286 research outputs found
Long-range interactions between substitutional nitrogen dopants in graphene: electronic properties calculations
Being a true two-dimensional crystal, graphene has special properties. In
particular, a point-like defect in graphene may have effects in the long range.
This peculiarity questions the validity of using a supercell geometry in an
attempt to explore the properties of an isolated defect. Still, this approach
is often used in ab-initio electronic structure calculations, for instance. How
does this approach converge with the size of the supercell is generally not
tackled for the obvious reason of keeping the computational load to an
affordable level. The present paper addresses the problem of substitutional
nitrogen doping of graphene. DFT calculations have been performed for 9x9 and
10x10 supercells. Although these calculations correspond to N concentrations
that differ by about 10%, the local densities of states on and around the
defects are found to depend significantly on the supercell size. Fitting the
DFT results by a tight-binding Hamiltonian makes it possible to explore the
effects of a random distribution of the substitutional N atoms, in the case of
finite concentrations, and to approach the case of an isolated impurity when
the concentration vanishes. The tight-binding Hamiltonian is used to calculate
the STM image of graphene around an isolated N atom. STM images are also
calculated for graphene doped with 0.5 % concentration of nitrogen. The results
are discussed in the light of recent experimental data and the conclusions of
the calculations are extended to other point defects in graphene
Mean-field approximation of the Hubbard model expressed in a many-body basis
The effective independent-particle (mean-field) approximation of the Hubbard
Hamiltonian is described in a many-body basis to develop a formal comparison
with the exact diagonalization of the full Hubbard model, using small atomic
chain as test systems. This allows for the development of an intuitive
understanding of the shortcomings of the mean-field approximation and of how
critical correlation effects are missed in this popular approach. The
description in the many-body basis highlights a potential ambiguity related to
the definition of the density of states. Specifically, satellite peaks are
shown to emerge in the mean-field approximation, in departure from the common
belief that they characterize correlation effects. The scheme emphasizes the
importance of correlation and how different many-body corrections can improve
the mean-field description. The pedagogical treatment is expected to make it
possible for researchers to acquire an improved understanding of many-body
effects as found in various areas related to electronic properties of molecules
and solids, which is highly relevant to current efforts in quantum information
and quantum computing
Semi-empirical many-body formalism of optical absorption in nanosystems and molecules
A computationally efficient Greenās function approach is developed to evaluate the optical properties of nanostructures within a semi-empirical Hubbard model. A GW formalism is applied on top of a tight-binding and mean-field approach. The use of the GW approximation includes key parts of the many-body physics that govern the optical response of nanostructures and molecules subjected to an external electromagnetic field and that is not included in the mean-field approximation. Such description of the electron-electron correlation yields computed spectra that compare significantly better with experiment for a subset of polycyclic aromatic hydrocarbons (PAHs) considered for illustrative purpose. More generally, the method is applicable to any structure whose electronic properties can be described in first approximation within a mean-field approach and is amenable for high-throughput studies aimed at screening materials with desired optical properties
Semi-empirical many-body formalism of optical absorption in nanosystems and molecules
A computationally efficient Green's function approach is developed to
evaluate the optical properties of nanostructures using a GW formalism applied
on top of a tight-binding and mean-field Hubbard model. The use of the GW
approximation includes key parts of the many-body physics that govern the
optical response of nanostructures and molecules subjected to an external
electromagnetic field. Such description of the electron-electron correlation
yields data that are in significantly improved agreement with experiments
performed on a subset of polycyclic aromatic hydrocarbons (PAHs) considered for
illustrative purpose. More generally, the method is applicable to any structure
whose electronic properties can be described in first approximation within a
mean-field approach and is amenable for high-throughput studies aimed at
screening materials with desired optical properties
Robust correlated magnetic moments in end-modified graphene nanoribbons
We conduct a theoretical examination of the electronic and magnetic
characteristics of end-modified 7-atom wide armchair graphene nanoribbons
(AGNRs). Our investigation is performed within the framework of a single-band
Hubbard model, beyond a mean-field approximation. First, we carry out a
comprehensive comparison of various approaches for accommodating
di-hydrogenation configurations at the AGNR ends. We demonstrate that the
application of an on-site potential to the modified carbon atom, coupled with
the addition of an electron, replicates phenomena such as the experimentally
observed reduction in the bulk-states (BS) gap. These results for the density
of states (DOS) and electronic densities align closely with those obtained
through a method explicitly designed to account for the orbital properties of
hydrogen atoms. Furthermore, our study enables a clear differentiation between
mean-field (MF) magnetic moments, which are spatially confined to the same
sites as the topological end-states (ES), and correlation-induced magnetic
moments, which exhibit localization along all edges of the AGNRs. Notably, we
find the robustness of these correlation-induced magnetic moments relative to
end modifications, within the scope of the method we employ
Optical modelling of 2D materials and multilayer systems: a complete picture
Bidimensional materials are, as their name suggests, ideally viewed as having
no thickness. The advent of multilayer stacks of 2D materials and combinations
of different materials in vertical van der Waals heterostructures highlights
however that these materials have a finite thickness. In this article, we show
how volume properties of stacked 2D layers can be calculated from boundary
conditions and conversely. We introduce the layer, surrounded by vacuum, as a
kind of transfer matrix with intrinsic parameters. This provides a link between
continuous and discrete media, and a connection with the interface reflection
and transmission coefficients calculated from microscopic models. We show how
to model hybrid systems and identify in the zero-thickness limit the intrinsic
parameters of the current sheet that represents the 2D material, namely the
in-plane surface susceptibility and the out-of-plane parameter
that corresponds to a displacement susceptibility. By considering
anisotropic layers under the assumption that TE and TM modes are not coupled,
we provide a unified vision of the different models used in optical
characterization of 2D materials, including ellipsometry. We build on this to
model 3D structures layer per layer and identify their effective permittivity
and propagation constants. We show that our model fits existing ellipsometric
data with the same reliability as the existing interface models, but with the
advantage that multilayer and monolayer systems are described in a same way
Anisotropy and effective medium approach in the optical response of two-dimensional material heterostructures
Two-dimensional (2D) materials offer a large variety of optical properties, from transparency to plasmonic excitation. They can be structured and combined to form heterostructures that expand the realm of possibility to manipulate light interactions at the nanoscale. Appropriate and numerically efficient models accounting for the high intrinsic anisotropy of 2D materials and heterostructures are needed. In this article, we retrieve the relevant intrinsic parameters that describe the optical response of a homogeneous 2D material from a microscopic approach. Well-known effective models for vertical heterostructure (stacking of different layers) are retrieved. We found that the effective optical response model of horizontal heterostructures (alternating nanoribbons) depends on the thickness. In the thin layer model, well adapted for 2D materials, a counterintuitive in-plane isotropic behavior is predicted. We confront the effective model formulation with exact reference calculations such as ab initio calculations for graphene, hexagonal boron nitride (hBN), as well as corrugated graphene with larger thickness but also with classical electrodynamics calculations that exactly account for the lateral structuration.</p
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