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2006-08-03
|
The intergalactic propagation of ultra-high energy cosmic ray nuclei
|
We investigate the propagation of ultra-high energy cosmic ray nuclei (A =
1-56) from cosmologically distant sources through the cosmic radiation
backgrounds. Various models for the injected composition and spectrum and of
the cosmic infra-red background are studied using updated photo-disintegration
cross-sections. The observational data on the spectrum and the composition of
ultra-high energy cosmic rays are jointly consistent with a model where all of
the injected primary cosmic rays are iron nuclei (or a mixture of heavy and
light nuclei).
|
0608085v4
|
2006-08-16
|
Pinning Down Gravitational Settling
|
We analyse high-resolution archival UVES data of turnoff and subgiant stars
in the nearby globular cluster NGC 6397 ([Fe/H] = -2). Balmer-profile analyses
are performed to derive reddening-free effective temperatures. Due to the
limited S/N and uncertainties related to blaze removal, we find the data
quality insufficient to exclude the existence of gravitational settling. If the
newly derived effective temperatures are taken as a basis for an abundance
analysis, the photospheric iron (Fe II) abundance in the turnoff stars is 0.11
dex lower than in the (well-mixed) subgiants.
|
0608338v1
|
2006-08-16
|
Sensitivity of p-Process Nucleosynthesis to Nuclear Reaction Rates in a 25 Solar Mass Supernova Model
|
The astrophysical p process, which is responsible for the origin of the
proton rich stable nuclei heavier than iron, was investigated using a full
nuclear reaction network for a type II supernova explosion when the shock front
passes through the O/Ne layer. Calculations were performed with a multi-layer
model adopting the seed of a pre-explosion evolution of a 25 solar mass star.
The reaction flux was calculated to determine the main reaction path and
branching points responsible for synthesizing the proton rich nuclei. In order
to investigate the impact of nuclear reaction rates on the predicted p-process
abundances, extensive simulations with different sets of collectively and
individually modified neutron-, proton-, alpha-capture and photodisintegration
rates have been performed. These results are not only relevant to explore the
nuclear physics related uncertainties in p-process calculations but are also
important for identifying the strategy and planning of future experiments.
|
0608341v1
|
2006-08-31
|
Galactic chemical evolution: Carbon through Zinc
|
We calculate the evolution of heavy element abundances from C to Zn in the
solar neighborhood adopting our new nucleosynthesis yields. Our yields are
calculated for wide ranges of metallicity (Z=0-Z_\odot) and the explosion
energy (normal supernovae and hypernovae), based on the light curve and spectra
fitting of individual supernovae. The elemental abundance ratios are in good
agreement with observations. Among the alpha-elements, O, Mg, Si, S, and Ca
show a plateau at [Fe/H] < -1, while Ti is underabundant overall. The observed
abundance of Zn ([Zn/Fe] ~ 0) can be explained only by the high energy
explosion models, which requires a large contribution of hypernovae. The
observed decrease in the odd-Z elements (Na, Al, and Cu) toward low [Fe/H] is
reproduced by the metallicity effect on nucleosynthesis. The iron-peak elements
(Cr, Mn, Co, and Ni) are consistent with the observed mean values at -2.5 <
[Fe/H] < -1$, and the observed trend at the lower metallicity can be explained
by the energy effect. We also show the abundance ratios and the metallicity
distribution functions of the Galactic bulge, halo, and thick disk. Our results
suggest that the formation timescale of the thick disk is ~ 1-3 Gyr.
|
0608688v1
|
2006-09-01
|
Oxygen abundances in the Galactic Bulge: evidence for fast chemical enrichment
|
AIMS: We spectroscopically characterize the Galactic Bulge to infer its star
formation timescale, compared to the other Galactic components, through the
chemical signature on its individual stars.
METHODS: We derived iron and oxygen abundances for 50 K giants in four fields
towards the Galactic bulge. High resolution (R=45,000) spectra for the target
stars were collected with FLAMES-UVES at the VLT.
RESULTS: Oxygen, as measured from the forbidden line at 6300 \AA, shows a
well-defined trend with [Fe/H], with [O/Fe] higher in bulge stars than in thick
disk ones, which were known to be more oxygen enhanced than thin disk stars.
CONCLUSIONS: These results support a scenario in which the bulge formed
before and more rapidly than the disk, and therefore the MW bulge can be
regarded as a prototypical old spheroid, with a formation history similar to
that of early-type (elliptical) galaxies.
|
0609052v2
|
2006-09-06
|
The Supernova -- Gamma-Ray Burst Connection
|
Observations show that at least some gamma-ray bursts (GRBs) happen
simultaneously with core-collapse supernovae (SNe), thus linking by a common
thread nature's two grandest explosions. We review here the growing evidence
for and theoretical implications of this association, and conclude that most
long-duration soft-spectrum GRBs are accompanied by massive stellar explosions
(GRB-SNe). The kinetic energy and luminosity of well-studied GRB-SNe appear to
be greater than those of ordinary SNe, but evidence exists, even in a limited
sample, for considerable diversity. The existing sample also suggests that most
of the energy in the explosion is contained in nonrelativistic ejecta
(producing the supernova) rather than in the relativistic jets responsible for
making the burst and its afterglow. Neither all SNe, nor even all SNe of Type
Ibc produce GRBs. The degree of differential rotation in the collapsing iron
core of massive stars when they die may be what makes the difference.
|
0609142v1
|
2006-09-14
|
The Formation of the Double Pulsar PSR J0737-3039A/B
|
Recent timing observations of the double pulsar J0737-3039A/B have shown that
its transverse velocity is extremely low, only 10 km/s, and nearly in the Plane
of the Galaxy. With this new information, we rigorously re-examine the history
and formation of this system, determining estimates of the pre-supernova
companion mass, supernova kick and misalignment angle between the pre- and
post-supernova orbital planes. We find that the progenitor to the recently
formed `B' pulsar was probably less than 2 MSun, lending credence to
suggestions that this object may not have formed in a normal supernova
involving the collapse of an iron core. At the same time, the supernova kick
was likely non-zero. A comparison to the history of the double-neutron-star
binary B1534+12 suggests a range of possible parameters for the progenitors of
these systems, which should be taken into account in future binary population
syntheses and in predictions of the rate and spatial distribution of short
gamma-ray burst events.
|
0609416v2
|
2006-09-19
|
On the Origin of the Type Ia Supernova Width-Luminosity Relation
|
Brighter Type Ia supernovae (SNe Ia) have broader, more slowly declining
B-band light curves than dimmer SNe Ia. We study the physical origin of this
width-luminosity relation (WLR) using detailed radiative transfer calculations
of Chandrasekhar mass SN Ia models. We find that the luminosity dependence of
the diffusion time (emphasized in previous studies) is in fact of secondary
relevance in understanding the model WLR. Instead, the essential physics
involves the luminosity dependence of the spectroscopic/color evolution of SNe
Ia. Following maximum-light, the SN colors are increasingly affected by the
development of numerous Fe II/Co II lines which blanket the B-band and, at the
same time, increase the emissivity at longer wavelengths. Because dimmer SNe Ia
are generally cooler, they experience an earlier onset of Fe III to Fe II
recombination in the iron-group rich layers of ejecta, resulting in a more
rapid evolution of the SN colors to the red. The faster B-band decline rate of
dimmer SNe Ia thus reflects their faster ionization evolution.
|
0609540v1
|
2006-09-23
|
Theoretical aspects of relativistic spectral features
|
The inner parts of black-hole accretion discs shine in X-rays which can be
monitored and the observed spectra can be used to trace strong gravitational
fields in the place of emission and along paths of light rays. This paper
summarizes several aspects of how the spectral features are influenced by
relativistic effects. We focus our attention onto variable and broad emission
lines, origin of which can be attributed to the presence of orbiting patterns
-- spots and spiral waves in the disc. We point out that the observed spectrum
can determine parameters of the central black hole provided the intrinsic local
emissivity is constrained by theoretical models.
|
0609645v1
|
2006-09-27
|
The XMM-Newton RGS spectrum of the high luminosity Seyfert 1 galaxy Markarian 509
|
We present a detailed analysis of the soft X-ray spectrum of the Seyfert 1
galaxy Markarian 509 taken with the XMM-Newton Reflection Grating Spectrometer.
An underlying power-law continuum and three warm absorber phases provide a good
fit to the data along with a number of broad and narrow emission lines. Our
three warm absorber phases each have different ionization parameters and column
densities. We identify a low ionization, log xi=0.89, high outflow velocity
phase producing an Fe M-shell UTA feature along with absorption from O VI and N
VI. There is an intermediate phase, log xi=2.14, showing absorption from H-like
carbon and nitrogen and He-like neon and oxygen. The third, high ionization,
log xi=3.26, low outflow velocity phase contains absorption from O VIII, Ne X
and highly ionized iron. All phases are blueshifted with respect to the
systemic velocity with flow velocities ranging from -60km/s to -510km/s. The
observed broad emission features have an RMS velocity of ~8000km/s for the C VI
and O VII lines.
|
0609746v1
|
2006-09-29
|
On SN 2003fg: The Probable Super-Chandrasekhar-Mass SN Ia
|
Howell et al. have reported the discovery of SN Ia SN 2003fg (SNLS-03D3bb)
and conclude that SN 2003fg is very likely a super-Chandrasekhar-mass SN Ia
perhaps with a mass of order 2 solar masses. Their work is the first strong
evidence that has been presented for a super-Chandrasekhar SN Ia. We have
performed an analysis of the SN 2003fg data using the Yoon & Langer binding
energy formula for a rotating super-Chandrasekhar-mass white dwarf (also used
by Howell et al.) included in a simple model of SNe Ia (which we call the SSC
model for Simple Super-Chandrasekhar model for SNe Ia) which assumes
spherically symmetric ejecta and relies on the approximations of an exponential
density profile for SN Ia ejecta and of a sharp boundary of the SN Ia
iron-peak-element core. Our results support the conclusion of Howell et al.: SN
2003fg is very probably super-Chandrasekhar and probably has mass of order 2
solar masses.
|
0609804v3
|
2006-10-13
|
r-Process Enhanced Halo Stars
|
Abundance observations indicate the presence of rapid-neutron capture (i.e.,
r-process) elements in old Galactic halo and globular cluster stars. These
observations provide insight into the nature of the earliest generations of
stars in the Galaxy -- the progenitors of the halo stars -- responsible for
neutron-capture synthesis of the heavy elements. The large star-to-star scatter
observed in the abundances of neutron-capture element/iron ratios at low
metallicities -- which diminishes with increasing metallicity or [Fe/H] --
suggests the formation of these heavy elements (presumably from certain types
of supernovae) was rare in the early Galaxy. The stellar abundances also
indicate a change from the r-process to the slow neutron capture (i.e., s-)
process at higher metallicities in the Galaxy and provide insight into Galactic
chemical evolution. Finally, the detection of thorium and uranium in halo and
globular cluster stars offers an independent age-dating technique that can put
lower limits on the age of the Galaxy, and hence the Universe.
