536 research outputs found
Tight-binding g-Factor Calculations of CdSe Nanostructures
The Lande g-factors for CdSe quantum dots and rods are investigated within
the framework of the semiempirical tight-binding method. We describe methods
for treating both the n-doped and neutral nanostructures, and then apply these
to a selection of nanocrystals of variable size and shape, focusing on
approximately spherical dots and rods of differing aspect ratio. For the
negatively charged n-doped systems, we observe that the g-factors for
near-spherical CdSe dots are approximately independent of size, but show strong
shape dependence as one axis of the quantum dot is extended to form rod-like
structures. In particular, there is a discontinuity in the magnitude of
g-factor and a transition from anisotropic to isotropic g-factor tensor at
aspect ratio ~1.3. For the neutral systems, we analyze the electron g-factor of
both the conduction and valence band electrons. We find that the behavior of
the electron g-factor in the neutral nanocrystals is generally similar to that
in the n-doped case, showing the same strong shape dependence and discontinuity
in magnitude and anisotropy. In smaller systems the g-factor value is dependent
on the details of the surface model. Comparison with recent measurements of
g-factors for CdSe nanocrystals suggests that the shape dependent transition
may be responsible for the observations of anomalous numbers of g-factors at
certain nanocrystal sizes.Comment: 15 pages, 6 figures. Fixed typos to match published versio
Ice-nucleating particle emissions from biomass combustion and the potential importance of soot aerosol
Ice-nucleating particles (INPs) are required for initial ice crystal formation in clouds at temperatures warmer than about -36°C and thus play a crucial role in cloud and precipitation formation. Biomass burning has been found to be a source of INPs in previous studies and is also a major contributor to atmospheric black carbon (BC) concentrations. This study focuses on isolating the BC contribution to the INP population associated with biomass combustion. Emissions of condensation mode INPs from a number of globally relevant biomass fuels were measured at -30°C and above water saturation as fires progressed from ignition to extinguishment in a laboratory setting. Number emissions of INPs were found to be highest during intense flaming combustion (modified combustion efficiency\u3e0.95). Overall, combustion emissions from 13 of 22 different biomass fuel types produced measurable INP concentrations for at least one replicate experiment. On average, all burns that produced measureable INPs had higher combustion efficiency, which is associated with higher BC emissions, than those that did not produce measureable INPs. Across all burns that produced measureable INPs, concentrations ranged from 0.1 to 10 cm-3, and the median emission factor was about 2 × 107 INPs per kilogram of fuel burned. For a subset of the burns, the contribution of refractory black carbon (rBC) to INP concentrations was determined by removing rBC via laser-induced incandescence. Reductions in INPs of 0-70% were observed, indicating an important contribution of rBC particles to INP concentrations for some burns, especially marsh grasses
Magnetotunneling spectroscopy of mesoscopic correlations in two-dimensional electron systems
An approach to experimentally exploring electronic correlation functions in
mesoscopic regimes is proposed. The idea is to monitor the mesoscopic
fluctuations of a tunneling current flowing between the two layers of a
semiconductor double-quantum-well structure. From the dependence of these
fluctuations on external parameters, such as in-plane or perpendicular magnetic
fields, external bias voltages, etc., the temporal and spatial dependence of
various prominent correlation functions of mesoscopic physics can be
determined. Due to the absence of spatially localized external probes, the
method provides a way to explore the interplay of interaction and localization
effects in two-dimensional systems within a relatively unperturbed environment.
We describe the theoretical background of the approach and quantitatively
discuss the behavior of the current fluctuations in diffusive and ergodic
regimes. The influence of both various interaction mechanisms and localization
effects on the current is discussed. Finally a proposal is made on how, at
least in principle, the method may be used to experimentally determine the
relevant critical exponents of localization-delocalization transitions.Comment: 15 pages, 3 figures include
Heirloom rice in Ifugao: an ‘anti-commodity’ in the process of commodification
We analyse the marketing of ‘heirloom rices’ produced in the Cordillera mountains of northern Luzon, the Philippines, as the commodification of a historical ‘anti-commodity’. We contend that, historically, rice was produced for social, cultural and spiritual purposes but not primarily for sale or trade. The Ifugaos were able to sustain terraced wet-rice cultivation within a system of ‘escape agriculture’ because they were protected from Spanish interference by the friction of terrain and distance. ‘Heirloom rice’ is a boundary concept that enables social entrepreneurs to commodify traditional landraces. We analyse the implications for local rice production and conservation efforts.Templeton Foundatio
Relationship between CD4 T cell turnover, cellular differentiation and HIV persistence during ART
