3,899 research outputs found
Glass transition in models with controlled frustration
A class of models with self-generated disorder and controlled frustration is
studied. Between the trivial case, where frustration is not present at all, and
the limit case, where frustration is present over every length scale, a region
with local frustration is found where glassy dynamics appears. We suggest that
in this region, the mean field model might undergo a p-spin like transition,
and increasing the range of frustration, a crossover from a 1-step replica
symmetry breaking to a continuous one might be observed.Comment: 4 pages, 6 figure
New Ground-Breaking Strategies in Bone Regeneration—In Memory of Nerio Ceroni
This editorial article is dedicated to the memory of the Nerio Ceroni, the grandfather
of the first author. Nerio Ceroni suffered from bone metastases and passed away in
January 2022
Vortex and half-vortex dynamics in a spinor quantum fluid of interacting polaritons
Spinorial or multi-component Bose-Einstein condensates may sustain fractional
quanta of circulation, vorticant topological excitations with half integer
windings of phase and polarization. Matter-light quantum fluids, such as
microcavity polaritons, represent a unique test bed for realising strongly
interacting and out-of-equilibrium condensates. The direct access to the phase
of their wavefunction enables us to pursue the quest of whether half vortices
---rather than full integer vortices--- are the fundamental topological
excitations of a spinor polariton fluid. Here, we are able to directly generate
by resonant pulsed excitations, a polariton fluid carrying either the half or
full vortex states as initial condition, and to follow their coherent evolution
using ultrafast holography. Surprisingly we observe a rich phenomenology that
shows a stable evolution of a phase singularity in a single component as well
as in the full vortex state, spiraling, splitting and branching of the initial
cores under different regimes and the proliferation of many vortex anti-vortex
pairs in self generated circular ripples. This allows us to devise the
interplay of nonlinearity and sample disorder in shaping the fluid and driving
the phase singularities dynamicsComment: New version complete with revised modelization, discussion and added
material. 8 pages, 7 figures. Supplementary videos:
https://drive.google.com/folderview?id=0B0QCllnLqdyBfmc2ai0yVF9fa2g2VnZodGUwemVkLThBb3BoOVRKRDJMS2dUdjlZdkRTQk
Jamming transition in granular media: A mean field approximation and numerical simulations
In order to study analytically the nature of the jamming transition in
granular material, we have considered a cavity method mean field theory, in the
framework of a statistical mechanics approach, based on Edwards' original idea.
For simplicity we have applied the theory to a lattice model and a transition
with exactly the same nature of the glass transition in mean field models for
usual glass formers is found. The model is also simulated in three dimensions
under tap dynamics and a jamming transition with glassy features is observed.
In particular two step decays appear in the relaxation functions and dynamic
heterogeneities resembling ones usually observed in glassy systems. These
results confirm early speculations about the connection between the jamming
transition in granular media and the glass transition in usual glass formers,
giving moreover a precise interpretation of its nature.Comment: 11 pages, 12 figure
Magnetic field structure in single late-type giants: The effectively single giant V390 Aur
We have studied the active giant V390 Aur using spectropolarimetry to obtain
direct and simultaneous measurements of the magnetic field and the activity
indicators in order to get a precise insight of its activity. We used the
spectropolarimeter NARVAL at the Bernard Lyot Telescope (Observatoire du Pic du
Midi, France) to obtain a series of Stokes I and Stokes V profiles. The Least
Square deconvolution (LSD) technique was applied to detect the Zeeman signature
of the magnetic field in each of our 13 observations and to measure its
longitudinal component. We could also monitor the CaII K & H and IR triplet, as
well as the H_alpha lines which are activity indicators. In order to
reconstruct the magnetic field geometry of V390 Aur, we applied the Zeeman
Doppler Imaging (ZDI) inversion method and present a map for the magnetic
field. Based on the obtained spectra, we also refined the fundamental
parameters of the star and the Li abundance. The ZDI revealed a structure in
the radial magnetic field consisting of a polar magnetic spot of positive
polarity and several negative spots at lower latitude. A high latitude belt is
present on the azimuthal field map, indicative of a toroidal field close to the
surface. It was found that the photometric period cannot fit the behaviour of
the activity indicators formed in the chromosphere. Their behaviour suggests
slower rotation compared to the photosphere, but our dataset is too short to be
able to estimate the exact periods for them.Accepted for publication in A&A All
these results can be explained in terms of an \alpha-\omega dynamo operation,
taking into account the stellar structure and rotation properties of V390 Aur
that we study using up to-date stellar models computed at solar metallicity.
