288 research outputs found
Modeling inelastic phonon scattering in atomic- and molecular-wire junctions
Computationally inexpensive approximations describing electron-phonon
scattering in molecular-scale conductors are derived from the non-equilibrium
Green's function method. The accuracy is demonstrated with a first principles
calculation on an atomic gold wire. Quantitative agreement between the full
non-equilibrium Green's function calculation and the newly derived expressions
is obtained while simplifying the computational burden by several orders of
magnitude. In addition, analytical models provide intuitive understanding of
the conductance including non-equilibrium heating and provide a convenient way
of parameterizing the physics. This is exemplified by fitting the expressions
to the experimentally observed conductances through both an atomic gold wire
and a hydrogen molecule.Comment: 5 pages, 3 figure
From tunneling to contact: Inelastic signals in an atomic gold junction
The evolution of electron conductance in the presence of inelastic effects is
studied as an atomic gold contact is formed evolving from a low-conductance
regime (tunneling) to a high-conductance regime (contact). In order to
characterize each regime, we perform density functional theory (DFT)
calculations to study the geometric and electronic structures, together with
the strength of the atomic bonds and the associated vibrational frequencies.
The conductance is calculated by first evaluating the transmission of electrons
through the system, and secondly by calculating the conductance change due to
the excitation of vibrations. As found in previous studies [Paulsson et al.,
Phys. Rev. B. 72, 201101(R) (2005)] the change in conductance due to inelastic
effects permits to characterize the crossover from tunneling to contact. The
most notorious effect being the crossover from an increase in conductance in
the tunneling regime to a decrease in conductance in the contact regime when
the bias voltage matches a vibrational threshold. Our DFT-based calculations
actually show that the effect of vibrational modes in electron conductance is
rather complex, in particular when modes localized in the contact region are
permitted to extend into the electrodes. As an example, we find that certain
modes can give rise to decreases in conductance when in the tunneling regime,
opposite to the above mentioned result. Whereas details in the inelastic
spectrum depend on the size of the vibrational region, we show that the overall
change in conductance is quantitatively well approximated by the simplest
calculation where only the apex atoms are allowed to vibrate. Our study is
completed by the application of a simplified model where the relevant
parameters are obtained from the above DFT-based calculations.Comment: 8 pages, 5 figure
Unified description of inelastic propensity rules for electron transport through nanoscale junctions
We present a method to analyze the results of first-principles based
calculations of electronic currents including inelastic electron-phonon
effects. This method allows us to determine the electronic and vibrational
symmeties in play, and hence to obtain the so-called propensity rules for the
studied systems. We show that only a few scattering states -- namely those
belonging to the most transmitting eigenchannels -- need to be considered for a
complete description of the electron transport. We apply the method on
first-principles calculations of four different systems and obtain the
propensity rules in each case.Comment: 4 pages, 4 figures, 1 table
http://link.aps.org/abstract/PRL/v100/e22660
Inelastic fingerprints of hydrogen contamination in atomic gold wire systems
We present series of first-principles calculations for both pure and hydrogen
contaminated gold wire systems in order to investigate how such impurities can
be detected. We show how a single H atom or a single H2 molecule in an atomic
gold wire will affect forces and Au-Au atom distances under elongation. We
further determine the corresponding evolution of the low-bias conductance as
well as the inelastic contributions from vibrations. Our results indicate that
the conductance of gold wires is only slightly reduced from the conductance
quantum G0=2e^2/h by the presence of a single hydrogen impurity, hence making
it difficult to use the conductance itself to distinguish between various
configurations. On the other hand, our calculations of the inelastic signals
predict significant differences between pure and hydrogen contaminated wires,
and, importantly, between atomic and molecular forms of the impurity. A
detailed characterization of gold wires with a hydrogen impurity should
therefore be possible from the strain dependence of the inelastic signals in
the conductance.Comment: 5 pages, 3 figures, Contribution to ICN+T2006, Basel, Switzerland,
