1,694 research outputs found
Large-scale Graphitic Thin Films Synthesized on Ni and Transferred to Insulators: Structural and Electronic Properties
We present a comprehensive study of the structural and electronic properties
of ultrathin films containing graphene layers synthesized by chemical vapor
deposition (CVD) based surface segregation on polycrystalline Ni foils then
transferred onto insulating SiO2/Si substrates. Films of size up to several
mm's have been synthesized. Structural characterizations by atomic force
microscopy (AFM), scanning tunneling microscopy (STM), cross-sectional
transmission electron microscopy (XTEM) and Raman spectroscopy confirm that
such large scale graphitic thin films (GTF) contain both thick graphite regions
and thin regions of few layer graphene. The films also contain many wrinkles,
with sharply-bent tips and dislocations revealed by XTEM, yielding insights on
the growth and buckling processes of the GTF. Measurements on mm-scale
back-gated transistor devices fabricated from the transferred GTF show
ambipolar field effect with resistance modulation ~50% and carrier mobilities
reaching ~2000 cm^2/Vs. We also demonstrate quantum transport of carriers with
phase coherence length over 0.2 m from the observation of 2D weak
localization in low temperature magneto-transport measurements. Our results
show that despite the non-uniformity and surface roughness, such large-scale,
flexible thin films can have electronic properties promising for device
applications.Comment: This version (as published) contains additional data, such as cross
sectional TEM image
Identification of amino acid determinants in CYP4B1 for optimal catalytic processing of 4-ipomeanol.
Mammalian CYP4B1 enzymes are cytochrome P450 mono-oxygenases that are responsible for the bioactivation of several exogenous pro-toxins including 4-ipomeanol (4-IPO). In contrast with the orthologous rabbit enzyme, we show here that native human CYP4B1 with a serine residue at position 427 is unable to bioactivate 4-IPO and does not cause cytotoxicity in HepG2 cells and primary human T-cells that overexpress these enzymes. We also demonstrate that a proline residue in the meander region at position 427 in human CYP4B1 and 422 in rabbit CYP4B1 is important for protein stability and rescues the 4-IPO bioactivation of the human enzyme, but is not essential for the catalytic activity of the rabbit CYP4B1 protein. Systematic substitution of native and p.S427P human CYP4B1 with peptide regions from the highly active rabbit enzyme reveals that 18 amino acids in the wild-type rabbit CYP4B1 protein are key for conferring high 4-IPO metabolizing activity. Introduction of 12 of the 18 amino acids that are also present at corresponding positions in other human CYP4 family members into the p.S427P human CYP4B1 protein results in a mutant human enzyme (P+12) that is as stable and as active as the rabbit wild-type CYP4B1 protein. These 12 mutations cluster in the predicted B-C loop through F-helix regions and reveal new amino acid regions important to P450 enzyme stability. Finally, by minimally re-engineering the human CYP4B1 enzyme for efficient activation of 4-IPO, we have developed a novel human suicide gene system that is a candidate for adoptive cellular therapies in humans
Development and geometry of isotropic and directional shrinkage crack patterns
We have studied shrinkage crack patterns which form when a thin layer of an
alumina/water slurry dries. Both isotropic and directional drying were studied.
The dynamics of the pattern formation process and the geometric properties of
the isotropic crack patterns are similar to what is expected from recent
models, assuming weak disorder. There is some evidence for a gradual increase
in disorder as the drying layer become thinner, but no sudden transition, in
contrast to what has been seen in previous experiments. The morphology of the
crack patterns is influenced by drying gradients and front propagation effects,
with sharp gradients having a strong orienting and ordering effect.Comment: 8 pages, 11 figures, 8 in jpg format, 3 in postscript. See also
http://mobydick.physics.utoronto.ca/mud.htm
Multiple Imputation Ensembles (MIE) for dealing with missing data
Missing data is a significant issue in many real-world datasets, yet there are no robust methods for dealing with it appropriately. In this paper, we propose a robust approach to dealing with missing data in classification problems: Multiple Imputation Ensembles (MIE). Our method integrates two approaches: multiple imputation and ensemble methods and compares two types of ensembles: bagging and stacking. We also propose a robust experimental set-up using 20 benchmark datasets from the UCI machine learning repository. For each dataset, we introduce increasing amounts of data Missing Completely at Random. Firstly, we use a number of single/multiple imputation methods to recover the missing values and then ensemble a number of different classifiers built on the imputed data. We assess the quality of the imputation by using dissimilarity measures. We also evaluate the MIE performance by comparing classification accuracy on the complete and imputed data. Furthermore, we use the accuracy of simple imputation as a benchmark for comparison. We find that our proposed approach combining multiple imputation with ensemble techniques outperform others, particularly as missing data increases
Quark Imaging in the Proton Via Quantum Phase-Space Distributions
We develop the concept of quantum phase-space (Wigner) distributions for
quarks and gluons in the proton. To appreciate their physical content, we
analyze the contraints from special relativity on the interpretation of elastic
form factors, and examine the physics of the Feynman parton distributions in
the proton's rest frame. We relate the quark Wigner functions to the
transverse-momentum dependent parton distributions and generalized parton
