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 mayoría 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&

Is oxidative stress MYC’s Achilles heel?

No abstract available

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