692 research outputs found
Effective temperature determinations of late-type stars based on 3D non-LTE Balmer line formation
Hydrogen Balmer lines are commonly used as spectroscopic effective
temperature diagnostics of late-type stars. However, the absolute accuracy of
classical methods that are based on one-dimensional (1D) hydrostatic model
atmospheres and local thermodynamic equilibrium (LTE) is still unclear. To
investigate this, we carry out 3D non-LTE calculations for the Balmer lines,
performed, for the first time, over an extensive grid of 3D hydrodynamic
STAGGER model atmospheres. For H, H, and H, we find
significant 1D non-LTE versus 3D non-LTE differences (3D effects): the outer
wings tend to be stronger in 3D models, particularly for H, while the
inner wings can be weaker in 3D models, particularly for H. For
H, we also find significant 3D LTE versus 3D non-LTE differences
(non-LTE effects): in warmer stars (K) the inner
wings tend to be weaker in non-LTE models, while at lower effective
temperatures (K) the inner wings can be stronger in
non-LTE models; the non-LTE effects are more severe at lower metallicities. We
test our 3D non-LTE models against observations of well-studied benchmark
stars. For the Sun, we infer concordant effective temperatures from H,
H, and H; however the value is too low by around 50K which could
signal residual modelling shortcomings. For other benchmark stars, our 3D
non-LTE models generally reproduce the effective temperatures to within
uncertainties. For H, the absolute 3D effects and non-LTE
effects can separately reach around 100K, in terms of inferred effective
temperatures. For metal-poor turn-off stars, 1D LTE models of H can
underestimate effective temperatures by around 150K. Our 3D non-LTE model
spectra are publicly available, and can be used for more reliable spectroscopic
effective temperature determinations.Comment: 19 pages, 10 figures, abstract abridged; accepted for publication in
Astronomy & Astrophysic
Kondo time scales for quantum dots - response to pulsed bias potentials
The response of a quantum dot in the Kondo regime to rectangular pulsed bias
potentials of various strengths and durations is studied theoretically. It is
found that the rise time is faster than the fall time, and also faster than
time scales normally associated with the Kondo problem. For larger values of
the pulsed bias, one can induce dramatic oscillations in the induced current
with a frequency approximating the splitting between the Kondo peaks that would
be present in steady state. The effect persists in the total charge transported
per pulse, which should facilitate the experimental observation of the
phenomenon.Comment: 5 pages with 4 encapsulated figures which come in separate postscript
files: latex file: text.tex figures: fig1.eps, fig2.eps, fig3.eps, fig4.ep
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
Electronic structure of small GaAs clusters
The electronic structure of small Ga_xAs_y clusters (x+y≤10) are calculated using the local density method. The calculation shows that even‐numbered clusters tend to be singlets, as opposed to odd‐numbered clusters which are open shell systems. This is in agreement with the experimental observations of even/odd alternations of the electron affinity and ionization potential. In the larger clusters, the atoms prefer an alternating bond arrangement; charge transfers are observed from Ga sites to As sites. This observation is also in agreement with recent chemisorption studies of ammonia on GaAs clusters. The close agreement between theoretical calculations and experimental results, together with the rich variation of electronic properties of GaAs clusters with composition makes GaAs clusters an ideal prototype system for the study of how electronic structure influences chemical reactivity
Benchmarking van der Waals Density Functionals with Experimental Data: Potential Energy Curves for H2 Molecules on Cu(111), (100), and (110) Surfaces
Detailed physisorption data from experiment for the H_2 molecule on low-index
Cu surfaces challenge theory. Recently, density-functional theory (DFT) has
been developed to account for nonlocal correlation effects, including van der
Waals (dispersion) forces. We show that the functional vdW-DF2 gives a
potential-energy curve, potential-well energy levels, and difference in lateral
corrugation promisingly close to the results obtained by resonant elastic
backscattering-diffraction experiments. The backscattering barrier is found
selective for choice of exchange-functional approximation. Further, the DFT-D3
and TS-vdW corrections to traditional DFT formulations are also benchmarked,
and deviations are analyzed.Comment: 15 pages, 9 figure
Concave Plasmonic Particles: Broad-Band Geometrical Tunability in the Near Infra-Red
Optical resonances spanning the Near and Short Infra-Red spectral regime were
exhibited experimentally by arrays of plasmonic nano-particles with concave
cross-section. The concavity of the particle was shown to be the key ingredient
for enabling the broad band tunability of the resonance frequency, even for
particles with dimensional aspect ratios of order unity. The atypical
flexibility of setting the resonance wavelength is shown to stem from a unique
interplay of local geometry with surface charge distributions
Plasmonic atoms and plasmonic molecules
The proposed paradigm of plasmonic atoms and plasmonic molecules allows one
to describe and predict the strongly localized plasmonic oscillations in the
clusters of nanoparticles and some other nanostructures in uniform way.
Strongly localized plasmonic molecules near the contacting surfaces might
become the fundamental elements (by analogy with Lego bricks) for a
construction of fully integrated opto-electronic nanodevices of any complexity
and scale of integration.Comment: 30 pages, 16 figure
Compilation of extended recursion in call-by-value functional languages
This paper formalizes and proves correct a compilation scheme for
mutually-recursive definitions in call-by-value functional languages. This
scheme supports a wider range of recursive definitions than previous methods.
We formalize our technique as a translation scheme to a lambda-calculus
featuring in-place update of memory blocks, and prove the translation to be
correct.Comment: 62 pages, uses pi
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