365 research outputs found
Backward diode composed of a metallic and semiconducting nanotube
The conditions necessary for a nanotube junction connecting a metallic and
semiconducting nanotube to rectify the current are theoretically investigated.
A tight binding model is used for the analysis, which includes the Hartree-Fock
approximation and the Green's function method.
It is found that the junction has a behavior similar to the backward diode if
the gate electrode is located nearby and the Fermi level of the semiconducting
tube is near the gap.
Such a junction would be advantageous since the required length for the
rectification could be reduced.Comment: 4 pages, RevTeX, uses epsf.st
Subband population in a single-wall carbon nanotube diode
We observe current rectification in a molecular diode consisting of a
semiconducting single-wall carbon nanotube and an impurity. One half of the
nanotube has no impurity, and it has a current-voltage (I-V) charcteristic of a
typical semiconducting nanotube. The other half of the nanotube has the
impurity on it, and its I-V characteristic is that of a diode. Current in the
nanotube diode is carried by holes transported through the molecule's
one-dimensional subbands. At 77 Kelvin we observe a step-wise increase in the
current through the diode as a function of gate voltage, showing that we can
control the number of occupied one-dimensional subbands through electrostatic
doping.Comment: to appear in Physical Review Letters. 4 pages & 3 figure
Band structures of periodic carbon nanotube junctions and their symmetries analyzed by the effective mass approximation
The band structures of the periodic nanotube junctions are investigated by
the effective mass theory and the tight binding model.
The periodic junctions are constructed by introducing pairs of a pentagonal
defect and a heptagonal defect periodically in the carbon nanotube.
We treat the periodic junctions whose unit cell is composed by two kinds of
metallic nanotubes with almost same radii, the ratio of which is between 0.7
and 1 .
The discussed energy region is near the undoped Fermi level where the channel
number is kept to two, so there are two bands.
The energy bands are expressed with closed analytical forms by the effective
mass theory with some assumptions, and they coincide well with the numerical
results by the tight binding model. Differences between the two methods are
also discussed. Origin of correspondence between the band structures and the
phason pattern discussed in Phys. Rev. B {\bf 53}, 2114, is clarified. The
width of the gap and the band are in inverse proportion to the length of the
unit cell, which is the sum of the lengths measured along the tube axis in each
tube part and along 'radial' direction in the junction part. The degeneracy and
repulsion between the two bands are determined only from symmetries.Comment: RevTeX, gif fil
Microplastics in Ghanaian coastal lagoon sediments: Their occurrence and spatial distribution
This is the author accepted manuscript.Each year millions of tonnes of plastic are produced worldwide and around 8 million tons are deposited into our marine environment. Rivers comprise the major conduit for plastic transport with their deltas, estuaries and coastal lagoons being the key interface between lotic aquatic and the oceanic environment. However, we have very little knowledge of the role of coastal lagoons in the plastic pollution pathways. We present the spatial and temporal distribution and abundance of microplastics in sediments from two coastal lagoons in Ghana, West Africa. Sediment cores were taken from Mukwei Lagoon, Kpeshie Lagoon and from the mangroves at Kpeshie Lagoon; areas approximately 5-15km East from the centre of Accra. Microplastics were detected in all samples with a decreasing trend recorded from West to East. All three sites recorded a similar depth profile for plastics: after an initial increase from the surface samples, there was a significant decrease in microplastic concentrations with depth.Academy of Medical Science
Size, Shape and Low Energy Electronic Structure of Carbon Nanotubes
A theory of the long wavelength low energy electronic structure of
graphite-derived nanotubules is presented. The propagating electrons are
described by wrapping a massless two dimensional Dirac Hamiltonian onto a
curved surface. The effects of the tubule size, shape and symmetry are included
through an effective vector potential which we derive for this model. The rich
gap structure for all straight single wall cylindrical tubes is obtained
analytically in this theory, and the effects of inhomogeneous shape
deformations on nominally metallic armchair tubes are analyzed.Comment: 5 pages, 3 postscript figure
The effect of absent blood flow on the zebrafish cerebral and trunk vasculature
The role of blood flow in vascular development is complex and context-dependent. In this study, we quantify the effect of the lack of blood flow on embryonic vascular development on two vascular beds, namely the cerebral and trunk vasculature in zebrafish. We perform this by analysing vascular topology, endothelial cell (EC) number, EC distribution, apoptosis, and inflammatory response in animals with normal blood flow or absent blood flow. We find that absent blood flow reduced vascular area and EC number significantly in both examined vascular beds, but the effect is more severe in the cerebral vasculature, and severity increases over time. Absent blood flow leads to an increase in non-EC-specific apoptosis without increasing tissue inflammation, as quantified by cerebral immune cell numbers and nitric oxide. Similarly, while stereotypic vascular patterning in the trunk is maintained, intra-cerebral vessels show altered patterning, which is likely to be due to vessels failing to initiate effective fusion and anastomosis rather than sprouting or path-seeking. In conclusion, blood flow is essential for cellular survival in both the trunk and cerebral vasculature, but particularly intra-cerebral vessels are affected by the lack of blood flow, suggesting that responses to blood flow differ between these two vascular beds
