25,536 research outputs found
Global-String and Vortex Superfluids in a Supersymmetric Scenario
The main goal of this work is to investigate the possibility of finding the
supersymmetric version of the U(1)-global string model which behaves as a
vortex-superfluid. To describe the superfluid phase, we introduce a
Lorentz-symmetry breaking background that, in an approach based on
supersymmetry, leads to a discussion on the relation between the violation of
Lorentz symmetry and explicit soft supersymmetry breakings. We also study the
relation between the string configuration and the vortex-superfluid phase. In
the framework we settle down in terms of superspace and superfields, we
actually establish a duality between the vortex degrees of freedom and the
component fields of the Kalb-Ramond superfield. We make also considerations
about the fermionic excitations that may appear in connection with the vortex
formation.Comment: 9 pages. This version presented the relation between Lorentz symmetry
violation by the background and the appearance of terms that explicitly break
SUS
Molecule survival in magnetized protostellar disk winds. II. Predicted H2O line profiles versus Herschel/HIFI observations
We investigate whether the broad wings of H2O emission identified with
Herschel towards low-mass Class 0 and Class 1 protostars may be consistent with
an origin in a dusty MHD disk wind, and the constraints it would set on the
underlying disk properties. We present synthetic H2O line profiles predictions
for a typical MHD disk wind solution with various values of disk accretion
rate, stellar mass, extension of the launching area, and view angle. We compare
them in terms of line shapes and intensities with the HIFI profiles observed by
the WISH Key Program. We find that a dusty MHD disk wind launched from 0.2--0.6
AU AU to 3--25 AU can reproduce to a remarkable degree the observed shapes and
intensities of the broad H2O component, both in the fundamental 557 GHz line
and in more excited lines. Such a model also readily reproduces the observed
correlation of 557 GHz line luminosity with envelope density, if the infall
rate at 1000 AU is 1--3 times the disk accretion rate in the wind ejection
region. It is also compatible with the typical disk size and bolometric
luminosity in the observed targets. However, the narrower line profiles in
Class 1 sources suggest that MHD disk winds in these sources, if present, would
have to be slower and/or less water rich than in Class 0 sources. In
conclusion, MHD disk winds appear as a valid (though not unique) option to
consider for the origin of the broad H2O component in low-mass protostars. ALMA
appears ideally suited to further test this model by searching for resolved
signatures of the warm and slow wide-angle molecular wind that would be
predicted.Comment: accepted for publication in A&
Some Comments on BPS systems
We look at simple BPS systems involving more than one field. We discuss the
conditions that have to be imposed on various terms in Lagrangians involving
many fields to produce BPS systems and then look in more detail at the simplest
of such cases. We analyse in detail BPS systems involving 2 interacting
Sine-Gordon like fields, both when one of them has a kink solution and the
second one either a kink or an antikink solution. We take their solitonic
static solutions and use them as initial conditions for their evolution in
Lorentz covariant versions of such models. We send these structures towards
themselves and find that when they interact weakly they can pass through each
other with a phase shift which is related to the strength of their interaction.
When they interact strongly they repel and reflect on each other. We use the
method of a modified gradient flow in order to visualize the solutions in the
space of fields.Comment: 27 pages, 17 figure
Fundamental Oscillation Periods of the Interlayer Exchange Coupling beyond the RKKY Approximation
A general method for obtaining the oscillation periods of the interlayer
exchange coupling is presented. It is shown that it is possible for the
coupling to oscillate with additional periods beyond the ones predicted by the
RKKY theory. The relation between the oscillation periods and the spacer Fermi
surface is clarified, showing that non-RKKY periods do not bear a direct
correspondence with the Fermi surface. The interesting case of a FCC(110)
structure is investigated, unmistakably proving the existence and relevance of
non-RKKY oscillations. The general conditions for the occurrence of non-RKKY
oscillations are also presented.Comment: 34 pages, 10 figures ; to appear in J. Phys.: Condens. Mat
Magnetic particles confined in a modulated channel: structural transitions tunable by tilting a magnetic field
The ground state of colloidal magnetic particles in a modulated channel are
investigated as function of the tilt angle of an applied magnetic field. The
particles are confined by a parabolic potential in the transversal direction
while in the axial direction a periodic substrate potential is present. By
using Monte Carlo (MC) simulations, we construct a phase diagram for the
different crystal structures as a function of the magnetic field orientation,
strength of the modulated potential and the commensurability factor of the
system. Interestingly, we found first and second order phase transitions
between different crystal structures, which can be manipulated by the
orientation of the external magnetic field. A re-entrant behavior is found
between two- and four-chain configurations, with continuous second order
transitions. Novel configurations are found consisting of frozen in solitons.
By changing the orientation and/or strength of the magnetic field and/or the
strength and the spatial frequency of the periodic substrate potential, the
system transits through different phases.Comment: Submitted to Phys. Rev. E (10 pages, 12 figures
Two-component mixture of charged particles confined in a channel: melting
The melting of a binary system of charged particles confined in a {\it
quasi}-one-dimensional parabolic channel is studied through Monte Carlo
simulations. At zero temperature the particles are ordered in parallel chains.
The melting is anisotropic and different melting temperatures are obtained
according to the spatial direction, and the different types of particles
present in the system. Melting is very different for the single-, two- and
four-chain configurations. A temperature induced structural phase transition is
found between two different four chain ordered states which is absent in the
mono-disperse system. In the mixed regime, where the two types of particles are
only slightly different, melting is almost isotropic and a thermally induced
homogeneous distribution of the distinct types of charges is observed.Comment: To appear in Journal of Physics: condensed matter ; (13 pages, 12
figures
Disordered Carbon nanotube alloys in the Effect Medium Super Cell Approximation
We investigate a disordered single-walled carbon nanotube (SWCNT) in an
effective medium super cell approximation (EMSCA).
First type of disorder that we consider is the presence of vacancies.
Our results show that the vacancies induce some bound states on their
neighbor host sites, leading to the creation of a band around the Fermi energy
in the SWCNT average density of states.Second type of disorder considered is a
substitutional alloy due to it's applications in
hetrojunctions. We found that for a fixed boron (nitrogen) concentration, by
increasing the nitrogen (boron) concentration the averaged semiconducting gap,
, decreases and at a critical concentration it disappears. A consequence
of our results for nano electronic devices is that by changing the
boron(nitrogen) concentration, one can make a semiconductor SWCNT with a
pre-determined energy gap.Comment: 4 page
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