1,411 research outputs found
Transverse Magnetoresistance of GaAs/AlGaAs Heterojunctions in the Presence of Parallel Magnetic Fields
We have calculated the resistivity of a GaAs\slash AlGaAs heterojunction in
the presence of both an in--plane magnetic field and a weak perpendicular
component using a semiclassical Boltzmann transport theory. These calculations
take into account fully the distortion of the Fermi contour which is induced by
the parallel magnetic field. The scattering of electrons is assumed to be due
to remote ionized impurities. A positive magnetoresistance is found as a
function of the perpendicular component, in good qualitative agreement with
experimental observations. The main source of this effect is the strong
variation of the electronic scattering rate around the Fermi contour which is
associated with the variation in the mean distance of the electronic states
from the remote impurities. The magnitude of the positive magnetoresistance is
strongly correlated with the residual acceptor impurity density in the GaAs
layer. The carrier lifetime anisotropy also leads to an observable anisotropy
in the resistivity with respect to the angle between the current and the
direction of the in--plane magnetic field.Comment: uuencoded file containing a 26 page RevTex file and 14 postscript
figures. Submitted to Phys. Rev.
KĆrero Mai: Kaiako experiences of synchronous online teaching and learning in New Zealand
Online teaching and learning programmes allow Äkonga who live in isolated areas, or who
have differing learning requirements, to study by distance. Maintaining student engagement
in the online environment is an important aspect. This article explores how kaiako (teachers)
can engage their Äkonga (students) better in online environments. The article has a particular
emphasis on supporting MÄori learners, who represent 25% of the New Zealand school
population. Five kaiako were interviewed about their experience of teaching New Zealand
secondary school students online. The study found that the kaiako had some awareness of
bicultural values and practices, but lacked confidence in embedding it in their online
teaching, which was limited to synchronous timetabled sessions with some communication
by text and email. The time provided for online students was considerably less than for the
secondary students in traditional classrooms
Optimisation of Bioluminescent Reporters for Use with Mycobacteria
BACKGROUND: Mycobacterium tuberculosis, the causative agent of tuberculosis, still represents a major public health threat in many countries. Bioluminescence, the production of light by luciferase-catalyzed reactions, is a versatile reporter technology with multiple applications both in vitro and in vivo. In vivo bioluminescence imaging (BLI) represents one of its most outstanding uses by allowing the non-invasive localization of luciferase-expressing cells within a live animal. Despite the extensive use of luminescent reporters in mycobacteria, the resultant luminescent strains have not been fully applied to BLI. METHODOLOGY/PRINCIPAL FINDINGS: One of the main obstacles to the use of bioluminescence for in vivo imaging is the achievement of reporter protein expression levels high enough to obtain a signal that can be detected externally. Therefore, as a first step in the application of this technology to the study of mycobacterial infection in vivo, we have optimised the use of firefly, Gaussia and bacterial luciferases in mycobacteria using a combination of vectors, promoters, and codon-optimised genes. We report for the first time the functional expression of the whole bacterial lux operon in Mycobacterium tuberculosis and M. smegmatis thus allowing the development of auto-luminescent mycobacteria. We demonstrate that the Gaussia luciferase is secreted from bacterial cells and that this secretion does not require a signal sequence. Finally we prove that the signal produced by recombinant mycobacteria expressing either the firefly or bacterial luciferases can be non-invasively detected in the lungs of infected mice by bioluminescence imaging. CONCLUSIONS/SIGNIFICANCE: While much work remains to be done, the finding that both firefly and bacterial luciferases can be detected non-invasively in live mice is an important first step to using these reporters to study the pathogenesis of M. tuberculosis and other mycobacterial species in vivo. Furthermore, the development of auto-luminescent mycobacteria has enormous ramifications for high throughput mycobacterial drug screening assays which are currently carried out either in a destructive manner using LuxAB or the firefly luciferase
Multibarrier tunneling
We study the tunneling through an arbitrary number of finite rectangular
opaque barriers and generalize earlier results by showing that the total
tunneling phase time depends neither on the barrier thickness nor on the
inter-barrier separation. We also predict two novel peculiar features of the
system considered, namely the independence of the transit time (for non
resonant tunneling) and the resonant frequency on the number of barriers
crossed, which can be directly tested in photonic experiments. A thorough
analysis of the role played by inter-barrier multiple reflections and a
physical interpretation of the results obtained is reported, showing that
multibarrier tunneling is a highly non-local phenomenon.Comment: RevTex, 7 pages, 1 eps figur
Evidence for charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction
High real-space-resolution atomic pair distribution functions of
La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using
high-energy x-ray powder diffraction to study the size and shape of the MnO_6
octahedron as a function of temperature and doping. In the paramagnetic
insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95
and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long
bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and
T=20K, we find a single Mn-O bond-length; however, as the metal-insulator
transition is approached either by increasing T or decreasing x, intensity
progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the
appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong
evidence that charge localized and delocalized phases coexist close to the
metal-insulator transition in the FM phase.Comment: 8 pages, 8 postscript figures, submitted to Phys. Rev.
