4,408 research outputs found
From vortex molecules to the Abrikosov lattice in thin mesoscopic superconducting disks
Stable vortex states are studied in large superconducting thin disks (for
numerical purposes we considered with radius R = 50 \xi). Configurations
containing more than 700 vortices were obtained using two different approaches:
the nonlinear Ginzburg-Landau (GL) theory and the London approximation. To
obtain better agreement with results from the GL theory we generalized the
London theory by including the spatial variation of the order parameter
following Clem's ansatz. We find that configurations calculated in the London
limit are also stable within the Ginzburg-Landau theory for up to ~ 230
vortices. For large values of the vorticity (typically, L > 100), the vortices
are arranged in an Abrikosov lattice in the center of the disk, which is
surrounded by at least two circular shells of vortices. A Voronoi construction
is used to identify the defects present in the ground state vortex
configurations. Such defects cluster near the edge of the disk, but for large L
also grain boundaries are found which extend up to the center of the disk.Comment: 15 pages, 10 figures, RevTex4, submitted to Phys. Rev.
A semiquantitative approach to the impurity-band-related transport properties of GaMnAs nanolayers
We investigate the spin-polarized transport of GaMnAs nanolayers in which a
ferromagnetic order exists below a certain transition temperature. Our
calculation for the self-averaged resistivity takes into account the existence
of an impurity band determining the extended ("metallic" transport) or
localized (hopping by thermal excitation) nature of the states at and near the
Fermi level. Magnetic order and resistivity are inter-related due to the
influence of the spin polarization of the impurity band and the effect of the
Zeeman splitting on the mobility edge. We obtain, for a given range of Mn
concentration and carrier density, a "metallic" behavior in which the transport
by extended carriers dominates at low temperature, and is dominated by the
thermally excited localized carriers near and above the transition temperature.
This gives rise to a conspicuous hump of the resistivity which has been
experimentally observed and brings light onto the relationship between
transport and magnetic properties of this material
Rubiaceae family: Flora Argentina, history and evolution
The Rubiaceae family in Argentina is represented by 41 genera and 146 species, of which 9 spp., 2 subspp., and 1 variety are endemic. The highest concentration and endemism are found in the NE ofthe country, in savannas, rainforests, riverside forests, followed by the NW of Argentina, from sea level to 1000-4000 m in the Andes. They also inhabit the heterogeneous landscape of the Center Region of the country, and few species grow in the Patagonian zone. This work is the contribution to the integral study of the Flora of our country. The beginning of knowledge of Argentina’s vegetation dates back to the mid 19th century, with foreign naturalists attracted by the virgin vegetation of the country. The Academy of Science and the first Institute of Botany were founded in Córdoba, where the herbarium and methodical study of native plants began. The first floristic works were carried out, which were enriched in the 20th century, with the improvement of instruments, development of new methods, which changed the analysis and interpretation of data. Fast means of transportation and communication facilitated the exchange of ideas and the increase in botanical expeditions. Despite the advances in the local botany, a comprehensive work on the country’s flora was still missing. It began with species checklists, regional catalogues and flora from various provinces. At the Instituto Botánica Darwinion (IBODA), the project “Flora Argentina” began methodically with the hand-picking of species bibliography, collection and identification activities. Professor Nélida Bacigalupo began to work there in 1949. She worked for more than 60 years and her publications about Rubiaceae taxa were the first in the country. When we met her, we had the privilege of valuing her predisposition and her wisdom as “Teacher”, which led to the transfer of her knowledge to three generations, for which we express our appreciation and gratitude
On the nature of the spin-polarized hole states in a quasi-two-dimensional GaMnAs ferromagnetic layer
A self-consistent calculation of the density of states and the spectral
density function is performed in a two-dimensional spin-polarized hole system
based on a multiple-scattering approximation. Using parameters corresponding to
GaMnAs thin layers, a wide range of Mn concentrations and hole densities have
been explored to understand the nature, localized or extended, of the
spin-polarized holes at the Fermi level for several values of the average
magnetization of the Mn ystem. We show that, for a certain interval of Mn and
