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2019-08-16
|
Itinerant ferromagnetism and intrinsic anomalous Hall effect in amorphous iron-germanium
|
The amorphous iron-germanium system ($a$-Fe$_x$Ge$_{1-x}$) lacks long-range
structural order and hence lacks a meaningful Brillouin zone. The magnetization
of \aFeGe is well explained by the Stoner model for Fe concentrations $x$ above
the onset of magnetic order around $x=0.4$, indicating that the local order of
the amorphous structure preserves the spin-split density of states of the
Fe-$3d$ states sufficiently to polarize the electronic structure despite
$\mathbf{k}$ being a bad quantum number. Measurements reveal an enhanced
anomalous Hall resistivity $\rho_{xy}^{\mathrm{AH}}$ relative to crystalline
FeGe; this $\rho_{xy}^{\mathrm{AH}}$ is compared to density functional theory
calculations of the anomalous Hall conductivity to resolve its underlying
mechanisms. The intrinsic mechanism, typically understood as the Berry
curvature integrated over occupied $\mathbf{k}$-states but shown here to be
equivalent to the density of curvature integrated over occupied energies in
aperiodic materials, dominates the anomalous Hall conductivity of
$a$-Fe$_x$Ge$_{1-x}$ ($0.38 \leq x \leq 0.61$). The density of curvature is the
sum of spin-orbit correlations of local orbital states and can hence be
calculated with no reference to $\mathbf{k}$-space. This result and the
accompanying Stoner-like model for the intrinsic anomalous Hall conductivity
establish a unified understanding of the underlying physics of the anomalous
Hall effect in both crystalline and disordered systems.
|
1908.06055v3
|
2019-08-21
|
Stellar population astrophysics (SPA) with the TNG. GIANO-B spectroscopy of red supergiants in Alicante 7 and Alicante 10
|
The Scutum complex in the inner disk of the Galaxy hosts a number of young
clusters and associations of red supergiant stars that are heavily obscured by
dust extinction. These stars are important tracers of the recent star formation
and chemical enrichment history in the inner Galaxy. Within the SPA Large
Programme at the TNG, we secured GIANO-B high-resolution (R=50,000) YJHK
spectra of 11 red supergiants toward the Alicante 7 and Alicante 10
associations near the RSGC3 cluster. Taking advantage of the full YJHK spectral
coverage of GIANO in a single exposure, we were able to measure several
hundreds of atomic and molecular lines that are suitable for chemical abundance
determinations. We also measured a prominent diffuse interstellar band at
lambda=1317.8 nm (vacuum). This provides an independent reddening estimate. The
radial velocities, Gaia proper motions, and extinction of seven red supergiants
in Alicante 7 and three in Alicante 10 are consistent with them being members
of the associations. One star toward Alicante 10 has kinematics and low
extinction that are inconsistent with a membership. By means of spectral
synthesis and line equivalent width measurements, we obtained chemical
abundances for iron-peak, CNO, alpha, other light, and a few neutron-capture
elements. We found average slightly subsolar iron abundances and solar-scaled
[X/Fe] abundance patterns for most of the elements, consistent with a thin-disk
chemistry. We found depletion of [C/Fe], enhancement of [N/Fe], and relatively
low 12C/13C<15, which is consistent with CN cycled material and possibly some
additional mixing in their atmospheres.
|
1908.07779v1
|
2019-08-28
|
Neutrino halo effect on collective neutrino oscillation in iron core-collapse supernova model of a 9.6 $M_{\odot}$ star
|
We extend the multi-angle computational framework and investigate the time
evolution of the neutrino halo on collective neutrino oscillation in the core
collapse of an iron core progenitor. We find that in the case of the $9.6\, \rm
M_\odot$ progenitor adopted in this work, there are windows of time when the
effects of neutrino halo and collective neutrino oscillation are not
simultaneously large. Inside the shock, the impact of the inward-scattered halo
neutrino cannot in general be neglected compared to the outward-propagating
neutrino flux. However, during early epochs, collective neutrino oscillation is
effectively shut down by multi-angle matter suppression. During the
intermediate epoch, collective neutrino oscillation is not suppressed, but its
onset radius is beyond the still relatively small explosion shock front where
the halo is prominent. We also find in the case of the $9.6\, \rm M_\odot$
progenitor the halo neutrinos induce a delay in the onset of collective
neutrino oscillations. This causes novel flavor conversions which sharpen
collective neutrino oscillation spectral features. We predict that the
inclusion of neutrino halo effects makes neutrino signals that are more clearly
distinct from thermal emission that when halo neutrinos are omitted.
|
1908.10594v2
|
2019-09-04
|
Multi-epoch X-ray spectral analysis of the narrow-line Seyfert 1 galaxy Mrk 478
|
A multi-epoch X-ray spectral and variability analysis is conducted for the
narrow-line Seyfert 1 (NLS1) active galactic nucleus (AGN) Mrk 478. All
available X-ray data from XMM-Newton and Suzaku satellites, spanning from 2001
to 2017, are modelled with a variety of physical models including partial
covering, soft-Comptonisation, and blurred reflection, to explain the observed
spectral shape and variability over the 16 years. All models are a similar
statistical fit to the data sets, though the analysis of the variability
between data sets favours the blurred reflection model. In particular, the
variability can be attributed to changes in flux of the primary coronal
emission. Different reflection models fit the data equally well, but differ in
interpretation. The use of reflionx predicts a low disc ionisation and power
law dominated spectrum, while relxill predicts a highly ionised and blurred
reflection dominated spectrum. A power law dominated spectrum might be more
consistent with the normal X-ray-to-UV spectral shape (aox). Both blurred
reflection models suggest a rapidly spinning black hole seen at a low
inclination angle, and both require a sub-solar (~0.5) abundance of iron. All
physical models require a narrow emission feature at 6.7 keV likely
attributable to Fe xxv emission, while no evidence for a narrow 6.4 keV line
from neutral iron is detected.
