447 research outputs found
Magnonic analog of the Edelstein effect in antiferromagnetic insulators
We investigate the nonequilibrium spin polarization due to a temperature gradient in antiferromagnetic insulators, which is the magnonic analog of the inverse spin-galvanic effect of electrons. We derive a linear-response theory of a temperature-gradient-induced spin polarization for collinear and noncollinear antiferromagnets, which comprises both extrinsic and intrinsic contributions. We apply our theory to several noncentrosymmetric antiferromagnetic insulators, i.e., to a one-dimensional antiferromagnetic spin chain, a single layer of kagome noncollinear antiferromagnet,e.g.,KFe3(OH)6(SO4)2,and a noncollinear breathing pyrochlore antiferromagnet, e.g., LiGaCr4O8. The shapes of our numerically evaluated response tensors agree with those implied by the magnetic symmetry. Assuming a realistic temperature gradient of 10 K/mm, we find two-dimensional spin densities of up to ∼10^6 hbar/cm^2 and three-dimensional bulk spin densities of up to ∼10^14 hbar/cm^3, encouraging an experimental detection
On the internal modes in sine-Gordon chain
We address the issue of internal modes of a kink of a discrete sine-Gordon
equation. The main point of the present study is to elucidate how the
antisymmetric internal mode frequency dependence enters the quasicontinuum
spectrum of nonlocalized waves. We analyze the internal frequency dependencies
as functions of both the number of cites and discreteness parameter and explain
the origin of spectrum peculiarity which arises after the frequency dependence
of antisymmetric mode returns back to the continuous spectrum at some nonzero
value of the intersite coupling.Comment: 5 pages, 3 figure
A connection between -ray and parsec-scale radio flares in the blazar 3C 273
We present a comprehensive 5-43 GHz VLBA study of the blazar 3C 273 initiated
after an onset of a strong -ray flare in this source. We have analyzed
the kinematics of new-born components, light curves, and position of the
apparent core to pinpoint the location of the -ray emission. Estimated
location of the -ray emission zone is close to the jet apex, 2 pc to 7
pc upstream from the observed 7 mm core. This is supported by ejection of a new
component. The apparent core position was found to be inversely proportional to
frequency. The brightness temperature in the 7 mm core reached values up to at
least K during the flare. This supports the dominance of particle
energy density over that of magnetic field in the 7 mm core. Particle density
increased during the radio flare at the apparent jet base, affecting
synchrotron opacity. This manifested itself as an apparent core shuttle along
the jet during the 7 mm flare. It is also shown that a region where optical
depth decreases from to spans over several parsecs along
the jet. The jet bulk flow speed estimated at the level of 12c on the basis of
time lags between 7 mm light curves of stationary jet features is 1.5 times
higher than that derived from VLBI apparent kinematics analysis.Comment: Accepted for publication in MNRAS. 17 pages, 15 figures, 10 tables,
with supplementary materials attache
Numerical model of the spatio-temporal dynamics in a water strider group
The water strider group demonstrates a very complex dynamics consisting of competition for the food items, territoriality and aggression to the conspecific individuals, escaping from the predators, etc. The situation is even more complex due to the presence of different instars, which in most water strider species live in the same habitat and occupy the same niche. The presented swarm model of water striders demonstrates the realistic population dynamics. For the swarm formation in the model, attraction and repulsion forces were used. Animal motion in the model takes into account inertia and kinetic energy dissipation effects. The model includes three different rates related to the growth of individuals: food appearance rate, food assimilation rate, and stored energy loss rate. The results of our modeling show that the size distribution of individuals seems to be an adequate measure for population status, and it has a characteristic shape for different model parameter combinations. Distribution of the distances between nearest neighbors is other important measure of the population density and its dynamics. Parameters of the model can be tuned in such a way, that the shape of both distributions in a steady phase coincides with that shape observed in a natural population, which helps to understand the factors leading to particular momentary distribution of both parameters (size and distance) in the population. From this point of view, the model can predict how both distributions can further develop from certain state depending on particular combination of factors
Decentralized Saddle-Point Problems with Different Constants of Strong Convexity and Strong Concavity
Large-scale saddle-point problems arise in such machine learning tasks as
GANs and linear models with affine constraints. In this paper, we study
distributed saddle-point problems (SPP) with strongly-convex-strongly-concave
smooth objectives that have different strong convexity and strong concavity
parameters of composite terms, which correspond to min and max variables, and
bilinear saddle-point part. We consider two types of first-order oracles:
deterministic (returns gradient) and stochastic (returns unbiased stochastic
gradient). Our method works in both cases and takes several consensus steps
between oracle calls
Decentralized convex optimization over time-varying graphs: a survey
Decentralized optimization over time-varying networks has a wide range of
