631 research outputs found
Stable two-dimensional solitary pulses in linearly coupled dissipative Kadomtsev-Petviashvili equations
A two-dimensional (2D) generalization of the stabilized Kuramoto -
Sivashinsky (KS) system is presented. It is based on the Kadomtsev-Petviashvili
(KP) equation including dissipation of the generic (Newell -- Whitehead --
Segel, NWS) type and gain. The system directly applies to the description of
gravity-capillary waves on the surface of a liquid layer flowing down an
inclined plane, with a surfactant diffusing along the layer's surface.
Actually, the model is quite general, offering a simple way to stabilize
nonlinear waves in media combining the weakly-2D dispersion of the KP type with
gain and NWS dissipation. Parallel to this, another model is introduced, whose
dissipative terms are isotropic, rather than of the NWS type. Both models
include an additional linear equation of the advection-diffusion type, linearly
coupled to the main KP-NWS equation. The extra equation provides for stability
of the zero background in the system, opening a way to the existence of stable
localized pulses. The consideration is focused on the case when the dispersive
part of the system of the KP-I type, admitting the existence of 2D localized
pulses. Treating the dissipation and gain as small perturbations and making use
of the balance equation for the field momentum, we find that the equilibrium
between the gain and losses may select two 2D solitons, from their continuous
family existing in the conservative counterpart of the model (the latter family
is found in an exact analytical form). The selected soliton with the larger
amplitude is expected to be stable. Direct simulations completely corroborate
the analytical predictions.Comment: a latex text file and 16 eps files with figures; Physical Review E,
in pres
Wetting transition of grain boundaries in the Sn-rich part of the Sn-Bi phase diagram
The microstructural evolution of tin-rich Sn-Bi alloys after the grain boundary wetting phase transition in the (liquid + beta-Sn) two-phase region of the Sn-Bi phase diagram was investigated. Three Sn-Bi alloys with 30.6, 23, and 10 wt% Bi were annealed between 139 and 215 A degrees C for 24 h. The micrographs of Sn-Bi alloys reveal that the small amount of liquid phase prevented the grain boundary wetting transition to occur during annealing close to the solidus line. The melted area of the grain boundary triple junctions and grain boundaries increased with increasing the annealing temperature. When the amount of liquid phase exceeded 34 wt% during annealing, increasing temperature has not affected the wetting behavior of grain boundaries noticeably and led only to the increase of the amount of liquid phase among solid grains in the microstructure. The XRD results show that the phase structure and crystallinity remained unchanged after quenching from various annealing temperatures
Resonant forcing of select degrees of freedom of multidimensional chaotic map dynamics
We study resonances of multidimensional chaotic map dynamics. We use the
calculus of variations to determine the additive forcing function that induces
the largest response, that is, the greatest deviation from the unperturbed
dynamics. We include the additional constraint that only select degrees of
freedom be forced, corresponding to a very general class of problems in which
not all of the degrees of freedom in an experimental system are accessible to
forcing. We find that certain Lagrange multipliers take on a fundamental
physical role as the efficiency of the forcing function and the effective
forcing experienced by the degrees of freedom which are not forced directly.
Furthermore, we find that the product of the displacement of nearby
trajectories and the effective total forcing function is a conserved quantity.
We demonstrate the efficacy of this methodology with several examples.Comment: 11 pages, 3 figure
A Conformal Field Theory for Eternal Inflation
We study a statistical model defined by a conformally invariant distribution
of overlapping spheres in arbitrary dimension d. The model arises as the
asymptotic distribution of cosmic bubbles in d+1 dimensional de Sitter space,
and also as the asymptotic distribution of bubble collisions with the domain
wall of a fiducial "observation bubble" in d+2 dimensional de Sitter space. In
this note we calculate the 2-,3-, and 4-point correlation functions of
exponentials of the "bubble number operator" analytically in d=2. We find that
these correlators, when carefully defined, are free of infrared divergences,
covariant under the global conformal group, charge conserving, and transform
with positive conformal dimensions that are related in a novel way to the
charge. Although by themselves these operators probably do not define a
full-fledged conformal field theory, one can use the partition function on a
sphere to compute an approximate central charge in the 2D case. The theory in
any dimension has a noninteracting limit when the nucleation rate of the
bubbles in the bulk is very large. The theory in two dimensions is related to
some models of continuum percolation, but it is conformal for all values of the
tunneling rate.Comment: 30 pages, 8 figure
Relating the Cosmological Constant and Supersymmetry Breaking in Warped Compactifications of IIB String Theory
It has been suggested that the observed value of the cosmological constant is
related to the supersymmetry breaking scale M_{susy} through the formula Lambda
\sim M_p^4 (M_{susy}/M_p)^8. We point out that a similar relation naturally
arises in the codimension two solutions of warped space-time varying
compactifications of string theory in which non-isotropic stringy moduli induce
a small but positive cosmological constant.Comment: 7 pages, LaTeX, references added and minor changes made, (v3) map
between deSitter and global cosmic brane solutions clarified, supersymmetry