|
0610412v1
|
2006-10-27
|
Computation of the Fourier parameters of RR Lyrae stars by template fitting
|
Due to the importance of accurate Fourier parameters, we devise a method that
is more appropriate for deriving these parameters on low-quality data than the
traditional Fourier fitting. Based on the accurate light curves of 248
fundamental mode RR Lyrae stars, we test the power of a full-fetched
implementation of the template method in the computation of the Fourier
decomposition. The applicability of the method is demonstrated also on datasets
of filter passbands different from that of the template set. We examine in more
detail the question of the estimation of Fourier- based iron abundance [Fe/H]
and average brightness. We get, for example, for light curves sampled randomly
in 30 data points with sigma=0.03 mag observational noise that optimized direct
Fourier fits yield sigma_[Fe/H]=0.33, whereas the template fits result in
sigma_[Fe/H]=0.18. Tests made on the RR Lyrae database of the Large Magellanic
Cloud (LMC) of the Optical Gravitational Lensing Experiment (OGLE) support the
applicability of the method on real photometric time series. These tests also
show that the dominant part of error in estimating the average brightness comes
from other sources, most probably from crowding effects, even for under-sampled
light curves.
|
0610823v1
|
2006-11-01
|
Amplification and variability of the AGN X-ray emission due to microlensing
|
We consider the contribution of microlensing to the AGN Fe K$\alpha$ line and
X-ray continuum amplification and variation. To investigate the variability of
the line and X-ray continuum, we studied the effects of microlensing on quasar
X-ray spectra produced by crossing of a microlensing pattern across a standard
relativistic accretion disk. To describe the disk emission we used a ray
tracing method considering both metrics, Schwarzschild and Kerr. We found that
the Fe K$\alpha$ and continuum may experience significant amplification by a
microlensing event (even for microlenses of very small mass). Also, we
investigate a contribution of microlensing to the X-ray variability of
high-redshifted QSOs, finding that cosmologically distributed deflector may
contribute significantly to the X-ray variability of high-redshifted QSOs
(z>2).
|
0611016v1
|
2006-11-23
|
Two Disk Components from a Gas Rich Disk-Disk Merger
|
We employ N-body, smoothed particle hydrodynamical simulations, including
detailed treatment of chemical enrichment, to follow a gas-rich merger which
results in a galaxy with disk morphology. We trace the kinematic, structural
and chemical properties of stars formed before, during, and after the merger.
We show that such a merger produces two exponential disk components, with the
older, hotter component having a scale-length 20% larger than the
later-forming, cold disk. Rapid star formation during the merger quickly
enriches the protogalactic gas reservoir, resulting in high metallicities of
the forming stars. These stars form from gas largely polluted by Type II
supernovae, which form rapidly in the merger-induced starburst. After the
merger, a thin disk forms from gas which has had time to be polluted by Type Ia
supernovae. Abundance trends are plotted, and we examine the proposal that
increased star formation during gas-rich mergers may explain the high
alpha-to-iron abundance ratios which exist in the relatively high-metallicity
thick disk component of the Milky Way.
|
0611748v1
|
2006-11-28
|
XMM-Newton study of 0.012<z<0.024 groups. I: Overview of the IGM thermodynamics
|
We study the thermodynamic properties of the hot gas in a sample of groups in
the 0.012-0.024 redshift range, using XMM-Newton observations. We present
measurements of temperature, entropy, pressure and iron abundance.
Non-parametric fits are used to derive the mean properties of the sample and to
study dispersion in the values of entropy and pressure. The scaling of the
entropy at 0.2r500 matches well the results of Ponman et al. (2003). However,
compared to cool clusters, the groups in our sample reveal larger entropy at
inner radii and a substantially flatter slope in the entropy in the outskirts,
compared to both the prediction of pure gravitational heating and to
observations of clusters. This difference corresponds to the systematically
flatter group surface brightness profiles, reported previously. The scaled
pressure profiles can be well approximated with a Sersic model with n=4. We
find that groups exhibit a systematically larger dispersion in pressure,
compared to clusters of galaxies, while the dispersion in entropy is similar.
|
0611846v2
|
2006-11-28
|
The Two-dimensional XMM-Newton Group Survey: z<0.012 groups
|
We present the results of the 2-dimensional XMM-Newton Group Survey (2dXGS),
an archival study of nearby galaxy groups. In this paper we consider eleven
nearby systems (z<0.012) in Mulchaey et al. (2003), which span a broad range in
X-ray luminosity from 10^40 to 10^43 ergs/s. We measure the iron abundance and
temperature distribution in these systems and derive pressure and entropy maps.
We find statistically significant evidence for structure in the entropy and
pressure of the gas component of seven groups on the 10-20% level. The
XMM-Newton data for the three groups with best statistics also suggest patchy
metalicity distributions within the central 20--50 kpc of the brightest group
galaxy, probed with 2-10 kpc resolution. This provides insights into the
processes associated with thermalization of the stellar mass loss. Analysis of
the global properties of the groups reveals a subclass of X-ray faint groups,
which are characterized by both higher entropy and lower pressure. We suggest
that the merger history of the central elliptical is responsible for both the
source and the observed thermodynamical properties of the hot gas of the X-ray
faint groups.
|
0611858v1
|
2006-12-12
|
Search for narrow energy-shifted lines in XMM-Newton AGN spectra
|
The detection of X-ray narrow spectral features in the 5-7 keV band is
becoming increasingly more common in AGN observations, thanks to the
capabilities of current X-ray satellites. Such lines, both in emission and in
absorption, are mostly interpreted as arising from Iron atoms. When observed
with some displacement from their rest frame position, these lines carry the
potential to study the motion of circumnuclear gas in AGN, providing a
diagnostic of the effects of the gravitational field of the central black hole.
These narrow features have been often found with marginal statistical
significance. We are carrying on a systematic search for narrow features using
spectra of bright type 1 AGNs available in the XMM-Newton archive. The aim of
this work is to characterise the occurrence of the narrow features phenomenon
on a large sample of objects and to estimate the significance of the features
through Monte Carlo simulations. The project and preliminary results are
presented.
|
0612317v1
|
2006-12-15
|
Non-LTE Models for Neutron Star Atmospheres and Supernova-Fallback Disks
|
We describe our recent progress in modeling supernova-fallback disks and
neutron star (NS) atmospheres.
We present a first detailed spectrum synthesis calculation of a SN-fallback
disk composed of iron. We assume a thin disk with a radial structure described
by the alpha-disk model. The vertical structure and emission spectrum are
computed self-consistently by solving the structure equations simultaneously
with the radiation transfer equations under non-LTE conditions. We describe the
properties of a specific disk model and discuss various effects on the emergent
UV/optical spectrum.
We investigate Compton scattering effects on the thermal spectrum of NSs. In
addition, we constructed a new generation of metal line-blanketed non-LTE model
atmospheres for NSs. It is compared to X-ray burst spectra of EXO0748-676. It
is possible that the gravitational redshift, deduced from absorption lines, is
lower (z=0.24) than hitherto assumed (z=0.35). Accordingly, this would result
in NS mass and radius lower limits of M>1.63Msun and R>13.8 km.
|
0612425v1
|
2006-12-28
|
Heating of Heavy Ions by Interplanetary Coronal Mass Ejection (ICME) Driven Collisionless Shocks
|
Shock heating and particle acceleration processes are some of the most
fundamental physical phenomena of plasma physics with countless applications in
laboratory physics, space physics, and astrophysics. This study is motivated by
previous observations of non-thermal heating of heavy ions in astrophysical
shocks (Korreck et al. 2004). Here, we focus on shocks driven by Interplanetary
Coronal Mass Ejections (ICMEs) which heat the solar wind and accelerate
particles. This study focuses specifically on the heating of heavy ions caused
by these shocks. Previous studies have focused only on the two dynamically
dominant species, H+ and He2+ . This study utilizes thermal properties measured
by the Solar Wind Ion Composition Spectrometer (SWICS) aboard the Advanced
Composition Explorer (ACE) spacecraft to examine heavy ion heating. This
instrument provides data for many heavy ions not previously available for
detailed study, such as Oxygen (O6+, O7+), Carbon (C5+, C6+), and Iron (Fe10+).
The ion heating is found to depend critically on the upstream plasma
|
0612767v1
|
2007-01-05
|
Ruling out a massive-assymptoic giant-branch star as the progenitor of supernova 2005cs
|
We calculate the predicted UBVRIJHK absolute magnitudes for models of
supernova progenitors and apply the result to the case of supernova 2005cs. We
agree with previous results that the initial mass of the star was of low,
around 6 to 8 M(sun). However such stars are thought to go through second
dredge-up to become AGB stars. We show that had this occurred to the progenitor
of 2005cs it would have been observed in JHK pre-explosion images. The
progenitor was not detected in these bands and therefore we conclude that it
was not an AGB star. Furthermore if some AGB stars do produce supernovae they
will have a clear signature in pre-explosion near-infrared images.
Electron-capture supernovae are thought to occur in AGB stars, hence the
implication is that 2005cs was not an electron-capture supernova but was the
collapse of an iron core.
|
0701152v1
|
2007-01-07
|
Modeling the X-ray fractional variability spectrum of Active Galactic Nuclei using multiple flares
|
Using Monte-Carlo simulations of X-ray flare distributions across the
accretion disk of active galactic nuclei (AGN), we obtain modeling results for
the energy-dependent fractional variability amplitude. Referring to previous
results of this model, we illustrate the relation between the shape of the
point-to-point fractional variability spectrum, F_pp, and the time-integrated
spectral energy distribution, F_E. The results confirm that the spectral shape
and variability of the iron Kalpha line are dominated by the flares closest to
the disk center.
|
0701164v1
|
2007-01-07
|
Ultrahigh energy cosmic rays as heavy nuclei from cluster accretion shocks
|
Large-scale accretion shocks around massive clusters of galaxies, generically
expected in the cold dark matter scenario of cosmological structure formation,
are shown to be plausible sources of the observed ultrahigh energy cosmic rays
(UHECRs) by accelerating a mixture of heavy nuclei including the iron group
elements. Current observations can be explained if the source composition at
injection for the heavier nuclei is somewhat enhanced from simple expectations
for the accreting gas. The proposed picture should be clearly testable by
current and upcoming facilities in the near future through characteristic
features in the UHECR spectrum, composition and anisotropy, in particular the
rapid increase of the average mass composition with energy from $10^{19}$ to
$10^{20}$ eV.
|
0701167v3
|
2007-01-14
|
Interstellar Iron and Silicon Depletions in Translucent Sight Lines
|
We report interstellar FeII and SiII column densities toward six translucent
sight lines (A_V >~ 1) observed with the Space Telescope Imaging Spectrograph
(STIS). The abundances were determined from the absorption of SiII] at 2335
Angstroms, and several weak Fe transitions including the first reported
detections of the 2234 Angstrom line. We derive an empirical f-value for the
FeII 2234 Angstrom transition of log(f lambda) = -1.54 +/- 0.05. The observed
sight lines sample a variety of extinction characteristics as indicated by
their R_V values, which range from 2.6 - 5.8. The dust-phase abundances of both
Si and Fe are positively correlated with the small-grain population (effective
radii smaller than a few hundred micron) toward the targets. The physical
conditions along the sight lines suggest that this relationship may be due to
differences in the survival of small particles in some interstellar
environments. The chemical composition of the small grains could either
resemble dust mantles or be silicate rich.
|
0701408v1
|
2007-01-19
|
Exploring the X-ray spectral variability of MCG-6-30-15 with XMM-Newton
|
We present a study of the spectral variability of the Seyfert I galaxy
MCG-6-30-15 based on the two long XMM-Newton observations from 2000 and 2001.
