74,936 research outputs found
Excitonic properties of F-centers in -alumina from First Principles Calculation
We use state-of-the art GW-BSE formalism to study electronic structure and
optical properties of oxygen vacancies in -alumina. Many body
perturbation theory within GW approximation in recent years have been used
extensively to study excited state properties of a wide range of systems.
Moreover, solving Bethe-Salpeter equation (BSE) enable us to capture excitonic
effects in a material. We compute the charge transition levels (CTLs) for
oxygen vacancies using DFT+GW formalism. We propose an alternative approach to
calculate these CTLs, which provides a more efficient way to perform
electrostatic correction required because of finite supercell sizes and
periodic boundary condition used in first principles calculations. We find that
oxygen vacancy in this material has deep donor levels, (+2/+1) at 2.5 eV and a
(+1/0) level at 3.8 eV above the VBM. We also study F-center absorption and
emission processes using constrained--DFT and BSE. Our calculated absorption
and emission energies are in excellent agreement with experimental results
Cyclic Density Functional Theory : A route to the first principles simulation of bending in nanostructures
We formulate and implement Cyclic Density Functional Theory (Cyclic DFT) -- a
self-consistent first principles simulation method for nanostructures with
cyclic symmetries. Using arguments based on Group Representation Theory, we
rigorously demonstrate that the Kohn-Sham eigenvalue problem for such systems
can be reduced to a fundamental domain (or cyclic unit cell) augmented with
cyclic-Bloch boundary conditions. Analogously, the equations of electrostatics
appearing in Kohn-Sham theory can be reduced to the fundamental domain
augmented with cyclic boundary conditions. By making use of this symmetry cell
reduction, we show that the electronic ground-state energy and the
Hellmann-Feynman forces on the atoms can be calculated using quantities defined
over the fundamental domain. We develop a symmetry-adapted finite-difference
discretization scheme to obtain a fully functional numerical realization of the
proposed approach. We verify that our formulation and implementation of Cyclic
DFT is both accurate and efficient through selected examples.
The connection of cyclic symmetries with uniform bending deformations
provides an elegant route to the ab-initio study of bending in nanostructures
using Cyclic DFT. As a demonstration of this capability, we simulate the
uniform bending of a silicene nanoribbon and obtain its energy-curvature
relationship from first principles. A self-consistent ab-initio simulation of
this nature is unprecedented and well outside the scope of any other systematic
first principles method in existence. Our simulations reveal that the bending
stiffness of the silicene nanoribbon is intermediate between that of graphene
and molybdenum disulphide. We describe several future avenues and applications
of Cyclic DFT, including its extension to the study of non-uniform bending
deformations and its possible use in the study of the nanoscale flexoelectric
effect.Comment: Version 3 of the manuscript, Accepted for publication in Journal of
the Mechanics and Physics of Solids,
http://www.sciencedirect.com/science/article/pii/S002250961630368
Scanning tunneling microscopy simulations of poly(3-dodecylthiophene) chains adsorbed on highly oriented pyrolytic graphite
We report on a novel scheme to perform efficient simulations of Scanning
Tunneling Microscopy (STM) of molecules weakly bonded to surfaces. Calculations
are based on a tight binding (TB) technique including self-consistency for the
molecule to predict STM imaging and spectroscopy. To palliate the lack of
self-consistency in the tunneling current calculation, we performed first
principles density-functional calculations to extract the geometrical and
electronic properties of the system. In this way, we can include, in the TB
scheme, the effects of structural relaxation upon adsorption on the electronic
structure of the molecule. This approach is applied to the study of
regioregular poly(3-dodecylthiophene) (P3DDT) polymer chains adsorbed on highly
oriented pyrolytic graphite (HOPG). Results of spectroscopic calculations are
discussed and compared with recently obtained experimental datComment: 15 pages plus 5 figures in a tar fil
QuantumATK: An integrated platform of electronic and atomic-scale modelling tools
QuantumATK is an integrated set of atomic-scale modelling tools developed
since 2003 by professional software engineers in collaboration with academic
researchers. While different aspects and individual modules of the platform
have been previously presented, the purpose of this paper is to give a general
overview of the platform. The QuantumATK simulation engines enable
electronic-structure calculations using density functional theory or
tight-binding model Hamiltonians, and also offers bonded or reactive empirical
force fields in many different parametrizations. Density functional theory is
implemented using either a plane-wave basis or expansion of electronic states
in a linear combination of atomic orbitals. The platform includes a long list
of advanced modules, including Green's-function methods for electron transport
simulations and surface calculations, first-principles electron-phonon and
electron-photon couplings, simulation of atomic-scale heat transport, ion
dynamics, spintronics, optical properties of materials, static polarization,
and more. Seamless integration of the different simulation engines into a
