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2023-05-12
|
Impact of errors in the magnetic field measurement on the precision determination of neutrino oscillation parameters at the proposed ICAL detector at INO
|
The magnetised iron calorimeter (ICAL) detector proposed at the India-based
Neutrino Observatory will be a 51 kton detector made up of 151 layers of 56 mm
thick soft iron with 40 mm air gap in between where the RPCs, the active
detectors, will be placed. The main goal of ICAL is to make precision
measurements of the neutrino oscillation parameters using the atmospheric
neutrinos as source. The charged current interactions of the atmospheric muon
neutrinos and anti-neutrinos in the detector produce charged muons. The
magnetic field, with a maximum value of $\sim$ 1.5 T in the central region of
ICAL, is a critical component since it will be used to distinguish the charges
and determine the momentum and direction of these muons. It is difficult to
measure the magnetic field inside the iron. The existing methods can only
estimate the internal field and hence will be prone to error. This paper
presents the first simulations study of the effect of errors in the measurement
of the magnetic field in ICAL on its physics potential, especially the neutrino
mass ordering and precision measurement of oscillation parameters in the 2--3
sector. The study is a GEANT4-based analysis, using measurements of the
magnetic field at the prototype ICAL detector. We find that there is only a
small effect on the determination of the mass ordering. While local
fluctuations in the magnetic field measurement are well-tolerated, calibration
errors must remain well within 5\% to retain good precision determination of
the parameters $\sin^2\theta_{23}$ and $\Delta m^2_{32}$.
|
2305.07291v2
|
2023-06-16
|
Tailoring defects and nanocrystal transformation for optimal heating power in bimagnetic $Co_yFe_{1-y}O@Co_xFe_{3-x}O_4$ particles
|
The effects of cobalt incorporation in spherical heterostructured iron oxide
nanocrystals (NCs) of sub-critical size have been explored by colloidal
chemistry methods. Synchrotron X-ray total scattering methods suggest that
cobalt (Co) substitution in rock salt iron oxide NCs tends to remedy its vacant
iron sites, offering a higher degree of resistance to oxidative conversion.
Self-passivation still creates a spinel-like shell, but with higher volume
fraction of the rock salt Co-containing phase in the core. The higher divalent
metal stoichiometry in the rock salt phase, with increasing Co content, results
in a population of unoccupied tetrahedral metal sites in the spinel part,
likely through oxidative shell creation, involving an ordered defect-clustering
mechanism, directly correlated to the core stabilization. To shed light on the
effects of Co-substitution and atomic-scale defects (vacant sites), Monte Carlo
simulations suggest that designed NCs, with desirable, enhanced magnetic
properties (cf. exchange bias and coercivity), are developed with
magnetocrystalline anisotropy raised at relatively low content of Co ions in
the lattice. Growth of optimally performing candidates combines also a strongly
exchange-coupled system, secured through a high volumetric ratio rock salt
phase, interfaced by a not so defective spinel shell. In view of these
requirements, Specific Absorption Rate (SAR) calculations demonstrate that the
sufficiently protected from oxidation rock salt core and preserved over time
heterostructure, play a key role in magnetically-mediated heating efficacies,
for potential use of such NCs in magnetic hyperthermia applications.
|
2306.09684v1
|
2023-06-22
|
Studying the mass sensitivity of air-shower observables using simulated cosmic rays
|
Using CORSIKA simulations, we investigate the mass sensitivity of cosmic-ray
air-shower observables for sites at the South Pole and Malarg\"ue, Argentina,
the respective locations of the IceCube Neutrino Observatory and the Pierre
Auger Observatory. Exact knowledge of observables from air-shower simulations
was used to study the event-by-event mass separation between proton, helium,
oxygen, and iron primary cosmic rays with a Fisher linear discriminant
analysis. Dependencies on the observation site as well as the energy and zenith
angle of the primary particle were studied in the ranges from
$10^{16.0}-10^{18.5}\,$eV and $0^\circ$ to $60^\circ$: they are mostly weak and
do not change the qualitative results. Promising proton-iron mass separation is
achieved using combined knowledge of all studied observables, also when typical
reconstruction uncertainties are accounted for. However, even with exact
measurements, event-by-event separation of intermediate-mass nuclei is
challenging and better methods than the Fisher discriminant and/or the
inclusion of additional observables will be needed. As an individual
observable, high-energy muons ($> 500\,$GeV) provide the best event-by-event
mass discrimination, but the combination of muons of any energy and
$X_{\text{max}}$ provides already a high event-by-event separation between
proton-iron primaries at both sites. We also confirm that the asymmetry and
width parameters of the air-shower longitudinal profile, $R$ and $L$, are mass
sensitive. Only $R$ seems to be suitable for event-by-event mass separation,
but $L$ can potentially be used to statistically determine the proton-helium
ratio. Overall, our results motivate the coincident measurement of several
air-shower observables, including at least $X_{\text{max}}$ and the sizes of
the muonic and electromagnetic shower components, for the next generation of
air-shower experiments.
|
2306.13246v2
|
2023-07-19
|
Efficiency, Accuracy, and Transferability of Machine Learning Potentials: Application to Dislocations and Cracks in Iron
|
Machine learning interatomic potentials (ML-IAPs) enable quantum-accurate,
classical molecular dynamics simulations of large systems, beyond reach of
density functional theory (DFT). Yet, their efficiency and ability to predict
systems larger than DFT supercells are not fully explored, posing a question
regarding transferability to large-scale simulations with defects (e.g.
dislocations, cracks). Here, we apply a three-step validation approach to
body-centered-cubic iron. First, accuracy and efficiency are assessed by
optimizing ML-IAPs based on four state-of-the-art ML packages. The Pareto front
of computational speed versus testing root-mean-square-error (RMSE) is
computed. Second, benchmark properties relevant to plasticity and fracture are
evaluated. Their average relative error Q with respect to DFT is found to
correlate with RMSE. Third, transferability of ML-IAPs to dislocations and
cracks is investigated by using per-atom model uncertainty quantification. The
core structures and Peierls barriers of screw, M111 and three edge dislocations
are compared with DFT. Traction-separation curve and critical stress intensity
factor (K_Ic) are also predicted. Cleavage on the pre-existing crack plane is
found to be the zero-temperature atomistic fracture mechanism of pure
body-centered-cubic iron under mode-I loading, independent of ML package and
training database. Quantitative predictions of dislocation glide paths and KIc
can be sensitive to database, ML package, cutoff radius, and are limited by DFT
accuracy. Our results highlight the importance of validating ML-IAPs by using
indicators beyond RMSE. Moreover, significant computational speed-ups can be
achieved by using the most efficient ML-IAP package, yet the assessment of the
accuracy and transferability should be performed with care.
|
2307.10072v2
|
2023-11-12
|
Star Formation in Self-gravitating Disks in Active Galactic Nuclei. III. Efficient Production of Iron and Infrared Spectral Energy Distributions
|
Strong iron lines are a common feature of the optical spectra of active
galactic nuclei (AGNs) and quasars from $z\sim 6-7$ to the local Universe, and
[Fe/Mg] ratios do not show cosmic evolution. During active episodes, accretion
disks surrounding supermassive black holes (SMBHs) inevitably form stars in the
self-gravitating part and these stars accrete with high accretion rates. In
this paper, we investigate the population evolution of accretion-modified stars
(AMSs) to produce irons and magnesium in AGNs. The AMSs as a new type of stars
are allowed to have any metallicity but without significant loss from stellar
winds since the winds are choked by the dense medium of the disks and return to
the core stars. Mass functions of the AMS population show a pile-up or cutoff
pile-up shape in top-heavy or top-dominant forms if the stellar winds are
strong, consistent with the narrow range of supernovae (SN) explosions driven
by the known pair-instability. This provides an efficient way to produce
metals. Meanwhile, SN explosions support an inflated disk as a dusty torus.