The X-ray spectrum and variability properties of the 2001 data have previously
been well described with a two-component model consisting of a variable power
law and a much less variable reflection component, containing a broad
relativistic iron line from the accretion disc around a rapidly rotating Kerr
black hole. The lack of variability of the reflection component has been
interpreted as an effect of strong gravitational light bending very close to
the central black hole. Using an improved reflection model, we fit the
two-component model to time-resolved spectra of both observations. Assuming
that the photon index of the power law is constant we reconfirm the old result
and show that this does not depend on the time-scale of the analysis.
|
0701567v1
|
2007-01-20
|
The origin of the ankle
|
The differential intensity of cosmic radiation shows a sequence of
depressions referred to as "knees" in a large energy band above 10^15 eV. The
global depression entailed in the complete spectrum with respect to the
extrapolated intensity based on low energy data amounts to a maximum factor of
8, occurring at 5 x 10^18 eV, where flux measurements exhibit a relative
minimum, referred to as the "ankle". It is demonstrated by a full simulation of
cosmic ray trajectories in the Galaxy that the intensity minimum around the
ankle energy is primarily due to the nuclear interactions of the cosmic ions
with the interstellar matter and to the galactic magnetic field. "Ankles"
signal the onset energies of the rectilinear propagation in the Milky Way at
the Earth, being for example, 4 x 10^18 eV for helium and 6 x 10^19 eV for
iron. The ankle, in spite of its notable importance at the Earth, is a local
perturbation of the universal spectrum which, between the knee and the ankle,
decreases by a round factor 10^9, regaining its unperturbed status above 10^19
eV.
|
0701593v1
|
2007-01-23
|
INTEGRAL Observations of the Vela Region Focusing on Vela X-1
|
The Vela region has been observed for 1.7 Ms in November 2005 by the INTEGRAL
satellite. We present preliminary spectral and temporal results of Vela X-1, an
eclipsing neutron star hosted in a wind-accreting high-mass X-ray binary
system. Using data from ISGRI, SPI and JEM-X, we firmly confirm the existence
of cyclotron resonant scattering features (CRSF) at ~27 keV and ~54 keV,
implying a neutron-star magnetic field of ~3 10^12 Gauss, and the presence of
an iron emission line at ~6.5 keV. During two strong flares those parameters
remained unchanged. Furthermore we measure the neutron-star spin period of
283.6 s, indicating a still constant trend.
|
0701641v1
|
2007-01-26
|
First ground based measurement of atmospheric Cherenkov light from cosmic rays
|
A recently proposed novel technique for the detection of cosmic rays with
arrays of Imaging Atmospheric Cherenkov Telescopes is applied to data from the
High Energy Stereoscopic System (H.E.S.S.). The method relies on the ground
based detection of Cherenkov light emitted from the primary particle prior to
its first interaction in the atmosphere. The charge of the primary particle (Z)
can be estimated from the intensity of this light, since it is proportional to
Z$^2$. Using H.E.S.S. data, an energy spectrum for cosmic-ray iron nuclei in
the energy range 13--200 TeV is derived. The reconstructed spectrum is
consistent with previous direct measurements and is one of the most precise so
far in this energy range.
|
0701766v1
|
2007-03-10
|
Constraining global parameters of accreting black holes by modeling magnetic flares
|
We present modeling results for the reprocessed radiation expected from
magnetic flares above AGN accretion disks. Relativistic corrections for the
orbital motion of the flare and for the curved space-time in the vicinity of
the black hole are taken into account. We investigate the local emission
spectra, as seen in a frame co-orbiting with the disk, and the observed spectra
at infinity. We investigate long-term flares at different orbital phases and
short-term flares for various global parameters of the accreting black hole.
Particular emphasis is put on the relation between the iron Kalpha line and the
Compton hump as these two features can be simultaneously observed by the Suzaku
satellite and later by Simbol-X.
|
0703240v1
|
2007-03-14
|
Absorption Measure Distribution of the Outflow in IRAS 13349+2438: Direct Observation of Thermal Instability?
|
We analyze the Chandra X-ray spectrum obtained with the HETGS grating
spectrometer of IRAS 13349+2438, which has one of the richest absorption
spectra of a quasar outflow. Absorption from almost all charge states of Fe is
detected. This allows for a detailed reconstruction of the absorption measure
distribution (AMD), which we define as the continuous distribution of column
density as a function of ionization parameter. We find a double peaked AMD for
IRAS 13349+2438 with a total (ionized) column density of N_H = (1.2 +- 0.3)*
10^{22} cm^-2} assuming solar iron abundance. For comparison, we perform a
similar analysis on the well studied HETGS spectrum of NGC 3783. Both sources
feature a deep minimum in column density consistent with no absorption from gas
at temperatures of 4.5 < log T < 5 (K). We interpret the minima as
observational evidence for thermal instability in this temperature regime.
|
0703351v1
|
2007-03-14
|
Delayed detonations in full-star models of Type Ia supernova explosions
|
Aims: We present the first full-star three-dimensional explosion simulations
of thermonuclear supernovae including parameterized deflagration-to-detonation
transitions that occur once the flame enters the distributed burning regime.
Methods: Treating the propagation of both the deflagration and the detonation
waves in a common front-tracking approach, the detonation is prevented from
crossing ash regions. Results: Our criterion triggers the detonation wave at
the outer edge of the deflagration flame and consequently it has to sweep
around the complex structure and to compete with expansion. Despite the impeded
detonation propagation, the obtained explosions show reasonable agreement with
global quantities of observed type Ia supernovae. By igniting the flame in
different numbers of kernels around the center of the exploding white dwarf, we
set up three different models shifting the emphasis from the deflagration phase
to the detonation phase. The resulting explosion energies and iron group
element productions cover a large part of the diversity of type Ia supernovae.
Conclusions: Flame-driven deflagration-to-detonation transitions, if
hypothetical, remain a possibility deserving further investigation.
|
0703378v1
|
2007-03-19
|
Stokes diagnostics of simulated solar magneto-convection
|
We present results of synthetic spectro-polarimetric diagnostics of radiative
MHD simulations of solar surface convection with magnetic fields. Stokes
profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared
spectral ranges emerging from the simulated atmosphere have been calculated in
order to study their relation to the relevant physical quantities and compare
with observational results. We have analyzed the dependence of the Stokes-I
line strength and width as well as of the Stokes-V signal and asymmetries on
the magnetic field strength. Furthermore, we have evaluated the correspondence
between the actual velocities in the simulation with values determined from the
Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles
(zero-crossing shift). We confirm that the line weakening in strong magnetic
fields results from a higher temperature (at equal optical depth) in the
magnetic flux concentrations. We also confirm that considerable Stokes-V
asymmetries originate in the peripheral parts of strong magnetic flux
concentrations, where the line of sight cuts through the magnetopause of the
expanding flux concentration into the surrounding convective donwflow.
|
0703490v1
|
2007-03-22
|
Statistical methods applied to composition studies of ultrahigh energy cosmic rays
|
The mass composition of high energy cosmic rays above $10^{17}$ eV is a
crucial issue to solve some open questions in astrophysics such as the
acceleration and propagation mechanisms. Unfortunately, the standard procedures
to identify the primary particle of a cosmic ray shower have low efficiency
mainly due to large fluctuations and limited experimental observables. We
present a statistical method for composition studies based on several
measurable features of the longitudinal development of the CR shower such as
$N_{max}$, $X_{max}$, asymmetry, skewness and kurtosis. Principal component
analysis (PCA) was used to evaluate the relevance of each parameter in the
representation of the overall shower features and a linear discriminant
analysis (LDA) was used to combine the different parameters to maximize the
discrimination between different particle showers. The new parameter from LDA
provides a separation between primary gammas, proton and iron nuclei better
than the procedures based on $X_{max}$ only. The method proposed here was
successfully tested in the energy range from $10^{17}$ to $10^{20}$ eV even
when limitations of shower track length were included in order to simulate the
field of view of fluorescence telescopes.
|
0703582v1
|
1993-05-11
|
Electronic Structure and Bonding in Epitaxially Stabilized Cubic Iron Silicides
|
We present an ab initio full-potential linearized augmented plane-wave
(FLAPW) study of the structural and electronic properties of the two bulk
unstable compounds FeSi (CsCl structure) and FeSi$_2$ (CaF$_2$ structure) which
have recently been grown by molecular beam epitaxy on Si(111). We obtain
equilibrium bulk lattice constants of 2.72 \AA\ and 5.32 \AA\ for FeSi and
FeSi$_2$, respectively. The density of states (DOS) of FeSi agrees well with
experiment, and shows metallic behavior. In agreement with a previous
calculation the DOS of FeSi$_2$ shows a large density of $d$-states at the
Fermi level, explaining the instability of the bulk phase. The electron charge
distributions reveal a small charge transfer from Si to Fe atomic spheres in
both compounds. While in FeSi the Fe-Si bond is indeed partially ionic, we show
that in FeSi$_2$ the electron distribution corresponds to a covalent charge
accumulation in the Fe-Si bond region. The reversed order of $d$-bands in FeSi
with respect to FeSi$_2$ is understood in terms of crystal field splitting and
Fe-Fe nearest neighbor $dd$-interactions in the CsCl structure, and a strong Si
$p$/Fe $d$ bonding in the fluorite structure, respectively.
|
9305007v1
|
1994-06-06
|
Volume Effect in the Landau Theory of Martensitic Phase Transitions in Cubic Crystals
|
An effect of the volume change upon proper ferroelastic (martensitic) phase
transitions in cubic crystals is considered. Corresponding terms in the
Ginzburg-Landau expansion of the Gibbs free energy are analyzed for the first-
as well as second-order phase transitions from cubic to tetragonal lattice
under the action of uniaxial and hydrostatic pressure. The pressure effect on
the critical temperature as well as on the phase transition anomalies of
isothermal compressibility and linear thermal expansion coefficient are studied
and recent experimental data on thermal expansion anomalies in V$_3$Si, In-Tl
and Ni-Al are discussed. The non-linearity of thermal expansion leads to the
special relation between the shear strain and volume change as a result of the
elastic energy minimization. This phenomenon can provide the transformation
from FCC lattice to BCC one, observed in the iron alloys.
|
9406030v2
|
1995-08-26
|
Magnetic Properties of Epitaxial and Polycrystalline Fe/Si Multilayers
|
Fe/Si multilayers with antiferromagnetic interlayer coupling have been grown
via ion-beam sputtering on both glass and single-crystal substrates. High-angle
x-ray diffraction measurements show that both sets of films have narrow Fe
peaks, implying a large crystallite size and crystalline iron silicide spacer
layers. Low-angle x-ray diffraction measurements show that films grown on glass
have rougher interfaces than those grown on single-crystal substrates. The
multilayers grown on glass have a larger remanent magnetization than the
multilayers grown on single-crystal substrates. The observation of
magnetocrystalline anisotropy in hysteresis loops and $(hkl)$ peaks in x-ray
diffraction demonstrates that the films grown on MgO and Ge are epitaxial. The
smaller remanent magnetization in Fe/Si multilayers with better layering
suggests that the remanence is not an intrinsic property.
|
9508117v1
|
1995-12-18
|
Soft X-ray Fluorescence Study of Buried Silicides in Antiferromagnetically Coupled Fe/Si Multilayer
|
Soft x-ray fluorescence spectroscopy has been employed to obtain information
about the Si-derived valence band states of Fe/Si multilayers. The valence band
spectra are quite different for films with and without antiferromagnetic
interlayer exchange coupling, demonstrating that these multilayers have
different silicide phases in their spacer layers. Comparison with previously
published fluorescence data on bulk iron silicides shows that the Fe
concentration in the silicide spacer layers is substantial. Near-edge x-ray
absorption data on antiferromagnetically coupled multilayers in combination
with the fluorescence data demonstrate unambiguously that the silicide spacer
layer in these films is metallic. These results on the electronic structure of
buried layers in a multilayer film exemplify the wide range of experiments made
possible by new high-brightness synchrotron sources.
|
9512126v1
|
1996-09-11
|
Exchange Coupling Between Iron Layers Separated by BCC Copper
|
The exchange coupling between Fe layers separated by BCC Cu is calculated for
Fe/Cu/Fe (001) trilayers. It is shown that the coupling is basically regulated
by three extrema of the bulk BCC Cu Fermi surface. The contributions from those
extrema are all of the same order of magnitude, but that associated with the
``belly'' at the $\Gamma$-point dominates. The calculated temperature
dependence of the coupling varies considerably with spacer layer thickness.
