1,644 research outputs found
Radiative Lifetimes of Single Excitons in Semiconductor Quantum Dots- Manifestation of the Spatial Coherence Effect
Using time correlated single photon counting combined with temperature
dependent diffraction limited confocal photoluminescence spectroscopy we
accurately determine, for the first time, the intrinsic radiative lifetime of
single excitons confined within semiconductor quantum dots. Their lifetime is
one (two) orders of magnitude longer than the intrinsic radiative lifetime of
single excitons confined in semiconductor quantum wires (wells) of comparable
confining dimensions. We quantitatively explain this long radiative time in
terms of the reduced spatial coherence between the confined exciton dipole
moment and the radiation electromagnetic field.Comment: 4 pages, 3 figure
Toy Models for Galaxy Formation versus Simulations
We describe simple useful toy models for key processes of galaxy formation in
its most active phase, at z > 1, and test the approximate expressions against
the typical behaviour in a suite of high-resolution hydro-cosmological
simulations of massive galaxies at z = 4-1. We address in particular the
evolution of (a) the total mass inflow rate from the cosmic web into galactic
haloes based on the EPS approximation, (b) the penetration of baryonic streams
into the inner galaxy, (c) the disc size, (d) the implied steady-state gas
content and star-formation rate (SFR) in the galaxy subject to mass
conservation and a universal star-formation law, (e) the inflow rate within the
disc to a central bulge and black hole as derived using energy conservation and
self-regulated Q ~ 1 violent disc instability (VDI), and (f) the implied steady
state in the disc and bulge. The toy models provide useful approximations for
the behaviour of the simulated galaxies. We find that (a) the inflow rate is
proportional to mass and to (1+z)^5/2, (b) the penetration to the inner halo is
~50% at z = 4-2, (c) the disc radius is ~5% of the virial radius, (d) the
galaxies reach a steady state with the SFR following the accretion rate into
the galaxy, (e) there is an intense gas inflow through the disc, comparable to
the SFR, following the predictions of VDI, and (f) the galaxies approach a
steady state with the bulge mass comparable to the disc mass, where the
draining of gas by SFR, outflows and disc inflows is replenished by fresh
accretion. Given the agreement with simulations, these toy models are useful
for understanding the complex phenomena in simple terms and for
back-of-the-envelope predictions.Comment: Resubmitted to MNRAS after responding to referee's comments; Revised
figure
Wavelet Decompositions of Nonrefinable Shift Invariant Spaces
AbstractThe motivation for this work is a recently constructed family of generators of shift invariant spaces with certain optimal approximation properties, but which are not refinable in the classical sense. We try to see whether, once the classical refinability requirement is removed, it is still possible to construct meaningful wavelet decompositions of dilates of the shift invariant space that are well suited for applications
Non-linear Stochastic Galaxy Biasing in Cosmological Simulations
We study the biasing relation between dark-matter halos or galaxies and the
underlying mass distribution, using cosmological -body simulations in which
galaxies are modelled via semi-analytic recipes. The nonlinear, stochastic
biasing is quantified in terms of the mean biasing function and the scatter
about it as a function of time, scale and object properties. The biasing of
galaxies and halos shows a general similarity and a characteristic shape, with
no galaxies in deep voids and a steep slope in moderately underdense regions.
At \sim 8\hmpc, the nonlinearity is typically \lsim 10 percent and the
stochasticity is a few tens of percent, corresponding to percent
variations in the cosmological parameter . Biasing
depends weakly on halo mass, galaxy luminosity, and scale. The time evolution
is rapid, with the mean biasing larger by a factor of a few at
compared to , and with a minimum for the nonlinearity and stochasticity at
an intermediate redshift. Biasing today is a weak function of the cosmological
model, reflecting the weak dependence on the power-spectrum shape, but the time
evolution is more cosmology-dependent, relecting the effect of the growth rate.
We provide predictions for the relative biasing of galaxies of different type
and color, to be compared with upcoming large redshift surveys. Analytic models
in which the number of objects is conserved underestimate the evolution of
biasing, while models that explicitly account for merging provide a good
description of the biasing of halos and its evolution, suggesting that merging
is a crucial element in the evolution of biasing.Comment: 27 pages, 21 figures, submitted to MNRA
Optical spectroscopy of single quantum dots at tunable positive, neutral and negative charge states
We report on the observation of photoluminescence from positive, neutral and
negative charge states of single semiconductor quantum dots. For this purpose
we designed a structure enabling optical injection of a controlled unequal
number of negative electrons and positive holes into an isolated InGaAs quantum
dot embedded in a GaAs matrix. Thereby, we optically produced the charge states
-3, -2, -1, 0, +1 and +2. The injected carriers form confined collective
'artificial atoms and molecules' states in the quantum dot. We resolve
spectrally and temporally the photoluminescence from an optically excited
quantum dot and use it to identify collective states, which contain charge of
one type, coupled to few charges of the other type. These states can be viewed
as the artificial analog of charged atoms such as H, H, H,
and charged molecules such as H and H. Unlike higher
dimensionality systems, where negative or positive charging always results in
reduction of the emission energy due to electron-hole pair recombination, in
our dots, negative charging reduces the emission energy, relative to the
charge-neutral case, while positive charging increases it. Pseudopotential
model calculations reveal that the enhanced spatial localization of the
hole-wavefunction, relative to that of the electron in these dots, is the
reason for this effect.Comment: 5 figure
- …