Furthermore, the evolving top-heavy initial mass functions (IMFs) lead to
bright luminosity in infrared bands in dusty regions. This contributes a new
component in infrared bands which is independent of the emissions from the
central part of accretion disks, appearing as a long-term trending of the NIR
continuum compared to optical variations. Moreover, the model can be further
tested through reverberation mapping of emission lines, including LIGO/LISA
detections of gravitational waves and signatures from spatially resolved
observations of GRAVITY+/VLTI.
|
2311.06782v1
|
2024-01-18
|
The 2022 Outburst of IGR J17091-3624: Connecting the exotic GRS 1915+105 to standard black hole X-ray binaries
|
While the standard X-ray variability of black hole X-ray binaries (BHXBs) is
stochastic and noisy, there are two known BHXBs that exhibit exotic
`heartbeat'-like variability in their light curves: GRS 1915+105 and IGR
J17091-3624. In 2022, IGR J17091-3624 went into outburst for the first time in
the NICER/NuSTAR era. These exquisite data allow us to simultaneously track the
exotic variability and the corresponding spectral features with unprecedented
detail. We find that as in typical BHXBs, the outburst began in the hard state,
then the intermediate state, but then transitioned to an exotic soft state
where we identify two types of heartbeat-like variability (Class V and a new
Class X). The flux-energy spectra show a broad iron emission line due to
relativistic reflection when there is no exotic variability, and absorption
features from highly ionized iron when the source exhibits exotic variability.
Whether absorption lines from highly ionized iron are detected in IGR
J17091-3624 is not determined by the spectral state alone, but rather is
determined by the presence of exotic variability; in a soft spectral state,
absorption lines are only detected along with exotic variability. Our finding
indicates that IGR J17091-3624 can be seen as a bridge between the most
peculiar BHXB GRS 1915+105 and `normal' BHXBs because it alternates between the
conventional and exotic behavior of BHXBs. We discuss the physical nature of
the absorbing material and exotic variability in light of this new legacy
dataset.
|
2401.10192v1
|
2024-03-15
|
Asteroid reflectance spectra from Gaia DR3: Near-UV in primitive asteroids
|
In the context of charge-coupled devices (CCDs), the ultraviolet (UV) region
has mostly remained unexplored after the 1990s. Gaia DR3 offers the community a
unique opportunity to explore tens of thousands of asteroids in the near-UV as
a proxy of the UV absorption. This absorption has been proposed in previous
works as a diagnostic of hydration, organics, and space weathering. Aims. In
this work, we aim to explore the potential of the NUV as a diagnostic region
for primitive asteroids using Gaia DR3. We used a corrective factor over the
blue part of Gaia spectra to erase the solar analog selection effect. We
identified an artificial relation between the band noise and slope and applied
a signal-to-noise ratio (S/N) threshold for Gaia bands. Meeting the quality
standards, we employed a Markov chain Monte Carlo (MCMC) algorithm to compute
the albedo threshold, maximizing primitive asteroid inclusion. Utilizing one-
and two-dimensional (1D and 2D) projections, along with
dimensionality-reduction methods (such as PCA and UMAP), we identified
primitive asteroid populations. We uncovered: (a) the first observational
evidence linking UV absorption to the 0.7 {\mu}m band, tied to hydrated
iron-rich phyllosilicates; and (b) a 2D space revealing a split in C-type
asteroids based on spectral features, including UV absorption. The computed
average depth (3.5 +- 1.0 %) and center (0.70 +- 0.03 {\mu}m) of the 0.7 {\mu}m
absorption band for primitive asteroids observed with Gaia is in agreement with
the literature values. In this paper, we shed light on the importance of the UV
absorption feature to discriminate among different mineralogies (i.e.,
iron-rich phyllosilicates vs. iron-poor) or to identify taxonomies that are
conflated in the visible (i.e., F-types vs. B-types). We have shown that this
is a promising region for diagnostic studies of the composition of primitive
asteroids.
|
2403.10321v1
|
1993-01-29
|
A Possible Forest of Emission Lines from Proto-galaxies
|
The possibility of detecting proto-galaxies in the UV band is pointed out,
assuming that galaxy formation occured at z $\sim 5-6$. It is shown that the
diffuse gas in collapsing galaxy sized objects with temperatures $\sim
10^{6\pm0.5}$ K, and with a modest amount of metallicity, should copiously
produce emission lines from highly ionized Iron atoms. The expected luminosity
from models of galaxy formation is compared with the sensitivity of HST.
|
9301012v1
|
1995-10-23
|
X-ray spectra from convective photospheres of neutron stars
|
We present first results of modeling convective photospheres of neutron
stars. We show that in photospheres composed of the light elements convection
arises only at relatively low effective temperatures (< 3-5*10^4 K), whereas in
the case of iron compositon it arises at < 3*10^ K. Convection changes the
depth dependence of the photosphere temperature and the shapes of the emergent
spectra. Thus, it should be taken into account for the proper interpretation of
EUV/soft-X-ray observations of the thermal radiation from neutron stars.
|
9510116v2
|
1997-01-22
|
Two-phase pair corona model for AGN: physical modelling and diagnostics
|
The predictions of the two-phase accretion disc-corona models for active
galactic nuclei are compared with observations. We discuss the possibility to
use X-ray spectral slopes, equivalent widths of the iron line, and the observed
flux-spectral index correlation as diagnostics of the X/gamma-ray source
compactness and geometry as well as of the cold disc temperature. As an example
of the application of the modelling tools, we use XSPEC to fit the broad-band
data of Seyfert 1 galaxy, IC4329A, with a theoretical spectrum from a
hemisphere-corona.
|
9701168v1
|
1997-03-04
|
RXTE Observation of the Seyfert 2 Galaxy NGC4507
|
Preliminary results of the RXTE observation of the Seyfert 2 Galaxy NGC4507
are presented. The observed broadband [4-100 keV] spectrum is intrinsically
hard (Gamma = 1.2); an iron line is detected with a relatively high equivalent
width (EW = 400 eV). The remaining calibration uncertanties are briefly
discussed, as well as the scientific implications of our results.
|
9703019v1
|
1997-05-12
|
The most Distant [OIII]-emitting Quasar PKS 1937-101 at redshift 3.8
|
We report the discovery of a high-z quasar with unambiguous [OIII]5007
emission; PKS 1937-101 at redshift 3.8. This quasar, however, shows little
evidence for rest-frame ultraviolet and optical FeII emission. It is thus shown
that PKS 1937-101 does not belong to a class of super iron-rich high-z quasars
reported by Elston, Thompson, & Hill (1994). The epoch of major star formation
in the host galaxy is discussed briefly.
|
9705075v1
|
1997-08-08
|
A depression before a bump in the highest energy cosmic ray spectrum
|
We re-examine the interaction of ultra high energy nuclei with the microwave
background radiation. We find that the giant dipole resonance leaves a new
signature in the differential energy spectrum of iron sources located around 3
Mpc: A depression before the bump which is followed by the expected cutoff.
|
9708082v2
|
1997-10-07
|
Fe I line shifts in the optical spectrum of the Sun
|
New improvements in the measurement of both the optical solar spectrum and
laboratory wavelengths for lines of neutral iron are combined to extract
central wavelength shifts for 1446 lines observed in the Sun. This provides the
largest available database of accurate solar wavelengths useful as a reference
for comparison with other solar-type stars. It is shown how the velocity shifts
correlate with line strength, approaching a constant value, close to zero, for
lines with equivalent widths larger than 200 mA.
|
9710066v1
|
1998-05-15
|
The double-lined spectroscopic binary alpha Andromedae: orbital elements and elemental abundances
|
We performed a spectroscopic study of the SB2 Mercury-Manganese star alpha
And. Our measurements of the secondary's radial velocities result in improved
orbital elements. The secondary shows abundances typical of the metallic-line
stars: a Ca deficiency, small overabundances of the iron-peak elements, and 1.0
dex overabundances of Sr and Ba.
|
9805205v1
|
1998-05-27
|
Strong gravity and X-ray spectroscopy
|
This paper reviews the effects of general relativity in an X-ray spectrum
reflected from a cold matter accreting onto a black hole. The spectrum consists
of the iron K$\alpha$ line and the Compton reflection. We sketch the overall
picture of radiative processes in the central parts of the accretion flow with
relation to the relativistic effects derived from the discrete features in the
X-ray spectrum. We discuss implications for detection of relativistic effects
and computational tools of spectral analysis.
|
9805328v1
|
1999-05-25
|
First results on characterization of Cerenkov images through combined use of Hillas, fractal and wavelet parameters
|
Based on Monte Carlo simulations using the CORSIKA code, it is shown that
Cerenkov images produced by ultrahigh energy $\gamma$-rays and cosmic ray
nuclei (proton, Neon and Iron) are fractal in nature. The resulting
multifractal and wavelet moments when employed in association with the
conventional Hillas parameters as inputs to a properly-trained artificial
neural network are found to provide more efficient primary characterization
scheme than the one based on the use of Hillas or fractal parameters alone.
|
9905312v1
|
1999-11-24
|
Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo
|
We report the discovery of molecular hydrogen absorption in a Galactic
high-velocity cloud (HVC) in the direction of the Large Magellanic Cloud. For
the same HVC we derive an iron abundance which is half of the solar value.
