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2007-11-19
|
The MINERvA Neutrino Interaction Experiment
|
The MINERvA neutrino interaction experiment in the NuMI beam at Fermilab will
measure several aspects of neutrino interactions in the few GeV energy region.
We will make cross section and form factor measurements using a fine-grained
fully active scintillator (CH) target, and also investigate nuclear effects on
neutrino interactions as well as hadron rescattering using integral nuclear
targets made of helium, carbon, iron, and lead. We will improve or add to
existing measurements and address some specific questions that are important
for current and upcoming neutrino oscillation experiments. This paper reports
on the progress toward the design, construction, and calibration of the
detector, which we expect will get its first neutrino beam in 2009.
|
0711.3029v1
|
2007-11-20
|
Beyond 1D: spectral line formation with 3D hydrodynamical model atmospheres of red giants
|
We present the results of realistic, 3D, hydrodynamical, simulations of
surface convection in red giant stars with varying effective temperatures and
metallicities. We use the convection simulations as time-dependent,
hydrodynamical, model atmospheres to compute spectral line profiles for a
number of ions and molecules under the assumption of local thermodynamic
equilibrium (LTE). We compare the results with the predictions of line
formation calculations based on 1D, hydrostatic, model stellar atmospheres in
order to estimate the impact of 3D models on the derivation of elemental
abundances. We find large negative 3D-1D LTE abundance corrections (typically
-0.5 to -1 dex) for weak low-excitation lines from molecules and neutral
species in the very low metallicity cases. Finally, we discuss the extent of
departures from LTE in the case of neutral iron spectral line formation.
|
0711.3186v1
|
2007-11-22
|
Stroemgren metallicity calibration: the m1, b-y relation
|
We performed a new calibration of the Stroemgren metallicity index m1 based
on the b-y color of cluster red giant stars. The current
Metallicity-Index-Color relation is not linear in the color range 0.40 < b-y <
1.0, but provides iron abundances of cluster and field red giants with an
accuracy of ~ 0.25 dex.
|
0711.3534v1
|
2007-11-29
|
Magnetoelectric Effects on Composite Nano Granular $Fe/TiO_{2-δ}$ Films
|
Employing a new experimental technique to measure magnetoelectric response
functions, we have measured the magnetoelectric effect in composite films of
nano granular metallic iron in anatase titanium dioxide at temperatures below
50 K. A magnetoelectric resistance is defined as the ratio of a transverse
voltage to bias current as a function of the magnetic field. In contrast to the
anomalous Hall resistance measured above 50 K, the magnetoelectic resistance
below 50 K is significantly larger and exhibits an even symmetry with respect
to magnetic field reversal $H\to -H$. The measurement technique required
attached electrodes in the plane of the film composite in order to measure
voltage as a function of bias current and external magnetic field. To our
knowledge, the composite films are unique in terms of showing magnetoelectric
effects at low temperatures, $<$ 50 K, and anomalous Hall effects at high
temperatures, $>$ 50 K.
|
0711.4776v1
|
2007-12-03
|
Temporal evolution of magnetic elements
|
We study the structure and evolution of the magnetic field of the quiet Sun
by investigating weak spectro-polarimetric signals. To this end, we observed a
quiet region close to the disk center with the German VTT in Tenerife, July 07,
2006. We recorded 38 scans of the same area. Each scan was eight arcsec wide
and observed within about 100 seconds. We used POLIS to simultaneously observe
Stokes profiles of the neutral iron lines at 630.15 and 630.25 nm, the Stokes-I
profile of the Ca II H line at 396.8 nm, and a continuum speckle channel at 500
nm. We witness two examples of magnetic flux cancellation of small-scale
opposite-polarity patches, followed by an enhanced chromospheric emission. In
each case, the two opposite-polarity patches gradually became smaller and,
within a few minutes, the smaller one completely disappeared. The larger patch
also diminished significantly. We provide evidence for a cancellation scenario
in the photosphere which leaves minor traces at the chromospheric level.
|
0712.0234v1
|
2007-12-05
|
$θ_{13}$, $δ$ and the neutrino mass hierarchy at a $γ=350$ double baseline Li/B $β$-Beam
|
We consider a $\beta$-Beam facility where $^8$Li and $^8$B ions are
accelerated at $\gamma = 350$, accumulated in a 10 Km storage ring and let
decay, so as to produce intense $\bar \nu_e$ and $\nu_e$ beams. These beams
illuminate two iron detectors located at $L \simeq 2000$ Km and $L \simeq 7000$
Km, respectively. The physics potential of this setup is analysed in full
detail as a function of the flux. We find that, for the highest flux ($10
\times 10^{18}$ ion decays per year per baseline), the sensitivity to
$\theta_{13}$ reaches $\sin^2 2 \theta_{13} \geq 2 \times10^{-4}$; the sign of
the atmospheric mass difference can be identified, regardless of the true
hierarchy, for $\sin^2 2 \theta_{13} \geq 4\times10^{-4}$; and, CP-violation
can be discovered in 70% of the $\delta$-parameter space for $\sin^2 2
\theta_{13} \geq 10^{-3}$, having some sensitivity to CP-violation down to
$\sin^2 2 \theta_{13} \geq 10^{-4}$ for $|\delta| \sim 90^\circ$.
|
0712.0796v3
|
2007-12-06
|
Editorial: Statistics and forensic science
|
Forensic science is usually taken to mean the application of a broad spectrum
of scientific tools to answer questions of interest to the legal system.
Despite such popular television series as CSI: Crime Scene Investigation and
its spinoffs--CSI: Miami and CSI: New York--on which the forensic scientists
use the latest high-tech scientific tools to identify the perpetrator of a
crime and always in under an hour, forensic science is under assault, in the
public media, popular magazines [Talbot (2007), Toobin (2007)] and in the
scientific literature [Kennedy (2003), Saks and Koehler (2005)]. Ironically,
this growing controversy over forensic science has occurred precisely at the
time that DNA evidence has become the ``gold standard'' in the courts, leading
to the overturning of hundreds of convictions many of which were based on
clearly less credible forensic evidence, including eyewitness testimony [Berger
(2006)].
|
0712.0974v1
|
2007-12-16
|
Evidence of magnetic field wrapping around penumbral filaments
|
We employ high-spatial resolution spectropolarimetric observations from the
Solar Optical Telescope on-board the Hinode spacecraft to investigate the fine
structure of the penumbral magnetic fields. The Stokes vector of two neutral
iron lines at 630 nm is inverted at every spatial pixel to retrieve the
depth-dependence of the magnetic field vector, line-of-sight velocity and
thermodynamic parameters. We show that the azimuthal angle of the magnetic
field vector has opposite sign on both sides above the penumbral filaments.
This is consistent with the wrapping of an inclined field around the horizontal
filaments. The wrapping effect is stronger for filaments with larger horizontal
extensions. In addition, we find that the external magnetic field can penetrate
into the intraspines, leading to non-radial magnetic fields inside them. These
findings shed some light on the controversial small-scale structure of the
sunspot penumbra.
|
0712.2548v1
|
2007-12-18
|
Defect-induced ferromagnetism in fullerenes
|
Based on the ab initio electronic structure calculations the picture of
ferromagnetism in polimerized C60 is proposed which seems to explain the whole
set of controversial experimental data. We have demonstrated that, in contrast
with cubic fullerene, in rhombohedral C60 the segregation of iron atoms is
energetically unprofitable which is a strong argument in favor of intrinsic
character of carbon ferromagnetism which can be caused by vacancies with
unpaired magnetic electrons. It is shown that: (i) energy formation of the
vacancies in the rhombohedral phase of C60 is essentially smaller than in the
cubic phase, (ii) there is a strong ferromagnetic exchange interactions between
carbon cages containing the vacancies, and (iii) the fusion of the magnetic
vacancies into nonmagnetic bivacancy is energetically profitable. The latter
can explain a fragility of the ferromagnetism.
|
0712.2928v2
|
2007-12-18
|
Metallicity estimates of Galactic Cepheids based on Walraven photometry
|
We present new empirical and theoretical calibrations of two photometric
metallicity indices based on Walraven photometry. The empirical calibration
relies on a sample of 48 Cepheids for which iron abundances based on high
resolution spectra are available in the literature. They cover a broad range in
metal abundance (-0.5 < [Fe/H] < +0.5) and the intrinsic accuracy of the
Metallicity Index Color (MIC) relations is better than 0.2 dex. The theoretical
calibration relies on a homogeneous set of scaled-solar evolutionary tracks for
intermediate-mass stars and on pulsation predictions concerning the topology of
the instability strip. The metal content of the adopted evolutionary tracks
ranges from Z=0.001 to Z=0.03 and the intrinsic accuracy of the MIC relations
is better than 0.1 dex.
|
0712.3006v1
|
2007-12-20
|
Determining element abundances of [WC]-type Central Stars for probing stellar evolution and nucleosynthesis
|
[WC]-type CSPNs are hydrogen-deficient Central Stars of Planetary Nebulae
showing strong stellar winds and a carbon-rich chemistry. We have analyzed new
high-resolution spectra of [WC]-type CSPNs with the Potsdam Wolf-Rayet (PoWR)
non-LTE expanding atmosphere models, using upgraded model atoms and atomic
data. Previous analyses are repeated on the basis of the current models which
account for iron-line blanketing. We especially focus on determining the
chemical composition, including some trace elements like nitrogen which are of
key importance for understanding the evolutionary origin of the
hydrogen-deficient Central Stars.
|
0712.3453v1
|
2007-12-21
|
Criteria for mixing rules application for inhomogeneous astrophysical grains
|
The analysis presented in this paper verifies which of the mixing rules are
best for real components of interstellar dust in possible wide range of
wavelengths.The DDA method with elements of different components with various
volume fractions has been used. We have considered 6 materials: ice, amorphous
carbon, graphite, SiC, silicates and iron, and the following mixing rules:
Maxwell-Garnett, Bruggeman, Looyenga, Hanay and Lichtenecker which must satisfy
rigorous bounds. The porous materials have also been considered. We have
assumed simplified spatial distribution, shape and size of inclusions. The
criteria given by \citet{draine1988} have been used to determine the range of
wavelengths for the considered mixtures in order to calculate the ${\rm
Q_{ext}}$ using the DDA. From all chosen mixing rules for the examined
materials in majority of cases (13 out of 20) the best results have been
obtained using the Lichtenecker mixing rule. In 5 cases this rule is better for
some volume fraction of inclusions.
|
0712.3796v3
|
2008-01-01
|
A Self-Consistent NLTE-Spectra Synthesis Model of FeLoBAL QSOs
|
We present detailed radiative transfer spectral synthesis models for the Iron
Low Ionization Broad Absorption Line (FeLoBAL) active galactic nuclei (AGN)
FIRST J121442.3+280329 and ISO J005645.1-273816. Detailed NLTE spectral
synthesis with a spherically symmetric outflow reproduces the observed spectra
very well across a large wavelength range. While exact spherical symmetry is
probably not required, our model fits are of high quality and thus very large
covering fractions are strongly implied by our results. We constrain the
kinetic energy and mass in the ejecta and discuss their implications on the
accretion rate. Our results support the idea that FeLoBALs may be an
evolutionary stage in the development of more ``ordinary'' QSOs.
|
0801.0321v2
|
2008-01-03
|
Spin orbit precession damping in transition metal ferromagnets
|
We provide a simple explanation, based on an effective field, for the
precession damping rate due to the spin-orbit interaction. Previous effective
field treatments of spin-orbit damping include only variations of the state
energies with respect to the magnetization direction, an effect referred to as
the breathing Fermi surface. Treating the interaction of the rotating spins
with the orbits as a perturbation, we include also changes in the state
populations in the effective field. In order to investigate the quantitative
differences between the damping rates of iron, cobalt, and nickel, we compute
the dependence of the damping rate on the density of states and the spin-orbit
parameter. There is a strong correlation between the density of states and the
damping rate. The intraband terms of the damping rate depend on the spin-orbit
parameter cubed while the interband terms are proportional to the spin-orbit
parameter squared. However, the spectrum of band gaps is also an important
quantity and does not appear to depend in a simple way on material parameters.
|
0801.0549v1
|
2008-01-04
|
Could the Ultra Metal-poor Stars be Chemically Peculiar and Not Related to the First Stars?
|
Chemically peculiar stars define a class of stars that show unusual elemental
abundances due to stellar photospheric effects and not due to natal variations.
