1,729 research outputs found
Phase transitions in the one-dimensional frustrated quantum XY model and Josephson-junction ladders
A one-dimensional quantum version of the frustrated XY (planar rotor) model
is considered which can be physically realized as a ladder of
Josephson-junctions at half a flux quantum per plaquette. This system undergoes
a superconductor to insulator transition at zero temperature as a function of
charging energy. The critical behavior is studied using a Monte Carlo transfer
matrix applied to the path-integral representation of the model and a
finite-size-scaling analysis of data on small system sizes. Depending on the
ratio between the interchain and intrachain couplings the system can have
single or double transitions which is consistent with the prediction that its
critical behavior should be described by the two-dimensional classical XY-Ising
model.Comment: 13 pages, Revtex, J. Appl. Phys. (to appear), Inpe-las-00
Phase transitions in a frustrated XY model with zig-zag couplings
We study a new generalized version of the square-lattice frustrated XY model
where unequal ferromagnetic and antiferromagnetic couplings are arranged in a
zig-zag pattern. The ratio between the couplings can be used to tune the
system, continuously, from the isotropic square-lattice to the
triangular-lattice frustrated XY model. The model can be physically realized as
a Josephson-junction array with two different couplings, in a magnetic field
corresponding to half-flux quanta per plaquette. Mean-field approximation,
Ginzburg-Landau expansion and finite-size scaling of Monte Carlo simulations
are used to study the phase diagram and critical behavior. Depending on the
value of , two separate transitions or a transition line in the
universality class of the XY-Ising model, with combined and U(1)
symmetries, takes place. In particular, the phase transitions of the standard
square-lattice and triangular-lattice frustrated XY models correspond to two
different cuts through the same transition line. Estimates of the chiral
() critical exponents on this transition line deviate significantly from
the pure Ising values, consistent with that along the critical line of the
XY-Ising model. This suggests that a frustrated XY model or Josephson-junction
array with a zig-zag coupling modulation can provide a physical realization of
the XY-Ising model critical line.Comment: 11 pages, 9 figures, RevTex, to appear in Phys. Rev.
Modelling Intermediate Age and Old Stellar Populations in the Infrared
We have investigated the spectro-photometric properties of the Asymptotic
Giant Branch (AGB) stars and their contribution to the integrated infrared
emission in simple stellar populations (SSP). Adopting analytical relations
describing the evolution of these stars in the HR diagram and empirical
relations for the mass-loss rate and the wind terminal velocity, we were able
to model the effects of the dusty envelope around these stars, with a minimal
number of parameters. We computed isochrones at different age and initial metal
content. We compare our models with existing infrared colors of M giants and
Mira stars and with IRAS PSC data. Contrary to previous models, in the new
isochrones the mass-loss rate, which establishes the duration of the AGB phase,
also determines the spectral properties of the stars. The contribution of these
stars to the integrated light of the population is thus obtained in a
consistent way. We find that the emission in the mid infrared is about one
order of magnitude larger when dust is taken into account in an intermediate
age population, irrespective of the particular mixture adopted. The dependence
of the integrated colors on the metallicity and age is discussed, with
particular emphasis on the problem of age-metallicity degeneracy. We show that,
contrary to the case of optical or near infrared colors, the adoption of a
suitable pass-band in the mid infrared allows a fair separation of the two
effects. We suggest intermediate redshift elliptical galaxies as possible
targets of this method of solving the age-metallicity dilemma. The new SSP
models constitute a first step in a more extended study aimed at modelling the
spectral properties of the galaxies from the ultraviolet to the far infrared.Comment: 16 pages, 10 figures, to appear in A&
Conformal Anomaly and Critical Exponents of the XY-Ising Model
We use extensive Monte Carlo transfer matrix calculations on infinite strips
of widths up to 30 lattice spacing and a finite-size scaling analysis to
obtain critical exponents and conformal anomaly number for the
two-dimensional -Ising model. This model is expected to describe the
critical behavior of a class of systems with simultaneous and
symmetries of which the fully frustrated model is a special case. The
effective values obtained for show a significant decrease with at
different points along the line where the transition to the ordered phase takes
place in a single transition. Extrapolations based on power-law corrections
give values consistent with although larger values can not be ruled
out. Critical exponents are obtained more accurately and are consistent with
previous Monte Carlo simulations suggesting new critical behavior and with
recent calculations for the frustrated model.Comment: 33 pages, 13 latex figures, uses RevTeX 3.
