258 research outputs found
Topological Defects from First Order Gauge Theory Phase Transitions
We investigate the mechanism by which topological defects form in first order
phase transitions with a charged order parameter. We show how thick
superconductor vortices and heavy cosmic strings form by trapping of magnetic
flux. In an external magnetic field, intermediate objects such as strips and
membranes of magnetic flux and chains of single winding defects are produced.
At non-zero temperature, a variety of spontaneous defects of different winding
numbers arise. In cosmology, our results mean that the magnetic flux thermal
fluctuations get trapped in a primordial multi-tension string network. The
mechanism may also apply to the production of cosmic-like strings in brane
collisions. In a thin type-I superconductor film, flux strips are found to be
meta-stable while thick vortices are stable up to some critical value of the
winding number which increases with the thickness of the film. In addition, a
non-dissipative Josephson-like current is obtained across the strips of
quantized magnetic flux.Comment: Corrections made on sections 4,5. Higher quality figures in published
versio
Existence of principal values of some singular integrals on Cantor sets, and Hausdorff dimension
Consider a standard Cantor set in the plane of Hausdorff dimension 1. If the
linear density of the associated measure vanishes, then the set of points
where the principal value of the Cauchy singular integral of exists has
Hausdorff dimension 1. The result is extended to Cantor sets in
of Hausdorff dimension and Riesz singular integrals of homogeneity
, 0 < < d : the set of points where the principal value of
the Riesz singular integral of exists has Hausdorff dimension . A
martingale associated with the singular integral is introduced to support the
proof.Comment: 14 pages, minor revision after the referee's report, to appear in
Pacific J. of Mat
Electromagnetic vacuum of complex media II: the Lamb shift and the total vacuum energy
We study the physical content of the electromagnetic vacuum energy of a
random medium made of atomic electric dipoles. First, we evaluate the
contribution of statistical fluctuations to the average total vacuum energy,
which is made out of the integration of the variations of the Lamb shift with
respect to the coupling constant. While the Lamb shift is a function of the
electrical susceptibility only, the vacuum energy is generally not. Second, we
make clear why the effective medium bulk energy does not account for the total
vacuum energy of a molecular dielectric. Consequently, the Lamb shift does not
derive from the effective medium bulk energy except at leading order in the
molecular density. The local field factors provide natural cutoffs for the
spectrum of the total vacuum energy at a wavelength of the order of the
correlation length. Third, we investigate to what extent shifts in the spectrum
of the dielectric constant may be attributed to the binding energy of a
dielectric. In particular, in the continuum approximation we have found a
relation between the electrostatic binding energy and the Lorentz-Lorenz shift.
Nonetheless, we conclude that the knowledge of the spectrum of the refractive
index is insufficient either to quantify the energy of radiative modes or to
estimate the electrostatic binding energy of molecular clusters.Comment: Comments added, some sections rewritte
Multiwavelength characterisation of an ACT-selected, lensed dusty star-forming galaxy at z=2.64
We present \ci\,(2--1) and multi-transition CO observations of a dusty
star-forming galaxy, ACT\,J2029+0120, which we spectroscopically confirm to lie
at \,=\,2.64. We detect CO(3--2), CO(5--4), CO(7--6), CO(8--7), and
\ci\,(2--1) at high significance, tentatively detect HCO(4--3), and place
strong upper limits on the integrated strength of dense gas tracers (HCN(4--3)
and CS(7--6)). Multi-transition CO observations and dense gas tracers can
provide valuable constraints on the molecular gas content and excitation
conditions in high-redshift galaxies. We therefore use this unique data set to
construct a CO spectral line energy distribution (SLED) of the source, which is
most consistent with that of a ULIRG/Seyfert or QSO host object in the taxonomy
of the \textit{Herschel} Comprehensive ULIRG Emission Survey. We employ RADEX
models to fit the peak of the CO SLED, inferring a temperature of T117 K
and cm, most consistent with a ULIRG/QSO object
and the presence of high density tracers. We also find that the velocity width
of the \ci\ line is potentially larger than seen in all CO transitions for this
object, and that the ratio is also larger
than seen in other lensed and unlensed submillimeter galaxies and QSO hosts; if
confirmed, this anomaly could be an effect of differential lensing of a shocked
