21,557 research outputs found
Coding for Racetrack Memories
Racetrack memory is a new technology which utilizes magnetic domains along a
nanoscopic wire in order to obtain extremely high storage density. In racetrack
memory, each magnetic domain can store a single bit of information, which can
be sensed by a reading port (head). The memory has a tape-like structure which
supports a shift operation that moves the domains to be read sequentially by
the head. In order to increase the memory's speed, prior work studied how to
minimize the latency of the shift operation, while the no less important
reliability of this operation has received only a little attention.
In this work we design codes which combat shift errors in racetrack memory,
called position errors. Namely, shifting the domains is not an error-free
operation and the domains may be over-shifted or are not shifted, which can be
modeled as deletions and sticky insertions. While it is possible to use
conventional deletion and insertion-correcting codes, we tackle this problem
with the special structure of racetrack memory, where the domains can be read
by multiple heads. Each head outputs a noisy version of the stored data and the
multiple outputs are combined in order to reconstruct the data. Under this
paradigm, we will show that it is possible to correct, with at most a single
bit of redundancy, deletions with heads if the heads are
well-separated. Similar results are provided for burst of deletions, sticky
insertions and combinations of both deletions and sticky insertions
Overview of the gene ontology task at BioCreative IV
Gene Ontology (GO) annotation is a common task among model organism databases (MODs) for capturing gene function data from journal articles. It is a time-consuming and labor-intensive task, and is thus often considered as one of the bottlenecks in literature curation. There is a growing need for semiautomated or fully automated GO curation techniques that will help database curators to rapidly and accurately identify gene function information in full-length articles. Despite multiple attempts in the past, few studies have proven to be useful with regard to assisting real-world GO curation. The shortage of sentence-level training data and opportunities for interaction between text-mining developers and GO curators has limited the advances in algorithm development and corresponding use in practical circumstances. To this end, we organized a text-mining challenge task for literature-based GO annotation in BioCreative IV. More specifically, we developed two subtasks: (i) to automatically locate text passages that contain GO-relevant information (a text retrieval task) and (ii) to automatically identify relevant GO terms for the genes in a given article (a concept-recognition task). With the support from five MODs, we provided teams with >4000 unique text passages that served as the basis for each GO annotation in our task data. Such evidence text information has long been recognized as critical for text-mining algorithm development but was never made available because of the high cost of curation. In total, seven teams participated in the challenge task. From the team results, we conclude that the state of the art in automatically mining GO terms from literature has improved over the past decade while much progress is still needed for computer-assisted GO curation. Future work should focus on addressing remaining technical challenges for improved performance of automatic GO concept recognition and incorporating practical benefits of text-mining tools into real-world GO annotation
Truncation of stellar disks in galaxies at z~1
We report here the first evidence for stellar disk truncation at high
redshift, based on surface photometry of a sample of 16 high redshift (0.6 < z
< 1.0) disk galaxies from the GOODS HST/ACS data. The radial profiles are best
fit by a double exponential profile. This result agrees with the profile of
disks in local galaxies. The cosmological surface brightness dimming at this
redshift range only allows us to detect galaxies with spatially ``early''
truncation, R_br/h_in <= 3.5. Six galaxies show the radial double exponential
structure, with an average value of R_br/h_in ~ 1.8. Such ``early'' truncated
galaxies are missing in local samples so far. This result opens the ground for
observing directly disk evolution through the study of the truncation radius as
a function of redshift.Comment: 4 pages, 1 figure, accepted for publication in the A&A Letter
The phase-space distribution of infalling dark matter subhalos
We use high-resolution numerical simulations to study the physical properties
of subhalos when they merge into their host halos. An improved algorithm is
used to identify the subhalos. We then examine their spatial and velocity
distributions in spherical and triaxial halo models. We find that the accretion
of satellites preferentially occurs along the major axis and perpendicular to
the spin axis of the host halo. Furthermore, the massive subhalos show a
stronger preference to be accreted along the major axis of the host halo than
the low-mass ones. Approximate fitting formulae are provided for the physical
properties of subhalos. Combined with analytical and semi-analytic techniques,
these empirical formulae provide a useful basis for studying the subsequent
evolution of subhalos and satellite galaxies in their hosts. Future studies
should however account for satellites that may not be undergoing the first
infall in their evolution.Comment: revised version in press in MN with added material and references, 21
