618 research outputs found
Integration of Action and Language Knowledge: A Roadmap for Developmental Robotics
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Diagrammatic Quantum Monte Carlo for Two-Body Problem: Exciton
We present a novel method for precise numerical solution of the irreducible
two-body problem and apply it to excitons in solids. The approach is based on
the Monte Carlo simulation of the two-body Green function specified by
Feynman's diagrammatic expansion. Our method does not rely on the specific form
of the electron and hole dispersion laws and is valid for any attractive
electron-hole potential. We establish limits of validity of the Wannier (large
radius) and Frenkel (small radius) approximations, present accurate data for
the intermediate radius excitons, and give evidence for the charge transfer
nature of the monopolar exciton in mixed valence materials.Comment: 4 pages, 5 figure
Excitonic Effects on Optical Absorption Spectra of Doped Graphene
We have performed first-principles calculations to study optical absorption
spectra of doped graphene with many-electron effects included. Both self-energy
corrections and electron-hole interactions are reduced due to the enhanced
screening in doped graphene. However, self-energy corrections and excitonic
effects nearly cancel each other, making the prominent optical absorption peak
fixed around 4.5 eV under different doping conditions. On the other hand, an
unexpected increase of the optical absorbance is observed within the infrared
and visible-light frequency regime (1 ~ 3 eV). Our analysis shows that a
combining effect from the band filling and electron-hole interactions results
in such an enhanced excitonic effect on the optical absorption. These unique
variations of the optical absorption of doped graphene are of importance to
understand relevant experiments and design optoelectronic applications.Comment: 15 pages, 5 figures; Nano Lett., Article ASAP (2011
Ab-initio calculation of the electronic and optical excitations in polythiophene: effects of intra- and interchain screening
We present an calculation of the electronic and optical excitations of an
isolated polythiophene chain as well as of bulk polythiophene. We use the GW
approximation for the electronic self-energy and include excitonic effects by
solving the electron-hole Bethe-Salpeter equation. The inclusion of interchain
screening in the case of bulk polythiophene drastically reduces both the
quasi-particle band gap and the exciton binding energies, but the optical gap
is hardly affected. This finding is relevant for conjugated polymers in
general.Comment: 4 pages, 1 figur
Molecular geometry optimization with a genetic algorithm
We present a method for reliably determining the lowest energy structure of
an atomic cluster in an arbitrary model potential. The method is based on a
genetic algorithm, which operates on a population of candidate structures to
produce new candidates with lower energies. Our method dramatically outperforms
simulated annealing, which we demonstrate by applying the genetic algorithm to
a tight-binding model potential for carbon. With this potential, the algorithm
efficiently finds fullerene cluster structures up to starting
from random atomic coordinates.Comment: 4 pages REVTeX 3.0 plus 3 postscript figures; to appear in Physical
Review Letters. Additional information available under "genetic algorithms"
at http://www.public.iastate.edu/~deaven
Optical absorption spectra of finite systems from a conserving Bethe-Salpeter equation approach
We present a method for computing optical absorption spectra by means of a
Bethe-Salpeter equation approach, which is based on a conserving linear
response calculation for electron-hole coherences in the presence of an
external electromagnetic field. This procedure allows, in principle, for the
determination of the electron-hole correlation function self-consistently with
the corresponding single-particle Green function. We analyze the general
approach for a "one-shot" calculation of the photoabsorption cross section of
finite systems, and discuss the importance of scattering and dephasing
contributions in this approach. We apply the method to the closed-shell
clusters Na_4, Na^+_9 and Na^+_(21), treating one active electron per Na atom.Comment: 9 pages, 3 figure
Embodied language learning and cognitive bootstrapping: methods and design principles
Co-development of action, conceptualization and social interaction mutually scaffold and support each other within a virtuous feedback cycle in the development of human language in children. Within this framework, the purpose of this article is to bring together diverse but complementary accounts of research methods that jointly contribute to our understanding of cognitive development and in particular, language acquisition in robots. Thus, we include research pertaining to developmental robotics, cognitive science, psychology, linguistics and neuroscience, as well as practical computer science and engineering. The different studies are not at this stage all connected into a cohesive whole; rather, they are presented to illuminate the need for multiple different approaches that complement each other in the pursuit of understanding cognitive development in robots. Extensive experiments involving the humanoid robot iCub are reported, while human learning relevant to developmental robotics has also contributed useful results.
Disparate approaches are brought together via common underlying design principles. Without claiming to model human language acquisition directly, we are nonetheless inspired by analogous development in humans and consequently, our investigations include the parallel co-development of action, conceptualization and social interaction. Though these different approaches need to ultimately be integrated into a coherent, unified body of knowledge, progress is currently also being made by pursuing individual methods
Dispersion of the dielectric function of a charge-transfer insulator
We study the problem of dielectric response in the strong coupling regime of
a charge transfer insulator. The frequency and wave number dependence of the
dielectric function and its inverse is the main object of consideration. We show that the
problem, in general, cannot be reduced to a calculation within the Hubbard
model, which takes into account only a restricted number of electronic states
near the Fermi energy. The contribution of the rest of the system to the
longitudinal response (i.e. to ) is essential
for the whole frequency range. With the use of the spectral representation of
the two-particle Green's function we show that the problem may be divided into
two parts: into the contributions of the weakly correlated and the Hubbard
subsystems. For the latter we propose an approach that starts from the
correlated paramagnetic ground state with strong antiferromagnetic
fluctuations. We obtain a set of coupled equations of motion for the
two-particle Green's function that may be solved by means of the projection
technique. The solution is expressed by a two particle basis that includes the
excitonic states with electron and hole separated at various distances. We
apply our method to the multiband Hubbard (Emery) model that describes layered
cuprates. We show that strongly dispersive branches exist in the excitonic
spectrum of the 'minimal' Emery model () and consider the
dependence of the spectrum on finite oxygen hopping and on-site
repulsion . The relationship of our calculations to electron energy loss
spectroscopy is discussed.Comment: 22 pages, 5 figure
X-ray Absorption Near-Edge Structure calculations with pseudopotentials. Application to K-edge in diamond and alpha-quartz
We present a reciprocal-space pseudopotential scheme for calculating X-ray
absorption near-edge structure (XANES) spectra. The scheme incorporates a
recursive method to compute absorption cross section as a continued fraction.
The continued fraction formulation of absorption is advantageous in that it
permits the treatment of core-hole interaction through large supercells
(hundreds of atoms). The method is compared with recently developed
Bethe-Salpeter approach. The method is applied to the carbon K-edge in diamond
and to the silicon and oxygen K-edges in alpha-quartz for which polarized XANES
spectra were measured. Core-hole effects are investigated by varying the size
of the supercell, thus leading to information similar to that obtained from
cluster size analysis usually performed within multiple scattering
calculations.Comment: 11 pages, 4 figure
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