Vortices in magnetically coupled superconducting layered systems

Pancake vortices in stacks of thin superconducting films or layers are considered. It is stressed that in the absence of Josephson coupling topological restrictions upon possible configurations of vortices are removed and various examples of structures forbidden in bulk superconductors are given. In particular, it is shown that vortices may skip surface layers in samples of less than a certain size R_c which might be macroscopic. The Josephson coupling suppresses R_c estimates

Accretion Processes for General Spherically Symmetric Compact Objects

We investigate the accretion process for different spherically symmetric space-time geometries for a static fluid. We analyse this procedure using the most general black hole metric ansatz. After that, we examine the accretion process for specific spherically symmetric metrics obtaining the velocity of the sound during the process and the critical speed of the flow of the fluid around the black hole. In addition, we study the behaviour of the rate of change of the mass for each chosen metric for a barotropic fluid.Comment: 10 pages, 15 figures, v2 accepted for publication in 'European Physical Journal C

Porcine colonization of the Americas: a 60k SNP story.

Made available in DSpace on 2018-07-01T01:24:17Z (GMT). No. of bitstreams: 1 hdy2012109a.pdf: 1129860 bytes, checksum: 44683cde430ba3f771242018513366b0 (MD5) Previous issue date: 2013-02-26bitstream/item/179305/1/hdy2012109a.pd

The extended tails of Palomar 5: A ten degree arc of globular cluster tidal debris

Using wide-field photometric data from the Sloan Digital Sky Survey (SDSS) we recently showed that the Galactic globular cluster Palomar 5 is in the process of being tidally disrupted. Its tidal tails were initially detected in a 2.5 degree wide band along the celestial equator. A new analysis of SDSS data for a larger field now reveals that the tails of Pal 5 have a much larger spatial extent and can be traced over an arc of 10 deg across the sky, corresponding to a projected length of 4 kpc at the distance of the cluster. The number of former cluster stars found in the tails adds up to about 1.2 times the number of stars in the cluster. The radial profile of stellar surface density in the tails follows approximately a power law r^gamma with -1.5 < gamma < -1.2. The stream of debris from Pal 5 is significantly curved, which demonstrates its acceleration by the Galactic potential. The cluster is presently near the apocenter but has repeatedly undergone disk crossings in the inner part of the Galaxy leading to strong tidal shocks. Our results suggest that the observed debris originates mostly from mass loss within the last 2 Gyrs. The cluster is likely to be destroyed after the next disk crossing, which will happen in about 100 Myr. (abridged)Comment: 44 pages, including 14 figures (Figs.1,3 & 14 with decreased resolution), accepted for publication in the Astronomical Journa

The Stellar Content and Star Formation History of the late-type spiral galaxy NGC 300 from Hubble Space Telescope observations

We present the first WFPC2 V, I photometry for the Sculptor Group galaxy NGC 300 in four fields ranging from the centre to the outer edge. We have made the first measurement of the star formation histories in two disk fields: the oldest stars were born at similar epochs and formation activity increased but at different mean rates. The main disk stellar population is predominantly old, consisting of RGB and AGB stars, based on a synthetic colour magnitude diagram analysis. Z is found to have been more metal poor than 0.006 (or 0.33Zsolar) with no evidence for significant change in the mean Z value over time in both disk fields. In the central region, we find a dearth of bright stars with respect to the two disk fields that cannot be explained by observational effects. Taken at face value, this finding would agree with the Davidge (1998) report of suppressed star formation there during the past 1Gyr with respect to his disk fields at larger radii; but the possibility of significant central extinction affecting our finding remains. We have also determined the first distance modulus estimate based on the tip of the red giant branch method: on the Cepheid distance scale of Ferrarese et al. (2000) we find (m-M)o = 26.56+/-0.07 (+/-0.13) mag; and a similar value from the Cepheid-independent empirical method by Lee et al. (1993), both in good agreement with the Cepheid distance determined by Freedman et al. (2001). A discrepancy between this and the theoretical calibration of the red giant branch tip magnitude method remains. Finally, we report a newly detected young (up to about 10Myr) stellar association of about average size (~140pc) in one of the disk fields.Comment: 47 pages, 12 figures (the quality of Figures 1 and 6 has been degraded), Corrected minor textual errors and two reference omissions to match the version to be published in The Astronomical Journa

