3,955 research outputs found
Arizona Prospector
https://digitalcommons.library.umaine.edu/mmb-vp/3024/thumbnail.jp
Classical Limit of Demagnetization in a Field Gradient
We calculate the rate of decrease of the expectation value of the transverse
component of spin for spin-1/2 particles in a magnetic field with a spatial
gradient, to determine the conditions under which a previous classical
description is valid. A density matrix treatment is required for two reasons.
The first arises because the particles initially are not in a pure state due to
thermal motion. The second reason is that each particle interacts with the
magnetic field and the other particles, with the latter taken to be via a
2-body central force. The equations for the 1-body Wigner distribution
functions are written in a general manner, and the places where quantum
mechanical effects can play a role are identified. One that may not have been
considered previously concerns the momentum associated with the magnetic field
gradient, which is proportional to the time integral of the gradient. Its
relative magnitude compared with the important momenta in the problem is a
significant parameter, and if their ratio is not small some non-classical
effects contribute to the solution.
Assuming the field gradient is sufficiently small, and a number of other
inequalities are satisfied involving the mean wavelength, range of the force,
and the mean separation between particles, we solve the integro- partial
differential equations for the Wigner functions to second order in the strength
of the gradient. When the same reasoning is applied to a different problem with
no field gradient, but having instead a gradient to the z-component of
polarization, the connection with the diffusion coefficient is established, and
we find agreement with the classical result for the rate of decrease of the
transverse component of magnetization.Comment: 22 pages, no figure
Quantum dynamics, dissipation, and asymmetry effects in quantum dot arrays
We study the role of dissipation and structural defects on the time evolution
of quantum dot arrays with mobile charges under external driving fields. These
structures, proposed as quantum dot cellular automata, exhibit interesting
quantum dynamics which we describe in terms of equations of motion for the
density matrix. Using an open system approach, we study the role of asymmetries
and the microscopic electron-phonon interaction on the general dynamical
behavior of the charge distribution (polarization) of such systems. We find
that the system response to the driving field is improved at low temperatures
(and/or weak phonon coupling), before deteriorating as temperature and
asymmetry increase. In addition to the study of the time evolution of
polarization, we explore the linear entropy of the system in order to gain
further insights into the competition between coherent evolution and
dissipative processes.Comment: 11pages,9 figures(eps), submitted to PR
Quasiparticle transport equation with collision delay. II. Microscopic Theory
For a system of non-interacting electrons scattered by neutral impurities, we
derive a modified Boltzmann equation that includes quasiparticle and virial
corrections. We start from quasiclassical transport equation for
non-equilibrium Green's functions and apply limit of small scattering rates.
Resulting transport equation for quasiparticles has gradient corrections to
scattering integrals. These gradient corrections are rearranged into a form
characteristic for virial corrections
Three-Dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for the
estimation of atmospheric parameters (Teff, logg, and [Fe/H]) for Galactic F-
and G-type stars. The ANNs are fed with medium-resolution (~ 1-2 A) non
flux-calibrated spectroscopic observations. From a sample of 279 stars with
previous high-resolution determinations of metallicity, and a set of (external)
estimates of temperature and surface gravity, our ANNs are able to predict Teff
with an accuracy of ~ 135-150 K over the range 4250 <= Teff <= 6500 K, logg
with an accuracy of ~ 0.25-0.30 dex over the range 1.0 <= logg <= 5.0 dex, and
[Fe/H] with an accuracy ~ 0.15-0.20 dex over the range -4.0 <= [Fe/H] <= +0.3.
Such accuracies are competitive with the results obtained by fine analysis of
high-resolution spectra. It is noteworthy that the ANNs are able to obtain
these results without consideration of photometric information for these stars.
