219 research outputs found
One-photon absorption by inorganic perovskite nanocrystals: A theoretical study
The one-photon absorption cross section of nanocrystals (NCs) of the
inorganic perovskite CsPbBr is studied theoretically using a multiband
envelope-function model combined with a treatment
of intercarrier correlation by many-body perturbation theory. A confined
exciton is described first within the Hartree-Fock (HF) approximation, and
correlation between the electron and hole is then included in leading order by
computing the first-order vertex correction to the electron-photon interaction.
The vertex correction is found to give an enhancement of the near-threshold
absorption cross section by a factor of up to 4 relative to the HF (mean-field)
value of the cross section, for NCs with an edge length -12 nm (regime of
intermediate confinement). The vertex-correction enhancement factors are found
to decrease with increasing exciton energy; the absorption cross section for
photons of energy eV (about 0.7 eV above threshold) is enhanced by
a factor of only 1.4-1.5 relative to the HF value. The
corrections to the absorption cross section are
also significant; they are found to increase the cross section at an energy
eV by about 30% relative to the value found in the effective-mass
approximation. The theoretical absorption cross section at eV,
assuming a Kane parameter eV, is found to be intermediate among the
set of measured values (which vary among themselves by nearly an order of
magnitude) and to obey a power-law dependence on the NC edge length , in good agreement with experiment. The
dominant contribution to the theoretical exponent 2.9 is shown to be the
density of final-state excitons. The main theoretical uncertainty in these
calculations is in the value of the Kane parameter .Comment: 15 pages, 8 figure
Charge-Induced Fragmentation of Sodium Clusters
The fission of highly charged sodium clusters with fissilities X>1 is studied
by {\em ab initio} molecular dynamics. Na_{24}^{4+} is found to undergo
predominantly sequential Na_{3}^{+} emission on a time scale of 1 ps, while
Na_{24}^{Q+} (5 \leq Q \leq 8) undergoes multifragmentation on a time scale
\geq 0.1 ps, with Na^{+} increasingly the dominant fragment as Q increases. All
singly-charged fragments Na_{n}^{+} up to size n=6 are observed. The observed
fragment spectrum is, within statistical error, independent of the temperature
T of the parent cluster for T \leq 1500 K. These findings are consistent with
and explain recent trends observed experimentally.Comment: To appear in Physical Review Letter
Epistatic Interactions in the Arabinose Cis-Regulatory Element
Changes in gene expression are an important mode of evolution; however, the proximate mechanism of these changes is poorly understood. In particular, little is known about the effects of mutations within cis binding sites for transcription factors, or the nature of epistatic interactions between these mutations. Here, we tested the effects of single and double mutants in two cis binding sites involved in the transcriptional regulation of the Escherichia coli araBAD operon, a component of arabinose metabolism, using a synthetic system. This system decouples transcriptional control from any posttranslational effects on fitness, allowing a precise estimate of the effect of single and double mutations, and hence epistasis, on gene expression. We found that epistatic interactions between mutations in the araBAD cis-regulatory element are common, and that the predominant form of epistasis is negative. The magnitude of the interactions depended on whether the mutations are located in the same or in different operator sites. Importantly, these epistatic interactions were dependent on the presence of arabinose, a native inducer of the araBAD operon in vivo, with some interactions changing in sign (e.g., from negative to positive) in its presence. This study thus reveals that mutations in even relatively simple cis-regulatory elements interact in complex ways such that selection on the level of gene expression in one environment might perturb regulation in the other environment in an unpredictable and uncorrelated manner
Simple Analytical Particle and Kinetic Energy Densities for a Dilute Fermionic Gas in a d-Dimensional Harmonic Trap
We derive simple analytical expressions for the particle density
and the kinetic energy density for a system of noninteracting
fermions in a dimensional isotropic harmonic oscillator potential. We test
the Thomas-Fermi (TF, or local-density) approximation for the functional
relation using the exact and show that it locally
reproduces the exact kinetic energy density , {\it including the shell
oscillations,} surprisingly well everywhere except near the classical turning
point. For the special case of two dimensions (2D), we obtain the unexpected
analytical result that the integral of yields the {\it
exact} total kinetic energy.Comment: 4 pages, 4 figures; corrected versio
Some exact results for a trapped quantum gas at finite temperature
We present closed analytical expressions for the particle and kinetic energy
spatial densities at finite temperatures for a system of noninteracting
fermions (bosons) trapped in a d-dimensional harmonic oscillator potential. For
d=2 and 3, exact expressions for the N-particle densities are used to calculate
perturbatively the temperature dependence of the splittings of the energy
levels in a given shell due to a very weak interparticle interaction in a
dilute Fermi gas. In two dimensions, we obtain analytically the surprising
result that the |l|-degeneracy in a harmonic oscillator shell is not lifted in
the lowest order even when the exact, rather than the Thomas-Fermi expression
for the particle density is used. We also demonstrate rigorously (in two
dimensions) the reduction of the exact zero-temperature fermionic expressions
to the Thomas-Fermi form in the large-N limit.Comment: 14 pages, 4 figures include
Optimizing Performance of Continuous-Time Stochastic Systems using Timeout Synthesis
We consider parametric version of fixed-delay continuous-time Markov chains
(or equivalently deterministic and stochastic Petri nets, DSPN) where
fixed-delay transitions are specified by parameters, rather than concrete
values. Our goal is to synthesize values of these parameters that, for a given
cost function, minimise expected total cost incurred before reaching a given
set of target states. We show that under mild assumptions, optimal values of
parameters can be effectively approximated using translation to a Markov
decision process (MDP) whose actions correspond to discretized values of these
parameters
Role of Fragment Higher Static Deformations in the Cold Binary Fission of Cf
We study the binary cold fission of Cf in the frame of a cluster
model where the fragments are born to their respective ground states and
interact via a double-folded potential with deformation effects taken into
account up to multipolarity . The preformation factors were
neglected. In the case when the fragments are assumed to be spherical or with
ground state quadrupole deformation, the -value principle dictates the
occurence of a narrow region around the double magic Sn, like in the
case of cluster radioactivity. When the hexadecupole deformation is turned on,
an entire mass-region of cold fission in the range 138 - 156 for the heavy
fragment arise, in agreement with the experimental observations.
This fact suggests that in the above mentioned mass-region, contrary to the
usual cluster radioactivity where the daughter nucleus is always a
neutron/proton (or both) closed shell or nearly closed shell spherical nucleus,
the clusterization mechanism seems to be strongly influenced by the
hexadecupole deformations rather than the -value.Comment: 10 pages, 12 figure
HARD PHOTON PRODUCTION IN NUCLEUS-NUCLEUS COLLISIONS AT 30 MeV/u AND 44 MeV/u
Doubly differential cross-sections for Bremsstrahlung production have been measured in the reactions 40Ar + 197Au at 30 MeV/u and 86Kr +12C, AgNat and 197Au at 44 MeV/u. A qualitative analysis of the characteristics of the γ-ray emission suggests strongly that the initial proton-neutron collisions are the main source of nuclear Bremsstrahlung
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