common platform allows for easy combination of different simulation methods
into complex workflows. Besides giving a general overview and presenting a
number of implementation details not previously published, we also present four
different application examples. These are calculations of the phonon-limited
mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model
simulation of lithium ion drift through a battery cathode in an external
electric field, and electronic-structure calculations of the
composition-dependent band gap of SiGe alloys.Comment: Submitted to Journal of Physics: Condensed Matte
Ab-initio transport properties of nanostructures from maximally-localized Wannier functions
We present a comprehensive first-principles study of the ballistic transport
properties of low dimensional nanostructures such as linear chains of atoms
(Al, C) and carbon nanotubes in presence of defects. A novel approach is
introduced where quantum conductance is computed from the combination of
accurate plane-wave electronic structure calculations, the evaluation of the
corresponding maximally-localized Wannier functions, and the calculation of
transport properties by a real-space Green's function method based on the
Landauer formalism. This approach is computationally very efficient, can be
straightforwardly implemented as a post-processing step in a standard
electronic-structure calculation, and allows to directly link the electronic
transport properties of a device to the nature of the chemical bonds, providing
insight onto the mechanisms that govern electron flow at the nanoscale.Comment: to be published in Phys. Rev. B (2003
Periodic Pulay method for robust and efficient convergence acceleration of self-consistent field iterations
Pulay's Direct Inversion in the Iterative Subspace (DIIS) method is one of
the most widely used mixing schemes for accelerating the self-consistent
solution of electronic structure problems. In this work, we propose a simple
generalization of DIIS in which Pulay extrapolation is performed at periodic
intervals rather than on every self-consistent field iteration, and linear
mixing is performed on all other iterations. We demonstrate through numerical
tests on a wide variety of materials systems in the framework of density
functional theory that the proposed generalization of Pulay's method
significantly improves its robustness and efficiency.Comment: Version 2 (with minor edits from version 1
Nanoscale Dielectric Capacitors Composed of Graphene and Boron Nitride Layers: A First Principles Study of High-Capacitance at Nanoscale
We investigate a nanoscale dielectric capacitor model consisting of
two-dimensional, hexagonal h-BN layers placed between two commensurate and
metallic graphene layers using self-consistent field density functional theory.
The separation of equal amounts of electric charge of different sign in
different graphene layers is achieved by applying electric field perpendicular
to the layers. The stored charge, energy, and the electric potential difference
generated between the metallic layers are calculated from the first-principles
for the relaxed structures. Predicted high-capacitance values exhibit the
characteristics of supercapacitors. The capacitive behavior of the present
nanoscale model is compared with that of the classical Helmholtz model, which
reveals crucial quantum size effects at small separations, which in turn recede
as the separation between metallic planes increases.Comment: Published version in The Journal of Physical Chemistry:
http://pubs.acs.org/doi/abs/10.1021/jp403706
Ab initio atomistic thermodynamics and statistical mechanics of surface properties and functions
Previous and present "academic" research aiming at atomic scale understanding
is mainly concerned with the study of individual molecular processes possibly
underlying materials science applications. Appealing properties of an
individual process are then frequently discussed in terms of their direct
importance for the envisioned material function, or reciprocally, the function
of materials is somehow believed to be understandable by essentially one
prominent elementary process only. What is often overlooked in this approach is
that in macroscopic systems of technological relevance typically a large number
of distinct atomic scale processes take place. Which of them are decisive for
observable system properties and functions is then not only determined by the
detailed individual properties of each process alone, but in many, if not most
cases also the interplay of all processes, i.e. how they act together, plays a
crucial role. For a "predictive materials science modeling with microscopic
understanding", a description that treats the statistical interplay of a large
number of microscopically well-described elementary processes must therefore be
applied. Modern electronic structure theory methods such as DFT have become a
standard tool for the accurate description of individual molecular processes.
Here, we discuss the present status of emerging methodologies which attempt to
achieve a (hopefully seamless) match of DFT with concepts from statistical
mechanics or thermodynamics, in order to also address the interplay of the
various molecular processes. The new quality of, and the novel insights that
can be gained by, such techniques is illustrated by how they allow the
description of crystal surfaces in contact with realistic gas-phase
environments.Comment: 24 pages including 17 figures, related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
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