751 research outputs found
Symmetry-induced interference effects in metalloporphyrin wires
Organo-metallic molecular structures where a single metallic atom is embedded
in the organic backbone are ideal systems to study the effect of strong
correlations on their electronic structure. In this work we calculate the
electronic and transport properties of a series of metalloporphyrin molecules
sandwiched by gold electrodes using a combination of density functional theory
and scattering theory. The impact of strong correlations at the central
metallic atom is gauged by comparing our results obtained using conventional
DFT and DFT+U approaches. The zero bias transport properties may or may not
show spin-filtering behavior, depending on the nature of the d state closest to
the Fermi energy. The type of d state depends on the metallic atom and gives
rise to interference effects that produce different Fano features. The
inclusion of the U term opens a gap between the d states and changes
qualitatively the conductance and spin-filtering behavior in some of the
molecules. We explain the origin of the quantum interference effects found as
due to the symmetry-dependent coupling between the d states and other molecular
orbitals and propose the use of these systems as nanoscale chemical sensors. We
also demonstrate that an adequate treatment of strong correlations is really
necessary to correctly describe the transport properties of metalloporphyrins
and similar molecular magnets
Seismology of beta Cephei stars: differentially-rotating models for interpreting the oscillation spectrum of nu-Eridani
A method for the asteroseismic analysis of beta Cephei stars is presented and
applied to the star nu Eridani. The method is based on the analysis of
rotational splittings, and their asymmetries using differentially-rotating
asteroseismic models. Models with masses around 7.13 M_sun, and ages around
14.9 Myr, were found to fit better 10 of the 14 observed frequencies, which
were identified as the fundamental radial mode and the three L=1 triplets g, p,
and p. The splittings and aymmetries found for these modes recover those
provided in the literature, except for p. For this last mode, all its
non-axysimmetric components are predicted by the models. Moreover, opposite
signs of the observed and predicted splitting asymmetries are found. If
identification is confirmed, this can be a very interesting source of
information about the internal rotation profile, in particular in the outer
regions of the star.
In general, the seismic models which include a description for shellular
rotation yield slightly better results as compared with those given by
uniformly-rotating models. Furthermore, we show that asymmetries are quite
dependent on the overshooting of the convective core, which make the present
technique suitable for testing the theories describing the angular momentum
redistribution and chemical mixing due to rotationally-induced turbulence.Comment: 11 pages, 9 figures, 8 tables. ApJ (in press
Unveiling the power spectra of Scuti stars with TESS. The temperature, gravity, and frequency scaling relation
Thanks to high-precision photometric data legacy from space telescopes like
CoRoT and Kepler, the scientific community could detect and characterize the
power spectra of hundreds of thousands of stars. Using the scaling relations,
it is possible to estimate masses and radii for solar-type pulsators. However,
these stars are not the only kind of stellar objects that follow these rules:
Scuti stars seem to be characterized with seismic indexes such as the
large separation (). Thanks to long-duration high-cadence TESS light
curves, we analysed more than two thousand of this kind of classical pulsators.
In that way, we propose the frequency at maximum power () as a
proper seismic index since it is directly related with the intrinsic
temperature, mass and radius of the star. This parameter seems not to be
affected by rotation, inclination, extinction or resonances, with the exception
of the evolution of the stellar parameters. Furthermore, we can constrain
rotation and inclination using the departure of temperature produced by the
gravity-darkening effect. This is especially feasible for fast rotators as most
of Scuti stars seem to be.Comment: 12 pages, 13 figures, accepted in Astronomy & Astrophysic
Determination of puberty in gilts: contrast of diagnostic methods
Background
Early onset of a gilt´s puberty is needed for adequate economic performance in farms, because it indicates her reproductive performance and longevity. Therefore, an effective diagnosis is needed. Our purpose was to compare different procedures (external characteristics, blood progesterone analysis and ultrasonography diagnosis) to detect puberty in 70 gilts (Topigs TN70; 240 days old) on farm conditions. Postmortem examination was the standard reference. Multiple logistic regression analysis was used to identify which combination of independent variables (predictors) best predicts the status of gilts.
