8,054 research outputs found
Electronic structure of the ferromagnetic superconductor UCoGe from first principles
The superconductor UCoGe is analyzed with electronic structure calculations
using Linearized Augmented Plane Wave method based on Density Functional
Theory. Ferromagnetic and antiferromagnetic calculations with and without
correlations (via LDA+U) were done. In this compound the Fermi level is
situated in a region where the main contribution to DOS comes from the U-5f
orbital. The magnetic moment is mainly due to the Co-3d orbital with a small
contribution from the U-5f orbital. The possibility of fully non-collinear
magnetism in this compound seems to be ruled out. These results are compared
with the isostructural compound URhGe, in this case the magnetism comes mostly
from the U-5f orbital
The loss of anisotropy in MgB2 with Sc substitution and its relationship with the critical temperature
The electrical conductivity anisotropy of the sigma-bands is calculated for
the (Mg,Sc)B2 system using a virtual crystal model. Our results reveal that
anisotropy drops with relatively little scandium content (< 30%); this
behaviour coincides with the lowering of Tc and the reduction of the Kohn
anomaly. This anisotropy loss is also found in the Al and C doped systems. In
this work it is argued that the anisotropy, or 2D character, of the sigma-bands
is an important parameter for the understanding of the high Tc found in MgB2
Influence of carbon on intraband scattering in Mg(B1-xCx)2
We report data on the Hall coefficient (RH) of the carbon substituted
Mg(B1-xCx)2 single crystals with x in the range from 0 to 0.1. The temperature
dependences of RH obtained for the substituted crystals differ systematically
at low temperatures, but all of them converge to the value of 1.8 x 10^-10
m^3/C at room temperature. The RH(T) data together with results of the
thermoelectric power and electrical resistivity measurements are interpreted
within a quasi-classical transport approach, where the presence of four
different conducting sheets is considered. The main influence of the carbon
substitution on the transport properties in the normal state is associated with
enhanced scattering rates, rather than modified concentration of charge
carriers. Presumably the carbon substitution increases the electron-impurity
scattering mainly in the pi band.Comment: 16 pages, 3 figure
Dynamics of the circumstellar gas in the Herbig Ae stars BF Orionis, SV Cephei, WW Vulpeculae and XY Persei
We present high resolution (lambda / Delta_lambda = 49000) echelle spectra of
the intermediate mass, pre-main sequence stars BF Ori, SV Cep, WW Wul and XY
Per. The spectra cover the range 3800-5900 angstroms and monitor the stars on
time scales of months and days. All spectra show a large number of Balmer and
metallic lines with variable blueshifted and redshifted absorption features
superimposed to the photospheric stellar spectra. Synthetic Kurucz models are
used to estimate rotational velocities, effective temperatures and gravities of
the stars. The best photospheric models are subtracted from each observed
spectrum to determine the variable absorption features due to the circumstellar
gas; those features are characterized in terms of their velocity, v, dispersion
velocity, Delta v, and residual absorption, R_max. The absorption components
detected in each spectrum can be grouped by their similar radial velocities and
are interpreted as the signature of the dynamical evolution of gaseous clumps
with, in most cases, solar-like chemical composition. This infalling and
outflowing gas has similar properties to the circumstellar gas observed in UX
Ori, emphasizing the need for detailed theoretical models, probably in the
framework of the magnetospheric accretion scenario, to understand the complex
environment in Herbig Ae (HAe) stars. WW Vul is unusual because, in addition to
infalling and outflowing gas with properties similar to those observed in the
other stars, it shows also transient absorption features in metallic lines with
no obvious counterparts in the hydrogen lines. This could, in principle,
suggest the presence of CS gas clouds with enhanced metallicity around WW Vul.
The existence of such a metal-rich gas component, however, needs to be
confirmed by further observations and a more quantitative analysis.Comment: 21 pages, 13 figures. Accepted for publication by Astronomy &
Astrophysic
The formation of volatile sulphur compounds in unclarified grape juice
The evolution of the sulphur gases OCS, CS2 and DMS was monitored during the clarification of freshly harvested juices from Müller-Turgau and Riesling. Immediately after crushing OCS and CS2 levels began to increase, substantially favoured by high temperatures. When fermentation started concentrations decreased. DMS was found only in fermenting juices
Molecular gas at supernova local environments unveiled by EDGE
CO observations allow estimations of the gas content of molecular clouds,
which trace the reservoir of cold gas fuelling star formation, as well as to
determine extinction via H column density, N(H). Here, we studied
millimetric and optical properties at 26 supernovae (SNe) locations of
different types in a sample of 23 nearby galaxies by combining molecular
CO (J = 1 0) resolved maps from the EDGE survey and
optical Integral Field Spectroscopy from the CALIFA survey. We found an even
clearer separation between type II and type Ibc SNe in terms of molecular gas
than what we found in the optical using H emission as a proxy for
current SF rate, which reinforces the fact that SNe Ibc are more associated
with SF-environments. While A at SN locations is similar for SNe II and SNe
Ibc, and higher compared to SNe Ia, N(H) is significantly higher for SNe
Ibc than for SNe II and SNe Ia. When compared to alternative extinction
estimations directly from SN photometry and spectroscopy, we find that our SNe
Ibc have also redder color excess but showed standard Na I D absorption
pseudo-equivalent widths (1 \AA). In some cases we find no extinction
when estimated from the environment, but high amounts of extinction when
measured from SN observations, which suggests that circumstellar material or
dust sublimation may be playing a role. This work serves as a benchmark for
future studies combining last generation millimeter and optical IFS instruments
to reveal the local environmental properties of extragalactic SNe.Comment: MNRAS accepted, 17 pages, 8 Figures, 4 Table
Hamilton-Jacobi Counterterms for Einstein-Gauss-Bonnet Gravity
The on-shell gravitational action and the boundary stress tensor are
essential ingredients in the study of black hole thermodynamics. We employ the
Hamilton-Jacobi method to calculate the boundary counterterms necessary to
remove the divergences and allow the study of the thermodynamics of
Einstein-Gauss-Bonnet black holes.Comment: 21 pages, LaTe
Expanding Aquatic Observations through Recreation
Accurate observations of the Earth system are required to understand how our planet is changing and to help manage its resources. The aquatic environment—including lakes, rivers, wetlands, estuaries, coastal and open oceans—is a fundamental component of the Earth system controlling key physical, biological, and chemical processes that allow life to flourish. Yet, this environment is critically undersampled in both time and space. New and cost-effective sampling solutions are urgently needed. Here, we highlight the potential to improve aquatic sampling by tapping into recreation. We draw attention to the vast number of participants that engage in aquatic recreational activities and argue, based on current technological developments and recent research, that the time is right to employ recreational citizens to improve large-scale aquatic sampling efforts. We discuss the challenges that need to be addressed for this strategy to be successful (e.g., sensor integration, data quality, and citizen motivation), the steps needed to realize its potential, and additional societal benefits that arise when engaging citizens in scientific sampling
Counterterms and dual holographic anomalies in CS gravity
The holographic Weyl anomaly associated to Chern-Simons gravity in 2n+1
dimensions is proportional to the Euler term in 2n dimensions, with no
contributions from the Weyl tensor. We compute the holographic energy-momentum
tensor associated to Chern-Simons gravity directly from the action, in an
arbitrary odd-dimensional spacetime. We show, in particular, that the
counterterms rendering the action finite contain only terms of the Lovelock
type.Comment: 10 pages, no figure
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