3,084 research outputs found
Functionality in single-molecule devices: Model calculations and applications of the inelastic electron tunneling signal in molecular junctions
We analyze how functionality could be obtained within single-molecule devices
by using a combination of non-equilibrium Green's functions and ab-initio
calculations to study the inelastic transport properties of single-molecule
junctions. First we apply a full non-equilibrium Green's function technique to
a model system with electron-vibration coupling. We show that the features in
the inelastic electron tunneling spectra (IETS) of the molecular junctions are
virtually independent of the nature of the molecule-lead contacts. Since the
contacts are not easily reproducible from one device to another, this is a very
useful property. The IETS signal is much more robust versus modifications at
the contacts and hence can be used to build functional nanodevices. Second, we
consider a realistic model of a organic conjugated molecule. We use ab-initio
calculations to study how the vibronic properties of the molecule can be
controlled by an external electric field which acts as a gate voltage. The
control, through the gate voltage, of the vibron frequencies and (more
importantly) of the electron-vibron coupling enables the construction of
functionality: non-linear amplification and/or switching is obtained from the
IETS signal within a single-molecule device.Comment: Accepted for publication in Journal of Chemical Physic
Non-equilibrium inelastic electronic transport: Polarization effects and vertex corrections to the self-consistent Born approximation
We study the effect of electron-vibron interactions on the inelastic
transport properties of single-molecule nanojunctions. We use the
non-equilibrium Green's functions technique and a model Hamiltonian to
calculate the effects of second-order diagrams (double-exchange DX and
dressed-phonon DPH diagrams) on the electron-vibration interaction and consider
their effects across the full range of parameter space. The DX diagram,
corresponding to a vertex correction, introduces an effective dynamical
renormalization of the electron-vibron coupling in both the purely inelastic
and the inelastic-resonant features of the IETS. The purely inelastic features
correspond to an applied bias around the energy of a vibron, while the
inelastic-resonant features correspond to peaks (resonance) in the conductance.
The DPH diagram affects only the inelastic resonant features. We also discuss
the circumstances in which the second-order diagrams may be approximated in the
study of more complex model systems.Comment: To be published in PR
The Circular Velocity Curve of the Milky Way from to kpc
We measure the circular velocity curve of the Milky Way with
the highest precision to date across Galactocentric distances of kpc. Our analysis draws on the -dimensional phase-space coordinates of
luminous red-giant stars, for which we previously determined
precise parallaxes using a data-driven model that combines spectral data from
APOGEE with photometric information from WISE, 2MASS, and Gaia. We derive the
circular velocity curve with the Jeans equation assuming an axisymmetric
gravitational potential. At the location of the Sun we determine the circular
velocity with its formal uncertainty to be with systematic uncertainties at the
level. We find that the velocity curve is gently but significantly declining at
, with a systematic uncertainty of
, beyond the inner kpc. We exclude the inner
kpc from our analysis due to the presence of the Galactic bar, which
strongly influences the kinematic structure and requires modeling in a
non-axisymmetric potential. Combining our results with external measurements of
the mass distribution for the baryonic components of the Milky Way from other
studies, we estimate the Galaxy's dark halo mass within the virial radius to be
and a local dark matter
density of .Comment: Accepted for publication in ApJ. All data can be downloaded here:
https://doi.org/10.5281/zenodo.146805
Structure and dynamics of Saturn's outer magnetosphere and boundary regions
In 1979-1981, the three USA spacecraft Pioneer 11 and Voyagers 1 and 2 discovered and explored the magnetosphere of Saturn to the limited extent possible on flyby trajectories. Considerable variation in the locations of the bow shock (BS) and magnetopause (MP) surfaces were observed in association with variable solar wind conditions and, during the Voyager 2 encounter, possible immersion in Jupiter's distant magnetic tail. The limited number of BS and MP crossings were concentrated near the subsolar region and the dawn terminator, and that fact, together with the temporal variability, makes it difficult to assess the three dimensional shape of the sunward magnetospheric boundary. The combined BS and MP crossing positions from the three spacecraft yield an average BS-to-MP stagnation point distance ratio of 1.29 +/- 0.10. This is near the 1.33 value for the Earth's magnetosphere, implying a similar sunward shape at Saturn. Study of the structure and dynamical behavior of the outer magnetosphere, both in the sunward hemisphere and the magnetotail region using combined plasma and magnetic field data, suggest that Saturn's magnetosphere is more similar to that of Earth than that of Jupiter
The Jovian magnetotail and its current sheet
Analyses of Voyager magnetic field measurements have extended the understanding of the structural and temporal characteristics of Jupiter's magnetic tail. The magnitude of the magnetic field in the lobes of the tail is found to decrease with Jovicentric distance approximately as r to he-1.4, compared with the power law exponent of -1.7 found for the rate of decrease along the Pioneer 10 outbound trajectory. Voyager observations of magnetic field component variations with Jovicentric distance in the tail do not support the uniform radial plasma outflow model derived from Pioneer data. Voyager 2 has shown that the azimuthal current sheet which surrounds Jupiter in the inner and middle magnetosphere extends tailward (in the anti-Sun direction) to a distance of at least 100 R sub J. In the tail this current sheet consists of a plasma sheet and embedded neutral sheet. In the region of the tail where the sheet is observed, the variation of the magnetic field as a result of the sheet structure and its 10 hr periodic motion is the dominant variation seen
A kiloparsec-scale nuclear stellar disk in the milky way as a possible explanation of the high velocity peaks in the galactic bulge
The Apache Point Observatory Galactic Evolution Experiment has measured the stellar velocities of red giant stars in the inner Milky Way. We confirm that the line of sight velocity distributions (LOSVDs) in the mid-plane exhibit a second peak at high velocities, whereas those at | b| =2^\circ do not. We use a high resolution simulation of a barred galaxy, which crucially includes gas and star formation, to guide our interpretation of the LOSVDs. We show that the data are fully consistent with the presence of a thin, rapidly rotating, nuclear disk extending to ∼1 kpc. This nuclear disk is orientated perpendicular to the bar and is likely to be composed of stars on x2 orbits. The gas in the simulation is able to fall onto such orbits, leading to stars populating an orthogonal disk
Integrated assurance assessment of a reconfigurable digital flight control system
The integrated application of reliability, failure effects and system simulator methods in establishing the airworthiness of a flight critical digital flight control system (DFCS) is demonstrated. The emphasis was on the mutual reinforcement of the methods in demonstrating the system safety
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