6,145 research outputs found
A pairwise maximum entropy model accurately describes resting-state human brain networks
The resting-state human brain networks underlie fundamental cognitive functions and consist of complex interactions among brain regions. However, the level of complexity of the resting-state networks has not been quantified, which has prevented comprehensive descriptions of the brain activity as an integrative system. Here, we address this issue by demonstrating that a pairwise maximum entropy model, which takes into account region-specific activity rates and pairwise interactions, can be robustly and accurately fitted to resting-state human brain activities obtained by functional magnetic resonance imaging. Furthermore, to validate the approximation of the resting-state networks by the pairwise maximum entropy model, we show that the functional interactions estimated by the pairwise maximum entropy model reflect anatomical connexions more accurately than the conventional functional connectivity method. These findings indicate that a relatively simple statistical model not only captures the structure of the resting-state networks but also provides a possible method to derive physiological information about various large-scale brain networks
The 95zr(n, gamma)96zr cross section from the surrogate ratio method and its effect on the s-process nucleosynthesis
The 95Zr(n,gamma)96Zr reaction cross section is crucial in the modelling of
s-process nucleosynthesis in asymptotic giant branch stars because it controls
the operation of the branching point at the unstable 95Zr and the subsequent
production of 96Zr. We have carried out the measurement of the 94Zr(18O,16O)
and 90Zr(18O,16O) reactions and obtained the gamma-decay probability ratio of
96Zr* and 92Zr* to determine the 95Zr(n,gamma)96Zr reaction cross sections with
the surrogate ratio method. Our deduced maxwellian-averaged cross section of
66+-16 mb at 30 keV is close to the value recommended by Bao et al. (2000), but
30% and more than a factor of two larger than the values proposed by Toukan &
Kappeler (1990) and Lugaro et al. (2014), respectively, and routinely used in
s-process models. We tested the new rate in stellar models with masses between
2 and 6 Msun and metallicities 0.014 and 0.03. The largest changes - up 80%
variations in 96Zr - are seen in models of mass 3-4 Msun, where the 22Ne
neutron source is mildly activated. The new rate can still provide a match to
data from meteoritic stardust silicon carbide grains, provided the maximum mass
of the parent stars is below 4 Msun, for a metallicity of 0.03.Comment: 10 pages, 6 figures, accepted for publication in Ap
Locking of the Rotation of Disk-Accreting Magnetized Stars
We investigate the rotational equilibrium state of a disk accreting
magnetized stars using axisymmetric magnetohydrodynamic (MHD) simulations. In
this ``locked'' state, the spin-up torque balances the spin-down torque so that
the net average torque on the star is zero. We investigated two types of
initial conditions, one with a relatively weak stellar magnetic field and a
high coronal density, and the other with a stronger stellar field and a lower
coronal density. We observed that for both initial conditions the rotation of
the star is locked to the rotation of the disk. In the second case, the radial
field lines carry significant angular momentum out of the star. However, this
did not appreciably change the condition for locking of the rotation of the
star. We find that in the equilibrium state the corotation radius is
related to the magnetospheric radius as for
case (1) and for case (2). We estimated periods of
rotation in the equilibrium state for classical T Tauri stars, dwarf novae and
X-ray millisecond pulsars.Comment: 10 pages, 9 figures. Accepted by ApJ, will appear in vol. 634, 2005
December
Comparative Evaluation of Action Recognition Methods via Riemannian Manifolds, Fisher Vectors and GMMs: Ideal and Challenging Conditions
We present a comparative evaluation of various techniques for action
recognition while keeping as many variables as possible controlled. We employ
two categories of Riemannian manifolds: symmetric positive definite matrices
and linear subspaces. For both categories we use their corresponding nearest
neighbour classifiers, kernels, and recent kernelised sparse representations.
