867 research outputs found
Structural changes at the semiconductor-insulator phase transition in the single layered La0.5Sr1.5MnO4 perovskite
The semiconductor-insulator phase transition of the single-layer manganite
La0.5Sr1.5MnO4 has been studied by means of high resolution synchrotron x-ray
powder diffraction and resonant x-ray scattering at the Mn K edge. We conclude
that a concomitant structural transition from tetragonal I4/mmm to orthorhombic
Cmcm phases drives this electronic transition. A detailed symmetry-mode
analysis reveals that condensation of three soft modes -Delta_2(B2u), X1+(B2u)
and X1+(A)- acting on the oxygen atoms accounts for the structural
transformation. The Delta_2 mode leads to a pseudo Jahn-Teller distortion (in
the orthorhombic bc-plane only) on one Mn site (Mn1) whereas the two X1+ modes
produce an overall contraction of the other Mn site (Mn2) and expansion of the
Mn1 one. The X1+ modes are responsible for the tetragonal superlattice
(1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of two
different Mn sites. A strong enhancement of the scattered intensity has been
observed for these superlattice reflections close to the Mn K edge, which could
be ascribed to some degree of charge disproportion between the two Mn sites of
about 0.15 electrons. We also found that the local geometrical anisotropy of
the Mn1 atoms and its ordering originated by the condensed Delta_2 mode alone
perfectly explains the resonant scattering of forbidden (1/4,1/4,0)-type
reflections without invoking any orbital ordering.Comment: 3 tables and 10 figures; accepted in Phys. Rev.
As lideranças e a diferenciação curricular inclusiva: que caminhos para a escola do século XXI?
Com este artigo pretende-se analisar num primeiro momento, o conceito de diferenciação curricular
no plano conceptual e dentro de um quadro de análise que permita aprofundar e clarificar o seu
sentido no contexto da escola para todos e no enquadramento da massificação da escolarização,
fazendo referência à globalização a qual traz novos aspectos para o debate em torno da finalidade
da escola. Analisa-se num segundo momento, o papel da liderança democrática e participativa no
desempenho do binómio curricular (currículo nacional e projectos curriculares) na medida em que
a articulação entre níveis centrais e de escola para o desenvolvimento da diferenciação curricular
inclusiva se constitui como meta indispensável para algo assim. Considera-se nesta perspectiva a
liderança propulsora da diferenciação curricular inclusiva na dimensão da praxis curricular das
escolas e dos professores. Por último relaciona-se esta problemática com a perspectiva da teoria
crítica no quadro de escolas entendidas como boas escolas
Tunable Graphene Electronics with Local Ultrahigh Pressure
We achieve fine tuning of graphene effective doping by applying ultrahigh
pressures (> 10 GPa) using Atomic Force Microscopy (AFM) diamond tips. Specific
areas in graphene flakes are irreversibly flattened against a SiO2 substrate.
Our work represents the first demonstration of local creation of very stable
effective p-doped graphene regions with nanometer precision, as unambiguously
verified by a battery of techniques. Importantly, the doping strength depends
monotonically on the applied pressure, allowing a controlled tuning of graphene
electronics. Through this doping effect, ultrahigh pressure modifications
include the possibility of selectively modifying graphene areas to improve
their electrical contact with metal electrodes, as shown by Conductive AFM.
Density Functional Theory calculations and experimental data suggest that this
pressure level induces the onset of covalent bonding between graphene and the
underlying SiO2 substrate. Our work opens a convenient avenue to tuning the
electronics of 2D materials and van der Waals heterostructures through pressure
with nanometer resolution
Integral-field near-infrared spectroscopy of two blue dwarf galaxies: NGC 5253 and He 2-10
We present integral field spectroscopy in the near infrared (NIR) of He 2-10
and NGC 5253, two well known nearby dwarf irregular galaxies showing high
star-formation rates. Our data provide an unprecedented detailed view of the
interstellar medium and star formation in these galaxies, allowing us to obtain
spatially resolved information from the NIR emission and absorption line
tracers. We study the spatial distribution and kinematics of different
components of the interstellar medium (ISM) mostly through the Bracket series
lines, the molecular hydrogen spectrum, [FeII] emission, and CO absorptions.
