1,740 research outputs found
First-principles approach to the charge-transport characteristics of monolayer molecular-electronics devices: Application to hexanedithiolate devices
We report on the development of an accurate first-principles computational scheme for the charge transport characteristics of molecular monolayer junctions and its application to hexanedithiolate (C6DT) devices. Starting from the Gaussian basis set density-functional calculations of a junction model in the slab geometry and corresponding two bulk electrodes, we obtain the transmission function using the matrix Green's function method and analyze the nature of transmission channels via atomic projected density of states. Within the developed formalism, by treating isolated molecules with the supercell approach, we can investigate the current-voltage characteristics of single and parallel molecular wires in a consistent manner. For the case of single C6DT molecules stretched between Au(111) electrodes, we obtain reasonable quantitative agreement of computed conductance with a recent scanning tunneling microscope experiment result. Comparing the charge transport properties of C6DT single molecules and their monolayer counterparts in the stretched and tilted geometries, we find that the effect of intermolecular coupling and molecule tilting on the charge transport characteristics is negligible in these devices. We contrast this behavior to that of the pi-conjugated biphenyldithiolate devices we have previously considered and discuss the relative importance of molecular cores and molecule-electrode contacts for the charge transport in those devices
Brief of Amicus Curiae Academic Authors and Legal Scholars in Support of Defendants Appellees and Affirmance, Nos. 12-14676-FF, 12-15147-FF (April 25, 2013)
Structure and band gaps of Ga-(V) semiconductors: The challenge of Ga pseudopotentials
Design of gallium pseudopotentials has been investigated for use in density functional calculations of zinc-blende-type cubic phases of GaAs, GaP, and GaN. A converged construction with respect to all-electron results is described. Computed lattice constants, bulk moduli, and band gaps vary significantly depending on pseudopotential construction or exchange-correlation functional. The Kohn-Sham band gap of the Ga-(V) semiconductors exhibits a distinctive and strong sensitivity to lattice constant, with near-linear dependence of gap on lattice constant for larger lattice constants and Gamma-X crossover that changes the slope of the dependence. This crossover occurs at approximate to 98, 101, and 95% deviation from the equilibrium lattice constant for GaAs, GaP, and GaN, respectively
Analysis of the Heyd-Scuseria-Ernzerhof density functional parameter space
The Heyd-Scuseria-Ernzerhof (HSE) density functionals are popular for their
ability to improve the accuracy of standard semilocal functionals such as
Perdew-Burke-Ernzerhof (PBE), particularly for semiconductor band gaps. They
also have a reduced computational cost compared to hybrid functionals, which
results from the restriction of Fock exchange calculations to small
inter-electron separations. These functionals are defined by an overall
fraction of Fock exchange and a length scale for exchange screening. We
systematically examine this two-parameter space to assess the performance of
hybrid screened exchange (sX) functionals and to determine a balance between
improving accuracy and reducing the screening length, which can further reduce
computational costs. Three parameter choices emerge as useful: "sX-PBE" is an
approximation to the sX-LDA screened exchange density functionals based on the
local density approximation (LDA); "HSE12" minimizes the overall error over all
tests performed; and "HSE12s" is a range-minimized functional that matches the
overall accuracy of the existing HSE06 parameterization but reduces the Fock
exchange length scale by half. Analysis of the error trends over parameter
space produces useful guidance for future improvement of density functionals.Comment: 11 pages, 7 figures, accepted to the Journal of Chemical Physic
Density functional theory and DFT+U study of transition metal porphines adsorbed on Au(111) surfaces and effects of applied electric fields
We apply Density Functional Theory (DFT) and the DFT+U technique to study the
adsorption of transition metal porphine molecules on atomistically flat Au(111)
surfaces. DFT calculations using the Perdew-Burke-Ernzerhof (PBE) exchange
correlation functional correctly predict the palladium porphine (PdP) low-spin
ground state. PdP is found to adsorb preferentially on gold in a flat geometry,
not in an edgewise geometry, in qualitative agreement with experiments on
substituted porphyrins. It exhibits no covalent bonding to Au(111), and the
binding energy is a small fraction of an eV. The DFT+U technique, parameterized
to B3LYP predicted spin state ordering of the Mn d-electrons, is found to be
crucial for reproducing the correct magnetic moment and geometry of the
isolated manganese porphine (MnP) molecule. Adsorption of Mn(II)P on Au(111)
substantially alters the Mn ion spin state. Its interaction with the gold
substrate is stronger and more site-specific than PdP. The binding can be
partially reversed by applying an electric potential, which leads to
significant changes in the electronic and magnetic properties of adsorbed MnP,
and ~ 0.1 Angstrom, changes in the Mn-nitrogen distances within the porphine
macrocycle. We conjecture that this DFT+U approach may be a useful general
method for modeling first row transition metal ion complexes in a
condensed-matter setting.Comment: 14 pages, 6 figure
The residual STL volume as a metric to evaluate accuracy and reproducibility of anatomic models for 3D printing: application in the validation of 3D-printable models of maxillofacial bone from reduced radiation dose CT images.
