14,230 research outputs found
Identification of fullerene-like CdSe nanoparticles from optical spectroscopy calculations
Semiconducting nanoparticles are the building blocks of optical nanodevices
as their electronic states, and therefore light absorption and emission, can be
controlled by modifying their size and shape. CdSe is perhaps the most studied
of these nanoparticles, due to the efficiency of its synthesis, the high
quality of the resulting samples, and the fact that the optical gap is in the
visible range. In this article, we study light absorption of CdSe
nanostructures with sizes up to 1.5 nm within density functional theory. We
study both bulk fragments with wurtzite symmetry and novel fullerene-like
core-cage structures. The comparison with recent experimental optical spectra
allows us to confirm the synthesis of these fullerene-like CdSe clusters
The planar-to-tubular structural transition in boron clusters from optical absorption
The optical response of the lowest energy isomers of the B_20 family is
calculated using time-dependent density functional theory within a real-space,
real-time scheme. Significant differences are found among the absorption
spectra of the clusters studied. We show that these differences can be easily
related to changes in the overall geometry. Optical spectroscopy is thus an
efficient tool to characterize the planar to tubular structural transition,
known to be present in these boron based systems
Generalized scalar field models with the same energy density and linear stability
We study how the properties of a Lagrangian density for a single real scalar
field in flat spacetime change with inclusion of an overall factor depending
only on the field. The focus of the paper is to obtain analytical results. So,
we show that even though it is possible to perform a field redefinition to get
an equivalent canonical model, it is not always feasible to write the canonical
model in terms of elementary functions. Also, we investigate the behavior of
the energy density and the linear stability of the solutions. Finally, we show
that one can find a class of models that present the same energy density and
the same stability potential.Comment: 6 pages, 4 figure
Magnetic response of carbon nanotubes from ab initio calculations
We present {\it ab initio} calculations of the magnetic susceptibility and of
the C chemical shift for carbon nanotubes, both isolated and in bundles.
These calculations are performed using the recently proposed gauge-including
projector augmented-wave approach for the calculation of magnetic response in
periodic insulating systems. We have focused on the semiconducting zigzag
nanotubes with diameters ranging from 0.6 to 1.6 nm. Both the susceptibility
and the isotropic shift exhibit a dependence with the diameter (D) and the
chirality of the tube (although this dependence is stronger for the
susceptibility). The isotropic shift behaves asymptotically as , where is a different constant for each family of nanotubes.
For a tube diameter of around 1.2 nm, a value normally found in experimental
samples, our results are in excellent agreement with experiments. Moreover, we
calculated the chemical shift of a double-wall tube. We found a diamagnetic
shift of the isotropic lines corresponding to the atoms of the inner tube due
to the effect of the outer tube. This shift is in good agreement with recent
experiments, and can be easily explained by demagnetizing currents circulating
the outer tube.Comment: 7 pages, 4 figure
Optimization of the ionization time of an atom with tailored laser pulses: a theoretical study
How fast can a laser pulse ionize an atom? We address this question by
considering pulses that carry a fixed time-integrated energy per-area, and
finding those that achieve the double requirement of maximizing the ionization
that they induce, while having the shortest duration. We formulate this
double-objective quantum optimal control problem by making use of the Pareto
approach to multi-objetive optimization, and the differential evolution genetic
algorithm. The goal is to find out how much a precise time-profiling of
ultra-fast, large-bandwidth pulses may speed up the ionization process with
respect to simple-shape pulses. We work on a simple one-dimensional model of
hydrogen-like atoms (the P\"oschl-Teller potential), that allows to tune the
number of bound states that play a role in the ionization dynamics. We show how
the detailed shape of the pulse accelerates the ionization process, and how the
presence or absence of bound states influences the velocity of the process
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
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