1,243 research outputs found
Systematic errors in global air-sea CO2 flux caused by temporal averaging of sea-level pressure
International audienceLong-term temporal averaging of meteorological data, such as wind speed and air pressure, can cause large errors in air-sea carbon flux estimates. Other researchers have already shown that time averaging of wind speed data creates large errors in flux due to the non-linear dependence of the gas transfer velocity on wind speed (Bates and Merlivat, 2001). However, in general, wind speed is negatively correlated with air pressure, and a given fractional change in the pressure of dry air produces an equivalent fractional change in the atmospheric partial pressure of carbon dioxide (<i>p</i>CO<sub>2air</sub>). Thus low pressure systems cause a drop in <i>p</i>CO<sub>2air</sub>, which together with the associated high winds, promotes outgassing/reduces uptake of CO<sub>2</sub> from the ocean. Here we quantify the errors in global carbon flux estimates caused by using monthly or climatological pressure data to calculate <i>p</i>CO<sub>2air</sub> (and thus ignoring the covariance of wind and pressure) over the period 1990?1999, using two common parameterisations for gas transfer velocity (Wanninkhof, 1992 (W92) and Wanninkhof and McGillis, 1999 (WM99)). Results show that on average, compared with estimates made using 6 hourly pressure data, the global oceanic sink is systematically overestimated by 7% (W92) and 10% (WM99) when monthly mean pressure is used, and 9% (W92) and 12% (WM99) when climatological pressure is used
Effects of J-gate potential and interfaces on donor exchange coupling in the Kane quantum computer architecture
We calculate the electron exchange coupling for a phosphorus donor pair in
silicon perturbed by a J-gate potential and the boundary effects of the silicon
host geometry. In addition to the electron-electron exchange interaction we
also calculate the contact hyperfine interaction between the donor nucleus and
electron as a function of the varying experimental conditions. Donor
separation, depth of the P nuclei below the silicon oxide layer and J-gate
voltage become decisive factors in determining the strength of both the
exchange coupling and the hyperfine interaction - both crucial components for
qubit operations in the Kane quantum computer. These calculations were
performed using an anisotropic effective-mass Hamiltonian approach. The
behaviour of the donor exchange coupling as a function of the device parameters
varied provides relevant information for the experimental design of these
devices.Comment: 15 pages, 15 figures. Accepted for Journal of Physics: Condensed
Matte
Voltage Control of Exchange Coupling in Phosphorus Doped Silicon
Motivated by applications to quantum computer architectures we study the
change in the exchange interaction between neighbouring phosphorus donor
electrons in silicon due to the application of voltage biases to surface
control electrodes. These voltage biases create electro-static fields within
the crystal substrate, perturbing the states of the donor electrons and thus
altering the strength of the exchange interaction between them. We find that
control gates of this kind can be used to either enhance, or reduce the
strength of the interaction, by an amount that depends both on the magnitude
and orientation of the donor separation.Comment: 5 Pages, 5 Figure
Molecular orbital calculations of two-electron states for P donor solid-state spin qubits
We theoretically study the Hilbert space structure of two neighbouring P
donor electrons in silicon-based quantum computer architectures. To use
electron spins as qubits, a crucial condition is the isolation of the electron
spins from their environment, including the electronic orbital degrees of
freedom. We provide detailed electronic structure calculations of both the
single donor electron wave function and the two-electron pair wave function. We
adopted a molecular orbital method for the two-electron problem, forming a
basis with the calculated single donor electron orbitals. Our two-electron
basis contains many singlet and triplet orbital excited states, in addition to
the two simple ground state singlet and triplet orbitals usually used in the
Heitler-London approximation to describe the two-electron donor pair wave
function. We determined the excitation spectrum of the two-donor system, and
study its dependence on strain, lattice position and inter donor separation.