|
1909.01897v1
|
2019-09-16
|
Ferrovolcanism on metal worlds and the origin of pallasites
|
As differentiated planetesimals cool, their cores can solidify from the
outside-in, as evidenced by paleomagnetic measurements and cooling rate
estimates of iron meteorites. The details of outside-in solidification and fate
of residual core melt are poorly understood. For a core primarily composed of
Fe and Ni alloyed with lighter constituent elements, like sulfur, such inward
core growth would likely be achieved by growth of solid FeNi dendrites. Growth
of FeNi dendrites results in interconnected pockets of residual melt that
become progressively enriched in sulfur up to a eutectic composition of 31 wt
percent sulfur as FeNi continues to solidify. Here we show that regions of
residual sulfur-enriched FeNi melt in the core attain sufficient excess
pressures to propagate via dikes into the mantle. Thus, core material will
intrude into the overlying rocky mantle or possibly even erupt onto the
plantesimals surface. We refer to these processes collectively as
ferrovolcanism. Our calculation show that ferrovolcanic surface eruptions are
more likely on bodies with mantles less than 50 km thick. We show that
intrusive ferromagmatism can produce pallasites, an enigmatic class of
meteorites composed of olivine crystals entrained in a matrix of FeNi metal.
Ferrovolcanic eruptions may explain the observations that Psyche has a bulk
density inconsistent with iron metorites yet shows evidence of a metallic
surface composition.
|
1909.07451v1
|
2019-09-24
|
Strongly correlated superconductor with polytypic 3D Dirac points
|
Topological superconductors should be able to provide essential ingredients
for quantum computing, but are very challenging to realize. Spin-orbit
interaction in iron-based superconductors opens the energy gap between the
$p$-states of pnictogen and $d$-states of iron very close to the Fermi level,
and such $p$-states have been recently experimentally detected. Density
functional theory predicts existence of topological surface states within this
gap in FeTe$_{1-x}$Se$_x$ making it an attractive candidate material. Here we
use synchrotron-based angle-resolved photoemission spectroscopy and band
structure calculations to demonstrate that FeTe$_{1-x}$Se$_x$ (x=0.45) is a
superconducting 3D Dirac semimetal hosting type-I and type-II Dirac points and
that its electronic structure remains topologically trivial. We show that the
inverted band gap in FeTe$_{1-x}$Se$_x$ can possibly be realized by further
increase of Te content, but strong correlations reduce it to a sub-meV size,
making the experimental detection of this gap and corresponding topological
surface states very challenging, not to mention exact matching with the Fermi
level. On the other hand, the $p-d$ and $d-d$ interactions are responsible for
the formation of extremely flat band at the Fermi level pointing to its
intimate relation with the mechanism of high-T$_c$ superconductivity in IBS.
|
1909.10806v2
|
2019-11-08
|
Oxygen Reduction Reaction and X-ray Photoelectron Spectroscopy of Sputtered Fe-N-C Films
|
Electrocatalysts for the oxygen reduction reaction (ORR) based on complexes
of iron and nitrogen in a carbon matrix (Fe-N-C) are a promising alternative to
platinum group metal (PGM) based catalysts in polymer electrolyte membrane
(PEM) fuel cells. Further improvements of Fe-N-C catalysts would benefit from
model thin film studies of activity and stability of catalytic sites, but
synthesis of Fe-N-C model thin films is challenging. Here we report on
synthesis and characterization of Fe-N-C thin films produced by co-sputtering
iron and carbon in a reactive nitrogen atmosphere onto removable glassy carbon
rotating disk electrode (RDE) tips. Scanning electron microscopy (SEM)
measurements indicate that the Fe-N-C films deposited at high temperature are
smoother than the films annealed at high temperature. ORR activity measured on
the thin Fe-N-C films is greater for both high-temperature samples than for the
room-temperature sample. From the analysis of X-ray photoelectron spectroscopy
(XPS) data, exposure of the films to high temperatures results in increased
graphitization of the carbon with the Fe-N-C films, and increased relative
amount of graphitic and hydrogenated nitrogen species. Overall the results of
this study demonstrate the feasibility of a thin film model system approach for
studying active sites in PGM-free catalysts.
|
1911.03368v1
|
2019-11-14
|
The Chemical Evolution of Iron-Peak Elements with Hypernovae
|
We calculate the mean evolution of the iron-peak abundance ratios
[(Cr,Mn,Co,Zn)/Fe] in the Galaxy, using modern supernova and hypernova chemical
yields and a Galactic Chemical Evolution code that assumes homogeneous chemical
evolution. We investigate a range of hypernova occurrence rates and are able to
produce a chemical composition that is a reasonable fit to the observed values
in metal-poor stars. This requires a hypernova occurence rate that is large
(50%) in the early Universe, decreasing throughout evolution to a value that is
within present day observational constraints (>~ 1%). A large hypernova
occurence rate is beneficial to matching the high [Zn/Fe] observed in the most
metal-poor stars, although including hypernovae with progenitor mass >= 60
solar masses is detrimental to matching the observed [(Mn,Co)/Fe] evolution at
low [Fe/H]. A significant contribution from HNe seems to be critical for
producing supersolar [(Co,Zn)/Fe] at low metallicity, though more work will
need to be done in order to match the most extreme values. We also emphasise
the need to update models for the enrichment sources at higher metallicity, as
the satisfactory recovery of the solar values of [(Cr,Mn,Co,Zn)/Fe] still
presents a challenge.
