669 research outputs found
Pair-distribution functions of the two-dimensional electron gas
Based on its known exact properties and a new set of extensive fixed-node
reptation quantum Monte Carlo simulations (both with and without backflow
correlations, which in this case turn out to yield negligible improvements), we
propose a new analytical representation of (i) the spin-summed
pair-distribution function and (ii) the spin-resolved potential energy of the
ideal two-dimensional interacting electron gas for a wide range of electron
densities and spin polarization, plus (iii) the spin-resolved pair-distribution
function of the unpolarized gas. These formulae provide an accurate reference
for quantities previously not available in analytic form, and may be relevant
to semiconductor heterostructures, metal-insulator transitions and quantum dots
both directly, in terms of phase diagram and spin susceptibility, and
indirectly, as key ingredients for the construction of new two-dimensional spin
density functionals, beyond the local approximation.Comment: 12 pages, 10 figures; misprints correcte
Monte Carlo approach of the islanding of polycrystalline thin films
We computed by a Monte Carlo method derived from the Solid on Solid model,
the evolution of a polycrystalline thin film deposited on a substrate during
thermal treatment. Two types of substrates have been studied: a single
crystalline substrate with no defects and a single crystalline substrate with
defects. We obtain islands which are either flat (i.e. with a height which does
not overcome a given value) or grow in height like narrow towers. A good
agreement was found regarding the morphology of numerical nanoislands at
equilibrium, deduced from our model, and experimental nanoislands resulting
from the fragmentation of YSZ thin films after thermal treatment.Comment: 20 pages, 7 figure
Electron-Phonon Coupling in Charged Buckminsterfullerene
A simple, yet accurate solution of the electron-phonon coupling problem in
C_{60} is presented. The basic idea behind it is to be found in the
parametrization of the ground state electronic density of the system calculated
making use of ab-initio methods, in term of sp hybridized orbitals.
This parametrization allows for an economic determination of the deformation
potential associated with the fullerene's normal modes. The resulting
electron-phonon coupling constants are used to calculate Jahn-Teller effects in
C_{60}^-, and multiple satellite peaks in the corresponding photoemission
reaction. Theory provides an accurate account of the experimental findings.Comment: 11 pages, 3 figures. Accepted for publication in Chem. Phys. Let
Acceleration Schemes for Ab-Initio Molecular Dynamics and Electronic Structure Calculations
We study the convergence and the stability of fictitious dynamical methods
for electrons. First, we show that a particular damped second-order dynamics
has a much faster rate of convergence to the ground-state than first-order
steepest descent algorithms while retaining their numerical cost per time step.
Our damped dynamics has efficiency comparable to that of conjugate gradient
methods in typical electronic minimization problems. Then, we analyse the
factors that limit the size of the integration time step in approaches based on
plane-wave expansions. The maximum allowed time step is dictated by the highest
frequency components of the fictitious electronic dynamics. These can result
either from the large wavevector components of the kinetic energy or from the
small wavevector components of the Coulomb potential giving rise to the so
called {\it charge sloshing} problem. We show how to eliminate large wavevector
instabilities by adopting a preconditioning scheme that is implemented here for
the first-time in the context of Car-Parrinello ab-initio molecular dynamics
simulations of the ionic motion. We also show how to solve the charge-sloshing
problem when this is present. We substantiate our theoretical analysis with
numerical tests on a number of different silicon and carbon systems having both
insulating and metallic character.Comment: RevTex, 9 figures available upon request, to appear in Phys. Rev.
Ab-initio Molecular Dynamics study of electronic and optical properties of silicon quantum wires: Orientational Effects
We analyze the influence of spatial orientation on the optical response of
hydrogenated silicon quantum wires. The results are relevant for the
interpretation of the optical properties of light emitting porous silicon. We
study (111)-oriented wires and compare the present results with those
previously obtained within the same theoretical framework for (001)-oriented
wires [F. Buda {\it et al.}, {\it Phys. Rev. Lett.} {\bf 69}, 1272, (1992)]. In
analogy with the (001)-oriented wires and at variance with crystalline bulk
silicon, we find that the (111)-oriented wires exhibit a direct gap at whose value is largely enhanced with respect to that found in bulk
silicon because of quantum confinement effects. The imaginary part of the
dielectric function, for the external field polarized in the direction of the
axis of the wires, shows features that, while being qualitatively similar to
those observed for the (001) wires, are not present in the bulk. The main
conclusion which emerges from the present study is that, if wires a few
nanometers large are present in the porous material, they are
optically active independently of their specific orientation.Comment: 14 pages (plus 6 figures), Revte
First-principle study of excitonic self-trapping in diamond
We present a first-principles study of excitonic self-trapping in diamond.
Our calculation provides evidence for self-trapping of the 1s core exciton and
gives a coherent interpretation of recent experimental X-ray absorption and
emission data. Self-trapping does not occur in the case of a single valence
exciton. We predict, however, that self-trapping should occur in the case of a
valence biexciton. This process is accompanied by a large local relaxation of
the lattice which could be observed experimentally.Comment: 12 pages, RevTex file, 3 Postscript figure
Gravitational Microlensing Events from the First Year of the Northern Galactic Plane Survey by the Zwicky Transient Facility
The Zwicky Transient Facility (ZTF) (Bellm et al. 2019; Graham et al. 2019; Masci et al. 2019) is currently surveying the entire northern sky, including dense Galactic plane fields. Here, we present preliminary results of the search for gravitational microlensing events in the ZTF data collected from the beginning of the survey (2018 March 20) through 2019 June 30
A fourfold coordinated point defect in silicon
Due to their technological importance, point defects in silicon are among the
best studied physical systems. The experimental examination of point defects
buried in bulk is difficult and evidence for the various defects usually
indirect. Simulations of defects in silicon have been performed at various
levels of sophistication ranging from fast force fields to accurate density
functional calculations. The generally accepted viewpoint from all these
studies is that vacancies and self interstitials are the basic point defects in
silicon. We challenge this point of view by presenting density functional
calculations that show that there is a new fourfold coordinated point defect in
silicon that is lower in energy
Transport and cooling of singly-charged noble gas ion beams
The transport and cooling of noble gas singly-charged ion beams by means of a
Radio Frequency Quadrupole Cooler Buncher (RFQCB) have been studied at the
LIMBE low energy beam line of the GANIL facility. Ions as light as
have been cooled and stored before their extraction in bunches using as
buffer gas. Bunches characteristics have been studied as a function of the
parameters of the device. Sizeable transmissions of up to 10 have been
obtained. A detailed study of the lifetime of ions inside the buncher has been
performed giving an estimate of the charge exchange cross-section. Results of a
microscopic Monte-Carlo transport code show reasonable agreement with
experimental data.Comment: 13 figure
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