761 research outputs found
Superconducting weak links in YBa2Cu3O7-δ an AC magnetic susceptibility study
AC magnetic susceptibility, X' and X'', and ac resistivity, ρ, of two different samples of superconducting YBa2Cu3O7-δ have been measured. The results show a strong non-linear behaviour and are very sensitive to the exciting amplitudes. From the analysis of the data and comparison with appropriate models and other superconducting compounds, evidence for the existence of weak superconducting links is inferred. Finally, the utility of acX measurements for detection of different superconducting phases is made evident. \u
Stellar populations in the Carina region: The Galactic plane at l = 291
Previous studies of the Carina region have revealed its complexity and
richness as well as a significant number of early-type stars. In many cases,
these studies only concentrated on the central region or were not homogeneous.
This latter aspect, in particular, is crucial because very different ages and
distances for key clusters have been claimed in recent years. The aim of this
work is to study in detail an area of the Galactic plane in Carina. We analyze
the properties of different stellar populations and focus on a sample of open
clusters and their population of YSOs and highly reddened early stars. We also
studied the stellar mass distribution in these clusters and the possible
scenario of their formation. Finally, we outline the Galactic spiral structure
in this direction. We obtained photometric data for six young open clusters
located in Carina at l = 291, and their adjacent stellar fields, which we
complemented with spectroscopic observations of a few selected targets. We also
culled additional information from the literature. Our results provide more
reliable estimates of distances, color excesses, masses, and ages of the
stellar populations in this direction. We estimate the basic parameters of the
studied clusters and find that they identify two overdensities of young stellar
populations. We find evidence of PMS populations inside them, with an apparent
coeval stellar formation in the most conspicuous clusters. We also discuss
apparent age and distance gradients in the direction NW-SE. We study the mass
distributions of several clusters in the region. They consistently show a
canonical IMF slope. We discover and characterise an abnormally reddened
massive stellar population. Spectroscopic observations of ten stars of this
latter population show that all selected targets were massive OB stars. Their
location is consistent with the position of the Car-Sag spiral arm.Comment: 15 pages, 13 figure
Mechanical topological insulator in zero dimensions
We study linear vibrational modes in finite isostatic Maxwell lattices, mechanical systems where the number of degrees of freedom matches the number of constraints. Recent progress in topological mechanics exploits the nontrivial topology of BDI class Hamiltonians in one dimenson and arising topological floppy modes at the edges. A finite frame, or zero-dimensional system, also exhibits a nonzero topological index according to the classification table. We construct mechanical insulating models in zero dimensions that complete the BDI classification in the available real space dimensions. We compute and interpret its nontrivial invariant Z2.N.L. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through The María de Maeztu Programme for Units of Excellence
in R&D (Grant No. MDM-2014-0377). This work has been funded by MINECO Grant No. FIS2015-64886-C5-5-
Thermogravimetry and neutron thermodiffractometry studies of the H-YBa2Cu3O7 system.
The high Tc superconducting oxide YBa2Cu3O7¿x reacts with hydrogen gas. Thermogravimetric, X-ray and neutron scattering experiments allow us to propose a two-step type of hydrogen bonding. Firstly, a few hydrogen atoms fill some oxygen vacancies and may favourably modify the electron state, giving rise to a slight increase in the critical temperature. Secondly, after a prolonged heating period, the collapse of the YBa2Cu3O7¿x type framework and of superconductivity were observed, and a new, highly hydrogenated material appeared
A Bayesian Method to Mitigate the Effects of Unmodelled Time-Varying Systematics for 21-cm Cosmology Experiments
Radio observations of the neutral hydrogen signal from the Cosmic Dawn and
Epoch of Reionisation have helped to provide constraints on the properties of
the first stars and galaxies. Since this global 21-cm cosmological signal from
the Cosmic Dawn is effectively constant on observing timescales and since
effects resulting from systematics will vary with time, the effects of these
systematics can be mitigated without the need for a model of the systematic. We
present a method to account for unmodelled time-varying systematics in 21-cm
radio cosmology experiments using a squared-exponential Gaussian process kernel
to account for correlations between time bins in a fully Bayesian way. We find
by varying the model parameters of a simulated systematic that the Gaussian
process method improves our ability to recover the signal parameters by
widening the posterior in the presence of a systematic and reducing the bias in
the mean fit parameters. When varying the amplitude of a model sinusoidal
systematic between 0.25 and 2.00 times the 21-cm signal amplitude and the
period between 0.5 and 4.0 times the signal width, we find on average a 5%
improvement in the root mean squared error of the fitted signal. We can use the
fitted Gaussian process hyperparameters to identify the presence of a
systematic in the data, demonstrating the method's utility as a diagnostic
tool. Furthermore, we can use Gaussian process regression to calculate a mean
fit to the residuals over time, providing a basis for producing a model of the
time-varying systematic.Comment: 11 pages, 13 figure
Microtubule rescue at midzone edges promotes overlap stability and prevents spindle collapse during anaphase B
During anaphase B, molecular motors slide interpolar microtubules to elongate the mitotic spindle, contributing to the separation of chromosomes. However, sliding of antiparallel microtubules reduces their overlap, which may lead to spindle breakage, unless microtubules grow to compensate sliding. How sliding and growth are coordinated is still poorly understood. In this study, we have used the 1ssion yeast S. pombe to measure microtubule dynamics during anaphase B. We report that the coordination of microtubule growth and sliding relies on promoting rescues at the midzone edges. This makes microtubules stable from pole to midzone, while their distal parts including the plus ends alternate between assembly and disassembly. Consequently, the midzone keeps a constant length throughout anaphase, enabling sustained sliding without the need for a precise regulation of microtubule growth speed. Additionally, we found that in S. pombe, which undergoes closed mitosis, microtubule growth speed decreases when the nuclear 22 membrane wraps around the spindle midzone
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