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2017-10-26
|
SDSS J090152.05+624342.6: A New "Overlapping-Trough" FeLoBAL Quasar At Z$\sim2$
|
We here report an identification of SDSS J090152.04+624342.6 as a new
"overlapping-trough" iron low-ionization broad absorption line quasar at
redshift of $z\sim2.1$. No strong variation of the broad absorption lines can
be revealed through the two spectra taken by the Sloan Digital Sky Survey with
a time interval of $\sim6$yr. Further optical and infrared spectroscopic study
on this object is suggested.
|
1710.09527v1
|
2017-11-03
|
Supernova β^- decay of nuclides 53Fe, 54Fe, 55Fe, and 56Fe in strongly screened plasma
|
The electron screening strong effect on the electron energy and threshold
energy of the beta decay reaction. in this paper, we study the $\beta^-$ decay
rates of some iron isotopes. The electron screening beta decay rates increase
by about two orders of magnitude. The strong screening beta decay rates due to
Q-value correction are by more than one order of magnitude higher than those of
without Q-value correction.
|
1711.01955v3
|
2017-11-29
|
The Memory Function Formalism: A Review
|
An introduction to the Zwanzig-Mori-G\"{o}tze-W\"{o}lfle memory function
formalism (or generalized Drude formalism) is presented. This formalism is used
extensively in analyzing the experimentally obtained optical conductivity of
strongly correlated systems like cuprates and Iron based superconductors etc.
For a broader perspective both the generalised Langevin equation approach and
the projection operator approach for the memory function formalism are given.
The G\"{o}tze-W\"{o}lfle perturbative expansion of memory function is presented
and its application to the computation of the dynamical conductivity of metals
is also reviewd. This review of the formalism contains all the mathematical
details for pedagogical purposes.
|
1711.10758v1
|
2017-11-30
|
A note on the contribution of multi-photon processes to radiative opacity
|
Recently, Bailey et al. performed iron opacity measurements on the Z machine
at Sandia National Laboratory in conditions close to the ones of the base of
the convective zone of the Sun. Such experiments have raised questions about
the physical models commonly used in opacity codes. To understand the
discrepancy between experiment and theory, More et al. investigated the role of
two-photon processes. In the present work we show, by a simple estimate and
using hydrogenic formulas, that due to the intensity of the backlight radiation
seen by the sample, such processes are likely to play an important role only
for highly excited states.
|
1711.11295v1
|
2017-12-10
|
Visible evidence to magnetism of graphene oxide
|
Graphene oxide continues to amaze scientific community for multiple
potentials in a broad span of applications such as catalysts, adsorbents,
oxidants, etc., determined by constant unveiling of its fantastic natures. Of
them, magnetism is not ultimately identified and directly observed by naked
eyes. Herein, we report graphene oxide directionally migrated and deposited
together simply under external magnetic field from common Nd-Fe-B magnet,
whereas the ferromagnetism of graphene oxide did not exhibit that strong as
iron. Therefore, we illustrated this interesting pathway to keep close to such
2D carbon materials and potentially promoted magnetic-oriented applications.
|
1712.03570v4
|
2017-12-22
|
Novel Method for Background Phase Removal on MRI Proton Resonance Frequency Measurements
|
MR images have a magnitude and a phase, but in almost all clinical
applications only the magnitude images are used, because the phase images have
a smooth but strong background signal that masks useful information. The phase
contains information such as the temperature during thermal ablation, and the
iron content of brain tissue. We describe a novel method to suppress the
background that is based on higher order edge detection and sparse image
representation.
|
1712.08651v1
|
2018-02-23
|
Electronic entropy in Green's-function calculations at finite temperatures
|
We revise critically existing approaches to evaluation of thermodynamic
potentials within the Green's function calculations at finite electronic
temperatures. We focus on the entropy and show that usual technical problems
related to the multivalued nature of the complex logarithm can be overcome.
This results in a simple expression for the electronic entropy, which does not
require any contour integration in the complex energy plane. Properties of the
developed formalism are discussed and its illustrating applications to selected
model systems and to bcc iron with disordered local magnetic moments are
presented as well.
|
1802.08495v1
|
2018-03-01
|
High Temperature Electronic Structure with KKR
|
Modeling high temperature (10's or 100's of eV), dense plasmas is challenging
due to the multitude of non-negligible physical effects including significant
partial ionization and multi-site effects. These effects cause the breakdown or
intractability of common methods and approximations used at low temperatures,
such as pseudopotentials or plane wave basis sets. Here we explore the KKR
Green's function method at these high temperature conditions. The method is
all-electron, does not rely on pseudopotentials, and uses a spherical harmonic
basis set, and so avoids the aforementioned limitations. It is found to be
accurate for solid density aluminum and iron plasmas compared when compared to
a plane wave method at low temperature, while being able to access high
temperatures.
|
1803.00596v1
|
2018-03-18
|
Effect of gap anisotropy on the spin resonance peak in the superconducting state of iron-based materials
|
Spin resonance in the superconducting state of Fe-based materials within the
multiorbital model with unequal anisotropic gaps on different Fermi surface
sheets is studied. On the basis of the model gap function and the one
calculated within the spin fluctuation theory of pairing, I show that the
resonance peak shifts to higher frequencies with increasing the zero-amplitude
gap magnitude. On the contrary, with increasing the gap anisotropy, it shifts
to lower frequencies and lose some intensity.
|
1803.06736v2
|
2018-05-03
|
Binarizer at SemEval-2018 Task 3: Parsing dependency and deep learning for irony detection
|
In this paper, we describe the system submitted for the SemEval 2018 Task 3
(Irony detection in English tweets) Subtask A by the team Binarizer. Irony
detection is a key task for many natural language processing works. Our method
treats ironical tweets to consist of smaller parts containing different
emotions. We break down tweets into separate phrases using a dependency parser.
We then embed those phrases using an LSTM-based neural network model which is
pre-trained to predict emoticons for tweets. Finally, we train a
fully-connected network to achieve classification.
|
1805.01112v1
|
2018-05-15
|
Nuclear EMC effect through $\barΛ/Λ$ production in semi-inclusive deep-inelastic scattering processes
|
We calculate $\Lambda$ and $\bar{\Lambda}$ hadron production cross sections
in charged lepton semi-inclusive deep-inelastic scattering off nuclear target
($A$, using iron $\mathrm{Fe}$ as an example) and deuteron ($D$) target. The
results show that the ratio
$(\bar{\Lambda}^{A}/\Lambda^{A})/(\bar{\Lambda}^{D}/\Lambda^{D})$ is sensitive
to the sea quark content of the nucleus. We adopt three different models to
take the nuclear EMC effect into account. The ratio
$(\bar{\Lambda}^{A}/\Lambda^{A})/(\bar{\Lambda}^{D}/\Lambda^{D})$ is predicted
to be different by these different models.
|
1805.05544v1
|
2018-05-16
|
Lattice distortions and/or intercalation as ways to induce magnetism in a-FeSi2: a theoretical study
|
The possibilities to induce magnetism in the non-magnetic bulk alpha-FeSi2 by
means of lattice distortions or intercalation with metal or non-metal ions of
light elements is investigated theoretically by combined ab initio and model
methods. We find that the distortions indeed can induce the formation of
magnetic moment on iron atoms in certain local environments; however, the
required strength of the distortions often is too large to be achieved in
experiments. For this reason we suggest using "chemical pressure" that is,
intercalating the alpha-FeSi2 films by light elements. We find that some of
such variants have promising characteristic.
|
1805.06189v1
|
2018-05-25
|
Pairing Mechanism in Hunds Metal Superconductors and the Universality of the Superconducting Gap to Critical Temperature Ratio
|
We analyze a simple model containing the physical ingredients of a Hund's
metal, the local spin fluctuations with power-law correlators,
$(\Omega_0/|\Omega|)^\gamma$, with $\gamma$ greater than one, interacting with
electronic quasiparticles. While the critical temperature and the gap change
significantly with varying parameters, the $2\Delta_{max}/k_BT_c$ remains close
to twice the BCS value in agreement with experimental observations in the
iron-based superconductors (FeSC).
|
1805.10280v2
|
2018-05-30
|
Charged and Neutral Current Pion Production in Neutrino-Nucleus Scattering
|
In this article, we present the charged and neutral current coherent pion
production in the neutrino-nucleus interaction in the resonance region using
the formalism based on the partially conserved axial current (PCAC) theorem
which relates the neutrino-nucleus cross section to the pion-nucleus elastic
cross section. The pion nucleus elastic cross section is calculated using the
Glauber model approach. We calculate the integrated cross sections for
neutrino-carbon, neutrino-iron and neutrino-oxygen scattering. The results of
integrated cross-section calculations are compared with the measured data
|
1805.11836v1
|
2018-06-08
|
The Role of Opacities in Stellar Pulsation
|
We examine the role of opacities in stellar pulsation with reference to
Cepheids and RR Lyraes, and examine the effect of augmented opacities on the
theoretical pulsation light curves in key temperature ranges. The temperature
ranges are provided by recent experimental and theoretical work that have
suggested that the iron opacities have been considerably underestimated. For
Cepheids, we find that the augmented opacities have noticeable effects in
certain period ranges (around $\log P \approx 1$) even though there is a
degeneracy with mixing length. We also find significant effects in theoretical
models of B-star pulsators.
|
1806.02996v1
|
2018-08-01
|
Long range dynamical coupling between magnetic adatoms mediated by a 2D topological insulator
|
We study the spin excitation spectra and the dynamical exchange coupling
between iron adatoms on a Bi bilayer nanoribbon. We show that the topological
character of the edge states is preserved in the presence of the magnetic
adatoms. Nevertheless, they couple significantly to the edge spin currents, as
witnessed by the large and long-ranged dynamical coupling we obtain in our
calcula- tions. The large effective magnetocrystalline anisotropy of the
magnetic adatoms combined with the transport properties of the topologically
protected edge states make this system a strong candidate for implementation of
spintronics devices and quantum information and/or computation protocols.
