On Nonconservative Stability Problems of Elastic Systems with Slight Damping

Equilibrium stability of linear two degree of freedom system with slight viscous damping subjected to nonconservative loadin

Transport properties of a molecule embedded in an Aharonov-Bohm interferometer

We theoretically investigate the transport properties of a molecule embedded in one arm of a mesoscopic Aharonov-Bohm interferometer. Due to the presence of phonons the molecule level position ($\epsilon_d$) and the electron-electron interaction ($U$) undergo a \emph{polaronic shift} which affects dramatically the electronic transport through the molecular junction. When the electron-phonon interaction is weak the linear conductance presents Fano-line shapes as long as the direct channel between the electrodes is opened. The observed Fano resonances in the linear conductance are originated from the interference between the spin Kondo state and the direct path. For strong enough electron-phonon interaction, the electron-electron interaction is renormalized towards negative values, {\it i.e.} becomes effectively attractive. This scenario favors fluctuations between the empty and doubly occupied charge states and therefore promotes a charge Kondo effect. However, the direct path between the contacts breaks the electron-hole symmetry which can efficiently suppress this charge Kondo effect. Nevertheless, we show that a proper tuning of the gate voltage is able to revive the Kondo resonance. Our results are obtained by using the Numerical Renormalization approximation to compute the electronic spectral function and the linear conductance.Comment: 17 pages, 12 figure

Integrability of N=6 Chern-Simons Theory at Six Loops and Beyond

We study issues concerning perturbative integrability of N=6 Chern-Simons theory at planar and weak `t Hooft coupling regime. By Feynman diagrammatics, we derive so called maximal-ranged interactions in the quantum dilatation generator, originating from homogeneous and inhomogeneous diagrams. These diagrams require proper regularization of not only ultraviolet but also infrared divergences. We first consider standard operator mixing method. We show that homogeneous diagrams are obtainable by recursive method to all orders. The method, however, is not easily extendable to inhomogeneous diagrams. We thus consider two-point function method and study both operator contents and spectrum of the quantum dilatation generator up to six loop orders. We show that, of two possible classes of operators, only one linear combination actually contributes. Curiously, this is exactly the same combination as in N=4 super Yang-Mills theory. We then study spectrum of anomalous dimension up to six loops. We find that the spectrum agrees perfectly with the prediction based on quantum integrability. In evaluating the six loop diagrams, we utilized remarkable integer-relation algorithm (PSLQ) developed by Ferguson, Baily and Arno.Comment: 1+39 pages, 12 figures, references added, minor structural changes, typos correcte

Structure and star formation in galaxies out to z=3: evidence for surface density dependent evolution and upsizing

We present an analysis of galaxies in the CDF-South. We find a tight relation to z=3 between color and size at a given mass, with red galaxies being small, and blue galaxies being large. We show that the relation is driven by stellar surface density or inferred velocity dispersion: galaxies with high surface density are red and have low specific star formation rates, and galaxies with low surface density are blue and have high specific star formation rates. Surface density and inferred velocity dispersion are better correlated with specific star formation rate and color than stellar mass. Hence stellar mass by itself is not a good predictor of the star formation history of galaxies. In general, galaxies at a given surface density have higher specific star formation rates at higher redshift. Specifically, galaxies with a surface density of 1-3 10^9 Msun/kpc^2 are "red and dead" at low redshift, approximately 50% are forming stars at z=1, and almost all are forming stars by z=2. This provides direct additional evidence for the late evolution of galaxies onto the red sequence. The sizes of galaxies at a given mass evolve like 1/(1+z)^(0.59 +- 0.10). Hence galaxies undergo significant upsizing in their history. The size evolution is fastest for the highest mass galaxies, and quiescent galaxies. The persistence of the structural relations from z=0 to z=2.5, and the upsizing of galaxies imply that a relation analogous to the Hubble sequence exists out to z=2.5, and possibly beyond. The star forming galaxies at z >= 1.5 are quite different from star forming galaxies at z=0, as they have likely very high gas fractions, and star formation time scales comparable to the orbital time.Comment: 20 pages, accepted for publication in ApJ, 2008, 68

Quartic double solids with ordinary singularities

We study the mixed Hodge structure on the third homology group of a threefold which is the double cover of projective three-space ramified over a quartic surface with a double conic. We deal with the Torelli problem for such threefolds.Comment: 14 pages, presented at the Conference Arnol'd 7