This allows us to determine how isolated the ground state singlet and triplet
orbitals are from the rest of the excited state Hilbert space. In addition to
calculating the energy spectrum, we are also able to evaluate the exchange
coupling between the two donor electrons, and the double occupancy probability
that both electrons will reside on the same P donor. These two quantities are
very important for logical operations in solid-state quantum computing devices,
as a large exchange coupling achieves faster gating times, whilst the magnitude
of the double occupancy probability can affect the error rate.Comment: 15 pages (2-column
Perineal resuturing versus expectant management following vaginal delivery complicated by a dehisced wound (PREVIEW): A pilot and feasibility randomised controlled trial
Objective: To establish the feasibility of conducting a definitive randomised controlled trial (RCT) comparing the effectiveness of resuturing versus expectant management for dehisced perineal wounds. Design: A multicentre pilot and feasibility RCT. Setting Ten UK maternity units from July 2011 to July 2013. Population: Eligible women with a dehisced perineal wound within 2 weeks of childbirth. Methods: The interventions were resuturing or expectancy. Randomisation was via web or telephone, stratified by participating centre. Blinding was not possible due to the nature of the interventions. Analysis was by intention-to-treat. Outcome: The primary outcome measure was wound healing at 6-8 weeks. Results: The study revealed a number of feasibility issues, particularly strong patient and clinician preference for treatment options at recruiting centres and the timing of the primary outcome measure. Thirty-four women were randomised (17 in each arm). Data from 33 women were analysed on an intention-to-treat analysis to obtain preliminary estimates of effect size. There was a difference in wound healing at 2 weeks favouring resuturing (OR 20.00, 95% CI 2.04 to 196.37, p=0.004). However, by 6-8 weeks all but one wound in both groups had healed. Conclusions: PREVIEW revealed a number of feasibility issues, which impacted on recruitment rate. These will have to be taken into account in the design of any future definitive study. In this feasibility study, resuturing was associated with quicker wound healing and women reported higher satisfaction rates with the outcome at 3 months
B-meson decay constants: a more complete picture from full lattice QCD
We extend the picture of -meson decay constants obtained in lattice QCD
beyond those of the , and to give the first full lattice QCD
results for the , and . We use improved NonRelativistic QCD
for the valence quark and the Highly Improved Staggered Quark (HISQ) action
for the lighter quarks on gluon field configurations that include the effect of
, and quarks in the sea with quark masses going down to
physical values. For the ratio of vector to pseudoscalar decay constants, we
find = 0.941(26), = 0.953(23) (both
less than 1.0) and = 0.988(27). Taking correlated
uncertainties into account we see clear indications that the ratio increases as
the mass of the lighter quark increases. We compare our results to those using
the HISQ formalism for all quarks and find good agreement both on decay
constant values when the heaviest quark is a and on the dependence on the
mass of the heaviest quark in the region of the . Finally, we give an
overview plot of decay constants for gold-plated mesons, the most complete
picture of these hadronic parameters to date.Comment: 20 pages, 9 figures. Minor updates to the discussion in several
places and some additional reference
Alternative selection of processing additives to enhance the lifetime of OPVs
The use of processing additives is known to accelerate the degradation of Organic Photovoltaics (OPVs) and therefore, this paper studies the impact of selecting alternative processing additives for PCPDTBT:PC71BM solar cells in order to improve the stability. The use of naphthalene-based processing additives has been undertaken, which is shown to reduce the initial power conversion efficiency by 23%-42%, primarily due to a decrease in the short-circuit current density, but also fill factor. However, the stability is greatly enhanced by using such additives, with the long term stability (T50%) enhanced by a factor of four. The results show that there is a trade-off between initial performance and stability to consider when selecting the initial process additives. XPS studies have provided some insight into the decreased degradation and show that using 1-chloronaphthalene (ClN) leads to reduced morphology changes and reduced oxidation of the thiophene-ring within the PCPDTBT backbone.HW would like to thank Bangor University for his 125 scholarship enabling his PhD study. JK would like to thank Sêr Cymru national research network in Advanced Engineering and Materials and the Newton Research Collaboration Programme scheme through the Royal Academy for Engineerin
Chemical and device degradation in PCPDTBT: PCBM solar cells using XPS and ToF-SIMS
Analysis of the degradation routes for PCPDTBT-based solar cells under illumination and in the presence of air have been conducted using a combination of X-ray Photoelectron Spectroscopy (XPS), Time-Of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and solar cell device data. After ageing, XPS studies show that PCPDTBT appears as an oxygen-containing polymer, with data indicating that a break-up in the aromatic rings, formation of sulphates at the thiophene ring, chain scission in the polymer backbone and also loss of side chains. XPS studies on active layers blends of PCPDTBT and PCBM also show significant changes in the vertical composition during ageing, with increased enrichment of PCPDTBT observed at the top surface and that the use of a processing additive (ODT) has a negative impact on the morphological stability. TOF-SIMS has been used to study electrode degradation during ageing experiments leads to migration of indium and tin ions into the active layer in non-inverted devices, but is eliminated for inverted devices.RAEng and EC financial support and Prof TJ Lewis (Bangor) for technical suppor
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