Thus, all evidence points to a Galactic origin for high-velocity cloud complex
in front of the LMC.
|
9911455v1
|
1999-11-30
|
Prompt Iron Enrichment, Two r-Process Components, and Abundances in Very Metal-Poor Stars
|
We present a model to explain the wide range of abundances for heavy
r-process elements (mass number A > 130) at low [Fe/H]. This model requires
rapid star formation and/or an initial population of supermassive stars in the
earliest condensed clots of matter to provide a prompt or initial Fe inventory.
Subsequent Fe and r-process enrichment was provided by two types of supernovae:
one producing heavy r-elements with no Fe on a rather short timescale and the
other producing light r-elements (A < or = 130) with Fe on a much longer
timescale.
|
9911526v1
|
2000-03-29
|
A Comparative Study of the Depth of Maximum of Simulated Air Shower Longitudinal Profiles
|
A comparative study of simulated air shower longitudinal profiles is
presented. An appropriate thinning level for the calculations is first
determined empirically. High statistics results are then provided, over a wide
energy range, (10^14.0 to 10^20.5 eV), for proton & iron primaries, using four
combinations of the MOCCA & CORSIKA program frameworks, and the SIBYLL & QGSJET
high energy hadronic interaction models. These results are compared to existing
experimental data. The way in which the first interaction controls Xmax is
investigated, as is the distribution of Xmax.
|
0003442v2
|
2000-04-20
|
X-rays from Radio-Galaxies: BeppoSAX Observations
|
We briefly review BeppoSAX observations of X-ray bright radio-galaxies. Their
X-ray spectra are quite varied, and perhaps surprisingly, any similarity
between radio-loud AGN and Seyfert galaxies is the exception rather than the
rule. When detected, reprocessing features (iron line and reflection) are
generally weak, suggesting two possible scenarios: either: (1) non-thermal
(jet?) radiation dilutes the X-ray emission from the disk in radio-loud
objects, or (2) the solid angle subtended by the X-ray reprocessing material is
smaller in radio-loud than in radio-quiet AGN due to different characteristics
of the accretion disk itself.
|
0004291v1
|
2000-05-08
|
Properties of Warm absorbers in Narrow-Line Seyfert 1 galaxies
|
We present a study of the X-ray properties of several NLS1 galaxies, focusing
on their warm absorbers. In the first part, we discuss properties of dusty and
dust-free warm absorbers in NLS1s, and study their potential contribution to
high-ionization optical iron lines. In the second part, we summarize our work
on the exceptional spectral variability of the NLS1 galaxy RX J0134.3-4258
(from Gamma about -4.4 in the ROSAT all-sky survey observation, to about -2.2
in our subsequent pointed observation).
|
0005172v1
|
2000-06-05
|
A pot of gold at the end of the cosmic "raynbow"?
|
We critically review the common belief that ultrahigh energy cosmic rays are
protons or atomic nuclei with masses not exceeding that of iron. We find that
heavier nuclei are indeed possible, and discuss possible sources and
acceleration mechanisms for such primaries. We also show detailed simulations
of extensive air showers produced by ``superheavy'' nuclei, and discuss
prospects for their detection in future experiments.
|
0006071v1
|
2000-06-19
|
Abundances and Kinematics of Field Halo and Disk Stars I: Observational Data and Abundance Analysis
|
We describe observations and abundance analysis of a high-resolution,
high-S/N survey of 168 stars, most of which are metal-poor dwarfs. We follow a
self-consistent LTE analysis technique to determine the stellar parameters and
abundances, and estimate the effects of random and systematic uncertainties on
the resulting abundances. Element-to-iron ratios are derived for key alpha,
odd, Fe-peak, r- and s-process elements. Effects of Non-LTE on the analysis of
Fe I lines are shown to be very small on the average. Spectroscopically
determined surface gravities are derived that are generally close to those
obtained from Hipparcos parallaxes.
|
0006260v1
|
2000-09-18
|
Age-metallicity relation and Chemical evolution of the LMC from UVES spectra of Globular Cluster giants
|
We report on the first high-resolution spectroscopy of 10 giants in LMC
Globular Clusters in a wide age range, obtained with the newly commissioned
spectrograph UVES at VLT UT2. These observations are used to derive oxygen and
iron content of these clusters, and the abundances are then used to cast a more
precise view, not only on the age-metallicity relation in the LMC, but also on
the chemical evolution of this dwarf irregular galaxy.
|
0009273v1
|
2000-09-20
|
The BeppoSAX observation of Mrk 766
|
The Narrow Line Seyfert 1 galaxy Mrk 766 has been observed by BeppoSAX on May
1997. The source was fairly variable, both in flux and spectrum, during the
observation. The variability is the largest around 2 keV, possibly due to
variations in the warm absorber properties. An oxygen line is clearly detected,
at least in the first part of the observation. An edge around 7.5 keV, coupled
with the lack of any detectable iron line, suggests either reprocessing of the
primary X-rays by a mildly ionized disc or absorption by a further, thicker and
more ionized material.
|
0009322v1
|
2000-11-24
|
Lithium-Beryllium-Boron and Oxygen in the early Galaxy
|
Oxygen is a much better evolutionary index than iron to follow the history of
Lithium-Beryllium-Boron (LiBeB) since it is the main producer of these light
elements at least in the early Galaxy. The O-Fe relation is crucial to the
determination of the exact physical process responsible for the LiBeB
production. Calculated nucleosynthetic yields of massive stars, estimates of
the energy cost of Be production, and above all recent observations reported in
this meeting seem to favor a mechanism in which fast nuclei enriched into He, C
and O arising from supernovae are accelerated in superbubbles and fragment on H
and He in the interstellar medium.
|
0011449v1
|
2001-02-27
|
Variable line profiles due to non-axisymmetric patterns in an accretion disc around a rotating black hole
|
We have explored spectral line profiles due to spiral patterns in accretion
discs around black holes. A parametrization was employed for the shape and
emissivity of spiral waves, which can be produced by non-axisymmetric
perturbations affecting the disc density and ionization structure. The effects
of the light-travel time, energy shift, and gravitational focusing near to a
rotating black hole were taken into account. A high-resolution ray-tracing code
was used to follow the time variations of the synthetic line profile. A variety
of expected spectral features were examined and the scheme applied to a broad
iron line observed in MCG-6-30-15.
|
0102460v1
|
2001-03-21
|
An iterative technique for solving equations of statistical equilibrium
|
Superlevel partitioning is combined with a simple relaxation procedure to
construct an iterative technique for solving equations of statistical
equilibrium. In treating an $N$-level model atom, the technique avoids the
$N^{3}$ scaling in computer time for direct solutions with standard linear
equation routines and also does not fail at large $N$ due to the accumulation
of round-off errors. In consequence, the technique allows detailed model atoms
with $N \ga 10^{3}$, such as those required for iron peak elements, to be
incorporated into diagnostic codes for analysing astronomical spectra. Tests
are reported for a 394-level Fe II ion and a 1266-level Ni I--IV atom.
|
0103338v1
|
2001-03-27
|
X-rays and accretion discs as probes of the strong gravity of black holes
|
The observations and interpretation of broad iron lines in the X-ray spectra
of Seyfert 1 galaxies are reviewed. The line profiles observed from
MCG--6-30-15 and NGC 3516 show extended red wings to the line explained by
large gravitational redshifts. The results are consistent with the emission
expected from an X-ray irradiated flat accretion disc orbiting very close to a
black hole. Results from XMM-Newton and Chandra are presented and the
possibility of broad oxygen lines discussed.
|
0103438v1
|
2001-06-13
|
Non-pulsing emission from X-ray pulsars
|
Results of GRANAT/ART-P observations of three X-ray pulsars in non-pulsing
states are presented: 1) a statistically significant non-pulsing flux with a
simple power-law spectrum was detected during the ``off''- state of Her X-1; 2)
a significant (13 $\sigma$) non-pulsing flux with a strong iron emission line
at energies ~ 6.7-6.9 keV was detected during the eclipse ingress of Cen X-3;
3) a weak non-pulsing flux was detected during the X-ray eclipse of Vela X-1,
which probably resulted from scattering of the pulsar emission in the stellar
wind of an optical star.
|
0106237v1
|
2001-11-04
|
Polarization of X-ray emission from the Sgr B2 cloud
|
The Sgr B2 giant molecular cloud is claimed to be an "X-ray reflection
nebula" - the reprocessing site of a powerful flare of the Sgr A* source,
occurred few hundred years ago. The shape of the X-ray spectrum and the
strength of the iron fluorescent line support this hypothesis. We argue that
the most clean test of the origin of X-rays from Sgr B2 would be a detection of
polarized emission from this source.
|
0111065v1
|
2001-12-17
|
Probing the Central Engine of the Narrow-Line Seyfert 1 Galaxies
|
The central engine of the the Narrow Line Seyfert 1 galaxies is being probed.
