949 research outputs found
Production of superconductor/carbon bicomponent fibers
Certain materials are unable to be drawn or spun into fiber form due to their improper melting characteristics or brittleness. However, fibrous samples of such materials are often necessary for the fabrication of intricate shapes and composites. In response to this problem, a unique process, referred to as the piggyback process, was developed to prepare fibrous samples of a variety of nonspinnable ceramics. In this technique, specially produced C-shaped carbon fibers serve as micromolds to hold the desired materials prior to sintering. Depending on the sintering atmosphere used, bicomponent or single component fibers result. While much has been demonstrated worldwide concerning the YBa2Cu3O(7-x) superconductor, fabrication into unique forms has proven quite difficult. However, a variety of intricate shapes are necessary for rapid commercialization of the superconducting materials. The potential for producing fibrous samples of the YBa2Cu3O(7-x) compound by the piggyback process is being investigated. Various organic and acrylic materials were investigated to determine suspending ability, reactivity with the YBa2Cu3O(7-x) compound during long term storage, and burn out characteristics. While many questions were answered with respect to the interfacial reactions between YBa2Cu3O(7-x) and carbon, much work is still necessary to improve the quality of the sintered material if the fibers produced are to be incorporated into useful composites or cables. Additional research is necessary to evaluate quality of the barrier layer during long soakings at the peak temperature; adjust the firing schedule to avoid microcracking and improve densification; and increase the solids loading in the superconductive suspension to decrease porosity
Proton-tetraneutron elastic scattering
We analyze the elastic scattering of protons on a 4n system. This was used as
part of the detection technique of a recent experiment [1] to search for the 4n
(tetraneutron) as a bound particle. We show that it is unlikely that this
process alone could yield the events reported in ref. [1], unless the 4n has an
anomalously large backward elastic scattering amplitude.Comment: 6 pages, 2 figures, accepted for publication in Phys. Rev.
Hippocampus and retrosplenial cortex combine path integration signals for successful navigation
The current study used fMRI in humans to examine goal-directed navigation in an open field environment. We designed a task that required participants to encode survey-level spatial information and subsequently navigate to a goal location in either first person, third person, or survey perspectives. Critically, no distinguishing landmarks or goal location markers were present in the environment, thereby requiring participants to rely on path integration mechanisms for successful navigation. We focused our analysis on mechanisms related to navigation and mechanisms tracking linear distance to the goal location. Successful navigation required translation of encoded survey-level map information for orientation and implementation of a planned route to the goal. Our results demonstrate that successful first and third person navigation trials recruited the anterior hippocampus more than trials when the goal location was not successfully reached. When examining only successful trials, the retrosplenial and posterior parietal cortices were recruited for goal-directed navigation in both first person and third person perspectives. Unique to first person perspective navigation, the hippocampus was recruited to path integrate self-motion cues with location computations toward the goal location. Last, our results demonstrate that the hippocampus supports goal-directed navigation by actively tracking proximity to the goal throughout navigation. When using path integration mechanisms in first person and third person perspective navigation, the posterior hippocampus was more strongly recruited as participants approach the goal. These findings provide critical insight into the neural mechanisms by which we are able to use map-level representations of our environment to reach our navigational goals
Directed Rh(I)-Catalyzed Asymmetric Hydroboration of Prochiral 1-Arylcycloprop-2-ene-1-carboxylic Acid Derivatives
This is the peer reviewed version of the following article: A. Edwards, M. Rubina, M. Rubin, Chem. Eur. J. 2018, 24, 1394., which has been published in final form at http://doi.org/10.1002/chem.201704443. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.A full account on rhodium-catalyzed asymmetric, directed hydroboration of functionalized prochiral cyclopropenes affording enantiomerically enriched cyclopropylboronates is reported. The scope and limitations of two alternate directing groups, ester and carboxamide, are evaluated. It was found that hydroboration of esters appeared to be more sensitive to substitution in the aromatic ring of the substrates. Specifically, ortho-halogens were detrimental for diastereo- and enantioselectivity, possibly because of additional coordination with rhodium. In contrast, more Lewis-basic amide directing groups allowed for stronger chelation to the transition metal, leading to consistently high diastereo- and enantioselectivity in hydroboration across a broader range of substrates
Nuclear Theory and Science of the Facility for Rare Isotope Beams
The Facility for Rare Isotope Beams (FRIB) will be a world-leading laboratory
for the study of nuclear structure, reactions and astrophysics. Experiments
with intense beams of rare isotopes produced at FRIB will guide us toward a
comprehensive description of nuclei, elucidate the origin of the elements in
the cosmos, help provide an understanding of matter in neutron stars, and
establish the scientific foundation for innovative applications of nuclear
science to society. FRIB will be essential for gaining access to key regions of
the nuclear chart, where the measured nuclear properties will challenge
established concepts, and highlight shortcomings and needed modifications to
current theory. Conversely, nuclear theory will play a critical role in
providing the intellectual framework for the science at FRIB, and will provide
invaluable guidance to FRIB's experimental programs. This article overviews the
broad scope of the FRIB theory effort, which reaches beyond the traditional
fields of nuclear structure and reactions, and nuclear astrophysics, to explore
exciting interdisciplinary boundaries with other areas.
