79 research outputs found
Time and energy-resolved two photon-photoemission of the Cu(100) and Cu(111) metal surfaces
We present calculations on energy- and time-resolved two-photon photoemission
spectra of images states in Cu(100) and Cu(111) surfaces. The surface is
modeled by a 1D effective potential and the states are propagated within a
real-space, real-time method. To obtain the energy resolved spectra we employ a
geometrical approach based on a subdivision of space into two regions. We treat
electronic inelastic effects by taking into account the scattering rates
calculated within a GW scheme. To get further insight into the decaying
mechanism we have also studied the effect of the variation of the classical
Hartree potential during the excitation. This effect turns out to be small.Comment: 11 pages, 7 figure
Quantifying the Plasmonic Character of Optical Excitations in a Molecular J-Aggregate
The definition of plasmon at the microscopic scale is far from being
understood. Yet, it is very important to recognize plasmonic features in
optical excitations, as they can inspire new applications and trigger new
discoveries by analogy with the rich phenomenology of metal nanoparticle
plasmons. Recently, the concepts of plasmonicity index and the generalized
plasmonicity index (GPI) have been devised as computational tools to quantify
the plasmonic nature of optical excitations. The question may arise whether any
strong absorption band, possibly with some sort of collective character in its
microscopic origin, shares the status of plasmon. Here we demonstrate that this
is not always the case, by considering a well-known class of systems
represented by J-aggregates molecular crystals, characterized by the intense J
band of absorption. By means of first-principles simulations, based on a
many-body perturbation theory formalism, we investigate the optical properties
of a J-aggregate made of push-pull organic dyes. We show that the effect of
aggregation is to lower the GPI associated with the J-band with respect to the
isolated dye one, which corresponds to a nonplasmonic character of the
electronic excitations. In order to rationalize our finding, we then propose a
simplified one-dimensional theoretical model of the J-aggregate. A useful
microscopic picture of what discriminates a collective molecular crystal
excitation from a plasmon is eventually obtained.Comment: Published by ACS under ACS AuthorChoice licens
Bonds, lone pairs, and shells probed by means of on-top dynamical correlations
The Electron Localization Function (ELF) by Becke and Edgecombe [J. Chem.
Phys. {\bf 92}, 5397 (1990)] is routinely adopted as a descriptor of atomic
shells and covalent bonds. Since the ELF and its related quantities find useful
exploitation also in the construction of modern density functionals, the
interest in complementing the ELF is linked to both the quests of improving
electronic structure descriptors and density functional approximations. The ELF
uses information which is available by considering parallel-spin electron pairs
in single-reference many-body states. In this work, we complement this
construction with information obtained by considering antiparallel-spin pairs
whose short-range correlations are modeled by a density functional
approximation. As a result, the approach requires only a contained
computational effort. Applications to a variety of systems show that, in this
way, we gain a spatial description of the bond in H (which is not available
with the ELF) together with some trends not optimally captured by the ELF in
other prototypical situations
Interplay between Intra- and Intermolecular Charge Transfer in the Optical Excitations of J-Aggregates
In a first-principles study based on density functional theory and many-body
perturbation theory, we address the interplay between intra- and intermolecular
interactions in a J-aggregate formed by push-pull organic dyes by investigating
its electronic and optical properties. We find that the most intense excitation
dominating the spectral onset of the aggregate, i.e., the J-band, exhibits a
combination of intramolecular charge transfer, coming from the push-pull
character of the constituting dyes, and intermolecular charge transfer, due to
the dense molecular packing. We also show the presence of a pure intermolecular
charge-transfer excitation within the J-band, which is expected to play a
relevant role in the emission properties of the J-aggregate. Our results shed
light on the microscopic character of optical excitations of J-aggregates and
offer new perspectives to further understand the nature of collective
excitations in organic semiconductors.Comment: published under ACS Authorchoice licens
An exact Coulomb cutoff technique for supercell calculations
We present a new reciprocal space analytical method to cutoff the long range
interactions in supercell calculations for systems that are infinite and
periodic in 1 or 2 dimensions, extending previous works for finite systems. The
proposed cutoffs are functions in Fourier space, that are used as a
multiplicative factor to screen the bare Coulomb interaction. The functions are
analytic everywhere but in a sub-domain of the Fourier space that depends on
the periodic dimensionality. We show that the divergences that lead to the
non-analytical behaviour can be exactly cancelled when both the ionic and the
Hartree potential are properly screened. This technique is exact, fast, and
very easy to implement in already existing supercell codes. To illustrate the
performance of the new scheme, we apply it to the case of the Coulomb
interaction in systems with reduced periodicity (as one-dimensional chains and
layers). For those test cases we address the impact of the cutoff in different
relevant quantities for ground and excited state properties, namely: the
convergence of the ground state properties, the static polarisability of the
system, the quasiparticle corrections in the GW scheme and in the binding
energy of the excitonic states in the Bethe-Salpeter equation. The results are
very promising.Comment: Submitted to Physical Review B on Dec 23rd 200
Optical properties of graphene nanoribbons: The role of many-body effects
We investigate from first principles the optoelectronic properties of nanometer-sized armchair graphene nanoribbons (GNRs). We show that many-body effects are essential to correctly describe both energy gaps and optical response. As a signature of the confined geometry, we observe strongly bound excitons dominating the optical spectra, with a clear family-dependent binding energy. Our results demonstrate that GNRs constitute one-dimensional nanostructures whose absorption and luminescence performance can be controlled by changing both family and edge termination.We investigate from first principles the optoelectronic properties of nanometer-sized armchair graphene nanoribbons (GNRs). We show that many-body effects are essential to correctly describe both energy gaps and optical response. As a signature of the confined geometry, we observe strongly bound excitons dominating the optical spectra, with a clear family-dependent binding energy. Our results demonstrate that GNRs constitute one-dimensional nanostructures whose absorption and luminescence performance can be controlled by changing both family and edge termination. © 2008 The American Physical Society
Halide Pb-free double–perovskites: ternary vs. quaternary stoichiometry
n view of their applicability in optoelectronics, we review here the relevant structural, electronic, and optical features of the inorganic Pb-free halide perovskite class. In particular, after discussing the reasons that have motivated their introduction in opposition to their more widely investigated organic-inorganic counterparts, we highlight milestones already achieved in their synthesis and characterization and show how the use of ab initio ground and excited state methods is relevant in predicting their properties and in disclosing yet unsolved issues which characterize both ternary and quaternary stoichiometry double-perovskites
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