We use the ASCA and RXTE data to model the X-ray primary continuum as well as
the reflected component and iron $K\alpha$ line. Since these are strongly
coupled, we obtain independent measurements of the disc ionization level and
the orientation dependent reflection amplitude. Using the available Optical/UV
data we also estimate the black hole masses and the $L/L_{Edd}$ ratios, which
are probably related to Boroson & Green eigenvector~1.
|
0112388v1
|
2002-06-01
|
On the Progenitors of Collapsars
|
We study the evolution of stars that may be the progenitors of common
(long-soft) GRBs. Bare rotating helium stars, presumed to have lost their
envelopes due to winds or companions, are followed from central helium ignition
to iron core collapse. Including realistic estimates of angular momentum
transport (Heger, Langer, & Woosley 2000) by non-magnetic processes and mass
loss, one is still able to create a collapsed object at the end with sufficient
angular momentum to form a centrifugally supported disk, i.e., to drive a
collapsar engine. However, inclusion of current estimates of magnetic torques
(Spruit 2002) results in too little angular momentum for collapsars.
|
0206005v1
|
2002-07-05
|
Cosmic ray drift, the second knee and galactic anisotropies
|
We show that the second knee in the cosmic ray spectrum (i.e. the steepening
occurring at $E\simeq 4\times 10^{17}$ eV) could be related to drift effects
affecting the heaviest nuclear component, the iron group nuclei, in a scenario
in which the knee at $3\times 10^{15}$ eV indicates the onset of drift effects
in the lighter proton component. We also study the anisotropies resulting from
diffusion and drift currents in the Galaxy, showing their potential relevance
to account for the AGASA observations at $E\sim 10^{18}$ eV, before the
extragalactic component becomes dominant.
|
0207143v1
|
2002-07-19
|
Carbon Isotopic Abundances in the Red Giants of Omega Centauri (NGC5139)
|
Carbon-12 and carbon-13 abundances are measured in eleven red-giant members
of the globular cluster Omega Centauri via observations of first-overtone CO
bands near 2.3 microns. The mean value for the entire sample is <12C/13C>= 4.3
+/-0.4, with nine giants equal, within the errors, to the equilibrium ration of
12C/13C= 3.5. No correlation is found within Omega Cen between 12C/13C and the
abundance of iron. The relation between 12C/13C and other abundance ratios,
such as [O/Fe], {Na/Fe], or [Al/Fe] are also discussed.
|
0207434v1
|
2002-07-20
|
Antimatter in the Universe
|
Different scenarios of baryogenesis are briefly reviewed from the point of
view of possibility of generation of cosmologically interesting amount of
antimatter. It is argued that creation of antimatter is possible and natural in
many models. In some models not only anti-helium may be produced but also a
heavier anti-elements and future observations of the latter would be critical
for discovery or establishing stronger upper limits on existence of antimatter.
Incidentally a recent observation of iron-rich quasar may present a support to
one special model of antimatter creation.
|
0207441v1
|
2002-11-12
|
Line Profiles from Different Accretion Engine Geometries
|
Line profiles represent an important probe of the geometry and dynamics of
the central accretion engines in AGN. Although a flat disk illuminated from
above can explain the relativistic Fe K alpha line profile in some cases,
current data, plausible disk physics, and future comparison to the iron and
other AGN lines invite some consideration of other geometries. General features
of line profiles calculated fom concave discs, warped discs, thick clumpy
(inflow/outflow) discs, and disks with spiral structure, are briefly
summarized.
|
0211236v1
|
2002-11-26
|
Spectral properties of the X-ray binary pulsar LMC X-4 during different intensity states
|
We present spectral variations of the binary X-ray pulsar LMC X-4 observed
with the RXTE/PCA during different phases of its 30.5 day long third period.
Only out of eclipse data were used for this study. The 3-25 keV spectrum,
modeled with high energy cut-off power-law and iron line emission is found to
show strong dependence on the intensity state. Correlations between the Fe line
emission flux and different parameters of the continuum are presented here.
|
0211558v1
|
2003-08-29
|
Iron Line Diagnostics for the GRS 1915+105 Black Hole
|
The properties of the broad Fe line detected in two BeppoSAX observations of
the microquasar GRS 1915+105 are summarized.
|
0308539v1
|
2003-12-09
|
The ionization equilibrium of iron in H II regions
|
We study the ionization equilibrium of Fe using photoionization models that
incorporate improved values for the ionization and recombination cross-sections
and the charge-exchange rates for the Fe ions. The previously available
photoionization models predict concentrations of Fe3+ which are a factor of 3-8
higher than the values inferred from emission lines of [Fe III] and [Fe IV].
Our new models reduce these discrepancies to factors of 2-5. We discuss the
possible reasons behind the remaining discrepancies and present an updated
ionization correction factor for obtaining the Fe abundance from the Fe++
abundance.
|
0312246v1
|
2004-01-16
|
Modeling AGN spectra with PHOENIX: a self-consistent approach
|
We find that spectra of certain Iron Low Ionization Broad Absorption Line
(FeLOBAL) QSOs, which are characterized by low--ionization emission and blue
shifted absorption lines, can be well matched with the spectral synthesis code
SYNOW. SYNOW is a resonance scattering code and assumes that line emission
comes from a single line forming region. This interpretation is novel as
traditionally line emission and absorption in BALQSOs are thought to come from
two different regions. We extend this analysis by using the detailed PHOENIX
code to model the spectra. We present a SYNOW fit and a preliminary model
result from PHOENIX.
|
0401346v1
|
2004-08-12
|
Chemical analysis of 24 dusty (pre-)main-sequence stars
|
We have analysed the chemical photospheric composition of 24 Herbig Ae/Be and
Vega-type stars in search for the lambda Bootis phenomenon. We present the
results of the elemental abundances of the sample stars. Some of the stars were
never before studied spectroscopically at optical wavelengths. We have
determined the projected rotational velocities of our sample stars.
Furthermore, we discuss stars that depict a (selective) depletion pattern in
detail. HD 4881 and HD 139614 seem to display an overall deficiency. AB Aur and
possibly HD 126367 have subsolar values for the iron abundance, but are almost
solar in silicon. HD 100546 is the only clear lambda Bootis star in our sample.
|
0408221v1
|
2004-09-10
|
The L to T Dwarf Transition
|
While the precise mechanism responsible for the L to T dwarf transition
remains unclear, it is clearly caused by changing cloud characteristics. Here
we briefly review data relevant to understanding the nature of the transition
and argue that changing atmospheric dynamics produce the transition by opening
holes through the global iron and silicate cloud decks. Other possibilities,
such as a sudden vertical collapse in these cloud decks are also considered.
