1,274 research outputs found
Two-dimensional Nanolithography Using Atom Interferometry
We propose a novel scheme for the lithography of arbitrary, two-dimensional
nanostructures via matter-wave interference. The required quantum control is
provided by a pi/2-pi-pi/2 atom interferometer with an integrated atom lens
system. The lens system is developed such that it allows simultaneous control
over atomic wave-packet spatial extent, trajectory, and phase signature. We
demonstrate arbitrary pattern formations with two-dimensional 87Rb wavepackets
through numerical simulations of the scheme in a practical parameter space.
Prospects for experimental realizations of the lithography scheme are also
discussed.Comment: 36 pages, 4 figure
A Home of One's Own: Aging and Homeownership in the United States in the late Nineteenth and Early Twentieth Centuries
One of the principal types of wealth accumulation in the United States has been real property, especially in the form of homes as the society became more urban and less agricultural. At present, almost two-thirds of all American households reside in owner-occupied structures. The present paper explores this phenomenon for the late nineteenth and early twentieth centuries from the standpoint of property accumulation over the life course. Age patterns of homeownership for urban and rural non-farm households are the central concern. Drawing on micro samples of the 1865 New York State census and the 1900 united States census, micro data on the 6,809 worker families residing in the united States in the 1889/90 U.S. Commissioner of Labor Survey, and published data from the 1890 and 1930 united States censuses, the incidence of homeownership by age of household head is described. The level of the ownership curve (by age) has risen over time, and its shape has changed. Differences by region and rural-urban residence are shown to have existed. Differentials between native and foreign-born whites narrowed from the late nineteenth century to circa 1930, but those by race (black versus white) persisted.
Buying the American Dream: Housing Demand in the United States in the Late Nineteenth Century
This paper examines homeownership and housing demand for a sample of approximately 6,800 urban, industrial workers in the United States for the period 1889/90. Using data from the Sixth and Seventh Annual Reports of the U.S. Commissioner of Labor, housing demand is viewed as a two part process: first, the "tenure choice" decision whether to own or rent; and, second, how much of either type of housing to purchase. Tenure choice and renter demand equations are estimated, using the concept of expected, rather than current income. Data limitations did not permit estimation of owner demand. The results indicate lower homeownership rates among American workers circa 1890 than later and significant effects on ownership of income, age of household head, region, industry, occupation, ethnicity, and family size and? composition. Rental prices and value/rent ratios had effects in the expected directions. Partial and full elasticities calculated for renter demand reveal downward biases if only current income is used to estimate housing demand. The results indicate that modern housing demand theory performs well with historical data.
Standardisation of prostate multiparametric MRI across a hospital network: a London experience.
OBJECTIVES: National guidelines recommend prostate multiparametric (mp) MRI in men with suspected prostate cancer before biopsy. In this study, we explore prostate mpMRI protocols across 14 London hospitals and determine whether standardisation improves diagnostic quality. METHODS: An MRI physicist facilitated mpMRI set-up across several regional hospitals, working together with experienced uroradiologists who judged diagnostic quality. Radiologists from the 14 hospitals participated in the assessment and optimisation of prostate mpMRI image quality, assessed according to both PiRADSv2 recommendations and on the ability to "rule in" and/or "rule out" prostate cancer. Image quality and sequence parameters of representative mpMRI scans were evaluated across 23 MR scanners. Optimisation visits were performed to improve image quality, and 2 radiologists scored the image quality pre- and post-optimisation. RESULTS: 20/23 mpMRI protocols, consisting of 111 sequences, were optimised by modifying their sequence parameters. Pre-optimisation, only 15% of T2W images were non-diagnostic, whereas 40% of ADC maps, 50% of high b-value DWI and 41% of DCE-MRI were considered non-diagnostic. Post-optimisation, the scores were increased with 80% of ADC maps, 74% of high b-value DWI and 88% of DCE-MRI to be partially or fully diagnostic. T2W sequences were not optimised, due to their higher baseline quality scores. CONCLUSIONS: Targeted intervention at a regional level can improve the diagnostic quality of prostate mpMRI protocols, with implications for improving prostate cancer detection rates and targeted biopsies
The Hall effect in star formation
Magnetic fields play an important role in star formation by regulating the
removal of angular momentum from collapsing molecular cloud cores. Hall
diffusion is known to be important to the magnetic field behaviour at many of
the intermediate densities and field strengths encountered during the
gravitational collapse of molecular cloud cores into protostars, and yet its
role in the star formation process is not well-studied. We present a
semianalytic self-similar model of the collapse of rotating isothermal
molecular cloud cores with both Hall and ambipolar diffusion, and similarity
solutions that demonstrate the profound influence of the Hall effect on the
dynamics of collapse.
The solutions show that the size and sign of the Hall parameter can change
the size of the protostellar disc by up to an order of magnitude and the
protostellar accretion rate by fifty per cent when the ratio of the Hall to
ambipolar diffusivities is varied between -0.5 <= eta_H / eta_A <= 0.2. These
changes depend upon the orientation of the magnetic field with respect to the
axis of rotation and create a preferred handedness to the solutions that could
be observed in protostellar cores using next-generation instruments such as
ALMA.
