1,073 research outputs found
Resistivity and Hall effect of LiFeAs: Evidence for electron-electron scattering
LiFeAs is unique among the broad family of FeAs-based superconductors,
because it is superconducting with a rather large K under
ambient conditions although it is a stoichiometric compound. We studied the
electrical transport on a high-quality single crystal. The resistivity shows
quadratic temperature dependence at low temperature giving evidence for strong
electron-electron scattering and a tendency towards saturation around room
temperature. The Hall constant is negative and changes with temperature, what
most probably arises from a van Hove singularity close to the Fermi energy in
one of the hole-like bands. Using band structure calculations based on angular
resolved photoemission spectra we are able to reproduce all the basic features
of both the resistivity as well as the Hall effect data.Comment: 6 pages, 3 figures included; V2 has been considerably revised and
contain a more detailed analysis of the Hall effect dat
Turbulent Mixing in the Interstellar Medium -- an application for Lagrangian Tracer Particles
We use 3-dimensional numerical simulations of self-gravitating compressible
turbulent gas in combination with Lagrangian tracer particles to investigate
the mixing process of molecular hydrogen (H2) in interstellar clouds. Tracer
particles are used to represent shock-compressed dense gas, which is associated
with H2. We deposit tracer particles in regions of density contrast in excess
of ten times the mean density. Following their trajectories and using
probability distribution functions, we find an upper limit for the mixing
timescale of H2, which is of order 0.3 Myr. This is significantly smaller than
the lifetime of molecular clouds, which demonstrates the importance of the
turbulent mixing of H2 as a preliminary stage to star formation.Comment: 10 pages, 5 figures, conference proceedings "Turbulent Mixing and
Beyond 2007
Using NuRadioMC to study the performance of UHE radio neutrino detectors
NuRadioMC is an open-source, Python-based simulation and reconstruction framework for radio detectors of ultra-high energy neutrinos and cosmic rays. Its modular design makes NuRadioMC suitable for use with a range of past, current and future detectors. In addition, the recent deployment
of a complete documentation as well as a pip release make NuRadioMC relatively easy to learn and use. Here, we outline the features currently available and under development in NuRadioMC, with a focus on its usage to simulate and study in-ice radio neutrino detectors
The alignment of molecular cloud magnetic fields with the spiral arms in M33
The formation of molecular clouds, which serve as stellar nurseries in
galaxies, is poorly understood. A class of cloud formation models suggests that
a large-scale galactic magnetic field is irrelevant at the scale of individual
clouds, because the turbulence and rotation of a cloud may randomize the
orientation of its magnetic field. Alternatively, galactic fields could be
strong enough to impose their direction upon individual clouds, thereby
regulating cloud accumulation and fragmentation, and affecting the rate and
efficiency of star formation. Our location in the disk of the Galaxy makes an
assessment of the situation difficult. Here we report observations of the
magnetic field orientation of six giant molecular cloud complexes in the
nearby, almost face-on, galaxy M33. The fields are aligned with the spiral
arms, suggesting that the large-scale field in M33 anchors the clouds.Comment: to appear in Natur
Evolution and Impact of Bars over the Last Eight Billion Years: Early Results from GEMS
Bars drive the dynamical evolution of disk galaxies by redistributing mass
and angular momentum, and they are ubiquitous in present-day spirals. Early
studies of the Hubble Deep Field reported a dramatic decline in the rest-frame
optical bar fraction f_opt to below 5% at redshifts z>0.7, implying that disks
at these epochs are fundamentally different from present-day spirals. The GEMS
bar project, based on ~8300 galaxies with HST-based morphologies and accurate
redshifts over the range 0.2-1.1, aims at constraining the evolution and impact
of bars over the last 8 Gyr. We present early results indicating that f_opt
remains nearly constant at ~30% over the range z=0.2-1.1,corresponding to
lookback times of ~2.5-8 Gyr. The bars detected at z>0.6 are primarily strong
with ellipticities of 0.4-0.8. Remarkably, the bar fraction and range of bar
sizes observed at z>0.6 appear to be comparable to the values measured in the
local Universe for bars of corresponding strengths. Implications for bar
evolution models are discussed.Comment: Submitted June 25, 2004. 10 pages 5 figures. To appear in Penetrating
Bars through Masks of Cosmic Dust: The Hubble Tuning Fork Strikes a New Note,