The calculated Rossby number also points to a very efficient dynamoComment: To appear in Astronomy & Astrophysics, 8 pages, 5 figure
A robust ransac-based planet radius estimation for onboard visual based navigation
Individual spacecraft manual navigation by human operators from ground station is expected to be an emerging problem as the number of spacecraft for space exploration increases. Hence, as an attempt to reduce the burden to control multiple spacecraft, future missions will employ smart spacecraft able to navigate and operate autonomously. Recently, image-based optical navigation systems have proved to be promising solutions for inexpensive autonomous navigation. In this paper, we propose a robust image processing pipeline for estimating the center and radius of planets and moons in an image taken by an on-board camera. Our custom image pre-processing pipeline is tailored for resource-constrained applications, as it features a computationally simple processing flow with a limited memory footprint. The core of the proposed pipeline is a best-fitting model based on the RANSAC algorithm that is able to handle images corrupted with Gaussian noise, image distortions, and frame drops. We report processing time, pixel-level error of estimated body center and radius and the effect of noise on estimated body parameters for a dataset of synthetic images
The close classical T Tauri binary V4046 Sgr: Complex magnetic fields & distributed mass accretion
We report here the first results of a multi-wavelength campaign focussing on
magnetospheric accretion processes within the close binary system V4046 Sgr,
hosting two partly-convective classical T Tauri stars of masses ~0.9 Msun and
age ~12 Myr. In this paper, we present time-resolved spectropolarimetric
observations collected in 2009 September with ESPaDOnS at the
Canada-France-Hawaii Telescope (CFHT) and covering a full span of 7d or ~2.5
orbital/rotational cycles of V4046 Sgr. Small circularly polarised Zeeman
signatures are detected in the photospheric absorption lines but not in the
accretion-powered emission lines of V4046 Sgr, thereby demonstrating that both
system components host large-scale magnetic fields weaker and more complex than
those of younger, fully-convective cTTSs of only a few Myr and similar masses.
Applying our tomographic imaging tools to the collected data set, we
reconstruct maps of the large-scale magnetic field, photospheric brightness and
accretion-powered emission at the surfaces of both stars of V4046 Sgr. We find
that these fields include significant toroidal components, and that their
poloidal components are mostly non-axisymmetric with a dipolar component of
50-100G strongly tilted with respect to the rotation axis; given the similarity
with fields of partly-convective main-sequence stars of similar masses and
rotation periods, we conclude that these fields are most likely generated by
dynamo processes. We also find that both stars in the system show cool spots
close to the pole and extended regions of low-contrast, accretion-powered
emission; it suggests that mass accretion is likely distributed rather than
confined in well defined high-contrast accretion spots, in agreement with the
derived magnetic field complexity.Comment: MNRAS in press (13 pages, 7 figures
Finite-size effects and intermittency in a simple aging system
We study the intermittent dynamics and the fluctuations of the dynamic
correlation function of a simple aging system. Given its size and its
coherence length , the system can be divided into independent
subsystems, where , and is the dimension of space.
Each of them is considered as an aging subsystem which evolves according to
an activated dynamics between energy levels.
We compute analytically the distribution of trapping times for the global
system, which can take power-law, stretched-exponential or exponential forms
according to the values of and the regime of times considered. An effective
number of subsystems at age , , can be defined, which
decreases as increases, as well as an effective coherence length,
, where characterizes the trapping
times distribution of a single subsystem. We also compute the probability
distribution functions of the time intervals between large decorrelations,
which exhibit different power-law behaviours as increases (or
decreases), and which should be accessible experimentally.
Finally, we calculate the probability distribution function of the two-time
correlator.
We show that in a phenomenological approach, where is replaced by the
effective number of subsystems , the same qualitative behaviour
as in experiments and simulations of several glassy systems can be obtained.Comment: 15 pages, 6 figures, published versio
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