hole densities, an increase on the magnetic order of the Mn ions come together
with a change of the nature of the states at the Fermi level. This fact
provides a delocalization of spin-polarized extended states anti-aligned to the
average Mn magnetization, and a higher spin-polarization of the hole gas. These
results are consistent with the occurrence of ferromagnetism with relatively
high transition temperatures observed in some thin film samples and
multilayered structures of this material.Comment: 3 page
Massive 70 micron quiet clumps I: evidence of embedded low/intermediate-mass star formation activity
Massive clumps, prior to the formation of any visible protostars, are the
best candidates to search for the elusive massive starless cores. In this work
we investigate the dust and gas properties of massive clumps selected to be 70
micron quiet, therefore good starless candidates. Our sample of 18 clumps has
masses 300 < M < 3000 M_sun, radius 0.54 < R < 1.00 pc, surface densities Sigma
> 0.05 g cm^-2 and luminosity/mass ratio L/M < 0.3. We show that half of these
70 micron quiet clumps embed faint 24 micron sources. Comparison with GLIMPSE
counterparts shows that 5 clumps embed young stars of intermediate stellar mass
up to ~5.5 M_sun. We study the clump dynamics with observations of N2H+ (1-0),
HNC (1-0) and HCO+ (1-0) made with the IRAM 30m telescope. Seven clumps have
blue-shifted spectra compatible with infall signatures, for which we estimate a
mass accretion rate 0.04 < M_dot < 2.0 x 10^-3 M_sun yr^-1, comparable with
values found in high-mass protostellar regions, and free-fall time of the order
of t_ff = 3 x 10^5 yr. The only appreciable difference we find between objects
with and without embedded 24 micron sources is that the infall rate appears to
increase from 24 micron dark to 24 micron bright objects. We conclude that all
70 micron quiet objects have similar properties on clump scales, independently
of the presence of an embedded protostar. Based on our data we speculate that
the majority, if not all of these clumps may already embed faint, low-mass
protostellar cores. If these clumps are to form massive stars, this must occur
after the formation of these lower mass stars.Comment: 44 pages, 11 Figures. Accepted for publication in MNRA
Mechanistic aspects of the linear stabilization of non-stationary electrochemical oscillations
The problem of non-stationarity in experimentally recorded time-series is common in many (electro)chemical systems. Underlying this non-stationarity is the slow drift in some uncontrollable parameter, and it occurs in spite of the fact that all controllable parameters are kept constant. Particularly for electrochemical systems, some of us have recently suggested [J. Phys. Chem. C, 144, (2010), 22262–22268] an empirical method to stabilize experimental time-series. The method was exemplified for the electro-oxidation of methanol and different patterns were satisfactorily stabilized. In this paper we further elaborate some mechanistic aspects of this method and test it for the electro-oxidation of formaldehyde, a system that has some resemblance with the electro-oxidation of methanol, but produces a richer dynamics. In terms of the reaction mechanism, we were able to describe the coupling and to separate the surface processes of the two sub-systems: the fast one (or the core-oscillator) and the slow one, responsible for the drift
Uma proposta de pesquisa para análise do relacionamento entre inovação e desempenho utilizando a abordagem multidimensional.
Exploring dynamic lighting, colour and form with smart textiles
This paper addresses an ongoing research, aiming at the development of smart
textiles that transform the incident light that passes through them – light transmittance – to
design dynamic light without acting upon the light source. A colour and shape change
prototype was developed with the objective of studying textile changes in time; to explore
temperature as a dynamic variable through electrical activation of the smart materials and
conductive threads integrated in the textile substrate; and to analyse the relation between textile
chromic and morphologic behaviour in interaction with light. Based on the experiments
conducted, results have highlighted some considerations of the dynamic parameters involved in
the behaviour of thermo-responsive textiles and demonstrated design possibilities to create
interactive lighting scenarios.This work is supported by FEDER funds through the Operational Programme for Competitiveness Factors – COMPETE and National Funds through FCT – Foundation for Science and Technology within the scope of the projects SFRH/BD/87196/2012, POCI-01-0145-FEDER-007136 and UID/CTM/00264. The authors also like to acknowledge Smart Textiles Design Lab for the support on the prototype development.info:eu-repo/semantics/publishedVersio
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