51 research outputs found
Ab initio GW electron-electron interaction effects in Quantum Transport
We present an ab initio approach to electronic transport in nanoscale systems
which includes electronic correlations through the GW approximation. With
respect to Landauer approaches based on density-functional theory (DFT), we
introduce a physical quasiparticle electronic-structure into a non-equilibrium
Green's function theory framework. We use an equilibrium non-selfconsistent
self-energy considering both full non-hermiticity and dynamical
effects. The method is applied to a real system, a gold mono-atomic chain. With
respect to DFT results, the conductance profile is modified and reduced by to
the introduction of diffusion and loss-of-coherence effects. The linear
response conductance characteristic appear to be in agreement with experimental
results.Comment: 5 pages, 4 figures, refused by PR
Monolayer and Bilayer Perfluoropentacene on Cu(111)
Perfluoropentacene (PFP), an n-type organic semiconductor, is deposited at monolayer and bilayer coverages on Cu(111). Scanning tunneling microscopy at various bias voltages is used to investigate the geometric and electronic structures of the layer. The appearances of the first layer and second layer differ, likely due to perturbation of the first layer electronic structure by the substrate. This has been previously observed for pentacene (Pn), the isostructural p-type organic semiconductor. The PFP film has a unit cell of (4, −3,3 4) relative to the substrate, which is larger than that of Pn/Cu(111), representing a half-integer increment in each direction
Amine-Gold Linked Single-Molecule Junctions: Experiment and Theory
The measured conductance distribution for single molecule benzenediamine-gold
junctions, based on 59,000 individual conductance traces recorded while
breaking a gold point contact in solution, has a clear peak at 0.0064 G
with a width of 40%. Conductance calculations based on density functional
theory (DFT) for 15 distinct junction geometries show a similar spread.
Differences in local structure have a limited influence on conductance because
the amine-Au bonding motif is well-defined and flexible. The average calculated
conductance (0.046 G) is seven times larger than experiment, suggesting
the importance of many-electron corrections beyond DFT
Orbital textures and charge density waves in transition metal dichalcogenides
Low-dimensional electron systems, as realized naturally in graphene or
created artificially at the interfaces of heterostructures, exhibit a variety
of fascinating quantum phenomena with great prospects for future applications.
Once electrons are confined to low dimensions, they also tend to spontaneously
break the symmetry of the underlying nuclear lattice by forming so-called
density waves; a state of matter that currently attracts enormous attention
because of its relation to various unconventional electronic properties. In
this study we reveal a remarkable and surprising feature of charge density
waves (CDWs), namely their intimate relation to orbital order. For the
prototypical material 1T-TaS2 we not only show that the CDW within the
two-dimensional TaS2-layers involves previously unidentified orbital textures
of great complexity. We also demonstrate that two metastable stackings of the
orbitally ordered layers allow to manipulate salient features of the electronic
structure. Indeed, these orbital effects enable to switch the properties of
1T-TaS2 nanostructures from metallic to semiconducting with technologically
pertinent gaps of the order of 200 meV. This new type of orbitronics is
especially relevant for the ongoing development of novel, miniaturized and
ultra-fast devices based on layered transition metal dichalcogenides
Infrared Nanoimaging of Hydrogenated Perovskite Nickelate Synaptic Devices
Solid-state devices made from correlated oxides such as perovskite nickelates
are promising for neuromorphic computing by mimicking biological synaptic
function. However, comprehending dopant action at the nanoscale poses a
formidable challenge to understanding the elementary mechanisms involved. Here,
we perform operando infrared nanoimaging of hydrogen-doped correlated
perovskite, neodymium nickel oxide (H-NdNiO3) devices and reveal how an applied
field perturbs dopant distribution at the nanoscale. This perturbation leads to
stripe phases of varying conductivity perpendicular to the applied field, which
define the macroscale electrical characteristics of the devices. Hyperspectral
nano-FTIR imaging in conjunction with density functional theory calculations
unveil a real-space map of multiple vibrational states of H-NNO associated with
OH stretching modes and their dependence on the dopant concentration. Moreover,
the localization of excess charges induces an out-of-plane lattice expansion in
NNO which was confirmed by in-situ - x-ray diffraction and creates a strain
that acts as a barrier against further diffusion. Our results and the
techniques presented here hold great potential to the rapidly growing field of
memristors and neuromorphic devices wherein nanoscale ion motion is
fundamentally responsible for function.Comment: 30 pages, 5 figures in the main text and 5 figures in the
Supplementary Materia
Goos-H\"{a}nchen-like shifts for Dirac fermions in monolayer graphene barrier
We investigate the Goos-H\"{a}nchen-like shifts for Dirac fermions in
transmission through a monolayer graphene barrier. The lateral shifts, as the
functions of the barrier's width and the incidence angle, can be negative and
positive in Klein tunneling and classical motion, respectively. Due to their
relations to the transmission gap, the lateral shifts can be enhanced by the
transmission resonances when the incidence angle is less than the critical
angle for total reflection, while their magnitudes become only the order of
Fermi wavelength when the incidence angle is larger than the critical angle.
These tunable beam shifts can also be modulated by the height of potential
barrier and the induced gap, which gives rise to the applications in
graphene-based devices.Comment: 5 pages, 5 figure
Highly Conducting pi-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes
We measure electronic conductance through single conjugated molecules bonded
to Au metal electrodes with direct Au-C covalent bonds using the scanning
tunneling microscope based break-junction technique. We start with molecules
terminated with trimethyltin end groups that cleave off in situ resulting in
formation of a direct covalent sigma bond between the carbon backbone and the
gold metal electrodes. The molecular carbon backbone used in this study consist
of a conjugated pi-system that has one terminal methylene group on each end,
which bonds to the electrodes, achieving large electronic coupling of the
electrodes to the pi-system. The junctions formed with the prototypical example
of 1,4-dimethylenebenzene show a conductance approaching one conductance
quantum (G0 = 2e2/h). Junctions formed with methylene terminated oligophenyls
with two to four phenyl units show a hundred-fold increase in conductance
compared with junctions formed with amine-linked oligophenyls. The conduction
mechanism for these longer oligophenyls is tunneling as they exhibit an
exponential dependence of conductance with oligomer length. In addition,
density functional theory based calculations for the Au-xylylene-Au junction
show near-resonant transmission with a cross-over to tunneling for the longer
oligomers.Comment: Accepted to the Journal of the American Chemical Society as a
Communication
Quantum Resistance Standard Based on Epitaxial Graphene
We report development of a quantum Hall resistance standard accurate to a few
parts in a billion at 300 mK and based on large area epitaxial graphene. The
remarkable precision constitutes an improvement of four orders of magnitude
over the best results obtained in exfoliated graphene and is similar to the
accuracy achieved in well-established semiconductor standards. Unlike the
traditional resistance standards the novel graphene device is still accurately
quantized at 4.2 K, vastly simplifying practical metrology. This breakthrough
was made possible by exceptional graphene quality achieved with scalable
silicon carbide technology on a wafer scale and shows great promise for future
large scale applications in electronics.Comment: Submitte
Spin Channels in Functionalized Graphene Nanoribbons
We characterize the transport properties of functionalized graphene
nanoribbons using extensive first-principles calculations based on density
functional theory (DFT) that encompass both monovalent and divalent ligands,
hydrogenated defects and vacancies. We find that the edge metallic states are
preserved under a variety of chemical environments, while bulk conducting
channels can be easily destroyed by either hydrogenation or ion or electron
beams, resulting in devices that can exhibit spin conductance polarization
close to unity.Comment: 14 pages, 5 figure
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