pages and 25 figure
Improving Pedagogical Content Knowledge On Rational Numbers Of Cambodian Teacher Trainers
Despite adequate facilities and several education reforms, most Cambodian teacher trainers fail to provide sufficient content knowledge and student-centered pedagogy. Many also lack the skills to diagnose preservice teachersâ misconceptions and to propose adequate solutions. Dictating lessons with little feedback or applied activities or having pre-service teachers copy off the board for extended periods, suggests lowquality instruction (Tandon & Fukao, 2015). To tackle this, the Flemish Association for Development Cooperation and Technical Assistance (VVOB- education for development)1 developed a 3-year (20142016) programme in close collaboration with the Cambodian Ministry of Education, Youth and Sport (MoEYS). The programme was rolled out in all primary teacher training colleges (PTTCs). One of the interventions in this programme aimed at improving both Pedagogical Content Knowledge (PCK) and Content Knowledge (CK) on rational numbers of mathematics teacher trainers, with a focus on 1) mathematics content knowledge, 2) the use of representations to enhance pre-service teachersâ understanding, 3) assessing pre-service teachersâ learning, and 4) addressing misconceptions. A total of 54 mathematics teacher trainers participated in this intervention. Their capacity was built through training, coaching, mentoring and try-outs with pre-service teachers. The impact of the intervention was measured through a pre-test post-test design, enriched by qualitative data collected during 97 lesson observations. After the intervention, 91% of the teacher trainers had significantly increased their score on the PCK test and 94 % had improved their teaching strategy in at least two of the three criteria of PCK. In this paper, the design and impact of the intervention are explained, and suggestions for further research are provided
The Alignment between Satellites and Central Galaxies: Theory vs. Observations
Recent studies have shown that the distribution of satellite galaxies is
preferentially aligned with the major axis of their central galaxy. The
strength of this alignment has been found to depend strongly on the colours of
the satellite and central galaxies, and only weakly on the mass of the halo in
which the galaxies reside. In this paper we study whether these alignment
signals, and their dependence on galaxy and halo properties, can be reproduced
in a hierarchical structure formation model of a CDM concordance
cosmology. To that extent we use a large -body simulation which we populate
with galaxies following a semi-analytical model for galaxy formation. We find
that if the orientation of the central galaxy is perfectly aligned with that of
its dark matter halo, then the predicted central-satellite alignment signal is
much stronger than observed. If, however, the minor axis of a central galaxy is
perfectly aligned with the angular momentum vector of its dark matter halo, we
can accurately reproduce the observed alignment strength as function of halo
mass and galaxy color. Although this suggests that the orientation of central
galaxies is governed by the angular momentum of their dark matter haloes, we
emphasize that any other scenario in which the minor axes of central galaxy and
halo are misaligned by (on average) will match the data
equally well. Finally, we show that dependence of the alignment strength on the
color of the central galaxy is most likely an artefact due to interlopers in
the group catalogue. The dependence on the color of the satellite galaxies, on
the other hand, is real and owes to the fact that red satellites are associated
with subhaloes that were more massive at their time of accretion.Comment: 13 Pages, 10 Figures, one figure replaced. added in discussion about
comparison with others results, Updated version to match accepted version to
MNRA
Investigation of methane oxidation by palladium-based catalyst via ReaxFF Molecular Dynamics simulation
Catalytic oxidations of methane over palladium-based nanoparticles, with and without oxygen coating, are investigated using ReaxFF Molecular Dynamics simulations. The simulation results show the complete dynamic process of the above catalytic reactions at the atomic level and help to reveal the underlying mechanisms both qualitatively and quantitatively. It is found that oxygen molecules are significantly easier to be adsorbed on both bare and oxygen-coated Pd surfaces compared with CH4. The presence of adsorbed O2 molecules on the surface blocks the active sites for CH4 adsorption on the oxygen-coated Pd surfaces. By comparing the adsorptive dissociation of CH4 over Pd nanoparticles with different levels of oxygen coverage, we find that it is much easier for the adsorptive dissociation of CH4 on oxygen-coated Pd nanoparticles than that on bare Pd nanoparticles at low temperatures. In contrast to the rapid dissociation of CH4 after adsorption, the dissociation of O2 requires much higher temperature than adsorption. Moreover, the CH4 dissociation rate increases with the rising temperature and is sensitive to the level of oxygen coverage on the surface. In addition, the activation energies for the adsorptive dissociation of CH4 are determined by fixed-temperature simulations from 400 to 1000 K through the changes of CH4 concentration and are found to be 3.27 and 2.28 kcal molâ1 on 0.3 and 0.7 ML oxygen-coated Pd nanoparticles, respectively, which are consistent with density functional theory calculations and experiments
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