11 research outputs found
Simple and efficient LCAO basis sets for the diffuse states in carbon nanostructures
We present a simple way to describe the lowest unoccupied diffuse states in
carbon nanostructures in density functional theory (DFT) calculations using a
minimal LCAO (linear combination of atomic orbitals) basis set. By comparing
plane wave basis calculations, we show how these states can be captured by
adding long-range orbitals to the standard LCAO basis sets for the extreme
cases of planar sp2 (graphene) and curved carbon (C60). In particular,
using Bessel functions with a long range as additional basis functions retain a
minimal basis size. This provides a smaller and simpler atom-centered basis set
compared to the standard pseudo-atomic orbitals (PAOs) with multiple
polarization orbitals or by adding non-atom-centered states to the basis.Comment: 3 pages, 3 figure
Manipulating the voltage drop in graphene nanojunctions using a gate potential
Graphene is an attractive electrode material to contact nanostructures down
to the molecular scale since it can be gated electrostatically. Gating can be
used to control the doping and the energy level alignment in the nanojunction,
thereby influencing its conductance. Here we investigate the impact of
electrostatic gating in nanojunctions between graphene electrodes operating at
finite bias. Using first principles quantum transport simulations, we show that
the voltage drop across \emph{symmetric} junctions changes dramatically and
controllably in gated systems compared to non-gated junctions. In particular,
for \emph{p}-type(\emph{n}-type) carriers the voltage drop is located close to
the electrode with positive(negative) polarity, i.e. the potential of the
junction is pinned to the negative(positive) electrode. We trace this behaviour
back to the vanishing density of states of graphene in the proximity of the
Dirac point. Due to the electrostatic gating, each electrode exposes different
density of states in the bias window between the two different electrode Fermi
energies, thereby leading to a non-symmetry in the voltage drop across the
device. This selective pinning is found to be independent of device length when
carriers are induced either by the gate or dopant atoms, indicating a general
effect for electronic circuitry based on graphene electrodes. We envision this
could be used to control the spatial distribution of Joule heating in graphene
nanostructures, and possibly the chemical reaction rate around high potential
gradients.Comment: 6 pages, 7 figure
Large-scale tight-binding simulations of quantum transport in ballistic graphene
Graphene has proven to host outstanding mesoscopic effects involving massless
Dirac quasiparticles travelling ballistically resulting in the current flow
exhibiting light-like behaviour. A new branch of 2D electronics inspired by the
standard principles of optics is rapidly evolving, calling for a deeper
understanding of transport in large-scale devices at a quantum level. Here we
perform large-scale quantum transport calculations based on a tight-binding
model of graphene and the non-equilibrium Green's function method and include
the effects of junctions of different shape, magnetic field, and
absorptive regions acting as drains for current. We stress the importance of
choosing absorbing boundary conditions in the calculations to correctly capture
how current flows in the limit of infinite devices. As a specific application
we present a fully quantum-mechanical framework for the "2D Dirac fermion
microscope" recently proposed by B{\o}ggild [Nat. Comm. 8, 10.1038
(2017)], tackling several key electron-optical effects therein predicted via
semiclassical trajectory simulations, such as electron beam collimation,
deflection and scattering off Veselago dots. Our results confirm that a
semiclassical approach to a large extend is sufficient to capture the main
transport features in the mesoscopic limit and the optical regime, but also
that a richer electron-optical landscape is to be expected when coherence or
other purely quantum effects are accounted for in the simulations.Comment: 12 pages, 10 figure
Ab initio current-induced molecular dynamics
We extend the ab initio molecular dynamics (AIMD) method based on density
functional theory to the nonequilibrium situation where an electronic current
is present in the electronic system. The dynamics is treated using the
semi-classical generalized Langevin equation. We demonstrate how the full
anharmonic description of the inter-atomic forces is important in order to
understand the current-induced heating and the energy distribution both in
frequency and in real space
Multi-scale approach to first-principles electron transport beyond 100 nm
Multi-scale computational approaches are important for studies of novel,
low-dimensional electronic devices since they are able to capture the different
length-scales involved in the device operation, and at the same time describe
critical parts such as surfaces, defects, interfaces, gates, and applied bias,
on a atomistic, quantum-chemical level. Here we present a multi-scale method
which enables calculations of electronic currents in two-dimensional devices
larger than 100 nm, where multiple perturbed regions described by density
functional theory (DFT) are embedded into an extended unperturbed region
described by a DFT-parametrized tight-binding model. We explain the details of
the method, provide examples, and point out the main challenges regarding its
practical implementation. Finally we apply it to study current propagation in
pristine, defected and nanoporous graphene devices, injected by chemically
accurate contacts simulating scanning tunneling microscopy probes
ElectronicStructureLibrary/flook: v0.7.0
Release of flook with new build-system. The build system is made about the smeka build system.
Now it is easier to create a build
Old Law Building Reading Room
Old Law Building Reading Roomhttps://scholarship.law.ufl.edu/uf-law-photo-gallery/1037/thumbnail.jp