384 research outputs found
Pressure effects in the triangular layered cobaltites NaxCoO2
We have measured transport properties as a function of temperature and
pressure up to 30GPa in the NaxCoO2 system. For the x=0.5 sample the transition
temperature at 53K increases with pressure, while paradoxically the sample
passes from an insulating to a metallic ground state. A similar transition is
observed in the x=0.31 sample under pressure. Compression on the x=0.75 sample
transforms the sample from a metallic to an insulating state. We discuss our
results in terms of interactions between band structure effects and Na+ order.Comment: 18 pages, 5 figure
Evidence for the coexistence of Dirac and massive carriers in a-(BEDT-TTF)2I3 under hydrostatic pressure
Transport measurements were performed on the organic layered compound \aI3
under hydrostatic pressure. The carrier types, densities and mobilities are
determined from the magneto-conductance of \aI3 . While evidence of
high-mobility massless Dirac carriers has already been given, we report here,
their coexistence with low-mobility massive holes. This coexistence seems
robust as it has been found up to the highest studied pressure. Our results are
in agreement with recent DFT calculations of the band structure of this system
under hydrostatic pressure. A comparison with graphene Dirac carriers has also
been done.Comment: 5 pages 5 figure
High pressure transport studies of the LiFeAs analogues CuFeTe2 and Fe2As
We have synthesized two iron-pnictide/chalcogenide materials, CuFeTe2 and
Fe2As, which share crystallographic features with known iron-based
superconductors, and carried out high-pressure electrical resistivity
measurements on these materials to pressures in excess of 30 GPa. Both
compounds crystallize in the Cu2Sb-type crystal structure that is
characteristic of LiFeAs (with CuFeTe2 exhibiting a disordered variant). At
ambient pressure, CuFeTe2 is a semiconductor and has been suggested to exhibit
a spin-density-wave transition, while Fe2As is a metallic antiferromagnet. The
electrical resistivity of CuFeTe2, measured at 4 K, decreases by almost two
orders of magnitude between ambient pressure and 2.4 GPa. At 34 GPa, the
electrical resistivity decreases upon cooling the sample below 150 K,
suggesting the proximity of the compound to a metal-insulator transition.
Neither CuFeTe2 nor Fe2As superconduct above 1.1 K throughout the measured
pressure range.Comment: 6 pages, 7 figure
Tifus murino en España durante el periodo 2004-2013
[ES] Objetivos: Estimar la incidencia y otros datos epidemiológicos del tifus murino en España.
Material y métodos: Se analizaron aquellos pacientes cuyo diagnóstico al alta hospitalaria entre los años 2004-2013 se hubiera codificado en el CMBD, según el CIE 9-MC, como 081.0 (tifus endémico murino portado por pulgas).
Resultados: Se diagnosticaron 56 pacientes (83,93% como diagnóstico principal y 16,07% como secundario) y 50 de ellos se detectaron en las Islas Canarias. La tasa de incidencia global por millón de habitantes y año, fue de 0,13 (IC95% de 0,10 a 0,17) en España.
La media de edad fue de 49,75 años y un rango de 8-84 años. El riesgo relativo (RR) hombre-mujer fue 1,7:1. La mayorÃa de casos se concentraron en el tercer trimestre del año.
Conclusiones: La incidencia del TM en España es baja y heterogénea. La enfermedad se detectó
principalmente en Canarias, varones y el tercer trimestre del año.
[EN] Objectives: To estimate the incidence and other epidemiological data of murine typhus in Spain.
Material and methods: We analyzed those patients whose diagnosis at hospital discharge between 2004-2013 years had been coded in the Hospitalization Minimum Data Set, according to the ICD 9-MC, as 081.0 (Typhus, Endemic Flea-Borne).
Main results: 56 patients were diagnosed (83.93% as main diagnosis and 16.07% as secondary) and 50 of them were detected in the Canary Islands. The global incidence rate per million inhabitants per year was 0.13 (CI95% from 0.10 to 0.17) in Spain.
The mean age was 49.75 years and a range of 8-84 years. The relative risk (RR) male-female was 1.7: 1. The majority of cases were concentrated in the third quarter of the year.
Conclusions: The incidence of TM in Spain is low and heterogeneous. The disease was detected mainly in the Canary Islands, males and the third quarter of the year
Retained strength of UHTCMCs after oxidation at 2278 K
In the frame of Horizon 2020 European C3HARME research project, the manufacture of ZrB2-based CMCs was developed through different processes: slurry infiltration and sintering, radio frequency chemical vapour infiltration (RF-CVI) and reactive metal infiltration (RMI). To assess the high temperature stability, room temperature bending strength was measured after oxidizing the samples at 2278 K and compared to the strength of the as-produced materials. Microstructures were analysed before and after the thermal treatment to assess the damage induced by the high temperature oxidation. Short fibre-reinforced composites showed the highest retained strength (>80%) and an unchanged stress–strain curve
Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry
The advent of Dirac materials has made it possible to realize two dimensional
gases of relativistic fermions with unprecedented transport properties in
condensed matter. Their photoconductive control with ultrafast light pulses is
opening new perspectives for the transmission of current and information. Here
we show that the interplay of surface and bulk transient carrier dynamics in a
photoexcited topological insulator can control an essential parameter for
photoconductivity - the balance between excess electrons and holes in the Dirac
cone. This can result in a strongly out of equilibrium gas of hot relativistic
fermions, characterized by a surprisingly long lifetime of more than 50 ps, and
a simultaneous transient shift of chemical potential by as much as 100 meV. The
unique properties of this transient Dirac cone make it possible to tune with
ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that
is impossible with conventional optoelectronic materials.Comment: Nature Communications, in press (12 pages, 6 figures
Testing approach to new fibre-reinforced UHTC materials in the C3HARME project
In space applications, very high temperatures are often a driving environmental condition for certain technologies. This is especially true for the engines of rockets and for re-entry systems where thermal protection is vital. With current state-of-the-art systems of both types, there are in some cases materials used which are simply consumed because temperatures are so high, there is no choice of a material able to withstand the conditions.