In this paper, we compare the elemental abundance patterns of the ultra
metal-poor stars with metallicities [Fe/H] $\sim -5 $ to those of a subclass of
chemically peculiar stars. These include post-AGB stars, RV Tauri variable
stars, and the Lambda Bootis stars, which range in mass, age, binarity, and
evolutionary status, yet can have iron abundance determinations as low as
[Fe/H] $\sim -5$. These chemical peculiarities are interpreted as due to the
separation of gas and dust beyond the stellar surface, followed by the
accretion of dust depleted-gas. Contrary to this, the elemental abundances in
the ultra metal-poor stars are thought to represent yields of the most
metal-poor supernova and, therefore, observationally constrain the earliest
stages of chemical evolution in the Universe. The abundance of the elements in
the photospheres of the ultra metal-poor stars appear to be related to the
condensation temperature of that element; if so, then their CNO abundances
suggest true metallicities of [X/H]~ -2 to -4, rather than their present
metallicities of [Fe/H] < -5.
|
0801.0752v1
|
2008-01-14
|
The Effects of Mass Loss on the Evolution of Chemical Abundances in Fm Stars
|
Self-consistent stellar models including all effects of atomic diffusion and
radiative accelerations as well as mass loss are evolved from the pre main
sequence for stars of 1.35-1.5, M$_{\odot}$ at solar metallicity (Z=0.02). A
mass loss rate similar to the solar mass loss rate is sufficient to reproduce
observations of the star $\tau$UMa. We discuss the effect of mass loss on the
iron convection zone that naturally appears beneath the main hydrogen
convection zone of these stars. We also find that the effects of mass loss can
be distinguished from those caused by turbulence, but are nevertheless able to
explain the particularities of the AmFm phenomenon.
|
0801.2122v1
|
2008-01-16
|
Observing the strong gravity regime of accreting black holes with Simbol-X
|
The X-ray reflection features of irradiated accretion disks around black
holes enable us to probe the effects of strong gravity. We investigate to which
precision the reflection signs, i.e. the iron K-line and the Comptonized hump,
can be observed with Simbol-X for nearby Seyfert galaxies. The simulations
presented include accurate computations of the local reprocessed spectra and
modifications due to general relativistic effects in the vicinity of the black
hole. We discuss the impact of global black hole parameters and of the
irradiation pattern of the disk on the resulting spectra as they will be
detected by the Simbol-X mission.
|
0801.2450v2
|
2008-01-17
|
Abundances of Planetary Nebula NGC2392
|
The spectra of the planetary nebula NGC2392 is reanalysed using spectral
measurements made in the mid-infrared with the Spitzer Space Telescope. The aim
is to determine the chemical composition of this object. We also make use of
IUE and ground based spectra. Abundances determined from the mid-infrared
lines, which are insensitive to electron temperature, are used as the basis for
the determination of the composition, which are found to differ somewhat from
earlier results. The abundances found, especially the low value of helium and
oxygen, indicate that the central star was originally of rather low mass.
Abundances of phosphorus, iron, silicon and chlorine have been determined for
the first time in this nebula. The variation of electron temperature in this
nebula is very clear reaching quite high values close to the center. The
temperature of the central star is discussed in the light of the high observed
stages of ionization. The nebular information indicates the spectrum of the
star deviates considerably from a blackbody.
|
0801.2767v1
|
2008-01-18
|
Spectra of W$^{39+}$-W$^{47+}$ in the 12 nm to 20 nm region observed with an EBIT light source
|
We observed spectra of highly ionized tungsten in the extreme ultraviolet
with an electron beam ion trap (EBIT) and a grazing incidence spectrometer at
the National Institute of Standards and Technology. Stages of ionization were
distinguished by varying the energy of the electron beam between 2.1 keV and
4.3 keV and correlating the energies with spectral line emergence. The spectra
were calibrated by reference lines of highly ionized iron produced in the EBIT.
Identification of the observed lines was aided by collisional-radiative
modeling of the EBIT plasma. Good quantitative agreement was obtained between
the modeling results and the experimental observations. Our line
identifications complement recent results for W$^{40+}$-W$^{45+}$ observed in a
tokamak plasma by P\"{u}tterich {\it et al} (\jpb {\bf 38}, 3071, 2005). For
most lines we agree with their assignment of ionization stage. Additionally, we
present new identifications for some allowed and forbidden lines of W$^{39+}$,
W$^{44+}$, W$^{46+}$, and W$^{47+}$. The uncertainties of our wavelengths range
from 0.002 nm to 0.010 nm.
|
0801.2817v1
|
2008-01-24
|
Accretion discs, coronae and jets in black hole binaries: prospects for Simbol-X
|
The phenomenology of accretion disc, coronnae and jets in X-ray binaries is
rather well established. However the structure of the accretion flow in the
various spectral states is still debated and the connection between the hot
flow and compact jet is far from being understood. Simbol-X should address
these two important questions in several ways. First, it will provide us with
the capability of producing high sensivity, broad band spectra and therefore
constrain simultaneously the shape and luminosity of all spectral components
(iron line, reflection bump, thermal disc and comptonised emission) which in
turn provides information on the geometry of the accretion flow. It will also
determine the exact contribution of jets to the X-ray band both in bright and
quiescent states. Finally it will shed new lights on the underlying mechanisms
triggering spectral state transitions by allowing us to follow in exquisite
details the rapid spectral evolution and its correlation with the radio jet
emission during those transitions.
|
0801.3767v1
|
2008-01-30
|
X-ray Emissions from Three-dimensional Magnetohydrodynamic Coronal Accretion Flows
|
We calculate the radiation spectrum and its time variability of the black
hole accretion disk-corona system based on the three-dimensional
magnetohydrodynamic simulation. In explaining the spectral properties of active
galactic nuclei (AGNs), it is often assumed that they consist of a
geometrically thin, optically thick disk and hot, optically thin corona
surrounding the thin disk. As for a model of the corona, we adopt the
simulation data of three-dimensional, non-radiative MHD accretion flows
calculated by Kato and coworkers, while for a thin disk we assume a standard
type disk. We perform Monte Carlo radiative transfer simulations in the corona,
taking into account the Compton scattering of soft photons from the thin disk
by hot thermal electrons and coronal irradiation heating of the thin disk,
which emits blackbody radiation. By adjusting the density parameter of the MHD
coronal flow, we can produce the emergent spectra which are consistent with
those of typical Seyfert galaxies. Moreover, we find rapid time variability in
X-ray emission spectra, originating from the density fluctuation produced by
the magnetorotational instability in the MHD corona. The features of reflection
component including iron fluorescent line emission are also briefly discussed.
|
0801.4613v1
|
2008-01-30
|
Cosmic Ray Velocity and Electric Charge Measurements in the AMS experiment
|
The Alpha Magnetic Spectrometer (AMS) is a particle physics detector designed
to measure charged cosmic ray spectra with energies up to the TeV region and
with high energy photon detection capability up to few hundred GeV. It will be
installed on the International Space Station (ISS) in 2008 and will operate for
more than three years. Due to its large acceptance, the flight duration and the
state-of-art of particle identification techniques, AMS will have a remarkable
sensitivity on antimatter and dark matter searches. The addition of different
detector systems provide AMS with complementary and redundant electric charge
and velocity measurements. The velocity of singly charged particles is expected
to be measured with a precision of 0.1% and charge separation up to iron is
attainable. The AMS capability of measuring a large range of electric charges
and accurate velocities, will largely contribute to a better understanding of
cosmic ray production, acceleration and propagation mechanisms in the galaxy.
|
0801.4656v1
|
2008-02-06
|
Chandra unveils a binary Active Galactic Nucleus in Mrk463
|
We analyse Chandra, XMM-Newton and HST data of the double-nucleus
Ultraluminous Infrared Galaxy (ULIRG), Mrk463. The Chandra detection of two
luminous ($\mathrm{L}_\mathrm{2-10 keV}=1.5\times10^{43}$ and
$3.8\times10^{42}$ erg cm$^{-2}$ s$^{-1}$), unresolved nuclei in Mrk~463
indicates that this galaxy hosts a binary AGN, with a projected separation of
$\simeq3.8$ kpc ($3.83\pm0.01$ arcsec). While the East nucleus was already
known to be a Seyfert 2 (and this is further confirmed by our Chandra detection
of a neutral iron line), this is the first unambiguous evidence in favour of
the AGN nature of the West nucleus. Mrk463 is therefore the clearest case so
far for a binary AGN, after NGC6240.
|
0802.0825v1
|
2008-02-12
|
Detection of supernovae neutrinos with neutrino-iron scattering
|
The $\nu_e-^{56}$Fe cross section is evaluated in the projected quasiparticle
random phase approximation (PQRPA). This model solves the puzzle observed in
RPA for nuclei with mass around $^{12}$C, because it is the only RPA model that
treats the Pauli principle correctly. The cross sections as a function of the
incident neutrino energy are compared with recent theoretical calculations of
similar models. The average cross section weighted with the flux spectrum
yields a good agreement with the experimental data. The expected number of
events in the detection of supernova neutrinos is calculated for the LVD
detector leading to an upper limit for the electron neutrino energy of
particular importance in this experiment
|
0802.1553v2
|
2008-02-12
|
Giant orbital moments are responsible for the anisotropic magnetoresistance of atomic contacts
|
We study here, both experimentally and theoretically, the anisotropy of
magnetoresistance in atomic contacts. Our measurements on iron break junctions
reveal an abrupt and hysteretic switch between two conductance levels when a
large applied field is continuously rotated. We show that this behaviour stems
from the coexistence of two metastable electronic states which result from the
anisotropy of electronic interactions responsible for the enhancement of
orbital magnetization. In both states giant orbital moments appear on the low
coordinated central atom in a realistic contact geometry. However they differ
by their orientation, parallel or perpendicular, with respect to the axis of
the contact. Our explanation is totally at variance with the usual model based
on the band structure of a monatomic linear chain, which we argue cannot be
applied to 3d ferromagnetic metals.
