17,612 research outputs found
Decelerated spreading in degree-correlated networks
While degree correlations are known to play a crucial role for spreading
phenomena in networks, their impact on the propagation speed has hardly been
understood. Here we investigate a tunable spreading model on scale-free
networks and show that the propagation becomes slow in positively (negatively)
correlated networks if nodes with a high connectivity locally accelerate
(decelerate) the propagation. Examining the efficient paths offers a coherent
explanation for this result, while the -core decomposition reveals the
dependence of the nodal spreading efficiency on the correlation. Our findings
should open new pathways to delicately control real-world spreading processes
Involvement of ras p2I in Neurotrophin-induced Response of Sensory, but Not Sympathetic Neurons
Little is known about the signal transduction mechanisms involved in the response to neurotrophins and other neurotrophic factors in neurons, beyond the activation of the tyrosine kinase activity of the neurotrophin receptors belonging to the trk family. We have previously shown that the introduction of the oncogene product ras p21 into the cytoplasm of chick embryonic neurons can reproduce the survival and neurite-outgrowth promoting effects of the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), and of ciliary neurotrophic factor (CNTF). To assess the potential signal- transducing role of endogenous ras p21, we introduced function-blocking anti-ras antibodies or their Fab fragments into cultured chick embryonic neurons. The BDNF-induced neurite outgrowth in E12 nodose ganglion neurons was reduced to below control levels, and the NGF- induced survival of E9 dorsal root ganglion (DRG) neurons was inhibited in a specific and dose-dependent fashion. Both effects could be reversed by saturating the epitope-binding sites with biologically inactive ras p21 before microinjection. Surprisingly, ras p21 did not promote the survival of NGF-dependent E12 chick sympathetic neurons, and the NGF-induced survival in these cells was not inhibited by the Fab-fragments. The survival effect of CNTF on ras-responsive ciliary neurons could not be blocked by anti-ras Fab fragments. These results indicate an involvement of ras p21 in the signal transduction of neurotrophic factors in sensory, but not sympathetic or ciliary neurons, pointing to the existence of different signaling pathways not only in CNTF-responsive, but also in neurotrophin-responsive neuronal populations
Magnetic-field asymmetry of nonlinear mesoscopic transport
We investigate departures of the Onsager relations in the nonlinear regime of
electronic transport through mesoscopic systems. We show that the nonlinear
current--voltage characteristic is not an even function of the magnetic field
due only to the magnetic-field dependence of the screening potential within the
conductor. We illustrate this result for two types of conductors: A quantum
Hall bar with an antidot and a chaotic cavity connected to quantum point
contacts. For the chaotic cavity we obtain through random matrix theory an
asymmetry in the fluctuations of the nonlinear conductance that vanishes
rapidly with the size of the contacts.Comment: 4 pages, 2 figures. Published versio
A PSPACE Construction of a Hitting Set for the Closure of Small Algebraic Circuits
In this paper we study the complexity of constructing a hitting set for the
closure of VP, the class of polynomials that can be infinitesimally
approximated by polynomials that are computed by polynomial sized algebraic
circuits, over the real or complex numbers. Specifically, we show that there is
a PSPACE algorithm that given n,s,r in unary outputs a set of n-tuples over the
rationals of size poly(n,s,r), with poly(n,s,r) bit complexity, that hits all
n-variate polynomials of degree-r that are the limit of size-s algebraic
circuits. Previously it was known that a random set of this size is a hitting
set, but a construction that is certified to work was only known in EXPSPACE
(or EXPH assuming the generalized Riemann hypothesis). As a corollary we get
that a host of other algebraic problems such as Noether Normalization Lemma,
can also be solved in PSPACE deterministically, where earlier only randomized
algorithms and EXPSPACE algorithms (or EXPH assuming the generalized Riemann
hypothesis) were known.
The proof relies on the new notion of a robust hitting set which is a set of
inputs such that any nonzero polynomial that can be computed by a polynomial
size algebraic circuit, evaluates to a not too small value on at least one
element of the set. Proving the existence of such a robust hitting set is the
main technical difficulty in the proof.
