770 research outputs found
Bridging quantum and classical plasmonics with a quantum-corrected model
Electromagnetic coupling between plasmonic resonances in metallic nanoparticles allows for engineering of the optical response and generation of strong localized near-fields. Classical electrodynamics fails to describe this coupling across sub-nanometer gaps, where quantum effects become important owing to non-local screening and the spill-out of electrons. However, full quantum simulations are not presently feasible for realistically sized systems. Here we present a novel approach, the quantum-corrected model (QCM), that incorporates quantum-mechanical effects within a classical electrodynamic framework. The QCM approach models the junction between adjacent nanoparticles by means of a local dielectric response that includes electron tunnelling and tunnelling resistivity at the gap and can be integrated within a classical electrodynamical description of large and complex structures. The QCM predicts optical properties in excellent agreement with fully quantum mechanical calculations for small interacting systems, opening a new venue for addressing quantum effects in realistic plasmonic systems
Host-plant acceptance on mineral soil and humus by the pine weevil Hylobius abietis (L.)
1 The pine weevil Hylobius abietis (L.) (Coleoptera, Curculionidae) is an economically important pest of conifer forest regeneration in Europe and Asia.
2 Soil scarification, which usually exposes mineral soil, is widely used to protect seedlings from weevil attack. However, the mechanism behind this protective effect is not yet fully understood.
3 Field experiments were conducted to determine the pine weevil's responses to visual and odour stimuli from seedlings when moving on mineral soil and on undisturbed humus surface.
4 One experiment measured the number of pine weevils approaching seedlings, with and without added host odour, on mineral soil and undisturbed humus. Seedlings with added host odour attracted more weevils on both soil types. Unexpectedly, somewhat more weevils approached seedlings surrounded by mineral soil.
5 In a similar experiment, feeding attacks on seedlings planted directly in the soil were recorded. Only half as many seedlings were attacked on mineral soil as on undisturbed humus.
6 In the first experiment, the weevils were trapped 2.5 cm from the bases of the seedlings' stems, whereas they could reach the seedlings in the experiment where seedlings were planted directly in the soil. We conclude that the pine weevils' decision on whether or not to feed on a seedling is strongly influenced by the surrounding soil type and that this decision is taken in the close vicinity of the seedling. The presence of pure mineral soil around the seedling strongly reduces the likelihood that an approaching pine weevil will feed on it
Hydrogenase biomimetics with redox-active ligands: Electrocatalytic proton reduction by [Fe2(CO)4(κ2-diamine)(μ-edt)] (diamine = 2,2′-bipy, 1,10-phen)
Diiron complexes bearing redox active diamine ligands have been studied as models of the active site of [FeFe]-hydrogenases. Heating [Fe2(CO)6(μ-edt)] (edt = 1,2-ethanedithiolate) with 2,2′-bipyridine (2,2′-bipy) or 1,10-phenanthroline (1,10-phen) in MeCN in the presence of Me3NO leads to the formation of [Fe2(CO)4(κ2-2,2′-bipy)(μ-edt)] (1-edt) and [Fe2(CO)4(κ2-1,10-phen)(μ-edt)] (2-edt), respectively, in moderate yields. In the solid state the diamine resides in dibasal sites, while both dibasal and apical–basal isomers are present in solution. Both stereoisomers protonate readily upon addition of strong acids. Cyclic voltammetry in MeCN shows that both complexes undergo irreversible oxidation and reduction, proposed to be a one- and two-electron process, respectively. The structures of neutral 2-edt and its corresponding one- and two-electron reduced species have been investigated by DFT calculations. In 2-edt− the added electron occupies a predominantly ligand-based orbital, and the iron–iron bond is maintained, being only slightly elongated. Addition of the second electron affords an open-shell triplet dianion where the second electron populates an Fe–Fe σ* antibonding orbital, resulting in effective scission of the iron–iron bond. The triplet state lies 4.2 kcal mol−1 lower in energy than the closed-shell singlet dianion whose HOMO correlates nicely with the LUMO of the neutral species 2-edt. Electrocatalytic proton reduction by both complexes has been studied in MeCN using CF3CO2H as the proton source. These catalysis studies reveal that while at high acid concentrations the active catalytic species is [Fe2(CO)4(μ-H)(κ2-diamine)(μ-edt)]+, at low acid concentrations the two complexes follow different catalytic mechanisms being associated with differences in their relative rates of protonation
Resonance Lifetimes from Complex Densities
The ab-initio calculation of resonance lifetimes of metastable anions
challenges modern quantum-chemical methods. The exact lifetime of the
lowest-energy resonance is encoded into a complex "density" that can be
obtained via complex-coordinate scaling. We illustrate this with one-electron
examples and show how the lifetime can be extracted from the complex density in
much the same way as the ground-state energy of bound systems is extracted from
its ground-state density
The SkyMapper search for extremely metal-poor stars in the Large Magellanic Cloud
We present results of a search for extremely metal-poor (EMP) stars in the
Large Magellanic Cloud, which can provide crucial information about the
properties of the first stars as well as on the formation conditions prevalent
during the earliest stages of star formation in dwarf galaxies. Our search
utilised SkyMapper photometry, together with parallax and proper motion cuts
(from Gaia), colour-magnitude cuts (by selecting the red giant branch region)
and finally a metallicity-sensitive cut. Low-resolution spectra of a sample of
photometric candidates were taken using the ANU 2.3m telescope/WiFeS
spectrograph, from which 7 stars with [Fe/H] -2.75 were identified, two
of which have [Fe/H] -3. Radial velocities, derived from the CaII
triplet lines, closely match the outer rotation curve of the LMC for the