Hall diffusion also determines the strength and position of the shocks that
bound the pseudo and rotationally-supported discs, and can introduce subshocks
that further slow accretion onto the protostar. In cores that are not initially
rotating Hall diffusion can even induce rotation, which could give rise to disc
formation and resolve the magnetic braking catastrophe. The Hall effect clearly
influences the dynamics of gravitational collapse and its role in controlling
the magnetic braking and radial diffusion of the field merits further
exploration in numerical simulations of star formation.Comment: 22 pages, 10 figures, accepted by MNRA
Coherent Matter Wave Transport in Speckle Potentials
This article studies multiple scattering of matter waves by a disordered
optical potential in two and in three dimensions. We calculate fundamental
transport quantities such as the scattering mean free path , the
Boltzmann transport mean free path \elltrb, and the Boltzmann diffusion
constant , using a diagrammatic Green functions approach. Coherent
multiple scattering induces interference corrections known as weak localization
which entail a reduced diffusion constant. We derive the corresponding
expressions for matter wave transport in an correlated speckle potential and
provide the relevant parameter values for a possible experimental study of this
coherent transport regime, including the critical crossover to the regime of
strong or Anderson localization.Comment: 33 pages, minor corrections, published versio
The Intrinsic Origin of Spin Echoes in Dipolar Solids Generated by Strong Pi Pulses
In spectroscopy, it is conventional to treat pulses much stronger than the
linewidth as delta-functions. In NMR, this assumption leads to the prediction
that pi pulses do not refocus the dipolar coupling. However, NMR spin echo
measurements in dipolar solids defy these conventional expectations when more
than one pi pulse is used. Observed effects include a long tail in the CPMG
echo train for short delays between pi pulses, an even-odd asymmetry in the
echo amplitudes for long delays, an unusual fingerprint pattern for
intermediate delays, and a strong sensitivity to pi-pulse phase. Experiments
that set limits on possible extrinsic causes for the phenomena are reported. We
find that the action of the system's internal Hamiltonian during any real pulse
is sufficient to cause the effects. Exact numerical calculations, combined with
average Hamiltonian theory, identify novel terms that are sensitive to
parameters such as pulse phase, dipolar coupling, and system size.
Visualization of the entire density matrix shows a unique flow of quantum
coherence from non-observable to observable channels when applying repeated pi
pulses.Comment: 24 pages, 27 figures. Revised from helpful referee comments. Added
new Table IV, new paragraphs on pages 3 and 1
Information overload : the differences that age makes
Information overload has long been studied as a phenomenon that causes problems at the personal, social and organisational level. This study investigates overload from a new angle, that of the influence of age on perceptions of information overload. A combination of questionnaires, interviews and diaries were used to gain insight into people’s perceptions towards information overload.
It was found that people of all ages suffer from information overload but young people are primarily affected by information literacy levels while older people are affected by technology. There was evidence of a link between age and technology use. A link was also found between job role and information overload and the impact technology has had on the quantity of information available. This research will benefit anyone, either individually or within an organisation, looking for ways to combat information overload. It identifies the influence of age on various factors and recommends actions that may be taken to reduce information overload. In particular, recommendations were made for further training in technology and information literacy. The paper is based on an approach not seen before in the literature as it investigates the effects of age on information overload by seeking to understand how perceptions towards information overload may differ between different age groups. It is anticipated that this paper will trigger further studies that could focus on the effect of job role on information overload and the likelihood of information addiction becoming a future concern
TeV Gamma-Ray Sources from a Survey of the Galactic Plane with Milagro
A survey of Galactic gamma-ray sources at a median energy of ~20 TeV has been
performed using the Milagro Gamma Ray Observatory. Eight candidate sources of
TeV emission are detected with pre-trials significance in the
region of Galactic longitude and latitude
. Four of these sources, including the Crab nebula
and the recently published MGRO J2019+37, are observed with significances
after accounting for the trials involved in searching the 3800
square degree region. All four of these sources are also coincident with EGRET
sources. Two of the lower significance sources are coincident with EGRET
sources and one of these sources is Geminga. The other two candidates are in
the Cygnus region of the Galaxy. Several of the sources appear to be spatially
extended. The fluxes of the sources at 20 TeV range from ~25% of the Crab flux
to nearly as bright as the Crab.Comment: Submitted to Ap
Spectrum and Morphology of the Two Brightest Milagro Sources in the Cygnus Region: MGRO J2019+37 and MGRO J2031+41
The Cygnus region is a very bright and complex portion of the TeV sky, host
to unidentified sources and a diffuse excess with respect to conventional
cosmic-ray propagation models. Two of the brightest TeV sources, MGRO J2019+37
and MGRO J2031+41, are analyzed using Milagro data with a new technique, and
their emission is tested under two different spectral assumptions: a power law
and a power law with an exponential cutoff. The new analysis technique is based
on an energy estimator that uses the fraction of photomultiplier tubes in the
observatory that detect the extensive air shower. The photon spectrum is
measured in the range 1 to 200 TeV using the last 3 years of Milagro data
(2005-2008), with the detector in its final configuration. MGRO J2019+37 is
detected with a significance of 12.3 standard deviations (), and is
better fit by a power law with an exponential cutoff than by a simple power
law, with a probability % (F-test). The best-fitting parameters for the
power law with exponential cutoff model are a normalization at 10 TeV of
, a spectral
index of and a cutoff energy of TeV. MGRO
J2031+41 is detected with a significance of 7.3, with no evidence of a
cutoff. The best-fitting parameters for a power law are a normalization of
and a
spectral index of . The overall flux is subject to an
30% systematic uncertainty. The systematic uncertainty on the power law
indices is 0.1. A comparison with previous results from TeV J2032+4130,
MGRO J2031+41 and MGRO J2019+37 is also presented.Comment: 11 pages, 10 figure
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