3,514 research outputs found
The Peruvian upwelling ecosystem: dynamics and interactions
Upwelling, Ecosystems, Fishery biology, Fishery oceanography, Conferences, Peru,
Electronic and atomic shell structure in aluminum nanowires
We report experiments on aluminum nanowires in ultra-high vacuum at room
temperature that reveal a periodic spectrum of exceptionally stable structures.
Two "magic" series of stable structures are observed: At low conductance, the
formation of stable nanowires is governed by electronic shell effects whereas
for larger contacts atomic packing dominates. The crossover between the two
regimes is found to be smooth. A detailed comparison of the experimental
results to a theoretical stability analysis indicates that while the main
features of the observed electron-shell structure are similar to those of
alkali and noble metals, a sequence of extremely stable wires plays a unique
role in Aluminum. This series appears isolated in conductance histograms and
can be attributed to "superdeformed" non-axisymmetric nanowires.Comment: 15 pages, 9 figure
Cluster-based density-functional approach to quantum transport through molecular and atomic contacts
We present a cluster-based density-functional approach to model charge
transport through molecular and atomic contacts. The electronic structure of
the contacts is determined in the framework of density functional theory, and
the parameters needed to describe transport are extracted from finite clusters.
A similar procedure, restricted to nearest-neighbor interactions in the
electrodes, has been presented by Damle et al. [Chem. Phys. 281, 171 (2002)].
Here, we show how to systematically improve the description of the electrodes
by extracting bulk parameters from sufficiently large metal clusters. In this
way we avoid problems arising from the use of nonorthogonal basis functions.
For demonstration we apply our method to electron transport through Au contacts
with various atomic-chain configurations and to a single-atom contact of Al.Comment: 18 pages, 13 figure
Verifiably Truthful Mechanisms
It is typically expected that if a mechanism is truthful, then the agents
would, indeed, truthfully report their private information. But why would an
agent believe that the mechanism is truthful? We wish to design truthful
mechanisms, whose truthfulness can be verified efficiently (in the
computational sense). Our approach involves three steps: (i) specifying the
structure of mechanisms, (ii) constructing a verification algorithm, and (iii)
measuring the quality of verifiably truthful mechanisms. We demonstrate this
approach using a case study: approximate mechanism design without money for
facility location
Older adults place lower value on choice relative to young adults
Choice is highly valued in modern society, from the supermarket to the hospital; however, it remains unknown
whether older and younger adults place the same value on increased choice. The current investigation tested
whether 53 older ( M age = 75.44 years) versus 53 younger adults ( M age = 19.58 years) placed lower value on increased
choice by examining the monetary amounts they were willing to pay for increased prescription drug coverage
options — important given the recently implemented Medicare prescription drug program. Results indicate
that older adults placed lower value on increasing choice sets relative to younger adults, who placed progressively
higher value on increasingly larger choice sets. These results are discussed regarding their implications for theory
and policy
Modeling elastic and photoassisted transport in organic molecular wires: length dependence and current-voltage characteristics
Using a pi-orbital tight-binding model, we study the elastic and
photoassisted transport properties of metal-molecule-metal junctions based on
oligophenylenes of varying lengths. The effect of monochromatic light is
modeled with an ac voltage over the contact. We first show how the low-bias
transmission function can be obtained analytically, using methods previously
employed for simpler chain models. In particular, the decay coefficient of the
off-resonant transmission is extracted by considering both a finite-length
chain and infinitely extended polyphenylene. Based on these analytical results,
we discuss the length-dependence of the linear-response conductance, the
thermopower, and the light-induced enhancement of the conductance in the limit
of weak intensity and low frequency. In general the conductance-enhancement is
calculated numerically as a function of the light frequency. Finally, we
compute the current-voltage characteristics at finite dc voltages, and show
that in the low-voltage regime, the effect of low-frequency light is to induce
current steps with a voltage separation determined by twice the frequency.
These effects are more pronounced for longer molecules. We study two different
profiles for the dc and ac voltages, and it is found that the results are
robust with respect to such variations. Although we concentrate here on the
specific model of oligophenylenes, the results should be qualitatively similar
for many other organic molecules with a large enough electronic gap.Comment: 15 pages, 11 figures, minor corrections to old versio
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