1,015 research outputs found
Energetics of positron states trapped at vacancies in solids
We report a computational first-principles study of positron trapping at
vacancy defects in metals and semiconductors. The main emphasis is on the
energetics of the trapping process including the interplay between the positron
state and the defect's ionic structure and on the ensuing annihilation
characteristics of the trapped state. For vacancies in covalent semiconductors
the ion relaxation is a crucial part of the positron trapping process enabling
the localization of the positron state. However, positron trapping does not
strongly affect the characteristic features of the electronic structure, e.g.,
the ionization levels change only moderately. Also in the case of metal
vacancies the positron-induced ion relaxation has a noticeable effect on the
calculated positron lifetime and momentum distribution of annihilating
electron-positron pairs.Comment: Submitted to Physical Review B on 17 April 2007. Revised version
submitted on 6 July 200
Modeling the momentum distributions of annihilating electron-positron pairs in solids
Measuring the Doppler broadening of the positron annihilation radiation or
the angular correlation between the two annihilation gamma quanta reflects the
momentum distribution of electrons seen by positrons in the
material.Vacancy-type defects in solids localize positrons and the measured
spectra are sensitive to the detailed chemical and geometric environments of
the defects. However, the measured information is indirect and when using it in
defect identification comparisons with theoretically predicted spectra is
indispensable. In this article we present a computational scheme for
calculating momentum distributions of electron-positron pairs annihilating in
solids. Valence electron states and their interaction with ion cores are
described using the all-electron projector augmented-wave method, and atomic
orbitals are used to describe the core states. We apply our numerical scheme to
selected systems and compare three different enhancement (electron-positron
correlation) schemes previously used in the calculation of momentum
distributions of annihilating electron-positron pairs within the
density-functional theory. We show that the use of a state-dependent
enhancement scheme leads to better results than a position-dependent
enhancement factor in the case of ratios of Doppler spectra between different
systems. Further, we demonstrate the applicability of our scheme for studying
vacancy-type defects in metals and semiconductors. Especially we study the
effect of forces due to a positron localized at a vacancy-type defect on the
ionic relaxations.Comment: Submitted to Physical Review B on September 1 2005. Revised
manuscript submitted on November 14 200
Anthropogenic aerosol forcing - insights from multiple estimates from aerosol-climate models with reduced complexity
This study assesses the change in anthropogenic aerosol forcing from the mid-1970s to the mid-2000s. Both decades had similar global-mean anthropogenic aerosol optical depths but substantially different global distributions. For both years, we quantify (i) the forcing spread due to model-internal variability and (ii) the forcing spread among models. Our assessment is based on new ensembles of atmosphere-only simulations with five state-of-the-art Earth system models. Four of these models will be used in the sixth Coupled Model Intercomparison Project (CMIP6; Eyring et al., 2016). Here, the complexity of the anthropogenic aerosol has been reduced in the participating models. In all our simulations, we prescribe the same patterns of the anthropogenic aerosol optical properties and associated effects on the cloud droplet number concentration. We calculate the instantaneous radiative forcing (RF) and the effective radiative forcing (ERF). Their difference defines the net contribution from rapid adjustments. Our simulations show a model spread in ERF from -0.4 to -0.9 W m(-2). The standard deviation in annual ERF is 0.3 W m(-2), based on 180 individual estimates from each participating model. This result implies that identifying the model spread in ERF due to systematic differences requires averaging over a sufficiently large number of years. Moreover, we find almost identical ERFs for the mid-1970s and mid-2000s for individual models, although there are major model differences in natural aerosols and clouds. The model-ensemble mean ERF is -0.54 W m(-2) for the pre-industrial era to the mid-1970s and -0.59 W m(-2) for the pre-industrial era to the mid-2000s. Our result suggests that comparing ERF changes between two observable periods rather than absolute magnitudes relative to a poorly constrained pre-industrial state might provide a better test for a model's ability to represent transient climate changes.Peer reviewe