molecular outflow.Comment: Accepted for publication in Ap
Spectroscopic and Mechanistic Studies of Heterodimetallic Forms of Metallo-β-lactamase NDM-1
In an effort to characterize the roles of each metal ion in metallo-β-lactamase NDM-1, heterodimetallic analogues (CoCo-, ZnCo-, and CoCd-) of the enzyme were generated and characterized. UV–vis, 1H NMR, EPR, and EXAFS spectroscopies were used to confirm the fidelity of the metal substitutions, including the presence of a homogeneous, heterodimetallic cluster, with a single-atom bridge. This marks the first preparation of a metallo-β-lactamase selectively substituted with a paramagnetic metal ion, Co(II), either in the Zn1 (CoCd-NDM-1) or in the Zn2 site (ZnCo-NDM-1), as well as both (CoCo-NDM-1). We then used these metal-substituted forms of the enzyme to probe the reaction mechanism, using steady-state and stopped-flow kinetics, stopped-flow fluorescence, and rapid-freeze-quench EPR. Both metal sites show significant effects on the kinetic constants, and both paramagnetic variants (CoCd- and ZnCo-NDM-1) showed significant structural changes on reaction with substrate. These changes are discussed in terms of a minimal kinetic mechanism that incorporates all of the data
Degradation versus self-assembly of block copolymer micelles
The stability of micelles self-assembled from block copolymers can be altered
by the degradation of the blocks. Slow degradation shifts the equilibrium size
distribution of block copolymer micelles and change their properties.
Quasi-equilibrium scaling theory shows that the degradation of hydrophobic
blocks in the core of micelles destabilize the micelles reducing their size,
while the degradation of hydrophilic blocks forming coronas of micelles favors
larger micelles and may, at certain conditions, induce the formation of
micelles from individual chains.Comment: Published in Langmuir http://pubs.acs.org/doi/pdf/10.1021/la204625
La estructura sísmica de la corteza de la Zona de Ossa Morena y su interpretación geológica
El experimento de sísmica de reflexión profunda IBERSEIS ha proporcionado una imagen de la corteza del Orógeno Varisco en el sudoeste de Iberia. Este artículo se centra en la descripción de la corteza de la Zona de Ossa Morena (OMZ), que está claramente dividida en una corteza superior, con reflectividad de buzamiento al NE, y una corteza inferior de pobre reflectividad. Las estructuras geológicas cartografiadas en superficie se correlacionan bien con la reflectividad de la corteza superior, y en la imagen sísmica se ven enraizar en la corteza media. Ésta está constituida por un cuerpo muy reflectivo, interpretado como una gran intrusión de rocas básicas. La imagen de las suturas que limitan la OMZ muestra el carácter fuertemente transpresivo de la colisión orogénica varisca registrada en el sudoeste de Iberia. La Moho actual es plana y, en consecuencia, no se observa la raíz del orógeno
VERTICO VII: Environmental quenching caused by suppression of molecular gas content and star formation efficiency in Virgo Cluster galaxies
We study how environment regulates the star formation cycle of 33 Virgo
Cluster satellite galaxies on 720 parsec scales. We present the first resolved
star-forming main sequence for cluster galaxies, dividing the sample based on
their global HI properties and comparing to a control sample of field galaxies.
HI-poor cluster galaxies have reduced star formation rate (SFR) surface
densities with respect to both HI-normal cluster and field galaxies (0.5 dex),
suggesting that mechanisms regulating the global HI content are responsible for
quenching local star formation. We demonstrate that the observed quenching in
HI-poor galaxies is caused by environmental processes such as ram pressure
stripping (RPS) simultaneously reducing molecular gas surface density and star
formation efficiency (SFE), compared to regions in HI-normal systems (by 0.38
and 0.22 dex, respectively). We observe systematically elevated SFRs that are
driven by increased molecular gas surface densities at fixed stellar mass
surface density in the outskirts of early-stage RPS galaxies, while SFE remains
unchanged with respect to the field sample. We quantify how RPS and starvation
affect the star formation cycle of inner and outer galaxy discs as they are
processed by the cluster. We show both are effective quenching mechanisms with
the key difference being that RPS acts upon the galaxy outskirts while
starvation regulates the star formation cycle throughout disc, including within
the truncation radius. For both processes, the quenching is caused by a
simultaneous reduction in molecular gas surface densities and SFE at fixed
stellar mass surface density.Comment: 17 pages, 1 table, 5 figures, accepted for publication in Ap
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