breaking discussion improved and references adde
Dimer coverings on the Sierpinski gasket with possible vacancies on the outmost vertices
We present the number of dimers on the Sierpinski gasket
at stage with dimension equal to two, three, four or five, where one of
the outmost vertices is not covered when the number of vertices is an
odd number. The entropy of absorption of diatomic molecules per site, defined
as , is calculated to be
exactly for . The numbers of dimers on the generalized
Sierpinski gasket with and are also obtained
exactly. Their entropies are equal to , , ,
respectively. The upper and lower bounds for the entropy are derived in terms
of the results at a certain stage for with . As the
difference between these bounds converges quickly to zero as the calculated
stage increases, the numerical value of with can be
evaluated with more than a hundred significant figures accurate.Comment: 35 pages, 20 figures and 1 tabl
Isolation of Bartonella species from rodents in Taiwan including a strain closely related to 'Bartonella rochalimae' from Rattus norvegicus
An increasing number of Bartonella species originally isolated from small mammals have been identified as emerging human pathogens. During an investigation of Bartonella infection in rodent populations carried out in Taiwan in 2006, a total of 58 rodents were tested. It was determined that 10.3% (6/58) of the animals were Bartonella bacteraemic. After PCR/RFLP analysis, four isolates were identified as Bartonella elizabethae and one isolate as Bartonella tribocorum. However, there was one specific isolate with an unrecognized PCR/RFLP pattern. After further sequence and phylogenetic analyses of the gltA, ftsZ and rpoB genes, and the 16S-23S rRNA intergenic spacer region, the results indicated that this specific isolate from Rattus norvegicus was closely related to human pathogenic 'Bartonella rochalimae'. Further studies need to be conducted to evaluate whether this rodent species could be a reservoir for 'B. rochalimae'
Effective Field Theory for Layered Quantum Antiferromagnets with Non-Magnetic Impurities
We propose an effective two-dimensional quantum non-linear sigma model
combined with classical percolation theory to study the magnetic properties of
site diluted layered quantum antiferromagnets like
LaCuMO (MZn, Mg). We calculate the staggered
magnetization at zero temperature, , the magnetic correlation length,
, the NMR relaxation rate, , and the N\'eel temperature,
, in the renormalized classical regime. Due to quantum fluctuations we
find a quantum critical point (QCP) at at lower doping than
the two-dimensional percolation threshold . We compare our
results with the available experimental data.Comment: Final version accepted for publication as a Rapid Communication on
Physical Review B. A new discussion on the effect of disorder in layered
quantum antiferromagnets is include
Non-perturbative calculations for the effective potential of the symmetric and non-Hermitian field theoretic model
We investigate the effective potential of the symmetric
field theory, perturbatively as well as non-perturbatively. For the
perturbative calculations, we first use normal ordering to obtain the first
order effective potential from which the predicted vacuum condensate vanishes
exponentially as in agreement with previous calculations. For the
higher orders, we employed the invariance of the bare parameters under the
change of the mass scale to fix the transformed form totally equivalent to
the original theory. The form so obtained up to is new and shows that all
the 1PI amplitudes are perurbative for both and regions. For
the intermediate region, we modified the fractal self-similar resummation
method to have a unique resummation formula for all values. This unique
formula is necessary because the effective potential is the generating
functional for all the 1PI amplitudes which can be obtained via and thus we can obtain an analytic calculation for the 1PI
amplitudes. Again, the resummed from of the effective potential is new and
interpolates the effective potential between the perturbative regions.
Moreover, the resummed effective potential agrees in spirit of previous
calculation concerning bound states.Comment: 20 page
Multicomponent theory of buoyancy instabilities in magnetized plasmas: The case of magnetic field parallel to gravity
We investigate electromagnetic buoyancy instabilities of the electron-ion
plasma with the heat flux based on not the magnetohydrodynamic (MHD) equations,
but using the multicomponent plasma approach when the momentum equations are
solved for each species. We consider a geometry in which the background
magnetic field, gravity, and stratification are directed along one axis. The
nonzero background electron thermal flux is taken into account. Collisions
between electrons and ions are included in the momentum equations. No
simplifications usual for the one-fluid MHD-approach in studying these
instabilities are used. We derive a simple dispersion relation, which shows
that the thermal flux perturbation generally stabilizes an instability for the
geometry under consideration. This result contradicts to conclusion obtained in
the MHD-approach. We show that the reason of this contradiction is the
simplified assumptions used in the MHD analysis of buoyancy instabilities and
the role of the longitudinal electric field perturbation which is not captured
by the ideal MHD equations. Our dispersion relation also shows that the medium
with the electron thermal flux can be unstable, if the temperature gradients of
ions and electrons have the opposite signs. The results obtained can be applied
to the weakly collisional magnetized plasma objects in laboratory and
astrophysics.Comment: Accepted for publication in Astrophysics & Space Scienc
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