eds. D. Block, K. Freeman, R. Groess, I. Puerari, & E.K. Block (Dordrecht:
Kluwer), in pres
Explaining Long-Distance Dispersal: Effects of Dispersal Distance on Survival and Growth in a Stream Salamander
Long-distance dispersal (LDD) may contribute disproportionately to range expansions, the creation of new evolutionary lineages, and species persistence in human-dominated landscapes. However, because data on the individual consequences of dispersal distance are extremely limited, we have little insight on how LDD is maintained in natural populations. I used six years of spatially explicit captureâmarkârecapture (CMR) data to test the prediction that individual performance increases with dispersal distance in the stream salamander Gyrinophilus porphyriticus. Dispersal distance was total distance moved along the 1-km study stream, ranging from 0 to 565 m. To quantify individual performance, I used CMR estimates of survival and individual growth rates based on change in body length. Survival and growth rates increased significantly with dispersal distance. These relationships were not confounded by pre-dispersal body condition or by ecological gradients along the stream. Individual benefits of LDD were likely caused by an increase in the upper limit of settlement site quality with dispersal distance. My results do not support the view that the fitness consequences of LDD are unpredictable and instead suggest that consistent evolutionary mechanisms may explain the prevalence of LDD in nature. They also highlight the value of direct CMR data for understanding the individual consequences of variation in dispersal distance and how that variation is maintained in natural populations
The HST Key Project on the Extragalactic Distance Scale. XXVIII. Combining the Constraints on the Hubble Constant
Since the launch of the Hubble Space Telescope nine years ago, Cepheid
distances to 25 galaxies have been determined for the purpose of calibrating
secondary distance indicators. A variety of these can now be calibrated, and
the accompanying papers by Sakai, Kelson, Ferrarese, and Gibson employ the full
set of 25 galaxies to consider the Tully-Fisher relation, the fundamental plane
of elliptical galaxies, Type Ia supernovae, and surface brightness
fluctuations.
When calibrated with Cepheid distances, each of these methods yields a
measurement of the Hubble constant and a corresponding measurement uncertainty.
We combine these measurements in this paper, together with a model of the
velocity field, to yield the best available estimate of the value of H_0 within
the range of these secondary distance indicators and its uncertainty.
The result is H_0 = 71 +/- 6 km/sec/Mpc. The largest contributor to the
uncertainty of this 67% confidence level result is the distance of the Large
Magellanic Cloud, which has been assumed to be 50 +/- 3 kpc
Study of Photon Dominated Regions in Cepheus B
Aim: The aim of the paper is to understand the emission from the photon
dominated regions in Cepheus B, estimate the column densities of neutral carbon
in bulk of the gas in Cepheus B and to derive constraints on the factors which
determine the abundance of neutral carbon relative to CO. Methods: This paper
presents 15'x15' fully sampled maps of CI at 492 GHz and 12CO 4-3 observed with
KOSMA at 1' resolution. The new observations have been combined with the FCRAO
12CO 1-0, IRAM-30m 13CO 2-1 and C18O 1-0 data, and far-infrared continuum data
from HIRES/IRAS. The KOSMA-tau spherical PDR model has been used to understand
the CI and CO emission from the PDRs in Cepheus B and to explain the observed
variation of the relative abundances of both C^0 and CO. Results: The emission
from the PDR associated with Cepheus B is primarily at V_LSR between -14 and
-11 km s^-1. We estimate about 23% of the observed CII emission from the
molecular hotspot is due to the ionized gas in the HII region. Over bulk of the
material the C^0 column density does not change significantly, (2.0+-1.4)x10^17
cm^-2, although the CO column density changes by an order of magnitude. The
observed \cbyco abundance ratio varies between 0.06 and 4 in Cepheus B. We find
an anti-correlation of the observed C/CO abundance ratio with the observed
hydrogen column density, which holds even when all previous observations
providing C/CO ratios are included. Here we show that this observed variation
of C/CO abundance with total column density can be explained only by clumpy
PDRs consisting of an ensemble of clumps. At high H2 column densities high mass
clumps, which exhibit low C/CO abundance, dominate, while at low column
densities, low mass clumps with high C/CO abundance dominate.Comment: 12 pages, 10 figures, Accepted for publication in A&
Function of Phytochrome A in Potato Plants as Revealed through the Study of Transgenic Plants
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