457 research outputs found
A pseudopotential study of electron-hole excitations in colloidal, free-standing InAs quantum dots
Excitonic spectra are calculated for free-standing, surface passivated InAs
quantum dots using atomic pseudopotentials for the single-particle states and
screened Coulomb interactions for the two-body terms. We present an analysis of
the single particle states involved in each excitation in terms of their
angular momenta and Bloch-wave parentage. We find that (i) in agreement with
other pseudopotential studies of CdSe and InP quantum dots, but in contrast to
k.p calculations, dot states wavefunction exhibit strong odd-even angular
momentum envelope function mixing (e.g. with ) and large
valence-conduction coupling. (ii) While the pseudopotential approach produced
very good agreement with experiment for free-standing, colloidal CdSe and InP
dots, and for self-assembled (GaAs-embedded) InAs dots, here the predicted
spectrum does {\em not} agree well with the measured (ensemble average over dot
sizes) spectra. (1) Our calculated excitonic gap is larger than the PL measure
one, and (2) while the spacing between the lowest excitons is reproduced, the
spacings between higher excitons is not fit well. Discrepancy (1) could result
from surface states emission. As for (2), agreement is improved when account is
taken of the finite size distribution in the experimental data. (iii) We find
that the single particle gap scales as (not ), that the
screened (unscreened) electron-hole Coulomb interaction scales as
(), and that the eccitonic gap sclaes as . These scaling
laws are different from those expected from simple models.Comment: 12 postscript figure
Construction of one-loop SYM effective action on the mixed branch in the harmonic superspace approach
We develop a systematic approach to construct the one-loop SYM
effective action depending on both vector multiplet and
hypermultiplet background fields. Beginning with the formulation of SYM theory in terms of harmonic superfields, we construct
the one-loop effective action using the covariant harmonic
supergraphs and calculate it in harmonic superfield form for
constant Abelian strength and corresponding constant hypermultiplet
fields. The hypermultiplet-dependent effective action is derived and given by
integral over the analytic subspace of harmonic superspace. We show that each
term in the Schwinger-De Witt expansion of the low-energy effective action is
written as integral over full superspace.Comment: 35 pages, JHEP styl
Height-diameter allometry of tropical forest trees
Tropical tree height-diameter (H:D) relationships may vary by forest type and region making large-scale estimates of above-ground biomass subject to bias if they ignore these differences in stem allometry. We have therefore developed a new global tropical forest database consisting of 39 955 concurrent H and D measurements encompassing 283 sites in 22 tropical countries. Utilising this database, our objectives were:
1. to determine if H:D relationships differ by geographic region and forest type (wet to dry forests, including zones of tension where forest and savanna overlap).
2. to ascertain if the H:D relationship is modulated by climate and/or forest structural characteristics (e.g. stand-level basal area, A).
3. to develop H:D allometric equations and evaluate biases to reduce error in future local-to-global estimates of tropical forest biomass.
Annual precipitation coefficient of variation (PV), dry season length (SD), and mean annual air temperature (TA) emerged as key drivers of variation in H:D relationships at the pantropical and region scales. Vegetation structure also played a role with trees in forests of a high A being, on average, taller at any given D. After the effects of environment and forest structure are taken into account, two main regional groups can be identified. Forests in Asia, Africa and the Guyana Shield all have, on average, similar H:D relationships, but with trees in the forests of much of the Amazon Basin and tropical Australia typically being shorter at any given D than their counterparts elsewhere. The region-environment-structure model with the lowest Akaike\u27s information criterion and lowest deviation estimated stand-level H across all plots to within amedian −2.7 to 0.9% of the true value. Some of the plot-to-plot variability in H:D relationships not accounted for by this model could be attributed to variations in soil physical conditions. Other things being equal, trees tend to be more slender in the absence of soil physical constraints, especially at smaller D. Pantropical and continental-level models provided less robust estimates of H, especially when the roles of climate and stand structure in modulating H:D allometry were not simultaneously taken into account
Imaging and spectroscopy of artificial-atom states in core/shell nanocrystal quantum dots
Current imaging scanning tunneling microscopy is used to observe the
electronic wavefunctions in InAs/ZnSe core/shell nanocrystals. Images taken at
a bias corresponding to the s conduction band state show that it is localized
in the central core region, while images at higher bias probing the p state
reveal that it extends to the shell. This is supported by optical and tunneling
spectroscopy data demonstrating that the s-p gap closes upon shell growth.