Although the ISM is mostly photo-excited, as derived by the [FeII]/Bry and H2
line ratios, some regions corresponding to non-thermal radio sources show a
[FeII]/Bry excess due to a significant contribution of SN driven shocks. In He
2-10 we find that the molecular gas clouds, as traced by CO(2-1) and H2
infrared line, show consistent morphologies and velocities when studied with
the two different tracers. Moreover, there is a clear association with the
youngest super star clusters as traced by the ionized gas. In the same galaxy
we observe a cavity depleted of gas, which is surrounded by some of the most
active regions of star formation, that we interpret as a signature of
feedback-induced star formation from older episodes of star formation. Finally,
we measured high turbulence in the ISM of both galaxies, sigma~30-80 km/s,
driven by the high star-formation activity.Comment: 12 pages, 10 figures, accepted for publication in A&
Spitzer view on the evolution of star-forming galaxies from z=0 to z~3
We use a 24 micron selected sample containing more than 8,000 sources to
study the evolution of star-forming galaxies in the redshift range from z=0 to
z~3. We obtain photometric redshifts for most of the sources in our survey
using a method based on empirically-built templates spanning from ultraviolet
to mid-infrared wavelengths. The accuracy of these redshifts is better than 10%
for 80% of the sample. The derived redshift distribution of the sources
detected by our survey peaks at around z=0.6-1.0 (the location of the peak
being affected by cosmic variance), and decays monotonically from z~1 to z~3.
We have fitted infrared luminosity functions in several redshift bins in the
range 0<z<~3. Our results constrain the density and/or luminosity evolution of
infrared-bright star-forming galaxies. The typical infrared luminosity (L*)
decreases by an order of magnitude from z~2 to the present. The cosmic star
formation rate (SFR) density goes as (1+z)^{4.0\pm0.2} from z=0 to z=0.8. From
z=0.8 to z~1.2, the SFR density continues rising with a smaller slope. At
1.2<z<3, the cosmic SFR density remains roughly constant. The SFR density is
dominated at low redshift (z<0.5) by galaxies which are not very luminous in
the infrared (L_TIR<1.e11 L_sun, where L_TIR is the total infrared luminosity,
integrated from 8 to 1000 micron). The contribution from luminous and
ultraluminous infrared galaxies (L_TIR>1.e11 L_sun) to the total SFR density
increases steadily from z~0 up to z~2.5, forming at least half of the
newly-born stars by z~1.5. Ultraluminous infrared galaxies (L_TIR>1.e12 L_sun)
play a rapidly increasing role for z>~1.3.Comment: 28 pages, 17 figures, accepted for publication in Ap
Orbital Ordering Structures in (Nd,Pr)0.5Sr0.5MnO3 Manganite Thin Films on Perovskite (011) Substrates
Structural study of orbital-ordered manganite thin films has been conducted
using synchrotron radiation, and a ground state electronic phase diagram is
made. The lattice parameters of four manganite thin films, Nd0.5Sr0.5MnO3
(NSMO) or Pr0.5Sr0.5MnO3 (PSMO) on (011) surfaces of SrTiO3 (STO) or
[(LaAlO3){0.3}(SrAl0.5Ta0.5O3){0.7}] (LSAT), were measured as a function of
temperature. The result shows, as expected based on previous knowledge of bulk
materials, that the films' resistivity is closely related to their structures.