\keywords{Nuclear Structure and Reactions. Nuclear
Astrophysics. Fundamental Interactions. High Performance
Computing. Rare Isotopes. Radioactive Beams.Comment: 20 pages, 7 figure
Mission operations and command assurance: Instilling quality into flight operations
Mission Operations and Command Assurance (MO&CA) is a Total Quality Management (TQM) task on JPL projects to instill quality in flight mission operations. From a system engineering view, MO&CA facilitates communication and problem-solving among flight teams and provides continuous process improvement to reduce the probability of radiating incorrect commands to a spacecraft. The MO&CA task has evolved from participating as a member of the spacecraft team to an independent team reporting directly to flight project management and providing system level assurance. JPL flight projects have benefited significantly from MO&CA's effort to contain risk and prevent rather than rework errors. MO&CA's ability to provide direct transfer of knowledge allows new projects to benefit from previous and ongoing flight experience
Tensor Product Approximation (DMRG) and Coupled Cluster method in Quantum Chemistry
We present the Copupled Cluster (CC) method and the Density matrix
Renormalization Grooup (DMRG) method in a unified way, from the perspective of
recent developments in tensor product approximation. We present an introduction
into recently developed hierarchical tensor representations, in particular
tensor trains which are matrix product states in physics language. The discrete
equations of full CI approximation applied to the electronic Schr\"odinger
equation is casted into a tensorial framework in form of the second
quantization. A further approximation is performed afterwards by tensor
approximation within a hierarchical format or equivalently a tree tensor
network. We establish the (differential) geometry of low rank hierarchical
tensors and apply the Driac Frenkel principle to reduce the original
high-dimensional problem to low dimensions. The DMRG algorithm is established
as an optimization method in this format with alternating directional search.
We briefly introduce the CC method and refer to our theoretical results. We
compare this approach in the present discrete formulation with the CC method
and its underlying exponential parametrization.Comment: 15 pages, 3 figure
A Coherent Nonlinear Optical Signal Induced by Electron Correlations
The correlated behavior of electrons determines the structure and optical
properties of molecules, semiconductor and other systems. Valuable information
on these correlations is provided by measuring the response to femtosecond
laser pulses, which probe the very short time period during which the excited
particles remain correlated. The interpretation of four-wave-mixing techniques,
commonly used to study the energy levels and dynamics of many-electron systems,
is complicated by many competing effects and overlapping resonances. Here we
propose a coherent optical technique, specifically designed to provide a
background-free probe for electronic correlations in many-electron systems. The
proposed signal pulse is generated only when the electrons are correlated,
which gives rise to an extraordinary sensitivity. The peak pattern in
two-dimensional plots, obtained by displaying the signal vs. two frequencies
conjugated to two pulse delays, provides a direct visualization and specific
signatures of the many-electron wavefunctions.Comment: 2 figure
Mission operations and command assurance: Flight operations quality improvements
Mission Operations and Command Assurance (MO&CA) is a Total Quality Management (TQM) task on JPL projects to instill quality in flight mission operations. From a system engineering view, MO&CA facilitates communication and problem-solving among flight teams and provides continuous solving among flight teams and provides continuous process improvement to reduce risk in mission operations by addressing human factors. The MO&CA task has evolved from participating as a member of the spacecraft team, to an independent team reporting directly to flight project management and providing system level assurance. JPL flight projects have benefited significantly from MO&CA's effort to contain risk and prevent rather than rework errors. MO&CA's ability to provide direct transfer of knowledge allows new projects to benefit from previous and ongoing flight experience
Production cross sections from 82Se fragmentation as indications of shell effects in neutron-rich isotopes close to the drip-line
Production cross sections for neutron-rich nuclei from the fragmentation of a
82Se beam at 139 MeV/u were measured. The longitudinal momentum distributions
of 126 neutron-rich isotopes of elements 11 <= Z <= 32 were scanned using an
experimental approach of varying the target thickness. Production cross
sections with beryllium and tungsten targets were determined for a large number
of nuclei including several isotopes first observed in this work. These are the
most neutron-rich nuclides of the elements 22 <= Z <= 25 (64Ti, 67V, 69Cr,
72Mn). One event was registered consistent with 70Cr, and another one with
75Fe. The production cross sections are correlated with Qg systematics to
reveal trends in the data. The results presented here confirm our previous
result from a similar measurement using a 76Ge beam, and can be explained with
a shell model that predicts a subshell closure at N = 34 around Z = 20. This is
demonstrated by systematic trends and calculations with the Abrasion-Ablation
model that are sensitive to separation energies.Comment: 13 pages, 11 figures, accepted to Phys.Rev.
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