SU(3) symmetry breaking in lower fp-shell nuclei

Results of shell-model calculations for lower fp-shell nuclei show that SU(3) symmetry breaking in this region is driven by the single-particle spin-orbit splitting. However, even though states of the yrast band exhibit SU(3) symmetry breaking, the results also show that the yrast band B(E2) values are insensitive to this fragmentation of the SU(3) symmetry; specifically, the quadrupole collectivity as measured by B(E2) transition strengths between low lying members of the yrast band remain high even though SU(3) appears to be broken. Results for $^{44,46,48}Ti$ and $^{48}Cr$ using the Kuo-Brown-3 two-body interaction are given to illustrate these observations.Comment: Updated to the published versio

Cross section measurements of 155,157Gd(n, γ) induced by thermal and epithermal neutrons

© SIF, Springer-Verlag GmbH Germany, part of Springer Nature 2019Neutron capture cross section measurements on 155Gd and 157Gd were performed using the time-of-flight technique at the n_TOF facility at CERN on isotopically enriched samples. The measurements were carried out in the n_TOF experimental area EAR1, at 185 m from the neutron source, with an array of 4 C6D6 liquid scintillation detectors. At a neutron kinetic energy of 0.0253 eV, capture cross sections of 62.2(2.2) and 239.8(8.4) kilobarn have been derived for 155Gd and 157Gd, respectively, with up to 6% deviation relative to values presently reported in nuclear data libraries, but consistent with those values within 1.6 standard deviations. A resonance shape analysis has been performed in the resolved resonance region up to 181 eV and 307 eV, respectively for 155Gd and 157Gd, where on average, resonance parameters have been found in good agreement with evaluations. Above these energies and up to 1 keV, the observed resonance-like structure of the cross section has been analysed and characterised. From a statistical analysis of the observed neutron resonances we deduced: neutron strength function of 2. 01 (28) × 10 - 4 and 2. 17 (41) × 10 - 4; average total radiative width of 106.8(14) meV and 101.1(20) meV and s-wave resonance spacing 1.6(2) eV and 4.8(5) eV for n + 155Gd and n + 157Gd systems, respectively.Peer reviewedFinal Accepted Versio

Democratic population decisions result in robust policy-gradient learning: A parametric study with GPU simulations

High performance computing on the Graphics Processing Unit (GPU) is an emerging field driven by the promise of high computational power at a low cost. However, GPU programming is a non-trivial task and moreover architectural limitations raise the question of whether investing effort in this direction may be worthwhile. In this work, we use GPU programming to simulate a two-layer network of Integrate-and-Fire neurons with varying degrees of recurrent connectivity and investigate its ability to learn a simplified navigation task using a policy-gradient learning rule stemming from Reinforcement Learning. The purpose of this paper is twofold. First, we want to support the use of GPUs in the field of Computational Neuroscience. Second, using GPU computing power, we investigate the conditions under which the said architecture and learning rule demonstrate best performance. Our work indicates that networks featuring strong Mexican-Hat-shaped recurrent connections in the top layer, where decision making is governed by the formation of a stable activity bump in the neural population (a "non-democratic" mechanism), achieve mediocre learning results at best. In absence of recurrent connections, where all neurons "vote" independently ("democratic") for a decision via population vector readout, the task is generally learned better and more robustly. Our study would have been extremely difficult on a desktop computer without the use of GPU programming. We present the routines developed for this purpose and show that a speed improvement of 5x up to 42x is provided versus optimised Python code. The higher speed is achieved when we exploit the parallelism of the GPU in the search of learning parameters. This suggests that efficient GPU programming can significantly reduce the time needed for simulating networks of spiking neurons, particularly when multiple parameter configurations are investigated. © 2011 Richmond et al