23,089 research outputs found
Viscoelastic model for the dynamic structure of binary systems
This paper presents the viscoelastic model for the Ashcroft-Langreth dynamic
structure factors of liquid binary mixtures. We also provide expressions for
the Bhatia-Thornton dynamic structure factors and, within these expressions,
show how the model reproduces both the dynamic and the self-dynamic structure
factors corresponding to a one-component system in the appropriate limits
(pseudobinary system or zero concentration of one component). In particular we
analyze the behavior of the concentration-concentration dynamic structure
factor and longitudinal current, and their corresponding counterparts in the
one-component limit, namely, the self dynamic structure factor and self
longitudinal current. The results for several lithium alloys with different
ordering tendencies are compared with computer simulations data, leading to a
good qualitative agreement, and showing the natural appearance in the model of
the fast sound phenomenon.Comment: 20 pages, 19 figures, submitted to PR
Integral field observations of the blue compact galaxy Haro14. Star formation and feedback in dwarf galaxies
(Abridged) Low-luminosity, gas-rich blue compact galaxies (BCG) are ideal
laboratories to investigate many aspects of the star formation in galaxies. We
study the morphology, stellar content, kinematics, and the nebular excitation
and ionization mechanism in the BCG Haro 14 by means of integral field
observations with VIMOS in the VLT. From these data we build maps in continuum
and in the brighter emission lines, produce line-ratio maps, and obtain the
velocity and velocity dispersion fields. We also generate the integrated
spectrum of the major HII regions and young stellar clusters identified in the
maps to determine reliable physical parameters and oxygen abundances. We find
as follows: i) the current star formation in Haro 14 is spatially extended with
the major HII regions placed along a linear structure, elongated in the
north-south direction, and in a horseshoe-like curvilinear feature that extends
about 760 pc eastward; the continuum emission is more concentrated and peaks
close to the galaxy center; ii) two different episodes of star formation are
present: the recent starburst, with ages 6 Myrs and the intermediate-age
clusters, with ages between 10 and 30 Myrs; these stellar components rest on a
several Gyr old underlying host galaxy; iii) the H/H pattern is
inhomogeneous, with excess color values varying from E(B-V)=0.04 up to
E(B-V)=1.09; iv) shocks play a significant role in the galaxy; and v) the
velocity field displays a complicated pattern with regions of material moving
toward us in the east and north galaxy areas. The morphology of Haro 14, its
irregular velocity field, and the presence of shocks speak in favor of a
scenario of triggered star formation. Ages of the knots are consistent with the
ongoing burst being triggered by the collective action of stellar winds and
supernovae originated in the central clusters.Comment: 18 pages, 17 figures. Accepted for publication in A&
Probing star formation and feedback in dwarf galaxies. Integral field view of the blue compact galaxy Tololo 1937-423
(Abridged) Blue compact galaxies (BCG) are gas-rich, low-mass, small systems
that form stars at unusually high rates. This makes them excellent laboratories
for investigating the process of star-formation (SF) at galactic scales and the
effects of massive stellar feedback on the interstellar (and intergalactic)
medium.
We analyzed the BCG Tololo 1937-423 using optical integral field spectroscopy
with VIMOS at the Very Large Telescope to probe its morphology, stellar
content, nebular excitation and ionization properties, and the kinematics of
its warm ionized gas.
We found that Tololo 1937-423 is currently undergoing an extended starburst,
with nine major clumps. The galaxy presents a single continuum peak
that is not cospatial with any knot in emission lines, indicating at least two
relatively recent episodes of SF. The inhomogeneous dust distribution reaches
its maximum roughly at the position of the continuum peak. We found shocked
regions in the galaxy outer regions and at the edges of the SF knots. The
oxygen abundance is similar in all the SF regions, suggesting a chemically
homogeneous ionized interstellar medium over spatial scales of several kpc. The
ionized gas kinematics displays an overall regular rotation around a
northwest-southeast axis.
