831 research outputs found
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 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
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
- …