4,365 research outputs found
Stable Irregular Dynamics in Complex Neural Networks
For infinitely large sparse networks of spiking neurons mean field theory
shows that a balanced state of highly irregular activity arises under various
conditions. Here we analytically investigate the microscopic irregular dynamics
in finite networks of arbitrary connectivity, keeping track of all individual
spike times. For delayed, purely inhibitory interactions we demonstrate that
the irregular dynamics is not chaotic but rather stable and convergent towards
periodic orbits. Moreover, every generic periodic orbit of these dynamical
systems is stable. These results highlight that chaotic and stable dynamics are
equally capable of generating irregular activity.Comment: 10 pages, 2 figure
On-axis spectroscopy of the host galaxies of 20 optically luminous quasars at z~0.3
We present the analysis of a sample of 20 bright low-redshift quasars
(M_B<-23 and z < 0.35) observed spectroscopically with the VLT. The FORS1
spectra, obtained in Multi Object Spectroscopy (MOS) mode, allow to observe
simultaneously the quasars and several reference stars used to spatially
deconvolve the data. Applying the MCS deconvolution method, we are able to
separate the individual spectra of the quasar and of the underlying host
galaxy. Contrary to some previous claims, we find that luminous quasars are not
exclusively hosted by massive ellipticals. Most quasar host galaxies harbour
large amounts of gas, irrespective of their morphological type. Moreover, the
stellar content of half of the hosts is a young Sc-like population, associated
with a rather low metallicity interstellar medium. A significant fraction of
the galaxies contain gas ionized at large distances by the quasar radiation.
This large distance ionization is always associated with signs of gravitational
interactions (as detected from images or disturbed rotation curves). The
spectra of the quasars themselves provide evidence that gravitational
interactions bring dust and gas in the immediate surrounding of the super
massive black hole, allowing to feed it. The quasar activity might thus be
triggered (1) in young gas-rich spiral galaxies by local events and (2) in more
evolved galaxies by gravitational interactions or collisions. The latter
mechanism gives rises to the most powerful quasars. Finally, we derive mass
models for the isolated spiral host galaxies and we show that the most reliable
estimators of the systemic redshift in the quasar spectrum are the tips of the
Ha and Hb lines.Comment: 30 pages, 19 figures, 9 tables, accepted for publication in MNRAS,
major revisio
Polarons in semiconductor quantum-dots and their role in the quantum kinetics of carrier relaxation
While time-dependent perturbation theory shows inefficient carrier-phonon
scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic
description of carrier-phonon interaction predicts fast carrier capture and
relaxation. The considered processes do not fulfill energy conservation in
terms of free-carrier energies because polar coupling of localized quantum-dot
states strongly modifies this picture.Comment: 6 pages, 6 figures, accepted for publication in Phys.Rev.
Influence of carrier-carrier and carrier-phonon correlations on optical absorption and gain in quantum-dot systems
A microscopic theory is used to study the optical properties of semiconductor
quantum dots. The dephasing of a coherent excitation and line-shifts of the
interband transitions due to carrier-carrier Coulomb interaction and
carrier-phonon interaction are determined from a quantum kinetic treatment of
correlation processes. We investigate the density dependence of both mechanisms
and clarify the importance of various dephasing channels involving the
localized and delocalized states of the system.Comment: 12 pages, 10 figure
Optical properties of self-organized wurtzite InN/GaN quantum dots: A combined atomistic tight-binding and full configuration interaction calculation
In this work we investigate the electronic and optical properties of
self-assembled InN/GaN quantum dots. The one-particle states of the
low-dimensional heterostructures are provided by a tight-binding model that
fully includes the wurtzite crystal structure on an atomistic level. Optical
dipole and Coulomb matrix elements are calculated from these one-particle wave
functions and serve as an input for full configuration interaction
calculations. We present multi-exciton emission spectra and discuss in detail
how Coulomb correlations and oscillator strengths are changed by the
piezoelectric fields present in the structure. Vanishing exciton and biexciton
ground state emission for small lens-shaped dots is predicted.Comment: 3 pages, 2 figure
Relaxation properties of the quantum kinetics of carrier-LO-phonon interaction in quantum wells and quantum dots
The time evolution of optically excited carriers in semiconductor quantum
wells and quantum dots is analyzed for their interaction with LO-phonons. Both
the full two-time Green's function formalism and the one-time approximation
provided by the generalized Kadanoff-Baym ansatz are considered, in order to
compare their description of relaxation processes. It is shown that the
two-time quantum kinetics leads to thermalization in all the examined cases,
which is not the case for the one-time approach in the intermediate-coupling
regime, even though it provides convergence to a steady state. The
thermalization criterion used is the Kubo-Martin-Schwinger condition.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.
Streaking temporal double slit interference by an orthogonal two-color laser field
We investigate electron momentum distributions from single ionization of Ar
by two orthogonally polarized laser pulses of different color. The two-color
scheme is used to experimentally control the interference between electron wave
packets released at different times within one laser cycle. This intracycle
interference pattern is typically hard to resolve in an experiment. With the
two-color control scheme these features become the dominant contribution to the
electron momentum distribution. Furthermore the second color can be used for
streaking of the otherwise interfering wave packets establishing a which-way
marker. Our investigation shows that the visibility of the interference fringes
depends on the degree of the which-way information determined by the
controllable phase between the two pulses.Comment: submitted to PR
Coulomb effects in semiconductor quantum dots
Coulomb correlations in the optical spectra of semiconductor quantum dots are
investigated using a full-diagonalization approach. The resulting multi-exciton
spectra are discussed in terms of the symmetry of the involved states.
Characteristic features of the spectra like the nearly equidistantly spaced
s-shell emission lines and the approximately constant p-shell transition
energies are explained using simplified Hamiltonians that are derived taking
into account the relative importance of various interaction contributions.
Comparisons with previous results in the literature and their interpretation
are made.Comment: 7 pages, 2 figure
Correlated Photon-Pair Emission from a Charged Single Quantum Dot
The optical creation and recombination of charged biexciton and trion
complexes in an (In,Ga)As/GaAs quantum dot is investigated by
micro-photoluminescence spectroscopy. Photon cross-correlation measurements
demonstrate the temporally correlated decay of charged biexciton and trion
states. Our calculations provide strong evidence for radiative decay from the
excited trion state which allows for a deeper insight into the spin
configurations and their dynamics in these systems.Comment: 5 pages, 3 figures, submitted for publicatio
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