Nuclear star formation in the quasar PG1126-041 from adaptive optics assisted spectroscopy
We present adaptive optics assisted spectroscopy of three quasars obtained
with NACO at VLT. The high angular resolution achieved with the adaptive optics
(~0.08"), joined to the diagnostic power of near-IR spectroscopy, allow us to
investigate the properties of the innermost 100 pc of these quasars. In the
quasar with the best adaptive optics correction, PG1126-041, we spatially
resolve the Pa-alpha emission within the nuclear 100 pc. The comparison with
higher excitation lines suggests that the narrow Pa-alpha emission is due to
nuclear star formation. The inferred intensity of the nuclear star formation
(13 M(sun)/yr) may account for most of the far-IR luminosity observed in this
quasar.Comment: 4 pages, 4 figures. Accepted for publication in A&
The Role of the Dust in Primeval Galaxies: A Simple Physical Model for Lyman Break Galaxies and Lyman Alpha Emitters
We explore the onset of star formation in the early Universe, exploiting the
observations of high-redshift Lyman-break galaxies (LBGs) and Lyman alpha
emitters (LAEs), in the framework of the galaxy formation scenario elaborated
by Granato et al. (2004) already successfully tested against the wealth of data
on later evolutionary stages. Complementing the model with a simple, physically
plausible, recipe for the evolution of dust attenuation in metal poor galaxies
we reproduce the luminosity functions (LFs) of LBGs and of LAEs at different
redshifts. This recipe yields a much faster increase with galactic age of
attenuation in more massive galaxies, endowed with higher star formation rates.
These objects have therefore shorter lifetimes in the LAE and LBG phases, and
are more easily detected in the dusty submillimeter bright phase. The short UV
bright lifetimes of massive objects strongly mitigate the effect of the fast
increase of the massive halo density with decreasing redshift, thus accounting
for the weaker evolution of the LBG LF, compared to that of the halo mass
function, and the even weaker evolution between z~6 and z~3 of the LAE LF. LAEs
are on the average expected to be younger, with lower stellar masses, and
associated to less massive halos than LBGs. Finally, we show that the
intergalactic medium can be completely reionized at redshift z~6-7 by massive
stars shining in protogalactic spheroids with halo masses from a few 10^10 to a
few 10^11 M_sun, showing up as faint LBGs with magnitude in the range
-17<M_1350<-20, without resorting to any special stellar initial mass function.Comment: 13 pages, 8 figures, uses REVTeX 4 + emulateapj.cls and apjfonts.sty.
Title changed and text revised following referee's comments. Accepted by Ap
An evolutionary model for GHz Peaked Spectrum Sources. Predictions for high frequency surveys
We have explored, in the general framework of the ``young source'' scenario,
evolutionary models for GHz Peaked Spectrum (GPS) galaxies and quasars which
reproduce the observed counts, redshift and peak frequency distributions of
currently available samples. Substantially different cosmological evolution
properties are found for the two populations: the quasar luminosity function
must evolve strongly up to , while the data on galaxies may be
consistent with no evolution. The models show that GPS sources (mostly quasars)
may comprise quite a significant fraction of bright (Jy) radio sources
at GHz if the intrinsic distribution of peak frequencies extends
up to GHz. In any case, however, their fraction decreases rapidly
with decreasing flux and their contribution to small scale fluctuations in the
frequency range covered by the forthcoming space missions MAP and Planck
Surveyor is expected to be minor.Comment: 7 pages, 4 figures, A&A accepte
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