622 research outputs found
Stochastic properties of systems controlled by autocatalytic reactions II
We analyzed the stochastic behavior of systems controlled by autocatalytic
reaction A+X -> X+X, X+X -> A+X, X -> B provided that the distribution of
reacting particles in the system volume is uniform, i.e. the point model of
reaction kinetics introduced in arXiv:cond-mat/0404402 can be applied. Assuming
the number of substrate particles A to be kept constant by a suitable
reservoir, we derived the forward Kolmogorov equation for the probability of
finding n=0,1,... autocatalytic particles X in the system at a given time
moment. We have shown that the stochastic model results in an equation for the
mean value of autocatalytic particles X which differs strongly from the kinetic
rate equation. It has been found that not only the law of the mass action is
violated but also the bifurcation point is disappeared in the well-known
diagram of X particle- vs. A particle-concentration. Therefore, speculations
about the role of autocatalytic reactions in processes of the "natural
selection" can be hardly supported.Comment: 17 pages, 6 figure
-meson in nuclear matter
The -nucleon (N) interactions are deduced from the heavy baryon
chiral perturbation theory up to the next-to-leading-order terms. Combining the
relativistic mean-field theory for nucleon system, we have studied the
in-medium properties of -meson. We find that all the elastic scattering
N interactions come from the next-to-leading-order terms. The N
sigma term is found to be about 280130 MeV. The off-shell terms are also
important to the in-medium properties of -meson. On application of the
latest determination of the N scattering length, the ratio of
-meson effective mass to its vacuum value is near , while
the optical potential is about MeV, at the normal nuclear density.Comment: 8 pages, 3 figures, to appear in PRC, many modification
LICOR-Liquid Columns' Resonances
The aim of the experiment LICOR was the investigation of the axial resonances oi cylindrical liquid columns supported by equal circular coaxiaJ disks. In preparation ot the D-2 experiment a âąheoreiical model has been developed, which exactly describes the small amplitude oscillations of finite cylindrical columns between coaxial circular disks. In addition, in terrestrial experiments the resonance frequencies of small liquid columns with up to 5 mm in diameter have been determined and investigations with density-matched liquids (silicon oil in a waierlmethanol mixture) have been performed. For the D-2 experiment LICOR the front disk and the rear disk lor use in the AFPM have been constructed and equipped with pressure sensors and the necessary electronics. The pressure exerted by the oscillating liquid column on trie supporting disks vsas as low as 10 Pa. Since the data downlink of the Materials Research Laboratory was just one signal oer second and channel, it was necessary to determine amplitude and phase of the pressure already in the LICOR disks. The D-2 experiment has been successfully performed. It has fully confirmed the theoretical models and remarkably supplements the experiments on small liquid columns and on density-matched columns
Microcavity controlled coupling of excitonic qubits
Controlled non-local energy and coherence transfer enables light harvesting
in photosynthesis and non-local logical operations in quantum computing. The
most relevant mechanism of coherent coupling of distant qubits is coupling via
the electromagnetic field. Here, we demonstrate the controlled coherent
coupling of spatially separated excitonic qubits via the photon mode of a solid
state microresonator. This is revealed by two-dimensional spectroscopy of the
sample's coherent response, a sensitive and selective probe of the coherent
coupling. The experimental results are quantitatively described by a rigorous
theory of the cavity mediated coupling within a cluster of quantum dots
excitons. Having demonstrated this mechanism, it can be used in extended
coupling channels - sculptured, for instance, in photonic crystal cavities - to
enable a long-range, non-local wiring up of individual emitters in solids
Physical Adsorption at the Nanoscale: Towards Controllable Scaling of the Substrate-Adsorbate van der Waals Interaction
The Lifshitz-Zaremba-Kohn (LZK) theory is commonly considered as the correct
large-distance limit for the van der Waals (vdW) interaction of adsorbates
(atoms, molecules, or nanoparticles) with solid substrates. In the standard
approximate form, implicitly based on "local" dielectric functions, the LZK
approach predicts universal power laws for vdW interactions depending only on
