25,377 research outputs found
Multi-excitons in self-assembled InAs/GaAs quantum dots: A pseudopotential, many-body approach
We use a many-body, atomistic empirical pseudopotential approach to predict
the multi-exciton emission spectrum of a lens shaped InAs/GaAs self-assembled
quantum dot. We discuss the effects of (i) The direct Coulomb energies,
including the differences of electron and hole wavefunctions, (ii) the exchange
Coulomb energies and (iii) correlation energies given by a configuration
interaction calculation. Emission from the groundstate of the exciton
system to the exciton system involving and
recombinations are discussed. A comparison with a simpler single-band,
effective mass approach is presented
A summary report on system effectiveness and optimization study
Report treats optimization and effectiveness separately. Report illustrates example of dynamic programming solution to system optimization. Computer algorithm has been developed to solve effectiveness problem and is included in report
Anomalous enhancement of a penguin hadronic matrix element in B->K eta'
We estimate the density matrix element for the pi^0, eta and eta' production
from the vacuum in the large-N_c limit. As a consequence, we find that the QCD
axial anomaly leads to highly non-trivial corrections to the usual flavour
SU(3) relations between B^0-> K^0 pi^0, B^0-> K^0 eta and B^0-> K^0 eta' decay
amplitudes. These corrections may explain why the B-> K eta' branching ratio is
about six times larger than the B-> K pi one.Comment: 5 pages, 1 figur
Dielectric Response of Periodic Systems from Quantum Monte Carlo Calculations
We present a novel approach that allows to calculate the dielectric response
of periodic systems in the quantum Monte Carlo formalism. We employ a many-body
generalization for the electric enthalpy functional, where the coupling with
the field is expressed via the Berry-phase formulation for the macroscopic
polarization. A self-consistent local Hamiltonian then determines the
ground-state wavefunction, allowing for accurate diffusion quantum Monte Carlo
calculations where the polarization's fixed point is estimated from the average
on an iterative sequence, sampled via forward-walking. This approach has been
validated for the case of an isolated hydrogen atom, and then applied to a
periodic system, to calculate the dielectric susceptibility of
molecular-hydrogen chains. The results found are in excellent agreement with
the best estimates obtained from the extrapolation of quantum-chemistry
calculations.Comment: 5 page 2figure
Shifts in hexapod diversification and what Haldane could have said
Data on species richness and taxon age are assembled for the extant hexapod orders (insects and their six-legged relatives). Coupled with estimates of phylogenetic relatedness, and simple statistical null models, these data are used to locate where, on the hexapod tree, significant changes in the rate of cladogenesis (speciation-minus-extinction rate) have occurred. Significant differences are found between many successive pairs of sister taxa near the base of the hexapod tree, all of which are attributable to a shift in diversification rate after the origin of the Neoptera (insects with wing flexion) and before the origin of the Holometabola (insects with complete metamorphosis). No other shifts are identifiable amongst supraordinal taxa. Whilst the Coleoptera have probably diversified faster than either of their putative sister lineages, they do not stand out relative to other closely related clades. These results suggest that any Creator had a fondness for a much more inclusive clade than the Coleoptera, definitely as large as the Eumetabola (Holometabola plus bugs and their relatives), and possibly as large as the entire Neoptera. Simultaneous, hence probable causative events are discussed, of which the origin of wing flexion has been the focus of much attention
Comparison of two methods for describing the strain profiles in quantum dots
The electronic structure of interfaces between lattice-mismatched
semiconductor is sensitive to the strain. We compare two approaches for
calculating such inhomogeneous strain -- continuum elasticity (CE, treated as a
finite difference problem) and atomistic elasticity (AE). While for small
strain the two methods must agree, for the large strains that exist between
lattice-mismatched III-V semiconductors (e.g. 7% for InAs/GaAs outside the
linearity regime of CE) there are discrepancies. We compare the strain profile
obtained by both approaches (including the approximation of the correct C_2
symmetry by the C_4 symmetry in the CE method), when applied to C_2-symmetric
InAs pyramidal dots capped by GaAs.Comment: To appear in J. Appl. Physic
A CLEAN-based Method for Deconvolving Interstellar Pulse Broadening from Radio Pulses
Multipath propagation in the interstellar medium distorts radio pulses, an
effect predominant for distant pulsars observed at low frequencies. Typically,
broadened pulses are analyzed to determine the amount of propagation-induced
pulse broadening, but with little interest in determining the undistorted pulse
shapes. In this paper we develop and apply a method that recovers both the
intrinsic pulse shape and the pulse broadening function that describes the
scattering of an impulse. The method resembles the CLEAN algorithm used in
synthesis imaging applications, although we search for the best pulse
broadening function, and perform a true deconvolution to recover intrinsic
pulse structre. As figures of merit to optimize the deconvolution, we use the
positivity and symmetry of the deconvolved result along with the mean square
residual and the number of points below a given threshold. Our method makes no
prior assumptions about the intrinsic pulse shape and can be used for a range
of scattering functions for the interstellar medium. It can therefore be
applied to a wider variety of measured pulse shapes and degrees of scattering
than the previous approaches. We apply the technique to both simulated data and
data from Arecibo observations.Comment: 9 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
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