The morphology of the galaxy and the two different episodes of SF suggest a
scenario of triggered (induced by supernova shock waves) SF acting in Tololo
1937-423. The inferred ages for the different SF episodes (~13-80 Myr for the
central post-starburst and 5-7 Myr for the ongoing SF) are consistent with
triggered SF, with the most recent SF episode caused by the collective effect
of stellar winds and supernova explosions from the central post-starburst. The
velocity dispersion pattern, with higher velocity dispersions found at the
edges of the SF regions, and shocked regions in the galaxy, also favor this
scenario.Comment: 16 pages, 18 figures. Accepted for publication in A&
Generalized Galilean Algebras and Newtonian Gravity
The non-relativistic versions of the generalized Poincar\'{e} algebras and
generalized -Lorentz algebras are obtained. This non-relativistic algebras
are called, generalized Galilean algebras type I and type II and denoted by
and
respectively. Using a generalized In\"{o}n\"{u}--Wigner contraction procedure
we find that the generalized Galilean algebras type I can be obtained from the
generalized Galilean algebras type II. The -expansion procedure allows us to
find the algebra from the Newton--Hooke
algebra with central extension. The procedure developed in Ref. \cite{newton}
allow us to show that the non-relativistic limit of the five dimensional
Einstein--Chern--Simons gravity is given by a modified version of the Poisson
equation. The modification could be compatible with the effects of Dark Matter,
which leads us to think that Dark Matter can be interpreted as a
non-relativistic limit of Dark Energy.Comment: 16 pages, no figures in 755 (2016) 433-43
Berry-phase blockade in single-molecule magnets
We formulate the problem of electron transport through a single-molecule
magnet (SMM) in the Coulomb blockade regime taking into account topological
interference effects for the tunneling of the large spin of a SMM. The
interference originates from spin Berry phases associated with different
tunneling paths. We show that in the case of incoherent spin states it is
essential to place the SMM between oppositely spin-polarized source and drain
leads in order to detect the spin tunneling in the stationary current, which
exhibits topological zeros as a function of the transverse magnetic field.Comment: 4 pages, Revtex 4, 4 EPS figure
Geometrical resonance in spatiotemporal systems
We generalize the concept of geometrical resonance to perturbed sine-Gordon,
Nonlinear Schrödinger and Complex Ginzburg-Landau equations. Using this
theory we can control different dynamical patterns. For instance, we can
stabilize breathers and oscillatory patterns of large amplitudes successfully
avoiding chaos. On the other hand, this method can be used to suppress
spatiotemporal chaos and turbulence in systems where these phenomena are
already present. This method can be generalized to even more general
spatiotemporal systems.Comment: 2 .epl files. Accepted for publication in Europhysics Letter
Propagating, evanescent, and localized states in carbon nanotube-graphene junctions
We study the electronic structure of the junctions between a single graphene
layer and carbon nanotubes, using a tight-binding model and the continuum
theory based on Dirac fermion fields. The latter provides a unified description
of different lattice structures with curvature, which is always localized at
six heptagonal carbon rings around each junction. When these are evenly spaced,
we find that it is possible to curve the planar lattice into armchair (6n,6n)
as well as zig-zag (6n,0) nanotubes. We show that the junctions fall into two
different classes, regarding the low-energy electronic behavior. One of them,
constituted by the junctions made of the armchair nanotubes and the zig-zag
(6n,0) geometries when n is a multiple of 3, is characterized by the presence
of two quasi-bound states at the Fermi level, which are absent for the rest of
the zig-zag nanotubes. These states, localized at the junction, are shown to
arise from the effective gauge flux induced by the heptagonal carbon rings,
which has a direct reflection in the local density of states around the
junction. Furthermore, we also analyze the band structure of the arrays of
junctions, finding out that they can also be classified into two different
groups according to the low-energy behavior. In this regard, the arrays made of
armchair and (6n,0) nanotubes with n equal to a multiple of 3 are characterized
by the presence of a series of flat bands, whose number grows with the length
of the nanotubes. We show that such flat bands have their origin in the
formation of states confined to the nanotubes in the array. This is explained
in the continuum theory from the possibility of forming standing waves in the
mentioned nanotube geometries, as a superposition of modes with opposite
momenta and the same quantum numbers under the C_6v symmetry of the junction.Comment: 13 pages, 8 figure
Four dimensional Lie symmetry algebras and fourth order ordinary differential equations
Realizations of four dimensional Lie algebras as vector fields in the plane
are explicitly constructed. Fourth order ordinary differential equations which
admit such Lie symmetry algebras are derived. The route to their integration is
described.Comment: 12 page
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