Individually, the amplitudes of these extrema contribution decrease with
temperature, each according to a different rate. Such an effect may cause an
actual increase of coupling with temperature for some Cu thicknesses.
|
9609104v1
|
1997-06-04
|
Monte Carlo Simulation of Magnetization Reversal in Fe Sesquilayers on W(110)
|
Iron sesquilayers grown at room temperature on W(110) exhibit a pronounced
coercivity maximum near a coverage of 1.5 atomic monolayers. On lattices which
faithfully reproduce the morphology of the real films, a kinetic Ising model is
utilized to simulate the domain-wall motion. Simulations reveal that the
dynamics is dominated by the second-layer islands, which act as pinning
centers. The simulated dependencies of the coercivity on the film coverage, as
well as on the temperature and the frequency of the applied field, are very
similar to those measured in experiments. Unlike previous micromagnetic models,
the presented approach provides insight into the dynamics of the domain-wall
motion and clearly reveals the role of thermal fluctuations.
|
9706036v2
|
1998-01-07
|
Monte Carlo simulation of magnetization reversal via domain-wall motion in Fe sesquilayers on W(110)
|
Iron sesquilayers are ultrathin films with coverages between one and two
atomic monolayers. They consist of an almost defect-free monolayer with compact
islands of a second atomic layer on top. This variation of the film thickness
results in a strong interaction between domain walls and the island structure.
It makes these systems an ideal laboratory to study the dynamics of domain
walls driven by weak external fields. We present computer simulations which
provide insight into the role of the thermally activated nucleation processes
by which a driven domain wall overcomes the obstacles created by the islands.
|
9801052v1
|
1998-04-03
|
First principles simulations of liquid Fe-S under Earth's core conditions
|
First principles electronic structure calculations, based upon density
functional theory within the generalized gradient approximation and ultra-soft
Vanderbilt pseudopotentials, have been used to simulate a liquid alloy of iron
and sulfur at Earth's core conditions. We have used a sulfur concentration of
$\approx 12 % $wt, in line with the maximum recent estimates of the sulfur
abundance in the Earth's outer core. The analysis of the structural, dynamical
and electronic structure properties has been used to report on the effect of
the sulfur impurities on the behavior of the liquid. Although pure sulfur is
known to form chains in the liquid phase, we have not found any tendency
towards polymerization in our liquid simulation. Rather, a net S-S repulsion is
evident, and we propose an explanation for this effect in terms of the
electronic structure. The inspection of the dynamical properties of the system
suggests that the sulfur impurities have a negligible effect on the viscosity
of Earth's liquid core.
|
9804035v1
|
1998-05-10
|
Assisted Tunneling in Ferromagnetic Junctions and Half-Metallic Oxides
|
Different mechanisms of spin-dependent tunneling are analyzed with respect to
their role in tunnel magnetoresistance (TMR). Microscopic calculation within a
realistic model shows that direct tunneling in iron group systems leads to
about a 30% change in resistance, which is close but lower than experimentally
observed values. The larger observed values of the tunnel magnetoresistance
(TMR) might be a result of tunneling involving surface polarized states. It is
found that tunneling via resonant defect states in the barrier radically
decreases the TMR by order of magnitude. One-magnon emission is shown to reduce
the TMR, whereas phonons increase the effect. The inclusion of both magnons and
phonons reasonably explains an unusual bias dependence of the TMR. The model
presented here is applied qualitatively to half-metallics with 100% spin
polarization, where one-magnon processes are suppressed and the change in
resistance in the absence of spin-mixing on impurities may be arbitrarily
large. Even in the case of imperfect magnetic configurations, the resistance
change can be a few 1000 percent. Examples of half-metallic systems are
CrO2/TiO2 and CrO2/RuO2.
|
9805114v1
|
1998-07-14
|
The hematite (alpha-Fe_2 O_3) (0001) surface: evidence for domains of distinct chemistry
|
Using spin-density functional theory we investigated various possible
structures of the hematite (0001) surface. Depending on the ambient oxygen
partial pressure, two geometries are found to be particularly stable under
thermal equilibrium: one being terminated by iron and the other by oxygen. Both
exhibit huge surface relaxations (-57 % for the Fe- and -79 % for the
O-termination) with important consequences for the surface electronic and
magnetic properties. With scanning tunneling microscopy we observe two
different surface terminations coexisting on single crystalline alpha-Fe_2 O_3
(0001) films, which were prepared in high oxygen pressures.
|
9807202v1
|
1998-08-08
|
The Influence of the Bulk Density on the Intergranular Properties of YBa2Cu3-xFexOy (0<x<0.01) Ceramics
|
The influence of the bulk density of YBa2Cu3-xFexOy (0 < x < 0.01) ceramics
on the intergranular superconducting (SC) properties was studied using the
temperature dependence of AC magnetic susceptibility measurements. It was found
that the simultaneous variation of the sample's density and the iron impurity
concentration does not influence effectively the onset temperature of the
superconducting state Tc(on). While only increasing of the sample's density
shifts the intergranular hysteresis losses peak temperature Tm(J) to the lower
values which connects with the decreasing of the Josephson magnetic vortices
pinning role. It was established that the shielding capability and Tm(J)
display a plateau with X in the 0.003 < X < 0..007 region which is due to the
monotonous decrease of the sample's density. It was shown that the shielding
capability at the T=78K for the sample with 3.8g/cm3 is two times higher than
that for the sample with the density of 5.0g/cm3. The possible interpretations
of the observed results are discussed.
|
9808086v1
|
1998-10-28
|
Interplay of Chemical Bonding and Magnetism in Fe_4N, Fe_3N, Fe_2N
|
Using spin density functional theory we have carried out a comparative study
of chemical bonding and magnetism in Fe_4N, Fe_3N and Fe_2N. All of these
compounds form close packed Fe lattices, while N occupies octahedral
interstitial positions. High spin fcc Fe and hypothetical FeN with rock salt
structure have been included in our study as reference systems. We find strong,
covalent Fe-N bonds as a result of a substantial \sigma-type p-d hybridisation,
with some charge transfer to N. Those Fe d orbitals which contribute to the p-d
bonds, do no longer participate in the exchange splitting of the Fe d bands.
Because of the large exchange fields, the majority spin d bands are always
fully occupied, while the minority spin d bands are close to half-filling, thus
optimizing the Fe d-d covalent bonding. As a consequence, in good approximation
the individual Fe moments decrease in steps of 0.5 \mu_B from fcc iron (2.7
\mu_B) via Fe_4N (2.7 and 1.97 mu_B}), \chem{Fe_3N} (1.99 \mu_B) to \zeta -
Fe_2N (1.43 \mu_B).
|
9810383v1
|
1998-12-16
|
Superconducting and Magnetic Properties of Nb/Pd_1-xFe_x/Nb Triple Layers
|
The superconducting and magnetic properties of Nb/Pd_1-xFe_x/Nb triple layers
with constant Nb layer thickness d_Nb=200 A and different interlayer
thicknesses are investigated. The thickness dependence of the magnetization and
of the superconducting transition temperature shows that for small iron
concentration x the Pd_1-xFe_x layer is likely to be in the paramagnetic state
for very thin films whereas ferromagnetic order is established for x>=0.13. The
parallel critical field B_c2II(T) exhibits a crossover from two-dimensional
(2D) behavior where the Nb films are coupled across the interlayer, towards a
2D behavior of coupled Nb films with increasing d_PdFe or x. This 2D-2D
crossover allows a determination of the penetration depth xi_F of Cooper pairs
into the Pd_1-xFe_x layer as a function of x. For samples with a ferromagnetic
interlayer xi_ is found to be independent of x.
|
9812278v1
|
1999-11-24
|
Kinetics of moisture absorption in mixtures for iron foundry
|
The moisture absorption in granulated materials used in foundry technologies
is analyzed. The absorption process has a diffusive behavior mainly. A simple
experimental technique, in which the wet weight increment was recorded as the
experimental parameter and an analytic method with computing procedure to find
the parameters characterizing the process was used. The determination of these
parameters by traditional methods is a very difficult task so, very refined and
expensive trials are needed. The fitting of the model permits to determine the
diffusion coefficient and the moisture concentration in the separation surface
between the sample and the environment. The concentration profiles are
established for different times. Finally, the possibility of occurrence of
superimposed diffusive processes in some materials is analyzed and the
diffusion coefficient and the amount of moisture incorporated by each process
are calculated.
|
9911400v1
|
2000-04-18
|
Metallic and nonmetallic double perovskites: A case study of A$_2$FeReO$_6$ (A= Ca, Sr, Ba)
|
We have investigated the structure and electronic properties of ferrimagnetic
double perovskites, A2FeReO6 (A= Ca, Sr, Ba). The A=Ba phase is cubic (Fm3m)
and metallic, while the A=Ca phase is monoclinic (P21/n) and nonmetallic.
57Fe Mossbauer spectroscopy shows that iron is present mainly in the
high-spin (S=5/2) Fe3+ state in the Ca compound, while it occurs in an
intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. It is
argued that a direct Re t2g - Re t2g interaction is the main cause for the
metallic character of the Ba compound; the high covalency of Ca-O bonds and the
monoclinic distortion (which lifts the degeneracy of t2g states) seem to
disrupt the Re-Re interaction in the case of the Ca compound, making it
non-metallic for the same electron count.
|
0004315v1
|
2000-05-05
|
Analysis of Dislocation Mechanism for Melting of Elements: Pressure Dependence
|
In the framework of melting as a dislocation-mediated phase transition we
derive an equation for the pressure dependence of the melting temperatures of
the elements valid up to pressures of order their ambient bulk moduli. Melting
curves are calculated for Al, Mg, Ni, Pb, the iron group (Fe, Ru, Os), the
chromium group (Cr, Mo, W), the copper group (Cu, Ag, Au), noble gases (Ne, Ar,
Kr, Xe, Rn), and six actinides (Am, Cm, Np, Pa, Th, U). These calculated
melting curves are in good agreement with existing data. We also discuss the
apparent equivalence of our melting relation and the Lindemann criterion, and
the lack of the rigorous proof of their equivalence. We show that the would-be
mathematical equivalence of both formulas must manifest itself in a new
relation between the Gr\"{u}neisen constant, bulk and shear moduli, and the
pressure derivative of the shear modulus.
|
0005118v1
|
2000-05-29
|
Electronic and Magnetic Properties of Febr2
|
Electronic and magnetic (e-m) properties of FeBr2 have been surprisingly well
described as originating from the Fe2+ ions and their fine electronic
structure. The fine electronic structure have been evaluated taking into
account the spin-orbit (s-o) coupling, crystal-field and inter-site
spin-dependent interactions. The required magnetic doublet ground state with an
excited singlet at D=2.8 meV results from the trigonal distortion. This effect
of the trigonal distortion and a large magnetic moment of iron, of 4.4 mB, can
be theoretically derived provided the s-o coupling is correctly taking into
account. The obtained good agreement with experimental data indicates on
extremaly strong correlations of the six 3d electrons in the Fe2+ ion yielding
their full localization and the insulating state. These calculations show that
for the meaningful analysis of e-m properties of FeBr2 the spin-orbit coupling
is essentially important and that the orbital moment (0.74 mB) is largely
unquenched (by the off-cubic trigonal distortion in the presence of the
spin-orbit coupling).
|
0005502v1
|
2000-08-08
|
Influence of carbon and nitrogen on electronic structure and hyperfine interactions in fcc iron-based alloys
|
Carbon and nitrogen austenites, modeled by Fe8N and Fe8C superstructures are
studied by full-potential LAPW method. Structure parameters, electronic and
magnetic properties as well as hyperfine interaction parameters are obtained.