the dimensionality of the interacting objects. However, recent experimental
findings are challenging the universality of this theoretical approach at
finite distances of relevance for nanoscale assembly. Here, we present a
combined analytical and numerical many-body study demonstrating that physical
adsorption can be significantly enhanced at the nanoscale. Regardless of the
band gap or the nature of the adsorbate specie, we find deviations from
conventional LZK power laws that extend to separation distances of up to 10--20
nanometers. Comparison with recent experimental observation of ultra
long-ranged vdW interactions in the delamination of graphene from a silicon
substrate reveals qualitative agreement with the present theory. The
sensitivity of vdW interactions to the substrate response and to the adsorbate
characteristic excitation frequency also suggests that adsorption strength can
be effectively tuned in experiments, paving the way to an improved control of
physical adsorption at the nanoscale
Impact of phonons on dephasing of individual excitons in deterministic quantum dot microlenses
Optimized light-matter coupling in semiconductor nanostructures is a key to
understand their optical properties and can be enabled by advanced fabrication
techniques. Using in-situ electron beam lithography combined with a
low-temperature cathodoluminescence imaging, we deterministically fabricate
microlenses above selected InAs quantum dots (QDs) achieving their efficient
coupling to the external light field. This enables to perform four-wave mixing
micro-spectroscopy of single QD excitons, revealing the exciton population and
coherence dynamics. We infer the temperature dependence of the dephasing in
order to address the impact of phonons on the decoherence of confined excitons.
The loss of the coherence over the first picoseconds is associated with the
emission of a phonon wave packet, also governing the phonon background in
photoluminescence (PL) spectra. Using theory based on the independent boson
model, we consistently explain the initial coherence decay, the zero-phonon
line fraction, and the lineshape of the phonon-assisted PL using realistic
quantum dot geometries
Retardation turns the van der Waals attraction into Casimir repulsion already at 3 nm
Casimir forces between surfaces immersed in bromobenzene have recently been
measured by Munday et al. Attractive Casimir forces were found between gold
surfaces. The forces were repulsive between gold and silica surfaces. We show
the repulsion is due to retardation effects. The van der Waals interaction is
attractive at all separations. The retardation driven repulsion sets in already
at around 3 nm. To our knowledge retardation effects have never been found at
such a small distance before. Retardation effects are usually associated with
large distances
Excitons in T-shaped quantum wires
We calculate energies, oscillator strengths for radiative recombination, and
two-particle wave functions for the ground state exciton and around 100 excited
states in a T-shaped quantum wire. We include the single-particle potential and
the Coulomb interaction between the electron and hole on an equal footing, and
perform exact diagonalisation of the two-particle problem within a finite basis
set. We calculate spectra for all of the experimentally studied cases of
T-shaped wires including symmetric and asymmetric GaAs/AlGaAs and
InGaAs/AlGaAs structures. We study in detail the
shape of the wave functions to gain insight into the nature of the various
states for selected symmetric and asymmetric wires in which laser emission has
been experimentally observed. We also calculate the binding energy of the
ground state exciton and the confinement energy of the 1D quantum-wire-exciton
state with respect to the 2D quantum-well exciton for a wide range of
structures, varying the well width and the Al molar fraction . We find that
the largest binding energy of any wire constructed to date is 16.5 meV. We also
notice that in asymmetric structures, the confinement energy is enhanced with
respect to the symmetric forms with comparable parameters but the binding
energy of the exciton is then lower than in the symmetric structures. For
GaAs/AlGaAs wires we obtain an upper limit for the binding energy
of around 25 meV in a 10 {\AA} wide GaAs/AlAs structure which suggests that
other materials must be explored in order to achieve room temperature
applications. There are some indications that
InGaAs/AlGaAs might be a good candidate.Comment: 20 pages, 10 figures, uses RevTeX and psfig, submitted to Physical
Review
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