|
0802.1595v1
|
2008-02-12
|
Electronic transport in iron atomic contacts: from the infinite wire to realistic geometries
|
We present a theoretical study of spin polarized transport in Fe atomic
contacts using a self-consistent tight-binding Hamiltonian in a non-orthogonal
$s$, $p$ and $d$ basis set, the spin-polarization being obtained from a
non-collinear Stoner-like model and the transmission probability from the
Fisher-Lee formula. The behaviour of an infinite perfect Fe wire is compared
with that of an infinite chain presenting geometric defects or magnetic walls
and with that of a finite chain connected to infinite one-dimensional or
three-dimensional leads. In the presence of defects or contacts the
transmission probability of $d$ electrons is much more affected than that of
$s$ electrons, in particular, contact effects may suppress some transmission
channels. It is shown that the behaviour of an infinite wire is never obtained
even in the limit of long chains connected to electrodes. The introduction of
the spin-orbit coupling term in the Hamiltonian enables us to calculate the
anisotropy of the magneto-resistance. Finally whereas the variation of the
magneto-resistance as a function of the magnetization direction is step-like
for an infinite wire, it becomes smooth in the presence of defects or contacts.
|
0802.1598v1
|
2008-02-28
|
On the Cepheid Metallicity Dichotomy
|
With the aid of stellar atmosphere models, we derive iron abundances [Fe/H]
from the OGLE B, V, I photometry on the Magellanic Cloud Cepheids. We show that
in both clouds the average metallicities of the first overtone variables are
lower than those of the fundamental ones (by ~0.2 and ~0.3 dex in the LMC and
SMC, respectively). Consequently, there is a correlation between the overall
[Fe/H] and luminosity; the lower luminosity stars tend to be also of lower
metallicity. These metallicity dependencies are concordant with the ones
derived for the two types of double-mode Cepheids from pulsation theory. Yet
another support of this dichotomy comes from the evolution theories that
require lower metallicities for blue-looping low-luminosity stars than for
high-luminosity ones. We also comment on the possibility of using
period-luminosity-color relations to derive more accurate metallicities.
|
0802.4166v1
|
2008-03-04
|
Magneto-Dielectric phenomena in charge and spin frustrated system of layered iron oxide
|
Dielectric and magnetic phenomena in spin and charge frustrated system RFe2O4
(R is a rare-earth metal ion) are studied. An electronic model for charge, spin
and orbital degrees in a pair of triangular-lattice planes is derived. We
analyze this model by utilizing the mean-field approximation and the
Monte-Carlo simulation in a finite size cluster. A three fold-type charge
ordered structure with charge imbalance between the planes is stabilized in
finite temperatures. This polar charge order is reinforced by spin ordering of
Fe ions. This novel magneto-dielectric phenomenon is caused by spin frustration
and charge-spin coupling in the exchange interaction. We show cross-correlation
effect in magnetic- and electric-field responses. Oxygen deficiency effect as
an impurity effect in a frustrated charge-spin coupled system is also examined.
|
0803.0394v1
|
2008-03-04
|
Light Vector Mesons in the Nuclear Medium
|
The light vector mesons ($\rho$, $\omega$, and $\phi$) were produced in
deuterium, carbon, titanium, and iron targets in a search for possible
in-medium modifications to the properties of the $\rho$ meson at normal nuclear
densities and zero temperature. The vector mesons were detected with the CEBAF
Large Acceptance Spectrometer (CLAS) via their decays to $e^{+}e^{-}$. The rare
leptonic decay was chosen to reduce final-state interactions. A combinatorial
background was subtracted from the invariant mass spectra using a
well-established event-mixing technique. The $\rho$ meson mass spectrum was
extracted after the $\omega$ and $\phi$ signals were removed in a nearly
model-independent way. Comparisons were made between the $\rho$ mass spectra
from the heavy targets ($A > 2$) with the mass spectrum extracted from the
deuterium target. With respect to the $\rho$-meson mass, we obtain a small
shift compatible with zero. Also, we measure widths consistent with standard
nuclear many-body effects such as collisional broadening and Fermi motion.
|
0803.0492v1
|
2008-03-05
|
Hall effect and magnetoresistance in the normal state of the superconductor LaO$_{0.9}$F$_{0.1-x}$FeAs
|
By using a two-step method, we successfully synthesized the iron based new
superconductor LaO$_{0.9}$F$_{0.1-x}$FeAs. The Hall effect and
magnetoresistance were measured in wide temperature region. A negative Hall
coefficient R_H has been found implying a dominant conduction by electron-like
charge carriers in this material. The charge carrier density determined at 100K
is about 9.8E20 cm-3, which is close to the cuprate superconductors. It is
further found that the magnetoresistance does not follow the Kohler's law.
Meanwhile, the Hall coefficient, the resistivity at 0T, and magnetoresistance
all have an anomalous property at about 240K, which may be induced by the
multi-band effect or by some exotic scattering mechanism.
|
0803.0623v3
|
2008-03-06
|
Properties of the ultraviolet flux of type Ia supernovae: an analysis with synthetic spectra of SN 2001ep and SN 2001eh
|
The spectral properties of type Ia supernovae in the ultraviolet (UV) are
investigated using the early-time spectra of SN 2001ep and SN 2001eh obtained
using the Hubble Space Telescope (HST). A series of spectral models is computed
with a Monte Carlo spectral synthesis code, and the dependence of the UV flux
on the elemental abundances and the density gradient in the outer layers of the
ejecta is tested. A large fraction of the UV flux is formed by reverse
fluorescence scattering of photons from red to blue wavelengths. This process,
combined with ionization shifts due to enhanced line blocking, can lead to a
stronger UV flux as the iron-group abundance in the outer layers is increased,
contrary to previous claims.
|
0803.0871v2
|
2008-03-06
|
Tight-Binding Theory of Manganese and Iron Oxides
|
The electronic structure is found to be understandable in terms of free-atom
term values and universal interorbital coupling parameters, since
self-consistent tight-binding calculations indicate that Coulomb shifts of the
d-state energies are small. Special-point averages over the bands are seen to
be equivalent to treatment of local octahedral clusters. The cohesive energy
per manganese for MnO, Mn2O3, and MnO2, in which manganese exists in valence
states Mn2+, Mn3+, and Mn4+, is very nearly the same and dominated by the
transfer of manganese s electrons to oxygen p states. There are small
corrections, one eV per Mn in all cases, from couplings of minority-spin
states. Transferring one majority-spin electron from an upper cluster state to
a nonbonding oxygen state adds 1.67 eV to the cohesion for Mn2O3, and two
transfers adds twice that for MnO2 . The electronic and magnetic properties are
consistent with this description and appear to be understandable in terms of
the same parameters.
|
0803.0994v1
|
2008-03-17
|
Point-Contact Spectroscopy of Iron-Based Layered Superconductor LaO$_{0.9}$F$_{0.1-δ}$FeAs
|
We present point-contact spectroscopy data for junctions between a normal
metal and the newly discovered F-doped superconductor
LaO$_{0.9}$F$_{0.1-\delta}$FeAs (F-LaOFeAs). A zero-bias conductance peak was
observed and its shape and magnitude suggests the presence of Andreev bound
states at the surface of F-LaOFeAs, which provides a possible evidence of an
unconventional pairing symmetry with a nodal gap function. The maximum gap
value $\Delta_0\approx3.9\pm0.7$meV was determined from the measured spectra,
in good agreement with the recent experiments on specific heat and lower
critical field.
|
0803.2405v3
|
2008-03-20
|
Bimodality in low luminosity E and S0 galaxies
|
Stellar population characteristics are presented for a sample of low
luminosity early-type galaxies (LLEs) in order to compare them with their more
luminous counterparts. Long-slit spectra of a sample of 10 LLEs were taken with
the ESO New Technology Telescope, selected for their low luminosities. Line
strengths were measured on the Lick standard system. Lick indices for these
LLEs were correlated with velocity dispersion (sigma), alongside published data
for a variety of Hubble types. The LLEs were found to fall below an
extrapolation of the correlation for luminous ellipticals and were consistent
with the locations of spiral bulges in plots of line strengths versus sigma.
Luminosity weighted average ages, metallicities and abundance ratios were
estimated from chi-squared fitting of 19 Lick indices to predictions from
simple stellar population models. The LLEs appear younger than luminous
ellipticals and of comparable ages to spiral bulges. These LLEs show a bimodal
metallicity distribution, consisting of a low metallicity group (possibly
misclassified dwarf spheroidal galaxies) and a high metallicity group (similar
to spiral bulges). Finally, they have low alpha-element to iron-peak abundance
ratios indicative of slow, extended star formation.
|
0803.3025v1
|
2008-03-22
|
Iron-based layered superconductor LaO$_{1-x}$F$_x$FeAs: an antiferromagnetic semimetal
|
We have studied the newly found superconductor compound LaO$_{1-x}$F$_x$FeAs
through the first-principles density functional theory calculations. We find
that the parent compound LaOFeAs is a quasi-2-dimensional antiferromgnetic
semimetal with most carriers being electrons and with a magnetic moment of
$2.3\mu_B$ located around each Fe atom on the Fe-Fe square lattice. Furthermore
this is a commensurate antiferromagnetic spin density wave due to the Fermi
surface nesting, which is robust against the F-doping. The observed
superconduction happens on the Fe-Fe antiferromagnetic layer, suggesting a new
superconductivity mechanism, mediated by the spin fluctuations. An abrupt
change on the Hall measurement is further predicted for the parent compound
LaOFeAs.
|
0803.3286v1
|
2008-03-23
|
Unconventional pairing originating from disconnected Fermi surfaces in superconducting LaFeAsO$_{1-x}$F$_x$}
|
For a newly discovered iron-based high $T_c$ superconductor
LaFeAsO$_{1-x}$F$_x$, we have constructed a minimal model, where inclusion of
all the five Fe $d$ bands is found to be necessary. Random-phase approximation
is applied to the model to investigate the origin of superconductivity. We
conclude that the multiple spin fluctuation modes arising from the nesting
across the disconnected Fermi surfaces realize an extended s-wave pairing,
while d-wave pairing can also be another candidate.
|
0803.3325v2
|
2008-03-24
|
Fast Variability of Nonthermal X-Ray Emission in Cassiopeia A: Probing Electron Acceleration in Reverse-Shocked Ejecta
|
Recent discovery of the year-scale variability in the synchrotron X-ray
emission of the supernova remnant (SNR) RX J1713.7-3946 has initiated our study
of multi-epoch X-ray images and spectra of the young SNR Cassiopeia A based on
the Chandra archive data taken in 2000, 2002, and 2004. We have found
year-scale time variations in the X-ray intensity for a number of X-ray
filaments or knots associated with the reverse-shocked regions. The X-ray
spectra of the variable filaments are characterized by a featureless continuum,
and described by a power law with a photon index within 1.9-2.3. The upper
limits on the iron K-line equivalent width are 110 eV, which favors a
synchrotron origin of the X-ray emission. The characteristic variability
timescale of 4 yr can be explained by the effects of fast synchrotron cooling
and diffusive shock acceleration with a plausible magnetic field of 1 mG. The
X-ray variability provides a new effective way of studying particle
acceleration at supernova shocks.
|
0803.3410v1
|
2008-03-24
|
Atomic Processes in Planetary Nebulae and H II Regions
|
Spectroscopic studies of Planetary Nebulae (PNe) and H {\sc ii} regions have
driven much development in atomic physics. In the last few years the
combination of a generation of powerful observatories, the development of ever
more sophisticated spectral modeling codes, and large efforts on mass
production of high quality atomic data have led to important progress in our
understanding of the atomic spectra of such astronomical objects. In this paper
I review such progress, including evaluations of atomic data by comparisons
with nebular spectra, detection of spectral lines from most iron-peak elements
and n-capture elements, observations of hyperfine emission lines and analysis
of isotopic abundances, fluorescent processes, and new techniques for
diagnosing physical conditions based on recombination spectra. The review is
directed toward atomic physicists and spectroscopists trying to establish the
current status of the atomic data and models and to know the main standing
issues.
|
0803.3457v1
|
2008-03-25
|
The effect of rotation on the abundances of the chemical elements of the A-type stars in the Praesepe cluster
|
We study how chemical abundances of late B-, A- and early F-type stars evolve
with time, and we search for correlations between the abundance of chemical
elements and other stellar parameters, such as effective temperature and Vsini.
