25,045 research outputs found
Elimination of the light shift in rubidium gas cell frequency standards using pulsed optical pumping
Changes in the intensity of the light source in an optically pumped, rubidium, gas cell frequency standard can produce corresponding frequency shifts, with possible adverse effects on the long-term frequency stability. A pulsed optical pumping apparatus was constructed with the intent of investigating the frequency stability in the absence of light shifts. Contrary to original expectations, a small residual frequency shift due to changes in light intensity was experimentally observed. Evidence is given which indicates that this is not a true light-shift effect. Preliminary measurements of the frequency stability of this apparatus, with this small residual pseudo light shift present, are presented. It is shown that this pseudo light shift can be eliminated by using a more homogeneous C-field. This is consistent with the idea that the pseudo light shift is due to inhomogeneity in the physics package (position-shift effect)
Infrared spectroscopy under multi-extreme conditions: Direct observation of pseudo gap formation and collapse in CeSb
Infrared reflectivity measurements of CeSb under multi-extreme conditions
(low temperatures, high pressures and high magnetic fields) were performed. A
pseudo gap structure, which originates from the magnetic band folding effect,
responsible for the large enhancement in the electrical resistivity in the
single-layered antiferromagnetic structure (AF-1 phase) was found at a pressure
of 4 GPa and at temperatures of 35 - 50 K. The optical spectrum of the pseudo
gap changes to that of a metallic structure with increasing magnetic field
strength and increasing temperature. This change is the result of the magnetic
phase transition from the AF-1 phase to other phases as a function of the
magnetic field strength and temperature. This result is the first optical
observation of the formation and collapse of a pseudo gap under multi-extreme
conditions.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev.
Orientational Melting in Carbon Nanotube Ropes
Using Monte Carlo simulations, we investigate the possibility of an
orientational melting transition within a "rope" of (10,10) carbon nanotubes.
When twisting nanotubes bundle up during the synthesis, orientational
dislocations or twistons arise from the competition between the anisotropic
inter-tube interactions, which tend to align neighboring tubes, and the torsion
rigidity that tends to keep individual tubes straight. We map the energetics of
a rope containing twistons onto a lattice gas model and find that the onset of
a free "diffusion" of twistons, corresponding to orientational melting, occurs
at T_OM > 160 K.Comment: 4 page LaTeX file with 3 figures (10 PostScript files
Needs Assessment of Barriers to Cervical Cancer Screening in Vietnamese American Health Care Providers
Vietnamese women living in the United States have a cervical cancer incidence rate that is five times that of White women. The low rate of cervical cancer screening among this high-risk population contributes to this disparity. In 2004, the National Cancer Institute collaborated with the Vietnamese American Medical Association to conduct a short needs assessment questionnaire (Pap Test Barriers Questionnaire for Health Care Providers) among its members to assess provider views about cervical cancer, barriers to Pap testing among Vietnamese women living in the United States, and types of patient education materials needed to help motivate Vietnamese women to receive a Pap test. Information from the questionnaire was used to inform development of a brochure and identify additional strategies to enhance outreach to Vietnamese women and providers. Almost all of the respondents (95%) thought that Pap tests were “very important” in the early detection of cervical cancer in Vietnamese women. In addition, knowledge about the importance of Pap tests was identified as the most influential factor for Vietnamese women not seeking a Pap test. Print materials that included both English and Vietnamese translations in the same publication were cited as a preferred communication tool. Further, health education through Vietnamese media was recommended as a primary strategy for reaching women with educational messages. Findings from this needs assessment contributes to a larger formative research effort to build NCI’s cervical cancer education program within its Office of Education and Special Initiatives
Astronomy in the Cloud: Using MapReduce for Image Coaddition
In the coming decade, astronomical surveys of the sky will generate tens of
terabytes of images and detect hundreds of millions of sources every night. The
study of these sources will involve computation challenges such as anomaly
detection and classification, and moving object tracking. Since such studies
benefit from the highest quality data, methods such as image coaddition
(stacking) will be a critical preprocessing step prior to scientific
investigation. With a requirement that these images be analyzed on a nightly
basis to identify moving sources or transient objects, these data streams
present many computational challenges. Given the quantity of data involved, the
computational load of these problems can only be addressed by distributing the
workload over a large number of nodes. However, the high data throughput
demanded by these applications may present scalability challenges for certain
storage architectures. One scalable data-processing method that has emerged in
recent years is MapReduce, and in this paper we focus on its popular
open-source implementation called Hadoop. In the Hadoop framework, the data is
partitioned among storage attached directly to worker nodes, and the processing
workload is scheduled in parallel on the nodes that contain the required input
data. A further motivation for using Hadoop is that it allows us to exploit
cloud computing resources, e.g., Amazon's EC2. We report on our experience
implementing a scalable image-processing pipeline for the SDSS imaging database
using Hadoop. This multi-terabyte imaging dataset provides a good testbed for
algorithm development since its scope and structure approximate future surveys.
First, we describe MapReduce and how we adapted image coaddition to the
MapReduce framework. Then we describe a number of optimizations to our basic
approach and report experimental results comparing their performance.Comment: 31 pages, 11 figures, 2 table
Electronic structures of doped anatase : (M=Co, Mn, Fe, Ni)
We have investigated electronic structures of a room temperature diluted
magnetic semiconductor : Co-doped anatase . We have obtained the
half-metallic ground state in the local-spin-density approximation(LSDA) but
the insulating ground state in the LSDA++SO incorporating the spin-orbit
interaction. In the stoichiometric case, the low spin state of Co is realized
with the substantially large orbital moment. However, in the presence of oxygen
vacancies near Co, the spin state of Co becomes intermediate. The
ferromagnetisms in the metallic and insulating phases are accounted for by the
double-exchange-like and the superexchange mechanism, respectively. Further,
the magnetic ground states are obtained for Mn and Fe doped ,
while the paramagnetic ground state for Ni-doped .Comment: 5 pages, 4 figure
Million-atom molecular dynamics simulation by order-N electronic structure theory and parallel computation
Parallelism of tight-binding molecular dynamics simulations is presented by
means of the order-N electronic structure theory with the Wannier states,
recently developed (J. Phys. Soc. Jpn. 69,3773 (2000)). An application is
tested for silicon nanocrystals of more than millions atoms with the
transferable tight-binding Hamiltonian. The efficiency of parallelism is
perfect, 98.8 %, and the method is the most suitable to parallel computation.
The elapse time for a system of atoms is 3.0 minutes by a
computer system of 64 processors of SGI Origin 3800. The calculated results are
in good agreement with the results of the exact diagonalization, with an error
of 2 % for the lattice constant and errors less than 10 % for elastic
constants.Comment: 5 pages, 3 figure
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