|
1808.00347v1
|
2018-08-07
|
Origin of superconductivity in Ni-doped SrFe_2As_2 , an Insight from DMFT
|
I describe the issues of the competing orders in normal state of a parent and
Ni-doped iron pnictide superconductor, SrF e 2 As 2 , using LMTO band structure
calculation plus multi orbital dynamical mean-field theory. Strong, electronic
correlations along with minimal antiferromagnetic instability create a
partially gapped Fermi surface, in qualitative agreement with earlier
experiments. A good semiquantitative similarity in both normal and ordered
state with the earlier experimental transport data is observed along with
suppression of magnetic ordering and explained within a correlated, mul-
tiorbital viewpoint. These results suggest that soft electronic fluctuation
mediate superconductivity in broad class of parent and underdoped 122 pnictides
with suppression of magnetic ordering.
|
1808.02298v1
|
2019-11-12
|
Classification of materials with phonon angular momentum and microscopic origin of angular momentum
|
We group materials into five symmetry classes and determine in which of these
classes phonons carry angular momentum in the Brillouin zone, away from a
high-symmetry point, line, or plane. In some materials phonons acquire angular
momentum via the forces induced by relative displacements of atoms out of their
equilibrium positions. However, for other materials, such as ferromagnetic
iron, phonon angular momentum arises from the forces induced by relative
velocities of atoms. These effects are driven by the spin-orbit interaction.
|
1911.05064v3
|
2020-01-27
|
Influence of Iron Losses on Switching Dynamics of an Electromagnet from Experiment and Simulation
|
The switching behaviour of electromagnets is determined by the inertia of the
armature, the stiffness of the return spring and the magnetostatic forces
between armature and yoke. For highly dynamic systems, hysteresis and eddy
current losses have a slowing effect. In this paper we consider the
experimentally observed behaviour of a switching magnet and compare it with
simulation results including hysteresis and eddy current losses.
|
2001.09638v1
|
2020-05-21
|
Cluster Dynamics Modeling of Niobium and Titanium Carbide Precipitates
|
Kinetics of niobium and titanium carbide precipitates in iron has been
simulated with cluster dynamics. The simulations, carried out in austenite and
ferrite for niobium carbides, respectively in austenite for titanium carbide,
were analyzed for dependency on temperature, solute concentration, and initial
cluster distribution. The results are presented for different temperatures and
solute concentrations and compared to available experimental data. They show
little impact of initial cluster distribution beyond a certain relaxation time
and that highly dilute alloys with only monomers present a significantly
different behavior than less dilute alloys or alloys with different initial
cluster distribution.
|
2005.10574v2
|
2020-07-12
|
Reconstruction of 3D Image of Nanorice Particle from Randomly Oriented Single-Shot Experimental Diffraction Patterns Using Angular Correlation Method
|
We reconstructed intensities in Fourier space and electron densities in the
real space for an azimuthally symmetric object Nanorice particle (Iron Oxide
nanoparticle) exposed in the ultrashort, bright and coherent x-ray free
electron laser (XFEL) pulses with random unknown orientations through the
method of angular correlations among intensities appeared in ninety eight 2D
diffraction patterns collected at Linac Coherent Light Source (LCLS).
|
2007.06030v1
|
2021-06-17
|
Effects of Covid-19 Pandemic on Chinese Commodity Futures Markets
|
In this study, empirical moments and the cointegration for all the liquid
commodity futures traded in the Chinese futures markets are analyzed for the
periods before and after Covid-19, which is important for trading strategies
such as pairs trading. The results show that the positive change in the average
returns of the products such as soybean, corn, corn starch, and iron ore
futures are significantly stronger than other products in the post Covid-19
era, whereas the volatility increased most for silver, petroleum asphalt and
egg futures after the pandemic started. The number of cointegrated pairs are
reduced after the pandemic indicating the differentiation in returns due to the
structural changes caused in the demand and supply conditions across
commodities.
|
2106.09250v1
|
2021-06-28
|
Strap-on magnets: a framework for rapid prototyping of magnets and beam lines
|
We describe a framework to assemble permanent-magnet cubes in 3D-printed
frames to construct dipole, quadrupole, and solenoid magnets, whose field, in
the absence of iron, can be calculated analytically in three spatial
dimensions. Rotating closely spaced dipoles and quadrupoles in opposite
directions allows us to adjust the integrated strength of a multipole.
Contributions of unwanted harmonics are calculated and found to be moderate. We
then combine multiple magnets to construct beam-line modules: chicane, triplet
cell, and solenoid focusing system.
|
2106.14676v1
|
2021-07-08
|
Warm Magnets
|
Warm magnets are magnets that function in normal ambient temperature
conditions. These types are mostly using a soft steel yoke for field
amplification and either Copper or Aluminium coils or permanent magnets to
generate the field. Magnets powered with such normal-conducting coils are often
also called classical, iron dominated or resistive magnets. Since decades these
types of magnets are the workhorse for most linear and circular accelerators
and beam transfer lines.
|
2107.03965v1
|
2021-11-08
|
Thermogravimetric study of the non-stoichiometric wüstite FeOx. Pseudo phases w i and w' i . New T-P-X equilibrium phase diagram
|
A statistical numerical analysis of thermogravimetric measurements
(temperature, mass variation, oxygen pressure) leads to a new equilibrium phase
diagram T-P-X of the iron monoxide or w\"ustite FeOx. Three varieties or pseudo
phases W1, W2, W3 are revealed above 911 {\textdegree}C, three others W'1, W'2,
W'3 below. This diagram displays their subfields, and shows 25 invariant points
between stable phases and 8 possible invariant points between metastable
phases. A thermodynamic approach is pointed out, which makes possible to
characterize the point defects and their clusters as components of the crystal
lattice.
|
2111.04346v1
|
2017-04-19
|
Remote Document Encryption - encrypting data for e-passport holders
|
We show how any party can encrypt data for an e-passport holder such that
only with physical possession of the e-passport decryption is possible. The
same is possible for electronic identity cards and driver licenses. We also
indicate possible applications. Dutch passports allow for 160 bit security,
theoretically giving sufficient security beyond the year 2079, exceeding
current good practice of 128 bit security. We also introduce the notion of RDE
Extraction PIN which effectively provides the same security as a regular PIN.
Our results ironically suggest that carrying a passport when traveling abroad
might violate export or import laws on strong cryptography.
|
1704.05647v3
|
2017-05-02
|
Computation of the unifying thread in high temperature superconductors from first principles quantum Monte Carlo
|
It has long been a challenge to describe the origin of unconventional
superconductivity. The two known examples with high Tc, based on iron and
copper, have very different electronic structures, while other materials with
similar electronic structure may not show superconductivity at all. In this
paper, the authors show that by using high accuracy diffusion Monte Carlo
calculations, the unconventional superconductors of both high Tc types form a
cluster at intermediate spin-charge coupling. The spin-charge coupling may
serve as a normal state marker for unconventional superconductivity, and
provides evidence that unconventional superconductivity is due to interaction
of charge with local spins in materials.
|
1705.01008v1
|
2017-06-09
|
Ultrasound attenuation in $s^\pm$-wave two-band superconductors
|
The two-band $s^{\pm}$-wave state is currently considered to be the most
promising candidate for newly discovered iron-based high-$T_c$ superconductors.
In this work we study theoretically the ultrasound attenuation in
$s^{\pm}$-wave two-band superconductors. The impurity effect is calculated
within the $\mathcal{T}$-matrix approximation. In particular, our theory
predict that, when the sizes of two order parameter are comparable, a
Hebel-Slichter peak may show up in the ultrasound attenuation versus
temperature curves. Our calculations also confirmed the presence of the
resonant impurity scattering at low temperature, observed previously by other
authors in the calculation of the NMR relaxation rate $1/T_{1}$.
|
1706.02963v1
|
2018-02-23
|
Electronic entropy in Green's-function calculations at finite temperatures
|
We revise critically existing approaches to evaluation of thermodynamic
potentials within the Green's function calculations at finite electronic
temperatures. We focus on the entropy and show that usual technical problems
related to the multivalued nature of the complex logarithm can be overcome.
This results in a simple expression for the electronic entropy, which does not
require any contour integration in the complex energy plane. Properties of the
developed formalism are discussed and its illustrating applications to selected
model systems and to bcc iron with disordered local magnetic moments are
presented as well.
|
1802.08495v1
|
2018-08-01
|
Long range dynamical coupling between magnetic adatoms mediated by a 2D topological insulator
|
We study the spin excitation spectra and the dynamical exchange coupling
between iron adatoms on a Bi bilayer nanoribbon. We show that the topological
character of the edge states is preserved in the presence of the magnetic
adatoms. Nevertheless, they couple significantly to the edge spin currents, as
witnessed by the large and long-ranged dynamical coupling we obtain in our
calcula- tions. The large effective magnetocrystalline anisotropy of the
magnetic adatoms combined with the transport properties of the topologically
protected edge states make this system a strong candidate for implementation of
spintronics devices and quantum information and/or computation protocols.
|
1808.00347v1
|
2018-08-07
|
Origin of superconductivity in Ni-doped SrFe_2As_2 , an Insight from DMFT
|
I describe the issues of the competing orders in normal state of a parent and
Ni-doped iron pnictide superconductor, SrF e 2 As 2 , using LMTO band structure
calculation plus multi orbital dynamical mean-field theory. Strong, electronic
correlations along with minimal antiferromagnetic instability create a
partially gapped Fermi surface, in qualitative agreement with earlier
experiments. A good semiquantitative similarity in both normal and ordered
state with the earlier experimental transport data is observed along with
suppression of magnetic ordering and explained within a correlated, mul-
tiorbital viewpoint. These results suggest that soft electronic fluctuation
mediate superconductivity in broad class of parent and underdoped 122 pnictides
with suppression of magnetic ordering.
|
1808.02298v1
|
2018-08-19
|
Size-dependent phase morphologies in LiFePO4 battery particles
|
Lithium iron phosphate (LiFePO$_4$) is the prototypical two-phase battery
material, whose complex patterns of lithium ion intercalation provide a testing
ground for theories of electrochemical thermodynamics. Using a depth-averaged
(a-b plane) phase-field model of coherent phase separation driven by Faradaic
reactions, we reconcile conflicting experimental observations of diamond-like
phase patterns in micron-sized platelets and surface-controlled patterns in
nanoparticles. Elastic analysis predicts this morphological transition for
particles whose a-axis dimension exceeds the bulk elastic stripe period. We
also simulate a rich variety of non-equilibrium patterns, influenced by
size-dependent spinodal points and electro-autocatalytic control of
thermodynamic stability.
|
1808.06164v1
|
2018-08-20
|
Production of Metal-free Diamond Nanoparticles
|
In this paper, the controlled production of high quality metal-free diamond
nanoparticles is demonstrated. Milling with tempered steel is shown to leave
behind iron oxide contamination which is difficult to remove. Milling with SiN
alleviates this issue but generates more non diamond carbon. Thus the choice of
milling materials is critically determined by the acceptable contaminants in
the ultimate application. The removal of metal impurities, present in all
commercially available nanoparticles, will open new possibilities towards the
production of customised diamond nanoparticles, covering the most demanding
quantum applications.
|
1808.06425v1
|
2018-10-14
|
Plasma opacity calculations using the Starrett and Saumon average-atom model with ion correlations
|
We present the opacities of iron, aluminum, and bromine plasmas calculated
using the Starrett and Saumon average-atom model allowing for ion correlations.
