308 research outputs found
Oxygen impurities in NiAl: Relaxation effects
We have used a full-potential linear muffin-tin orbital method to calculate
the effects of oxygen impurities on the electronic structure of NiAl. Using the
supercell method with a 16-atom supercell we have investigated the cases where
an oxygen atom is substitutionally placed at either a nickel or an aluminum
site. Full relaxation of the atoms within the supercell was allowed. We found
that oxygen prefers to occupy a nickel site over an aluminum site with a site
selection energy of 138 mRy (21,370 K). An oxygen atom placed at an aluminum
site is found to cause a substantial relaxation of its nickel neighbors away
from it. In contrast, this steric repulsion is hardly present when the oxygen
atom occupies the nickel site and is surrounded by aluminum neighbors. We
comment on the possible relation of this effect to the pesting degradation
phenomenon (essentially spontaneous disintegration in air) in nickel
aluminides.Comment: To appear in Phys. Rev. B (Aug. 15, 2001
Ballistic electron transport in stubbed quantum waveguides: experiment and theory
We present results of experimental and theoretical investigations of electron
transport through stub-shaped waveguides or electron stub tuners (ESTs) in the
ballistic regime. Measurements of the conductance G as a function of voltages,
applied to different gates V_i (i=bottom, top, and side) of the device, show
oscillations in the region of the first quantized plateau which we attribute to
reflection resonances. The oscillations are rather regular and almost periodic
when the height h of the EST cavity is small compared to its width. When h is
increased, the oscillations become less regular and broad depressions in G
appear. A theoretical analysis, which accounts for the electrostatic potential
formed by the gates in the cavity region, and a numerical computation of the
transmission probabilities successfully explains the experimental observations.
An important finding for real devices, defined by surface Schottky gates, is
that the resonance nima result from size quantization along the transport
direction of the EST.Comment: Text 20 pages in Latex/Revtex format, 11 Postscript figures. Phys.
Rev. B,in pres
Spontaneous Coherence and Collective Modes in Double-Layer Quantum Dot Systems
We study the ground state and the collective excitations of
parabolically-confined double-layer quantum dot systems in a strong magnetic
field. We identify parameter regimes where electrons form maximum density
droplet states, quantum-dot analogs of the incompressible states of the bulk
integer quantum Hall effect. In these regimes the Hartree-Fock approximation
and the time-dependent Hartree-Fock approximations can be used to describe the
ground state and collective excitations respectively. We comment on the
relationship between edge excitations of dots and edge magneto-plasmon
excitations of bulk double-layer systems.Comment: 20 pages (figures included) and also available at
http://fangio.magnet.fsu.edu/~jhu/Paper/qdot_cond.ps, replaced to fix figure
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Brand origin identification by consumers: A classification perspective
The authors apply a classification perspective to (1) examine the extent to which consumers can identify the correct country of origin (COO) of different brands of consumer durables, (2) investigate the factors facilitating/hindering correct COO identification, and (3) trace the implications of correct/incorrect COO identification on brand evaluation. The results from a U.K. sample indicate that consumers' ability to classify brands correctly according to their origin is limited and also reveal substantial differences in the classification of different brands to their COO. Moreover, the key antecedent of correct COO identification is consumer ethnocentrism, with sociodemographics (e.g., age, gender) also playing a role. Finally, the authors find that though there are differences in brand evaluations depending on whether the correct COO was identified, such differences are not observed for all brands investigated
Ecological management of cereal stemborers in African smallholder agriculture through behavioural manipulation
1. Africa faces serious challenges in feeding its rapidly growing human
population owing to the poor productivity of maize and sorghum, the most important
staple crops formillions of smallholder farmers in the continent,with yields being among
the lowest in the world.
2. A complex of lepidopterous stemborers attack cereals in Africa. However, their
effective control is difficult, largely as a result of the cryptic and nocturnal habits of
moths, and protection provided by host stem for immature pest stages.Moreover, current
control measures are uneconomical and impractical for resource-poor farmers.
3. An ecological approach, based on companion planting, known as ‘push–pull’,
provides effective management of these pests, and involves combined use of inter- and
trap cropping systems where stemborers are attracted and trapped on trap plants with
added economic value (‘pull’), and are driven away from the cereal crop by antagonistic
intercrops (‘push’).
4. Novel defence strategies inducible by stemborer oviposition have recently been
discovered, leading to the attraction of egg and larval parasitoids, in locally adapted
maize lines but not in elite hybrids. We also established that landscape complexity did
not improve the ecosystem service of biological control, but rather provided a disservice
by acting as a ‘source’ of stemborer pests colonising the crop.