Evolutionary distances in the twilight zone -- a rational kernel approach
Phylogenetic tree reconstruction is traditionally based on multiple sequence
alignments (MSAs) and heavily depends on the validity of this information
bottleneck. With increasing sequence divergence, the quality of MSAs decays
quickly. Alignment-free methods, on the other hand, are based on abstract
string comparisons and avoid potential alignment problems. However, in general
they are not biologically motivated and ignore our knowledge about the
evolution of sequences. Thus, it is still a major open question how to define
an evolutionary distance metric between divergent sequences that makes use of
indel information and known substitution models without the need for a multiple
alignment. Here we propose a new evolutionary distance metric to close this
gap. It uses finite-state transducers to create a biologically motivated
similarity score which models substitutions and indels, and does not depend on
a multiple sequence alignment. The sequence similarity score is defined in
analogy to pairwise alignments and additionally has the positive semi-definite
property. We describe its derivation and show in simulation studies and
real-world examples that it is more accurate in reconstructing phylogenies than
competing methods. The result is a new and accurate way of determining
evolutionary distances in and beyond the twilight zone of sequence alignments
that is suitable for large datasets.Comment: to appear in PLoS ON
On a universal photonic tunnelling time
We consider photonic tunnelling through evanescent regions and obtain general
analytic expressions for the transit (phase) time (in the opaque barrier
limit) in order to study the recently proposed ``universality'' property
according to which is given by the reciprocal of the photon frequency.
We consider different physical phenomena (corresponding to performed
experiments) and show that such a property is only an approximation. In
particular we find that the ``correction'' factor is a constant term for total
internal reflection and quarter-wave photonic bandgap, while it is
frequency-dependent in the case of undersized waveguide and distributed Bragg
reflector. The comparison of our predictions with the experimental results
shows quite a good agreement with observations and reveals the range of
applicability of the approximated ``universality'' property.Comment: RevTeX, 8 pages, 4 figures, 1 table; subsection added with a new
experiment analyzed, some other minor change
Prenatal hypoxia induces increased cardiac contractility on a background of decreased capillary density.
Background: Chronic hypoxia in utero (CHU) is one of the most common insults to fetal development and may be associated with poor cardiac recovery from ischaemia-reperfusion injury,yet the effects on normal cardiac mechanical performance are poorly understood.
Methods: Pregnant female wistar rats were exposed to hypoxia (12% oxygen, balance nitrogen)for days 10â20 of pregnancy. Pups were born into normal room air and weaned normally. At 10 weeks of age, hearts were excised under anaesthesia and underwent retrograde 'Langendorff' perfusion. Mechanical performance was measured at constant filling pressure (100 cm H2O) with intraventricular balloon. Left ventricular free wall was dissected away and capillary density estimated following alkaline phosphatase staining. Expression of SERCA2a and Nitric Oxide Synthases (NOS) proteins were estimated by immunoblotting.
Results: CHU significantly increased body mass (P < 0.001) compared with age-matched control rats but was without effect on relative cardiac mass. For incremental increases in left ventricular balloon volume, diastolic pressure was preserved. However, systolic pressure was significantly greater following CHU for balloon volume = 50 ÎŒl (P < 0.01) and up to 200 ÎŒl (P < 0.05). For higher balloon volumes systolic pressure was not significantly different from control. Developed pressures were correspondingly increased relative to controls for balloon volumes up to 250 ÎŒl (P < 0.05).Left ventricular free wall capillary density was significantly decreased in both epicardium (18%; P <0.05) and endocardium (11%; P < 0.05) despite preserved coronary flow. Western blot analysis revealed no change to the expression of SERCA2a or nNOS but immuno-detectable eNOS protein was significantly decreased (P < 0.001) in cardiac tissue following chronic hypoxia in utero.