We show that the use of earlier average-atom ion-correlation model of Rozsnyai,
as has recently been done in the solar opacity calculations, overestimates the
effect of ion correlations on plasma opacities. The reason for this
overestimation is discussed.
|
1810.06027v1
|
2019-01-03
|
A magnetic tight-binding model : surface properties of transition metals and cobalt nanoparticules
|
The magnetic and surface properties of some transition metals have been
investigated through the tight-binding approximation including Coulomb
correlations. These surface properties are derived from a charge neutrality
rule restricted to the d-band leading to a charge distribution including sp
surface states in agreement with a Linear Muffin-Tin Orbital (LMTO)
calculation. This new approach describes the local magnetism, surface energies
and work functions without recourse to the total energy. Our investigation
focuses on fcc cobalt, bcc iron, fcc nickel and fcc platinum surfaces with an
exploration of fcc cobalt nanoparticles.
|
1901.00841v4
|
2019-01-28
|
X-ray diffraction analysis to support phase identification in FeSe and Fe$_7$Se$_8$ epitaxial thin films
|
X-ray diffraction (XRD) data and analysis for epitaxial iron selenide thin
films grown by pulsed laser deposition (PLD) are presented. The films contain
${\beta}$-FeSe and Fe$_7$Se$_8$ phases in a double epitaxy configuration with
the ${\beta}$-FeSe phase (001) oriented on the (001) MgO growth substrate.
Fe$_7$Se$_8$ simultaneously takes on two different epitaxial orientations in
certain growth conditions, exhibiting both (101)- and (001)- orientations. Each
of these orientations are verified with the presented XRD data. Additionally,
XRD data used to determine the PLD target composition as well as mosaic
structure of the ${\beta}$-FeSe phase are shown.
|
1901.09794v1
|
2019-07-17
|
Role of interactions in the energy of the spin resonance peak in Fe-based superconductors
|
We consider the spin response within the five-orbital model for iron-based
superconductors and study two cases: equal and unequal gaps in different bands.
In the first case, the spin resonance peak in the superconducting state appears
below the characteristic energy scale determined by the gap magnitude,
$2\Delta_L$. In the second case, the energy scale corresponds to the sum of
smaller and larger gap magnitudes, $\Delta_L + \Delta_S$. Increasing the values
of the Hubbard interaction and the Hund's exchange, we observe a shift of the
spin resonance energy to lower frequencies.
|
1907.08133v1
|
2019-10-07
|
Thermodynamic dislocation theory for polycrystals under tension/compression
|
Starting from the assumption that all possible orientations of grains are
equally probable, we prove that the geometric factor of thermodynamic
dislocation theory for polycrystals subjected to axially symmetric tension or
compression must be equal to 2. We then use large-scale least-square analysis
to identify the physics based parameters of this theory and show that the
simulated stress-strain curves for OFHC copper, ARMCO iron and 4340 steel agree
well with the experiments of Johnson and Cook.
|
1910.02642v2
|
2019-10-25
|
Portable system for the prediction of anemia based on the ocular conjunctiva using Artificial Intelligence
|
Anemia is a major health burden worldwide. Examining the hemoglobin level of
blood is an important way to achieve the diagnosis of anemia, but it requires
blood drawing and a blood test. In this work we propose a non-invasive, fast,
and cost-effective screening test for iron-deficiency anemia in Peruvian young
children. Our initial results show promising evidence for detecting
conjunctival pallor anemia and Artificial Intelligence techniques with photos
taken with a popular smartphone.
|
1910.12399v1
|
2020-03-04
|
Classical Nova Carinae 2018: Discovery of circumbinary iron and oxygen
|
We present time-lapse spectroscopy of a classical nova explosion commencing 9
days after discovery. These data reveal the appearance of a transient feature
in Fe II and [O I]. We explore different models for this feature and conclude
that it is best explained by a circumbinary disc shock-heated following the
classical nova event. Circumbinary discs may play an important role in novae in
accounting for the absorption systems known as THEA, the transfer of angular
momentum, and the possible triggering of the nova event itself.
|
2003.02008v1
|
2020-08-21
|
Multipole Orders in a Chain Ferrate Na2FeSe
|
Fundamental block and staggered orders of magnetic Fe multipoles in Na2FeSe2
are classified by their symmetry and magnetoelectric properties. Our structure
model incorporates ferromagnetic or antiferromagnetic coupling between chains.
The ferrate salt is valued in studies of highly correlated electrons as the
only iron selenide known to possess chain-like structural units hosting ferrous
cations. Axial and polar (Dirac) multipoles are compulsory in the electronic
structure since Fe ions exhibit enantiomorphic symmetry in the parent
K2ZnO2-type compound. Calculated Bragg diffraction patterns for neutrons and
x-rays reveal specific contributions from both multipole types.
|
2008.09374v1
|
2020-09-07
|
Anomalous In-plane Magnetic Anisotropy in Strain-mediated Converse Magnetoelectric Coupling
|
Magnetic axis rotation (MAR) in ferromagnetic (FM) layers is crucial for
strain-mediated converse magnetoelectric coupling. Employing the density
functional theory (DFT), we computationally study the magnetic anisotropy of
selected deformed FM materials such as body-centered iron. The results show
that the short axis is more energy-favorable at high in-plane strain difference
than previously predicted phenomenologically. This anomalous trend and the
complex energy behaviors at different strain conditions explain why
spin-lattice dynamics (SLD) simulation does not produce in-plane MAR and imply
couplings between different energy terms together with high order coefficient
contributions.
|
2009.03045v2
|
2020-10-27
|
Thermodynamic approach to electric quadrupole moments
|
Higher-rank electric/magnetic multipole moments are attracting attention
these days as candidate order parameters for exotic material phases. However,
quantum-mechanical formulation of those multipole moments is still an ongoing
issue. In this paper, we propose a thermodynamic definition of electric
quadrupole moments as a measure of symmetry breaking, following previous
studies of orbital magnetic dipole moments and magnetic quadrupole moments. The
obtained formulas are illustrated with a model of orbital-ordered nematic
phases of iron-based superconductors.
|
2010.13999v1
|
2020-11-18
|
Spin superfluidity in noncollinear antiferromagnets
|
We explore the spin superfluid transport in exchange interaction dominated
three-sublattice antiferromagnets. The system in the long-wavelength regime is
described by an $SO(3)$ invariant field theory. Additional corrections from
Dzyaloshinskii-Moriya interactions or anisotropies can break the symmetry;
however, the system still approximately holds a $U(1)$-rotation symmetry. Thus,
the power-law spatial decay signature of spin superfluidity is identified in a
nonlocal-measurement setup where the spin injection is described by the
generalized spin-mixing conductance. We suggest iron jarosites as promising
material candidates for realizing our proposal.
|
2011.09102v2
|
2020-12-22
|
Unusual Scaling of Kondo Spin Relaxation
|
The relation between the Kondo spin relaxation rate $ {\tau_{sK}}^{-1} $ and
the Kondo momentum relaxation rate $ {\tau_{eK}}^{-1} $ is explored by using
nonlocal spin valves with submicron copper channels that contain dilute iron
impurities. A linear relation between $ {\tau_{sK}}^{-1} $ and $
{\tau_{eK}}^{-1} $ is established under varying temperatures. However, under
varying impurity concentrations, ${\tau_{sK}}^{-1}$ remains nearly constant
despite variation of $ {\tau_{eK}}^{-1} $ by a factor of 10. This surprising
relation can be understood by considering spin relaxation through overlapping
Kondo screening clouds and supports the physical existence of the elusive Kondo
clouds.
|
2012.11781v1
|
2020-12-23
|
Magnetic Moment Tensor Potentials for collinear spin-polarized materials reproduce different magnetic states of bcc Fe
|
We present the magnetic Moment Tensor Potentials (mMTPs), a class of
machine-learning interatomic potentials, accurately reproducing both
vibrational and magnetic degrees of freedom as provided, e.g., from
first-principles calculations. The accuracy is achieved by a two-step
minimization scheme that coarse-grains the atomic and the spin space. The
performance of the mMTPs is demonstrated for the prototype magnetic system bcc
iron, with applications to phonon calculations for different magnetic states,
and molecular dynamics simulations with fluctuating magnetic moments.
|
2012.12763v2
|
2021-03-08
|
All-order relativistic computations for atoms and molecules using an explicitly correlated Gaussian basis
|
A variational solution procedure is reported for the many-particle no-pair
Dirac-Coulomb-Breit Hamiltonian aiming at a parts-per-billion (ppb) convergence
of the atomic and molecular energies, described within the fixed nuclei
approximation. The procedure is tested for nuclear charge numbers from $Z=1$
(hydrogen) to $28$ (iron). Already for the lowest $Z$ values, a significant
difference is observed from leading-order Foldy-Woythusen perturbation theory,
but the observed deviations are smaller than the estimated self-energy and
vacuum polarization corrections.
|
2103.04923v1
|
2021-03-14
|
Nuclear effects in neutrino-nucleus DIS and probe for short range correlation
|
We investigate charged-current neutrino-nucleus deep inelastic scattering
with particular interests in relationship of short range correlation (SRC) and
EMC effect. The structure functions $F^A_2(x,Q^2)$, $x F^A_3(x,Q^2)$ and ratios
of differential cross sections are presented where the nuclei $A$ are chosen to
be carbon, iron and lead. We propose a kind of universal modification functions
which would provide a nontrivial test of SRC universality on the platform of
neutrino-nucleus scattering and improve our understanding of nucleon structure
substantially.
|
2103.07873v1
|
2021-04-05
|
Two-magnon Raman scattering in antiferromagnetic phases of frustrated spin models on the honeycomb lattice
|
We calculate the two-magnon Raman scattering spectra in antiferromagnetic
phases of several frustrated spin models defined on the honeycomb lattice.
