptions to identify corresponding seed points in the two images. The matching algorithm is designed such that fragmentation and merging of ice floes are taken into account by accepting partial matches. The technique has been demonstrated to work on synthetic test patterns and real image pairs from SEASAT in times ranging from .5 to 0.7 seconds for 128 x 128 images
Fluctuations and phase transitions in Larkin-Ovchinnikov liquid crystal states of population-imbalanced resonant Fermi gas
Motivated by a realization of imbalanced Feshbach-resonant atomic Fermi
gases, we formulate a low-energy theory of the Fulde-Ferrell and the
Larkin-Ovchinnikov (LO) states and use it to analyze fluctuations, stability,
and phase transitions in these enigmatic finite momentum-paired superfluids.
Focusing on the unidirectional LO pair-density wave state, that spontaneously
breaks the continuous rotational and translational symmetries, we show that it
is characterized by two Goldstone modes, corresponding to a superfluid phase
and a smectic phonon. Because of the liquid-crystalline "softness" of the
latter, at finite temperature the 3d state is characterized by a vanishing LO
order parameter, quasi-Bragg peaks in the structure and momentum distribution
functions, and a "charge"-4, paired Cooper-pairs, off-diagonal-long-range
order, with a superfluid-stiffness anisotropy that diverges near a transition
into a nonsuperfluid state. In addition to conventional integer vortices and
dislocations the LO superfluid smectic exhibits composite half-integer
vortex-dislocation defects. A proliferation of defects leads to a rich variety
of descendant states, such as the "charge"-4 superfluid and Fermi-liquid
nematics and topologically ordered nonsuperfluid states, that generically
intervene between the LO state and the conventional superfluid and the
polarized Fermi-liquid at low and high imbalance, respectively. The fermionic
sector of the LO gapless superconductor is also quite unique, exhibiting a
Fermi surface of Bogoliubov quasiparticles associated with the Andreev band of
states, localized on the array of the LO domain-walls.Comment: 56 pages, 21 figure
Representations of Materials for Machine Learning
High-throughput data generation methods and machine learning (ML) algorithms
have given rise to a new era of computational materials science by learning
relationships among composition, structure, and properties and by exploiting
such relations for design. However, to build these connections, materials data
must be translated into a numerical form, called a representation, that can be
processed by a machine learning model. Datasets in materials science vary in
format (ranging from images to spectra), size, and fidelity. Predictive models
vary in scope and property of interests. Here, we review context-dependent
strategies for constructing representations that enable the use of materials as
inputs or outputs of machine learning models. Furthermore, we discuss how
modern ML techniques can learn representations from data and transfer chemical
and physical information between tasks. Finally, we outline high-impact
questions that have not been fully resolved and thus, require further
investigation.Comment: 20 pages, 5 figures, To Appear in Annual Review of Materials Research
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