The precise role of CD4 T cell turnover in maintaining HIV persistence during antiretroviral therapy (ART) has not yet been well characterized. In resting CD4 T cell subpopulations from 24 HIV-infected ART-suppressed and 6 HIV-uninfected individuals, we directly measured cellular turnover by heavy water labeling, HIV reservoir size by integrated HIV-DNA (intDNA) and cell-associated HIV-RNA (caRNA), and HIV reservoir clonality by proviral integration site sequencing. Compared to HIV-negatives, ART-suppressed individuals had similar fractional replacement rates in all subpopulations, but lower absolute proliferation rates of all subpopulations other than effector memory (TEM) cells, and lower plasma IL-7 levels (p = 0.0004). Median CD4 T cell half-lives decreased with cell differentiation from naïve to TEM cells (3 years to 3 months, p<0.001). TEM had the fastest replacement rates, were most highly enriched for intDNA and caRNA, and contained the most clonal proviral expansion. Clonal proviruses detected in less mature subpopulations were more expanded in TEM, suggesting that they were maintained through cell differentiation. Earlier ART initiation was associated with lower levels of intDNA, caRNA and fractional replacement rates. In conclusion, circulating integrated HIV proviruses appear to be maintained both by slow turnover of immature CD4 subpopulations, and by clonal expansion as well as cell differentiation into effector cells with faster replacement rates
Cetacean <i>Morbillivirus</i>: Current knowledge and future directions
We review the molecular and epidemiological characteristics of cetacean morbillivirus (CeMV) and the diagnosis and pathogenesis of associated disease, with six different strains detected in cetaceans worldwide. CeMV has caused epidemics with high mortality in odontocetes in Europe, the USA and Australia. It represents a distinct species within the Morbillivirus genus. Although most CeMV strains are phylogenetically closely related, recent data indicate that morbilliviruses recovered from Indo-Pacific bottlenose dolphins (Tursiops aduncus), from Western Australia, and a Guiana dolphin (Sotalia guianensis), from Brazil, are divergent. The signaling lymphocyte activation molecule (SLAM) cell receptor for CeMV has been characterized in cetaceans. It shares higher amino acid identity with the ruminant SLAM than with the receptors of carnivores or humans, reflecting the evolutionary history of these mammalian taxa. In Delphinidae, three amino acid substitutions may result in a higher affinity for the virus. Infection is diagnosed by histology, immunohistochemistry, virus isolation, RT-PCR, and serology. Classical CeMV-associated lesions include bronchointerstitial pneumonia, encephalitis, syncytia, and lymphoid depletion associated with immunosuppression. Cetaceans that survive the acute disease may develop fatal secondary infections and chronic encephalitis. Endemically infected, gregarious odontocetes probably serve as reservoirs and vectors. Transmission likely occurs through the inhalation of aerosolized virus but mother to fetus transmission was also reported
Measurement of a small atmospheric ratio
From an exposure of 25.5~kiloton-years of the Super-Kamiokande detector, 900
muon-like and 983 electron-like single-ring atmospheric neutrino interactions
were detected with momentum MeV/, MeV/, and
with visible energy less than 1.33 GeV. Using a detailed Monte Carlo
simulation, the ratio was measured to be , consistent with previous results from the
Kamiokande, IMB and Soudan-2 experiments, and smaller than expected from
theoretical models of atmospheric neutrino production.Comment: 14 pages with 5 figure
From chemical gardens to chemobrionics
Chemical gardens are perhaps the best example in chemistry of a
self-organizing nonequilibrium process that creates complex
structures. Many different chemical systems and materials can
form these self-assembling structures, which span at least 8
orders of magnitude in size, from nanometers to meters. Key to
this marvel is the self-propagation under fluid advection of
reaction zones forming semipermeable precipitation membranes
that maintain steep concentration gradients, with osmosis and
buoyancy as the driving forces for fluid flow. Chemical gardens
have been studied from the alchemists onward, but now in the
21st century we are beginning to understand how they can lead
us to a new domain of self-organized structures of semipermeable
membranes and amorphous as well as polycrystalline solids
produced at the interface of chemistry, fluid dynamics, and
materials science. We propose to call this emerging field
chemobrionics
A Coin Vibrational Motor Swimming at Low Reynolds Number
Low-cost coin vibrational motors, used in haptic feedback, exhibit rotational internal motion inside a rigid case. Because the motor case motion exhibits rotational symmetry, when placed into a fluid such as glycerin, the motor does not swim even though its oscillatory motions induce steady streaming in the fluid. However, a piece of rubber foam stuck to the curved case and giving the motor neutral buoyancy also breaks the rotational symmetry allowing it to swim. We measured a 1 cm diameter coin vibrational motor swimming in glycerin at a speed of a body length in 3 seconds or at 3 mm/s. The swim speed puts the vibrational motor in a low Reynolds number regime similar to bacterial motility, but because of the oscillations of the motor it is not analogous to biological organisms. Rather the swimming vibrational motor may inspire small inexpensive robotic swimmers that are robust as they contain no external moving parts. A time dependent Stokes equation planar sheet model suggests that the swim speed depends on a steady streaming velocity V stream ~ Re 1/2s U 0 where U 0 is the velocity of surface oscillations, and streaming Reynolds number Re s = U 20/(ων) for motor angular frequency ω and fluid kinematic viscosity ν
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