Results
Puberty (46/70 gilts; 65.71%) was characterized by the presence of follicles larger than 6 mm, corpus albicans, corpus rubrum, and corpus luteum (postmortem examination). Vaginal length, body condition, backfat, carcass weight and progesterone blood concentration were significantly higher in pubertal than prepubertal gilts (P < 0.05). Two types of ultrasonography equipment (DELTA and W3) were compared and performed by the same senior technician (V1). The results obtained by two technicians with different levels of experience (V1 and V2, a junior technician) using W3 were also compared. Ultrasonography provided better results than other diagnostic techniques, although the effectiveness of the ultrasonography changed with technological improvements and with increased expertise of technicians. The most accurate results were found by V1/DELTA (Nagelkerke´s R2 = 0.846; Sensitivity = 0.956; Specificity = 0.958; Positive predictive value = 0.978; Negative predictive value = 0.920; Area under ROC curve = 0.957). Results using the W3 equipment could be improved when used in conjunction with vaginal length (V1; Nagelkerke´s R2 = 0.834; Sensitivity = 0.933; Specificity = 0.958; Positive predictive value = 0.977; Negative predictive value = 0.885; Area under ROC curve = 0.972) or progesterone concentration (V2; Nagelkerke´s R2 = 0.780; Sensitivity = 0.955; Specificity = 0.826; Positive predictive value = 0.915; Negative predictive value = 0.905; Area under ROC curve = 0.970).
Conclusions
Ultrasonography provided better results than other diagnostic techniques. The effectiveness of the ultrasonography changes with technological improvements and with increased expertise of technicians. Results using the W3 equipment could be improved when used along with vaginal length (V1) or progesterone concentration (V2). Accuracy parameters are a guide to choose puberty diagnosis, but the farms must also evaluate effect on gilts, ease and cost of administration
Asteroseismology with the WIRE satellite. I. Combining Ground- and Space-based Photometry of the Delta Scuti Star Epsilon Cephei
We have analysed ground-based multi-colour Stromgren photometry and
single-filter photometry from the star tracker on the WIRE satellite of the
delta scuti star Epsilon Cephei. The ground-based data set consists of 16
nights of data collected over 164 days, while the satellite data are nearly
continuous coverage of the star during 14 days. The spectral window and noise
level of the satellite data are superior to the ground-based data and this data
set is used to locate the frequencies. However, we can use the ground-based
data to improve the accuracy of the frequencies due to the much longer time
baseline. We detect 26 oscillation frequencies in the WIRE data set, but only
some of these can be seen clearly in the ground-based data. We have used the
multi-colour ground-based photometry to determine amplitude and phase
differences in the Stromgren b-y colour and the y filter in an attempt to
identify the radial degree of the oscillation frequencies. We conclude that the
accuracies of the amplitudes and phases are not sufficient to constrain
theoretical models of Epsilon Cephei. We find no evidence for rotational
splitting or the large separation among the frequencies detected in the WIRE
data set. To be able to identify oscillation frequencies in delta scuti stars
with the method we have applied, it is crucial to obtain more complete coverage
from multi-site campaigns with a long time baseline and in multiple filters.