Our proof uses anti-concentration results for polynomials, basic tools from
algebraic geometry and the existential theory of the reals
Current induced rotational torques in the skyrmion lattice phase of chiral magnets
In chiral magnets without inversion symmetry, the magnetic structure can form
a lattice of magnetic whirl lines, a two-dimensional skyrmion lattice,
stabilized by spin-orbit interactions in a small range of temperatures and
magnetic fields. The twist of the magnetization within this phase gives rise to
an efficient coupling of macroscopic magnetic domains to spin currents. We
analyze the resulting spin-transfer effects, and, in particular, focus on the
current induced rotation of the magnetic texture by an angle. Such a rotation
can arise from macroscopic temperature gradients in the system as has recently
been shown experimentally and theoretically. Here we investigate an alternative
mechanism, where small distortions of the skyrmion lattice and the transfer of
angular momentum to the underlying atomic lattice play the key role. We employ
the Landau-Lifshitz-Gilbert equation and adapt the Thiele method to derive an
effective equation of motion for the rotational degree of freedom. We discuss
the dependence of the rotation angle on the orientation of the applied magnetic
field and the distance to the phase transition.Comment: 11 pages, 6 figures; minor changes, published versio
Searching for young Jupiter analogs around AP Col: L-band high-contrast imaging of the closest pre-main sequence star
The nearby M-dwarf AP Col was recently identified by Riedel et al. 2011 as a
pre-main sequence star (age 12 - 50 Myr) situated only 8.4 pc from the Sun. The
combination of its youth, distance, and intrinsically low luminosity make it an
ideal target to search for extrasolar planets using direct imaging. We report
deep adaptive optics observations of AP Col taken with VLT/NACO and Keck/NIRC2
in the L-band. Using aggressive speckle suppression and background subtraction
techniques, we are able to rule out companions with mass m >= 0.5 - 1M_Jup for
projected separations a>4.5 AU, and m >= 2 M_Jup for projected separations as
small as 3 AU, assuming an age of 40 Myr using the COND theoretical
evolutionary models. Using a different set of models the mass limits increase
by a factor of ~2. The observations presented here are the deepest
mass-sensitivity limits yet achieved within 20 AU on a star with direct
imaging. While Doppler radial velocity surveys have shown that Jovian bodies
with close-in orbits are rare around M-dwarfs, gravitational microlensing
studies predict that ~17% of these stars host massive planets with orbital
separations of 1-10 AU. Sensitive high-contrast imaging observations, like
those presented here, will help to validate results from complementary
detection techniques by determining the frequency of gas giant planets on wide
orbits around M-dwarfs.Comment: Accepted for publication in ApJ, 6 pages text ApJ style (incl.
references), 4 figures, 1 tabl
Upper bounds for the number of orbital topological types of planar polynomial vector fields "modulo limit cycles"
The paper deals with planar polynomial vector fields. We aim to estimate the
number of orbital topological equivalence classes for the fields of degree n.
An evident obstacle for this is the second part of Hilbert's 16th problem. To
circumvent this obstacle we introduce the notion of equivalence modulo limit
cycles. This paper is the continuation of the author's paper in [Mosc. Math. J.
1 (2001), no. 4] where the lower bound of the form 2^{cn^2} has been obtained.
Here we obtain the upper bound of the same form. We also associate an equipped
planar graph to every planar polynomial vector field, this graph is a complete
invariant for orbital topological classification of such fields.Comment: 23 pages, 5 figure
Transition from antibunching to bunching in cavity QED
The photon statistics of the light emitted from an atomic ensemble into a
single field mode of an optical cavity is investigated as a function of the
number of atoms. The light is produced in a Raman transition driven by a pump
laser and the cavity vacuum [M.Hennrich et al., Phys. Rev. Lett. 85, 4672
(2000)], and a recycling laser is employed to repeat this process continuously.
For weak driving, a smooth transition from antibunching to bunching is found
for about one intra-cavity atom. Remarkably, the bunching peak develops within
the antibunching dip. For saturated driving and a growing number of atoms, the
bunching amplitude decreases and the bunching duration increases, indicating
the onset of Raman lasing.Comment: 4 pages, 4 figure
Role of HLA Class II-specific alloreactive T cells in biliary epithelium injury associated with liver transplant rejection
Construction of an isotropic cellular automaton for a reaction-diffusion equation by means of a random walk
We propose a new method to construct an isotropic cellular automaton
corresponding to a reaction-diffusion equation. The method consists of
replacing the diffusion term and the reaction term of the reaction-diffusion
equation with a random walk of microscopic particles and a discrete vector
field which defines the time evolution of the particles. The cellular automaton
thus obtained can retain isotropy and therefore reproduces the patterns found
in the numerical solutions of the reaction-diffusion equation. As a specific
example, we apply the method to the Belousov-Zhabotinsky reaction in excitable
media
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