We compare against traditional action recognition techniques based on Gaussian
mixture models and Fisher vectors (FVs). We evaluate these action recognition
techniques under ideal conditions, as well as their sensitivity in more
challenging conditions (variations in scale and translation). Despite recent
advancements for handling manifolds, manifold based techniques obtain the
lowest performance and their kernel representations are more unstable in the
presence of challenging conditions. The FV approach obtains the highest
accuracy under ideal conditions. Moreover, FV best deals with moderate scale
and translation changes
Cyanobacterial Diversity and a New Acaryochloris-Like Symbiont from Bahamian Sea-Squirts
Symbiotic interactions between ascidians (sea-squirts) and microbes are poorly understood. Here we characterized the cyanobacteria in the tissues of 8 distinct didemnid taxa from shallow-water marine habitats in the Bahamas Islands by sequencing a fragment of the cyanobacterial 16S rRNA gene and the entire 16S–23S rRNA internal transcribed spacer region (ITS) and by examining symbiont morphology with transmission electron (TEM) and confocal microscopy (CM). As described previously for other species, Trididemnum spp. mostly contained symbionts associated with the Prochloron-Synechocystis group. However, sequence analysis of the symbionts in Lissoclinum revealed two unique clades. The first contained a novel cyanobacterial clade, while the second clade was closely associated with Acaryochloris marina. CM revealed the presence of chlorophyll d (chl d) and phycobiliproteins (PBPs) within these symbiont cells, as is characteristic of Acaryochloris species. The presence of symbionts was also observed by TEM inside the tunic of both the adult and larvae of L. fragile, indicating vertical transmission to progeny. Based on molecular phylogenetic and microscopic analyses, Candidatus Acaryochloris bahamiensis nov. sp. is proposed for this symbiotic cyanobacterium. Our results support the hypothesis that photosymbiont communities in ascidians are structured by host phylogeny, but in some cases, also by sampling location
Electron-Transport Properties of Na Nanowires under Applied Bias Voltages
We present first-principles calculations on electron transport through Na
nanowires at finite bias voltages. The nanowire exhibits a nonlinear
current-voltage characteristic and negative differential conductance. The
latter is explained by the drastic suppression of the transmission peaks which
is attributed to the electron transportability of the negatively biased plinth
attached to the end of the nanowire. In addition, the finding that a voltage
drop preferentially occurs on the negatively biased side of the nanowire is
discussed in relation to the electronic structure and conduction.Comment: 4 pages, 6 figure
The first observed stellar occultations by the irregular satellite Phoebe (Saturn IX) and improved rotational period
peer reviewedWe report six stellar occultations by Phoebe (Saturn IX), an irregular satellite of Saturn, obtained between mid-2017 and mid-2019. The 2017 July 6 event was the first stellar occultation by an irregular satellite ever observed. The occultation chords were compared to a 3D shape model of the satellite obtained from Cassini observations. The rotation period available in the literature led to a sub-observer point at the moment of the observed occultations where the chords could not fit the 3D model. A procedure was developed to identify the correct sub-observer longitude. It allowed us to obtain the rotation period with improved precision compared to the currently known value from literature. We show that the difference between the observed and the predicted sub-observer longitude suggests two possible solutions for the rotation period. By comparing these values with recently observed rotational light curves and single- chord stellar occultations, we can identify the best solution for Phoebe's rotational period as 9.27365 ± 0.00002 h. From the stellar occultations, we also obtained six geocentric astrometric positions in the ICRS as realized by the Gaia DR2 with uncertainties at the 1-mas level
Spin-density-functional theory of circular and elliptical quantum dots
Using spin-density-functional theory, we study the electronic states of a
two-dimensional parabolic quantum dot with up to N=58 electrons. We observe a
shell structure for the filling of the dot with electrons. Hund's rule
determines the spin configuration of the ground state, but only up to 22
electrons. At specific N, the ground state is degenerate, and a small
elliptical deformation of the external potential induces a rotational
charge-density-wave (CDW) state. Previously identified spin-density-wave (SDW)
states are shown to be artifacts of broken spin symmetry in density-functional
theory.Comment: 10 pages, 3 figure
Probing the Shape of Quantum Dots with Magnetic Fields
A tool for the identification of the shape of quantum dots is developed. By
preparing a two-electron quantum dot, the response of the low-lying excited
states to a homogeneous magnetic field, i.e. their spin and parity
oscillations, is studied for a large variety of dot shapes. For any geometric
configuration of the confinement we encounter characteristic spin singlet -
triplet crossovers. The magnetization is shown to be a complementary tool for
probing the shape of the dot.Comment: 11 pages, 4 figure
Experimental and theoretical evidence for pressure-induced metallization in FeO with the rock-salt type structure
Electrical conductivity of FeO was measured up to 141 GPa and 2480 K in a
laserheated diamond-anvil cell. The results show that rock-salt (B1) type
structured FeO metallizes at around 70 GPa and 1900 K without any structural
phase transition. We computed fully self-consistently the electronic structure
and the electrical conductivity of B1 FeO as a function of pressure and
temperature, and found that although insulating as expected at ambient
condition, B1 FeO metallizes at high temperatures, consistent with experiments.
The observed metallization is related to spin crossover
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