Analysis of electron-positron momentum spectra of metallic alloys as supported by first-principles calculations
Electron-positron momentum distributions measured by the coincidence Doppler
broadening method can be used in the chemical analysis of the annihilation
environment, typically a vacancy-impurity complex in a solid. In the present
work, we study possibilities for a quantitative analysis, i.e., for
distinguishing the average numbers of different atomic species around the
defect. First-principles electronic structure calculations self-consistently
determining electron and positron densities and ion positions are performed for
vacancy-solute complexes in Al-Cu, Al-Mg-Cu, and Al-Mg-Cu-Ag alloys. The
ensuing simulated coincidence Doppler broadening spectra are compared with
measured ones for defect identification. A linear fitting procedure, which uses
the spectra for positrons trapped at vacancies in pure constituent metals as
components, has previously been employed to find the relative percentages of
different atomic species around the vacancy [A. Somoza et al. Phys. Rev. B 65,
094107 (2002)]. We test the reliability of the procedure by the help of
first-principles results for vacancy-solute complexes and vacancies in
constituent metals.Comment: Submitted to Physical Review B on September 19 2006. Revised version
submitted on November 8 2006. Published on February 14 200
Positron localization effects on the Doppler broadening of the annihilation line: Aluminum as a case study
The coincidence Doppler broadening (CDB) technique is widely used to measure one-dimensional momentum distributions of annihilation photons, with the aim of obtaining information on the chemical environment of open-volume defects. However, the quantitative analysis of CDB spectra needs to include also purely geometrical effects. A demonstration is given here, on the basis of CDB spectra measured in quenched and in deformed pure aluminum. The comparison of the experimental results with ab initio computations shows that the observed differences come from the difference in free volume seen by positrons trapped in quenched-in vacancies or in vacancylike defects associated to dislocations. The computation reproduces accurately all details of CDB spectra, including the peak near the Fermi break, which is due to the zero-point motion of the confined positron.Peer reviewe
BVOC–aerosol–climate feedbacks investigated using NorESM
Both higher temperatures and increased CO2 concentrations are
(separately) expected to increase the emissions of biogenic volatile organic
compounds (BVOCs). This has been proposed to initiate negative climate
feedback mechanisms through increased formation of secondary organic aerosol
(SOA). More SOA can make the clouds more reflective, which can provide a
cooling. Furthermore, the increase in SOA formation has also been proposed to
lead to increased aerosol scattering, resulting in an increase in diffuse
radiation. This could boost gross primary production (GPP) and further
increase BVOC emissions. In this study, we have used the Norwegian Earth
System Model (NorESM) to investigate both these feedback mechanisms. Three
sets of experiments were set up to quantify the feedback with respect to (1)Â doubling
the CO2, (2)Â increasing temperatures corresponding to a doubling of
CO2 and (3)Â the combined effect of both doubling CO2 and a
warmer climate. For each of these experiments, we ran two simulations, with
identical setups, except for the BVOC emissions. One simulation was run with
interactive BVOC emissions, allowing the BVOC emissions to respond to changes
in CO2 and/or climate. In the other simulation, the BVOC emissions
were fixed at present-day conditions, essentially turning the feedback off.
The comparison of these two simulations enables us to investigate each step
along the feedback as well as estimate their overall relevance for the future
climate.
We find that the BVOC feedback can have a significant impact on the climate.
The annual global BVOC emissions are up to 63 % higher when the feedback
is turned on compared to when the feedback is turned off, with the largest
response when both CO2 and climate are changed. The higher BVOC
levels lead to the formation of more SOA mass (max 53 %) and result in
more particles through increased new particle formation as well as larger
particles through increased condensation. The corresponding changes in the
cloud properties lead to a −0.43 W m−2 stronger net cloud forcing.