Conductance of Distorted Carbon Nanotubes
We have calculated the effects of structural distortions of armchair carbon
nanotubes on their electrical transport properties. We found that the bending
of the nanotubes decreases their transmission function in certain energy ranges
and leads to an increased electrical resistance. Electronic structure
calculations show that these energy ranges contain localized states with
significant - hybridization resulting from the increased curvature
produced by bending. Our calculations of the contact resistance show that the
large contact resistances observed for SWNTs are likely due to the weak
coupling of the NT to the metal in side bonded NT-metal configurations.Comment: 5 pages RevTeX including 4 figures, submitted to PR
Phonon dispersion and electron-phonon interaction in peanut-shaped fullerene polymers
We reveal that the periodic radius modulation peculiar to one-dimensional
(1D) peanut-shaped fullerene (C) polymers exerts a strong influence on
their low-frequency phonon states and their interactions with mobile electrons.
The continuum approximation is employed to show the zone-folding of phonon
dispersion curves, which leads to fast relaxation of a radial breathing mode in
the 1D C polymers. We also formulate the electron-phonon interaction
along the deformation potential theory, demonstrating that only a few set of
electron and phonon modes yields a significant magnitude of the interaction
relevant to the low-temperature physics of the system. The latter finding gives
an important implication for the possible Peierls instability of the C
polymers suggested in the earlier experiment.Comment: 9 pages, 8 figure
Resonant Andreev reflections in superconductor-carbon-nanotube devices
Resonant Andreev reflection through superconductor-carbon-nanotube devices
was investigated theoretically with a focus on the superconducting proximity
effect. Consistent with a recent experiment, we find that for high transparency
devices on-resonance, the Andreev current is characterized by a large value and
a resistance dip; low-transparency off-resonance devices give the opposite
result. We also give evidence that the observed low-temperature transport
anomaly may be a natural result of Andreev reflection process
Charting a course for smartphones and wearables to transform population health research
The use of data from smartphones and wearable devices has huge potential for population health research, given the high level of device ownership; the range of novel health-relevant data types available from consumer devices; and the frequency and duration with which data are, or could be, collected. Yet, the uptake and success of large-scale mobile health research in the last decade have not met this intensely promoted opportunity. We make the argument that digital person-generated health data are required and necessary to answer many top priority research questions, using illustrative examples taken from the James Lind Alliance Priority Setting Partnerships. We then summarize the findings from 2 UK initiatives that considered the challenges and possible solutions for what needs to be done and how such solutions can be implemented to realize the future opportunities of digital person-generated health data for clinically important population health research. Examples of important areas that must be addressed to advance the field include digital inequality and possible selection bias; easy access for researchers to the appropriate data collection tools, including how best to harmonize data items; analysis methodologies for time series data; patient and public involvement and engagement methods for optimizing recruitment, retention, and public trust; and methods for providing research participants with greater control over their data. There is also a major opportunity, provided through the linkage of digital person-generated health data to routinely collected data, to support novel population health research, bringing together clinician-reported and patient-reported measures. We recognize that well-conducted studies need a wide range of diverse challenges to be skillfully addressed in unison (eg, challenges regarding epidemiology, data science and biostatistics, psychometrics, behavioral and social science, software engineering, user interface design, information governance, data management, and patient and public involvement and engagement). Consequently, progress would be accelerated by the establishment of a new interdisciplinary community where all relevant and necessary skills are brought together to allow for excellence throughout the life cycle of a research study. This will require a partnership of diverse people, methods, and technologies. If done right, the synergy of such a partnership has the potential to transform many millions of people’s lives for the bette
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