7,888 research outputs found
Hydrodynamic Simulations of Counterrotating Accretion Disks
Hydrodynamic simulations have been used to study accretion disks consisting
of counterrotating components with an intervening shear layer(s).
Configurations of this type can arise from the accretion of newly supplied
counterrotating matter onto an existing corotating disk. The grid-dependent
numerical viscosity of our hydro code is used to simulate the influence of a
turbulent viscosity of the disk. Firstly, we consider the case where the gas
well above the disk midplane rotates with angular rate +\Omega(r) and that well
below has the same properties but rotates with rate -\Omega(r). We find that
there is angular momentum annihilation in a narrow equatorial boundary layer in
which matter accretes supersonically with a velocity which approaches the
free-fall velocity and the average accretion speed of the disk can be
enormously larger than that for a conventional \alpha-disk rotating in one
direction. Secondly, we consider the case of a corotating accretion disk for
rr_t. In this case we observed, that
matter from the annihilation layer lost its stability and propagated inward
pushing matter of inner regions of the disk to accrete. Thirdly, we
investigated the case where counterrotating matter inflowing from large radial
distances encounters an existing corotating disk. Friction between the
inflowing matter and the existing disk is found to lead to fast boundary layer
accretion along the disk surfaces and to enhanced accretion in the main disk.
These models are pertinent to the formation of counterrotating disks in
galaxies and possibly in Active Galactic Nuclei and in X-ray pulsars in binary
systems.Comment: LaTeX, 18 pages, to appear in Ap
Microscopic Theory of Skyrmions in Quantum Hall Ferromagnets
We present a microscopic theory of skyrmions in the monolayer quantum Hall
ferromagnet. It is a peculiar feature of the system that the number density and
the spin density are entangled intrinsically as dictated by the W
algebra. The skyrmion and antiskyrmion states are constructed as W-rotated states of the hole-excited and electron-excited states,
respectively. They are spin textures accompanied with density modulation that
decreases the Coulomb energy. We calculate their excitation energy as a
function of the Zeeman gap and compared the result with experimental data.Comment: 15 pages (to be published in PRB
Characterisation of the Medipix3 detector for 60 and 80 keV electrons
In this paper we report quantitative measurements of the imaging performance for the current generation of hybrid pixel detector, Medipix3, used as a direct electron detector. We have measured the modulation transfer function and detective quantum efficiency at beam energies of 60 and 80 keV. In single pixel mode, energy threshold values can be chosen to maximize either the modulation transfer function or the detective quantum efficiency, obtaining values near to, or exceeding those for a theoretical detector with square pixels. The Medipix3 charge summing mode delivers simultaneous, high values of both modulation transfer function and detective quantum efficiency. We have also characterized the detector response to single electron events and describe an empirical model that predicts the detector modulation transfer function and detective quantum efficiency based on energy threshold. Exemplifying our findings we demonstrate the Medipix3 imaging performance recording a fully exposed electron diffraction pattern at 24-bit depth together with images in single pixel and charge summing modes. Our findings highlight that for transmission electron microscopy performed at low energies (energies <100 keV) thick hybrid pixel detectors provide an advantageous architecture for direct electron imaging
Scalable Similarity Search for Molecular Descriptors
Similarity search over chemical compound databases is a fundamental task in
the discovery and design of novel drug-like molecules. Such databases often
encode molecules as non-negative integer vectors, called molecular descriptors,
which represent rich information on various molecular properties. While there
exist efficient indexing structures for searching databases of binary vectors,
solutions for more general integer vectors are in their infancy. In this paper
we present a time- and space- efficient index for the problem that we call the
succinct intervals-splitting tree algorithm for molecular descriptors (SITAd).
Our approach extends efficient methods for binary-vector databases, and uses
ideas from succinct data structures. Our experiments, on a large database of
over 40 million compounds, show SITAd significantly outperforms alternative
approaches in practice.Comment: To be appeared in the Proceedings of SISAP'1
Medipix3 Demonstration and understanding of near ideal detector performance for 60 & 80 keV electrons
In our article we report first quantitative measurements of imaging
performance for the current generation of hybrid pixel detector, Medipix3, as
direct electron detector. Utilising beam energies of 60 & 80 keV, measurements
of modulation transfer function (MTF) and detective quantum efficiency (DQE)
have revealed that, in single pixel mode (SPM), energy threshold values can be
chosen to maximize either the MTF or DQE, obtaining values near to, or even
exceeding, those for an ideal detector. We have demonstrated that the Medipix3
charge summing mode (CSM) can deliver simultaneous, near ideal values of both
MTF and DQE. To understand direct detection performance further we have
characterized the detector response to single electron events, building an
empirical model which can predict detector MTF and DQE performance based on
energy threshold. Exemplifying our findings we demonstrate the Medipix3 imaging
performance, recording a fully exposed electron diffraction pattern at 24-bit
depth and images in SPM and CSM modes. Taken together our findings highlight
that for transmission electron microscopy performed at low energies (energies
<100 keV) thick hybrid pixel detectors provide an advantageous and alternative
architecture for direct electron imagin
Singular Laplacian Growth
The general equations of motion for two dimensional Laplacian growth are
derived using the conformal mapping method. In the singular case, all
singularities of the conformal map are on the unit circle, and the map is a
degenerate Schwarz-Christoffel map. The equations of motion describe the
motions of these singularities. Despite the typical fractal-like outcomes of
Laplacian growth processes, the equations of motion are shown to be not
particularly sensitive to initial conditions. It is argued that the sensitivity
of this system derives from a novel cause, the non-uniqueness of solutions to
the differential system. By a mechanism of singularity creation, every solution
can become more complex, even in the absence of noise, without violating the
growth law. These processes are permitted, but are not required, meaning the
equation of motion does not determine the motion, even in the small.Comment: 8 pages, Latex, 4 figures, Submitted to Phys. Rev.
- …