830 research outputs found
Homogeneous Conformal String Backgrounds
We present exact solutions characterised by Bianchi-type I,II,III,V,VI
four-dimensional metric, space-independent dilaton, and vanishing torsion
background, for the low energy string effective action with zero central charge
deficit. We show that, in such a context, curvature singularities cannot be
avoided, except for the trivial case of flat spacetime and constant dilaton. We
also provide a further example of the failure of the standard prescription for
connecting conformal string backgrounds through duality transformations
associated to non-semisimple, non-Abelian isometry group.Comment: 20 pages, latex, no figures, to appear in Class. Q. Gra
Isotropization of Bianchi-Type Cosmological Solutions in Brans-Dicke Theory
The cosmic, general analitic solutions of the Brans--Dicke Theory for the
flat space of homogeneous and isotropic models containing perfect, barotropic,
fluids are seen to belong to a wider class of solutions --which includes
cosmological models with the open and the closed spaces of the
Friedmann--Robertson--Walker metric, as well as solutions for models with
homogeneous but anisotropic spaces corresponding to the Bianchi--Type metric
clasification-- when all these solutions are expressed in terms of reduced
variables. The existence of such a class lies in the fact that the scalar
field, , times a function of the mean scale factor or ``volume element'',
, which depends on time and on the barotropic index of the
equation of state used, can be written as a function of a ``cosmic time''
reduced in terms of another function of the mean scale factor depending itself
again on the barotropic index but independent of the metrics here employed.
This reduction procedure permites one to analyze if explicitly given
anisotropic cosmological solutions ``isotropize'' in the course of their time
evolution. For if so can happen, it could be claimed that there exists a
subclass of solutions that is stable under anisotropic perturbations.Comment: 15 pages, Late
Physical activity and smoking habit in adolescent students
El objetivo de este estudio fue detectar en qué medida afecta la actividad
física y el hábito tabáquico en estudiantes de Enseñanza Secundaria. Se
estudiaron 168 adolescentes no fumadores y fumadores recién iniciados. Se
midieron peso, talla, presión arterial, frecuencia cardíaca, actividad física, fuerza,
flexibilidad y resistencia y una prueba espirométrica. Tanto en chicas y chicos
fumadores, se evidenciaron peores resultados en la mayoría de los parámetros
espirométricos (FEV1, FEF25-75%, FVC) y un envejecimiento prematuro del
pulmón, más acentuado en chicas. La actividad físico-deportiva moderada se
asocia a adolescentes que menos fuman y tienen más facilidad para abandonar
el hábito tabáquicoThe aim of the study was to detect how physical education and smoking habits
affect secondary school students, 168 non smokers and newly initiated were
studied. Weight, height, heart rate, blood pressure, physical activity, strength,
flexibility and endurance, and spirometric tests were analyzed. In both, female
and male smokers, obtained worse results in the mayority of spirometric
parameters (FEV1, FEF25-75%, FVC) and premature lung ageing, more pointed in
females. Moderate physical-sporting activity is associated to adolescents who
smoke less and quit smoking more easil
Note: Small anaerobic chamber for optical spectroscopy
The study of oxygen-sensitive biological samples requires an effective control of the atmosphere in which they are housed. In this aim however, no commercial anaerobic chamber is adequate to solely enclose the sample and small enough to fit in a compact spectroscopic system with which analysis can be performed. Furthermore, spectroscopic analysis requires the probe beam to pass through the whole chamber, introducing a requirement for adequate windows. In response to these challenges, we present a 1 l anaerobic chamber that is suitable for broad-band spectroscopic analysis. This chamber has the advantage of (1) providing access, via a septum, to the sample and (2) allows the sample position to be adjusted while keeping the chamber fixed and hermetic during the experiment
Three-dimensional electronic instabilities in polymerized solid A1C60
The low-temperature structure of A1C60 (A=K, Rb) is an ordered array of
polymerized C60 chains, with magnetic properties that suggest a non-metallic
ground state. We study the paramagnetic state of this phase using
first-principles electronic-structure methods, and examine the magnetic
fluctuations around this state using a model Hamiltonian. The electronic and
magnetic properties of even this polymerized phase remain strongly three
dimensional, and the magnetic fluctuations favor an unusual three-dimensional
antiferromagnetically ordered structure with a semi-metallic electronic
spectrum.Comment: REVTeX 3.0, 10 pages, 4 figures available on request from
[email protected]
A microfluidic flow-cell for the study of the ultrafast dynamics of biological systems
The study of biochemical dynamics by ultrafast spectroscopic methods is often restricted by the limited amount of liquid sample available, while the high repetition rate of light sources can induce photodamage. In order to overcome these limitations, we designed a high flux, sub-ml, capillary flow-cell. While the 0.1 mm thin window of the 0.5 mm cross-section capillary ensures an optimal temporal resolution and a steady beam deviation, the cell-pump generates flows up to ∼0.35 ml/s that are suitable to pump laser repetition rates up to ∼14 kHz, assuming a focal spot-diameter of 100 μm. In addition, a decantation chamber efficiently removes bubbles and allows, via septum, for the addition of chemicals while preserving the closed atmosphere. The minimal useable amount of sample is ∼250 μl
Isotropization of Bianchi type models and a new FRW solution in Brans-Dicke theory
Using scaled variables we are able to integrate an equation valid for
isotropic and anisotropic Bianchi type I, V, IX models in Brans-Dicke (BD)
theory. We analyze known and new solutions for these models in relation with
the possibility that anisotropic models asymptotically isotropize, and/or
possess inflationary properties. In particular, a new solution of curve
() Friedmann-Robertson-Walker (FRW) cosmologies in Brans-Dicke theory
is analyzed.Comment: 15 pages, 4 postscript figures, to appear in Gen. Rel. Grav., special
issue dedicated in honour of Prof. H. Dehne
Superconductivity in a spin liquid - a one dimensional example
We study a one-dimensional model of interacting conduction electrons with a
two-fold degenerate band away from half filling. The interaction includes an
on-site Coulomb repulsion and Hund's rule coupling. We show that such
one-dimensional system has a divergent Cooper pair susceptibility at T = 0,
provided the Coulomb interaction between electrons on the same orbital and
the modulus of the Hund's exchange integral are larger than the
interorbital Coulomb interaction. It is remarkable that the superconductivity
can be achieved for {\it any} sign of . The opening of spectral gaps makes
this state stable with respect to direct electron hopping between the orbitals.
The scaling dimension of the superconducting order parameter is found to be
between 1/4 (small ) and 1/2 (large ).Comment: 11 pages, Latex, no figure
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