Any acceptable model of the L to T transition must ultimately connect changing
cloud properties to the underlying atmospheric dynamics.
|
0409267v1
|
2004-10-27
|
An Iron-Rich Sun and Its Source of Energy
|
Mass-fractionation enriches light elements and the lighter isotopes of each
element at the solar surface, making a photosphere that is 91 percent H and 9
percent He. The solar interior consists mostly of elements that comprise 99
percent of ordinary meteorites (Fe, O, Ni, Si, S, Mg and Ca) elements made in
the deep interior of a supernova. Solar energy arises from a series of nuclear
reactions triggered by neutron-emission from the collapsed supernova core on
which the Sun formed. Solar mass-fractionation, solar neutrinos, and the annual
solar-wind outpouring of 3 E43 H atoms from the solar surface are by-products
of solar luminosity.
|
0410646v1
|
2004-11-04
|
Filamentary jets as a cosmic-ray "Zevatron"
|
Strong, anisotropic turbulence reflecting magnetized filaments is considered,
to model the diffusive acceleration of particles by shock waves in active
galactic nucleus jets. We address that at knot A of the nearby M87 jet, the
shock involving the filamentary turbulence can accelerate an iron nucleus to
zetta-eV (ZeV; 10^{21} eV) ranges. A smaller value of the particle diffusion
coefficient is found to be essential to achieve a ZeV cosmic-ray accelerator, a
"Zevatron."
|
0411101v3
|
2005-01-23
|
Supernova Rates in Galaxy Clusters
|
Measurements of SN rates in different environments and redshifts can shed
light on the nature of SN-Ia progenitors, star formation history, and chemical
enrichment history. I summarize some recent work by our group in this area, and
discuss the implications. The current evidence favors production of most of the
iron in the ICM (and perhaps everywhere) by core-collapse SNe, rather than
SNe-Ia. These SNe may have been produced by the first, top-heavy-IMF,
generation of stars that reionized the Universe. Improved rate measurements can
sharpen the picture, and I describe our recent efforts in this direction.
|
0501492v1
|
2005-02-24
|
The polarized electron target as a new solar-neutrino detector
|
In this paper, we analyze the scattering of solar neutrinos on the polarized
electron target, and predict how the effect of parity violation in weak
interactions may help to distinguish neutrino signal from detector background.
We indicate that the knowledge of the Sun motion across the sky is sufficient
to predict the day/night asymmetry in the $(\nu_ee^-)$ scattering on the
polarized electron target. To make this detection feasible, the polarized
electron target for solar neutrinos needs to be build from magnetic materials,
e.g. from ferromagnetic iron foils, paramagnetic scintillator crystals or
scintillating ferrofluids.
|
0502522v1
|
2005-04-12
|
ASCA and XMM-Newton Observations of A2029
|
The X-ray data of A2029 obtained with XMM-Newton show no evidence of an
embedded AGN in the central region of this cluster, which was suggested from
the analysis of restored ASCA image data, although some hot spots are seen
within or around the central cD galaxy. The absence of AGN at the cluster
center is consistentent with the result of Chandra observations. Radial
profiles of the iron abundance and the 2D (surface) temperature obtained from
the XMM-Newton data are in good agreement with the Chandra data as a whole.
|
0504268v1
|
2005-07-12
|
Disentangling the composite continuum of symbiotic binaries I. S-type systems
|
We describe a method of disentangling the composite,0.12-5 microns continuum
of symbiotic binaries.The observed SED is determined by the IUE/HST archival
spectra and flux-points corresponding to the optical UBVRI and infrared JHKLM
photometric measurements. The modeled SED is given by superposition of fluxes
from the cool giant, hot stellar source and nebula including the effect of the
Rayleigh scattering process and considering influence of the iron curtain
absorptions.We applied this method to 21 S-type symbiotic stars during
quiescence, activity and eclipses.
|
0507272v1
|
2005-09-08
|
HE1327-2326, an unevolved star with [Fe/H]<-5.0. I. A Comprehensive Abundance Analysis
|
We present the elemental abundances of HE1327-2326, the most iron-deficient
star known, determined from a comprehensive analysis of spectra obtained with
the Subaru Telescope High Dispersion Spectrograph.
|
0509206v1
|
2005-12-05
|
Nonideal strongly magnetized plasmas of neutron stars and their electromagnetic radiation
|
We study the equation of state, polarization and radiation properties for
nonideal, strongly magnetized plasmas which compose outer envelopes of magnetic
neutron stars. Detailed calculations are performed for partially ionized
hydrogen atmospheres and for condensed hydrogen or iron surfaces of these
stars. This is a companion paper to astro-ph/0511803
|
0512088v1
|
2005-12-27
|
Observations of H-alpha, iron, and oxygen lines in B, Be, and shell stars
|
We have carried out a spectroscopic survey of several B, Be, and shell stars
in optical and near-infrared regions. Line profiles of the H-alpha line and of
selected Fe II and O I lines are presented.
|
0512619v1
|
2006-05-06
|
Abundances of s-process elements in planetary nebulae: Br, Kr & Xe
|
We identify emission lines of post-iron peak elements in very high
signal-to-noise spectra of a sample of planetary nebulae. Analysis of lines
from ions of Kr and Xe reveals enhancements in most of the PNe, in agreement
with the theories of s-process in AGB star. Surprisingly, we did not detect
lines from Br even though s-process calculations indicate that it should be
produced with Kr at detectable levels.
|
0605181v1
|
2006-10-01
|
HST and FUSE Spectroscopy of the DAO-type Central Star LS V+4621
|
The DAO-type white dwarf LS V+4621 is the hydrogen-rich central star of the
possible planetary nebula Sh 2-216. We have taken high-resolution, high-S/N
ultraviolet spectra with STIS aboard the HST and FUSE in order to constrain its
photospheric parameters. A detailed spectral analysis by means of
state-of-the-art NLTE model-atmosphere techniques is presented which includes
the determination the individual abundances of iron-group elements.
|
0610019v1
|
2006-10-07
|
General Relativity effects and line emission
|
General Relativity effects (gravitational redshift, light bending, ...)
strongly modify the characteristics of the lines emitted close to the Black
Hole in Active Galactic Nuclei and Galactic Black Hole systems. These effects
are reviewed and illustrated, with particular emphasis on line emission from
the accretion disc. Methods, based on the iron line, to measure the two
astrophysically relevant parameters of a Black Hole, the mass and spin, are
briefly discussed.
|
0610224v1
|
2006-11-16
|
A Sr-Rich Star on the Main Sequence of Omega Centauri
|
Abundance ratios relative to iron for carbon, nitrogen, strontium and barium
are presented for a metal-rich main sequence star ([Fe/H]=--0.74) in the
globular cluster omega Centauri. This star, designated 2015448, shows depleted
carbon and solar nitrogen, but more interestingly, shows an enhanced abundance
ratio of strontium [Sr/Fe] ~ 1.6 dex, while the barium abundance ratio is
[Ba/Fe]<0.6 dex. At this metallicity one usually sees strontium and barium
abundance ratios that are roughly equal. Possible formation scenarios of this
peculiar object are considered.
|
0611509v1
|
2006-11-29
|
Spectropolarimetric diagnostics of thermonuclear supernova explosions
|
Even at extragalactic distances, the shape of supernova ejecta can be
effectively diagnosed by spectropolarimetry. We present here results for 17
Type Ia supernovae that allow a statistical study of the correlation among the
geometric structures and other observable parameters of Type Ia supernovae.
These observations suggest that their ejecta typically consist of a smooth,
central iron rich core and an outer layer with chemical asymmetries. The degree
of this peripheral asphericity is correlated with the light-curve decline rate
of Type Ia supernovae. These observations lend strong support to
delayed-detonation models of Type Ia supernovae.
|
0611902v1
|
2007-03-20
|
Limits on iron-dominated fallback disk in SN 1987A
|
The non-detection of a point source in SN1987A imposes an upper limit for the
optical luminosity of L=2L_sun. This limits the size of a possible fallback
disk around the stellar remnant. Assuming a steady-state thin disk with
blackbody emission requires a disk smaller than 100,000 km if the accretion
rate is at 30% of the Eddington rate (Graves et al. 2005). We have performed
detailed non-LTE radiation transfer calculations to model the disk spectrum
more realistically. It turns out that the observational limit on the disk
extension becomes even tighter, namely 70,000 km.
