83,776 research outputs found
Bifurcation analysis and phase diagram of a spin-string model with buckled states
We analyze a one-dimensional spin-string model, in which string oscillators
are linearly coupled to their two nearest neighbors and to Ising spins
representing internal degrees of freedom. String-spin coupling induces a
long-range ferromagnetic interaction among spins that competes with a spin-spin
antiferromagnetic coupling. As a consequence, the complex phase diagram of the
system exhibits different flat rippled and buckled states, with first or second
order transition lines between states. The two-dimensional version of the model
has a similar phase diagram, which has been recently used to explain the
rippled to buckled transition observed in scanning tunnelling microscopy
experiments with suspended graphene sheets. Here we describe in detail the
phase diagram of the simpler one-dimensional model and phase stability using
bifurcation theory. This gives additional insight into the physical mechanisms
underlying the different phases and the behavior observed in experiments.Comment: 15 pages, 7 figure
Calcium Triplet Synthesis
We present theoretical equivalent widths for the sum of the two strongest
lines of the Calcium Triplet, CaT index, in the near-IR, using evolutionary
techniques and the most recent models and observational data for this feature
in individual stars. We compute the CaT index for Single Stellar Populations
(instantaneous burst, standard Salpeter-type IMF) at four metallicities,
Z=0.004, 0.008, 0.02 (solar) and 0.05, and ranging in age from very young
bursts of star formation (few Myr) to old stellar populations, up to 17 gyr,
representative of globular clusters, elliptical galaxies and bulges of spirals.
The interpretation of the observed equivalent widths of CaT in different
stellar systems is discussed. Composite-population models are also computed as
a tool to interpret the CaT detections in star-forming regions, in order to
disantangle between the component due to Red Supergiants stars, RSG, and the
underlying, older, population. CaT is found to be an excellent
metallicity-indicator for populations older than 1 Gyr, practically independent
of the age. We discuss its application to remove the age- metallicity
degeneracy, characteristic of all studies of galaxy evolution based on the
usual integrated indices (both broad band colors and narrow band indices). The
application of the models computed here to the analysis of a sample of
elliptical galaxies will be discussed in a forthcoming paper (Gorgas et al.
1998).Comment: 17 pages, 7 figures, to be published in A&
Quantum Decoherence of Photons in the Presence of Hidden U(1)s
Many extensions of the standard model predict the existence of hidden sectors
that may contain unbroken abelian gauge groups. We argue that in the presence
of quantum decoherence photons may convert into hidden photons on sufficiently
long time scales and show that this effect is strongly constrained by CMB and
supernova data. In particular, Planck-scale suppressed decoherence scales D ~
E^2/M_Pl (characteristic for non-critical string theories) are incompatible
with the presence of even a single hidden U(1). The corresponding bounds on the
decoherence scale are four orders of magnitude stronger than analogous bounds
derived from solar and reactor neutrino data and complement other bounds
derived from atmospheric neutrino data.Comment: 8 pages, 9 figure
Arguments towards the construction of a matrix model groundstate
We discuss the existence and uniqueness of wavefunctions for inhomogenoeus
boundary value problems associated to x^2y^2-type matrix model on a bounded
domain of R^2. Both properties involve a combination of the Cauchy-Kovalewski
Theorem and a explicit calculations.Comment: 3 pages, Latex Proceedings for the XIX Simposio Chileno de Fisica,
SOCHIFI 2014 Conference, 26-28 November 2014, held at Concepcion U., Chil
Galaxy evolution across the optical emission-line diagnostic diagrams?
The discovery of the M-sigma relation, the local galaxy bimodality, and the
link between black-hole and host-galaxy properties, have raised the question
whether AGN play a role in galaxy evolution. Several theoretical models
implement AGN feedback to explain the observed galaxy luminosity function, and
possibly the color and morphological transformation of spiral galaxies into
passive ellipticals. To understand the importance of AGN feedback, a study of
the AGN populations in the radio-optical domain is crucial. A mass sequence
linking star-forming galaxies and AGN has been already noted in previous works,
and it is now investigated as possible evolutionary sequence. We observed a
sample of 119 intermediate-redshift (0.04<z<0.4) SDSS-FIRST radio emitters with
the Effelsberg 100-m telescope at 4.85 and 10.45 GHz and obtained spectral
indices. We find indications of spectral index flattening in high-metallicity
star-forming galaxies, composite galaxies, and Seyferts. This "flattening
sequence" along the [NII]-based emission-line diagnostic diagram is consistent
with the hardening of galaxy ionizing field, due to nuclear activity. After
combining our data with FIRST measurements at 1.4 GHz, we find that the
three-point radio spectra of Seyferts and LINERs show substantial differences,
attributable to small radio core components and larger (arcsecond sized)
jet/lobe components, respectively. A visual inspection of FIRST images seems to
confirm this hypothesis. Galaxies along this sequence are hypothesized to be
transitioning from the active star-forming galaxies (blue cloud) to the passive
elliptical galaxies (red sequence). This supports the suggestion that AGN play
a role in shutting down star-formation, and allow the transition from one
galaxy class to the other.Comment: 20 pages, 19 figures, accepted for publication in A&
The supermembrane with central charges:(2+1)-D NCSYM, confinement and phase transition
The spectrum of the bosonic sector of the D=11 supermembrane with central
charges is shown to be discrete and with finite multiplicities, hence
containing a mass gap. The result extends to the exact theory our previous
proof of the similar property for the SU(N) regularised model and strongly
suggest discreteness of the spectrum for the complete Hamiltonian of the
supermembrane with central charges. This theory is a quantum equivalent to a
symplectic non-commutative super-Yang-Mills in 2+1 dimensions, where the
space-like sector is a Riemann surface of positive genus. In this context, it
is argued how the theory in 4D exhibits confinement in the N=1 supermembrane
with central charges phase and how the theory enters in the quark-gluon plasma
phase through the spontaneous breaking of the centre. This phase is interpreted
in terms of the compactified supermembrane without central charges.Comment: 33 pages, Latex. In this new version, several changes have been made
and various typos were correcte
On the groundstate of octonionic matrix models in a ball
In this work we examine the existence and uniqueness of the groundstate of a
SU(N)x G2 octonionic matrix model on a bounded domain of R^N. The existence and
uniqueness argument of the groundstate wavefunction follows from the
Lax-Milgram theorem. Uniqueness is shown by means of an explicit argument which
is drafted in some detail.Comment: Latex, 6 page
Massless ground state for a compact SU(2) matrix model in 4D
We show the existence and uniqueness of a massless supersymmetric ground
state wavefunction of a SU(2) matrix model in a bounded smooth domain with
Dirichlet boundary conditions. This is a gauge system and we provide a new
framework to analyze the quantum spectral properties of this class of
supersymmetric matrix models subject to constraints which can be generalized
for arbitrary number of colors.Comment: 12 pages, Latex. Somme clarifications. Minor changes. Version to
appear at NP
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