These include the N\'{e}el antiferromagnetic phase of a $J_1$-$J_2$-$J_3$ model
and the stripe phase of the Heisenberg-Kitaev model. We show that both the
magnetic frustration and the anisotropy of interactions may significantly
affect the Raman spectra. We further discuss the implications of our results to
the magnetic excitations of the iron-based compound BaFe$_2$Se$_2$O and show
how the magnetic interactions can be extracted from fit to the Raman spectrum.
|
2104.01903v1
|
2021-04-08
|
Ferromagnetism in a Semiconductor with Mobile Carriers via Low-Level Nonmagnetic Doping
|
We show that doped cubic iron pyrite, which is a diamagnetic semiconductor,
becomes ferromagnetic when $p$-type doped. We furthermore find that this
material can exhibit high spin polarization both for tunneling and transport
devices. These results are based on first principles electronic structure and
transport calculations. This illustrates the use of $p$-type doping without
magnetic impurities as a strategy for obtaining ferromagnetic semiconducting
behavior, with implications for spintronic applications that require both
magnetic ordering and good mobility. This is a combination that has been
difficult to achieve by doping semiconductors with magnetic impurities. We show
that phosphorus and arsenic may be effective dopants for achieving this
behavior.
|
2104.03877v1
|
2021-09-30
|
Pairing enhanced by local orbital fluctuations in a model for monolayer FeSe
|
The pairing mechanism in different classes of correlated materials, including
iron based superconductors, is still under debate. For FeSe monolayers, uniform
nematic fluctuations have been shown in a lattice Monte Carlo study to play a
potentially important role. Here, using dynamical mean field theory
calculations for the same model system, we obtain a similar phase diagram and
provide an alternative interpretation of the superconductivity in terms of
local orbital fluctuations and phase rigidity. Our study clarifies the relation
between the superconducting order parameter, superfluid stiffness and orbital
fluctuations, and provides a link between the spin/orbital freezing theory of
unconventional superconductivity and theoretical works considering the role of
nematic fluctuations.
|
2109.15312v1
|
2021-12-16
|
Investigating the Longitudinal Development of EAS with Ultra High Energies
|
The simulation of the extensive air showers was performed by investigating
the longitudinal development parameters (N and Xmax) by using AIRES system
version 19.04.0. The simulation was performed at the energy range (10^18-10^20
eV) for different primary particles (such as primary proton and iron nuclei)
and different zenith angles. The longitudinal development curves of EAS are
fitted using Gaussian function that gave a new parameters for different primary
particles and different zenith angles at the energy range (10^18-10^20 eV).
|
2201.00401v1
|
2022-01-13
|
The FACET Project: Forward Aperture CMS ExTension to search for new Long-Lived Particles
|
FACET is a proposed new subsysem for CMS to search for portals such as dark
photons, dark higgs, heavy neutral leptons and axion-like particles in the very
forward direction at the High Luminosity LHC. Such particles can penetrate up
to 50 m of iron and then decay inside a 14 m$^3$ vacuum pipe made by enlarging
an 18 m long section of the LHC pipe to a radius of 50 cm.
|
2201.05224v1
|
2022-01-28
|
Theory of spin-excitation anisotropy in the nematic phase of FeSe obtained from RIXS measurements
|
Recent resonant inelastic x-ray scattering (RIXS) experiments have detected a
significant high-energy spin-excitation anisotropy in the nematic phase of the
enigmatic iron-based superconductor FeSe, whose origin remains controversial.
We apply an itinerant model previously used to describe the spin-excitation
anisotropy as measured by neutron scattering measurements, with magnetic
fluctuations included within the RPA approximation. The calculated RIXS cross
section exhibits overall agreement with the RIXS data, including the high
energy spin-excitation anisotropy.
|
2201.12008v2
|
2022-04-28
|
Spin-glass-like behaviour in SmFeAsO0.8F0.2
|
The iron-based oxypnictide superconductor SmFeAsO0.8F0.2 was synthesized
under high-pressure and investigated by measuring dc magnetic susceptibility.
The zero-field cooled (ZFC) magnetic susceptibility confirmed the bulk
superconductivity of the sample with a critical temperature of Tc = 50 K and a
significant jump in the magnetization at 4.3 K, usually attributed to the
antiferromagnetic ordering of Sm3+ ions in this system. Since the occurrence of
the jump depends on the cooling history, our data strongly suggest a
spin-glass-like behaviour.
|
2204.13473v2
|
2022-05-16
|
Feedback of non-local $d_{xy}$ nematicity on the magnetic anisotropy in FeSe
|
We analyze theoretically the magnetic anisotropy in the nematic phase of FeSe
by computing the spin and the orbital susceptibilities from the microscopic
multiorbital model. In particular, we take into account both the $xz/yz$ and
the recently proposed non-local $xy$ nematic ordering and show that the latter
one could play a crucial role in reproducing the experimentally-measured
temperature dependence of the magnetic anisotropy. This provides a direct
fingerprint of the different nematic scenarios on the magnetic properties of
FeSe.
|
2205.07718v1
|
2022-08-05
|
Exploring the bouncing cosmological models in symmetric teleparallel gravity
|
In this study, cosmological models with perfect fluid and a gravitational
framework f(Q) will be examined. In this modified theory of gravity, the
gravitational force has the form f(Q), where Q stands for the non-metricity
scalar. I create two bouncing cosmological models, one in which the Lagrangian
f(Q) is assumed to have a linear dependence on Q and the other in which it has
a polynomial functional form. It has been discovered that the parameters of the
individual models largely determine how they will behave. The resulting models'
equation of state (EoS) parameter captures the universe's ironic behavior. It
should be highlighted that the built-in cosmological models go against the
energy requirements. The models' kinematical and physical characteristics are
discussed.
|
2208.03171v1
|
2022-10-01
|
India Based Neutrino Observatory, Physics Reach and Status Report
|
The India-based Neutrino Observatory (INO) is a proposed underground facility
located in India that will primarily house the magnetised Iron CALorimeter
(ICAL) detector to study atmospheric neutrinos produced by interactions of
cosmic rays with Earth's atmosphere. The physics goal is to to make precision
measurements of the neutrino mixing and oscillation parameters through such a
study. We present here the results from detailed simulations studies, as well
as a status report on the project. In particular, we highlight the sensitivity
of ICAL to the open issue of the neutrino mass ordering, which can be
determined {\it independent of the CP phase} at ICAL.
|
2210.00206v2
|
2022-10-31
|
Superconducting Magnet System Concept with a Mechanical Energy Transfer in the Magnetic Field
|
There is an interest to design superconducting magnet systems working in a
persistent current mode. These systems continuously generate magnetic field
with disconnected power source working like permanent magnet devices. In this
paper proposed a magnet system concept based on the direct mechanical energy
transfer in the magnetic field. Short circuited superconducting coils do not
have current leads and power source. To pump the mechanical energy in the
superconducting coil used a magnetizer which magnetically coupled with the
coil. The mechanical removing the magnetizer from the magnet induces a
persistent current in the superconducting coil which generates the magnetic
field. Iron dominated magnet system concept was investigated using OPERA3d code
which confirmed a visibility of proposed approach.
|
2211.00102v1
|
2022-10-31
|
HTS Dipole Magnet with a Mechanical Energy Transfer in the Magnetic Field
|
There were designed and successfully tested at Fermilab several high
temperature superconducting (HTS) model magnets for particle accelerators. Some
of them worked in a persistent current mode continuously generating magnetic
field in the iron dominated magnet gap. In the paper was investigated a novel
HT dipole magnet concept with a mechanical energy transfer in the magnetic
field. To pump the energy in the superconducting HTS dipole magnet used a
detachable magnetizer. The HTS dipole magnet was build and successfully tested
at a liquid nitrogen temperature. Discussed the magnet design, test results,
the proposed approach limits, and efficiency.
|
2211.00105v1
|
2022-11-02
|
A brief perspective of high temperature superconductivity in the cuprates: Strong correlations combined with superexchange match experiment
|
High temperature superconductivity encompasses the cuprates, nickelates, iron
pnictides, and LaH$_x$ compounds. The first three groups of compounds involve
in the pairing electrons, which are strongly to moderately correlated, whereas
in the last class of systems specific phonon excitations. In this overview we
concentrate first on the (semi)quantitative theory of high T$_{C}$
superconductivity in the cuprates based on our original vibrational approach
beyond the renormalized mean field theory. The model we explore mainly is
$t$-$J$-$U$ model containing both the superexchange (kinetic energy) combined
with strong interelectronic correlations. Selected equilibrium and
dynamic-excitation properties are analyzed briefly. General questions regarding
the pseudogap and two--dimensional character of those systems are raised.