|
0703520v1
|
1995-07-07
|
Nonlinear magneto-optical Kerr spectra of thin ferromagnetic iron films calculated with ab initio theory
|
Using a spin-polarized full-potential linear muffin-tin orbital method we
present calculations of the nonlinear magneto-optical Kerr effect for thin bcc
Fe films within a slab geometry. Film layer dependent contributions to the Kerr
spectrum are determined. Thus, we calculate the magneto-optical Kerr spectra
for the linear and nonlinear case. Our results show clearly that the Kerr
spectra of thin films are characteristicly different from those at surfaces of
bulk materials. In the case of linear Kerr spectra of Au/Fe(bcc)/Au(001) films
our theoretical results are in good agreement with observed frequency- and
thickness-dependent spectra.
|
9507018v1
|
1997-03-04
|
Magnetic and thermodynamic properties of Sr_{2}LaFe_{3}O_{9}
|
Using a Dirac-Heisenberg Hamiltonian with biquadratic exchange interactions,
we study the effect of iron disproportionation on the magnetic ordering, and
describe the first-order magnetic transition occurring in the perovskite
Sr_{2}LaFe_{3}O_{9}. Upon fitting the experimental data, we give an estimate of
the exchange integrals for the antiferromagntic and ferromagnetic interactions,
in agreement with previous works on kindered compounds. Spin-wave theory yields
a magnon spectrum with a gapless antiferromagnetic mode together with two
gapped ferromagnetic ones.
|
9703044v1
|
1998-05-07
|
The first principles calculation of transport coefficients
|
We demonstrate the practical feasibility of calculating transport
coefficients such as the viscosity of liquids completely from first principles
using the Green-Kubo relations. Results presented for liquid aluminum are shown
to have a statistical error of only ca. 5%. The importance of such calculations
is illustrated by results for a liquid iron-sulfur alloy under Earth's core
conditions, which indicate that the viscosity of the liquid outer core is not
substantially higher than that of typical liquid metals under ambient
conditions.
|
9805082v1
|
1999-04-29
|
First- principle calculations of magnetic interactions in correlated systems
|
We present a novel approach to calculate the effective exchange interaction
parameters based on the realistic electronic structure of correlated magnetic
crystals in local approach with the frequency dependent self energy. The analog
of ``local force theorem'' in the density functional theory is proven for
highly correlated systems. The expressions for effective exchange parameters,
Dzialoshinskii- Moriya interaction, and magnetic anisotropy are derived. The
first-principle calculations of magnetic excitation spectrum for ferromagnetic
iron, with the local correlation effects from the numerically exact QMC-scheme
is presented.
|
9904428v1
|
1999-08-24
|
Tunable Charge Density Wave Transport in a Current-Effect Transistor
|
The collective charge density wave (CDW) conduction is modulated by a
transverse single-particle current in a transistor-like device. Nonequilibrium
conditions in this geometry lead to an exponential reduction of the depinning
threshold, allowing the CDWs to slide for much lower bias fields. The results
are in excellent agreement with a recently proposed dynamical model in which
''wrinkles'' in the CDW wavefronts are ''ironed'' by the transverse current.
The experiment might have important implications for other driven periodic
media, such as moving vortex lattices or ''striped phases'' in high-Tc
superconductors.
|
9908339v1
|
1999-08-26
|
Magnons in real materials from density-functional theory
|
We present an implementation of the adiabatic spin-wave dynamics of Niu and
Kleinman. This technique allows to decouple the spin and charge excitations of
a many-electron system using a generalization of the adiabatic approximation.
The only input for the spin-wave equations of motion are the energies and Berry
curvatures of many-electron states describing frozen spin spirals. The latter
are computed using a newly developed technique based on constrained
density-functional theory, within the local spin density approximation and the
pseudo-potential plane-wave method. Calculations for iron show an excellent
agreement with experiments.
|
9908386v1
|
2000-11-29
|
Giant Magnetoresistance at the Interface of Iron Thin Films
|
Ag/Fe/Ag and Cr/Fe/Cr trilayers with a single $25 nm$ thick ferromagnetic
layer exhibit giant magnetoresistance (GMR) type behavior. The resistance
decreases for parallel and transversal magnetic field alignements with a
Langevin-type magnetic field dependence up to B=12 T. The phenomenon is
explained by a granular interface structure. Results on Fe/Ag multilayers are
also interpreted in terms of a granular interface magnetoresistance.
|
0011491v1
|
2002-05-30
|
Spin-dependent properties of a two-dimensional electron gas with ferromagnetic gates
|
A theoretical prediction of the spin-dependent electron self-energy and
in-plane transport of a two-dimensional electron gas in proximity with a
ferromagnetic gate is presented. The application of the predicted
spin-dependent properties is illustrated by the proposal of a device
configuration with two neighboring ferromagnetic gates which produces a
magnetoresistance effect on the channel current generated by nonmagnetic source
and drain contacts. Specific results are shown for a silicon inversion layer
with iron gates. The gate leakage current is found to be beneficial to the spin
effects.
|
0205651v2
|
2002-07-10
|
Different routes to charge disproportionation in perovskites-type Fe oxides
|
Iron perovskites CaFeO_3 and La_{0.33}Sr_{0.67}FeO_3 show charge
disproportionation, resulting in charge-ordered states with Fe^{3+}:Fe^{5+}
=1:1 and =2:1, respectively. We have made photoemission and unrestricted
Hartree-Fock band-structure calculation of CaFeO_3 and compared it with
La_{0.33}Sr_{0.67}FeO_3. With decreasing temperature, a gradual decrease of the
spectral weight near the Fermi level occurred in CaFeO_3 as in
La_{0.33}Sr_{0.67}FeO_3 although lattice distortion occurs only in CaFeO_3.
Hartree-Fock calculations have indicated that both the breathing and tilting
distortions are necessary to induce the charge disproportionation in CaFeO_3,
while no lattice distortion is necessary for the charge disproportionation in
La_{0.33}Sr_{0.67}FeO_3.
|
0207250v1
|
2002-07-26
|
Ballistic versus diffusive magnetoresistance of a magnetic point contact
|
The quasiclassical theory of a nanosize point contacts (PC) between two
ferromagnets is developed. The maximum available magnetoresistance values in PC
are calculated for ballistic versus diffusive transport through the area of a
contact. In the ballistic regime the magnetoresistance in excess of few
hundreds percents is obtained for the iron-group ferromagnets. The necessary
conditions for realization of so large magnetoresistance in PC, and the
experimental results by Garcia et al are discussed
|
0207648v1
|
2002-11-01
|
Exploring dynamical magnetism with time-dependent density-functional theory: from spin fluctuations to Gilbert damping
|
We use time-dependent spin-density-functional theory to study dynamical
magnetic phenomena. First, we recall that the local-spin-density approximation
(LSDA) fails to account correctly for magnetic fluctuations in the paramagnetic
state of iron and other itinerant ferromagnets. Next, we construct a
gradient-dependent density functional that does not suffer from this problem of
the LSDA. This functional is then used to derive, for the first time, the
phenomenological Gilbert equation of micromagnetics directly from
time-dependent density-functional theory. Limitations and extensions of Gilbert
damping are discussed on this basis, and some comparisons with phenomenological
theories and experiments are made.
|
0211021v1
|
2003-05-06
|
Probing the shape of atoms in real space
|
The structure of single atoms in real space is investigated by scanning
tunneling microscopy. Very high resolution is possible by a dramatic reduction
of the tip-sample distance. The instabilities which are normally encountered
when using small tip-sample distances are avoided by oscillating the tip of the
scanning tunneling microscope vertically with respect to the sample. The
surface atoms of Si(111)-(7 x 7) with their well-known electronic configuration
are used to image individual samarium, cobalt, iron and silicon atoms. The
resulting images resemble the charge density corresponding to 4f, 3d and 3p
atomic orbitals.
|
0305103v1
|
2003-06-21
|
The Effect of Disorder on a Quantum Phase Transition
|
The conductivity and magnetization of Fe1-xCoxS2 were measured to investigate
quantum critical behavior in disordered itinerant magnets. Small x (<0.001) is
required to convert insulating iron pyrite into a metal, followed by a
paramagnetic-to-ferromagnetic metal transition at x = 0.032+/-0.004. Singular
contributions are discovered that are distinct from those at either
metal-insulator or magnetic transitions. Our data reveal that disorder and low
carrier density associated with proximity to a metal-insulator transition
fundamentally modifies the critical behavior of the magnetic transition.
|
0306541v1
|
2003-09-09
|
Finite-Temperature Micromagnetics of Hysterisis for Misaligned Single Iron Nanopillars
|
We present micromagnetic results for the hysterisis of a single magnetic
nanopillar which is misaligned with respect to the applied magnetic field. We
provide results for both a one dimensional stack of magnetic rotors and of full
micromagnetic simulations. The results are compared to the Stoner-Wohlfarth
model.
|
0309237v1
|
2004-01-12
|
Generalized Friedel sum rule and anomalies of the mobility due to the resonant scattering of electrons by donor impurities in semiconductors
|
On the basis of Friedel approach the theoretical description of the effects
of resonance scattering of conduction electrons by donor impurities in
semiconductors with allowance for the stabilization of electron concentration
in coinciding the Fermi energy with the resonance level energy has been
developed. It has been shown that such a stabilization gives rise to the
appearance of maximum in concentration dependence and to its related anomalies
of temperature dependences of electron mobility. The advantage of the
application of the approach based on the proposed theory to the interpretation
of experimental data on mercury selenide with iron impurities is discussed.
