577 research outputs found
Controlling the morphological and electro-optical properties of polymer dispersed Liquid crystals
Polymer dispersed liquid crystals have shown potential as the basis for a new display screen technology which can be used to produce flexible displays. The work presented here investigates the properties during the synthesis process of PDLCs, which affect the electro-optical properties of the films produced. An investigation is performed through the use of small angle light scattering, and small angle neutron scattering, into the thermodynamic phase properties of the initial liquid crystal monomer solution from which the PDLCs are formed. Through the use of small angle light scattering and ESEM the phase separation mechanism is determined and the morphological properties of films formed under different thermodynamic conditions are investigated and linked to the electro-optical properties of test films. It is shown that due to the rod like nature of liquid crystal molecules that the monomer/liquid crystal phase properties cannot be explained by classic polymer solution thermodynamics. Phase separation is shown to occur by one of two mechanisms depending upon whether or not a crosslinking monomer species is present, either a gel phase separation mechanism or a viscoelastic mechanism. Depending upon the initial thermodynamic properties of the system the formation of a polymer network can either act to promote (resulting in a large droplet morphology size) or suppress (resulting in small impure liquid crystal droplets) phase separation. This in turn is linked to the effectiveness of films formed under given conditions as potential display devices. With thanks to EPSRC and Sony MSL, Stuttgart for fundin
Collective Properties of Excitons in Presence of a Two-Dimensional Electron Gas
We have studied the collective properties of two-dimensional (2D) excitons
immersed within a quantum well which contains 2D excitons and a two-dimensional
electron gas (2DEG). We have also analyzed the excitations for a system of 2D
dipole excitons with spatially separated electrons and holes in a pair of
quantum wells (CQWs) when one of the wells contains a 2DEG. Calculations of the
superfluid density and the Kosterlitz-Thouless (K-T) phase transition
temperature for the 2DEG-exciton system in a quantum well have shown that the
K-T transition temperature increases with increasing exciton density and that
it might be possible to have fast long range transport of excitons. The
superfluid density and the K-T transition temperature for dipole excitons in
CQWs in the presence of a 2DEG in one of the wells increases with increasing
inter-well separation.Comment: 10 pages, 1 figure. accepted by Solid State Communication
A Robust Classification of Galaxy Spectra: Dealing with Noisy and Incomplete Data
Over the next few years new spectroscopic surveys (from the optical surveys
of the Sloan Digital Sky Survey and the 2 degree Field survey through to
space-based ultraviolet satellites such as GALEX) will provide the opportunity
and challenge of understanding how galaxies of different spectral type evolve
with redshift. Techniques have been developed to classify galaxies based on
their continuum and line spectra. Some of the most promising of these have used
the Karhunen and Loeve transform (or Principal Component Analysis) to separate
galaxies into distinct classes. Their limitation has been that they assume that
the spectral coverage and quality of the spectra are constant for all galaxies
within a given sample. In this paper we develop a general formalism that
accounts for the missing data within the observed spectra (such as the removal
of sky lines or the effect of sampling different intrinsic rest wavelength
ranges due to the redshift of a galaxy). We demonstrate that by correcting for
these gaps we can recover an almost redshift independent classification scheme.