|
1911.05901v2
|
2019-11-21
|
Weakness of Correlation Effect Manifestation in BaNi$_2$As$_2$: ARPES and LDA+DMFT study
|
The electronic spectral function of BaNi$_2$As$_2$ is investigated using both
the angle-resolved photoemission spectroscopy (ARPES) and a combined
computational scheme of local density approximation together with dynamical
mean-field theory (LDA+DMFT). In contrast to well studied isostructural iron
arsenide high temperature superconductors, the BaNi$_2$As$_2$ demonstrate weak
correlation effects although Ni-3d elections have even lager on-site
interaction than Fe-3d ones. LDA+DMFT effective mass enhancement for bands
crossing the Fermi level is found to be only about $1.2$ which agrees well with
ARPES data. This reduction of the correlation manifestation with respect to
iron pnictides comes from the increase of 3d-orbital filling, when going from
Fe to Ni. The electron correlations cause remarkable reconstruction of the bare
BaNi$_2$As$_2$ LDA band structure below $-0.8$ eV due to self-energy effect. A
simplified toy model to understand weakness of correlation effects in
BaNi$_2$As$_2$ and to describe the LDA+DMFT self-energy shape is discussed. For
more realistic comparison of LDA+DMFT spectral function maps with ARPES data we
take into account several experimental features: the photoemission
cross-section, the experimental energy and angular resolutions and the
photo-hole lifetime effects. Thus presented here LDA+DMFT calculations with
experimental features included provide nearly qualitative agreement with ARPES
data and assure the observation of a dramatic apparent decrease of the
correlation strength compared to the Fe compounds.
|
1911.09300v2
|
2020-01-03
|
Disappearance of the Fe K$α$ emission line in Ultra Compact X-ray Binaries 4U 1543-624 and Swift J1756.9-2508
|
We investigate the long-term variability of the K$\alpha$ line of iron in the
spectra of two Ultra Compact X-ray Sources (UCXBs) with C/O-rich donors. We
revisit archival observations by five different X-ray telescopes, over a
~twenty year period. Adopting physically motivated models for the spectral
continuum, we probe the long-term evolution of the source emission in a
self-consistent manner enabling physical interpretation of potential
variability in the primary X-ray emission continuum and/or any emission lines
from reflection off the accretion disk. We find that the spectral shape and
flux of the source emission (for both objects) has remained almost constant
throughout all the observations, displaying only minor variability in some
spectral parameters and the source flux (largest variation is a ~25% drop in
the flux of Swift J1756.9-2508). We note a striking variability of the Fe
K$\alpha$ line which fluctuates from a notable equivalent width of ~66-100 eV
in 4U 1543-624 and ~170 eV in Swift J1756.9-2508, to non-detections with upper
limits of 2-8 eV. We argue that the disappearance of the iron line is due to
the screening of the Fe K$\alpha$ line by the overabundant oxygen in the
C/O-rich UCXBs. This effect is cancelled when oxygen becomes fully ionized in
the inner disk region, resulting in the variability of the Fe K$\alpha$ line in
an otherwise unaltered spectral shape. This finding supports earlier
predictions on the consequences of H-poor, C/O-rich accretion disk on
reflection induced fluorescent lines in the spectra of UCXBs.
|
2001.00713v2
|
2020-01-06
|
Are Faint Supernovae Responsible for Carbon-Enhanced Metal-Poor Stars?
|
Mixing and fallback models in faint supernova models are supposed to
reproduce the abundance patterns of observed carbon-enhanced metal-poor (CEMP)
stars in the Galactic halo. A fine tuning of the model parameters for
individual stars is required to reproduce the observed ratios of carbon to
iron. We focus on extremely metal-poor stars formed out of the ejecta from the
mixing and fallback models using a chemical evolution model. Our chemical
evolution models take into account the contribution of individual stars to
chemical enrichment in host halos together with their evolution in the context
of the hierarchical clustering. Parametrized models of mixing and fallback
models for Pop. III faint supernovae are implemented in the chemical evolution
models with merger trees to reproduce the observed CEMP stars. A variety of
choices for model parameters on star formation and metal-pollution by faint
supernovae is unable to reproduce the observed stars with [Fe/H] < -4 and [C/H]
> -2, which are the majority of CEMP stars among the lowest metallicity stars.
Only possible solution is to form stars from small ejecta mass, which produces
an inconsistent metallicity distribution function. We conclude that not all the
CEMP stars are explicable by the mixing and fallback models. We also tested the
contribution of binary mass transfers from AGB stars that are also supposed to
reproduce the abundances of known CEMP stars. This model reasonably reproduces
the distribution of carbon and iron abundances simultaneously only if we assume
that long-period binaries are favored at [Fe/H] < -3.5.
|
2001.01420v1
|
2020-02-09
|
A detailed study on the reflection component for the Black Hole Candidate MAXI J1836-194
|
We present a detailed spectral analysis of the black hole candidate MAXI
J1836-194. The source was caught in the intermediate state during its 2011
outburst by Suzaku and RXTE. We jointly fit the X-ray data from these two
missions using the relxill model to study the reflection component, and a steep
inner emissivity profile indicating a compact corona as the primary source is
required in order to achieve a good fit. In addition, a reflection model with a
lamp-post configuration (relxilllp), which is normally invoked to explain the
steep emissivity profile, gives a worse fit and is excluded at 99% confidence
level compared to relxill. We also explore the effect of the ionization
gradient on the emissivity profile by fitting the data with two relativistic
reflection components, and it is found that the inner emissivity flattens.