applications in distributed learning, signal processing and various distributed
control problems. The agents of the distributed system locally hold
optimization objectives and can communicate to their immediate neighbors over a
network that changes from time to time. In this paper, we survey
state-of-the-art results and describe the techniques for optimization over
time-varying graphs. We also give an overview of open questions in the field
and formulate hypotheses and directions for future work
Opacity, variability and kinematics of AGN jets
Synchrotron self-absorption in active galactic nuclei (AGN) jets manifests
itself as a time delay between flares observed at high and low radio
frequencies. It is also responsible for the observing frequency dependent
change in size and position of the apparent base of the jet, aka the core shift
effect, detected with very long baseline interferometry (VLBI). We measure the
time delays and the core shifts in 11 radio-loud AGN to estimate the speed of
their jets without relying on multi-epoch VLBI kinematics analysis. The 158
GHz total flux density time lags are obtained using Gaussian process
regression, the core shift values are measured using VLBI observations and
adopted from the literature. A strong correlation is found between the apparent
core shift and the observed time delay. Our estimate of the jet speed is higher
than the apparent speed of the fastest VLBI components by the median
coefficient of 1.4. The coefficient ranges for individual sources from 0.5 to
20. We derive Doppler factors, Lorentz factors and viewing angles of the jets,
as well as the corresponding de-projected distance from the jet base to the
core. The results support evidence for acceleration of the jets with bulk
motion Lorentz factor on de-projected scales
of 0.5500 parsecs.Comment: Accepted by MNRAS; 11 pages, 11 figures, 3 table
The frictional and adhesive properties of the shells of terrestrial hairy snails
Hair‐like structures of the shell in some terrestrial snails have been suggested to represent an adaptive advantage. One of the recently proposed explanations was the hypothesis that the possession of hairs facilitates the snails' adherence to the leaves of the plants during foraging, when humidity levels are high. In order to obtain some reliable data to test this contact mechanics hypothesis, the present paper deals with the frictional and adhesive properties of the smooth and hairy shells of two species of terrestrial mollusks, Trochulus hispidus and T. villosulus. Additionally, actively generated traction force of the snails was measured on different substrates. The conducted experiments revealed that in wet conditions the hairiness indeed increases friction and that adhesion through the water film is high enough to retain the weight of the snail. However, possession of long hairs decreases the adhesion, in comparison with the animals with short hairs. Our data suggest that hair‐like structures require more energy expenditure during locomotion and only provide the advantage of higher adhesion on wet substrates. Obtained maximal traction forces were about 30 times higher than friction forces of the shell surface dragged over the substrate. Neither wetting condition of the substrate nor surface roughness had considerable effect on the traction forces of snails. Therefore, the function of the hair‐like structures seems not to be related with snail locomotion and adhesion
FingerFlex: Inferring Finger Trajectories from ECoG signals
Motor brain-computer interface (BCI) development relies critically on neural
time series decoding algorithms. Recent advances in deep learning architectures
allow for automatic feature selection to approximate higher-order dependencies
in data. This article presents the FingerFlex model - a convolutional
encoder-decoder architecture adapted for finger movement regression on
electrocorticographic (ECoG) brain data. State-of-the-art performance was
achieved on a publicly available BCI competition IV dataset 4 with a
correlation coefficient between true and predicted trajectories up to 0.74. The
presented method provides the opportunity for developing fully-functional
high-precision cortical motor brain-computer interfaces.Comment: 6 pages, 3 figures, 4 tables. Preprint. Under revie
Jumping mechanism in the marsh beetles (Coleoptera: Scirtidae)
The jumping mechanism with supporting morphology and kinematics is described in the marsh beetle Scirtes hemisphaericus (Coleoptera: Scirtidae). In marsh beetles, the jump is performed by the hind legs by the rapid extension of the hind tibia. The kinematic parameters of the jump are: 139-1536 m s-2 (acceleration), 0.4-1.9 m s-1 (velocity), 2.7-8.4 ms (time to take-off), 0.2-5.4 × 10-6 J (kinetic energy) and 14-156 (g-force). The power output of a jumping leg during the jumping movement is 3.5 × 103 to 9.6 × 103 W kg-1. A resilin-bearing elastic extensor ligament is considered to be the structure that accumulates the elastic strain energy. The functional model of the jumping involving an active latching mechanism is proposed. The latching mechanism is represented by the conical projection of the tibial flexor sclerite inserted into the corresponding socket of the tibial base. Unlocking is triggered by the contraction of flexor muscle pulling the tibial flexor sclerite backwards which in turn comes out of the socket. According to the kinematic parameters, the time of full extension of the hind tibia, and the value of the jumping leg power output, this jumping mechanism is supposed to be latch-mediated spring actuation using the contribution of elastically stored strain energy
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