We have also explored the impact of the signal-to-noise ratio (S/N) on the
behavior of ANNs, and conclude that, when analyzed with ANNs trained on spectra
of commensurate S/N, it is possible to extract physical parameter estimates of
similar accuracy with stellar spectra having S/N as low as 13. Taken together,
these results indicate that the ANN approach should be of primary importance
for use in present and future large-scale spectroscopic surveys.Comment: 51 pages, 11 eps figures, uses aastex; to appear in Ap
Quantized charge transport through a static quantum dot using a surface acoustic wave
We present a detailed study of the surface acoustic wave mediated quantized
transport of electrons through a split gate device containing an impurity
potential defined quantum dot within the split gate channel. A new regime of
quantized transport is observed at low RF powers where the surface acoustic
wave amplitude is comparable to the quantum dot charging energy. In this regime
resonant transport through the single-electron dot state occurs which we
interpret as turnstile-like operation in which the traveling wave amplitude
modulates the entrance and exit barriers of the quantum dot in a cyclic fashion
at GHz frequencies. For high RF powers, where the amplitude of the surface
acoustic wave is much larger than the quantum dot energies, the quantized
acoustoelectric current transport shows behavior consistent with previously
reported results. However, in this regime, the number of quantized current
plateaus observed and the plateau widths are determined by the properties of
the quantum dot, demonstrating that the microscopic detail of the potential
landscape in the split gate channel has a profound influence on the quantized
acoustoelectric current transport.Comment: 9 page
Long-Term Results of Complex Abdominal Aortic Aneurysm Open Repair
This study investigated the long-term outcomes of patients treated with open surgical repair for complex abdominal aortic aneurysms (c-AAAs). A total of 119 patients with c-AAAs undergoing repair between January 2010 and June 2016 in a high-volume aortic center were included. The long-term imaging follow-up consisted of yearly abdominal ultrasound examinations and 5-year computed tomography angiography. At a median follow-up of 76 months (IQR 38 months), forty-three deaths (37%) and three (2.5%) aortic-related deaths were observed. Long-term chronic renal decline was observed in fifty (43.8%) patients, significantly correlated with post-operative acute kidney injury. During the follow-up, five reinterventions (4.3%) were performed. The present study suggests that open c-AAA repair can be performed with acceptable operative risk with durable results. To achieve the best possible long-term outcome, the open surgery repair of complex AAA should be performed in high-volume aortic centers and tailored to the patient
Mycobacterium tuberculosis Transmission from Human to Canine
A 71-year-old woman from Tennessee, USA with a 3-week history of a productive, nonbloody cough was evaluated. Chest radiograph showed infiltrates and atelectasis in the upper lobe of the right lung. A tuberculosis (TB) skin test resulted in a 14-mm area of induration. Sputum stained positive for acid-fast bacilli (AFB) and was positive for Mycobacterium tuberculosis by DNA probe and culture. Treatment was initiated with isoniazid, rifampicin, and pyrazinamide. After 14 days of daily, directly observed therapy, the patient complained of nausea, vomiting and diarrhoea. Treatment adjustments were made, and therapy was completed 11 months later with complete recovery. Six months after the patient\u27s TB diagnosis, she took her three and a half-year-old male Yorkshire Terrier to a veterinary clinic with cough, weight loss, and vomiting of several months\u27 duration. Initial sputum sample was negative on AFB staining. Eight days after discharge from a referral veterinary teaching hospital with a presumptive diagnosis of TB, the dog was euthanized due to urethral obstruction. Liver and tracheobronchial lymph node specimens collected at necropsy were positive for M. tuberculosis complex by polymerase chain reaction. The M. tuberculosis isolates from the dog and its owner had an indistinguishable 10-band pattern by IS6110-based restriction fragment length polymorphism genotyping
Targeted deletion of miR-132/-212 impairs memory and alters the hippocampal transcriptome
miR-132 and miR-212 are structurally related microRNAs that have been found to exert powerful modulatory effects within the central nervous system (CNS). Notably, these microRNAs are tandomly processed from the same noncoding transcript, and share a common seed sequence: thus it has been difficult to assess the distinct contribution of each microRNA to gene expression within the CNS. Here, we employed a combination of conditional knockout and transgenic mouse models to examine the contribution of the miR-132/-212 gene locus to learning and memory, and then to assess the distinct effects that each microRNA has on hippocampal gene expression. Using a conditional deletion approach, we show that miR-132/-212 double-knockout mice exhibit significant cognitive deficits in spatial memory, recognition memory, and in tests of novel object recognition. Next, we utilized transgenic miR-132 and miR-212 overexpression mouse lines and the miR-132/-212 double-knockout line to explore the distinct effects of these two miRNAs on the transcriptional profile of the hippocampus. Illumina sequencing revealed that miR-132/-212 deletion increased the expression of 1138 genes; Venn analysis showed that 96 of these genes were also downregulated in mice overexpressing miR-132. Of the 58 genes that were decreased in animals overexpressing miR-212, only four of them were also increased in the knockout line. Functional gene ontology analysis of downregulated genes revealed significant enrichment of genes related to synaptic transmission, neuronal proliferation, and morphogenesis, processes known for their roles in learning, and memory formation. These data, coupled with previous studies, firmly establish a role for the miR-132/-212 gene locus as a key regulator of cognitive capacity. Further, although miR-132 and miR-212 share a seed sequence, these data indicate that these miRNAs do not exhibit strongly overlapping mRNA targeting profiles, thus indicating that these two genes may function in a complex, nonredundant manner to shape the transcriptional profile of the CNS. The dysregulation of miR-132/-212 expression could contribute to signaling mechanisms that are involved in an array of cognitive disorders
- …