102 research outputs found
VLBI Imaging of ICRF Sources in the Southern Hemisphere using Geodetic and Astrometric Observations
The present International Celestial Reference Frame (ICRF), the ICRF-3 is based
on a catalogue of 4536 quasar positions obtained from Very Long Baseline Interferometry
(VLBI) radio measurements. This radio frame is crucial for many applications, from
measurements of Earth’s orientation in space to spacecraft navigation and measurements
of sea-level rise. However, the deficit in ICRF source density in the South and lack of
dedicated imaging campaigns in the South, to monitor structural changes, remain a big
concern. These ICRF sources can exhibit spatially extended emission structures that can
have a significant effect on astrometric VLBI measurements. The Celestial Reference
Frame Deep South (CRDS) is a dedicated astrometric VLBI programme to observe
Southern ICRF sources on a regular basis. In an effort to improve the situation in the
South, these CRDS sessions have recently been optimized for VLBI imaging. In this
thesis, I present VLBI images and source structure analysis results for southern ICRF
sources observed in four of these CRDS sessions. For some of these sources, I present
the very first high-resolution radio images. I also present results from source structure
analysis and a corresponding assessment of astrometric quality, and I also present results
from efforts to increase the ICRF source density in the South.Mathematical SciencesPh. D. (Astronomy
Band engineered bilayer Haldane model: Evidence of multiple topological phase transitions
We have studied the evolution of the topological properties of a
band-engineered AB-stacked bilayer honeycomb structure in the presence of a
Haldane flux. Without a Haldane flux, band engineering makes the band touching
points (the so-called Dirac points) move towards each other and eventually
merge into one at an intermediate point in the Brillouin zone.
Here the dispersion is linear along one direction and quadratic along the
other. In the presence of a Haldane flux, the system acquires topological
properties, and finite Chern numbers can be associated with the pairs of the
conduction and the valence bands. The valence band closer to the Fermi level
() possesses Chern numbers equal to and , while the one
further away from corresponds to Chern numbers . The conduction
bands are associated with similar properties, except their signs are reversed.
The Chern lobes shrink in the band-engineered model, and we find evidence of
multiple topological phase transitions, where the Chern numbers discontinuously
jump from to , to , to to and
to . These transitions are supported by the presence or absence of
the chiral edge modes in a nanoribbon bilayer geometry and the vanishing of the
plateau in the anomalous Hall conductivity. Different phases are further
computed for different hopping amplitudes across the layers, which shows the
shrinking of the Chern lobes for large interlayer tunneling
Topological features of Haldane model on a dice lattice: role of flat band on transport properties
We study the topological properties of a Haldane model on a band deformed
dice lattice, which has three atoms per unit cell (call them as A, B and C) and
the spectrum comprises of three bands, including a flat band. The bands are
systematically deformed with an aim to study the evolution of topology and the
transport properties. The deformations are induced through hopping anisotropies
and are achieved in two distinct ways. In one of them, the hopping amplitudes
between the sites of B and C sublattices and those between A and B sublattices
are varied along a particular direction, and in the other, the hopping between
the sites of A and B sublattices are varied (keeping B-C hopping unaltered)
along the same direction. The first case retains some of the spectral features
of the familiar dice lattice and yields Chern insulating lobes in the phase
diagram with till a certain critical deformation. The topological
features are supported by the presence of a pair of chiral edge modes at each
edge of a ribbon and the plateaus observed in the anomalous Hall conductivity
support the above scenario. Whereas, a selective tuning of only the A-B hopping
amplitudes distorts the flat band and has important ramifications on the
topological properties of the system. The insulating lobes in the phase diagram
have distinct features compared to the case above, and there are dips observed
in the Hall conductivity near the zero bias. The dip widens as the hopping
anisotropy is made larger, and thus the scenario registers significant
deviation from the familiar plateau structure observed in the anomalous Hall
conductivity. However, a phase transition from a topological to a trivial
insulating region demonstrated by the Chern number changing discontinuously
from to zero beyond a certain critical hopping anisotropy remains a
common feature in the two cases
Understanding the State of LGBTQIA+ Healthcare and Support in Camden County
4.5% of American adults identify as lesbian, gay, or bisexual and about 1.4 million adults identify as transgender.
This demographic is impacted by many social determinants of health and health disparities, particularly for transgender patients.
33% of LGBTQIA+ patients ranging from a pool of 28,000 surveyed patients have had a negative experience with their health providers, and 8% of them had to educate their physicians about their needs due to physicians’ lack of knowledge about this demographic.
Research suggests that there is a reluctance to access mental health services in the LGBTQIA+ community due to homophobia,, difficulties disclosing sexual and gender identity, and fears of being misunderstood
Observationally inferred dark matter phase-space distribution and direct detection experiments
We present a detailed analysis of the effect of an observationally determined
dark matter (DM) velocity distribution function (VDF) of the Milky Way (MW) on
DM direct detection rates. We go beyond local kinematic tracers and use
rotation curve data up to 200 kpc to construct a MW mass model and
self-consistently determine the local phase-space distribution of DM. This
approach mitigates any incomplete understanding of local dark matter-visible
matter degeneracies that can affect the determination of the VDF. Comparing
with the oft used Standard Halo Model (SHM), which assumes an isothermal VDF,
we look at how the tail of the empirically determined VDF alters our
interpretation of the present direct detection WIMP DM cross section exclusion
limits. While previous studies have suggested a very large difference (of more
than an order of magnitude) in the bounds at low DM masses, we show that
accounting for the detector response at low threshold energies, the difference
is still significant although less extreme. The change in the number of signal
events, when using the empirically determined DM VDF in contrast to the SHM
VDF, is most prominent for low DM masses for which the shape of the recoil
energy spectrum depends sensitively on the detector threshold energy as well as
detector response near the threshold. We demonstrate that these trends carry
over to the respective DM exclusion limits, modulo detailed understanding of
the experimental backgrounds. With the unprecedented precision of astrometric
data in the GAIA era, use of observationally determined DM phase-space will
become a critical and necessary ingredient for DM searches. We provide an
accurate fit to the current best observationally determined DM VDF (and
self-consistent local DM density) for use in analyzing current DM direct
detection data by the experimental community.Comment: 13 pages, 10 figures, 4 tables, version accepted by PR
Successful use of conjunctival flaps to prolong survival of type I keratoprosthesis in severe bilateral chemical burns: two case reports [version 1; peer review: 2 approved]
Introduction: This report describes the use of conjunctival flaps to enable the survival of type I keratoprosthesis (KPro) in two cases of bilateral severe total limbal stem cell deficiency (LSCD) following chemical burns. Presentation of case: Two patients had a history of bilateral chemical injury with lime. On examination, the presenting vision was light perception to hand motions and both cases had conjunctivalized ocular surfaces with symblepharon. A modified technique of type I keratoprosthesis was used, where the conjunctivalized corneal pannus was dissected and lifted off as an inferior fornix-based conjunctival flap. This was followed by a standard surgical technique of type I KPro. The flap was then secured over the device and optical opening was made two weeks later. Both the patients had stable ocular surfaces with visual acuity of 20/20 at 2–7 years of follow-up. Discussion: In patients with total LSCD with adnexal involvement, type I KPro has unsatisfactory long-term survival because of the risk of repeated epithelial breakdowns and stromal ulceration. With the innovative approach described in this report, type I KPro can be successfully used for sustainable visual improvement in the presence of severe ocular surface disease and symblepharon. Conclusion: Conjunctival flaps can be used along with type I KPros to improve long-term survival and give sustainable visual outcomes in cases of bilateral corneal blindness due to advanced ocular surface damage
- …