|
2211.01024v1
|
2022-12-01
|
Numerical simulation of non-central collisions of spherical magnets
|
We present a computational model of non-central collisions of two spherical
neodymium-iron-boron magnets, suggested as a demonstration of angular momentum
conservation. Our program uses an attractive dipole-dipole force and a
repulsive contact force to solve the Newtonian equations of motion for the
magnets. We confirm the conservation of angular momentum and study the changes
in energy throughout the interaction. Using the exact expression for the
dipole-dipole force, including non-central terms, we correctly model the final
rotational frequencies, which is not possible with a simple power-law
approximation.
|
2212.00876v1
|
2022-12-19
|
Equivalence of Active and Passive Gravitational Mass Tested with Lunar Laser Ranging
|
LLR measures the distance between observatories on Earth and retro-reflectors
on Moon since 1969. In this paper, we study the possible violation of the
equality of passive and active gravitational mass ($m_{a}/m_{p}$), for
Aluminium (Al) and Iron (Fe), using LLR data. Our new limit of
$3.9\cdot10^{-14}$ is about 100 times better than that of Bartlett and Van
Buren [1986] reflecting the benefit of the many years of LLR data.
|
2212.09407v1
|
2022-12-27
|
Tuning quantum paramagnetism and d-wave superconductivity in single-layer iron chalcogenides by chemical pressure
|
By substituting S into single-layer FeSe/SrTiO3, chemical pressure is applied
to tune its paramagnetic state that is modeled as an incoherent superposition
of spin-spiral states. The resulting electronic bands resemble an ordered
checkerboard antiferromagnetic structure, consistent with angle-resolved
photoemission spectroscopy measurements. Scanning tunneling spectroscopy
reveals a gap evolving from U-shaped for FeSe to V-shaped for FeS with
decreasing size, attributed to a d-wave superconducting state for which nodes
emerge once the gap size is smaller than the effective spin-orbit coupling.
|
2212.13603v2
|
2023-02-03
|
An Equilibrated Error Estimator for the 2D/1D MSFEM T-Formulation of the Eddy Current Problem
|
The 2D/1D multiscale finite element method (MSFEM) is an efficient way to
simulate rotating machines in which each iron sheet is exposed to the same
field. It allows the reduction of the three dimensional sheet to a two
dimensional cross-section by resolving the dependence along the thickness of
the sheet with a polynomial expansion. This work presents an equilibrated error
estimator based on flux equilibration and the theorem of Prager and Synge for
the T-formulation of the eddy current problem in a 2D/1D MSFEM setting. The
estimator is shown to give both a good approximation of the total error and to
allow for adaptive mesh refinement by correctly estimating the local error
distribution.
|
2302.01601v1
|
2023-03-09
|
Full-waveform tomography reveals iron spin crossover in Earth lower mantle
|
Joint interpretation of bulk and shear wave speeds constrains the chemistry
of the deep mantle. At all depths, the diversity of wave speeds cannot be
explained by an isochemical mantle. Between 1000 and 2500 km depth,
hypothetical mantle models containing an electronic spin crossover in (Mg,Fe)O
provide a significantly better fit to the wave-speed distributions, as well as
more realistic temperatures and silica contents, than models without a spin
crossover. Below 2500 km, wave speed distributions are explained by enrichment
in silica towards the core-mantle-boundary. This silica enrichment may
represent the fractionated remains of an ancient basal magma ocean.
|
2303.05476v1
|
2023-05-07
|
Surface supercurrent diode effect
|
We propose a new type of supercurrent diode effect on the surface of a
superconductor with surface states under in-plane magnetic fields. Surface
supercurrent diode effect can lead to a perfect supercurrent diode in a
considerably wide range of fields. For comparison, the conventional
supercurrent diode effect due to the spin-orbit coupling in a two-dimensional
superconductor cannot be perfect in usual cases. Candidates such as the (001)
surface of iron-based superconductors BaFe$_{2-x}$Co$_x$As$_2$ are discussed.
Calculations are performed under the Ginzburg-Landau formalism.
|
2305.04219v2
|
2023-05-12
|
DFT+U Type Strong Correlation Functional Derived from Multiconfigurational Wavefunction Theory
|
We present a DFT+U-type functional for strong correlation, derived from
multiconfigurational wavefunction theory. The reference system experiences
electron-electron interactions only in DFT+U-type atomic states, yielding a
block-localized configuration interaction Hamiltonian which depends on the
atomic state occupancies and the promotion energies of doubly excited
determinants. Simple approximations for the promotion energies recover the
flat-plane condition and provide beyond-zero-sum accuracy for iron
spin-crossover complexes.
|
2305.07736v2
|
2023-05-15
|
Easily Computed Marginal Likelihoods from Posterior Simulation Using the THAMES Estimator
|
We propose an easily computed estimator of marginal likelihoods from
posterior simulation output, via reciprocal importance sampling, combining
earlier proposals of DiCiccio et al (1997) and Robert and Wraith (2009). This
involves only the unnormalized posterior densities from the sampled parameter
values, and does not involve additional simulations beyond the main posterior
simulation, or additional complicated calculations. It is unbiased for the
reciprocal of the marginal likelihood, consistent, has finite variance, and is
asymptotically normal. It involves one user-specified control parameter, and we
derive an optimal way of specifying this. We illustrate it with several
numerical examples.
|
2305.08952v1
|
2023-08-01
|
Overview of superconductivity in field-cooled magnetic materials
|
Considerable experimental skills have been accumulated in the preparation of
field-cooled (FC) magnetic materials. This stimulates the search for FC
magnetic materials that are superconductors.
The article overviews the recent proposed mechanism of superconductivity in
field-cooled magnetic materials.
It is based on previously published results for magnon-induced
superconductivity in field-cooled spin-1/2 antiferromagnets $[PRB96,214409]$
(arXiv:1712.02983), Sequence of superconducting states in field cooled
$FeCr_2S_4$ $[JPCM33,495604]$
(arXiv:2111.02765) and Partial order induced superconductivity in $Fe^{2+}$
iron. $[EPL126,47001]$ (arXiv:1902.02290).
|
2308.00470v1
|
2023-10-31
|
Generating site saturation mutagenesis libraries and transferring them to broad host range plasmids using type IIS restriction enzymes
|
Protein engineering is an established method for tailoring enzymatic
reactivity. A commonly used method is directed evolution, where the mutagenesis
and natural selection process is mimicked and accelerated in the laboratory.
Here, we describe a reliable method for generating saturation mutagenesis
libraries by golden gate cloning in a broad host range plasmid containing the
pBBR1 replicon. The applicability is demonstrated by generating a mutant
library of the iron nitrogenase gene cluster (anfHDGK) of Rhodobacter
capsulatus, which is subsequently screened for the improved formation of
molecular hydrogen.
|
2311.00138v1
|
2023-11-21
|
Comment on "Ironing, sweeping, and multidimensional screening''
|
In their study of price discrimination for a monopolist selling heterogeneous
products to consumers having private information about their own
multidimensional types, Rochet and Chon\'e (1998) discovered a new form of
screening in which consumers with intermediate types are bunched together into
isochoice groups of various dimensions incentivized to purchase the same
product. They analyzed a particular example involving customer types
distributed uniformly over the unit square. For this example, we prove that
their proposed solution is not selfconsistent, and we indicate how consistency
can be restored.
|
2311.13012v2
|
2024-01-04
|
Multiband Quantum Materials
|
Quantum materials are defined by the emergence of new properties resulting
from collective quantum effects and by holding promise for their quantum
applications. Novel superconductors, from high-Tc cuprates and iron-based
superconductors to twisted monolayers, exhibit a higher level of emergent
complexity, with a multiband electronic structure playing a pivotal role in
their comprehension and potential applications. Here, we provide a brief
overview of key multiband effects in these superconductors and topological
semimetals, offering guidelines for the theory-assisted development of new
quantum materials and devices.
|
2401.02237v1
|
2024-01-07
|
Mid-infrared evidence for iron-rich dust in the multi-ringed inner disk of HD 144432
|
Context. Rocky planets form by the concentration of solid particles in the
inner few au regions of planet-forming disks. Their chemical composition
reflects the materials in the disk available in the solid phase at the time the
planets were forming. Aims. We aim to constrain the structure and dust
composition of the inner disk of the young star HD 144432, using an extensive
set of infrared interferometric data taken by the Very Large Telescope
Interferometer (VLTI), combining PIONIER, GRAVITY, and MATISSE observations.
Methods. We introduced a new physical disk model, TGMdust, to image the
interferometric data, and to fit the disk structure and dust composition. We
also performed equilibrium condensation calculations with GGchem. Results. Our
best-fit model has three disk zones with ring-like structures at 0.15, 1.3, and
4.1 au. Assuming that the dark regions in the disk at ~0.9 au and at ~3 au are
gaps opened by planets, we estimate the masses of the putative gap-opening
planets to be around a Jupiter mass. We find evidence for an optically thin
emission ($\tau<0.4$) from the inner two disk zones ($r<4$ au) at $\lambda>3\
\mu$m. Our silicate compositional fits confirm radial mineralogy gradients. To
identify the dust component responsible for the infrared continuum emission, we
explore two cases for the dust composition, one with a silicate+iron mixture
and the other with a silicate+carbon one. We find that the iron-rich model
provides a better fit to the spectral energy distribution. Conclusions. We
propose that in the warm inner regions ($r<5$ au) of typical planet-forming
disks, most if not all carbon is in the gas phase, while iron and iron sulfide
grains are major constituents of the solid mixture along with forsterite and
enstatite. Our analysis demonstrates the need for detailed studies of the dust
in inner disks with new mid-infrared instruments such as MATISSE and JWST/MIRI.
|
2401.03437v1
|
2024-01-16
|
The effect of iron layer thickness on the interlayer exchange coupling in Fe/MgO (001) superlattices
|
We describe the effect of the Fe layer thickness on the antiferromagnetic
interlayer exchange coupling in [Fe/MgO]$_N$ superlattices. An increase in
coupling strength with increasing Fe layer thickness is observed, which
highlights the need of including the extension of both the layers when
discussing the interlayer exchange coupling in superlattices.