From this classification we can derive an optimal interpolation that
reconstructs the underlying galaxy spectral energy distributions in the regions
of missing data. This provides a simple and effective mechanism for building
galaxy spectral energy distributions directly from data that may be noisy,
incomplete or drawn from a number of different sources.Comment: 20 pages, 8 figures. Accepted for publication in A
Magneto-infrared modes in InAs-AlSb-GaSb coupled quantum wells
We have studied a series of InAs/GaSb coupled quantum wells using
magneto-infrared spectroscopy for high magnetic fields up to 33T within
temperatures ranging from 4K to 45K in both Faraday and tilted field
geometries. This type of coupled quantum wells consists of an electron layer in
the InAs quantum well and a hole layer in the GaSb quantum well, forming the
so-called two dimensional electron-hole bilayer system. Unlike the samples
studied in the past, the hybridization of the electron and hole subbands in our
samples is largely reduced by having narrower wells and an AlSb barrier layer
interposed between the InAs and the GaSb quantum wells, rendering them weakly
hybridized. Previous studies have revealed multiple absorption modes near the
electron cyclotron resonance of the InAs layer in moderately and strongly
hybridized samples, while only a single absorption mode was observed in the
weakly hybridized samples. We have observed a pair of absorption modes
occurring only at magnetic fields higher than 14T, which exhibited several
interesting phenomena. Among which we found two unique types of behavior that
distinguishes this work from the ones reported in the literature. This pair of
modes is very robust against rising thermal excitations and increasing magnetic
fields alligned parallel to the heterostructures. While the previous results
were aptly explained by the antilevel crossing gap due to the hybridization of
the electron and hole wavefunctions, i.e. conduction-valence Landau level
mixing, the unique features reported in this paper cannot be explained within
the same concept. The unusual properties found in this study and their
connection to the known models for InAs/GaSb heterostructures will be
disccused; in addition, several alternative ideas will be proposed in this
paper and it appears that a spontaneous phase separation can account for most
of the observed features
A composite HII region luminosity function in H alpha of unprecedented statistical weight
Context. Statistical properties of HII region populations in disk galaxies
yield important clues to the physics of massive star formation.
Aims. We present a set of HII region catalogues and luminosity functions for
a sample of 56 spiral galaxies in order to derive the most general form of
their luminosity function.
Methods. HII region luminosity functions are derived for individual galaxies
which, after photometric calibration, are summed to form a total luminosity
function comprising 17,797 HII regions from 53 galaxies.
Results. The total luminosity function, above its lower limit of
completeness, is clearly best fitted by a double power law with a significantly
steeper slope for the high luminosity portion of the function. This change of
slope has been reported in the literature for individual galaxies, and occurs
at a luminosity of log L = 38.6\pm0.1 (L in erg/s) which has been termed the
Stromgren luminosity. A steep fall off in the luminosity function above log L =
40 is also noted, and is related to an upper limit to the luminosities of
underlying massive stellar clusters. Detailed data are presented for the
individual sample galaxies.
Conclusions. The luminosity functions of HII regions in spiral galaxies show
a two slope power law behaviour, with a significantly steeper slope for the
high luminosity branch. This can be modelled by assuming that the high
luminosity regions are density bounded, though the scenario is complicated by
the inhomogeneity of the ionized interstellar medium. The break, irrespective
of its origin, is of potential use as a distance indicator for disc galaxies.Comment: Accepted for publication as a Letter in Astronomy & Astrophysics.
Latex with postscript figures. Online-only tables and figures are included in
this preprint. The HII region catalogues for 56 galaxies will be published
electronically on the CDS but are available also on request from the author
Identifying Ultra-Cool Dwarfs at Low Galactic Latitudes: A Southern Candidate Catalogue
We present an Ultra-Cool Dwarf (UCD) catalogue compiled from low southern
Galactic latitudes and mid-plane, from a cross-correlation of the 2MASS and
SuperCOSMOS surveys. The catalogue contains 246 members identified from 5042
sq. deg. within 220 deg. <= l <= 360 deg. and 0 deg. < l <= 30 deg., for |b| <=
15 deg. Sixteen candidates are spectroscopically confirmed in the near-IR as
UCDs with spectral types from M7.5V to L9. Our catalogue selection method is
presented enabling UCDs from ~M8V to the L-T transition to be selected down to
a 2MASS limiting magnitude of Ks ~= 14.5 mag. This method does not require
candidates to have optical detections for catalogue inclusion. An optimal set
of optical/near-IR and reduced proper-motion selection criteria have been
defined that includes: an Rf and Ivn photometric surface gravity test, a dual
Rf-band variability check, and an additional photometric classification scheme
to selectively limit contaminants. We identify four candidates as possible
companions to nearby Hipparcos stars -- observations are needed to identify
these as potential benchmark UCD companions. We also identify twelve UCDs
within a possible distance 20 pc, three are previously unknown of which two are
estimated within 10 pc, complimenting the nearby volume-limited census of UCDs.
An analysis of the catalogue spatial completeness provides estimates for
distance completeness over three UCD MJ ranges, while Monte-Carlo simulations
provide an estimate of catalogue areal completeness at the 75 per cent level.