distributions, emphasizing the physical role of the skewness parameter. We show
that the Wigner functions allow to visualize quantum quarks and gluons using
the language of the classical phase space. We present two examples of the quark
Wigner distributions and point out some model-independent features.Comment: 20 pages with 3 fiture
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Evaluating forecasts of extreme events for hydrological applications: an approach for screening unfamiliar performance measures
Many different performance measures have been developed to evaluate field predictions in meteorology. However, a researcher or practitioner encountering a new or unfamiliar measure may have difficulty in interpreting its results, which may lead to them avoiding new measures and relying on those that are familiar. In the context of evaluating forecasts of extreme events for hydrological applications, this article aims to promote the use of a range of performance measures. Some of the types of performance measures that are introduced in order to demonstrate a six-step approach to tackle a new measure. Using the example of the European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble precipitation predictions for the Danube floods of July and August 2002, to show how to use new performance measures with this approach and the way to choose between different performance measures based on their suitability for the task at hand is shown. Copyright © 2008 Royal Meteorological Societ
Maternal Responsivity Predicts Language Development in Young Children With Fragile X Syndrome
The relationship between early maternal responsivity and later child communication outcomes in young children with fragile X syndrome was investigated. Data were obtained from 55 motherâchild dyads over a 36-month period. Performance data were obtained at each measurement point from video observations of four different contexts. These were coded for (a) child communication behaviors, (b) parent responsivity, and (c) behavior management behaviors. Results indicate that early maternal responsivity predicts the level of four important child language outcomes at 36 months of age after controlling for child developmental level and autism symptomology
Unraveling hadron structure with generalized parton distributions
The generalized parton distributions, introduced nearly a decade ago, have
emerged as a universal tool to describe hadrons in terms of quark and gluonic
degrees of freedom. They combine the features of form factors, parton densities
and distribution amplitudes--the functions used for a long time in studies of
hadronic structure. Generalized parton distributions are analogous to the
phase-space Wigner quasi-probability function of non-relativistic quantum
mechanics which encodes full information on a quantum-mechanical system. We
give an extensive review of main achievements in the development of this
formalism. We discuss physical interpretation and basic properties of
generalized parton distributions, their modeling and QCD evolution in the
leading and next-to-leading orders. We describe how these functions enter a
wide class of exclusive reactions, such as electro- and photo-production of
photons, lepton pairs, or mesons. The theory of these processes requires and
implies full control over diverse corrections and thus we outline the progress
in handling higher-order and higher-twist effects. We catalogue corresponding
results and present diverse techniques for their derivations. Subsequently, we
address observables that are sensitive to different characteristics of the
nucleon structure in terms of generalized parton distributions. The ultimate
goal of the GPD approach is to provide a three-dimensional spatial picture of
the nucleon, direct measurement of the quark orbital angular momentum, and
various inter- and multi-parton correlations.Comment: 370 pages, 62 figures; Dedicated to Anatoly V. Efremov on occasion of
his 70th anniversar
Human embryonic stem cell-derived neurons establish region-specific, long-range projections in the adult brain
While the availability of pluripotent stem cells has opened new prospects for generating neural donor cells for nervous system repair, their capability to integrate with adult brain tissue in a structurally relevant way is still largely unresolved. We addressed the potential of human embryonic stem cell-derived long-term self-renewing neuroepithelial stem cells (lt-NES cells) to establish axonal projections after transplantation into the adult rodent brain. Transgenic and species-specific markers were used to trace the innervation pattern established by transplants in the hippocampus and motor cortex. In vitro, lt-NES cells formed a complex axonal network within several weeks after the initiation of differentiation and expressed a composition of surface receptors known to be instrumental in axonal growth and pathfinding. In vivo, these donor cells adopted projection patterns closely mimicking endogenous projections in two different regions of the adult rodent brain. Hippocampal grafts placed in the dentate gyrus projected to both the ipsilateral and contralateral pyramidal cell layers, while axons of donor neurons placed in the motor cortex extended via the external and internal capsule into the cervical spinal cord and via the corpus callosum into the contralateral cortex. Interestingly, acquisition of these region-specific projection profiles was not correlated with the adoption of a regional phenotype. Upon reaching their destination, human axons established ultrastructural correlates of synaptic connections with host neurons. Together, these data indicate that neurons derived from human pluripotent stem cells are endowed with a remarkable potential to establish orthotopic long-range projections in the adult mammalian brain
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