PACS: 72.20.Dp; 72.20.Fr Keywords: semiconductors, impurities in
semiconductors, electron states, electron transport
|
0401174v1
|
2004-02-13
|
Intersite elastic coupling and invar effect
|
The invar phenomenon (very small thermal expansion in some iron alloys or
compounds) is usually explained by the thermally-induced transitions between
different spin states of Fe, having different atomic volumes. We consider these
processes taking into account elastic interaction between Fe atoms in different
spin states. Inclusion of these interactions explains why thermal expansion may
be close to zero in a broad temperature interval and thus gives rise to the
invar effect.
|
0402374v1
|
2004-04-08
|
Projective Dynamics in Realistic Models of Nanomagnets
|
The free-energy extrema governing the magnetization-reversal process for a
model of an iron nanopillar are investigated using the projective dynamics
method. Since the time evolution of the model is computationally intensive, one
could question whether sufficient statistics can be generated, with current
resources, to evaluate the position of the metastable configuration.
Justification for the fine-grained discretization of the model that we use here
is given, and it is shown that tractable results can be obtained for this
system on realistic time scales.
|
0404198v1
|
2004-10-06
|
Spin and Valence-Fluctuation Mediated Superconductivity in Pressurized Fe and CeCu2(Si/Ge)2
|
We review the evidence supporting valence-fluctuation mediated
superconductivity in CeCu2Si2 and CeCu2Ge2, where Tc reaches 2.4 K at high
pressure. In these systems the valence and magnetic critical points, at p_V and
p_c respectively, are well separated. Characteristic signatures associated with
both phenomena are distinct. In contrast, the valence and spin fluctuation
regions appear much closer in most Ce based compounds. Concerning d-transition
metals, superconductivity in pure iron emerges in the pressure window 15-30 GPa
with the onset of Tc up to almost 3 K. All relevant observations point to
unconventional superconductivity, likely mediated by ferromagnetic spin
fluctuations.
|
0410143v1
|
2005-01-17
|
Jahn-Teller stabilization of a "polar" metal oxide surface: Fe3O4(001)
|
Using ab initio thermodynamics we compile a phase diagram for the surface of
Fe3O4(001) as a function of temperature and oxygen pressures. A hitherto
ignored polar termination with octahedral iron and oxygen forming a wave-like
structure along the [110]-direction is identified as the lowest energy
configuration over a broad range of oxygen gas-phase conditions. This novel
geometry is confirmed in a x-ray diffraction analysis. The stabilization of the
Fe3O4(001)-surface goes together with dramatic changes in the electronic and
magnetic properties, e.g., a halfmetal-to-metal transition.
|
0501402v1
|
2005-07-06
|
Inverse Magnetoresistance of Molecular Junctions
|
We present calculations of spin-dependent electron transport through single
organic molecules bridging pairs of iron nanocontacts. We predict the
magnetoresistance of these systems to switch from positive to negative with
increasing applied bias for both conducting and insulating molecules. This
novel inverse magnetoresistance phenomenon is robust, does not depend on the
presence of impurities, and is unique to molecular and atomic nanoscale
magnetic junctions. Its physical origin is identified and its relevance to
experiment and to potential technological applications is discussed.
|
0507126v2
|
2005-09-09
|
Correlated hybridization in transition metal complexes
|
We apply local orbital basis density functional theory (using SIESTA) coupled
with a mapping to the Anderson impurity model to estimate the Coulomb assisted
or correlated hybridization between transition metal d-orbitals and ligand
sp-orbitals for a number of molecular complexes. We find remarkably high values
which can have several physical implications including: (i) renormalization of
effective single band or multiband Hubbard model parameters for the cuprates
and, potentially, elemental iron, and (ii) spin polarizing molecular
transistors.
|
0509259v4
|
2005-09-12
|
Linear scaling Krylov subspace method for large scale {\it ab initio} electronic structure calculations of metals
|
An efficient and robust linear scaling method is presented for large scale
{\it ab initio} electronic structure calculations of a wide variety of
materials including metals. The detailed short range and the effective long
range contributions to the electronic structure are taken into account by
solving an embedded cluster defined in a Krylov subspace, which provides rapid
convergence for not only insulators but also metals.
As an illustration of the method, we present a large scale calculation based
on density functional theory for a palladium cluster with a single iron
impurity.
|
0509291v1
|
2005-12-19
|
Experimental Evidence for Crossed Andreev Reflection
|
We report on electronic transport properties of mesoscopic
superconductor-ferromagnet spin-valve structures. Two ferromagnetic iron leads
form planar tunnel contacts to a superconducting aluminum wire, where the
distance of the two contacts is of the order of the coherence length of the
aluminum. We observe a negative non-local resistance which can be explained by
crossed Andreev reflection, a process where an electron incident from one of
the leads gets reflected as a hole into the other, thereby creating a pair of
spatially separated, entangled particles.
|
0512445v1
|
2005-12-19
|
Spin injection into a short DNA chain
|
Quantun spin transport through a short DNA chain connected to ferromagnetic
electrodes has been investigated by the transfer matrix method. We describe the
system by a tight-binding model where the parameters are extracted from the
experimental data and realistic metal energy bands. For ferromagnetic iron
electrodes, the magnetoresistance of a 30-basepair Poly(G)-Poly(C) DNA is found
to be lower than 10% at a bias of < 4 V, but can rach up to 20% at a bias of 5
V. In the presence of the spin-flip mechanism, the magnetoresistance is
significantly enhanced when the spin-flip coupling is weak but as the coupling
becomes stronger the decreasing magnetoresistance develops an oscillatory
behavior.
|
0512473v1
|
2005-12-22
|
Point defect concentrations in metastable Fe-C alloys
|
Point defect species and concentrations in metastable Fe-C alloys are
determined using density functional theory and a constrained free-energy
functional. Carbon interstitials dominate unless iron vacancies are in
significant excess, whereas excess carbon causes greatly enhances vacancy
concentration. Our predictions are amenable to experimental verification; they
provide a baseline for rationalizing complex microstructures known in hardened
and tempered steels, and by extension other technological materials created by
or subjected to extreme environments.
|
0512598v2
|
2005-12-31
|
Physical nature of fcc-bcc martensitic transformation in iron based alloys
|
The summary of the models offered by the author revealing features of the
physical mechanisms controlling processes of martensite crystal formation is
resulted. The rapid growth of a cooling martensite crystal is considered as a
self-organized process controlled by the quasi-longitudinal lattice
displacement waves (DW). It is shown, that processes of the heterogeneous
nucleation and wave growth have the genetic connection in case of spontaneous
fcc-bcc martensitic transformation. The exposition of strain martensite
formation is considered in the context of a cryston model.
|
0601002v1
|
2006-02-03
|
Low Ghz loss in sputtered epitaxial Fe
|
We show that sputtered, pure epitaxial iron films can have high-frequency
loss as low as, or lower than, any known metallic ferromagnetic
heterostructure. Minimum 34 Ghz ferromagnetic resonance (FMR) linewidths of 41
Oe are demonstrated, some ~ 5-10 % lower than the previous minimum reported for
molecular beam epitaxially (MBE) deposited Fe. Intrinsic and extrinsic damping
have been separated over 0-40 Ghz, giving a lower bound for intrinsic LL(G)
relaxation rates of lambda or G = 85 MHz (alpha = 0.0027) and extrinsic 50 Mhz.