BackgroundThe effects of reduced radiation dose CT for the generation of maxillofacial bone STL models for 3D printing is currently unknown. Images of two full-face transplantation patients scanned with non-contrast 320-detector row CT were reconstructed at fractions of the acquisition radiation dose using noise simulation software and both filtered back-projection (FBP) and Adaptive Iterative Dose Reduction 3D (AIDR3D). The maxillofacial bone STL model segmented with thresholding from AIDR3D images at 100 % dose was considered the reference. For all other dose/reconstruction method combinations, a "residual STL volume" was calculated as the topologic subtraction of the STL model derived from that dataset from the reference and correlated to radiation dose.ResultsThe residual volume decreased with increasing radiation dose and was lower for AIDR3D compared to FBP reconstructions at all doses. As a fraction of the reference STL volume, the residual volume decreased from 2.9 % (20 % dose) to 1.4 % (50 % dose) in patient 1, and from 4.1 % to 1.9 %, respectively in patient 2 for AIDR3D reconstructions. For FBP reconstructions it decreased from 3.3 % (20 % dose) to 1.0 % (100 % dose) in patient 1, and from 5.5 % to 1.6 %, respectively in patient 2. Its morphology resembled a thin shell on the osseous surface with average thickness <0.1 mm.ConclusionThe residual volume, a topological difference metric of STL models of tissue depicted in DICOM images supports that reduction of CT dose by up to 80 % of the clinical acquisition in conjunction with iterative reconstruction yields maxillofacial bone models accurate for 3D printing
A 490 GHz planar circuit balanced Nb-AlO-Nb quasiparticle mixer for radio astronomy: Application to quantitative local oscillator noise determination
This article presents a heterodyne experiment which uses a 380-520 GHz planar
circuit balanced Nb--Nb
superconductor-insulator-superconductor (SIS) quasiparticle mixer with 4-8 GHz
instantaneous intermediate frequency (IF) bandwidth to quantitatively determine
local oscillator (LO) noise. A balanced mixer is a unique tool to separate
noise at the mixer's LO port from other noise sources. This is not possible in
single-ended mixers. The antisymmetric IV characteristic of a SIS mixer further
helps to simplify the measurements. The double-sideband receiver sensitivity of
the balanced mixer is 2-4 times the quantum noise limit over the
measured frequencies with a maximum LO noise rejection of 15 dB. This work
presents independent measurements with three different LO sources that produce
the reference frequency but also an amount of near-carrier noise power which is
quantified in the experiment as a function of the LO and IF frequency in terms
of an equivalent noise temperature . In a second experiment we use only
one of two SIS mixers of the balanced mixer chip, in order to verify the
influence of near-carrier LO noise power on a single-ended heterodyne mixer
measurement. We find an IF frequency dependence of near-carrier LO noise power.
The frequency-resolved IF noise temperature slope is flat or slightly negative
for the single-ended mixer. This is in contrast to the IF slope of the balanced
mixer itself which is positive due to the expected IF roll-off of the mixer.
This indicates a higher noise level closer to the LO's carrier frequency. Our
findings imply that near-carrier LO noise has the largest impact on the
sensitivity of a receiver system which uses mixers with a low IF band, for
example superconducting hot-electron bolometer (HEB) mixers.Comment: 13 pages, 8 figures, 2 tables, see manuscript for complete abstrac
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