105 research outputs found
Brownian motion of black holes in stellar systems with non-Maxwellian distribution for the stars field
A massive black hole at the center of a dense stellar system, such as a
globular cluster or a galactic nucleus, is subject to a random walk due
gravitational encounters with nearby stars. It behaves as a Brownian particle,
since it is much more massive than the surrounding stars and moves much more
slowly than they do. If the distribution function for the stellar velocities is
Maxwellian, there is a exact equipartition of kinetic energy between the black
hole and the stars in the stationary state. However, if the distribution
function deviates from a Maxwellian form, the strict equipartition cannot be
achieved. The deviation from equipartition is quantified in this work by
applying the Tsallis q-distribution for the stellar velocities in a
q-isothermal stellar system and in a generalized King model.Comment: Presented at XXVI Int. Astronomical Union General Assembly, Symp.
238, Prague, Czech Republic, Aug 21-25 200
Logarithmic diffusion and porous media equations: a unified description
In this work we present the logarithmic diffusion equation as a limit case
when the index that characterizes a nonlinear Fokker-Planck equation, in its
diffusive term, goes to zero. A linear drift and a source term are considered
in this equation. Its solution has a lorentzian form, consequently this
equation characterizes a super diffusion like a L\'evy kind. In addition is
obtained an equation that unifies the porous media and the logarithmic
diffusion equations, including a generalized diffusion equation in fractal
dimension. This unification is performed in the nonextensive thermostatistics
context and increases the possibilities about the description of anomalous
diffusive processes.Comment: 5 pages. To appear in Phys. Rev.
Nonlinear equation for anomalous diffusion: unified power-law and stretched exponential exact solution
The nonlinear diffusion equation is analyzed here, where , and , and are real parameters.
This equation unifies the anomalous diffusion equation on fractals ()
and the spherical anomalous diffusion for porous media (). Exact
point-source solution is obtained, enabling us to describe a large class of
subdiffusion (), normal diffusion () and
superdiffusion (). Furthermore, a thermostatistical basis
for this solution is given from the maximum entropic principle applied to the
Tsallis entropy.Comment: 3 pages, 2 eps figure
Thermal Conductivity of superconducting (TMTSF)_2ClO_4: evidence for a nodeless gap
We report on the first measurements of thermal conductivity in the
superconducting state of (TMTSF)_2ClO_4. The electronic contribution to heat
transport is found to decrease rapidly below T_c, indicating the absence of
low-energy electronic excitations. We argue that this result provides strong
evidence for a nodeless superconducting gap function but does not exclude a
possible unconventional order parameter.Comment: 4 pages including 4 figures, submitted to Phys. Rev. Let
Nonlinear anomalous diffusion equation and fractal dimension: Exact generalized gaussian solution
In this work we incorporate, in a unified way, two anomalous behaviors, the
power law and stretched exponential ones, by considering the radial dependence
of the -dimensional nonlinear diffusion equation where , ,
, and are real parameters and is a time-dependent
source. This equation unifies the O'Shaugnessy-Procaccia anomalous diffusion
equation on fractals () and the spherical anomalous diffusion for
porous media (). An exact spherical symmetric solution of this
nonlinear Fokker-Planck equation is obtained, leading to a large class of
anomalous behaviors. Stationary solutions for this Fokker-Planck-like equation
are also discussed by introducing an effective potential.Comment: Latex, 6 pages. To appear in Phys. Rev.
On-chain electrodynamics of metallic (TMTSF)_2 X salts: Observation of Tomonaga-Luttinger liquid response
We have measured the electrodynamic response in the metallic state of three
highly anisotropic conductors, (TMTSF)_2 X, where X=PF_6, AsF_6, or ClO_4, and
TMTSF is the organic molecule tetramethyltetraselenofulvalene. In all three
cases we find dramatic deviations from a simple Drude response. The optical
conductivity has two features: a narrow mode at zero frequency, with a small
spectral weight, and a mode centered around 200 cm^{-1}, with nearly all of the
spectral weight expected for the relevant number of carriers and single
particle bandmass. We argue that these features are characteristic of a nearly
one-dimensional half- or quarter-filled band with Coulomb correlations, and
evaluate the finite energy mode in terms of a one-dimensional Mott insulator.
At high frequencies (\hbar\omega > t_\perp, the transfer integral perpendicular
to the chains), the frequency dependence of the optical conductivity
\sigma_1(\omega) is in agreement with calculations based on an interacting
Tomonaga-Luttinger liquid, and is different from what is expected for an
uncorrelated one-dimensional semiconductor. The zero frequency mode shows
deviations from a simple Drude response, and can be adequately described with a
frequency dependent mass and relaxation rate.Comment: 12 pages, 7 figures, RevTeX; minor corrections to text and
references; To be published in Phys. Rev. B, 15 July 199
Small-q phonon-mediated superconductivity in organic \kappa-BEDT-TTF compounds
We propose a mew picture for superconductivity in
salts arguing that small-{\bf q} electron-phonon scattering dominates the
pairing. We reproduce the distinct X-shaped d-wave gap reported recently by
magnetooptic measurements and we argue that the softness of the momentum
structure of the gap and the near degeneracy of s- and d-wave gap states may be
at the origin of the experimental controversy about the gap symmetry. We show
that a magnetic field applied parallel to the planes may induce extended
gapless-regions on the FS accounting for the experimental signatures of a
Fulde-Ferrel-Larkin-Ovchinikov state and it may induce gap symmetry transitions
as well.Comment: 4 pages and 7 figure
Gadd45α activity is the principal effector of Shigella mitochondria-dependent epithelial cell death in vitro and ex vivo
Modulation of death is a pathogen strategy to establish residence and promote survival in host cells and tissues. Shigella spp. are human pathogens that invade colonic mucosa, where they provoke lesions caused by their ability to manipulate the host cell responses. Shigella spp. induce various types of cell death in different cell populations. However, they are equally able to protect host cells from death. Here, we have investigated on the molecular mechanisms and cell effectors governing the balance between survival and death in epithelial cells infected with Shigella. To explore these aspects, we have exploited both, the HeLa cell invasion assay and a novel ex vivo human colon organ culture model of infection that mimics natural conditions of shigellosis. Our results definitely show that Shigella induces a rapid intrinsic apoptosis of infected cells, via mitochondrial depolarization and the ensuing caspase-9 activation. Moreover, for the first time we identify the eukaryotic stress-response factor growth arrest and DNA damage 45α as a key player in the induction of the apoptotic process elicited by Shigella in epithelial cells, revealing an unexplored role of this molecule in the course of infections sustained by invasive pathogens
- …