5. Here we review and provide new data on the direct and indirect effects of the
push–pull approach on stemborers and their natural enemies, including the mechanisms
involved, and highlight opportunities for exploiting intrinsic plant defences and natural
ecosystem services in pest management in smallholder farming systems in Africa
Supporting both learning and research in a UK post-1992 university library: a case study
Nationally, there has been debate on the role of research within higher education and increased interest in the teaching/research nexus. A team of Academic Liaison Librarians at Anglia Polytechnic University was awarded funding to investigate the extent to which learning resources overlap with research resources, whether researcher/teachers encourage their students to use the resources they use themselves and how far electronic resources have affected the relationship between learning and research materials. Semistructured interviews were carried out with 21 academics who are both teachers and researchers. They proved to be committed to using research in their teaching. Students were encouraged to engage with research through the recommendation of resources, seminar discussion, and researchers’ own work for reading and illustrating methodologies. Respondents claimed to be making significant use of the APU library website, online databases and journals. The majority of them were also recommending the same resources to their students. Convenience, speed and variety of information sources were quoted as some of the advantages of the new e-environment. A loss of a relationship with librarians and with the physical library was cited as an example of negative effects of the electronic resource environment
Nucleon Charge and Magnetization Densities from Sachs Form Factors
Relativistic prescriptions relating Sachs form factors to nucleon charge and
magnetization densities are used to fit recent data for both the proton and the
neutron. The analysis uses expansions in complete radial bases to minimize
model dependence and to estimate the uncertainties in radial densities due to
limitation of the range of momentum transfer. We find that the charge
distribution for the proton is significantly broad than its magnetization
density and that the magnetization density is slightly broader for the neutron
than the proton. The neutron charge form factor is consistent with the Galster
parametrization over the available range of Q^2, but relativistic inversion
produces a softer radial density. Discrete ambiguities in the inversion method
are analyzed in detail. The method of Mitra and Kumari ensures compatibility
with pQCD and is most useful for extrapolating form factors to large Q^2.Comment: To appear in Phys. Rev. C. Two new figures and accompanying text have
been added and several discussions have been clarified with no significant
changes to the conclusions. Now contains 47 pages including 21 figures and 2
table
Inhaled corticosteroids in COPD and onset of type 2 diabetes and osteoporosis: matched cohort study
Some studies suggest an association between onset and/or poor control of type 2 diabetes mellitus and inhaled corticosteroid (ICS) therapy for chronic obstructive pulmonary disease (COPD), and also between increased fracture risk and ICS therapy; however, study results are contradictory and these associations remain tentative and incompletely characterized. This matched cohort study used two large UK databases (1983–2016) to study patients (≥ 40 years old) initiating ICS or long-acting bronchodilator (LABD) for COPD from 1990–2015 in three study cohorts designed to assess the relation between ICS treatment and (1) diabetes onset (N = 17,970), (2) diabetes progression (N = 804), and (3) osteoporosis onset (N = 19,898). Patients had ≥ 1-year baseline and ≥ 2-year outcome data. Matching was via combined direct matching and propensity scores. Conditional proportional hazards regression, adjusting for residual confounding after matching, was used to compare ICS vs. LABD and to model ICS exposures. Median follow-up was 3.7–5.6 years/treatment group. For patients prescribed ICS, compared with LABD, the risk of diabetes onset was significantly increased (adjusted hazard ratio 1.27; 95% CI, 1.07–1.50), with overall no increase in risk of diabetes progression (adjusted hazard ratio 1.04; 0.87–1.25) or osteoporosis onset (adjusted hazard ratio 1.13; 0.93–1.39). However, the risks of diabetes onset, diabetes progression, and osteoporosis onset were all significantly increased, with evident dose–response relationships for all three outcomes, at mean ICS exposures of 500 µg/day or greater (vs. < 250 µg/day, fluticasone propionate–equivalent). Long-term ICS therapy for COPD at mean daily exposure of ≥ 500 µg is associated with an increased risk of diabetes, diabetes progression, and osteoporosis
Ultrafast carrier relaxation and vertical-transport phenomena in semiconductor superlattices: A Monte Carlo analysis
The ultrafast dynamics of photoexcited carriers in semiconductor superlattices is studied theoretically on the basis of a Monte Carlo solution of the coupled Boltzmann transport equations for electrons and holes. The approach allows a kinetic description of the relevant interaction mechanisms such as intra- miniband and interminiband carrier-phonon scattering processes. The energy relaxation of photoexcited carriers, as well as their vertical transport, is investigated in detail. The effects of the multiminiband nature of the superlattice spectrum on the energy relaxation process are discussed with particular emphasis on the presence of Bloch oscillations induced by an external electric field. The analysis is performed for different superlattice structures and excitation conditions. It shows the dominant role of carrier-polar-optical-phonon interaction in determining the nature of the carrier dynamics in the low-density limit. In particular, the miniband width, compared to the phonon energy, turns out to be a relevant quantity in predicting the existence of Bloch oscillations
CD4+ T cell-mediated recognition of a conserved cholesterol-dependent cytolysin epitope generates broad antibacterial immunity
CD4+ T cell-mediated immunity against Streptococcus pneumoniae (pneumococcus) can protect against recurrent bacterial colonization and invasive pneumococcal diseases (IPDs). Although such immune responses are common, the pertinent antigens have remained elusive. We identified an immunodominant CD4+ T cell epitope derived from pneumolysin (Ply), a member of the bacterial cholesterol-dependent cytolysins (CDCs). This epitope was broadly immunogenic as a consequence of presentation by the pervasive human leukocyte antigen (HLA) allotypes DPB1∗02 and DPB1∗04 and recognition via architecturally diverse T cell receptors (TCRs). Moreover, the immunogenicity of Ply427–444 was underpinned by core residues in the conserved undecapeptide region (ECTGLAWEWWR), enabling cross-recognition of heterologous bacterial pathogens expressing CDCs. Molecular studies further showed that HLA-DP4-Ply427–441 was engaged similarly by private and public TCRs. Collectively, these findings reveal the mechanistic determinants of near-global immune focusing on a trans-phyla bacterial epitope, which could inform ancillary strategies to combat various life-threatening infectious diseases, including IPDs
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