Shapes of the current images resemble atom-like envelope wavefunctions of the
quantum dot calculated within a particle in a box model.Comment: to be published in Physical Review Letter
Electron and hole states in quantum-dot quantum wells within a spherical 8-band model
In order to study heterostructures composed both of materials with strongly
different parameters and of materials with narrow band gaps, we have developed
an approach, which combines the spherical 8-band effective-mass Hamiltonian and
the Burt's envelope function representation. Using this method, electron and
hole states are calculated in CdS/HgS/CdS/H_2O and CdTe/HgTe/CdTe/H_2O
quantum-dot quantum-well heterostructures. Radial components of the wave
functions of the lowest S and P electron and hole states in typical quantum-dot
quantum wells (QDQWs) are presented as a function of radius. The 6-band-hole
components of the radial wave functions of an electron in the 8-band model have
amplitudes comparable with the amplitude of the corresponding 2-band-electron
component. This is a consequence of the coupling between the conduction and
valence bands, which gives a strong nonparabolicity of the conduction band. At
the same time, the 2-band-electron component of the radial wave functions of a
hole in the 8-band model is small compared with the amplitudes of the
corresponding 6-band-hole components. It is shown that in the CdS/HgS/CdS/H_2O
QDQW holes in the lowest states are strongly localized in the well region
(HgS). On the contrary, electrons in this QDQW and both electron and holes in
the CdTe/HgTe/CdTe/H_2O QDQW are distributed through the entire dot. The
importance of the developed theory for QDQWs is proven by the fact that in
contrast to our rigorous 8-band model, there appear spurious states within the
commonly used symmetrized 8-band model.Comment: 15 pages, 5 figures, E-mail addresses: [email protected],
[email protected]
Involvement of Iron in Biofilm Formation by Staphylococcus aureus
Staphylococcus aureus is a human pathogen that forms biofilm on catheters and medical implants. The authors' earlier study established that 1,2,3,4,6-penta-O-galloyl-β-D-glucopyranose (PGG) inhibits biofilm formation by S. aureus by preventing the initial attachment of the cells to a solid surface and reducing the production of polysaccharide intercellular adhesin (PIA). Our cDNA microarray and MALDI-TOF mass spectrometric studies demonstrate that PGG treatment causes the expression of genes and proteins that are normally expressed under iron-limiting conditions. A chemical assay using ferrozine verifies that PGG is a strong iron chelator that depletes iron from the culture medium. This study finds that adding FeSO4 to a medium that contains PGG restores the biofilm formation and the production of PIA by S. aureus SA113. The requirement of iron for biofilm formation by S. aureus SA113 can also be verified using a semi-defined medium, BM, that contains an iron chelating agent, 2, 2′-dipyridyl (2-DP). Similar to the effect of PGG, the addition of 2-DP to BM medium inhibits biofilm formation and adding FeSO4 to BM medium that contains 2-DP restores biofilm formation. This study reveals an important mechanism of biofilm formation by S. aureus SA113
Nanotechnology for catalysis and solar energy conversion
This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: 'high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing' to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as described by Spanopoulos et al 'Next generation' solar cells using multiple exciton generation (MEG) from hot carriers, described in the article by Nozik and Beard, could lead to remarkable improvement in photovoltaic efficiency by using quantization effects in semiconductor nanostructures (quantum dots, wires or wells). These challenges will not be met without simultaneous improvement in nanoscale characterization methods. Terahertz spectroscopy, discussed in the article by Milot et al is one example of a method that is overcoming the difficulties associated with nanoscale materials characterization by avoiding electrical contacts to nanoparticles, allowing characterization during device operation, and enabling characterization of a single nanoparticle. Besides experimental advances, computational science is also meeting the challenges of nanomaterials synthesis. The article by Kohlstedt and Schatz discusses the computational frameworks being used to predict structure–property relationships in materials and devices, including machine learning methods, with an emphasis on organic photovoltaics. The contribution by Megarity and Armstrong presents the 'electrochemical leaf' for improvements in electrochemistry and beyond. In addition, biohybrid approaches can take advantage of efficient and specific enzyme catalysts. These articles present the nanoscience and technology at the forefront of renewable energy development that will have significant benefits to society
Background field formalism and construction of effective action for N=2, d=3 supersymmetric gauge theories
We review the background field method for three-dimensional Yang-Mills and
Chern-Simons models in N=2 superspace. Superfield proper time (heat kernel)
techniques are developed and exact expressions of heat kernels for constant
backgrounds are presented. The background field method and heat kernel
techniques are applied for evaluating the low-energy effective actions in N=2
supersymmetric Yang-Mills and Chern-Simons models as well as in N=4 and N=8 SYM
theories.Comment: 1+30 pages, dedicated to the 60 year Jubilee of Professor D.I.
Kazakov; references added. arXiv admin note: substantial text overlap with
arXiv:1010.496
Fifteen essential science advances needed for effective restoration of the world's forest landscapes
There has never been a more pressing and opportune time for science and practice to collaborate towards restoration of the world's forests. Multiple uncertainties remain for achieving successful, long-term forest landscape restoration (FLR). In this article, we use expert knowledge and literature review to identify knowledge gaps that need closing to advance restoration practice, as an introduction to a landmark theme issue on FLR and the UN Decade on Ecosystem Restoration. Aligned with an Adaptive Management Cycle for FLR, we identify 15 essential science advances required to facilitate FLR success for nature and people. They highlight that the greatest science challenges lie in the conceptualization, planning and assessment stages of restoration, which require an evidence base for why, where and how to restore, at realistic scales. FLR and underlying sciences are complex, requiring spatially explicit approaches across disciplines and sectors, considering multiple objectives, drivers and trade-offs critical for decision-making and financing. The developing tropics are a priority region, where scientists must work with stakeholders across the Adaptive Management Cycle. Clearly communicated scientific evidence for action at the outset of restoration planning will enable donors, decision makers and implementers to develop informed objectives, realistic targets and processes for accountability. This article paves the way for 19 further articles in this theme issue, with author contributions from across the world. This introduction article is part of the theme issue ‘Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration’
Ultrafast optical generation of coherent phonons in CdTe1-xSex quantum dots
We report on the impulsive generation of coherent optical phonons in
CdTe0.68Se0.32 nanocrystallites embedded in a glass matrix. Pump probe
experiments using femtosecond laser pulses were performed by tuning the laser
central energy to resonate with the absorption edge of the nanocrystals. We
identify two longitudinal optical phonons, one longitudinal acoustic phonon and
a fourth mode of a mixed longitudinal-transverse nature. The amplitude of the
optical phonons as a function of the laser central energy exhibits a resonance
that is well described by a model based on impulsive stimulated Raman
scattering. The phases of the coherent phonons reveal coupling between
different modes. At low power density excitations, the frequency of the optical
coherent phonons deviates from values obtained from spontaneous Raman
scattering. This behavior is ascribed to the presence of electronic impurity
states which modify the nanocrystal dielectric function and, thereby, the
frequency of the infrared-active phonons
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