17,692 research outputs found
On the Temperley-Lieb reflection matrices
This work concerns the boundary integrability of the spin-s
Temperley-Lieb model. A systematic computation method is
used to constructed the solutions of the boundary Yang-Baxter equations. For
half-integer, a general free parameter solution is presented.
It turns that for integer, the general solution has free
parameters. Moreover, some particular solutions are discussed.Comment: LaTex 17 page
Exact Lyapunov Exponent for Infinite Products of Random Matrices
In this work, we give a rigorous explicit formula for the Lyapunov exponent
for some binary infinite products of random real matrices. All
these products are constructed using only two types of matrices, and ,
which are chosen according to a stochastic process. The matrix is singular,
namely its determinant is zero. This formula is derived by using a particular
decomposition for the matrix , which allows us to write the Lyapunov
exponent as a sum of convergent series. Finally, we show with an example that
the Lyapunov exponent is a discontinuous function of the given parameter.Comment: 1 pages, CPT-93/P.2974,late
Beyond Mean-Field Low-Lying Excitations of Dipolar Bose Gases
We theoretically investigate various beyond mean-field effects on Bose gases
at zero temperature featuring the anisotropic and long-range dipole-dipole
interaction in addition to the isotropic and short-range contact interaction.
Within the realm of the Bogoliubov-de Gennes theory, we consider static
properties and low-lying excitations of both homogeneous and harmonically
trapped dipolar bosonic gases. For the homogeneous system, the condensate
depletion, the ground-state energy, the equation of state, and the speed of
sound are discussed in detail. Making use of the local density approximation,
we extend these results in order to study the properties of a dipolar Bose gas
in a harmonic trap and in the regime of large particle numbers. After deriving
the equations of motion for the general case of a triaxial trap, we analyze the
influence of quantum fluctuations on important properties of the gas, such as
the equilibrium configuration and the low-lying excitations in the case of a
cylinder-symmetric trap. In addition to the monopole and quadrupole oscillation
modes, we also discuss the radial quadrupole mode. We find that the latter
acquires a quantum correction exclusively due to the dipole-dipole interaction.
As a result, we identify the radial quadrupole as a reasonably accessible
source for the signature of dipolar many-body effects and stress the enhancing
character that dipolar interactions have for quantum fluctuations in the other
oscillation modes.Comment: Version published in PR
Accelerating Cold Dark Matter Cosmology ()
A new kind of accelerating flat model with no dark energy that is fully
dominated by cold dark matter (CDM) is investigated. The number of CDM
particles is not conserved and the present accelerating stage is a consequence
of the negative pressure describing the irreversible process of gravitational
particle creation. A related work involving accelerating CDM cosmology has been
discussed before the SNe observations [Lima, Abramo & Germano, Phys. Rev. D53,
4287 (1996)]. However, in order to have a transition from a decelerating to an
accelerating regime at low redshifts, the matter creation rate proposed here
includes a constant term of the order of the Hubble parameter. In this case,
does not need to be small in order to solve the age problem and the
transition happens even if the matter creation is negligible during the
radiation and part of the matter dominated phase. Therefore, instead of the
vacuum dominance at redshifts of the order of a few, the present accelerating
stage in this sort of Einstein-de Sitter CDM cosmology is a consequence of the
gravitational particle creation process. As an extra bonus, in the present
scenario does not exist the coincidence problem that plagues models with
dominance of dark energy. The model is able to harmonize a CDM picture with the
present age of the universe, the latest measurements of the Hubble parameter
and the Supernovae observations.Comment: 9 pages, 6 figures, typos corrected, references added, discussion in
Appendix B extende
Exact solution of A-D Temperley-Lieb Models
We solve for the spectrum of quantum spin chains based on representations of
the Temperley-Lieb algebra associated with the quantum groups {\cal U}_q(X_n }
for X_n = A_1,B_n,C_nD_n$. We employ a generalization of the coordinate
Bethe-Ansatz developed previously for the deformed biquadratic spin one chain.
As expected, all these models have equivalent spectra, i.e. they differ only in
the degeneracy of their eigenvalues. This is true for finite length and open
boundary conditions. For periodic boundary conditions the spectra of the lower
dimensional representations are containded entirely in the higher dimensional
ones. The Bethe states are highest weight states of the quantum group, except
for some states with energy zero
Awaking the vacuum in relativistic stars
Void of any inherent structure in classical physics, the vacuum has revealed
to be incredibly crowded with all sorts of processes in relativistic quantum
physics. Yet, its direct effects are usually so subtle that its structure
remains almost as evasive as in classical physics. Here, in contrast, we report
on the discovery of a novel effect according to which the vacuum is compelled
to play an unexpected central role in an astrophysical context. We show that
the formation of relativistic stars may lead the vacuum energy density of a
quantum field to an exponential growth. The vacuum-driven evolution which would
then follow may lead to unexpected implications for astrophysics, while the
observation of stable neutron-star configurations may teach us much on the
field content of our Universe.Comment: To appear in Phys. Rev. Let
Cosmological Inhomogeneities with Bose-Einstein Condensate Dark Matter
We consider the growth of cosmological perturbations to the energy density of
dark matter during matter domination when dark matter is a scalar field that
has undergone Bose-Einstein condensation. We study these inhomogeneities within
the framework of both Newtonian gravity, where the calculation and results are
more transparent, and General Relativity. The direction we take is to derive
analytical expressions, which can be obtained in the small pressure limit.
Throughout we compare our results to those of the standard cosmology, where
dark matter is assumed pressureless, using our analytical expressions to
showcase precise differences. We find, compared to the standard cosmology, that
Bose-Einstein condensate dark matter leads to a scale factor, gravitational
potential and density contrast that increase at faster rates.Comment: 17 pages, 2 figures; typos corrected, references adde
Potencial de isolados nativos de rizobactérias no controle biológico da fusariose na cultura do maracujazeiro.
On the shape of barchan dunes
Barchans are crescent-shaped sand dunes forming in aride regions with
unidirectional wind and limited sand supply. We report analytical and numerical
results for dune shapes under different environmental conditions as obtained
from the so-called `minimal model' of aeolian sand dunes. The profiles of
longitudinal vertical slices (i.e. along the wind direction) are analyzed as a
function of wind speed and sand supply. Shape transitions can be induced by
changes of mass, wind speed and sand supply. Within a minimal extension of the
model to the transverse direction the scale-invariant profile of transverse
vertical cuts can be derived analytically.Comment: to appear in J. Phys.: Condens. Matter 17 (2005
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