We estimate a UCD space density of Rho (total) = (6.41+-3.01)x10^3/pc^3 over
the range of 10.5 <= MJ ~< 14.9, similar to values measured at higher Galactic
latitudes (|b| ~> 10 deg.) in the field population and obtained from more
robust spectroscopically confirmed UCD samples.Comment: MNRAS accepted April 2012. Contains 30 figures and 11 tables. Tables
2 and 6 to be published in full and on-line only. The on-line tables can also
be obtained by contacting the author
The sub-stellar birth rate from UKIDSS
We present a new sample of mid-L to mid-T dwarfs with effective temperatures of 11001700 K selected from the UKIDSS Large Area Survey (LAS) and confirmed with infrared spectra from X-shooter/Very Large Telescope. This effective temperature range is especially sensitive to the formation history of Galactic brown dwarfs and allows us to constrain the form of the sub-stellar birth rate, with sensitivity to differentiate between a flat (stellar like) birth rate and an exponentially declining form. We present the discovery of 63 new L and T dwarfs from the UKIDSS LAS DR7, including the identification of 12 likely unresolved binaries, which form the first complete sub-set from our programme, covering 495 square degrees of sky, complete to J = 18.1. We compare our results for this sub-sample with simulations of differing birth rates for objects of masses 0.10-0.03 M-circle dot and ages 1-10 Gyr. We find that the more extreme birth rates (e. g. a halo type form) can likely be excluded as the true form of the birth rate. In addition, we find that although there is substantial scatter we find a preference for a mass function, with a power-law index a in the range -1 <alpha <0 that is consistent (within the errors) with the studies of late T dwarfs.Peer reviewe
Effects of acute versus chronic hypoxia on DNA damage responses and genomic instability.
Questions exist concerning the effects of acute versus chronic hypoxic conditions on DNA replication and genomic stability that may influence tumorigenesis. Severe hypoxia causes replication arrest independent of S-phase checkpoint, DNA damage response, or transformation status. Arrests occur during both the initiation and elongation phases of DNA replication, correlated with a rapid decrease in available deoxynucleotide triphosphates. With fluctuating oxygen tensions in tumors, arrested hypoxic cells may undergo rapid reperfusion and reoxygenation that leads to reoxygenation-induced DNA damage. In cells subjected to chronic hypoxia, we found that replicative restart was inhibited along with numerous replication factors, including MCM6 and RPA, the latter of which limits the hypoxia-induced DNA damage response. In contrast, in cells where replicative restart occurred, it was accompanied by extensive reoxygenation-induced DNA damage and compromised DNA repair. We found that cells reoxygenated after acute hypoxia underwent rapid p53-dependent apoptosis. Our findings suggest that cells lacking functional p53 are more susceptible to genomic instability and potentially tumorigenesis if they experience reoxygenation after acute exposure to hypoxia
A composite H region luminosity function in Hα of unprecedented statistical weight
ABSTRACT Context. Statistical properties of H region populations in disk galaxies yield important clues to the physics of massive star formation. Aims. We present a set of H region catalogues and luminosity functions for a sample of 56 spiral galaxies in order to derive the most general form of their luminosity function. Methods. H region luminosity functions are derived for individual galaxies which, after photometric calibration, are summed to form a total luminosity function comprising 17,797 H regions from 53 galaxies. Results. The total luminosity function, above its lower limit of completeness, is clearly best fitted by a double power law with a significantly steeper slope for the high luminosity portion of the function. This change of slope has been reported in the literature for individual galaxies, and occurs at a luminosity of log L = 38.6 ± 0.1 (L in erg s −1 ) which has been termed the Strömgren luminosity. A steep fall off in the luminosity function above log L = 40 is also noted, and is related to an upper limit to the luminosities of underlying massive stellar clusters. Detailed data are presented for the individual sample galaxies. Conclusions. The luminosity functions of H regions in spiral galaxies show a two slope power law behaviour, with a significantly steeper slope for the high luminosity branch. This can be modelled by assuming that the high luminosity regions are density bounded, though the scenario is complicated by the inhomogeneity of the ionized interstellar medium. The break, irrespective of its origin, is of potential use as a distance indicator for disc galaxies
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