Calculations prove that Fe-C austenite can be successfully modeled by ordered
Fe8C superstructure. The results show that chemical Fe-C bond in Fe8C has
higher covalent part than in Fe8N. Detailed analysis of electric field gradient
formation for both systems is performed. The calculation of electric field
gradient allow us to carry out a good interpretation of Moessbauer spectra for
Fe-C and Fe-N systems.
|
0008129v1
|
2000-11-23
|
Magnetization in Molecular Iron Rings
|
The organometallic ring molecules Fe_6 and Fe_10 are leading examples of a
class of nanoscopic molecular magnets, which have been of intense recent
interest both for their intrinsic magnetic properties and as candidates for the
observation of macroscopic quantum coherent phenomena. Torque magnetometry
experiments have been performed to measure the magnetization in single crystals
of both systems. We provide a detailed interpretation of these results, with a
view to full characterization of the material parameters. We present both the
most accurate numerical simulations performed to date for ring molecules, using
Exact Diagonalization and Density Matrix Renormalization Group techniques, and
a semiclassical description for purposes of comparison. The results permit
quantitative analysis of the variation of critical fields with angle, of the
nature and height of magnetization and torque steps, and of the width and
rounding of the plateau regions in both quantities.
|
0011403v1
|
2001-01-31
|
Langevin Simulation of Thermally Activated Magnetization Reversal in Nanoscale Pillars
|
Numerical solutions of the Landau-Lifshitz-Gilbert micromagnetic model
incorporating thermal fluctuations and dipole-dipole interactions (calculated
by the Fast Multipole Method) are presented for systems composed of nanoscale
iron pillars of dimension 9 nm x 9 nm x 150 nm. Hysteresis loops generated
under sinusoidally varying fields are obtained, while the coercive field is
estimated to be 1979 $\pm$ 14 Oe using linear field sweeps at T=0 K. Thermal
effects are essential to the relaxation of magnetization trapped in a
metastable orientation, such as happens after a rapid reversal of an external
magnetic field less than the coercive value. The distribution of switching
times is compared to a simple analytic theory that describes reversal with
nucleation at the ends of the nanomagnets. Results are also presented for
arrays of nanomagnets oriented perpendicular to a flat substrate. Even at a
separation of 300 nm, where the field from neighboring pillars is only $\sim$ 1
Oe, the interactions have a significant effect on the switching of the magnets.
|
0101477v2
|
2001-09-28
|
Density functional simulation of small Fe nanoparticles
|
We calculate from first principles the electronic structure, relaxation and
magnetic moments in small Fe particles, applying the numerical local orbitals
method in combination with norm-conserving pseudopotentials. The accuracy of
the method in describing elastic properties and magnetic phase diagrams is
tested by comparing benchmark results for different phases of crystalline iron
to those obtained by an all-electron method. Our calculations for the
bipyramidal Fe_5 cluster qualitatively and quantitatively confirm previous
plane-wave results that predicted a non-collinear magnetic structure. For
larger bcc-related (Fe_35) and fcc-related (Fe_38, Fe_43, Fe_62) particles, a
larger inward relaxation of outer shells has been found in all cases,
accompanied by an increase of local magnetic moments on the surface to beyond 3
mu_B.
|
0109540v2
|
2001-10-09
|
Decay of spin-Peierls state in CuGeO3:Fe. The case of a strong disorder
|
Influence of doping by iron impurity on spin-Peierls state in CuGeO3 is
studied. ESR measurements for the frequency/temperature domain 60-450 GHz/
1.8-300 K and specific heat data obtained for the interval 6-20 K show that
insertion of 1% of Fe completely destroy both spin-Peierls and
antiferromagnetic orders. Damping of long-range magnetic order is accompanied
by onset at T<20 K of power asymptotics for magnetic susceptibility
chi~T^-alpha and magnetic part of specific heat cm~T^(1-alpha), with the index
alpha=0.35-0.37. This effect is characteristic to the limit of strong disorder
for doped CuGeO3 and may reflect formation of the Griffiths phase at low
temperatures in CuGeO3:Fe.
|
0110177v1
|
2001-11-27
|
Complementary approaches to the ab initio calculation of melting properties
|
Several research groups have recently reported {\em ab initio} calculations
of the melting properties of metals based on density functional theory, but
there have been unexpectedly large disagreements between results obtained by
different approaches. We analyze the relations between the two main approaches,
based on calculation of the free energies of solid and liquid and on direct
simulation of the two coexisting phases. Although both approaches rely on the
use of classical reference systems consisting of parameterized empirical
interaction models, we point out that in the free energy approach the final
results are independent of the reference system, whereas in the current form of
the coexistence approach they depend on it. We present a scheme for correcting
the predictions of the coexistence approach for differences between the
reference and {\em ab initio} systems. To illustrate the practical operation of
the scheme, we present calculations of the high-pressure melting properties of
iron using the corrected coexistence approach, which agree closely with earlier
results from the free energy approach. A quantitative assessment is also given
of finite-size errors, which we show can be reduced to a negligible size.
|
0111510v1
|
2001-12-03
|
Electron correlation effects and ferromagnetism in Iron
|
Electron correlation effects in Fe are analyzed using a first principles
LCAO-scheme. In our approach, we first use a local orbital DFT-LDA solution to
introduce a Hubbard Hamiltonian without fitting parameters. In a second step,
we introduce a many-body solution to this Hamiltonian using a DMF
approximation. Our analysis shows that magnetism in Fe is an effect associated
with the first atomic Hund's rule. Moreover, we also find important correlation
effects in the Fe-spin polarized DOS. The photoemision spectra is explained
using a value of U_eff as large as 4 eV, provided the satellite peaks appearing
around 3-5 eV below the Fermi energy are interpreted appropriately.
|
0112038v1
|
2001-12-07
|
Proton Spin Relaxation Induced by Quantum Tunneling in Fe8 Molecular Nanomagnet
|
The spin-lattice relaxation rate $T_{1}^{-1}$ and NMR spectra of $^1$H in
single crystal molecular magnets of Fe8 have been measured down to 15 mK. The
relaxation rate $T_1^{-1}$ shows a strong temperature dependence down to 400
mK. The relaxation is well explained in terms of the thermal transition of the
iron state between the discreet energy levels of the total spin S=10. The
relaxation time $T_1$ becomes temperature independent below 300 mK and is
longer than 100 s. In this temperature region stepwise recovery of the
$^1$H-NMR signal after saturation was observed depending on the return field of
the sweep field. This phenomenon is attributed to resonant quantum tunneling at
the fields where levels cross and is discussed in terms of the Landau-Zener
transition.
|
0112111v1
|
2002-08-01
|
Magnetic shape-memory effects in La2-xSrxCuO4 crystals
|
The magnetic field affects the motion of electrons and the orientation of
spins in solids, but it is believed to have little impact on the crystal
structure. This common perception has been challenged recently by ferromagnetic
shape-memory alloys, where the spin-lattice coupling is so strong that
crystallographic axes even in a fixed sample are forced to rotate, following
the direction of moments. One would, however, least expect any structural
change to be induced in antiferromagnets where spins are antiparallel and give
no net moment. Here we report on such unexpected magnetic shape-memory effects
that take place ironically in one of the best-studied 2D antiferromagnets,
La2-xSrxCuO4 (LSCO). We find that lightly-doped LSCO crystals tend to align
their b axis along the magnetic field, and if the crystal orientation is fixed,
this alignment occurs through the generation and motion of crystallographic
twin boundaries. Both resistivity and magnetic susceptibility exhibit curious
switching and memory effects induced by the crystal-axes rotation; moreover,
clear kinks moving over the crystal surfaces allow one to watch the crystal
rearrangement directly with a microscope or even bare eyes.
|
0208013v1
|
2002-08-12
|
Fe/V and Fe/Co (001) superlattices: growth, anisotropy, magnetisation and magnetoresistance
|
Some physical properties of bcc Fe/V and Fe/Co (001) superlattices are
reviewed. The dependence of the magnetic anisotropy on the in-plane strain
introduced by the lattice mismatch between Fe and V is measured and compared to
a theoretical derivation. The dependence of the magnetic anisotropy (and
saturation magnetisation) on the layer thickness ratio Fe/Co is measured and a
value for the anisotropy of bcc Co is derived from extrapolation. The
interlayer exchange coupling of Fe/V superlattices is studied as a function of
the layer thickness V (constant Fe thickness) and layer thickness of Fe
(constant V thickness). A region of antiferromagnetic coupling and GMR is found
for V thicknesses 12-14 monolayers. However, surprisingly, a 'cutoff' of the
antiferromagnetic coupling and GMR is found when the iron layer thickness
exceeds about 10 monolayers.
|
0208229v1
|
2002-08-22
|
Spin fluctuations, electron-phonon coupling and superconductivity in near-magnetic elementary metals; Fe,Co,Ni and Pd
|
An investigation of possibilities for superconductivity mediated by spin
fluctuations in some elementary metals is motivated by the recent discovery of
superconductivity in the hcp high-pressure phase of iron.
The electronic structure, the electron-phonon coupling ($\lambda_{ph}$) and
the coupling due to spin-fluctuations ($\lambda_{sf}$) are calculated for
different phases and different volumes for four elementary metals. The results
show that such possibilities are best for systems near, but on the non-magnetic
side of, a magnetic instability. Fcc Ni, which show stable magnetism over a
wide pressure range, is not interesting in this respect. Ferro- and
antiferro-magnetic fluctuations in hcp Fe contribute to a relatively strong
coupling in the pressure range where superconductivity is observed. The absence
of fluctuations at large q-vectors makes fcc Pd only moderately interesting
despite its large exchange enhancement for q=0. Fcc Co at high pressure ($\sim$
0.5 Mbar) behaves as an improved version of Pd, where the fluctuations extend
to larger q. The estimations of T$_C$, which reproduce the experimental
situation in Fe quite well, suggest a measurable T$_C$ for the high-pressure
phase of fcc Co, while the estimate is lower for the ambient-pressure phase of
fcc Pd.
|
0208424v1
|
2002-08-30
|
Itinerant Ferromagnetism in the Periodic Anderson Model
|
We introduce a novel mechanism for itinerant ferromagnetism, based on a
simple two-band model. The model includes an uncorrelated and dispersive band
hybridized with a second band which is narrow and correlated. The simplest
Hamiltonian containing these ingredients is the Periodic Anderson Model (PAM).