Observed superlattice reflections indicate that NSMO thin films have an
antiferro-orbital-ordered phase as their low-temperature phase while PSMO film
on LSAT has a ferro-orbital-ordered phase, and that on STO has no
orbital-ordered phase. A metallic ground state was observed only in films
having a narrow region of A-site ion radius, while larger ions favor
ferro-orbital-ordered structure and smaller ions stabilize
antiferro-orbital-ordered structure. The key to the orbital-ordering transition
in (011) film is found to be the in-plane displacement along [0-1 1] direction.Comment: 19pages, 11 figure
Multiwavelength study of the star formation in the bar of NGC 2903
NGC 2903 is a nearby barred spiral with an active starburst in the center and
Hii regions distributed along its bar. We aim to analyse the star formation
properties in the bar region of NGC 2903 and study the links with the typical
bar morphological features. A combination of space and ground-based data from
the far-ultraviolet to the sub-millimeter spectral ranges is used to create a
panchromatic view of the NGC 2903 bar. We produce two catalogues: one for the
current star formation regions, as traced by the halpha compact emission, and a
second one for the ultraviolet (UV) emitting knots, containing positions and
luminosities. From them we have obtained ultraviolet colours, star formation
rates, dust attenuation and halpha EWs, and their spatial distribution have
been analysed. Stellar cluster ages have been estimated using stellar
population synthesis models (Starburst99). NGC 2903 is a complex galaxy, with a
very different morphology on each spectral band. The CO(J=1-0) and the 3.6
micron emission trace each other in a clear barred structure, while the halpha
leads both components and it has an s-shape distribution. The UV emission is
patchy and does not resemble a bar. The UV emission is also characterised by a
number of regions located symmetrically with respect to the galaxy center,
almost perpendicular to the bar, in a spiral shape covering the inner ~2.5 kpc.
These regions do not show a significant halpha nor 24 micron emission. We have
estimated ages for these regions ranging from 150 to 320 Myr, being older than
the rest of the UV knots, which have ages lower than 10 Myr. The SFR calculated
from the UV emission is ~0.4 M/yr, compatible with the SFR as derived
from halpha calibrations (M/yr).Comment: Accepted for publication in A&
Integral field spectroscopy in the near infrared of NGC 3125-A and SBS 0335-052
We present integral field spectroscopy in the near infrared of the nearby
dwarf starburst galaxies NGC 3125-A and of the low metallicity dwarf galaxy SBS
0335-052. The use of adaptive optics in the observations produces sub-arcsecond
angular resolution. We pinpoint the star forming cores of both galaxies,
identify relevant ISM components such as dust, photo ionized gas, shock excited
gas and molecular gas. We relate these components to the large scale star
formation process of the galaxies. In particular we find the emission of the
near infrared lines of H2 and especially [FeII] does not coincide with the HII
region in NGC 3125. We have the first clear detection of [FeII] in SBS
0335-052
AEGIS: New Evidence Linking Active Galactic Nuclei to the Quenching of Star Formation
Utilizing Chandra X-ray observations in the All-wavelength Extended Groth
Strip International Survey (AEGIS) we identify 241 X-ray selected Active
Galactic Nuclei (AGNs, L > 10^{42} ergs/s) and study the properties of their
host galaxies in the range 0.4 < z < 1.4. By making use of infrared photometry
from Palomar Observatory and BRI imaging from the Canada-France-Hawaii
Telescope, we estimate AGN host galaxy stellar masses and show that both
stellar mass and photometric redshift estimates (where necessary) are robust to
the possible contamination from AGNs in our X-ray selected sample. Accounting
for the photometric and X-ray sensitivity limits of the survey, we construct
the stellar mass function of X-ray selected AGN host galaxies and find that
their abundance decreases by a factor of ~2 since z~1, but remains roughly flat
as a function of stellar mass. We compare the abundance of AGN hosts to the
rate of star formation quenching observed in the total galaxy population. If
the timescale for X-ray detectable AGN activity is roughly 0.5-1 Gyr--as
suggested by black hole demographics and recent simulations--then we deduce
that the inferred AGN "trigger" rate matches the star formation quenching rate,
suggesting a link between these phenomena. However, given the large range of
nuclear accretion rates we infer for the most massive and red hosts, X-ray
selected AGNs may not be directly responsible for quenching star formation.Comment: 12 pages. Submitted to ApJ. Comments welcom
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