majority of the candidates in our sample. Therefore, our targets are robustly
members of the LMC based on their 6D phase-space information (coordinates,
spectrophotometric distance, proper motions and radial velocities), and they
constitute the most metal-poor stars so far discovered in this galaxy.Comment: Accepted for publication in MNRA
3D Stagger model atmospheres with FreeEOS I. Exploring the impact of microphysics on the Sun
Three-dimensional radiation-hydrodynamics (3D RHD) simulations of stellar
surface convection provide valuable insights into many problems in solar and
stellar physics. However, almost all 3D near-surface convection simulations to
date are based on solar-scaled chemical compositions, which limit their
application on stars with peculiar abundance patterns. To overcome this
difficulty, we implement the robust and widely-used FreeEOS equation of state
and our Blue opacity package into the Stagger 3D radiation-magnetohydrodynamics
code. We present a new 3D RHD model of the solar atmosphere, and demonstrate
that the mean stratification as well as the distributions of key physical
quantities are in good agreement with those of the latest Stagger solar model
atmosphere. The new model is further validated by comparing against solar
observations. The new model atmospheres reproduce the observed flux spectrum,
continuum centre-to-limb variation, and hydrogen line profiles at a
satisfactory level, thereby confirming the realism of the model and the
underlying input physics. These implementations open the prospect for studying
other stars with different -element abundance, carbon-enhanced
metal-poor stars and population II stars with peculiar chemical compositions
using 3D Stagger model atmospheres.Comment: 24 pages, 20 figures, accepted for publication in A&
Many Body Theory of Charge Transfer in Hyperthermal Atomic Scattering
We use the Newns-Anderson Hamiltonian to describe many-body electronic
processes that occur when hyperthermal alkali atoms scatter off metallic
surfaces. Following Brako and Newns, we expand the electronic many-body
wavefunction in the number of particle-hole pairs (we keep terms up to and
including a single particle-hole pair). We extend their earlier work by
including level crossings, excited neutrals and negative ions. The full set of
equations of motion are integrated numerically, without further approximations,
to obtain the many-body amplitudes as a function of time. The velocity and
work-function dependence of final state quantities such as the distribution of
ion charges and excited atomic occupancies are compared with experiment. In
particular, experiments that scatter alkali ions off clean Cu(001) surfaces in
the energy range 5 to 1600 eV constrain the theory quantitatively. The
neutralization probability of Na ions shows a minimum at intermediate
velocity in agreement with the theory. This behavior contrasts with that of
K, which shows ... (7 figures, not included. Figure requests:
[email protected])Comment: 43 pages, plain TeX, BUP-JBM-
Active quantum plasmonics
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license.The ability of localized surface plasmons to squeeze light and engineer nanoscale electromagnetic fields through electron-photon coupling at dimensions below the wavelength has turned plasmonics into a driving tool in a variety of technological applications, targeting novel and more efficient optoelectronic processes. In this context, the development of active control of plasmon excitations is a major fundamental and practical challenge. We propose a mechanism for fast and active control of the optical response of metallic nanostructures based on exploiting quantum effects in subnanometric plasmonic gaps. By applying an external dc bias across a narrow gap, a substantial change in the tunneling conductance across the junction can be induced at optical frequencies, which modifies the plasmonic resonances of the system in a reversible manner. We demonstrate the feasibility of the concept using time-dependent density functional theory calculations. Thus, along with two-dimensional structures, metal nanoparticle plasmonics can benefit from the reversibility, fast response time, and versatility of an active control strategy based on applied bias. The proposed electrical manipulation of light using quantum plasmonics establishes a new platform for many practical applications in optoelectronics.J.A. acknowledges support from the Spanish Ministry of Economy and Competitiveness through projects FIS2013-41184-P and 2015CD0010 of the Consejo Superior de Investigaciones Científicas scientific cooperation program for development
“I-COOP LIGHT” 2015. P.N. acknowledges support from the Robert A. Welch Foundation
(grant C-1222) and the Air Force Office of Science and Research (grant FA9550-15-1-0022). M.Z. acknowledges financial support from the Departamento Administrativo de Ciencia, Tecnología e Innovación–COLCIENCIAS and Facultad de Ciencias from Universidad de los Andes.Peer Reviewe
A leed analysis of the (2×1)H-Ni(110) structure
A monolayer of H atoms adsorbed on Ni(110) below 180 K forms a (2×1) structure. The unit cell exhibits a glide symmetry plane and contains two adsorbed atoms. Based on a quantitative comparison between experimental and calculated LEED I/V spectra using standard R-factors the following structure was derived: On the clean Ni(110) surface the separation between the first two atomic layers, d12, is contracted by 8.5%±1.5% with respect to the bulk value; those between the second and third and the third and fourth layer, d23 and d34, are expanded by 3.5%±1.5% and 1%±1.5%, respectively—in agreement with recent other results. In the presence of the H adlayer the contraction of d12 is reduced to 4.5%±1.5%, while the expansion of d23 is not affected within the limits of accuracy. The third interlayer spacing d34 returns to its bulk value. The H atoms occupy threefold-coordinated sites formed by two Ni atoms from the first layer and one Ni atom from the second layer which confirms previous more qualitative conclusions based on He diffraction and vibrational spectroscopy. The bond lengths between H and its neighbouring Ni atoms were determined to be equal, namely 1.72±0.1 Å
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