Using quantum Monte Carlo and analytical methods, we show that the PAM and an
extension of it contain the new mechanism and exhibit a non-saturated
ferromagnetic ground state in the intermediate valence regime. We propose that
the mechanism, which does not assume an intra atomic Hund's coupling, is
present in both the iron group and in some f electron compounds like
Ce(Rh_{1-x} Ru_x)_3 B_2, La_x Ce_{1-x} Rh_3 B_2 and the uranium
monochalcogenides US, USe, and UTe.
|
0208604v1
|
2002-11-25
|
Calculation of thermal expansion coefficient of Fe/sub 3/Al with the addition of transition metal elements
|
The addition of transition metal elements can significantly modify physical
properties of intermetalic compounds. We studied the influence of Molybdenum
and Vanadium additives on thermal expansion coefficient (CTE) of Fe/sub 3/Al
and FeAl over the wide range of temperatures. The site preference of both
transition metals was determined by full-potential LMTO method within the
grandcanonical formalism. At low temperatures CTEs were found directly from the
FP-LMTO calculations by incorporating them into the Debye model of a solid. The
obtained thermal expansion for pure Fe/sub 3/Al and FeAl is within 10% of its
experimentally measured values. At high temperatures we performed molecular
dynamics simulations based on our many-body atomistic potentials. The
parameters were fitted to reproduce the total energy of a crystal under various
types of deformations obtained by FP-LMTO method and were tested with respect
to different structures and vacancy formation energies. Our calculations show
that addition of V decreases the CTEs of both iron-aluminides, while the
addition of Mo makes Fe/sub 3/Al DO3 structure unstable.
|
0211569v1
|
2002-12-17
|
Anomalous temperature dependence of the dephasing time in mesoscopic Kondo wires
|
We present measurements of the magnetoconductance of long and narrow quasi
one-dimensional gold wires containing magnetic iron impurities in a temperature
range extending from $15 $mK to $4.2 $K. The dephasing rate extracted from the
weak antilocalisation shows a pronounced plateau in a temperature region of
$300 $mK - $800 $mK, associated with the phase breaking due to the Kondo
effect. Below the Kondo temperature the dephasing rate decreases linearly with
temperature, in contradiction with standard Fermi-liquid theory. Our data
suggest that the formation of a spin glass due to the interactions between the
magnetic moments are responsible for the observed anomalous temperature
dependence.
|
0212420v1
|
2003-01-22
|
Tunneling splitting of magnetic levels in Fe8 detected by 1H NMR cross relaxation
|
Measurements of proton NMR and the spin lattice relaxation rate 1/T1 in the
octanuclear iron (III) cluster [Fe8(N3C6H15)6O2(OH)12][Br8 9H2O], in short Fe8,
have been performed at 1.5 K in a powder sample aligned along the main
anisotropy z axis, as a function of a transverse magnetic field (i.e.,
perpendicular to the main easy axis z). A big enhancement of 1/T1 is observed
over a wide range of fields (2.5-5 T), which can be attributed to the tunneling
dynamics; in fact, when the tunneling splitting of the pairwise degenerate
m=+-10 states of the Fe8 molecule becomes equal to the proton Larmor frequency
a very effective spin lattice relaxation channel for the nuclei is opened. The
experimental results are explained satisfactorily by considering the
distribution of tunneling splitting resulting from the distribution of the
angles in the hard xy plane for the aligned powder, and the results of the
direct diagonalization of the model Hamiltonian.
|
0301415v1
|
2003-02-13
|
Properties of superconducting MgB_2 wires: "in-situ" versus "ex-situ" reaction technique
|
We have fabricated a series of iron-sheathed superconducting wires prepared
by the powder-in-tube technique from (MgB_2)_{1-x}:(Mg+2B)_x initial powder
mixtures taken with different proportions, so that x varies from 0 to 1. It
turned out that "ex-situ" prepared wire (x = 0) has considerable disadvantages
compared to all the other wires in which "in-situ" assisted (0 < x < 1) or pure
"in-situ" (x = 1) preparation was used due to weaker inter-grain connectivity.
As a result, higher critical current densities J_c were measured over the
entire range of applied magnetic fields B_a for all the samples with x > 0.
Pinning of vortices in MgB_2 wires is shown to be due to grain boundaries.
J_c(B_a) behavior is governed by an interplay between the transparency of grain
boundaries and the amount of "pinning" grain boundaries. Differences between
thermo-magnetic flux-jump instabilities in the samples and a possible threat to
practical applications are also discussed.
|
0302249v1
|
2003-03-07
|
Macroscopic and Microscopic Investigation on the History Dependence of the Mechanical Behaviour of Powders
|
As an example for history dependent mechanical behaviour of cohesive powders
experiments and computer simulations of uniaxial consolidation are compared.
Some samples were precompacted transversally to the consolidation direction and
hence had a different history. The experiments were done with two carbonyl iron
powders, for which the average particle diameters differed by a factor of ca.
2. Whereas the particle diameter was the only characteristic length in the
simulations, the evaluation of the experimental data indicates that at least a
second characteristic length must be present.
|
0303132v1
|
2003-05-19
|
Andreev bound states in normal and ferromagnet/high-Tc superconducting tunnel junctions
|
Ag/BSCCO and Fe/Ag/BSCCO planar tunnel junctions were constructed in order to
study experimentally the effect of an exchange potential on the spin polarized
current transported through Andreev bound states appearing at the interface
with a superconductor with broken time reversal pairing symmetry. The zero bias
conductance peak (ZBCP) resulting from the Andreev bound states (ABS) is split
into two symmetric peaks shifted at finite energies when the counterlectrode is
normal. Four asymmetric peaks are observed when the ferromagnetic spin
polarized charge reservoir is added, due to the combined effect of a
spin-filtering exchange energy in the barrier, which is a spin dependent
phenomenon, and the spin independent effect of a broken time reversal symmetry
(BTRS). The polarization in the iron layer leads to asymmetry. Due to the shift
of ABS peaks to finite energies, the conductance at zero energy behaves as
predicted by recent theoretical developments for pure d-wave junctions without
Andreev reflections.
|
0305446v1
|
2003-06-11
|
Electron Coherence in Mesoscopic Kondo Wires
|
We present measurements of the magnetoresistance of long and narrow quasi
one-dimensional gold wires containing magnetic iron impurities. The electron
phase coherence time extracted from the weak antilocalisation shows a
pronounced plateau in a temperature region of 300 mK - 800 mK, associated with
the phase breaking due to the Kondo effect. Below the Kondo temperature, the
phase coherence time increases, as expected in the framework of Kondo physics.
At much lower temperatures, the phase coherence time saturates again, in
contradiction with standard Fermi liquid theory. In the same temperature
regime, the resistivity curve displays a characteristic maximum at zero
magnetic field, associated with the formation of a spin glass state. We argue
that the interactions between the magnetic moments are responsible for the low
temperature saturation of the phase coherence time.
|
0306276v1
|
2003-09-17
|
Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect
|
Double perovskite, (Sr1-xNdx)2FeMoO6, was doped with electrons through
partial substitution of divalent Sr by trivalent Nd (0 < x < 0.2). The Fe
valence and the degree of B-site order were probed by 57Fe Mossbauer
spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms
occupying wrong sites, i.e. antisite disorder. It had very little effect on the
Fe valence: a small but visible increase in the isomer shift was seen for the
mixed-valent FeII/III atoms occupying the right site indicating a slight
movement towards divalency of these atoms, which was more than counterbalanced
by the increase in the fraction of antisite Fe atoms with III valence state. It
is therefore argued that the bulk of the electron doping is received by
antisite Mo atoms, which - being surrounded by six MoV/VI atoms - prefer the
lower IV/V valence state. Thus under Nd substitution, the charge-neutrality
requirement inflicts a lattice disorder such that low-valent MoIV/V can exist.
|
0309394v1
|
2003-11-21
|
First-principle molecular dynamics with ultrasoft pseudopotentials: parallel implementation and application to extended bio-inorganic system
|
We present a plane-wave ultrasoft pseudopotential implementation of
first-principle molecular dynamics, which is well suited to model large
molecular systems containing transition metal centers. We describe an efficient
strategy for parallelization that includes special features to deal with the
augmented charge in the contest of Vanderbilt's ultrasoft pseudopotentials. We
also discuss a simple approach to model molecular systems with a net charge
and/or large dipole/quadrupole moments. We present test applications to
manganese and iron porphyrins representative of a large class of biologically
relevant metallorganic systems. Our results show that accurate
Density-Functional Theory calculations on systems with several hundred atoms
are feasible with access to moderate computational resources.
|
0311507v1
|
2004-01-28
|
Importance of Thermal Disorder on the Properties of Alloys: Origin of Paramagnetism and Structural Anomalies in Iron-Aluminum
|
The bcc-based Fe_{1-x}Al_{x} exhibit interesting magnetic and anomalous
structural properties as a function of composition and sample processing
conditions arising from thermal or off-stoichiometric chemical disorder, and,
although well studied, these properties are not understood. In stoichiometric
B2 FeAl, including the effects of partial long-range order (i.e., thermal
antisites), we find the observed paramagnetic response (with non-zero local
moments), in contrast to past investigations which find ferromagnetism based on
local density approximation (LDA) to density functional theory or which find a
non-magnetic state from LDA+U, both of which are inconsistent with experiment.
Moreover, from this magneto-chemical coupling, we are able to determine the
origins of the lattice constant anomalies found in Fe_{1-x}Al_{x} for x=25-50,
as observed from various processing conditions.
|
0401587v1
|
2004-01-30
|
Magnetoelectric Effects in Ferromagnetic Metal-Piezoelectric Oxide Layered Structures
|
Frequency dependence of magnetoelectric (ME) coupling is investigated in
trilayers of ferromagnetic alloy and piezoelectric lead zirconate titanate
(PZT). The ferromagnetic phases studied include permendur, a soft magnet with
high magnetostriction, iron, nickel, and cobalt. Low frequency data on ME
voltage coefficient versus bias magnetic field indicate strong coupling only
for trilayers with permendure or Ni. Measurements of frequency dependence of ME
voltage reveal a giant ME coupling at electromechanical resonance. The ME
interactions for transverse fields is an order of magnitude stronger than for
longitudinal fields. The maximum voltage coefficient of 90 V/cm Oe at resonance
is measured for samples with nickel or permendure and is three orders of
magnitude higher than low-frequency values.
|
0401648v1
|
2004-04-15
|
Evidence for crossed Andreev reflection in superconductor-ferromagnet hybrid structures
|
We have measured the non-local resistance of aluminum-iron spin-valve
structures fabricated by e-beam lithography and shadow evaporation. The sample
geometry consists of an aluminum bar with two or more ferromagnetic wires
forming point contacts to the aluminum at varying distances from each other. In
the normal state of aluminum, we observe a spin-valve signal which allows us to
control the relative orientation of the magnetizations of the ferromagnetic
contacts. In the superconducting state, at low temperatures and excitation
voltages well below the gap, we observe a spin-dependent non-local resistance
which decays on a smaller length scale than the normal-state spin-valve signal.
The sign, magnitude and decay length of this signal is consistent with
predictions made for crossed Andreev reflection (CAR).
|
0404360v2
|
2004-04-27
|
Phase Separation in Li$_x$FePO$_4$ Induced by Correlation Effects
|
We report on a significant failure of LDA and GGA to reproduce the phase
stability and thermodynamics of mixed-valence Li$_x$FePO$_4$ compounds.
Experimentally, Li$_x$FePO$_4$ compositions ($0 \leq x \leq 1$) are known to be
unstable and phase separate into Li FePO$_4$ and FePO$_4$. However,
first-principles calculations with LDA/GGA yield energetically favorable
intermediate compounds an d hence no phase separation. This qualitative failure
of LDA/GGA seems to have its origin in the LDA/GGA self-interaction which de
localizes charge over the mixed-valence Fe ions, and is corrected by explicitly
considering correlation effects in this material. This is demonstrated with
LDA+U calculations which correctly predict phase separation in Li$_x$FePO$_4$
for $U-J \gtrsim 3.5$eV. T he origin of the destabilization of intermediate
compounds is identified as electron localization and charge ordering at
different iron sites. Introduction of correlation also yields more accurate
electrochemical reaction energies between FePO$_4$/Li$_x$FePO$_ 4$ and
Li/Li$^+$ electrodes.
|
0404631v2
|
2004-05-02
|
Spin Dynamics and Multiple Reflections in Ferromagnetic Film in Contact with Normal Metal Layers
|
Spin dynamics of a metallic ferromagnetic film imbedded between normal metal
layers is studied using the spin-pumping theory of Tserkovnyak et al. [Phys.
