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$_{\infty }$-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

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

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.

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

Effect of in-plane magnetic field on magnetic phase transitions in nu=2 bilayer quantum Hall systems

By using the effective bosonic spin theory, which is recently proposed by Demler and Das Sarma [ Phys. Rev. Lett. 82, 3895 (1999) ], we analyze the effect of an external in-plane magnetic field on the magnetic phase transitions of the bilayer quantum Hall system at filling factor nu=2. It is found that the quantum phase diagram is modified by the in-plane magnetic field. Therefore, quantum phase transitions can be induced simply by tilting the magnetic field. The general behavior of the critical tilted angle for different layer separations and interlayer tunneling amplitudes is shown. We find that the critical tilted angles being calculated agree very well with the reported values. Moreover, a universal critical exponent for the transition from the canted antiferromagnetic phase to the ferromagnetic phase is found to be equal to 1/2 within the present effective theory.Comment: RevTeX, 4 pages with 3 EPS figures include

Arc-like distribution of high CO(J=3-2)/CO(J=1-0) ratio gas surrounding the central star cluster of the supergiant HII region NGC 604

We report the discovery of a high CO(J=3-2)/CO(J=1-0) ratio gas with an arc-like distribution (``high-ratio gas arc'') surrounding the central star cluster of the supergiant HII region NGC 604 in the nearby spiral galaxy M 33, based on multi-J CO observations of a 5' $\times$ 5' region of NGC 604 conducted using the ASTE 10-m and NRO 45-m telescopes. The discovered ``high-ratio gas arc'' extends to the south-east to north-west direction with a size of $\sim$ 200 pc. The western part of the high-ratio gas arc closely coincides well with the shells of the HII regions traced by H$\alpha$ and radio continuum peaks. The CO(J=3-2)/CO(J=1-0) ratio, R_{3-2/1-0}, ranges between 0.3 and 1.2 in the observed region, and the R_{3-2/1-0} values of the high-ratio gas arc are around or higher than unity, indicating very warm (T_kin > 60 K) and dense (n(H_2) > 10^{3-4} cm^{-3}) conditions of the high-ratio gas arc. We suggest that the dense gas formation and second-generation star formation occur in the surrounding gas compressed by the stellar wind and/or supernova of the first-generation stars of NGC 604, i.e., the central star cluster of NGC 604.Comment: 4 pages, 4 figures. The Astrophysical Journal Letters, in pres

Spontaneous alloying in binary metal microclusters - A molecular dynamics study -

Microcanonical molecular dynamics study of the spontaneous alloying(SA), which is a manifestation of fast atomic diffusion in a nano-sized metal cluster, is done in terms of a simple two dimensional binary Morse model. Important features observed by Yasuda and Mori are well reproduced in our simulation. The temperature dependence and size dependence of the SA phenomena are extensively explored by examining long time dynamics. The dominant role of negative heat of solution in completing the SA is also discussed. We point out that a presence of melting surface induces the diffusion of core atoms even if they are solid-like. In other words, the {\it surface melting} at substantially low temperature plays a key role in attaining the SA.Comment: 15 pages, 12 fgures, Submitted to Phys.Rev.