A Morphological-type dependence in the mu_0-log(h) plane of Spiral galaxy disks

We present observational evidence for a galaxy `Type' dependence to the location of a spiral galaxy's disk parameters in the mu_0-log(h) (central disk surface-brightness - disk scale-length) plane. With a sample of ~40 Low Surface Brightness galaxies (both bulge- and disk-dominated) and ~80 High Surface Brightness galaxies, the early-type disk galaxies (<=Sc) tend to define a bright envelope in the mu_0-log(h) plane, while the late-type (>=Scd) spiral galaxies have, in general, smaller and fainter disks. Below the defining surface brightness threshold for a Low Surface Brightness galaxy (i.e. more than 1 mag fainter than the 21.65 B-mag arcsec^(-2) Freeman value), the early-type spiral galaxies have scale-lengths greater than 8-9 kpc, while the late-type spiral galaxies have smaller scale-lengths. All galaxies have been modelled with a seeing-convolved Sersic r^(1/n) bulge and exponential disk model. We show that the trend of decreasing bulge shape parameter (n) with increasing Hubble type and decreasing bulge-to-disk luminosity ratio, which has been observed amongst the High Surface Brightness galaxies, extends to the Low Surface Brightness galaxies, revealing a continuous range of structural parameters.Comment: To be published in ApJ. Inc. three two-part figure

Enhancement of the Josephson current by an exchange field in superconductor-ferromagnet structures

We calculate the dc Josephson current for two superconductor-ferromagnet (S/F) bilayers separated by a thin insulating film. It is demonstrated that the critical Josephson current $I_{c}$ in the junction strongly depends on the relative orientation of the effective exchange field $h$ of the bilayers. We found that in the case of an antiparallel orientation, $I_{c}$ increases at low temperatures with increasing $h$ and at zero temperature has a singularity when $h$ equals the superconducting gap $\Delta$. This striking behavior contrasts suppression of the critical current by the magnetic moments aligned in parallel and is an interesting new effect of the interplay between superconductors and ferromagnets.Comment: to be published in PR

A Deep Survey of the Fornax dSph I: Star Formation History

Based on a deep imaging survey, we present the first homogeneous star formation history (SFH) of the Fornax dwarf spheroidal (dSph) galaxy. We have obtained two-filter photometry to a depth of B ~ 23 over the entire surface of Fornax, the brightest dSph associated with the Milky Way, and derived its SFH using a CMD-fitting technique. We show that Fornax has produced the most complex star formation and chemical enrichment histories of all the Milky Way dSphs. This system has supported multiple epochs of star formation. A significant number of stars were formed in the early Universe, however the most dominant population are the intermediate age stars. This includes a strong burst of star formation approximately 3 to 4 Gyr ago. Significant population gradients are also evident. Similar to other dSphs, we have found that recent star formation was concentrated towards the centre of the system. Furthermore, we show that the central region harboured a faster rate of chemical enrichment than the outer parts of Fornax. At the centre, the ancient stars (age > 10 Gyr) display a mean metallicity of [Fe/H] ~ -1.4, with evidence for three peaks in the metallicity distribution. Overall, enrichment in Fornax has been highly efficient: the most recent star formation burst has produced stars with close to solar metallicity. Our results support a scenario in which Fornax experienced an early phase of rapid chemical enrichment producing a wide range of abundances. Star formation gradually decreased until ~4 Gyr ago, when Fornax experienced a sudden burst of strong star formation activity accompanied by substantial chemical enrichment. Weaker star forming events followed, and we have found tentative evidence for stars with ages less than 100 Myr.Comment: 13 pages, 13 figures, accepted for publication in Ap

BactMAP:An R package for integrating, analyzing and visualizing bacterial microscopy data

High-throughput analyses of single-cell microscopy data are a critical tool within the field of bacterial cell biology. Several programs have been developed to specifically segment bacterial cells from phase-contrast images. Together with spot and object detection algorithms, these programs offer powerful approaches to quantify observations from microscopy data, ranging from cell-to-cell genealogy to localization and movement of proteins. Most segmentation programs contain specific post-processing and plotting options, but these options vary between programs and possibilities to optimize or alter the outputs are often limited. Therefore, we developed BactMAP (Bacterial toolbox for Microscopy Analysis &amp; Plotting), a command-line based R package that allows researchers to transform cell segmentation and spot detection data generated by different programs into various plots. Furthermore, BactMAP makes it possible to perform custom analyses and change the layout of the output. Because BactMAP works independently of segmentation and detection programs, inputs from different sources can be compared within the same analysis pipeline. BactMAP complies with standard practice in R which enables the use of advanced statistical analysis tools, and its graphic output is compatible with ggplot2, enabling adjustable plot graphics in every operating system. User feedback will be used to create a fully automated Graphical User Interface version of BactMAP in the future. Using BactMAP, we visualize key cell cycle parameters in Bacillus subtilis and Staphylococcus aureus, and demonstrate that the DNA replication forks in Streptococcus pneumoniae dissociate and associate before splitting of the cell, after the Z-ring is formed at the new quarter positions. BactMAP is available from https://veeninglab.com/bactmap