In order to overcome these restraints, Ultra-high-temperature ceramics (UHTC) provide an option in terms of the pure temperature capability of the material. However, bulk UHTC materials so far suffered from the problem of rather low mechanical properties and especially size limitations due to thermal shock sensitivity. If this situation could be improved by introducing a fibre reinforcement and combining the advantages of UHTC with the robustness of Ceramic Matrix Composites (CMC), the resulting UHTCMC materials could open up a whole new world of possibilities for new designs and flight regimes of the systems mentioned.
The C3HARME project aims at exactly that. The goal is to develop new fibre-reinforced UHTC types and to significantly improve the maturity of them to a level of TRL 5-6. In order to do so, the thermo-mechanical characterization of samples and components is of great importance. A multitude of different tests are done to get the full data set of all the relevant properties from room temperature up to very high temperatures as they are encountered in the application scenario. The talk will give an overview about the characterization strategy and the individual tests and facilities applied with some of the results already obtained
The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55
The quantitative study of the physical properties and chemical abundances of
large samples of massive blue stars at different metallicities is a powerful
tool to understand the nature and evolution of these objects. Their analysis
beyond the Milky Way is challenging, nonetheless it is doable and the best way
to investigate their behavior in different environments. Fulfilling this task
in an objective way requires the implementation of automatic analysis
techniques that can perform the analyses systematically, minimizing at the same
time any possible bias.
As part of the ARAUCARIA project we carry out the first quantitative
spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55
at 1.94 Mpc away. By applying the methodology developed in this work, we derive
their stellar parameters, chemical abundances and provide a characterization of
the present-day metallicity of their host galaxy.
Based on the characteristics of the stellar atmosphere/line formation code
FASTWIND, we designed and created a grid of models for the analysis of massive
blue supergiant stars. Along with this new grid, we implemented a spectral
analysis algorithm. Both tools were specially developed to perform fully
consistent quantitative spectroscopic analyses of low spectral resolution of
B-type supergiants in a fast and objective way.
We present the main characteristics of our FASTWIND model grid and perform a
number of tests to investigate the reliability of our methodology. The
automatic tool is applied afterward to a sample of 12 B-type supergiant stars
in NGC55, deriving the stellar parameters and abundances. The results indicate
that our stars are part of a young population evolving towards a red supergiant
phase. The derived chemical composition hints to an average metallicity similar
to the one of the Large Magellanic Cloud, with no indication of a spatial trend
across the galaxy.Comment: 19 pages, 12 figures and 9 tables. Accpeted for publication in A&
A Survey of Local Group Galaxies Currently Forming Stars: III. A Search for Luminous Blue Variables and Other H-alpha Emission-Lined Stars
We describe a search for H-alpha emission-lined stars in M31, M33, and seven
dwarfs in or near the Local Group (IC 10, NGC 6822, WLM, Sextans B, Sextans A,
Pegasus and the Phoenix dwarf) using interference filter imaging with the KPNO
and CTIO 4-m telescope and Mosaic cameras. The survey is aimed primarily at
identifying new Luminous Blue Variables (LBVs) from their spectroscopic
similarity to known LBVs, avoiding the bias towards photometric variability,
which may require centuries to manifest itself if LBVs go through long
quiescent periods. Followup spectroscopy with WIYN confirms that our survey
detected a wealth of stars whose spectra are similar to the known LBVs. We
"classify" the spectra of known LBVs, and compare these to the spectra of the
new LBV candidates. We demonstrate spectacular spectral variability for several
of the new LBV candidates, such as AM2, previously classified as a Wolf-Rayet
star, which now shows FeI, FeII and Balmer emission lines but neither the NIII
4634,42 nor HeII 4686 emission that it did in 1982. Profound spectral changes
are also noted for other suspected and known LBVs. Several of the LBV
candidates also show >0.5 mag changes in V over the past 10-20 years. The
number of known or suspected LBVs is now 24 in M31, 37 in M33, 1 in NGC 6822,
and 3 in IC 10. We estimate that the total number of LBVs in M31 and M33 may be
several hundred, in contrast to the 8 known historically through large-scale
photometric variability. This has significant implications for the time scale
of the LBV phase. We also identify a few new WRs and peculiar emission-lined
objects.Comment: Accepted by the Astronomical Journal. Version with higher quality
figures may be downloaded from http://www.lowell.edu/users/massey/has.pdf.g
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