We have observed a large number of B-, A- and F-type stars belonging to open
clusters of different ages. In this paper we concentrate on the Praesepe
cluster (log t = 8.85), for which we have obtained high resolution, high
signal-to-noise ratio spectra of sixteen normal A- and F-type stars and one Am
star, using the SOPHIE spectrograph of the Observatoire de Haute-Provence. For
all the observed stars, we have derived fundamental parameters and chemical
abundances. In addition, we discuss another eight Am stars belonging to the
same cluster, for which the abundance analysis had been presented in a previous
paper. We find a strong correlation between peculiarity of Am stars and Vsini.
The abundance of the elements underabundant in Am stars increases with Vsini,
while it decreases for the overabundant elements. Chemical abundances of
various elements appear correlated with the iron abundance.
|
0803.3540v1
|
2008-03-26
|
Realistic calculations of nuclear disappearance lifetimes induced by neutron-antineutron oscillations
|
Realistic calculations of nuclear disappearance lifetimes induced by
neutron-antineutron oscillations are reported for oxygen and iron, using
antineutron nuclear potentials derived from a recent comprehensive analysis of
antiproton atomic X-ray and radiochemical data. A lower limit of 3.3 x 10E8 s
on the neutron-antineutron oscillation time is derived from the
Super-Kamiokande I new lower limit of 1.77 x 10E32 yr on the neutron lifetime
in oxygen. Antineutron scattering lengths in carbon and nickel, needed in trap
experiments using ultracold neutrons, are calculated from updated antinucleon
optical potentials at threshold, with results shown to be largely model
independent.
|
0803.3696v3
|
2008-03-29
|
Superconductivity at 52 K in iron-based F-doped layered quaternary compound Pr[O1-xFx]FeAs
|
Since the discovery of copper oxide superconductor in 1986 [1], extensive
efforts have been devoted to the search of new high-Tc superconducting
materials, especially high-Tc systems other than cuprates. The recently
discovered quaternary superconductor La[O1-xFx]FeAs with the superconducting
critical transition Tc of 26 K [2], which has a much simple layered structure
compared with cuprates, has attracted quick enthusiasm and is going to become a
new high-Tc system [3-6]. Here we report the discovery of bulk
superconductivity in the praseodymium-arsenide oxides Pr[O1-xFx]FeAs with an
onset drop of resistivity as high as 52 K, and the unambiguous zero-resistivity
and Meissner transition at low temperature, which will place these quaternary
compounds to another high-Tc superconducting system explicitly.
|
0803.4283v1
|
2008-04-01
|
Structural identification of cubic iron-oxide nanocrystal mixtures: X-ray powder diffraction versus quasi-kinematic transmission electron microscopy
|
Two novel (and proprietary) strategies for the structural identification of a
nanocrystal from either a single high-resolution (HR) transmission electron
microscopy (TEM) image or a single precession electron diffraction pattern are
proposed and their advantages discussed in comparison to structural
fingerprinting from powder X-ray diffraction patterns. Simulations for cubic
magnetite and maghemite nanocrystals are used as examples. This is an expanded
and updated version of a conference paper that has been published in Suppl.
Proc. of TMS 2008, 137th Annual Meeting & Exhibition, Volume 1, Materials
Processing and Properties, pp. 25-32.
|
0804.0063v1
|
2008-04-06
|
Mott State and Quantum Critical Points in Rare-Earth Oxypnictides $RO_{1-x}F_xFeAs$ ($R=La, Sm, Nd, Pr, Ce$)
|
We investigate the magnetic phase diagram of the newly discovered iron-based
high temperature oxypnictide superconductors of the type RO$_{1-x}$F$_x $FeAs,
with rare earths R=La, Sm, Nd, Pr and Ce by means of {\it ab initio} SGGA and
SGGA+U density functional computations. We find undoped LaOFeAs to be a Mott
insulator when incorporating electronic correlations via SGGA+U for any
physically relevant value of $U$. The doped compounds are according to SGGA
conductors with a transition from an antiferromagnetic to a non-magnetic state
at a hole doping of concentration $x_c$=0.075 for R=Nd, Pr and at electron
doping $x_c$=0.25 for Ce and 0.6 for Sm. Superconductivity in these rare-earth
oxypnictides thus appears in the vicinity of a magnetic quantum critical point
where electronic correlations are expected to play an important role because of
the vicinity of a Mott insulating state at zero doping.
|
0804.0866v3
|
2008-04-07
|
The Wall and The Ball: A Study of Domain Referent Spreadsheet Errors
|
The Cell Error Rate in simple spreadsheets averages about 2% to 5%. This CER
has been measured in domain free environments. This paper compares the CERs
occurring in domain free and applied domain tasks. The applied domain task
requires the application of simple linear algebra to a costing problem. The
results show that domain referent knowledge influences participants' approaches
to spreadsheet creation and spreadsheet usage. The conclusion is that
spreadsheet error making is influenced by domain knowledge and domain
perception. Qualitative findings also suggest that spreadsheet error making is
a part of overall human behaviour, and ought to be analyzed against this wider
canvas.
|
0804.0943v1
|
2008-04-10
|
Theory of magnetic excitations in iron-based layered superconductors
|
Based on the effective four-band model we analyze the spin response in the
normal and superconducting states of the Fe-pnictide superconductors. While the
normal state spin excitations are dominated by the continuum of the
interorbital antiferromagnetic fluctuations and the intraband spin density wave
fluctuations, the unconventional superconductivity yields different feedback.
The resonance peak in form of the well-defined spin exciton occurs {\it only}
for the interband scattering at the antiferromagnetic momentum ${\bf Q}_{AFM}$
for the $s_\pm$ (extended s-wave) superconducting order parameter and it
disappears rapidly for ${\bf q} < {\bf Q}_{AFM}$. The resonance feature is
extremely weak for the $d_{x^2 -y^2}$-wave order parameter due to specific
Fermi surface topology of these compounds. The essential difference between
$s_\pm$-wave and $d_{x^2 -y^2}$-wave symmetries for the magnetic excitations
can be used for experimental determination of the superconducting wave function
symmetry.
|
0804.1793v2
|
2008-04-16
|
Superconductivity and Phase Diagram in the Iron-based Arsenic-oxides ReFeAsO1-delta (Re = rare earth metal) without F-Doping
|
Here we report a new class of superconductors prepared by high pressure
synthesis in the quaternary family ReFeAsO1-delta (Re = Sm, Nd, Pr, Ce, La)
without fluorine doping. The onset superconducting critical temperature (Tc) in
these compounds increases with the reduction of Re atom size, and the highest
Tc obtained so far is 55 K in SmFeAsO1-delta. For the NdFeAsO1-delta system
with different oxygen concentration a dome-shaped phase diagram was found.
|
0804.2582v3
|
2008-04-21
|
Probing protoneutron star density profile from neutrino signals
|
Supernovae of Type II is a phenomenon that occurs at the end of evolution of
massive stars when the iron core of the star exceeds a mass limit. After
collapse of the core under gravity the shock wave alone does not succeed in
expelling the mass of the star and in this sense the role of neutrinos is the
most important mechanism to do so. During the emission of neutrinos flavor
conversion is possible, related the phenomenon of oscillations, which however
depends directly on the particular density profile of the medium. In this paper
we present results of numerical simulations of neutrino flavor conversion in
protoneutron stars and after collapse. The probabilities of survival for a
given flavor in a complete three-flavors framework is estimated through an
algorithm which conserves unitarity to a high degree of accuracy. The
sensitivity of the results to the different adopted models for the protoneutron
star structure is examined in detail demonstrating how the neutrino signal
could be used to check the validity of models.
|
0804.3247v1
|
2008-04-23
|
Superconductivity at 53.5 K in GdFeAsO1-delta
|
Here we report the fabrication and superconductivity of the iron-based
arsenic-oxide GdFeAsO1-delta compound with oxygen-deficiency, which has an
onset resistivity transition temperature at 53.5 K. This material has a same
crystal structure as the newly discovered high-Tc ReFeAsO1-delta family (Re =
rare earth metal) and a further reduced crystal lattice, while the Tc starts to
decrease compared with the SmFeAsO1-delta system.
|
0804.3727v3
|
2008-04-28
|
Thorium-doping induced superconductivity up to 56 K in Gd1-xThxFeAsO
|
Following the discovery of superconductivity in an iron-based arsenide
LaO1-xFxFeAs with a superconducting transition temperature (Tc) of 26 K[1], Tc
was pushed up surprisingly to above 40 K by either applying pressure[2] or
replacing La with Sm[3], Ce[4], Nd[5] and Pr[6]. The maximum Tc has climbed to
55 K, observed in SmO1-xFxFeAs[7, 8] and SmFeAsO1-x[9]. The value of Tc was
found to increase with decreasing lattice parameters in LnFeAsO1-xFx (Ln stands
for the lanthanide elements) at an apparently optimal doping level. However,
the F- doping in GdFeAsO is particularly difficult[10,11] due to the lattice
mismatch between the Gd2O2 layers and Fe2As2 layers. Here we report observation
of superconductivity with Tc as high as 56 K by the Th4+ substitution for Gd3+
in GdFeAsO. The incorporation of relatively large Th4+ ions relaxes the lattice
mismatch, hence induces the high temperature superconductivity.
|
0804.4290v2
|
2008-05-02
|
Commensurate Spin Density Wave in LaOFeAs: A Local Probe Study
|
We present a detailed study on the magnetic order in the undoped mother
compound LaOFeAs of the recently discovered Fe-based superconductor
LaO$_{1-x}$F$_x$FeAs. In particular, we present local probe measurements of the
magnetic properties of LaOFeAs by means of $^{57}$Fe M\"ossbauer spectroscopy
and muon spin relaxation in zero external field along with magnetization and
resistivity studies. These experiments prove a commensurate static magnetic
order with a strongly reduced ordered moment of 0.25(5) $\mu_B$ at the iron
site below T_N = 138 K, well separated from a structural phase transition at
T_N = 156 K. The temperature dependence of the sublattice magnetization is
determined and compared to theory. Using a four-band spin density wave model
both, the size of the order parameter and the quick saturation below T_N are
reproduced.
|
0805.0264v2
|
2008-05-06
|
Flux-lattice melting in LaO$_{1-x}$F$_{x}$FeAs: first-principles prediction
|
We report the theoretical study of the flux-lattice melting in the novel
iron-based superconductor $LaO_{0.9}F_{0.1}FeAs$ and
$LaO_{0.925}F_{0.075}FeAs$. Using the Hypernetted-Chain closure and an
efficient algorithm, we calculate the two-dimensional one-component plasma pair
distribution functions, static structure factors and direct correlation
functions at various temperatures. The Hansen-Verlet freezing criterion is
shown to be valid for vortex-liquid freezing in type-II superconductors.