Rev. Lett. 88, 117601 (2002)]. The scattering matrix for this structure is
obtained using a spin-dependent potential with quantum well in the
ferromagnetic region. Owing to multiple reflections in the well, the excess
Gilbert damping and the gyromagnetic ratio exhibit quantum oscillations as a
function of the thickness of the ferromagnetic film. The wavelength of the
oscillations is given by the depth of the quantum well. For iron film imbedded
between gold layers, the amplitude of the oscillations of the Gilbert damping
is in an order of magnitude agreement with the damping observed by Urban et al.
[Phys. Rev. Lett. 87, 217204 (2001)]. The results are compared with the linear
response theory of Mills [Phys. Rev. B 68, 0144419 (2003)].
|
0405020v1
|
2004-05-06
|
Theory of Metal-Insulator Transition in PrRu4P12 and PrFe4P12
|
All symmetry allowed couplings between the 4f^2-electron ground state doublet
of trivalent praseodymium in PrRu4P12 and PrFe4P12 and displacements of the
phosphorus, iron or ruthenium ions are considered. Two types of displacements
can change the crystal lattice from body-centred cubic to simple orthorhombic
or to simple cubic. The first type lowers the point group symmetry from
tetrahedral to orthorhombic, while the second type leaves it unchanged, with
corresponding space group reductions Im3 --> Pmmm and Im3 --> Pm3 respectively.
In former case, the lower point-group symmetry splits the degeneracy of the
4f^2 doublet into states with opposite quadrupole moment, which then leads to
anti-quadrupolar ordering, as in PrFe4P12. Either kind of displacement may
conspire with nesting of the Fermi surface to cause the metal-insulator or
partial metal-insulator transition observed in PrFe4P12 and PrRu4P12. We
investigate this scenario using band-structure calculations, and it is found
that displacements of the phosphorus ions in PrRu4P12 (with space group
reduction Im3 --> Pm3) open a gap everywhere on the Fermi surface.
|
0405127v1
|
2004-05-07
|
A linear response approach to the calculation of the effective interaction parameters in the LDA+U method
|
In this work we reexamine the LDA+U method of Anisimov and coworkers in the
framework of a plane-wave pseudopotential approach. A simplified
rotational-invariant formulation is adopted. The calculation of the Hubbard U
entering the expression of the functional is discussed and a linear response
approach is proposed that is internally consistent with the chosen definition
for the occupation matrix of the relevant localized orbitals. In this way we
obtain a scheme whose functionality should not depend strongly on the
particular implementation of the model in ab-initio calculations. We
demonstrate the accuracy of the method, computing structural and electronic
properties of a few systems including transition and rare-earth correlated
metals, transition metal monoxides and iron-silicate.
|
0405160v1
|
2004-05-10
|
Self-assembly of iron nanoclusters on the Fe3O4(111) superstructured surface
|
We report on the self-organized growth of a regular array of Fe nanoclusters
on a nanopatterned magnetite surface. Under oxidizing preparation conditions
the (111) surface of magnetite exhibits a regular superstructure with
three-fold symmetry and a 42 A periodicity. This superstructure represents an
oxygen terminated (111) surface, which is reconstructed to form a periodically
strained surface. This strain patterned surface has been used as a template for
the growth of an ultrathin metal film. A Fe film of 0.5 A thickness was
deposited on the substrate at room temperature. Fe nanoclusters are formed on
top of the surface superstructure creating a regular array with the period of
the superstructure. We also demonstrate that at least the initial stage of Fe
growth occurs in two-dimensional mode. In the areas of the surface where the
strain pattern is not formed, random nucleation of Fe was observed.
|
0405201v2
|
2004-06-24
|
Resonant magnetic X-ray scattering spectra in SDW Cr -- ab initio study -----
|
Using ab-initio band structure calculation based on the local density
approximation, Cr K-edge resonant X-ray magnetic scattering spectra are
analyzed in the spin density wave (SDW) state of chromium. We perform band
structure calculation, assuming an ideal bcc lattice structure with the lattice
constant observed at the spin-flip temperature T_{SF} and a commensurate SDW
state with the propagation vector close to the observed value at T_{SF}. Taking
account of the spin-orbit interaction, we obtain the orbital moment on each Cr
site induced in proportion to the local spin moment, which is quite small, at
most a tenth of those in nickel or iron. In spite of the tiny 3d orbital
moment, the orbital polarization is found to have large fluctuations as a
function of energy. We obtain the scattering intensity at the Cr K-edge on the
SDW magnetic Bragg spot, which shows resonant enhancement in good agreement
with the experiment. The 3d orbital polarization is found to be highly
correlated with the intensity of the resonant main peak, indicating that the 4p
orbital polarization is mainly induced by the 3d orbital polarization through
the p-d hybridization.
|
0406576v1
|
2004-09-15
|
Effect of Nano Scale Fe Doping on Superconducting Properties of MgB2
|
Iron is an important sheath material for fabrication of MgB2 wires. However,
the effect of Fe doping on the superconducting properties of MgB2 remains
controversial. In this work, we present results of nano-scale Fe particle
doping in to MgB2. The Fe doping experiments were performed using both bulk and
thin film form. It was found that Fe doping did not affect the lattice
parameters of MgB2, as evidenced by the lack of change in the XRD peak
positions for MgB2. Because of the high reactivity of nano-scale Fe particles,
Fe doping is largely in the form of FeB at low doping level while Fe2B was
detected at 10wt% doping by both XRD and TEM. There is no evidence for Fe
substitution for Mg. The transition temperature decreased modestly with
increasing Fe doping levels. The Jc(H) performance was severely depressed at
above 3wt% doping level. The detrimental effect of nano-scale Fe doping on both
Tc and Jc(H) is attributable to the grain decoupling as a result of magnetic
scattering of Fe-containing dopants at grain boundaries.
|
0409408v1
|
2004-10-17
|
Low-temperature anomaly in heat capacity due to overlapping the spectrums in molecular crystals
|
Investigations of dynamic and thermodynamic properties for a molecular
crystal tris-hexafluoroacetylacetonate-iron $Fe(O_2C_5HF_6)_3$ are presented.
Heat capacity $C_p(T)$ has been measured by adiabatic calorimetry method in the
temperature range $4.8-321 K$. An anomaly with a maximum at $T_c = 44.6 K$ has
been discovered. Intermolecular vibrations spectrum was calculated by lattice
dynamics method in quasiharmonic approximation. Intramolecular frequencies are
found by solving the Schr\"odinger equation in approach of small harmonic
oscillations. In the frequency interval $\approx 30-70 cm^{-1}$ overlapping the
spectrums intra- and intermolecular oscillations has been found. The good
agreement for calculated and experimental $C_p(T)$ occurred to be possible for
two sets of the force constants. These sets describe two phases above and below
$T_c$. The difference between phases is connected with freezing of rotation
$CF_3$ groups. It has been concluded that the interaction between different
modes leads to phase transition and anomaly in heat capacity.
|
0410428v1
|
2004-11-27
|
Point defect dynamics in bcc metals
|
We present an analysis of the time evolution of self-interstitial atom and
vacancy (point defect) populations in pure bcc metals under constant
irradiation flux conditions. Mean-field rate equations are developed in
parallel to a kinetic Monte Carlo (kMC) model. When only considering the
elementary processes of defect production, defect migration, recombination and
absorption at sinks, the kMC model and rate equations are shown to be
equivalent and the time evolution of the point defect populations is analyzed
using simple scaling arguments. We show that the typically large mismatch of
the rates of interstitial and vacancy migration in bcc metals can lead to a
vacancy population that grows as the square root of time. The vacancy cluster
size distribution under both irreversible and reversible attachment can be
described by a simple exponential function. We also consider the effect of
highly mobile interstitial clusters and apply the model with parameters
appropriate for vanadium and $\alpha-$iron.
|
0411680v1
|
2004-12-23
|
Modeling truncated hemoglobin vibrational dynamics
|
We present a study on the near equilibrium dynamics of two small proteins in
the family of truncated hemoglobins, developed under the framework of a
Gaussian network approach. Effective beta carbon atoms are taken into account
besides C-alphas for all residues but glycines in the coarse-graining
procedure, without leading to an increase in the degrees of freedom
(betaGaussian Model). Normalized covariance matrix and deformation along
slowest modes with collective character are analyzed, pointing out
anti-correlations between functionally relevant sites for the proteins under
study. In particular we underline the functional motions of an extended
tunnel-cavity system running inside the protein matrix, which provide a pathway
for small ligands binding with the iron in the heme group. We give a rough
estimate of the order of magnitude of the relaxation times of the slowest two
overdamped modes and compare results with previous studies on globins.
|
0412661v2
|
2005-02-23
|
Charge order in Fe2OBO3: An LSDA+U study
|
Charge ordering in the low-temperature monoclinic structure of iron oxoborate
(Fe2OBO3) is investigated using the local spin density approximation (LSDA)+U
method. While the difference between t_{2g} minority occupancies of Fe^{2+} and
Fe^{3+} cations is large and gives direct evidence for charge ordering, the
static "screening" is so effective that the total 3d charge separation is
rather small. The occupied Fe^{2+} and Fe^{3+} cations are ordered alternately
within the chain which is infinite along the a-direction. The charge order
obtained by LSDA+U is consistent with observed enlargement of the \beta angle.
An analysis of the exchange interaction parameters demonstrates the
predominance of the interribbon exchange interactions which determine the whole
L-type ferrimagnetic spin structure.
|
0502555v1
|
2005-03-07
|
Thermal enhancement of the antiferromagnetic exchange coupling between Fe epilayers separated by a crystalline ZnSe spacer
|
We have put into evidence the existence of an antiferromagnetic coupling
between iron epilayers separated by a ZnSe crystalline semiconductor. The
effect has been observed for ZnSe spacers thinner than 4 nm at
room-temperature. The coupling constant increases linearly with temperature
with a constant slope of ~5.5x 10-9 J/m2K. The mechanisms that may explain such
exchange interaction are discussed in the manuscript. It results that
thermally-induced effective exchange coupling mediated by spin-dependent on and
off resonant tunnelling of electrons via localized mid-gap defect states in the
ZnSe spacer layer appears to be the most plausible mechanism to induce the
antiferromagnetic coupling.
|
0503148v1
|
2005-03-14
|
Gutzwiller-Correlated Wave Functions: Application to Ferromagnetic Nickel
|
Ferromagnetic Nickel is the most celebrated iron group metal with pronounced
discrepancies between the experimental electronic properties and predictions of
density functional theories. In this work, we show in detail that the recently
developed multi-band Gutzwiller theory provides a very good description of the
quasi-particle band structure of nickel. We obtain the correct exchange
splittings and we reproduce the experimental Fermi-surface topology. The
correct (111)-direction of the magnetic easy axis and the right order of
magnitude of the magnetic anisotropy are found. Our theory also reproduces the
experimentally observed change of the Fermi-surface topology when the magnetic
moment is oriented along the (001)-axis. In addition to the numerical study, we
give an analytical derivation for a much larger class of variational
wave-functions than in previous investigations. In particular, we cover cases
of superconductivity in multi-band lattice systems.
