5,471 research outputs found
DMRG study of the Bond Alternating \textbf{S}=1/2 Heisenberg ladder with Ferro-Antiferromagnetic couplings
We obtain the phase diagram in the parameter space and an
accurate estimate of the critical line separating the different phases. We show
several measuments of the magnetization, dimerization, nearest neighbours
correlation, and density of energy in the different zones of the phase diagram,
as well as a measurement of the string order parameter proposed as the non
vanishing phase order parameter characterizing Haldane phases. All these
results will be compared in the limit with the behaviour of the
Bond Alternated Heisenberg Chain (BAHC). The analysis of our
data supports the existence of a dimer phase separated by a critical line from
a Haldane one, which has exactly the same nature as the Haldane phase in the
BAHC.Comment: Version 4. 8 pages, 15 figures (12 figures in document
Bosonization on a lattice: The emergence of the higher harmonics
6 págs.; 2 figs.A general and transparent procedure to bosonize fermions placed on a lattice is presented. Harmonics higher than kF in the one-particle Green function are shown to appear due to the compact character of real electron bands. Quantitative estimations of the role of higher harmonics are made possible by this bosonization technique. © 1995 The American Physical Society.J.F. was supported by DEYCIT Project No. PB93-1248.Peer Reviewe
Instability of wormholes supported by a ghost scalar field. I. Linear stability analysis
We examine the linear stability of static, spherically symmetric wormhole
solutions of Einstein's field equations coupled to a massless ghost scalar
field. These solutions are parametrized by the areal radius of their throat and
the product of the masses at their asymptotically flat ends. We prove that all
these solutions are unstable with respect to linear fluctuations and possess
precisely one unstable, exponentially in time growing mode. The associated time
scale is shown to be of the order of the wormhole throat divided by the speed
of light. The nonlinear evolution is analyzed in a subsequent article.Comment: 12 pages, 1 figure, minor changes, to appear in Classical and Quantum
Gravit
Li depletion in solar analogues with exoplanets: Extending the sample
We want to study the effects of the formation of planets and planetary
systems on the atmospheric Li abundance of planet host stars. In this work we
present new determinations of lithium abundances for 326 Main Sequence stars
with and without planets in the T range 5600-5900 K. 277 stars
come from the HARPS sample, the remaining targets have been observed with a
variety of high resolution spectrographs. We confirm significant differences in
the Li distribution of solar twins (T = T 80 K,
log g = log g 0.2 and [Fe/H] = [Fe/H] 0.2): the
full sample of planet host stars (22) shows Li average values lower than
"single" stars with no detected planets (60). If we focus in subsamples with
narrower ranges in metallicity and age, we observe indications of a similar
result though it is not so clear for some of the studied subsamples.
Furthermore, we compare the observed spectra of several couples of stars with
very similar parameters which show different Li abundances up to 1.6 dex.
Therefore we show that neither age, nor mass nor metallicity of a parent star
is the only responsible for enhanced Li depletion in solar analogues. We
conclude that another variable must account for that difference and suggest
that this could be the presence of planets which causes additional rotationally
induced mixing in the external layers of planet host stars. Moreover, we find
indications that the amount of depletion of Li in planet host solar-type stars
is higher when the planets are more massive than Jupiter.Comment: 16 pages, accepted for publication in A&
Bosonization on the lattice: the emergence of the higher harmonics
A general and transparent procedure to bosonize fermions placed on a lattice
is presented. Harmonics higher than are shown to appear in the
one-paticle Green function, due to the compact character of real electron
bands. Quantitative estimations of the role of these higher harmonics are made
possible by the bosonization technique presented here.Comment: Pages: 15 (REVTEX 3.0) plus 4 postscript figures appended at the end
of the tex
The cosmic evolution of the spatially-resolved star formation rate and stellar mass of the CALIFA survey
We investigate the cosmic evolution of the absolute and specific star
formation rate (SFR, sSFR) of galaxies as derived from a spatially-resolved
study of the stellar populations in a set of 366 nearby galaxies from the
CALIFA survey. The analysis combines GALEX and SDSS images with the 4000 break,
H_beta, and [MgFe] indices measured from the datacubes, to constrain parametric
models for the SFH, which are then used to study the cosmic evolution of the
star formation rate density (SFRD), the sSFR, the main sequence of star
formation (MSSF), and the stellar mass density (SMD). A delayed-tau model,
provides the best results, in good agreement with those obtained from
cosmological surveys. Our main results from this model are: a) The time since
the onset of the star formation is larger in the inner regions than in the
outer ones, while tau is similar or smaller in the inner than in the outer
regions. b) The sSFR declines rapidly as the Universe evolves, and faster for
early than for late type galaxies, and for the inner than for the outer regions
of galaxies. c) SFRD and SMD agree well with results from cosmological surveys.
At z< 0.5, most star formation takes place in the outer regions of late spiral
galaxies, while at z>2 the inner regions of the progenitors of the current E
and S0 are the major contributors to SFRD. d) The inner regions of galaxies are
the major contributor to SMD at z> 0.5, growing their mass faster than the
outer regions, with a lookback time at 50% SMD of 9 and 6 Gyr for the inner and
outer regions. e) The MSSF follows a power-law at high redshift, with the slope
evolving with time, but always being sub-linear. f) In agreement with galaxy
surveys at different redshifts, the average SFH of CALIFA galaxies indicates
that galaxies grow their mass mainly in a mode that is well represented by a
delayed-tau model, with the peak at z~2 and an e-folding time of 3.9 Gyr.Comment: 23 pages, 16 figures, 6 tables, accepted for publication in Astronomy
& Astrophysics. *Abridged abstract
Adaptação ao cultivo e valorização de Lavandula luisieri
Foram estudadas diversas populações de Lavandula luisieri (Rozeira) Rivas-Martínez
endémicas da Península Ibérica (Beira Interior, Castilla-la-Mancha e Andaluzia; acessos
de Casal da Fraga, Mata, Penamacor e Vila Velha de Ródão – Portugal; Toledo e
Sevilha – Espanha), comparando as populações relativamente ao óleo essencial
(quantitativamente e qualitativamente), assim como, a sua actividade insecticida sobre
pragas fitófagas (Spodoptera littoralis, Mysus persicae e Leptinotarsa decemlineata).
Em função dos seus componentes escolheram-se as duas populações mais interessantes
em termos da sua actividade biológica (Penamacor e Toledo) e instalaram-se dois
campos produtivos em Castelo Branco e Saragoça, por forma a inferir sobre a influência
do local de produção na qualidade do material vegetal obtido. Será fornecida
informação técnica sobre a instalação destes campos
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