These results may indicate that the ionization state of the disc is not
constant. All the models above require a supersolar iron abundance higher than
4.5. However, we find that the high-density version of reflionx can describe
the same spectra even with solar iron abundance well. A moderate rotating black
hole (a* = 0.84-0.94) is consistently obtained by our models, which is in
agreement with previously reported values.
|
2002.03315v1
|
2020-02-11
|
AstroSat/LAXPC view of GX 17+2: Spectral Evolution along the Z-track
|
In this paper, we present the first results obtained using $\sim$ 50 ks
observations of the bright low-mass X-ray binary (LMXB) GX 17+2 with Large Area
X-ray Proportional Counter (LAXPC) onboard {\it AstroSat}. The source traced
out a complete Z-track in the hardness intensity diagram (HID). The spectra at
different sections of the Z-diagram are well described by either a combination
of a thermal Comptonization component, a power-law and a relativistic iron line
or a model consisting of a thermal disk component, a single temperature
blackbody, a power-law and a relativistic iron line. Fitting the spectra with
both phenomenological models suggests that the power-law component is strong in
the horizontal branch (HB), becomes weaker as the source moves down the normal
branch (NB) and then again becomes stronger as the sources moves up the flaring
branch (FB). However, we find that the strength of the power-law component is
model dependent, although the trend in the variation of the power-law strength
along the Z-track is similar. A simple model composed by a Comptonized emission
and power-law component, convolved with the ionized reflection, also describes
the spectra very well.
A normal branch oscillation (NBO) with a centroid frequency 7.42$\pm$0.23 Hz,
quality factor (Q) $\sim$ 4.88, rms 1.41$\pm$0.29\% and significance
5.1$\sigma$ is detected at the middle of the NB. The parameters of the
Comptonized emission show a systematic evolution along the Z-diagram. The
optical depth of the corona increases as the source moves up along the FB,
suggesting possible trigger of an outflow or dumping of the disc material in to
the corona by radiation pressure.
|
2002.04489v1
|
2020-02-18
|
The Solar wind prevents re-accretion of debris after Mercury's giant impact
|
The planet Mercury possesses an anomalously large iron core, and a
correspondingly high bulk density. Numerous hypotheses have been proposed in
order to explain such a large iron content. A long-standing idea holds that
Mercury once possessed a larger silicate mantle which was removed by a giant
impact early in the the Solar system's history. A central problem with this
idea has been that material ejected from Mercury is typically re-accreted onto
the planet after a short (~Myr) timescale. Here, we show that the primordial
Solar wind would have provided sufficient drag upon ejected debris to remove
them from Mercury-crossing trajectories before re-impacting the planet's
surface. Specifically, the young Sun likely possessed a stronger wind, fast
rotation and strong magnetic field. Depending upon the time of the giant
impact, the ram pressure associated with this wind would push particles outward
into the Solar system, or inward toward the Sun, on sub-Myr timescales,
depending upon the size of ejected debris. Accordingly, the giant impact
hypothesis remains a viable pathway toward the removal of planetary mantles,
both on Mercury and extrasolar planets, particularly those close to young stars
with strong winds.
|
2002.07847v2
|
2020-02-20
|
Chemical Diversity of Super-Earths As a Consequence of Formation
|
Recent observations of rocky super-Earths have revealed an apparent wider
distribution of Fe/Mg ratios, or core to mantle ratios, than the planets in our
Solar System. This study aims to understand how much of the chemical diversity
in the super-Earth population can arise from giant impacts during planetary
formation. Planet formation simulations have only recently begun to treat
collisions more realistically in an attempt to replicate the planets in our
Solar System. We investigate planet formation more generally by simulating the
formation of rocky super-Earths with varying initial conditions using a version
of SyMBA, a gravitational N-body code, that incorporates realistic collisions.
We track the maximum plausible change in composition after each impact. The
final planets span a range of Fe/Mg ratios similar to the Solar System planets,
but do not completely match the distribution in super-Earth data. We only form
a few planets with minor iron-depletion, suggesting other mechanisms are at
work. The most iron-rich planets have a lower Fe/Mg ratio than Mercury, and are
less enriched than planets such as Kepler-100b. This indicates that further
work on our understanding of planet formation and further improvement of
precision of mass and radius measurements are required to explain planets at
the extremes of this Fe/Mg distribution.
|
2002.09042v1
|
2020-04-17
|
Thermodynamic signatures of an antiferromagnetic quantum critical point inside a superconducting dome
|
Recent experiments in unconventional superconductors, and in particular
iron-based materials, have reported evidence of an antiferromagnetic quantum
critical point (AFM-QCP) emerging inside the superconducting dome of the phase
diagram. Fluctuations associated with such an AFM-QCP are expected to promote
unusual temperature dependencies of thermodynamic quantities. Here, we compute
the $T$ dependence of the specific heat $C(T)$ deep inside a fully gapped
$s^{+-}$ superconducting state as the AFM-QCP is approached. We find that, at
the AFM-QCP, the specific heat $C(T)$ vanishes quadratically with temperature,
as opposed to the typical exponential suppression seen in fully-gapped BCS
superconductors. This robust result is due to a non-analytic contribution to
the free-energy arising from the general form of the bosonic (AFM) propagator
in the SC state. Away from the AFM-QCP, as temperature is lowered, $C(T)$ shows
a crossover from a $T^2$ behavior to an exponential behavior, with the
crossover temperature scale set by the value of the superconducting gap and the
distance to the QCP. We argue that these features in the specific heat can be
used to unambiguously determine the existence of AFM-QCPs inside the
superconducting domes of iron-based and other fully gapped unconventional
superconductors.