|
2401.08336v1
|
2023-11-10
|
Automatic extraction and 3D reconstruction of split wire from point cloud data based on improved DPC algorithm
|
In order to solve the problem of point cloud data splitting improved by DPC
algorithm, a research on automatic separation and 3D reconstruction of point
cloud data split lines is proposed. First, the relative coordinates of each
point in the cloud point are calculated. Second, it is planned to develop a
relative ensemble-based DPC swarm algorithm for analyzing the number of
separation lines to determine all parts in the cloud content. Finally, fit each
separator using the least squares method. iron. The cloud point of the
resulting split subconductors has a clear demarcation line, and the distance
between adjacent split subconductors is 0.45 m, divided by the four vertices of
the square.
|
2401.08587v1
|
2024-02-20
|
Electron-phonon coupling in ferromagnetic Fe-Co alloys from first principles
|
We calculate from first principles the electron-phonon coupling strength in
ferromagnetic iron-cobalt Fe$_{1-x}$Co$_x$ alloys for compositions ranging from
$x=0$ to $x=0.75$. We find strong, spin-dependent variation of the
electron-phonon coupling strength with alloy composition. The minimum of the
electron-phonon interaction is found near the composition $x=0.25$. We analyze
the variation of the electron-phonon interaction with composition, as a
function of electron spin, density of states, electron-phonon matrix elements,
and phonon frequencies. Our results are in good qualitative agreement with
magnetization dynamics experiments.
|
2402.12650v1
|
2024-02-20
|
Enhancement of the critical current by surface irregularities in Fe-based superconductors
|
The critical current $I_c$ of single crystals of the iron pnictide
superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$, has been studied through
measurements of magnetic hysteresis cycles. We show that the introduction of
surface irregularities in the $\mu$m scale significantly increase $I_c$,
primarily near the irreversibility magnetic field $H_{irr}$, where the surface
currents are the main contribution to $I_c$. Such an increase is consistent
with a theoretical estimate for the maximum non-dissipative current that a
rough surface can sustain, based on Mathieu-Simon continuum theory for the
vortex state.
|
2402.12933v1
|
2024-03-28
|
X-ray reflectometric studies of nanoparticulate hematite films to decouple the rough and smooth behaviors of it and crystallographic and morphological properties concerning fatty acid chain length
|
In this study, the use of X-Ray reflectometry technique signifies the types
of rough and smooth surfaces of hematite film prepared from different fatty
acid derivatives of the iron salt. Followed by this, the film morphology and
crystallographic properties concerning different fatty acid chain length have
been discussed.
|
2403.19065v1
|
1995-06-14
|
ASCA PV observations of the Seyfert 1 galaxy MCG-6-30-15 : rapid variability of the warm absorber
|
We present a detailed re-analysis of the two {\it ASCA} Performance
Verification observations of the nearby Seyfert 1 galaxy MCG-6-30-15.
Confirming the results of Fabian et al. (1994), we find definite evidence for
the {\sc O\,vii} and {\sc O\,viii} K-shell absorption edges of the warm
absorber and a doubling of the warm absorber column density within the 3 weeks
separating the two observations. No intra-day {\it flux-correlated} variability
of the warm absorber is found. However, we report the discovery of an `event'
in which the warm absorber parameters temporarily change for
\sim10\,000\thinspace s before returning to their original values. Possible
interpretations are discussed but a contradiction remains: the constancy of the
ionization state of the warm absorber argues that it lies at large distances
from the central source whereas the short term change in column density argues
for small distances. Fluorescent iron emission is examined. As found by Fabian
et al. (1994), the iron line is broad and strong (equivalent width
\sim300\thinspace eV). The line profile is also suggestive of it being skewed.
Such a line would be expected from a relativistic accretion disk. We also find
very rapid primary X-ray variability. Assuming relativistic beaming to be
unimportant, the derived efficiency is comparable to the maximum obtainable
from accretion onto a Schwarzschild black hole. Correlated variability outside
of the energy range of {\it ASCA} might exceed this maximum, thus requiring
efficient accretion onto a Kerr hole.
|
9506086v1
|
1996-03-18
|
The X-Ray Spectral Variability of Mrk 766
|
Analysis results from ASCA and ROSAT observations of the narrow-line Seyfert
1 galaxy Mrk 766 are reported. In the ASCA observation we observed rapid
variability with a doubling time scale of 1000 seconds. A spectral variability
event was observed in which the spectrum softened and hardened above and below
~1 keV, respectively, as the flux increased. The spectra could be modeled with
5 components: a power law, warm absorber, iron K(alpha) line and soft excess
component flux. The spectral variability resulted from a highly significant
change in the intrinsic photon law index from Gamma ~1.6 to ~2.0, an increase
in the warm absorber ionization, and a marginally significant decrease in the
soft component normalization. A ~100 eV equivalent width narrow iron K(alpha)
line was detected in the high state spectrum. Spectral hardening during flux
increases was observed in three ROSAT observations.
The change in intrinsic photon index and disappearance of the soft excess
component in the ASCA spectra can be explained as a transition from a first
order pair reprocessed spectrum to a pair cascade brought about by a sudden
increase in the injected electron Lorentz factor. The change in the ionization
of the warm absorber, though model dependent, could correspond to the increase
in flux at the oxygen edges resulting from the spectral index change. The ROSAT
spectral variability can be interpreted by variable intensity hard power law
and a relatively nonvarying soft component, possibly primary disk emission.
These results are compared with those reported from other narrow-line Seyfert 1
galaxies.
|
9603091v1
|
1996-04-23
|
The Luminosity-Metallicity Relation for Bulges of Spiral Galaxies
|
Optical integrated spectra of bulges have been obtained for 28 spiral
galaxies. By applying an appropriate aperture size for each galaxy, the
unavoidable contamination of disk starlight has been carefully minimized and
set to $\sim$ 1/7 of the total stellar light. The sample covers a wide range in
bulge luminosity and morphology. The Mg$_2$ index shows a tight correlation
with the bulge intrinsic luminosity, similar - and extended at fainter
magnitudes - to the relationship known for ellipticals. Other features such as
CaIIK, CN, G-band, and TiO show similar trend with the bulge luminosity. On the
contrary, the Fe5270 and Fe5335 iron lines show a constant equivalent width -
within some dispersion - for all bulges. A simple application of a stellar
population synthesis model suggests that magnesium should be most enhanced with
respect to iron in the brightest bulges. Concerning the structural parameters,
bulges do occupy the same region in the fundamental plane as ellipticals. In
conclusion, the present study favors the idea that the bulk of stars in bulges
have formed with only moderate or negligible influence of the disk material,
likely by very similar processes as those driving the formation of elliptical
galaxies.
|
9604132v1
|
1996-06-17
|
The variable iron K emission line in MCG-6-30-15
|
We report on the variability of the iron K emission line in the Seyfert 1
galaxy MCG--6-30-15 during a four-day ASCA observation. The line consists of a
narrow core at an energy of about 6.4 keV, and a broad red wing extending to
below 5 keV, which are interpreted as line emission arising from the inner
parts of an accretion disk. The narrow core correlates well with the continuum
flux whereas the broad wing weakly anti-correlates. When the source is
brightest, the line is dominated by the narrow core, whilst during a deep
minimum, the narrow core is very weak and a huge red tail appears. However, at
other times when the continuum shows rather rapid changes, the broad wing is
more variable than the narrow core, and shows evidence for correlated changes
contrary to its long time scale behaviour. The peculiar line profile during the
deep minimum spectrum suggests that the line emitting region is very close to a
central spinning (Kerr) black hole where enormous gravitational effects
operate.
|
9606103v1
|
1996-08-03
|
New beryllium observations in low-metallicity stars
|
We present observations of the Be II 313.0 nm resonance doublet in 14 halo
and old disk stars with metallicities ranging from [Fe/H]=-0.4 to about -3.0
obtained with the CASPEC spectrograph of the ESO 3.6m telescope at a FWHM about
8.6 km/s resolution. Abundances are derived by means of the synthetic spectra
technique employing Kurucz (1993) atmospheric models, with enhanced
alpha-elements and no overshooting. The derived abundances together with those
available in literature show that for -2.7 < [Fe/H] < -0.8 Be correlates
linearly with iron [Be] proportional to 1.07(+/- 0.08)[Fe/H], giving strength
to previous results. However, a steeper correlation is still possible at
metallicities lower than [Fe/H]<-1.4 with [Be] proportional to 1.6(+/-
0.44)[Fe/H]. When iron is replaced with oxygen, Be is found tracking closely
oxygen up to solar values, without signs of breaking in correspondence of the
onset of the Galactic disk. No evidence of intrinsic dispersion is found, ought
to the large errors involved in the Be abundance determinations, but for three
stars (HD 106516, HD 3795, HD 211998) a significant upper limit in the Be
abundance can be placed at about 1 dex below the mean trend of the Be-Fe
relation. For such stars non conventional mixing is required to explain Be
depletion. Be observations can be used to discriminate strongly Li-depleted
stars. These are the stars which show less Li than that expected by high energy
cosmic rays production as deduced from Be observations. The available Be
observations imply that some of the stars which contribute to the scatter in
the Li-Fe diagramme are Li-depleted stars.
|
9608011v1
|
1996-09-15
|
Multiwavelength observations of isolated neutron stars as a tool to probe the properties of their surfaces
|
We show that an analysis of multiwavelength observations of isolated neutron
stars based on neutron star atmosphere models can be used not only to evaluate
the neutron star effective temperature, but also to determine chemical
composition of its surface. To demonstrate how this new method can be applied
to a specific object, we chose the old isolated neutron star candidate
RXJ1856.5-3754, whose soft X-ray radiation has been studied recently by Walter
et al. (1996). We fitted the soft X-ray spectrum of this object with neutron
star atmosphere models of different chemical compositions and used these fits
to calculate the source spectrum over a broad wavelength range. We showed, in
particular, that the optical/UV flux expected from this object depends
drastically on the composition of its surface. In particular, the neutron star
covered with hydrogen would be 5-6 magnitudes brighter than the neutron star
with an iron surface. The object should also be observable with $EUVE$; the EUV
flux is expected to be almost twice higher for the iron surface than for the
hydrogen one. Thus, multiwavelength observations of this object would enable
one to examine, for the first time, chemical composition of the neutron star
surface. The method proposed can be applied to other nearby isolated neutron
stars.
|
9609097v2
|
1996-10-16
|
An X-ray spectral study of 24 type-1 AGN
|
I present a study of the X-ray spectral properties of a sample containing 24
type-1 active galactic nuclei using the medium spectral resolution of ``ASCA''.
