3,270 research outputs found
Self-consistent and environment-dependent Hamiltonian for quantum-mechanics materials simulations.
I will report the development of a semi-empirical self-consistent and environment-dependent model Hamiltonian, which is intended to treat large systems in the order of 10000 atoms. This covers a range of important physical phenomena that are too large to be treated with first-principles calculations. Our model features an aggressive treatment of environment-dependent effects, which are known to limit the accuracy of two-center models which do not include them. Specifically, we account for multi-center integrals, and we use a full iterative treatment of the self-consistency problem, which addressed the important role of charge redistribution. Our results indicate that our treatment is superior to other semi-empirical models that treat environment-dependency in a more phenomenological manner, and either ignore the charge redistribution, or treat it not at equal footing as the environment-dependency. The feasibility of this methodology has been tested for silicon
An Analysis of the Broadband (22-3900 MHz) Radio Spectrum of HB3 (G132.7+1.3): The Detection of Thermal Radio Emission from an Evolved Supernova Remnant?
We present an analysis of the broadband radio spectrum (from 22 to 3900 MHz)
of the Galactic supernova remnant (SNR) HB3 (G132.7+1.3). Published
observations have revealed that a curvature is present in the radio spectrum of
this SNR, indicating that a single synchrotron component appears is
insufficient to adequately fit the spectrum. We present here a fit to this
spectrum using a combination of a synchrotron component and a thermal
bremsstrahlung component. We discuss properties of this latter component and
estimate the ambient density implied by the presence of this component to be n
\~ 10 cm^-3. We have also analyzed extracted X-ray spectra from archived {\it
ASCA} GIS observations of different regions of HB3 to obtain independent
estimates of the density of the surrounding interstellar medium (ISM). From
this analysis, we have derived electron densities of 0.1-0.4 f^-1/2 cm^-3 for
the ISM for the three different regions of the SNR, where f is the volume
filling factor. By comparing these density estimates with the estimate derived
from the thermal bremsstrahlung component, we argue that the radio thermal
bremsstrahlung emission is emitted from a thin shell enclosing HB3. The
presence of this thermal bremsstrahlung component in the radio spectrum of HB3
suggests that this SNR is in fact interacting with an adjacent molecular cloud
associated with the HII region W3. By extension, we argue that the presence of
thermal emission at radio wavelengths may be a useful tool for identifying
interactions between SNRs and molecular clouds, and for estimating the ambient
density near SNRs using radio continuum data.Comment: 5 pages, 2 figures, accepted for ApJ
EMBEDDING CULTURE IN RELATION TO THE SHOTGUN AT THE UNITED STATES NAVAL ACADEMY
The objective of this study was to determine if the randomization of members in an organization can result in a new organizational culture. This study was conducted following the 2021 shotgun of the Brigade of Midshipmen at the United States Naval Academy, where 75 percent of Midshipmen were redistributed into new companies. Different aspects of organizational culture were researched, including the way cultures form, transformational leadership, different configuration models, and Schein’s embedding mechanisms. Focus groups were held for the Midshipmen and their leadership to discuss how the shotgun impacted their company’s culture, if at all. Despite the wide variety of experiences from the Midshipmen, three common themes emerged. Embedding a new culture requires a cohesive environment to create a change, a promulgation of a clear set of goals is needed to align the personnel with the organization, and that establishing a sense of accountability is critical. It was also discovered that a culture would establish itself regardless of the intentionality presented by its members. Limitations of this study included no pre-shotgun data, the voluntary nature of the focus groups, and the sheer amount of data synthesizing required. Recommendations for further attempts of this shotgun should include more oversight to ensure current issues are corrected beforehand, and that new cultures align with the larger organization.Lieutenant, United States NavyLieutenant, United States NavyApproved for public release. Distribution is unlimited
Global Cosmological Parameters Determined Using Classical Double Radio Galaxies
A sample of 20 powerful extended radio galaxies with redshifts between zero
and two were used to determine constraints on global cosmological parameters.
Data for six radio sources were obtained from the VLA archive, analyzed, and
combined with the sample of 14 radio galaxies used previously by Guerra & Daly
to determine cosmological parameters. The results are consistent with our
previous results, and indicate that the current value of the mean mass density
of the universe is significantly less than the critical value. A universe with
of unity is ruled out at 99.0% confidence, and the best fitting
values of in matter are and
assuming zero space curvature and zero cosmological
constant, respectively. Note that identical results obtain when the low
redshift bin, which includes Cygnus A, is excluded; these results are
independent of whether the radio source Cygnus A is included. The method does
not rely on a zero-redshift normalization.
The radio properties of each source are also used to determine the density of
the gas in the vicinity of the source, and the beam power of the source. The
six new radio sources have physical characteristics similar to those found for
the original 14 sources. The density of the gas around these radio sources is
typical of gas in present day clusters of galaxies. The beam powers are
typically about .Comment: 39 pages includes 21 figures, accepted to Ap
Modelling the Galactic Magnetic Field on the Plane in 2D
We present a method for parametric modelling of the physical components of
the Galaxy's magnetised interstellar medium, simulating the observables, and
mapping out the likelihood space using a Markov Chain Monte-Carlo analysis. We
then demonstrate it using total and polarised synchrotron emission data as well
as rotation measures of extragalactic sources. With these three datasets, we
define and study three components of the magnetic field: the large-scale
coherent field, the small-scale isotropic random field, and the ordered field.
In this first paper, we use only data along the Galactic plane and test a
simple 2D logarithmic spiral model for the magnetic field that includes a
compression and a shearing of the random component giving rise to an ordered
component. We demonstrate with simulations that the method can indeed constrain
multiple parameters yielding measures of, for example, the ratios of the
magnetic field components. Though subject to uncertainties in thermal and
cosmic ray electron densities and depending on our particular model
parametrisation, our preliminary analysis shows that the coherent component is
a small fraction of the total magnetic field and that an ordered component
comparable in strength to the isotropic random component is required to explain
the polarisation fraction of synchrotron emission. We outline further work to
extend this type of analysis to study the magnetic spiral arm structure, the
details of the turbulence as well as the 3D structure of the magnetic field.Comment: 18 pages, 11 figures, updated to published MNRAS versio
Reliability Equivalence in Public Transport Contexts
This work provides initial investigation into whether equivalence between the mean-variance and scheduling approaches to transport reliability can be applied in the context high frequency public transport services. Each of these approaches is briefly outlined and the current work is framed by previous research attempting to demonstrate equivalence: both theoretically and empirically. The basic assumptions underpinning the theoretical approach to equivalence are explored and then re-formulated based upon which variables are likely to be known. The concept of headway is introduced to the theoretical approach using notation from previous research in order to represent public transport services. An empirical illustration of the method is undertaken using smart card data obtained from the London Underground metro system. The data are combined with timetable data and a previously developed method for estimating passenger preferred arrival times, which in turn allows the theoretical equivalence between mean-variance and scheduling approaches to be tested empirically. This is initially performed for a single origin-destination (OD) pair and then for 23 other ODs of varying headways. The example using a single OD demonstrates that even for a high frequency metro service, application of the theoretical equivalence is problematic, with variable parameters substantially affected. In the case of many ODs, a linear relationship is observed between the ratio of public transport to standard scheduling parameters and headway, suggesting the theoretical equivalence becomes less viable as headway increases. At the lowest values of headway, it is concluded that the equivalence remains problematic and further work is required before equivalence between the mean variance and scheduling approaches can be implemented in the public transport context
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