|
2004.08300v3
|
2020-04-22
|
Crystal structures of Fe-gluconate
|
Fe-gluconate, Fe(C_6H_11O_7_2xH_2O is a well-known material widely used for
iron supplementation. On the other hand, it is used in food industry as a
coloring agent, in cosmetic industry for skin and nail conditioning and
metallurgy. Despite of wide range of applications its physical properties were
not studied extensively. In this study, Fe-gluconate with three different
amount of water viz. x=2 (fully hydrated, 0 < x < 2 (intermediate) and x=0
(dry) was investigated by means of X-ray diffraction (XRD) and M\"ossbauer
spectroscopic (MS) methods. The former in the temperature range of 20-300 K,
and the latter at 295 K. Based on the XRD measurements crystallographic
structures were determined: monoclinic (space group I2) for the hydrated sample
and triclinic (space group P1) for the dry sample. The partially hydrated
sample was two-phased. Unit cells parameters for both structures show strong,
very complex and non-monotonic temperature dependences. M\"ossbauer
spectroscopic measurements gave evidence that iron in all samples exist in form
of Fe(II) and Fe(III) ions. The amount of the latter equals to ca.30% in the
hydrated sample and to ca.20% in the dry one.
|
2004.10535v3
|
2020-04-24
|
Magnetism driven by strong electronic correlation in the heavily carrier-doped iron oxypnictide LaFeAsO$_{0.49}$H$_{0.51}$
|
The magnetism of the second antiferromagnetic phase (AF2) arising in the
iron-based LaFeAsO$_{1-x}$H$_{x}$ superconductor for $x\gtrsim0.4$ was
investigated by muon spin rotation measurements under hydrostatic pressure up
to 2.6 GPa. The N\'eel temperature ($T_{\rm N}$) obtained for a sample with
$x=0.51$ exhibits considerably greater sensitivity to pressure than that in the
pristine antiferromagnetic phase (AF1, $x\lesssim0.06$). Moreover, while the
AF1 phase is always accompanied by the structural transition (from tetragonal
to orthorhombic) at a temperature ($T_{\rm s}$) which is slightly higher than
$T_{\rm N}$, the AF2 phase prevails at higher pressures above $\sim$1.5 GPa
where the structural transition is suppressed ($T_{\rm s}=0$). These features
indicate that the microscopic origin of the AF2 phase is distinct from that of
AF1, suggesting that electronic correlation plays important role in the former
phase. We argue that the orbital-selective Mott transition is a plausible
scenario to account for the observed pressure dependence of $T_{\rm N}$ and
$T_{\rm s}$ in the AF2 phase.
|
2004.11547v2
|
2020-05-16
|
Vacancy-driven non-cubic local structure and magnetic anisotropy tailoring in Fe$_x$O-Fe$_{3-δ}$O$_4$ nanocrystals
|
In contrast to bulk materials, nanoscale crystal growth is critically
influenced by size- and shape-dependent properties. However, it is challenging
to decipher how stoichiometry, in the realm of mixed-valence elements, can act
to control physical properties, especially when complex bonding is implicated
by short and long-range ordering of structural defects. Here, solution-grown
iron-oxide nanocrystals (NCs) of the pilot wustite system are found to convert
into iron-deficient rock-salt and ferro-spinel sub-domains, but attain a
surprising tetragonally distorted local structure. Cationic vacancies within
chemically uniform NCs are portrayed as the parameter to tweak the underlying
properties. These lattice imperfections are shown to produce local
exchange-anisotropy fields that reinforce the nanoparticles magnetization and
overcome the influence of finite-size effects. The concept of atomic-scale
defect control in subcritical size NCs, aspires to become a pathway to
tailor-made properties with improved performance for hyperthermia heating over
defect-free NCs.
|
2005.07947v1
|
2020-05-28
|
Rocklines as Cradles for Refractory Solids in the Protosolar Nebula
|
In our solar system, terrestrial planets and meteoritical matter exhibit
various bulk compositions. To understand this variety of compositions,
formation mechanisms of meteorites are usually investigated via a thermodynamic
approach that neglect the processes of transport throughout the protosolar
nebula. Here, we investigate the role played by rocklines
(condensation/sublimation lines of refractory materials) in the innermost
regions of the protosolar nebula to compute the composition of particles
migrating inward the disk as a function of time. To do so, we utilize a
one-dimensional accretion disk model with a prescription for dust and vapor
transport, sublimation and recondensation of refractory materials (ferrosilite,
enstatite, fayalite, forsterite, iron sulfide, metal iron and nickel). We find
that the diversity of the bulk composition of cosmic spherules, chondrules and
chondrites can be explained by their formation close to rocklines, suggesting
that solid matter is concentrated in the vicinity of these
sublimation/condensation fronts. Although our model relies a lot on the number
of considered species and the availability of thermodynamic data governing
state changes, it suggests that rocklines played a major role in the formation
of small and large bodies in the innermost regions of the protosolar nebula.