Swept frequency measurements indicate the potential for integrated frequency
domain devices with Q>100 at 30-40 Ghz.
|
0602094v1
|
2006-06-01
|
Structure peculiarities of cementite and their influence on the magnetic characteristics
|
The iron carbide $Fe_3C$ is studied by the first-principle density functional
theory. It is shown that the crystal structure with the carbon disposition in a
prismatic environment has the lowest total energy and the highest energy of
magnetic anisotropy as compared to the structure with carbon in an octahedron
environment. This fact explains the behavior of the coercive force upon
annealing of the plastically deformed samples. The appearance of carbon atoms
in the octahedron environment can be revealed by Mossbauer experiment.
|
0606024v1
|
2007-02-06
|
Comment on Resonant X-ray diffraction studies on the charge ordering in magnetite
|
In a recent letter, E. Nazarenko et al [Phys. Rev. Lett. 97, 056403 (2006)
and cond-mat/0606596] have investigated the low temperature phase of magnetite
by means of resonant X-ray scattering. The paper puts forward the quantitative
determination of an effective charge ordering (CO) of 0.24 electron among the
octahedral iron atoms in the insulating phase. The comment puts in evidence
that the analysis performed by Nazarenko et al is wrong and that some of their
conclusions are unsupported .
|
0702135v2
|
2001-02-23
|
The fate of classical tensor inhomogeneities in pre-big-bang string cosmology
|
In pre-big-bang string cosmology one uses a phase of dilaton-driven inflation
to stretch an initial (microscopic) spatial patch to the (much larger) size of
the big-bang fireball. We show that the dilaton-driven inflationary phase does
not naturally iron out the initial classical tensor inhomogeneities unless the
initial value of the string coupling is smaller than 10^(-35).
|
0102102v1
|
1995-08-02
|
Search for Magnetic Monopoles Trapped in Matter
|
There have been many searches for magnetic monopoles in flight, but few for
monopoles in matter. We have searched for magnetic monopoles in meteorites,
schists, ferromanganese nodules, iron ores and other materials. The detector
was a superconducting induction coil connected to a SQUID (Superconducting
Quantum Interference Device) with a room temperature bore 15 cm in diameter. We
tested a total of more than 331 kg of material including 112 kg of meteorites.
We found no monopole and conclude the overall monopole/nucleon ratio in the
samples is $<1.2 \times 10^{-29}$ with a 90\% confidence level.
|
9508003v1
|
1998-12-11
|
The OPERA Experiment
|
OPERA(Oscillation Project with Emulsion-tRacking Apparatus) is a new detector
concept, iron(lead)-emulsion for a long-baseline neutrino oscillation
experiment. This experiment would perform an appearance search for nu_mu-nu_tau
oscillation in the parameter region indicated by the atmospheric neutrino
anomaly. OPERA can run at the Gran Sasso Laboratory in the proposed NGS
(Neutrino to Gran Sasso) Beam from CERN.
|
9812015v3
|
1999-03-22
|
Study of the Hadron Shower Profiles with the Tile Hadron Calorimeter
|
The lateral and longitudinal profiles of the hadronic showers detected by
iron-scintillator tile hadron calorimeter with longitudinal tile configuration
have been investigated. The results are based on 100 GeV pion beam data. Due to
the beam scan provided many different beam impact locations with cells it is
succeeded to obtain detailed picture of transverse shower behavior. The
underlying radial energy densities for four depths and for overall calorimeter
have been reconstructed. The three-dimensional hadronic shower parametrisation
have been suggested.
|
9903052v1
|
1999-08-20
|
Precision Calibration of the NuTeV Calorimeter
|
NuTeV is a neutrino-nucleon deep-inelastic scattering experiment at Fermilab.
The detector consists of an iron-scintillator sampling calorimeter interspersed
with drift chambers, followed by a muon toroidal spectrometer. We present
determinations of response and resolution functions of the NuTeV calorimeter
for electrons, hadrons, and muons over an energy range of 4.8 to 190 GeV. The
absolute hadronic energy scale is determined to an accuracy of 0.43%. We
compare our measurements to predictions from calorimeter theory and GEANT3
simulations.
|
9908056v1
|
1999-12-23
|
Recent atmospheric neutrino results from Soudan 2
|
An updated measurement of the atmospheric nu_mu/nu_e ratio-of-ratios,
0.68+-0.11+-0.06, has been obtained using a 4.6-kty exposure of the Soudan-2
iron tracking calorimeter. The L/E distributions have been analyzed for effects
of nu_mu -> nu_x oscillations, and an allowed region in the Delta m^2 vs. sin^2
2 theta plane has been determined.
|
9912060v1
|
2001-09-18
|
Nuclear transparency from quasielastic A(e,e'p) reactions uo to Q^2=8.1 (GeV/c)^2
|
The quasielastic (e,e$^\prime$p) reaction was studied on targets of
deuterium, carbon, and iron up to a value of momentum transfer $Q^2$ of 8.1
(GeV/c)$^2$. A nuclear transparency was determined by comparing the data to
calculations in the Plane-Wave Impulse Approximation. The dependence of the
nuclear transparency on $Q^2$ and the mass number $A$ was investigated in a
search for the onset of the Color Transparency phenomenon. We find no evidence
for the onset of Color Transparency within our range of $Q^2$. A fit to the
world's nuclear transparency data reflects the energy dependence of the free
proton-nucleon cross section.
|
0109027v1
|
2002-01-10
|
A method for detecting $ν_τ$ appearance in the spectra of quasielastic CC events
|
A method for detecting the transition \omutau in long-baseline accelerator
experiments, that consists in comparing the far-to-near ratios of the spectra
of quasielastic CC events generated by high- and low-energy beams of muon
neutrinos, is proposed. The test may be accessible to big water Cherenkov
detectors and iron--scintillator calorimeters, and is limited by statistics
rather than systematics.
|
0201019v1
|
2003-07-02
|
NuTeV Cross Section and Structure Function Measurements
|
The NuTeV experiment has obtained a unique high statistics sample of neutrino
and antineutrino interactions using its high-energy sign-selected beam.
Charged-current neutrino and anti-neutrino differential cross sections are
extracted. Neutrino-Iron structure functions, F_2(x,Q^2) and xF_3(x,Q^2), are
determined by fitting the y-dependence of the differential cross sections.
NuTeV has precise understanding of its hadron and muon energy scales, which
improves the systematic precision of this measurement.
|
0307005v2
|
2005-07-02
|
The MINOS Detectors
|
The Main Injector Neutrino Oscillation Search (MINOS) experiment's primary
goal is the precision measurement of the neutrino oscillation parameters in the
atmospheric neutrino sector. This long-baseline experiment uses Fermilab's NuMI
beam, measured with a Near Detector at Fermilab, and again 735 km later using a
Far Detector in the Soudan Mine Underground Lab in northern Minnesota. The
detectors are magnetized iron/scintillator calorimeters. The Far Detector has
been operational for cosmic ray and atmospheric neutrino data from July of
2003, the Near Detector from September 2004, and the NuMI beam started in early
2005. This poster presents details of the two detectors.
|
0507018v1
|
2005-07-09
|
NuTeV Structure Function Measurement
|
The NuTeV experiment obtained high statistics samples of neutrino and
anti-neutrino charged current events during the 1996-1997 Fermilab fixed target
run. The experiment combines sign-selected neutrino and anti-neutrino beams and
the upgraded CCFR iron-scintillator neutrino detector. A precision continuous
calibration beam was used to determine the muon and hadron energy scales to a
precision of 0.7% and 0.43% respectively. The structure functions F_2(x,Q^2)
and xF_3(x,Q^2) obtained by fitting the y-dependence of the sum and the
difference of the neutrino and anti-neutrino differential cross sections are
presented.
|
0507040v1
|
1996-07-10
|
QCD analysis of the CCFR data for $xF_3$ and Higher--Twist Contribution
|
The QCD analysis of the $xF_3$ structure function measured in deep-inelastic
scattering of neutrinos and antineutrinos on an iron target at the Fermilab
Tevatron is done in 1--, 2-- and 3--loop order of QCD. The x dependence of the
higher--twist contribution is evaluated. The experimental value of
higher--twist corrections to the Gross--Llewellyn Smith sum rule is discussed.
|
9607275v2
|
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