|
0503332v1
|
2005-03-15
|
Direct evidence of the orbital contribution to the magnetic moment in AA'FeReO6 double perovskites
|
Spin and orbital magnetic moments of Re in AA'FeReO6 double perovskites (A,A'
= Ba, Sr, Ca) have been directly probed employing XMCD spectroscopy at the Re
L2,3-edges. A considerable orbital magnetic moment is observed in all the
studied compounds despite octahedral coordination. Relative orbital to spin
contribution per Re atom rises with lattice distortion from mL/mS = -0.28 to
-0.34 for AA'=Ba2FeReO6 and Ca2FeReO6, respectively. A preliminary XMCD
measurements at the Fe L2,3-edges reveals also a significant orbital moment of
iron in Ca2FeReO6. The relation of the results to the magnetic properties of
the compounds is discussed.
|
0503358v1
|
2005-04-13
|
Growth Mechanisms and Oxidation-Resistance of Gold-Coated Iron Nanoparticles
|
We report the chemical synthesis of Fe-core/Au-shell nanoparticles by a
reverse micelle method, and the investigation of their growth mechanisms and
oxidation-resistant characteristics. The core-shell structure and the presence
of the Fe & Au phases have been confirmed by transmission electron microscopy,
energy dispersive spectroscopy, X-ray diffraction, Mossbauer spectroscopy, and
inductively coupled plasma techniques. Additionally, atomic-resolution
Z-contrast imaging and electron energy loss spectroscopy (EELS) in a scanning
transmission electron microscope (STEM) have been used to study details of the
growth processes. The Au-shell grows by nucleating on the Fe-core surface
before coalescing. The magnetic moments of such nanoparticles, in the loose
powder form, decrease over time due to oxidation. The less than ideal
oxidation-resistance of the Au shell may have been caused by the rough Au
surfaces. However, in the pressed pellet form, electrical transport
measurements show that the particles are fairly stable, as the resistance of
the pellet does not change appreciably over time.
|
0504314v1
|
2005-04-26
|
Theoretical estimates for proton-NMR spin-lattice relaxation rates of heterometallic spin rings
|
Heterometallic molecular chromium wheels are fascinating new magnetic
materials. We reexamine the available experimental susceptibility data on MCr7
wheels in terms of a simple isotropic Heisenberg Hamiltonian for M=Fe, Ni, Cu,
and Zn and find in that FeCr7 needs to be described with an iron-chromium
exchange that is different from all other cases. In a second step we model the
behavior of the proton spin lattice relaxation rate as a function of applied
magnetic field for low temperatures as it is measured in Nuclear Magnetic
Resonance (NMR) experiments. It appears that CuCr7 and NiCr7 show an
unexpectedly reduced relaxation rate at certain level crossings.
|
0504658v2
|
2005-07-06
|
Partially ferromagnetic electromagnet for trapping and cooling neutral atoms to quantum degeneracy
|
We have developed a compact partially ferromagnetic electromagnet to produce
a Ioffe-Pritchard trap for neutral atoms. Our structure permits strong magnetic
confinement with low power consumption. Compared to the previous iron-core
electromagnet, it allows for easy compensation of remnant fields and very high
stability, along with cost-effective realization and compactness. We describe
and characterize our apparatus and demonstrate trapping and cooling of Rb 87
atoms to quantum degeneracy. Pure Bose-Einstein condensates containing 10^6
atoms are routinely realized on a half-minute cycle. In addition we test the
stability of the magnetic trap by producing atom lasers.
|
0507129v2
|
2005-08-19
|
Spin-waves in antiferromagnetic single crystal LiFePO$_4$
|
Spin-wave dispersions in the antiferromagnetic state of single crystal
LiFePO$_4$ were determined by inelastic neutron scattering measurements. The
dispersion curves measured from the (010) reflection along both {\it a}$^\ast$
and {\it b}$^\ast$ reciprocal-space directions reflect the anisotropic coupling
of the layered Fe$^{2+}$ (S = 2) spin-system. The spin-wave dispersion curves
were theoretically modeled using linear spin-wave theory by including in the
spin-Hamiltonian in-plane nearest- and next-nearest-neighbor interactions ({\it
J}$_1$ and {\it J}$_2$), inter-plane nearest-neighbor interactions ({\it
J}$_\bot$) and a single-ion anisotropy ({\it D}). A weak (010) magnetic peak
was observed in elastic neutron scattering studies of the same crystal
indicating that the ground state of the staggered iron moments is not along
(010) direction, as previously reported from polycrystalline samples studies,
but slightly rotated away from this axis.
|
0508480v1
|
2005-08-27
|
Raman spectroscopy and field emission measurements on catalytically grown carbon nanotubes
|
We used microcontact printing to pattern a silicon surface with an
iron-containing catalytic solution. Multi-wall carbon nanotubes were
subsequently grown on the patterned areas by chemical vapor deposition at
temperatures between 650 and 1000C. We demonstrate that the diameter of the
catalytically grown multi-wall nanotubes increases with the deposition
temperature. Raman spectroscopy has been used to investigate the crystalline
character of the obtained structures and it is found that the fraction of the
nano-crystalline shell increases with the temperatures. The measurement of the
field emission properties shows a correlation between the tube diameter and the
emission field values.
|
0508657v2
|
2005-09-14
|
Magnetic ordering in trigonal chain compounds
|
We present electronic structure calculations for the one-dimensional magnetic
chain compounds Ca_3CoRhO_6 and Ca_3FeRhO_6. The calculations are based on
density functional theory and the local density approximation. We use the
augmented spherical wave (ASW) method. The observed alternation of low- and
high-spin states along the Co-Rh and Fe-Rh chains is related to differences in
the oxygen coordination of the transition metal sites. Due to strong
hybridization the O 2p states are polarized, giving rise to extended localized
magnetic moments centered at the high-spin sites. Strong metal-metal overlap
along the chains leads to a substantial contribution of the low-spin Rh
4d_{3z^2-r^2} orbitals to the exchange coupling of the extended moments.
Interestingly, this mechanism holds for both compounds, even though the
coupling is ferromagnetic for the cobalt and antiferromagnetic for the iron
compound. However, our results allow to understand the different types of
coupling from the filling dependence of the electronic properties.
|
0509374v1
|
2005-10-27
|
High spin polarization in the ferromagnetic filled skutterudites KFe4Sb12 and NaFe4Sb12
|
The spin polarization of ferromagnetic alkali-metal iron antimonides KFe4Sb12
and NaFe4Sb12 is studied by point-contact Andreev reflection using
superconducting Nb and Pb tips. From these measurements an intrinsic transport
spin polarization Pt of 67% and 60% for the K and Na compound, respectively, is
inferred which establishes these materials as a new class of highly spin
polarized ferromagnets. The results are in accord with band structure
calculations within the local spin density approximation (LSDA) that predict
nearly 100% spin polarization in the density of states. We discuss the impact
of calculated Fermi velocities and spin fluctuations on Pt.
|
0510736v1
|
2005-11-21
|
Electronic structure study by means of X-ray spectroscopy and theoretical calculations of the "ferric star" single molecule magnet
|
The electronic structure of the single molecule magnet system
M[Fe(L)2]3*4CHCl3 (M=Fe,Cr; L=CH3N(CH2CH2O)2) has been studied using X-ray
photoelectron spectroscopy, X-ray absorption spectroscopy, soft X-ray emission
spectroscopy, and density functional calculations. There is good agreement
between theoretical calculations and experimental data. The valence band mainly
consists of three bands between 2 eV and 30 eV. Both theory and experiments
show that the top of the valence band is dominated by the hybridization between
Fe 3d and O 2p bands. From the shape of the Fe 2p spectra it is argued that Fe
in the molecule is most likely in the 2+ charge state. Its neighboring atoms
(O,N) exhibit a magnetic polarisation yielding effective spin S=5/2 per iron
atom, giving a high spin state molecule with a total S=5 effective spin for the
case of M = Fe.
|
0511522v2
|
2006-02-08
|
Investigation of the presence of charge order in magnetite by measurement of the spin wave spectrum
|
Inelastic neutron scattering results on magnetite (Fe3O4) show a large
splitting in the acoustic spin wave branch, producing a 7 meV gap midway to the
Brillouin zone boundary at q = (0,0,1/2) and E = 43 meV. The splitting occurs
below the Verwey transition temperature, where a metal-insulator transition
occurs simultaneously with a structural transformation, supposedly caused by
the charge ordering on the iron sublattice. The wavevector (0,0,1/2)
corresponds to the superlattice peak in the low symmetry structure. The
dependence of the magnetic superexchange on changes in the crystal structure
and ionic configurations that occur below the Verwey transition affect the spin
wave dispersion. To better understand the origin of the observed splitting, we
have constructed a series of Heisenberg models intended to reproduce the
pairwise variation of the magnetic superexchange arising from both small
crystalline distortions and charge ordering. We find that none of the models
studied predicts the observed splitting, whose origin may arise from
charge-density wave formation or magnetoelastic coupling.
|
0602214v1
|
2006-02-15
|
A spin triplet supercurrent through the half-metallic ferromagnet CrO2
|
In general, conventional superconductivity should not occur in a ferromagnet,
though it has been seen in iron under pressure. Moreover, theory predicts that
the current is always carried by pairs of electrons in a spin singlet state, so
conventional superconductivity decays very rapidly when in contact with a
ferromagnet, which normally prohibits the existence of singlet pairs. It has
been predicted that this rapid spatial decay would not occur when spin triplet
superconductivity could be induced in the ferromagnet. Here we report a
Josephson supercurrent through the strong ferromagnet CrO2, from which we infer
that it is a spin triplet supercurrent. Our experimental setup is different
from those envisaged in the earlier predictions, but we conclude that the
underlying physical explanation for our result is a conversion from spin
singlet to spin triplets at the interface. The supercurrent can be switched
with the direction of the magnetization, analogous to spin valve transistors,
and therefore could enable magnetization-controlled Josephson junctions.
|
0602359v1
|
2006-02-16
|
Ferromagnetic resonance in systems with competing uniaxial and cubic anisotropies
|
We develop a model for ferromagnetic resonance in systems with competing
uniaxial and cubic anisotropies. This model applies to (i) magnetic materials
with both uniaxial and cubic anisotropies, and (ii) magnetic nanoparticles with
effective core and surface anisotropies; We numerically compute the resonance
frequency as a function of the field and the resonance field as a function of
the direction of the applied field for an arbitrary ratio of cubic-to-uniaxial
anisotropy. We also provide some approximate analytical expressions in the case
of weak cubic anisotropy. We propose a method that uses these expressions for
estimating the uniaxial and cubic anisotropy constants, and for determining the
relative orientation of the cubic anisotropy axes with respect to the crystal
principle axes. This method is applicable to the analysis of experimental data
of resonance type measurements for which we give a worked example of an iron
thin film with mixed anisotropy.
|
0602369v3
|
2006-02-18
|
Scaling Behavior of Portevin-Le Chatelier Effect
|
The scaling behavior of the Portevin-Le Chatelier (PLC) effect is studied by
deforming a substitutional alloy, Al-2.5%Mg and an interstitial alloy, low
carbon steel (0.15%C, 0.33%Mn, 0.04%P, 0.05%S, 0.15%Si and rest Iron) at room
temperature for a wide range of strain rates. To reveal the exact scaling
nature, the time series data of true stress vs. time, obtained during the
tensile deformation (corrected for drift due to strain hardening by polynomial
fitting method), are analyzed by two complementary methods: the standard
deviation analysis and the diffusion entropy analysis. From these analyses we
could establish that in the entire span of strain rates, PLC effect showed Levy
walk type of scaling property.
|
0602436v1
|
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