1,021 research outputs found
Phase Transition in Ferromagnetic Ising Models with Non-Uniform External Magnetic Fields
In this article we study the phase transition phenomenon for the Ising model
under the action of a non-uniform external magnetic field. We show that the
Ising model on the hypercubic lattice with a summable magnetic field has a
first-order phase transition and, for any positive (resp. negative) and bounded
magnetic field, the model does not present the phase transition phenomenon
whenever , where is the external
magnetic field.Comment: 11 pages. Published in Journal of Statistical Physics - 201
Particle Motion and Electromagnetic Fields of Rotating Compact Gravitating Objects with Gravitomagnetic Charge
The exact solution for the electromagnetic field occuring when the
Kerr-Taub-NUT compact object is immersed (i) in an originally uniform magnetic
field aligned along the axis of axial symmetry (ii) in dipolar magnetic field
generated by current loop has been investigated. Effective potential of motion
of charged test particle around Kerr-Taub-NUT gravitational source immersed in
magnetic field with different values of external magnetic field and NUT
parameter has been also investigated. In both cases presence of NUT parameter
and magnetic field shifts stable circular orbits in the direction of the
central gravitating object. Finally we find analytical solutions of Maxwell
equations in the external background spacetime of a slowly rotating magnetized
NUT star. The star is considered isolated and in vacuum, with monopolar
configuration model for the stellar magnetic field.Comment: 18 pages, 6 figures, new results in section 2 added, section 3 is
revised, 3 references are adde
Percolation on two- and three-dimensional lattices
In this work we apply a highly efficient Monte Carlo algorithm recently
proposed by Newman and Ziff to treat percolation problems. The site and bond
percolation are studied on a number of lattices in two and three dimensions.
Quite good results for the wrapping probabilities, correlation length critical
exponent and critical concentration are obtained for the square, simple cubic,
HCP and hexagonal lattices by using relatively small systems. We also confirm
the universal aspect of the wrapping probabilities regarding site and bond
dilution.Comment: 15 pages, 6 figures, 3 table
An Inhomogeneous Model Universe Behaving Homogeneously
We present a new model universe based on the junction of FRW to flat
Lemaitre-Tolman-Bondi (LTB) solutions of Einstein equations along our past
light cone, bringing structures within the FRW models. The model is assumed
globally to be homogeneous, i.e. the cosmological principle is valid. Local
inhomogeneities within the past light cone are modeled as a flat LTB, whereas
those outside the light cone are assumed to be smoothed out and represented by
a FRW model. The model is singularity free, always FRW far from the observer
along the past light cone, gives way to a different luminosity distance
relation as for the CDM/FRW models, a negative deceleration parameter near the
observer, and correct linear and non-linear density contrast. As a whole, the
model behaves like a FRW model on the past light cone with a special behavior
of the scale factor, Hubble and deceleration parameter, mimicking dark energy.Comment: 23 pages, 19 figures, published version in GR
Is the Universe Inflating? Dark Energy and the Future of the Universe
We consider the fate of the observable universe in the light of the discovery
of a dark energy component to the cosmic energy budget. We extend results for a
cosmological constant to a general dark energy component and examine the
constraints on phenomena that may prevent the eternal acceleration of our patch
of the universe. We find that the period of accelerated cosmic expansion has
not lasted long enough for observations to confirm that we are undergoing
inflation; such an observation will be possible when the dark energy density
has risen to between 90% and 95% of the critical. The best we can do is make
cosmological observations in order to verify the continued presence of dark
energy to some high redshift. Having done that, the only possibility that could
spoil the conclusion that we are inflating would be the existence of a
disturbance (the surface of a true vacuum bubble, for example) that is moving
toward us with sufficiently high velocity, but is too far away to be currently
observable. Such a disturbance would have to move toward us with speed greater
than about 0.8c in order to spoil the late-time inflation of our patch of the
universe and yet avoid being detectable.Comment: 7 pages, 7 figure
Tunable local polariton modes in semiconductors
We study the local states within the polariton bandgap that arise due to deep
defect centers with strong electron-phonon coupling. Electron transitions
involving deep levels may result in alteration of local elastic constants. In
this case, substantial reversible transformations of the impurity polariton
density of states occur, which include the appearance/disappearance of the
polariton impurity band, its shift and/or the modification of its shape. These
changes can be induced by thermo- and photo-excitation of the localized
electron states or by trapping of injected charge carriers. We develop a simple
model, which is applied to the center in . Further possible
experimental realizations of the effect are discussed.Comment: 7 pages, 3 figure
Quintessence Cosmology and the Cosmic Coincidence
Within present constraints on the observed smooth energy and its equation of
state parameter, it is important to find out whether the smooth energy is
static (cosmological constant) or dynamic (quintessence). The most dynamical
quintessence fields observationally allowed are now still fast-rolling and no
longer satisfy the tracker approximation if the equation of state parameter
varies moderately with cosmic scale. We are optimistic about distinguishing
between a cosmological constant and appreciably dynamic quintessence, by
measuring average values for the effective equation of state parameter.