July-August 200
Memory and Modularity in Cell-Fate Decision Making
Genetically identical cells sharing an environment can display markedly different phenotypes. It is often unclear how much of this variation derives from chance, external signals, or attempts by individual cells to exert autonomous phenotypic programs. By observing thousands of cells for hundreds of consecutive generations under constant conditions, we dissect the stochastic decision between a solitary, motile state and a chained, sessile state in Bacillus subtilis. The motile state is memoryless, exhibiting no autonomous control over the time spent in the state, whereas chaining is tightly timed. Timing enforces coordination among related cells in the multicellular state. Further, we show that the three-protein regulatory circuit governing the decision is modular, as initiation and maintenance of chaining are genetically separable functions. As stimulation of the same initiating pathway triggers biofilm formation, we argue that autonomous timing allows a trial commitment to multicellularity that external signals could extend
Cost-effectiveness of rosuvastatin in comparison with generic atorvastatin and simvastatin in a Swedish population at high risk of cardiovascular events
Background: To assess the long-term cost-effectiveness of rosuvastatin therapy compared with generic simvastatin and generic atorvastatin in reducing the incidence of cardiovascular events and mortality in a Swedish population with Framingham risk ≥20%. Methods: A probabilistic Monte Carlo simulation model based on data from JUPITER (the Justification for the Use of statins in Prevention: an Intervention Trial Evaluating Rosuvastatin) was used to estimate the long-term cost-effectiveness of rosuvastatin 20 mg daily versus simvastatin or atorvastatin 40 mg for the prevention of cardiovascular death and morbidity. The three-stage model included cardiovascular event prevention simulating the 4 years of JUPITER, initial prevention beyond the trial, and subsequent cardiovascular event prevention. A Swedish health care payer perspective (direct costs only) was modeled for a lifetime horizon, with 2008/2009 as the costing period. Univariate and probabilistic sensitivity analyses were performed. Results: The incremental cost per quality-adjusted life-year (QALY) gained with rosuvastatin 20 mg over simvastatin or atorvastatin 40 mg ranged from SEK88,113 (rosuvastatin 20 mg versus simvastatin 40 mg; Framingham risk ≥30%; net avoidance of 34 events/1000 patients) to SEK497,542 (versus atorvastatin 40 mg: Framingham risk ≥20%; net avoidance of 11 events/1000 patients) over a lifetime horizon. Probabilistic sensitivity analyses indicated that at a willingness-to-pay threshold of SEK500,000/QALY, rosuvastatin 20 mg would be cost-effective for approximately 75%–85% of simulations relative to atorvastatin or simvastatin 40 mg. Sensitivity analyses indicated the findings to be robust. Conclusion: Rosuvastatin 20 mg is cost-effective over a lifetime horizon compared with generic simvastatin or atorvastatin 40 mg in patients at high cardiovascular risk in Sweden
Inelastic transport theory from first-principles: methodology and applications for nanoscale devices
We describe a first-principles method for calculating electronic structure,
vibrational modes and frequencies, electron-phonon couplings, and inelastic
electron transport properties of an atomic-scale device bridging two metallic
contacts under nonequilibrium conditions. The method extends the
density-functional codes SIESTA and TranSIESTA that use atomic basis sets. The
inelastic conductance characteristics are calculated using the nonequilibrium
Green's function formalism, and the electron-phonon interaction is addressed
with perturbation theory up to the level of the self-consistent Born
approximation. While these calculations often are computationally demanding, we
show how they can be approximated by a simple and efficient lowest order
expansion. Our method also addresses effects of energy dissipation and local
heating of the junction via detailed calculations of the power flow. We
demonstrate the developed procedures by considering inelastic transport through
atomic gold wires of various lengths, thereby extending the results presented
in [Frederiksen et al., Phys. Rev. Lett. 93, 256601 (2004)]. To illustrate that
the method applies more generally to molecular devices, we also calculate the
inelastic current through different hydrocarbon molecules between gold
electrodes. Both for the wires and the molecules our theory is in quantitative
agreement with experiments, and characterizes the system-specific mode
selectivity and local heating.Comment: 24 pages, 17 figure
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