Flux-lattice meting lines for $LaO_{0.9}F_{0.1}FeAs$ and
$LaO_{0.925}F_{0.075}FeAs$ are predicted through the combination of the density
functional theory and the mean-field substrate approach.
|
0805.0632v2
|
2008-05-06
|
Pulsation-Initiated Mass Loss in Luminous Blue Variables: A Parameter Study
|
Luminous blue variables (LBVs) are characterized by semi-periodic episodes of
enhanced mass-loss, or outburst. The cause of these outbursts has thus far been
a mystery. One explanation is that they are initiated by kappa-effect
pulsations in the atmosphere caused by an increase in luminosity at
temperatures near the so-called ``iron bump'' (T ~ 200,000 K), where the Fe
opacity suddenly increases. Due to a lag in the onset of convection, this
luminosity can build until it exceeds the Eddington limit locally, seeding
pulsations and possibly driving some mass from the star. We present some
preliminary results from a parameter study focusing on the conditions necessary
to trigger normal S-Dor type (as opposed to extreme eta-Car type) outbursts. We
find that as Y increases or Z decreases, the pulsational amplitude decreases
and outburst-like behavior, indicated by a large, sudden increase in
photospheric velocity, becomes likes likely.
|
0805.0764v1
|
2008-05-07
|
Processing and Characterization of Precision Microparts from Nickel-based Materials
|
The objective of this research was to study the influence of electroplating
parameters on electrodeposit characteristics for the production of nickel (Ni)
and nickel-iron (Ni-Fe) microparts by photoelectroforming. The research focused
on the most relevant parameter for industry, which is the current density,
because it determines the process time and the consumed energy. The results of
the Ni and Ni-Fe characterisations can be divided into two aspects closely
linked with each other ; the morphology and the hardness.
|
0805.0866v1
|
2008-05-09
|
Evidence for electromagnetic granularity in the polycrystalline iron-based superconductor LaO0.89F0.11FeAs
|
The new rare-earth arsenate superconductors are layered, low carrier density
compounds with many similarities to the high-Tc cuprates. An important question
is whether they also exhibit weak-coupling across randomly oriented
grain-boundaries. In this work we show considerable evidence for such
weak-coupling by study of the dependence of magnetization in bulk and powdered
samples. Bulk sample magnetization curves show very little hysteresis while
remanent magnetization shows almost no sample size dependence, even after
powdering. We conclude that these samples exhibit substantial electromagnetic
granularity on a scale approximating the grain size, though we cannot yet
determine whether this is intrinsic or extrinsic.
|
0805.1282v1
|
2008-05-12
|
MOST detects SPBe pulsations in HD 127756 & HD 217543: Asteroseismic rotation rates independent of vsini
|
The MOST (Microvariability and Oscillations of Stars) satellite has
discovered SPBe (Slowly Pulsating Be) oscillations in the stars HD 127756
(B1/B2 Vne) and HD 217543 (B3 Vpe). For HD 127756, 30 significant frequencies
are identified from 31 days of nearly continuous photometry; for HD 217543, up
to 40 significant frequencies from 26 days of data. In both cases, the
oscillations fall into three distinct frequency ranges, consistent with models
of the stars. The variations are caused by nonradial g-modes (and possibly
r-modes) distorted by rapid rotation and excited by the opacity mechanism near
the iron opacity bump. A comparison of pulsation models and observed frequency
groups yields a rotation frequency for each star, independently of vsini. The
rotation rates of these stars, as well as those of the SPBe stars previously
discovered by MOST, HD 163868 and $\beta$ CMi, are all close to their critical
values.
|
0805.1720v1
|
2008-05-14
|
Intrinsic Properties of Stoichiometric LaOFeP
|
DC and ac magnetization, resistivity, specific heat, and neutron diffraction
data reveal that stoichiometric LaOFeP is metallic and non-superconducting
above T = 0.35 K, with gamma = 12.5 mJ/mol*K. Neutron diffraction data at room
temperature and T = 10 K are well described by the stoichiometric, tetragonal
ZrCuSiAs structure and show no signs of structural distortions or long range
magnetic ordering, to an estimated detectability limit of 0.07 uB/Fe. We
propose a model, based on the shape of the iron-pnictide tetrahedron, that
explains the differences between LaOFeP and LaOFeAs, the parent compound of the
recently discovered high-Tc oxyarsenides, which, in contrast, shows both
structural and spin density wave (SDW) transitions.
|
0805.2149v1
|
2008-05-14
|
Theory of the Magnetic Moment in Iron Pnictides
|
We show that the combined effects of spin-orbit, monoclinic distortion, and
p-d hybridization in tetrahedrally coordinated Fe in LaOFeAs invalidates the
naive Hund's rule filling of the Fe d-levels. The two highest occupied levels
have one electron each but as a result of the p-d hybridization have very
different on-site repulsions. As a result, electrons in the upper level are
more itinerant while those in the lower level are more localized. It is the
xy-projection of the spin in the lower level that orders antiferromagnetically
as the z-components of the spins in the two levels is shown to be vanishingly
small in the ground state. The resulting magnetic moment is highly anisotropic
with an in-plane value of $0.25-0.35\mu_B$ per Fe and a z-projection of
$0.06\mu_B$, both of which are in agreement with experiment. As a consequence,
we arrive the minimal model that describes the electronic properties of
LaOFeAs.
|
0805.2167v2
|
2008-05-14
|
MuSR studies of RE(O,F)FeAs (RE = La, Nd, Ce) and LaOFeP systems: possible incommensurate/stripe magnetism and superfluid density
|
Muon spin relaxation (MuSR) measurements in iron oxy-pnictide systems have
revealed: (1) commensurate long-range order in undoped LaOFeAs; (2) Bessel
function line shape in La(O0.97F0.03)FeAs which indicates possible
incommensurate or stripe magnetism; (3) anomalous weak magnetism existing in
superconducting LaOFeP, Ce(O0.84F0.16)FeAs, and Nd(O0.88F0.12)FeAs but absent
in superconducting La(O0.92F0.08)FeAs; and (4) scaling of superfluid density
and Tc in the Ce, La, and Nd-FeAs superconductors following a nearly linear
relationship found in cuprates.
|
0805.2186v1
|
2008-05-19
|
Suzaku Observation of Group of Galaxies NGC 507: Temperature and Metal Distributions in the Intra-cluster Medium
|
Temperature and abundance distributions of the intra-cluster medium (ICM) in
the NGC 507 group of galaxies were studied with Suzaku. Observed concentric
annular spectra were well-represented by a two temperature model for ICM, and
we found steeper abundance gradients for Mg, Si, S, and Fe compared with O in
the central region. Abundance ratios of alpha-elements to iron were found to be
similar to those in other groups and poor clusters. We calculated metal
mass-to-light ratios for Fe, O and Mg (IMLR, OMLR, MMLR) for NGC 507, and
values for different systems were compared. Hotter and richer systems tend to
show higher values of IMLR, OMLR, and MMLR. OMLR and MMLR were measured to an
outer region for the first time with Suzaku, while IMLR was consistent with
that with ASCA. We also looked into 2-dimensional map of the hardness ratio,
but found no significant deviation from the circular symmetry.
|
0805.2771v1
|
2008-05-19
|
Impact of impellers on the axisymmetric magnetic mode in the VKS2 dynamo experiment
|
In the VKS2 (von K\'arm\'an Sodium 2) successful dynamo experiment of
September 2006, the magnetic field that was observed showed a strong
axisymmetric component, implying that non axisymmetric components of the flow
field were acting. By modeling the induction effect of the spiraling flow
between the blades of the impellers in a kinematic dynamo code, we find that
the axisymmetric magnetic mode is excited and becomes dominant in the vicinity
of the dynamo threshold. % The control parameters are the magnetic Reynolds
number of the mean flow, the coefficient measuring the induction effect,
$\alpha$, and the type of boundary conditions (vacuum for steel impellers and
normal field for soft iron impellers). We show that using realistic values of
$\alpha$, the observed critical magnetic Reynolds number, $Rm^c \approx 32$,
can be reached easily with ferromagnetic boundary conditions. We conjecture
that the dynamo action achieved in this experiment may not be related to the
turbulence in the bulk of the flow, but rather to the alpha effect induced by
the impellers.
|
0805.2805v2
|
2008-05-19
|
Electromagnetic properties and electronic structure of iron-based layered superconductor LaOFeP
|
Structural, electronic, and magnetic properties of undoped and aliovalent-ion
(Ca, F)-doped LaOFeP, which undergo superconducting transitions at transition
temperatures (Tc) 4-7 K [Kamihara et al, J. Am. Chem. Soc. 128, 10012, (2006)],
were investigated. Tc of the samples varied from 2.4 to 5.5 K in the undoped
samples, and was increased up to ~7 K by Ca- and F-doping. The Tc increases are
correlated with a decrease in the lattice volume. LaOFeP exhibits paramagnetism
in the normal conducting state. Photoemission spectroscopy combined with
first-principle band calculations clarified that Fe 3d (dz2 + (dxz, dyz))
orbitals hybridized with P 3p to form a Fermi surface. The band calculations
also suggest that the 3d electron of the Fe in LaOFeP is basically in the
low-spin configuration, and that the spin moment of LaOFeP is almost quenched,
leading to the paramagnetism of the itinerant electrons.
|
0805.2983v1
|
2008-05-21
|
A Numerical Renormalization Group Study of the Superconducting and Spin Density Wave Instabilities in MFeAsO$_{1-x}$F$_x$ Compounds
|
We apply the fermion renormalization group method, implemented numerically by
Honerkamp et.al., to a two-band model of FeAs-based materials. At half filling
we find the $(\pi,0)$ or $(0,\pi)$ spin density wave order and a sub-dominant
superconducting pairing tendency. Due to a topological reason, the spin density
wave gap has nodes on the fermi surfaces. Away from half filling we find an
unconventional s-wave and a sub-dominant $d_{x^2-y^2}$ pairing instability. The
former has $s$ symmetry around the hole fermi surface but exhibits
$s+d_{x^2-y^2}$ symmetry around the electron pockets where the 90 degree
rotation is broken. The pairing mechanism is inter-pocket pair hopping.
Interestingly, the same interaction also drives the antiferromagnetism.
|
0805.3343v3
|
2008-05-22
|
Pseudogap Value in the Energy Spectrum of LaOFeAs: Fixed Spin Moment Treatment
|
The experimental data available up to date in literature corresponding to the
paramagnetic - spin density wave transition in nonsuperconducting LaOFeAs are
discussed. In particular, we pay attention that upon spin density wave
transition there is a relative decrease of the density of states on the Fermi
level and a pseudogap formation. The values of these quantities are not
properly described in frames of the density functional theory. The agreement of
them with experimental estimations becomes more accurate with the use of fixed
spin moment procedure when iron spin moment is set to experimental value.