However, reconstructing the quintessence potential from observations of any
scale dependence appears problematic in the near future. For our flat universe,
at present dominated by smooth energy in the form of either a cosmological
constant (LCDM) or quintessence (QCDM), we calculate the asymptotic collapsed
mass fraction to be maximal at the observed smooth energy/matter ratio.
Identifying this collapsed fraction as a conditional probability for habitable
galaxies, we infer that the prior distribution is flat. Interpreting this prior
as a distribution over theories, rather than as a distribution over
unobservable subuniverses, leads us to heuristic predictions about the class of
future quantum cosmology theories and the static or quasi-static nature of the
smooth energy.Comment: Typos corrected, as presented at Cosmo-01 Workshop, Rovaniemi,
Finland and accepted for publication in Physical Review D. 9 pages, 4 figure
Absorption and quasinormal modes of classical fields propagating on 3D and 4D de Sitter spacetime
We extensively study the exact solutions of the massless Dirac equation in 3D
de Sitter spacetime that we published recently. Using the Newman-Penrose
formalism, we find exact solutions of the equations of motion for the massless
classical fields of spin s=1/2,1,2 and to the massive Dirac equation in 4D de
Sitter metric. Employing these solutions, we analyze the absorption by the
cosmological horizon and de Sitter quasinormal modes. We also comment on the
results given by other authors.Comment: 31 page
Unconstrained Hamiltonian Formulation of SU(2) Gluodynamics
SU(2) Yang-Mills field theory is considered in the framework of the
generalized Hamiltonian approach and the equivalent unconstrained system is
obtained using the method of Hamiltonian reduction. A canonical transformation
to a set of adapted coordinates is performed in terms of which the
Abelianization of the Gauss law constraints reduces to an algebraic operation
and the pure gauge degrees of freedom drop out from the Hamiltonian after
projection onto the constraint shell. For the remaining gauge invariant fields
two representations are introduced where the three fields which transform as
scalars under spatial rotations are separated from the three rotational fields.
An effective low energy nonlinear sigma model type Lagrangian is derived which
out of the six physical fields involves only one of the three scalar fields and
two rotational fields summarized in a unit vector. Its possible relation to the
effective Lagrangian proposed recently by Faddeev and Niemi is discussed.
Finally the unconstrained analog of the well-known nonnormalizable groundstate
wave functional which solves the Schr\"odinger equation with zero energy is
given and analysed in the strong coupling limit.Comment: 20 pages REVTEX, no figures; final version to appear in Phys. Rev. D;
minor changes, notations simplifie
let-7 microRNAs regulate microglial function and suppress glioma growth through Toll-like receptor 7
Microglia express Toll-like receptors (TLRs) that sense pathogen- and host-derived factors, including single-stranded RNA. In the brain, let-7 microRNA (miRNA) family members are abundantly expressed, and some have recently been shown to serve as TLR7 ligands. We investigated whether let-7 miRNA family members differentially control microglia biology in health and disease. We found that a subset of let-7 miRNA family members function as signaling molecules to induce microglial release of inflammatory cytokines, modulate antigen presentation, and attenuate cell migration in a TLR7-dependent manner. The capability of the let-7 miRNAs to control microglial function is sequence specific, mapping to a let-7 UUGU motif. In human and murine glioblastoma/glioma, let-7 miRNAs are differentially expressed and reduce murine GL261 glioma growth in the same sequence-specific fashion through microglial TLR7. Taken together, these data establish let-7 miRNAs as key TLR7 signaling activators that serve to regulate the diverse functions of microglia in health and glioma
- …