Our model gives insights on the mechanisms that might have contributed to the
formation of Mercury's large core.
|
2005.14116v3
|
2020-06-12
|
SDSS J124043.01+671034.68: The partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?
|
The white dwarf SDSS J124043.01+671034.68 (SDSS J1240+6710) was previously
found to have an oxygen-dominated atmosphere with significant traces of neon,
magnesium, and silicon. A possible origin via a violent late thermal pulse or
binary interactions have been suggested to explain this very unusual
photospheric composition. We report the additional detection of carbon, sodium,
and aluminium in far-ultraviolet and optical follow-up spectroscopy. No
iron-group elements are detected, with tight upper limits on iron, cobalt and
nickel, suggesting that the star underwent partial oxygen burning, but failed
to ignite silicon burning. Modelling the spectral energy distribution and
adopting the distance based on the Gaia parallax, we infer a low white dwarf
mass, M(wd)=0.41+/-0.05Msun. The large space velocity of SDSS J1240+6710,
computed from the Gaia proper motion and its radial velocity, is compatible
with a Galactic rest-frame velocity of ~250km/s in the opposite direction with
respect to the Galactic rotation, strongly supporting a binary origin of this
star. We discuss the properties of SDSS J1240+6710 in the context of the
recently identified survivors of thermonuclear supernovae, the D6 and LP 40-365
stars, and conclude that it is unlikely related to either of those two groups.
We tentatively suggest that SDSS J1240+6710 is the partially burned remnant of
a low-mass white dwarf that underwent a thermonuclear event.
|
2006.07381v1
|
2020-06-15
|
Opacity Driven Convection and Variability in Accretion Disks around Supermassive Black Holes
|
We study the structure of accretion disks around supermassive black holes in
the radial range $30\sim 100$ gravitational radii, using a three dimensional
radiation magneto-hydrodynamic simulation. For typical conditions in this
region of Active Galactic Nuclei (AGN), the Rosseland mean opacity is expected
to be larger than the electron scattering value. We show that the iron opacity
bump causes the disk to be convective unstable. Turbulence generated by
convection puffs up the disk due to additional turbulent pressure support and
enhances the local angular momentum transport. This also results in strong
fluctuations in surface density and heating of the disk. The opacity drops with
increasing temperature and convection is suppressed. The disk cools down and
the whole process repeats again. This causes strong oscillations of the disk
scale height and luminosity variations by more than a factor of $\approx 3-6$
over a few years' timescale. Since the iron opacity bump will move to different
locations of the disk for black holes with different masses and accretion
rates, we suggest that this is a physical mechanism that can explain the
variability of AGN with a wide range of amplitudes over a time scale of years
to decades.
|
2006.08657v1
|
2020-07-05
|
Size and temperature dependent magnetization of iron nanoclusters
|
The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and
8 nm, is investigated by means of spin dynamics (SD) simulations coupled to
molecular dynamics (MD-SD), using a distance-dependent exchange interaction.
Finite-size effects in the total magnetization as well as the influence of the
free surface and the surface/core proportion of the nanoclusters are analyzed
in detail for a wide temperature range, going beyond the cluster and bulk Curie
temperatures. Comparison is made with experimental data and with theoretical
models based on the mean-field Ising model adapted to small clusters, and
taking into account the influence of low coordinated spins at free surfaces.
Our results for the temperature dependence of the average magnetization per
atom M(T), including the thermalization of the transnational lattice degrees of
freedom, are in very good agreement with available experimental measurements on
small Fe nanoclusters. In contrast, significant discrepancies with experiment
are observed if the translational degrees of freedom are artificially frozen.
The finite-size effects on M(T) are found to be particularly important near the
cluster Curie temperature. Simulated magnetization above the Curie temperature
scales with cluster size as predicted by models assuming short-range magnetic
ordering (SRMO). Analytical approximations to the magnetization as a function
of temperature and size are proposed.
|
2007.02230v2
|
2020-07-06
|
On the Remarkable Superconductivity of FeSe and its Close Cousins
|
Emergent electronic phenomena in iron-based superconductors have been at the
forefront of condensed matter physics for more than a decade. Much has been
learned about the origin and intertwined roles of ordered phases, including
nematicity, magnetism, and superconductivity, in this fascinating class of
materials. In recent years, focus has been centered on the peculiar and highly
unusual properties of FeSe and its close cousins. This family of materials has
attracted considerable attention due to the discovery of unexpected
superconducting gap structures, a wide range of superconducting critical
temperatures, and evidence for nontrivial band topology, including associated
spin-helical surface states and vortex-induced Majorana bound states. Here, we
review superconductivity in iron chalcogenide superconductors, including bulk
FeSe, doped bulk FeSe, FeTe$_{1-x}$Se$_x$, intercalated FeSe materials, and
monolayer FeSe and FeTe$_{1-x}$Se$_x$ on SrTiO$_3$. We focus on the
superconducting properties, including a survey of the relevant experimental
studies, and a discussion of the different proposed theoretical pairing
scenarios. In the last part of the paper, we review the growing recent evidence
for nontrivial topological effects in FeSe-related materials, focusing again on
interesting implications for superconductivity.
|
2007.02966v2
|
2020-07-11
|
Site preference of Fe atoms in the olivine (Fe$_x$Mg$_{2-x}$)SiO$_4$ and its surface
|
Olivine is involved in many natural reactions and industrial reactions as a
catalyst. The catalytic ability is highly possible rely on the Fe$^{2+}$ in
olivine. We use density functional theory calculation and thermodynamics to
investigate the site preference of Fe atom in olivine which composition from
iron-rich to iron-poor and its surfaces. The Fe$^{2+}$ always shows its high
spin (quintet) state which has larger ion radius than Mg$^{2+}$ in olivine
crystal and surfaces. The Fe$^{2+}$ inside the surface slab prefers the smaller
M1 site than M2 site by enlarging the metal-oxygen octahedra when occupied the
metal site as in the bulk system. Energy contribution of entropies accumulation
caused temperature raise stops this preference at the temperature where a
cation order-disorder distribution energy crossover happen in olivine. Surface
exposed site provide Fe$^{2+}$ large space due its unsaturated nature. This
lead a higher level of preference of Fe$^{2+}$ to the surface site than any
metal site inside the crystal no matter M1 or M2 site is exposed. This indicate
the Fe$^{2+}$ in the bulk system can diffuse to a metal site exposed on the
surface driven by the energy difference. Many reactions can use the on surface
Fe$^{2+}$ as a catalyst because of the active chemical behavior of Fe.