This effect becomes about 50 % stronger when the model is run with
reduced anthropogenic aerosol emissions, indicating that the feedback will
become even more important as we decrease aerosol and precursor emissions. We
do not find a boost in GPP due to increased aerosol scattering on a global
scale. Instead, the fate of the GPP seems to be controlled by the BVOC effects
on the clouds. However, the higher aerosol scattering associated with the
higher BVOC emissions is found to also contribute with a potentially
important enhanced negative direct forcing (−0.06 W m−2). The global
total aerosol forcing associated with the feedback is −0.49 W m−2,
indicating that it has the potential to offset about 13 % of the forcing
associated with a doubling of CO2.</p
Positron unveiling high mobility graphene stack interfaces in Li-ion cathodes
Carbon-based coatings in Li-ion battery cathodes improve electron conductivity and enable rapid charging. However, the mechanism is not well understood. Here, we address this question by using positrons as non-destructive probes to investigate nano-interfaces within cathodes. We calculate the positron annihilation lifetime in a graphene stack LiCoO2 heterojunction using an ab initio method with a non-local density approximation to accurately describe the electron-positron correlation. This ideal heterostructure represents the standard carbon-based coating performed on cathode nanoparticles to improve the conduction properties of the cathode. We characterize the interface between LiCoO2 and graphene as a p-type Schottky junction and find positron surface states. The intensity of the lifetime component for these positron surface states serves as a descriptor for positive ion ultra-fast mobility. Consequently, optimizing the carbon layer by enhancing this intensity and by analogizing Li-ion adatoms on graphene layers with positrons at surfaces can improve the design of fast-charging channels.Carbon layers in Li-ion battery cathodes are important for fast charging but the underlying mechanism is still not well understood. Here, ab initio calculations of the positron annihilation lifetime in graphene stack LiCoO2 heterojunction gives insights into ultra-fast ion mobility
Brightening of the global cloud field by nitric acid and the associated radiative forcing
Clouds cool Earth's climate by reflecting 20% of the incoming solar energy, while also trapping part of the outgoing radiation. The effect of human activities on clouds is poorly understood, but the present-day anthropogenic cooling via changes of cloud albedo and lifetime could be of the same order as warming from anthropogenic addition in CO<sub>2</sub>. Soluble trace gases can increase water condensation to particles, possibly leading to activation of smaller aerosols and more numerous cloud droplets. We have studied the effect of nitric acid on the aerosol indirect effect with the global aerosol-climate model ECHAM5.5-HAM2. Including the nitric acid effect in the model increases cloud droplet number concentrations globally by 7%. The nitric acid contribution to the present-day cloud albedo effect was found to be −0.32 W m<sup>−2</sup> and to the total indirect effect −0.46 W m<sup>−2</sup>. The contribution to the cloud albedo effect is shown to increase to −0.37 W m<sup>−2</sup> by the year 2100, if considering only the reductions in available cloud condensation nuclei. Overall, the effect of nitric acid can play a large part in aerosol cooling during the following decades with decreasing SO<sub>2</sub> emissions and increasing NO<sub>x</sub> and greenhouse gases
Future biogeochemical forcing in Eastern Siberia: cooling or warming?
Over-proportional warming in the northern high latitudes, and large carbon stocks in boreal and (sub)arctic ecosystems have raised concerns as to whether substantial positive climate feedbacks from biogeochemical process responses should be expected. Such feedbacks occur if increasing temperatures lead to e.g., a net release of CO2 or CH4. However, temperature-enhanced emissions of biogenic volatile organic compounds (BVOC) have been shown to contribute to a cooling feedback via growth of secondary organic aerosol (SOA), and related aerosol forcings. Combining measurements in Eastern Siberia with model-based estimates of vegetation and permafrost dynamics, BVOC emissions and aerosol growth, we show here that the additional climate forcing from changes in ecosystem CO2 balance and BVOC-SOA interactions nearly cancel on a regional scale. The interactions between emissions and vegetation dynamics that underlie individual forcing estimates are complex and highlight the importance of addressing ecosystem-climate feedbacks in consistent, process-based model frameworks that account for a multitude of system processes
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