93 research outputs found
Exact energy spectrum of a two-temperature kinetic Ising model
The exact energy spectrum is developed for a two temperature kinetic Ising
spin chain, and its dual reaction diffusion system with spatially alternating
pair annihilation and creation rates. Symmetries of the system
pseudo-Hamiltonian that enable calculation of the spectrum are also used to
derive explicit state vectors for small system sizes, and to make observations
regarding state vectors in the general case. Physical consequences of the
surprisingly simple form for the eigenvalues are also discussed
Stable three-dimensional spinning optical solitons supported by competing quadratic and cubic nonlinearities
We show that the quadratic interaction of fundamental and second harmonics in
a bulk dispersive medium, combined with self-defocusing cubic nonlinearity,
give rise to completely localized spatiotemporal solitons (vortex tori) with
vorticity s=1. There is no threshold necessary for the existence of these
solitons. They are found to be stable against small perturbations if their
energy exceeds a certain critical value, so that the stability domain occupies
about 10% of the existence region of the solitons. We also demonstrate that the
s=1 solitons are stable against very strong perturbations initially added to
them. However, on the contrary to spatial vortex solitons in the same model,
the spatiotemporal solitons with s=2 are never stable.Comment: latex text, 10 ps and 2 jpg figures; Physical Review E, in pres
Vortex stability in nearly two-dimensional Bose-Einstein condensates with attraction
We perform accurate investigation of stability of localized vortices in an
effectively two-dimensional ("pancake-shaped") trapped BEC with negative
scattering length. The analysis combines computation of the stability
eigenvalues and direct simulations. The states with vorticity S=1 are stable in
a third of their existence region, , where is
the number of atoms, and is the corresponding collapse
threshold. Stable vortices easily self-trap from arbitrary initial
configurations with embedded vorticity. In an adjacent interval, , the unstable vortex
periodically splits in two fragments and recombines. At , the fragments do not recombine, as each one collapses by
itself. The results are compared with those in the full 3D Gross-Pitaevskii
equation. In a moderately anisotropic 3D configuration, with the aspect ratio
, the stability interval of the S=1 vortices occupies
of their existence region, hence the 2D limit provides for a reasonable
approximation in this case. For the isotropic 3D configuration, the stability
interval expands to 65% of the existence domain. Overall, the vorticity
heightens the actual collapse threshold by a factor of up to 2. All vortices
with are unstable.Comment: 21 pages, 8 figures, to appear in Physical Review
Cooperative sequential adsorption models on a Cayley tree: analytical results and applications
We present a class of cooperative sequential adsorption models on a Cayley
tree with constant and variable attachment rates and their possible
applications for ionic self-assembly of thin films and drug encapsulation of
nanoparticles. Using the empty interval method, and generalizing results known
from reaction-diffusion processes on Cayley trees, we calculate a variety of
quantities such as time-dependent surface coverage and time-dependent
probabilities of certain particle configurations
Stochastic epidemic-type model with enhanced connectivity: exact solution
We present an exact analytical solution to a one-dimensional model of the
Susceptible-Infected-Recovered (SIR) epidemic type, with infection rates
dependent on nearest-neighbor occupations. We use a quantum mechanical
approach, transforming the master equation via a quantum spin operator
formulation. We calculate exactly the time-dependent density of infected,
recovered and susceptible populations for random initial conditions, and
compare our results with a low connectivity SIR model reported by Schuetz et
al.. Our results compare well to those of previous work, validating the model
as a useful tool for additional and extended studies in this important area.
Our model also provides exact solutions for the n-point correlation functions,
and can be extended to more complex epidemic type models
Non-equilibrium statistical mechanics: a solvable model
A two-temperature linear spin model is presented that allows an easily
understandable introduction to non-equilibrium statistical physics. The model
is one that includes the concepts that are typical of more realistic
non-equilibrium models but that allows straightforward steady state solutions
and, for small systems, development of the full time dependence for
configuration probabilities. The model is easily accessible to upper-level
undergraduate students, and also provides a good check for computer models of
larger systems
The relationship between gut microbiota and spontaneous bacterial peritonitis in patients with liver cirrhosis - a literature review
Gut microbiota is an essential component in the pathogenesis of liver cirrhosis and its complications. There is a direct relationship between the gut and the liver called the gutliver axis through which bacteria can reach the liver through the portal venous blood. However, it remains unclear how bacteria leave the intestine and reach the fluid collection in the abdomen. A series of mechanisms have been postulated to be involved in the pathogenesis of spontaneous bacterial peritonitis (SBP) and other complications of liver cirrhosis, including bacterial translocation, bacterial overgrowth, altered intestinal permeability and dysfunctional immunity. The hepatic function may also be affected by the alteration of intestinal microbiota composition. Current treatment in SBP is antibiotic therapy, but lately, probiotics have been the useful treatment suggested to improve the intestinal barrier and prevent bacterial translocation. However, studies are contradictory regarding their usefulness. In this review, we will summarize the literature data on the pathogenesis of spontaneous bacterial peritonitis concerning the existence of a relationship with the microbiota and the useful use of probiotics
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