This is important when planning photometric and spectroscopic ground-based
support for future satellite missions like COROT and KEPLER.Comment: 13 pages, 12 figures, 4 tables. Fig. 4 reduced in quality. Accepted
by A&
Preparing the COROT space mission: new variable stars in the galactic Anticenter direction
The activities related to the preparation of the asteroseismic, photometric
space mission COROT are described. Photoelectric observations, wide--field CCD
photometry, uvbyB calibrations and further time--series have been obtained at
different observatories and telescopes. They have been planned to complete the
COROT programme in the direction of the galactic Anticenter. In addition to
suitable asteroseismic targets covering the different evolutionary stages
between ZAMS and TAMS, we discovered several other variable stars, both
pulsating and geometrical. We compared results on the incidence of variability
in the galactic Center and Anticenter directions. Physical parameters have been
obtained and evolutionary tracks fitting them have been calculated. The
peculiarities of some individual stars alre pointed out. Paper based on
observations collected at the San Pedro Martir, Sierra Nevada, Teide, La Silla,
Haute-Provence and Roque de Los Muchachos (Telescopio Nazionale Galileo and
Mercator telescopes) observatories.Comment: 13 pages, 9 figures. Accepted for The Astronomical Journal (2005 May
volume
Synthetic approaches to phosphasugars (2-oxo-1,2- oxaphosphacyclanes) using the anomeric alkoxyl radical β‑fragmentation reaction as the key step
The anomeric alkoxyl radical β-fragmentation (ARF) of carbohydrates possessing an electron-withdrawing group (EWG) at C2, promoted by PhI(OAc)2/I2, gives rise to an acyclic iodide through which a pentavalent atom of phosphorus can be introduced via the Arbuzov reaction. After selective hydrolysis and subsequent cyclization, the phosphonate or phosphinate intermediates can be converted into 2- deoxy-1-phosphahexopyranose and 2-deoxy-1-phosphapentopyranose sugars. The ARF of carbohydrates with an electron-donor group (EDG) at C2 proceeds by a radical-polar crossover mechanism, and the cyclization occurs by nucleophilic attack of a conveniently positioned phosphonate or phosphinate group to the transient oxocarbenium ion. This alternative methodology leads to 5-phosphasugars with a 4-deoxy-5-phosphapentopyranose framework. The structure and conformation of the 2-oxo-1,2-oxaphosphinane and 2-oxo-1,2-oxaphospholane ring systems in different carbohydrate models have been studied by NMR and X-ray crystallography
The Frequency Ratio Method for the seismic modelling of gamma Doradus stars. II The role of rotation
The effect of rotation on the Frequency Ratio Method (Moya et al. 2005) is
examined. Its applicability to observed frequencies of rotating gamma Doradus
stars is discussed taking into account the following aspects: the use of a
perturbative approach to compute adiabatic oscillation frequencies; the effect
of rotation on the observational Brunt-Vaisala integral determination and
finally, the problem of disentangling multiplet-like structures from frequency
patterns due to the period spacing expected for high-order gravity modes in
asymptotic regime. This analysis reveals that the FRM produces reliable results
for objects with rotational velocities up to 70 kms/s, for which the FRM
intrinsic error increases one order of magnitude with respect to the typical
FRM errors given in Moya et al. (2005). Our computations suggest that, given
the spherical degree "l" identification, the FRM may be discriminating for m =
0 modes, in the sense that the method avoids any misinterpretation induced by
the presence of rotationally split multiplet-like structures, which reinforces
the robustness of the method. However, if "l" is unknown, such discrimination
is not ensured. In order to check the FRM in presence of slow-moderate
rotation, we have applied it to the three observed frequencies of the slowly
rotating (vsini = 29 km/s) gamma Doradus star HD48501.Comment: A&A, in pres
Odd non-linear conductivity under spatial inversion in chiral Tellurium
Electrical transport in non-centrosymmetric materials departs from the
well-established phenomenological Ohm's law. Instead of a linear relation
between current and electric field, a non-linear conductivity emerges along
specific crystallographic directions. This non-linear transport is
fundamentally related to the lack of spatial inversion symmetry. However, the
experimental implications of an inversion symmetry operation on the non-linear
conductivity remain to be explored. Here, we report on a large, non-linear
conductivity in chiral Tellurium. By measuring samples with opposite
handedness, we demonstrate that the non-linear transport is odd under spatial
inversion. Furthermore, by applying an electrostatic gate, we modulate the
non-linear output by a factor of 300, reaching the highest reported value
excluding engineered heterostructures. Our results establish chiral Te as an
ideal compound not just to study the fundamental interplay between crystal
structure, symmetry operations and non-linear transport, but also to develop
wireless rectifiers and energy-harvesting chiral devices.Comment: 7 pages, 4 figures, Supplemental Materia
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