Strong electron correlations which are not included into the present
calculation scheme may lead both to the decrease of spin moment and
renormalization of energy spectrum in the vicinity of the Fermi level for
correct description of discussed characteristics.
|
0805.3453v1
|
2008-05-22
|
The Araucaria Project: the Local Group Galaxy WLM--Distance and metallicity from quantitative spectroscopy of blue Supergiants
|
The quantitative analysis of low resolution spectra of A and B supergiants is
used to determine a distance modulus of 24.99 +/- 0.10 mag (995 +/- 46 Kpc) to
the Local Group galaxy WLM. The analysis yields stellar effective temperatures
and gravities, which provide a distance through the Flux weighted
Gravity--Luminosity Relationship (FGLR). Our distance is 0.07 mag larger than
the most recent results based on Cepheids and the tip of the RGB. This
difference is within the 1-sigma overlap of the typical uncertainties quoted in
these photometric investigations. In addition, non-LTE spectral synthesis of
the rich metal line spectra (mostly iron, chromium and titanium) of the A
supergiants is carried out, which allows the determination of stellar
metallicities. An average metallicity of -0.87 +/- 0.06 dex with respect to
solar metallicity is found.
|
0805.3555v1
|
2008-05-30
|
Chemical Abundances in 35 Metal-Poor Stars. I. Basic Data
|
We carried out a homogeneous abundance study for various elements, including
$\alpha$-elements, iron peak elements and $n$-capture elements for 35
metal-poor stars with a wide metallicity range
($-3.0\lesssim$[Fe/H]$\lesssim-0.5$). High-resolution ($R\simeq30$k), high
signal-to-noise($S/N\geq110$) spectra with a wavelength range of 3800 to 10500
\AA using the Bohyunsan Optical Echelle Spectrograph (BOES). Equivalent widths
were measured by means of the Gaussian-fitting method for numerous isolated
weak lines of elements. Atmospheric parameters were determined by a
self-consistent LTE analysis technique using Fe I and Fe II lines. In this
study, we present the EWs of lines and atmospheric parameters for 35 metal-poor
stars.
|
0805.4663v1
|
2008-06-03
|
Critical Fields and Anisotropy of NdO0.82F0.18FeAs Single Crystals
|
The newly discovered iron-based superconductors have stimulated enormous
interests in the field of superconductivity. Since the new superconductor is a
layered system, the anisotropy is a parameter with the first priority to know.
Meanwhile any relevant message about the critical fields (upper critical field
and irreversibility line) are essentially important. By using flux method, we
have successfully grown the single crystals NdO0.82F0.18FeAs at ambient
pressure. Resistive measurements reveal a surprising discovery that the
anisotropy \Gamma = (mc/mab)^{1/2} is below 5, which is much smaller than the
theoretically calculated results. The data measured up to 400 K show a
continuing curved feature which prevents a conjectured linear behavior for an
unconventional metal. The upper critical fields determined based on the
Werthamer-Helfand-Hohenberg formula are H_{c2}^{H||ab}(T=0 K) = 304 T and
H_{c2}^{H||c}(T=0 K)=62-70 T, indicating a very encouraging application of the
new superconductors.
|
0806.0532v1
|
2008-06-04
|
Symmetry of superconducting states with two orbitals on a tetragonal lattice: application to $LaO_{1-x}F_{x}FeAs$
|
We use group theory to classify the superconducting states of systems with
two orbitals on a tetragonal lattice. The orbital part of the superconducting
gap function can be either symmetric or anti-symmetric. For the orbital
symmetric state, the parity is even for spin singlet and odd for spin triplet;
for the orbital anti-symmetric state, the parity is odd for spin singlet and
even for spin triplet. The gap basis functions are obtained with the use of the
group chain scheme by taking into account the spin-orbit coupling. In the weak
pairing limit, the orbital anti-symmetric state is only stable for the
degenerate orbitals. Possible application to iron-based superconductivity is
discussed.
|
0806.0712v3
|
2008-06-04
|
Electronic structure of oxygen-free 38K superconductor Ba1-xKxFe2As2 in comparison with BaFe2As2 from first principles
|
Based on first-principles FLAPW-GGA calculation, we have investigated
electronic structure of newly discovered oxygen-free 38K superconductor
Ba1-xKxFe2As2 in comparison with parent phase - tetragonal ternary iron
arsenide BaFe2As2. The density of states, magnetic properties, near-Fermi bands
compositions, together with Sommerfeld coefficients and molar Pauli
paramagnetic susceptibility are evaluated. The results allow us to classify
these systems as quasi-two-dimensional ionic metals, where the conduction is
strongly anisotropic, only happening on the (Fe-As) layers. According to our
calculations, at the hole doping of BaFe2As2 the density of states at the Fermi
level grows, and this can be a possible factor of occurrence of
superconductivity for Ba1-xKxFe2As2. On the other hand, Ba1-xKxFe2As2 lays at
the border of magnetic instability and the pairing interactions might involve
magnetic or orbital fluctuations.
|
0806.0750v1
|
2008-06-04
|
Valence band and core level photoemission spectros-copy of LaFeAsO$_{1-x}$F$_{x}$
|
We have investigated the electronic structure of LaFeAsO$_{1-x}$F$_{x}$ (x =
0; 0.1; 0.2) by angle-integrated photoemission spectroscopy and local density
approximation (LDA) based band structure calculations. The valence band
consists of a low energy peak at E = -0.25 eV and a broad structure around E =
-5 eV in qualitative agreement with LDA. From the photon energy dependence of
these peaks we conclude that the former derives almost exclusively from Fe 3d
states. This constitutes experimental evidence for the strong iron character of
the relevant states in a broad window around EF and confirms theoretical
predictions.
|
0806.0833v2
|
2008-06-06
|
Superconductivity in hole-doped (Sr$_{1-x}$K$_x$)Fe$_2$As$_2$
|
A series of layered (Sr$_{1-x}$K$_x$)Fe$_2$As$_2$ compounds with nominal x=0
to 0.40 are synthesized by solid state reaction method. Similar to other parent
compounds of iron-based pnictide superconductors, the pure SrFe$_2$As$_2$ shows
a strong resistivity anomaly near 210 K, which was ascribed to the
spin-density-wave instability. The anomaly temperature is much higher than
those observed in LaOFeAs and BaFe$_2$As$_2$, the two prototype parent
compounds with ZrCuSiAs- and ThCr$_2$Si$_2$-type structures. K-doping strongly
suppresses this anomaly and induces superconductivity. Like in the case of
K-doped BaFe$_2$As$_2$, sharp superconducting transitions at T$_c\sim$38 K was
observed. We performed the Hall coefficient measurement, and confirmed that the
dominant carriers are hole-type. The carrier density is enhanced by a factor of
3 in comparison to F-doped LaOFeAs superconductor.
|
0806.1209v1
|
2008-06-09
|
Non-LTE Spectral Analysis of Extremely Hot Post-AGB Stars: Constraints for Evolutionary Theory
|
Spectral analysis by means of Non-LTE model-atmosphere techniques has arrived
at a high level of sophistication: fully line-blanketed model atmospheres which
consider opacities of all elements from H to Ni allow the reliable
determination of photospheric parameters of hot, compact stars. Such models
provide a crucial test of stellar evolutionary theory: recent abundance
determinations of trace elements like, e.g., F, Ne, Mg, P, S, Ar, Fe, and Ni
are suited to investigate on AGB nucleosynthesis. E.g., the strong Fe depletion
found in hydrogen-deficient post-AGB stars is a clear indication of an
efficient s-process on the AGB where Fe is transformed into Ni or even heavier
trans iron-group elements. We present results of recent spectral analyses based
on high-resolution UV observations of hot stars.
|
0806.1432v1
|
2008-06-16
|
Structural and magnetic phase diagram of CeFeAsO1-xFx and its relationship to high-temperature superconductivity
|
We use neutron scattering to study the structural and magnetic phase
transitions in the iron pnictides CeFeAsO1-xFx as the system is tuned from a
semimetal to a high-transition-temperature (high-Tc) superconductor through
Fluorine (F) doping x. In the undoped state, CeFeAsO develops a structural
lattice distortion followed by a stripe like commensurate antiferromagnetic
order with decreasing temperature. With increasing Fluorine doping, the
structural phase transition decreases gradually while the antiferromagnetic
order is suppressed before the appearance of superconductivity, resulting an
electronic phase diagram remarkably similar to that of the high-Tc copper
oxides. Comparison of the structural evolution of CeFeAsO1-xFx with other
Fe-based superconductors reveals that the effective electronic band width
decreases systematically for materials with higher Tc. The results suggest that
electron correlation effects are important for the mechanism of high-Tc
superconductivity in these Fe pnictides.
|
0806.2528v1
|
2008-06-16
|
Antiferromagnetic transition in EuFe$_2$As$_2$: A possible parent compound for superconductors
|
Ternary iron arsenide EuFe$_2$As$_2$ with ThCr$_2$Si$_2$-type structure has
been studied by magnetic susceptibility, resistivity, thermopower, Hall and
specific heat measurements. The compound undergoes two magnetic phase
transitions at about 200 K and 20 K, respectively. The former was found to be
accompanied with a slight drop in magnetic susceptibility (after subtracting
the Curie-Weiss paramagnetic contribution), a rapid decrease in resistivity, a
large jump in thermopower and a sharp peak in specific heat with decreasing
temperature, all of which point to a spin-density-wave-like antiferromagnetic
transition. The latter was proposed to be associated with an A-type
antiferromagnetic ordering of Eu$^{2+}$ moments. Comparing with the physical
properties of the iso-structural compounds BaFe$_2$As$_2$ and SrFe$_2$As$_2$,
we expect that superconductivity could be induced in EuFe$_2$As$_2$ through
appropriate doping.
|
0806.2591v2
|
2008-06-16
|
Electronic structure and exotic exchange splitting in spin-density-wave states of BaFe$_2$As$_2$
|
The magnetic properties in the parent compounds are often intimately related
to the microscopic mechanism of superconductivity. Here we report the first
direct measurements on the electronic structure of a parent compound of the
newly discovered iron-based superconductor, BaFe$_2$As$_2$, which provides a
foundation for further studies. We show that the energy of the spin density
wave (SDW) in BaFe$_2$As$_2$ is lowered through exotic exchange splitting of
the band structure, rather than Fermi surface nesting of itinerant electrons.
This clearly demonstrates that a metallic SDW state could be solely induced by
interactions of local magnetic moments, resembling the nature of
antiferromagnetic order in cuprate parent compounds.
|
0806.2627v2
|
2008-06-22
|
Thermodynamic properties of Ba1-xMxFe2As2 (M = La and K)
|
The specific heat $C(T)$ of BaFe$_2$As$_2$ single crystal, electron-doped
Ba$_{0.7}$La$_{0.3}$Fe$_2$As$_2$ and hole-doped Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$
polycrystals were measured. For undoped BaFe$_2$As$_2$ single crystal, a very
sharp specific heat peak was observed at 136 K. This is attributed to the
structural and antiferromagnetic transitions occurring at the same temperature.