The sample consists of 20 radio-quiet objects and 4 radio-loud objects. A
simple power-law continuum absorbed by Galactic material provides a very poor
description of the spectra of most objects. Deviations from the power-law form
are interpreted in terms of X-ray reprocessing/absorption processes. In
particular, at least half of the objects show K-shell absorption edges of warm
oxygen (O vii and O viii) characteristic of optically-thin, photoionized
material along the line-of-sight to the central engine, the so-called warm
absorber. It is found that objects with significant optical reddening display
deep O vii edges. Coupled with other evidence, this suggests the existence of
dusty warm plasma. A radiatively driven outflow originating from the molecular
torus is probably the source of this plasma. Rapid variability of the warm
absorber also points to there being another component closer to the central
source and probably situated within the broad line region.
Spectral features at energies characteristic of cold fluorescent iron
K$\alpha$ emission are common. Radio-quiet objects have iron emission well
described as originating from either the inner regions of an accretion disk or,
in a small number of cases, from the molecular torus. Two of the radio-loud
objects (3C~120 and 3C~382) have a much broader feature which presents problems
for the relativistic disk model. The presence of radio-jets may be important in
forming this spectral feature.
|
9610127v1
|
1996-12-10
|
An Investigation of Neutrino-Driven Convection and the Core Collapse Supernova Mechanism Using Multigroup Neutrino Transport
|
We investigate neutrino-driven convection in core collapse supernovae and its
ramifications for the explosion mechanism. We begin with an ``optimistic'' 15
solar mass precollapse model, which is representative of the class of stars
with compact iron cores. This model is evolved through core collapse and bounce
in one dimension using multigroup (neutrino-energy--dependent) flux-limited
diffusion (MGFLD) neutrino transport and Lagrangian hydrodynamics, providing
realistic initial conditions for the postbounce convection and evolution. Our
two-dimensional simulation begins at 106 ms after bounce at a time when there
is a well-developed gain region, and proceeds for 400 ms. We couple
two-dimensional (PPM) hydrodynamics to one-dimensional MGFLD neutrino
transport. At 225 ms after bounce we see large-scale convection behind the
shock, characterized by high-entropy, mushroom-like, expanding upflows and
dense, low-entropy, finger-like downflows. The upflows reach the shock and
distort it from sphericity. The radial convection velocities become supersonic
just below the shock, reaching magnitudes in excess of 10^9 cm/sec. Eventually,
however, the shock recedes to smaller radii, and at about 500 ms after bounce
there is no evidence in our simulation of an explosion or of a developing
explosion. Failure in our ``optimistic'' 15 solar mass Newtonian model leads us
to conclude that it is unlikely, at least in our approximation, that
neutrino-driven convection will lead to explosions for more massive stars with
fatter iron cores or in cases in which general relativity is included.
|
9612107v1
|
1997-01-08
|
A new chemo-evolutionary population synthesis model for early-type galaxies. II: Observations and Results
|
We present here the results of applying a new chemo-evolutionary stellar
population model developed by ourselves in a previous paper (Vazdekis et al.
1996) to new high quality observational data of the nuclear regions of two
representative elliptical galaxies and the bulge of the Sombrero galaxy. Here
we fit in detail about 20 absorption lines and 6 optical and near-infrared
colors following two approaches: fitting a single-age single-metallicity model
and fitting our full chemical evolutionary model. We find that all of the iron
lines are weaker than the best fitting models predict, indicating that the
iron-abundance is anomalous and deficient. We also find that the Ca_I index at
4227 A is much lower than predicted by the models. We can obtain good fits for
all the other lines and observed colors with models of old and metal-rich
stellar populations, and can show that the observed radial gradients are due to
metallicity decreasing outward. We find that good fits are obtained both with
fully evolutionary models and with single-age single-metallicity models. This
is due to the fact that in the evolutionary model more than 80% of stars form
with in 1.5 Gyr after the formation of the galaxies. The fact that slightly
better fits are obtained with evolutionary models indicates these galaxies
contain a small spread in metallicity.
|
9701036v1
|
1997-02-24
|
A New Approach to Determine the Initial Mass Function in the Solar Neighborhood
|
Oxygen to iron abundance ratios of metal-poor stars provide information on
nucleosynthesis yields from massive stars which end in Type II supernova
explosions. Using a standard model of chemical evolution of the Galaxy we have
reproduced the solar neighborhood abundance data and estimated the oxygen and
iron yields of genuine SN II origin. The estimated yields are compared with the
theoretical yields to derive the relation between the lower and upper mass
limits in each generation of stars and the IMF slope. Independently of this
relation, we furthermore derive the relation between the lower mass limit and
the IMF slope from the stellar mass to light ratio in the solar neighborhood.
These independent relations unambiguously determine the upper mass limit of
$m_u=50 \pm 10 M_sun$ and the IMF slope index of 1.3 - 1.6 above 1 M_sun. This
upper mass limit corresponds to the mass beyond which stars end as black holes
without ejecting processed matter into the interstellar medium. We also find
that the IMF slope index below 0.5 M_sun cannot be much shallower than 0.8.
|
9702205v2
|
1997-07-15
|
Rotating Astrophysical Systems and a Gauge Theory Approach to Gravity
|
We discuss three applications of a gauge theory of gravity to rotating
astrophysical systems. The theory employs gauge fields in a flat Minkowski
background spacetime to describe gravitational interactions. The iron
fluorescence line observed in AGN is discussed, assuming that the line
originates from matter in an accretion disk around a Kerr (rotating) black
hole. Gauge-theory gravity, expressed in the language of Geometric Algebra,
allows very efficient numerical calculation of photon paths. From these paths
we are able to infer the line shape of the iron line. Comparison with
observational data allows us to constrain the black hole parameters, and, for
the first time, infer an emissivity profile for the accretion disk. The
topological constraints imposed by gauge-theory gravity are exploited to
investigate the nature of the Kerr singularity. This reveals a simple physical
picture of a ring of matter moving at the speed of light which surrounds a
sheet of pure isotropic tension. Implications for the end-points of collapse
processes are discussed. Finally we consider rigidly-rotating cosmic strings.
It is shown that a solution in the literature has an unphysical stress-energy
tensor on the axis. Well defined solutions are presented for an ideal
two-dimensional fluid. The exterior vacuum solution admits closed timelike
curves and exerts a confining force.
|
9707165v1
|
1997-10-28
|
Chemical abundances in the young galaxy at z=2.309 towards PHL 957
|
We present high-resolution UES spectra of the quasar PHL 957 obtained for
studying the foreground Damped Ly-alpha galaxy at z=2.309. Measurements of
absorption lines lead to accurate abundance determinations of Fe, S and N which
complement measurements of Zn, Cr and Ni already available for this system. We
find [Fe/H]= -2.0 (+/-0.1), [S/H]= -1.54 (+/-0.06) and [N/H]=-2.76 (+/-0.07).
The ratio [Fe/Zn]=-0.44 provides evidence that about 74% of iron and about 28%
of zinc are locked into dust grains with a dust-to-gas ratio of about 3% of the
Galactic one. The total iron content in both gas and dust in the DLA system is
[Fe/H]=-1.4. This confirms a rather low metallicity in the galaxy, which is in
the early stages of its chemical evolution. The detection of SII allows us to
measure the SII/ZnII ratio, which is a unique diagnostic tool for tracing back
its chemical history, since it is not affected by the presence of dust.
Surprisingly, the resulting relative abundance is [S/Zn]=0.0 (+/-0.1), at
variance with the overabundance found in the Galactic halo stars with similar
metallicity. We emphasize that the [S/Zn] ratio is solar in all the three DLA
absorbers with extant data. Upper limits are also found for Mn, Mg, O and P
and, once the dust depletion is accounted for, we obtain [Mg/Fe]_{c} <+0.2,
[O/Fe]_{c}< 0.4, [Mn/Fe]_{c}<+0.0 and [P/Fe]_{c}<-0.7. The [alpha/Fe] values do
not support a Galactic halo-like abundances implying that the chemical
evolution of this young galaxy is not reproducing our own Galaxy's evolution.
|
9710314v1
|
1997-12-02
|
Detecting Compton Reflection and a Broad Iron Line in MCG-5-23-16 with the Rossi X-ray Timing Explorer
|
We report the detection with the Rossi X-ray Timing Explorer of a Compton
reflection signature in the Seyfert galaxy MCG-5-23-16. RXTE also resolves the
Fe K-alpha fluorescence line with FWHM ~48,000 km s^{-1}. This measurement
provides the first independent confirmation of ASCA detections in Seyfert
galaxies of broad Fe K-alpha lines that are thought to be the signature of
emission from the inner regions of an accretion disk orbiting a black hole.