Meanwhile this energetics preference should be considered in the future model
to explain the natural observed zoning olivine have a high Fe edge and low Fe
center. These microscopic understanding can be essential to many olivine
related geochemical and astrochemical reactions.
|
2007.05851v3
|
2020-07-19
|
A geometric approach to separate the effects of magnetic susceptibility and chemical shift/exchange in a phantom with isotropic magnetic susceptibility
|
Purpose: To separate the effects of magnetic susceptibility and chemical
shift/exchange in a phantom with isotropic magnetic susceptibility. To generate
a chemical shift/exchange-corrected quantitative susceptibility mapping (QSM)
result.
Theory and Methods: Magnetic susceptibility and chemical shift/exchange are
the properties of a material. Both are known to induce the resonance frequency
shift in MRI. In current QSM, the susceptibility is reconstructed from the
frequency shift, ignoring the contribution of the chemical shift/exchange. In
this work, a simple geometric approach, which averages the frequency shift maps
from three orthogonal B0 directions to generate a chemical shift/exchange map,
is developed using the fact that the average nullifies the (isotropic)
susceptibility effects. The resulting chemical shift/exchange map is subtracted
from the total frequency shift, producing a frequency shift map solely from
susceptibility. Finally, this frequency shift map is reconstructed to a
susceptibility map using a QSM algorithm. The proposed method is validated in
numerical simulations and applied to phantom experiments with olive oil, bovine
serum albumin, ferritin, and iron oxide solutions.
Results: Both simulations and experiments confirm that the method
successfully separates the contributions of the susceptibility and chemical
shift/exchange, reporting the susceptibility and chemical shift/exchange of
olive oil (susceptibility: 0.62 ppm, chemical shift: -3.60 ppm), bovine serum
albumin (susceptibility: -0.059 ppm, chemical shift: 0.008 ppm), ferritin
(susceptibility: 0.125 ppm, chemical shift: -0.005 ppm), and iron oxide
(susceptibility: 0.30 ppm, chemical shift: -0.039 ppm) solutions.
Conclusion: The proposed method successfully separates the susceptibility and
chemical shift/exchange in phantoms with isotropic magnetic susceptibility.
|
2007.09614v1
|
2020-07-21
|
Interplay of itinerant magnetism and reentrant spin-glass behavior in Fe$_{x}$Cr$_{1-x}$
|
When suppressing the itinerant antiferromagnetism in chromium by doping with
the isostructual itinerant ferromagnet iron, a dome of spin-glass behavior
emerges around a putative quantum critical point at an iron concentration $x
\approx 0.15$. Here, we report a comprehensive investigation of polycrystalline
samples of Fe$_{x}$Cr$_{1-x}$ in the range $0.05 \leq x \leq 0.30$ using x-ray
powder diffraction, magnetization, ac susceptibility, and neutron
depolarization measurements, complemented by specific heat and electrical
resistivity data for $x = 0.15$. Besides antiferromagnetic ($x < 0.15$) and
ferromagnetic regimes ($0.15 \leq x$), we identify a dome of reentrant
spin-glass behavior at low temperatures for $0.10 \leq x \leq 0.25$ that is
preceded by a precursor phenomenon. Neutron depolarization indicates an
increase of the size of ferromagnetic clusters with increasing $x$ and the
Mydosh parameter $\phi$, inferred from the ac susceptibility, implies a
crossover from cluster-glass to superparamagnetic behavior. Taken together,
these findings consistently identify Fe$_{x}$Cr$_{1-x}$ as an
itinerant-electron system that permits to study the evolution of spin-glass
behavior of gradually varying character in unchanged crystalline environment.
|
2007.10644v1
|
2021-02-03
|
Stellar astrophysics in the near UV with VLT-CUBES
|
Alongside future observations with the new European Extremely Large Telescope
(ELT), optimised instruments on the 8-10m generation of telescopes will still
be competitive at 'ground UV' wavelengths (3000-4000 A). The near UV provides a
wealth of unique information on the nucleosynthesis of iron-peak elements,
molecules, and neutron-capture elements. In the context of development of the
near-UV CUBES spectrograph for ESO's Very Large Telescope (VLT), we are
investigating the impact of spectral resolution on the ability to estimate
chemical abundances for beryllium and more than 30 iron-peak and heavy
elements. From work ahead of the Phase A conceptual design of CUBES, here we
present a comparison of the elements observable at the notional resolving power
of CUBES (R~20,000) to those with VLT-UVES (R~40,000). For most of the
considered lines signal-to-noise is a more critical factor than resolution. We
summarise the elements accessible with CUBES, several of which (e.g. Be, Ge,
Hf) are now the focus of quantitative simulations as part of the ongoing Phase
A study.
|
2102.02205v1
|
2021-02-07
|
Crystal structure and properties of iron-based spin-chain compound Ba9Fe3Se15
|
We report the synthesis of a new quasi one-dimensional (1D) iron selenide.