$C(T)$ of the electron-doped non-superconducting
Ba$_{0.7}$La$_{0.3}$Fe$_2$As$_2$ also shows a small peak at 120 K, indicating a
similar but weaker structural/antiferromagnetic transition. For the hole-doped
superconducting Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$, a clear peak of $C/T$ was
observed at $T_c$ = 36 K, which is the highest peak seen at superconducting
transition for iron-based high-$T_c$ superconductors so far. The electronic
specific heat coefficient $\gamma$ and Debye temperature $\Theta_D$ of these
compounds were obtained from the low temperature data.
|
0806.3573v2
|
2008-06-24
|
Electronic Structure and Electron Correlation in LaFeAsO_{1-x}F_x and LaFePO_{1-x}F_x
|
Photoemission spectroscopy is used to investigate the electronic structure of
the newly discovered iron-based superconductors LaFeAsO_{1-x}F_x and
LaFePO_{1-x}F_x. Line shapes of the Fe 2p core-level spectra suggest an
itinerant character of Fe 3d electrons. The valence-band spectra are generally
consistent with band-structure calculations except for the shifts of Fe
3d-derived peaks toward the Fermi level. From spectra taken in the Fe 3p -> 3d
core-absorption region, we have obtained the experimental Fe 3d partial density
of states, and explained it in terms of a band-structure calculation with a
phenomenological self-energy correction, yielding a mass renormalization factor
of ~< 2.
|
0806.3860v1
|
2008-06-26
|
High Energy Observations of AGN Jets and their Future Prospects
|
In next five years, dramatic progress is anticipated for the AGN studies, as
we have two important missions to observe celestial sources in the high energy
regime: GLAST and Suzaku. In this talk, I will summarize recent highlights in
studies of AGN jets, focusing on the high-sensitivity X-ray observations that
may shed new light on the forthcoming GLAST era. I will especially present some
examples from most recent Suzaku observations of blazars, which provides
important hints for the shock acceleration in sub-pc scale jets, as well as
particle content in jets. Then I will focus on the neutral iron-line feature
observed in some broad line radio galaxies, as a probe of jet launching and/or
the disk-jet connection. Finally, I will discuss new results of large scale
(kpc to Mpc) jets recently resolved with Chandra X-ray observatory.
Simultaneous monitoring observations in various wavelengths will be
particularly valuable for variable blazar sources, allowing the cross
correlations of time series as well as detailed modeling of the spectral
evolution between the X-ray and gamma-ray energy bands. Possible impacts of
these new observations across the electromagnetic spectrum on various spatial
scales are discussed to challenge the long-standing mystery of AGN jet sources.
|
0806.4240v1
|
2008-07-03
|
Coulomb Parameter U and Correlation Strength in LaFeAsO
|
First principles constrained density functional theory scheme in Wannier
functions formalism has been used to calculate Coulomb repulsion U and Hund's
exchange J parameters for iron 3d electrons in LaFeAsO. Results strongly depend
on the basis set used in calculations: when O-2p, As-4p, and Fe-3d orbitals and
corresponding bands are included, computation results in U=3-4 eV, however,
with the basis set restricted to Fe-3d orbitals and bands only, computation
gives parameters corresponding to F^0=0.8 eV, J=0.5 eV. LDA+DMFT (the Local
Density Approximation combined with the Dynamical Mean-Field Theory)
calculation with this parameters results in weakly correlated electronic
structure that is in agreement with X-ray experimental spectra.
|
0807.0547v1
|
2008-07-04
|
Pressure Effect on Superconductivity of Iron-based Arsenic-oxide ReFeAsO0.85 (Re=Sm and Nd)
|
Here we report pressure effect on superconducting transition temperature (Tc)
of ReFeAsO0.85 (Re= Sm and Nd) system without fluorine doping. In-situ
measurements under high pressure showed that Tc of the two compounds decrease
monotonously over the pressure range investigated. The pressure coefficients
dTc/dP in SmFeAsO0.85 and Nd FeAsO0.85 were different, revealing the important
influence of the deformation in layers on Tc. Theoretical calculations
suggested that the electron density of states decrease with increasing
pressure, following the same trend of experimental data.
|
0807.0675v2
|
2008-07-06
|
Momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals measured by angle resolved photoemission spectroscopy
|
We use angle resolved photoemission spectroscopy (ARPES) to study the
momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals.
We find that the Gamma hole pocket is fully gapped below the superconducting
transition temperature. The value of the superconducting gap is 15 +- 1.5 meV
and its anisotropy around the hole pocket is smaller than 20% of this value.
This is consistent with an isotropic or anisotropic s-wave symmetry of the
order parameter or exotic d-wave symmetry with nodes located off the Fermi
surface sheets. This is a significant departure from the situation in the
cuprates, pointing to possibility that the superconductivity in the iron
arsenic based system arises from a different mechanism.
|
0807.0815v1
|
2008-07-07
|
Optical spectroscopy of superconducting Ba{0.55}K{0.45}Fe2As2: evidence for strong coupling to low energy bosons
|
Optical spectroscopy on single crystals of the new iron arsenide
superconductor Ba{0.55}K{0.45}Fe2As2 shows that the infrared spectrum consists
of two major components: a strong metallic Drude band and a well separated mid
infrared absorption centered at 0.7 eV. It is difficult to separate the two
components unambiguously but several fits of Lorentzian peaks suggest a model
with a Drude peak having a plasma frequency of 1.8 to 2.1 eV and a midinfrared
peak with a plasma frequency of 2.5 eV. In contrast to the cuprate
superconductors the scattering rate obtained from the extended Drude model
saturates at 150 meV as compared to 500 meV for a typical cuprate. Detailed
analysis of the frequency dependent scattering rate shows that the charge
carriers interact with broad bosonic spectrum with a peak at 25 meV and a
coupling constant lambda =approx 2 at low temperature. As the temperature
increases this coupling weakens to lambda=0.6 at ambient temperature. This
suggests a bosonic spectrum that is similar to what is seen in the lower Tc
cuprates.
|
0807.1040v1
|
2008-07-08
|
Possibility of Unconventional Pairing Due to Coulomb Interaction in Fe-Based Pnictide Superconductors: Perturbative Analysis of Multi-Band Hubbard Models
|
Possibility of unconventional pairing due to Coulomb interaction in
iron-pnictide superconductors is studied by applying a perturbative approach to
realistic 2- and 5-band Hubbard models. The linearized Eliashberg equation is
solved by expanding the effective pairing interaction perturbatively up to
third order in the on-site Coulomb integrals. The numerical results for the
5-band model suggest that the eigenvalues of the Eliashberg equation are
sufficiently large to explain the actual high Tc for realistic values of
Coulomb interaction and the most probable pairing state is spin-singlet s-wave
without any nodes just on the Fermi surfaces, although the superconducting
order parameter changes its sign between the small Fermi pockets. On the other
hand the 2-band model is quite insufficient to explain the actual high Tc.
|
0807.1168v2
|
2008-07-09
|
The chemical evolution of Manganese in different stellar systems
|
Aims. To model the chemical evolution of manganese relative to iron in three
different stellar systems: the solar neighbourhood, the Galactic bulge and the
Sagittarius dwarf spheroidal galaxy, and compare our results with the recent
and homogeneous observational data. Methods. We adopt three chemical evolution
models well able to reproduce the main properties of the solar vicinity, the
galactic Bulge and the Sagittarius dwarf spheroidal. Then, we compare different
stellar yields in order to identify the best set to match the observational
data in these systems. Results. We compute the evolution of manganese in the
three systems and we find that in order to reproduce simultaneously the [Mn/Fe]
versus [Fe/H] in the Galactic bulge, the solar neighbourhood and Sagittarius,
the type Ia SN Mn yield must be metallicity-dependent. Conclusions. We conclude
that the different histories of star formation in the three systems are not
enough to reproduce the different behaviour of the [Mn/Fe] ratio, unlike the
situation for [alpha/Fe]; rather, it is necessary to invoke
metallicity-dependent type Ia SN Mn yields, as originally suggested by
McWilliam, Rich & Smecker-Hane in 2003.
|
0807.1463v1
|
2008-07-09
|
Lattice and magnetic instabilities in CaFe2As2: A single crystal neutron diffraction study
|
Neutron diffraction measurements of a high quality single crystal of CaFe2As2
are reported. A sharp transition was observed between the high temperature
tetragonal and low temperature orthorhombic structures at TS = 172.5K (on
cooling) and 173.5K (on warming). Coincident with the structural transition we
observe a rapid, but continuous, ordering of the Fe moments, in a commensurate
antiferromagnetic structure is observed, with a saturated moment of
0.80(5)muB/Fe directed along the orthorhombic a-axis. The hysteresis of the
structural transition is 1K between cooling and warming and is consistent with
previous thermodynamic, transport and single crystal x-ray studies. The
temperature onset of magnetic ordering shifts rigidly with the structural
transition providing the clearest evidence to date of the coupling between the
structural and magnetic transitions in this material and the broader class of
iron arsenides.
|
0807.1525v3
|
2008-07-10
|
Structural Chemistry, Spin Order, and the Distinction Between the Cuprate and Pnictide High-Temperature Superconductors
|
In the cuprate and iron-pnictide systems, valence changes induce
high-temperature superconductivity while the local structural chemistry and
local spin order both independently generate the attractive interactions
responsible for the high transition temperature. We argue that together they
favor d-wave singlet superconductivity in the cuprates but s-wave singlet in
the pnictides. This difference arises from the existence of a large on-site
repulsion between carriers in the cuprates largely absent in the pnictides.
Fluorine is responsible for raising Tc significantly in some pnictides and in
the cuprates to 155K-168K, the highest achieved at ambient pressure. We propose
an experimental procedure for finding and fabricating the fluorinated cuprate
phase having that exceptional property.
|
0807.1673v5
|
2008-07-15
|
Physical and kinematical properties of the X-ray absorber in the broad absorption line quasar APM 08279+5255
|
We have re-analyzed the X-ray spectra of the gravitational lensed
high-redshift BAL QSO APM 08279+5255, observed with the XMM-Newton and Chandra
observatories. Previous studies (Hasinger et al. 2002; Chartas et al. 2002)
detected unusual, highly-ionized iron absorption features, but differed in
their interpretation of these features, regarding the kinematical and
ionization structure. We seek one physical model that can be successfully
applied to both observations. For the first time we have performed detailed
photoionization modeling on the X-ray spectrum of APM 08279+5255. The absorbing
gas in APM 08279+5255 can be represented by a two-absorbers model with column
densities N_H(1)~7x10^{22} cm^-2, N_H(2)~6x10^{22} cm^-2, and ionization
parameters logxi(1)~1.5 and logxi(2)~3, with one of them (the high-ionization
component) outflowing at v~0.18(\pm 0.01)c, carrying large amount of gas out of
the system. We find that the Chandra spectrum of APM 08279+5255 requires the
same Fe/O ratio overabundance (previously) indicated by the XMM-Newton
observation, showing that both absorber components underwent similar chemical
evolution and/or have similar origin.
|
0807.2372v1
|
2008-07-16
|
Columnar magnetic structure coupled with orthorhombic distortion in the antiferromagnetic iron arsenide SrFe$_2$As$_2$
|
Neutron powder diffraction experiments were carried out on polycrystalline
SrFe$_2$As$_2$ in order to determine the magnetic structure and its
relationship with the crystallographic one. Below $T_0$=205 K, magnetic
reflections appear simultaneously with the onset of the orthorhombic
distortion. From a detailed Rietveld analysis, the magnetic propagation vector
of SrFe$_2$As$_2$ is determined to be {\textit{\textbf{q}}}=(1 0 1); the
coupling of Fe moments is antiferromagnetic along the longer $a$ direction
within the Fe-As layer, and the interlayer coupling is antiferromagnetic as
well. The size of the Fe magnetic moment is deduced to be 1.01(3) ${\mu}_{\rm
B}$ with an orientation parallel to the a-axis. The temperature dependence of
the magnetic moment shows excellent agreement with not only that of the muon
precession frequency but also with that of the structural distortion, revealing
the strong coupling of the columnar magnetic order and the structural
distortion in SrFe$_2$As$_2$.