Under the assumption that reflection arises from an isotropic source located
above a neutral accretion disk, and using a theoretical model that accounts for
the dependence of the reflected spectrum on inclination angle, we derive a 90%
confidence range for the disk inclination of i = 50 to 81 degrees. The large
inclination is consistent with that expected from the unified model for
MCG-5-23-16 based on its Seyfert 1.9 classification. If we assume that the
high-energy cutoff in the incident spectrum lies at energies larger than a few
hundred keV, then the equivalent width of the Fe K-alpha line is much larger
than predicted for the amount of reflection. This implies either an enhanced
iron abundance, a covering factor of reflecting material > 0.5, or a cutoff in
the incident spectrum at energies between ~60 and ~200 keV.
|
9712035v2
|
1997-12-03
|
Resonance Averaged Photoionization Cross Sections for Astrophysical Models
|
We present ground state photoionization cross sections of atoms and ions
averaged over resonance structures for photoionization modeling of
astrophysical sources. The detailed cross sections calculated in the
close-coupling approximation using the R-matrix method, with resonances
delineated at thousands of energies, are taken from the Opacity Project
database TOPbase and the Iron Project, including new data for the low
ionization stages of iron Fe I--V. The resonance-averaged cross sections are
obtained by convolving the detailed cross sections with a Gaussian distribution
over the autoionizing resonances. This procedure is expected to minimize errors
in the derived ionization rates that could result from small uncertainties in
computed positions of resonances, while preserving the overall resonant
contribution to the cross sections in the important near threshold regions. The
detailed photoionization cross sections at low photon energies are complemented
by new relativistic distorted-wave calculations for Z<= 12, and from
central-field calculations for Z>12 at high energies, including inner-shell
ionization. The effective cross sections are then represented by a small number
of points that can be readily interpolated linearly for practical applications;
a Fortran subroutine and data are available. The present numerically averaged
cross sections are compared with analytic fits that do not accurately represent
the effective cross sections in regions dominated by resonances.
|
9712037v1
|
1997-12-25
|
Broadening of the Iron emission line in MCG-6-30-15 by Comptonization
|
We show that the Iron K emission line from MCG-6-30-15 could be broadened due
to Comptonization by a surrounding highly ionized cloud with radius $\sim
10^{14}$ cms. We calculate the temperature of the cloud to be $\sim0.21\kev$,
provided a reasonable estimate of the UV flux is made. The X-ray/$\gamma$-ray
emission observed from the source is compatible with this model. Such a cloud
should be highly ionized and strong absorption edges are not expected from the
source (Fabian et al 1995).
For a $\onlyten{6}\msol$ black hole the size of the could corresponds to
about 300 Schwarzschild radius. The intrinsic line could then be emitted far
from the black hole and gravitational red-shift and Doppler effects would be
negligible. If the black hole mass is much larger than $\onlyten{6}\msol$,
gravitational/Doppler red-shifts would also contribute significantly to the
broadening.
We argue that the broad red wing observed in the source does not by itself
imply emission from regions close ($R < 5 r_s$) to the black hole. However,
Comptonization cannot produce a double peak. The presence of such a feature is
a clear sign of inner disk emission influenced by gravitational and Doppler
effects, perhaps broadened by the Comptonization. We note that simultaneous
broad band (2-100 keV) study of this source can also reveal (or rule out) the
presence of such a Comptonizing cloud.
|
9712327v1
|
1998-02-11
|
The 6.4 keV Fluorescent Iron Line from Cluster Cooling Flows
|
For the inner region of the cooling flow (e.g. within the radius of
$\sim$50--100 kpc) the Thomson optical depth of the hot gas in a massive
cooling flow can be as large as $\sim 0.01$. Assuming that the cooling time in
the inner region is few times shorter than the life time of the cluster, the
Thomson depth of the accumulated cold gas can be higher accordingly (if most of
the gas remains in the form of clouds). The illumination of the cold clouds by
the X-ray emission of the hot gas should lead to the appearance of a 6.4 keV
iron fluorescent line, with an equivalent width proportional to $\tau_T$. The
equivalent width only weakly depends on the detailed properties of the clouds,
e.g. on the column density of individual clouds, as long as the column density
is less than few $10^{23} cm^{-2}$. Another effect also associated exclusively
with the cold gas is a flux in the Compton shoulder of bright X-ray emission
lines. It also scales linearly with the Thomson optical depth of the cold gas.
With the new generation of X-ray telescopes, combining large effective area and
high spectral resolution, the mass of the cold gas in cooling flows (and it's
distribution) can be measured.
|
9802130v1
|
1998-04-23
|
The broad-band X-ray spectrum of Mrk 3
|
We have used non-simultaneous Ginga, ASCA and ROSAT observations to
investigate the complex X-ray spectrum of the Seyfert 2 galaxy Mrk 3. We find
that the composite spectrum can be well described in terms of a heavily cut-off
hard X-ray continuum, iron K_alpha emission and a soft X-ray excess, with
spectral variability confined to changes in the continuum normalisation and the
flux in the iron line. Previous studies have suggested that the power-law
continuum in Mrk 3 is unusually hard. We obtain a canonical value for the
energy index of the continuum (i.e. alpha approx 0.7) when a warm absorber
(responsible for an absorption edge observed near 8 keV) is included in the
spectral model. Alternatively, the inclusion of a reflection component yields a
comparable power-law index. The soft-excess flux cannot be modelled solely in
terms of pure electron scattering of the underlying power-law continuum.
However, a better fit to the spectral data is obtained if we include the
effects of both emission and absorption in a partially photoionized scattering
medium. In particular the spectral feature prominent at $\sim 0.9$ keV could
represent O VIII recombination radiation produced in a hot photoionized medium.
We discuss our results in the context of other recent studies of the soft X-ray
spectra of Seyfert 2 galaxies.
|
9804247v1
|
1998-05-05
|
Chemical composition and fundamental parameters of roAp stars
|
Element abundances of three roAp stars, HD 166473, HD 203932, and HD 217522,
were determined using Kurucz model atmospheres with metal abundances scaled to
solar ones and the results were compared with data from the literature
concerning three further roAp stars, normal B and A stars and two lambda Bootis
stars. Up to 38 elements could be identified and therefore, this work
represents the most complete chemical investigation hitherto published, which
can be summarized as follows:
- all investigated roAp stars have a similar abundance pattern,
- the overabundances of rare earth and other heavy elements are comparable to
cool non-pulsating Ap-stars,
- iron belongs to the most deficient and cobalt to the most enhanced elements
in the group of the iron peak elements, and
- the light elements carbon, nitrogen, and oxygen are less abundant than in
atmospheres with abundances scaled to the Sun.
Beside an unexpected possible relation between effective temperature and
metallicity of roAp stars, no outstanding differences from non-pulsating Ap
stars could be detected. This statement, however, suffers from the lack of
comparably detailed investigations of the latter.
|
9805037v1
|
1998-07-01
|
Magnetic Dipole Microwave Emission from Dust Grains
|
Thermal fluctuations in the magnetization of interstellar grains will produce
magnetic dipole emission at frequencies below ~100 GHz. We show how to
calculate absorption and emission from small particles composed of magnetic
materials. The Kramers-Kronig relations for a dusty medium are generalized to
include the possibility of magnetic grains. The frequency-dependent magnetic
permeability is discussed for candidate grain materials, including iron and
magnetite.
We calculate emission spectra for various interstellar grain candidates.
While paramagnetic grains or magnetite grains cannot account for the observed
"anomalous" emission from dust in the 14-90 GHz range, stronger magnetic dipole
emission will result if a fraction of the grain material is ferromagnetic, as
could be the case given the high Fe content of interstellar dust. The observed
emission from dust near 90 GHz implies that not more than 5% of interstellar Fe
is in the form of metallic iron grains or inclusions (e.g., in "GEMS").
However, we show that if most interstellar Fe is in a moderately ferromagnetic
material, it could contribute a substantial fraction of the observed 14-90 GHz
emission, perhaps comparable to the contribution from spinning ultrasmall dust
grains. The two emission mechanisms can be distinguished by measuring the
emission from dark clouds. The expected polarization of magnetic dipole
emission is discussed
|
9807009v1
|
1998-08-03
|
Cloud model of the mean quasar spectrum
|
We assume a distribution of clouds optically thick for electron scattering
(OTCM) which are moderately optically thin for absorption and we consider them
as a model of the mean quasar spectrum of Laor et al. (1997). We show that the
model is particularly sensitive to the value of the ionization parameter $\xi$
and that for $\xi \sim 500$ the model well reproduces the optical/UV/X-ray mean
quasar spectrum, in agreement with the estimates of the ionization parameter
based on the energy of the iron $K_{\alpha}$ line. We cannot definitively
reject synchrotron emission as a source of primary radiation but we favor the
model in which the hard X-ray emission is produced by Compton scattering of
soft photons in a central hot medium surrounded by cool clouds. In such a model
clouds are located typically at the distance of $\sim 12 R_{Schw}$, with the
covering factor about 0.88 and the radius of hot plasma is $\sim 9 R_{Schw}$.
The model explains optical/UV emission as predominantly due to the dark sides
of the clouds and the soft X-ray emission as due to the reflection by the
irradiated sides of the clouds. Therefore, atomic features are expected in
these bands although they are hardly present in the observational data. The
kinematical effects connected with the cloud motion affect those features but
do not remove them. The level of the primary emission required to model the
mean quasar spectrum is too low to reproduce the equivalent width of the iron
K$_{\alpha}$ line correctly but more detailed computations may resolve this
problem.
|
9808014v1
|
1998-08-10
|
The Enrichment History of Hot Gas in Poor Galaxy Groups
|
We have analyzed the ASCA SIS and GIS data for seventeen groups and
determined the average temperature and abundance of the hot x-ray emitting gas.
For groups with gas temperatures less than 1.5 keV we find that the abundance
is correlated with the gas temperature and luminosity. We have also determined
the abundance of the alpha-elements and iron independently for those groups
with sufficient counts. We find that for the cool groups (i.e. kT <1.5 keV) the
ratio of alpha-elements to iron is ~1, about half that seen in clusters.
Spectral fits with the S, Si and Fe abundances allowed to vary separately
suggest the S/Fe ratio is similar to that seen in clusters while the Si/Fe
ratio in groups is half the value determined for richer systems. The mass of
metals per unit blue luminosity drops rapidly in groups as the temperature
drops. There are two possible explanations for this decrease. One is that the
star formation in groups is very different from that in rich clusters. The
other explanation is that groups lose much of their enriched material via winds
during the early evolution of ellipticals. If the latter is true, we find that
poor groups will have contributed significantly (roughly 1/3 of the metals) to
the enrichment of the intergalactic medium.
|
9808085v1
|
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