Conclusion: These data offer potential mechanisms for poor recovery following ischaemia, including decreased coronary flow reserve and impaired angiogenesis with subsequent detrimental effects of post-natal cardiac performance
The Impact of Duty Hours on Resident Self Reports of Errors
BACKGROUND: Resident duty hour limitations aim, in part, to reduce medical errors. Residentsâ perceptions of the impact of duty hours on errors are unknown. OBJECTIVE: To determine residentsâ self-reported contributing factors, frequency, and impact of hours worked on suboptimal care practices and medical errors. DESIGN: Cross-sectional survey. SUBJECTS: 164 Internal Medicine Residents at the University of California, San Francisco. MEASUREMENTS AND RESULTS: Residents were asked to report the frequency and contributing factors of suboptimal care practices and medical errors, and how duty hours impacted these practices and aspects of resident work-life. One hundred twenty-five residents (76%) responded. The most common suboptimal care practices were working while impaired by fatigue and forgetting to transmit information during sign-out. In multivariable models, residents who felt overwhelmed with work (pâ=â0.02) and who reported spending >50% of their time in nonphysician tasks (pâ=â0.002) were more likely to report suboptimal care practices. Residents reported work-stress (a composite of fatigue, excessive workload, distractions, stress, and inadequate time) as the most frequent contributing factor to medical errors. In multivariable models, only engaging in suboptimal practices was associated with self-report of higher risk for medical errors (pâ<â0.001); working more than 80Â hours per week was not associated with suboptimal care or errors. CONCLUSION: Our findings suggest that administrative load and work stressors are more closely associated with resident reports of medical errors than the number of hours work. Efforts to reduce resident duty hours may also need to address the nature of residentsâ work to reduce errors
Overexpression of Mcl-1 exacerbates lymphocyte accumulation and autoimmune kidney disease in lpr mice
Cell death by apoptosis has a critical role during embryonic development and in maintaining tissue homeostasis. In mammals,
there are two converging apoptosis pathways: the âextrinsicâ pathway, which is triggered by engagement of cell surface âdeath
receptorsâ such as Fas/APO-1; and the âintrinsicâ pathway, which is triggered by diverse cellular stresses, and is regulated by prosurvival
and pro-apoptotic members of the Bcl-2 family of proteins. Pro-survival Mcl-1, which can block activation of the proapoptotic
proteins, Bax and Bak, appears critical for the survival and maintenance of multiple haemopoietic cell types. To
investigate the impact on haemopoiesis of simultaneously inhibiting both apoptosis pathways, we introduced the vavP-Mcl-1
transgene, which causes overexpression of Mcl-1 protein in all haemopoietic lineages, into Faslpr/lpr mice, which lack functional
Fas and are prone to autoimmunity. The combined mutations had a modest impact on myelopoiesis, primarily an increase in the
macrophage/monocyte population in Mcl-1tg/lpr mice compared with lpr or Mcl-1tg mice. The impact on lymphopoiesis was
striking, with a marked elevation in all major lymphoid subsets, including the non-conventional double-negative (DN) T cells
(TCRÎČ+
CD4â
CD8â
B220+
) characteristic of Faslpr/lpr mice. Of note, the onset of autoimmunity was markedly accelerated in Mcl-1tg/lpr
mice compared with lpr mice, and this was preceded by an increase in immunoglobulin (Ig)-producing cells and circulating
autoantibodies. This degree of impact was surprising, given the relatively mild phenotype conferred by the vavP-Mcl-1 transgene
by itself: a two- to threefold elevation of peripheral B and T cells, no significant increase in the non-conventional DN T-cell
population and no autoimmune disease. Comparison of the phenotype with that of other susceptible mice suggests that the
development of autoimmune disease in Mcl-1tg/lpr mice may be influenced not only by Ig-producing cells but also other
haemopoietic cell types
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