|
0807.2608v1
|
2008-07-18
|
Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering
|
Recent investigations of the superconducting iron-arsenide families have
highlighted the role of pressure, be it chemical or mechanical, in fostering
superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced
transition to a non-magnetic, volume "collapsed" tetragonal phase, which
becomes superconducting at lower temperature. Spin-polarized total-energy
calculations on the collapsed structure reveal that the magnetic Fe moment
itself collapses, consistent with the absence of magnetic order in neutron
diffraction.
|
0807.3032v3
|
2008-07-20
|
Electronic structure of LaFe1-xCoxAsO from first principle calculations
|
Based on the first-principles calculations, we have investigated the
geometry, binding properties, density of states and band structures of the
novel superconductor LaFe1-xCoxAsO and its parent compounds with the ZrCuSiAs
structure. We demonstrate that La-O bond and TM-As (TM=Fe or Co) bond are both
strongly covalent, while the LaO and TMAs layers have an almost ionic
interaction through the Bader charge analysis. Partial substitution of iron
with cobalt modify the Fermi level from a steep edge to a flat slope, which
explains why in this system Co doping suppresses the spin density wave (SDW)
transition.
|
0807.3153v2
|
2008-07-24
|
Evidence for Strong Itinerant Spin Fluctuations in the Normal State of CeFeAsO(0.89)F(0.11) Iron-Oxypnictides
|
The electronic structure in the normal state of CeFeAsO0.89F0.11 oxypnictide
superconductors has been investigated with x-ray absorption and photoemission
spectroscopy. All the data exhibit signatures of Fe d-electron itinerancy.
Exchange multiplets appearing in the Fe 3s core level indicate the presence of
itinerant spin fluctuations. These findings suggest that the underlying physics
and the origin of superconductivity in these materials are likely to be quite
different from those of the cuprate high-temperature superconductors. These
materials provide opportunities for elucidating the role of magnetic
fluctuations in high-temperature superconductivity.
|
0807.3781v2
|
2008-07-24
|
Modeling UX Ursae Majoris: An abundance of challenges
|
We present a system model for optical and far UV spectra of the nova-like
variable UX UMa involving a white dwarf, secondary star, gas stream, hot spot
and accretion disk using our code BINSYN and based on an initially adopted
system distance. Calculated SED intensity data successfully fit successive
tomographically-extracted annuli longward of the Balmer limit but require a
postulated `iron curtain' shortward of the Balmer limit that is applied to the
annulus section closest to the secondary star, while postulated recombination
emission fills in the model SED shortward of the Balmer limit and is applied to
the annulus section more remote from the secondary star. The same model fits
$UBV$ 1954 light curves by Walker and Herbig. Fits to $HST$ $FOS$ spectra are
approximate but require assumed time-variable changes in the SED. Comparable
effects, possibly involving variable absorption, afflict $FUSE$ spectra.
|
0807.3920v1
|
2008-07-25
|
Superconductivity and Crystal Structures of (Ba1-xKx)Fe2As2 (x = 0 - 1)
|
We report on doping dependencies of structural parameters and superconducting
transition temperatures in the solid solution (Ba1-xKx)Fe2As2. As the main
effect of doping on the crystal structure, we find linear decreasing As-Fe-As
bond angles and Fe-Fe distances, equivalent to an elongation of the FeAs4
tetrahedra along [001]. The structural changes are intimately coupled to the
electronic states at the Fermi level, because the most relevant Fe-3dx2-y2
orbitals are strongly affected by the As-Fe-As bond angle. Superconductivity is
present over the whole doping range in (Ba1-xKx)Fe2As2 with a maximum Tc of 38
K at x ~ 0.4. The superconducting transitions in the orthorhombic compounds
(Ba0.9K0.1)Fe2As2 (Tc ~ 3 K) and (Ba0.8K0.2)Fe2As2 (Tc ~ 25 K) is strong
evidence for the coexistence of superconductivity with the structurally
distorted and potentially magnetically ordered state in the BaFe2As2 family of
iron arsenide superconductors.
|
0807.4096v2
|
2008-07-28
|
Superconductivity at 27 K in tetragonal FeSe under high pressure
|
A huge enhancement of the superconducting transition temperature Tc was
observed in tetragonal FeSe superconductor under high pressure. The onset
temperature became as high as 27 K at 1.48 GPa and the pressure coefficient
showed a huge value of 9.1 K/GPa. The upper critical field Hc2 was estimated to
be ~ 72 T at 1.48 GPa. Because of the high Hc2, FeSe system may be a candidate
for application as superconducting wire rods. Moreover, the investigation of
superconductivity on simple structured FeSe may provide important clues to the
mechanism of superconductivity in iron-based superconductors.
|
0807.4315v2
|
2008-07-29
|
Anisotropic pairing in the iron pnictides
|
We determine the anisotropy of the spin fluctuation induced pairing gap on
the Fermi surface of the FeAs based superconductors as function of the exchange
and Hund's coupling $J_{H}$. We find that for sufficiently large $J_{H}$,
nearly commensurate magnetic fluctuations yield a fully gapped
$s^{\pm}$-pairing state with small anisotropy of the gap amplitude on each
Fermi surface sheet, but significant variations of the gap amplitude for
different sheets of the Fermi surface. In particular, we obtain the large
variation of the gap amplitude on different Fermi surface sheets, as seen in
ARPES experiments. For smaller values of Hund's coupling incommensurate
magnetic fluctuations yield an $s^{\pm}$-pairing state with line nodes. Such a
state is also possible once the anisotropy of the material is reduced and three
dimensional effects come into play.
|
0807.4566v2
|
2008-07-31
|
Evidence for Unconventional Superconductivity in Arsenic-Free Iron-Based Superconductor FeSe : A ^77Se-NMR Study
|
We report the results of $^{77}$Se--nuclear magnetic resonance (NMR) in
$\alpha$-FeSe, which exhibits a similar crystal structure to the
LaFeAsO$_{1-x}$F$_x$ superconductor and shows superconductivity at 8 K. The
nuclear-spin lattice relaxation rate $1/T_1$ shows $T^3$ behavior below the
superconducting transition temperature $T_c$ without a coherence peak. The
$T_1T=$ const. behavior, indicative of the Fermi liquid state, can be seen in a
wide temperature range above $T_c$. The superconductivity in $\alpha$-FeSe is
also an unconventional one as well as LaFeAsO$_{1-x}$F$_x$ and related
materials. The FeAs layer is not essential for the occurrence of the
unconventional superconductivity.
|
0808.0040v3
|
2008-08-03
|
Magnetic and Transport Studies on Electron-doped CeFeAsO1-xFx Superconductor
|
The magnetic and transport behaviors of cerium substituted iron oxy-arsenide
superconductor with x = 0.1 to 0.4 fluoride (F) doping have been investigated
in this report. Temperature dependent susceptibility and resistivity
measurements showed the 0.1 F-doped sample (CeFeAsO0.9F0.1) has a
superconducting transition temperature (Tc) of around 30 K. With increasing
doping beyond x = 0.2 Tc saturates to around 40 K. Temperature dependent
susceptibility measured in different magnetic fields for the under-doped sample
showed Meissner effect in low field and the diamagnetism is still visible up to
1 Tesla, with an obvious magnetic transition below 5 K, perhaps originating
from magnetic ordering of the rare earth cerium. The corresponding field
dependent resistance versus temperature measurements indicated a broadening of
less than 3 K for Tc at mid-point by increasing the field to 5 Tesla indicating
rather low anisotropy. An estimated upper critical field of more than 48 Tesla
and accordingly an estimated maximum coherence length of 2.6 nm were obtained
confirming the high upper critical field with a short coherence length for this
superconductor.
|
0808.0296v1
|
2008-08-04
|
Tellurium substitution effect on superconductivity of the alpha-phase Iron Selenide
|
We have carried out a systematic study of the PbO-type compound
FeSe_{1-x}Te_x (x = 0~1), where Te substitution effect on superconductivity is
investigated. It is found that superconducting transition temperature reaches a
maximum of Tc=15.2K at about 50% Te substitution. The pressure-enhanced Tc of
FeSe0.5Te0.5 is more than 10 times larger than that of FeSe. Interestingly,
FeTe is no longer superconducting. A low temperature structural distortion
changes FeTe from triclinic symmetry to orthorhombic symmetry. We believe that
this structural change breaks the magnetic symmetry and suppresses
superconductivity in FeTe.
|
0808.0474v2
|
2008-08-06
|
The correlation of black hole mass with metallicity index of host spheroid
|
We investigate the correlation between the mass of the supermassive black
holes (SMBHs) and metal abundance, using existing data sets. The SMBH mass
$M_{bh}$ is well correlated with integrated stellar feature of Mgb. For 28
galaxies, the best-fit $M_{bh}$-Mgb relation has a small scatter, which is an
equivalent level with other well-known relation, such as a correlation between
the stellar velocity dispersion and the mass. An averaged iron index $<$Fe$>$
also positively correlates with $M_{bh}$, but the best-fit $M_{bh}$-$<$Fe$>$
relation has a larger scatter. The difference comes from the synthesis and
evolution mechanisms, and may be important for the SMBH and star formation
history in the host spheroid.
|
0808.0769v1
|
2008-08-06
|
Granularity and vortex dynamics in LaO_0.92F_0.08FeAs as probed by harmonics of the AC magnetic susceptibility
|
Fundamental and higher harmonics of the AC magnetic susceptibility have been
measured on a LaO_0.92F_0.08FeAs sample as a function of the temperature, at
various amplitudes and frequencies of the AC magnetic field, with a small
superimposed DC field parallel to the AC field. The granularity of the sample
has been investigated and the inter-grain and intra-grain contributions have
been clearly individuated looking at both the first and third harmonics. The
vortex dynamics has been also analyzed, and a comparison with the magnetic
behavior of both the MgB_2 and the cuprate superconductors has been performed.
Some vortex dissipative phenomena, i.e. the thermally activated flux flow and
the flux creep, have been detected in the presented measurements, similar to
what obtained on YBCO. Nevertheless, although the general behavior is similar,
several differences have been also evidenced between these different classes of
superconductors, mainly in the third harmonics. We infer that different vortex
dynamics have to be included into the analysis of the magnetic response in this
iron-based new material.
|
0808.0804v1
|
2008-08-10
|
59-Co and 75-As NMR Investigation of Electron-Doped High Tc Superconductor BaFe(1.8)Co(0.2)As(2) (Tc = 22K)
|
We report an NMR investigation of the superconductivity in BaFe(2)As(2)
induced by Co doping (Tc=22K). We demonstrate that Co atoms form an alloy with
Fe atoms and donate carriers without creating localized moments. Our finding
strongly suggests that the underlying physics of iron-pnictide superconductors
is quite different from the widely accepted physical picture of high Tc
cuprates as doped Mott insulators. We also show a crossover of electronic
properties into a low temperature pseudo-gap phase with a pseudo-gap Delta
560K, where chi(spin) constant and resisitivty T. The NMR Knight shift below Tc
decreases for both along the c-axis and ab-plane, and is consistent with the
singlet pairing scenario.
|
0808.1420v2
|
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