2,402 research outputs found
Approximation of quantum control correction scheme using deep neural networks
We study the functional relationship between quantum control pulses in the
idealized case and the pulses in the presence of an unwanted drift. We show
that a class of artificial neural networks called LSTM is able to model this
functional relationship with high efficiency, and hence the correction scheme
required to counterbalance the effect of the drift. Our solution allows
studying the mapping from quantum control pulses to system dynamics and then
analysing the robustness of the latter against local variations in the control
profile.Comment: 6 pages, 3 figures, Python code available upon request. arXiv admin
note: text overlap with arXiv:1803.0516
Influence of annealing parameters on the ferromagnetic properties of optimally passivated (Ga,Mn)As epilayers
The influence of annealing parameters - temperature and time - on the
magnetic properties of As-capped (Ga,Mn)As epitaxial thin films have been
investigated. The dependence of the transition temperature (Tc) on annealing
time marks out two regions. The Tc peak behavior, characteristic of the first
region, is more pronounced for thick samples, while for the second
(`saturated') region the effect of the annealing time is more pronounced for
thin samples. A right choice of the passivation medium, growth conditions along
with optimal annealing parameters routinely yield Tc-values of ~ 150 K and
above, regardless of the thickness of the epilayers.Comment: 5 pages, 3 figure
The slimming effect of advection on black-hole accretion flows
At super-Eddington rates accretion flows onto black holes have been described
as slim (aspect ratio ) or thick (H/R >1) discs, also known as
tori or (Polish) doughnuts. The relation between the two descriptions has never
been established, but it was commonly believed that at sufficiently high
accretion rates slim discs inflate, becoming thick. We wish to establish under
what conditions slim accretion flows become thick. We use analytical equations,
numerical 1+1 schemes, and numerical radiative MHD codes to describe and
compare various accretion flow models at very high accretion rates.We find that
the dominant effect of advection at high accretion rates precludes slim discs
becoming thick. At super-Eddington rates accretion flows around black holes can
always be considered slim rather than thick.Comment: 8 pages, 5 figures. Astronomy & Astrophysics, in pres
Ferromagnetism and interlayer exchange coupling in short period (Ga,Mn)As/GaAs superlattices
Magnetic properties of (Ga,Mn)As/GaAs superlattices are investigated. The
structures contain magnetic (Ga,Mn)As layers, separated by thin layers of
non-magnetic GaAs spacer. The short period GaMnAs/GaAs
superlattices exhibit a paramagnetic-to-ferromagnetic phase transition close to
60K, for thicknesses of (Ga,Mn)As down to 23 \AA. For
GaMnAs/GaAs superlattices of similar dimensions, the Curie
temperature associated with the ferromagnetic transition is found to oscillate
with the thickness of non magnetic spacer. The observed oscillations are
related to an interlayer exchange interaction mediated by the polarized holes
of the (Ga,Mn)As layers.Comment: REVTeX 4 style; 4 pages, 2 figure
Magnetic properties of GaMnAs single layers and GaInMnAs superlattices investigated at low temperature and high magnetic field
Magnetotransport properties of GaMnAs single layers and InGaMnAs/InGaAs
superlattice structures were investigated at temperatures from 4 K to 300 K and
magnetic fields up to 23 T to study the influence of carriers confinement
through different structures. Both single layers and superlattice structures
show paramagnetic-to-ferromagnetic phase transition. In GaMnAs/InGaAs
superlattice beside the Curie temperature (Tc ~ 40 K), a new phase transition
is observed close to 13 K.Comment: 8 pages, 5 figures, Proceedings of the XXXII International School on
the Physics of Semiconducting Compounds, Jaszowiec 2003, Polan
Cubic anisotropy in high homogeneity thin (Ga,Mn)As layers
Historically, comprehensive studies of dilute ferromagnetic semiconductors,
e.g., -type (Cd,Mn)Te and (Ga,Mn)As, paved the way for a quantitative
theoretical description of effects associated with spin-orbit interactions in
solids, such as crystalline magnetic anisotropy. In particular, the theory was
successful in explaining {\em uniaxial} magnetic anisotropies associated with
biaxial strain and non-random formation of magnetic dimers in epitaxial
(Ga,Mn)As layers. However, the situation appears much less settled in the case
of the {\em cubic} term: the theory predicts switchings of the easy axis
between in-plane and directions as a
function of the hole concentration, whereas only the
orientation has been found experimentally. Here, we report on the observation
of such switchings by magnetization and ferromagnetic resonance studies on a
series of high-crystalline quality (Ga,Mn)As films. We describe our findings by
the mean-field - Zener model augmented with three new ingredients. The
first one is a scattering broadening of the hole density of states, which
reduces significantly the amplitude of the alternating carrier-induced
contribution. This opens the way for the two other ingredients, namely the
so-far disregarded single-ion magnetic anisotropy and disorder-driven
non-uniformities of the carrier density, both favoring the
direction of the apparent easy axis. However, according to our results, when
the disorder gets reduced a switching to the orientation
is possible in a certain temperature and hole concentration range.Comment: 12 pages, 9 figure
GRMHD simulations of visibility amplitude variability for Event Horizon Telescope images of Sgr A*
Synthesis imaging of the black hole in the center of the Milky Way, Sgr A*,
with the Event Horizon Telescope (EHT) rests on the assumption of a stationary
image. We explore the limitations of this assumption using high-cadence GRMHD
simulations of Sgr A*. We employ analytic models that capture the basic
characteristics of the images to understand the origin of the variability in
the simulated visibility amplitudes. We find that, in all simulations, the
visibility amplitudes for baselines oriented perpendicular to the spin axis of
the black hole typically decrease smoothly over baseline lengths that are
comparable to those of the EHT. On the other hand, the visibility amplitudes
for baselines oriented parallel to the spin axis show significant structure
with one or more minima. This suggests that fitting EHT observations with
geometric models will lead to reasonably accurate determination of the
orientation of the black-hole on the plane of the sky. However, in the
disk-dominated models, the locations and depths of the minima in the visibility
amplitudes depend primarily on the width and asymmetry of the crescent-like
images and are highly variable. In the jet-dominated models, the locations of
the minima are determined by the separation of the two image components but
their depths depend primarily on the relative brightness of the two components
and are also variable. This suggests that using time-independent models to
infer additional black-hole parameters, such as the shadow size or the spin
magnitude, will be severely affected by the variability of the accretion flow.Comment: replaced to match published version, new figure added, results
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