Ba9Fe3Se15 was synthesized at high temperature and high pressure of 5.5 GPa and
systematically studied via structural, magnetic and transport measurements at
ambient and at high-pressures. Ba9Fe3Se15 crystallizes in a monoclinic
structure and consists of face-sharing FeSe6 octahedral chains along the c
axis. At ambient pressure it exhibits an insulating behavior with a band gap
~460 meV and undergoes a ferrimagnet-like phase transition at 14 K. Under high
pressure, a complete metallization occurs at ~29 GPa, which is accompanied by a
spin state crossover from high spin (HS) state to low spin (LS) state. The LS
appears for pressures P >36 GPa.
|
2102.03708v1
|
2021-02-07
|
The phonon mechanism explanation of the superconductivity dichotomy between FeSe and FeS monolayers on STO and other substrates
|
It was observed recently (K. Shigekawa et al, PNAS 116, 2470 (2019)) that
while monolayer iron chalcigenide FeSe on SrTiO3 (STO) substrate has a very
high critical temperature, its chemical and structural "twin" material FeS=STO
has a very low Tc if any. To explain this the substrate interfacial phonon
model of superconductivity in iron chalcogenides is further developed. The main
glue is the oxygen ion (60mev) vibrations longitudinal optical (LO) mode. The
mode propagates mainly in the TiO2 layer adjacent to the monolayer (and
genrally present also in similar highly polarized ionic crystals like BaTiO3;
rutile, anatase). It has stronger electron - phonon coupling to electron gas in
FeSe than a well known (100mev) harder LO mode. It is shown that while (taking
into account screened Coulomb repulsion efects) the critical temperature of
FeSe on STO and TiO2 is above 65K, it becomes less than 5K for FeS due to two
factors suppressing the electron - phonon coupling. The efective mass in the
later is twice smaller and in addition the distance between the electron gas in
FeSe to the vibrating substrate oxygen atoms is 15% smaller than in FeS
reducinng the central peak in electron-phonon interaction. The theory is
extended to other ionic insulating substrates.
|
2102.03854v1
|
2021-02-09
|
Testing a Prototype 1U CubeSat on a Stratospheric Balloon Flight
|
High-altitude balloon experiments are becoming very popular among
universities and research institutes as they can be used for testing
instruments eventually intended for space, and for simple astronomical
observations of Solar System objects like the Moon, comets, and asteroids,
difficult to observe from the ground due to atmosphere. Further, they are one
of the best platforms for atmospheric studies. In this experiment, we build a
simple 1U CubeSat and, by flying it on a high-altitude balloon to an altitude
of about 30 km, where the total payload weighted 4.9 kg and examine how some
parameters, such as magnetic field, humidity, temperature or pressure, vary as
a function of altitude. We also calibrate the magnetometer to remove the hard
iron and soft iron errors. Such experiments and studies through a stratospheric
balloon flights can also be used to study the performance of easily available
commercial sensors in extreme conditions as well. We present the results of the
first flight, which helped us study the functionality of the various sensors
and electronics at low temperatures reaching about -40 degrees Celsius. Further
the motion of the payload has been tracked throughout this flight. This
experiment took place on 8 March 2020 from the CREST campus of the Indian
Institute of Astrophysics, Bangalore. Using the results from this flight, we
identify and rectify the errors to obtain better results from the subsequent
flights.
|
2102.04847v1
|
2021-02-21
|
Meteoritic Proteins with Glycine, Iron and Lithium
|
We report that polymer amide [1] with a protein backbone of mainly glycine
units and iron is present in the CV3 meteorites Acfer 086, Allende and KABA.
The evidence for this is from particles of these meteorites after Folch
extraction being analyzed by MALDI mass spectrometry and from the 3D physical
structures that form in the various Folch solvent phases. The two physical
forms we observe are branching rods and entrapping spheres on the 100micron
scale. Two potential molecular structures of polymer amide are presented in
this report. One we term hemolithin of mass 2320Da, contains glycine,
hydroxy-glycine, Fe, O and Li. The other, we term hemoglycin of mass 1494Da is
of glycine, hydroxy-glycine, Fe and O. Hemoglycin is connected covalently in
triplets by silicon to form a triskelion. Analysis of the complete spectrum of
isotopes associated with each molecular fragment shows very high 2H enhancement
above terrestrial averaging 25,700 parts per thousand (sigma = 3,500, n=15),
confirming extra-terrestrial origin and hence the existence of these molecules
within the asteroid parent body of the CV3 meteorite class. The hemoglycin
triskelia join via silicon bonds into an extended lattice, as seen in mass
spectrometry of lattice fragments at m/z 4641 and above [2]. The identification
of hemolithin and hemoglycin required careful methodology at room temperature
with the minimum of steps up to mass spectrometry analysis ensuring that only
the laser step fragmented the molecules. The 3D structures were imaged through
permanently closed Folch extraction V-vials at a magnification of 10.
|
2102.10700v1
|
2021-02-22
|
Estimating SARS-CoV-2 Infections from Deaths, Confirmed Cases, Tests, and Random Surveys
|
There are many sources of data giving information about the number of
SARS-CoV-2 infections in the population, but all have major drawbacks,
including biases and delayed reporting. For example, the number of confirmed
cases largely underestimates the number of infections, deaths lag infections
substantially, while test positivity rates tend to greatly overestimate
prevalence. Representative random prevalence surveys, the only putatively
unbiased source, are sparse in time and space, and the results come with a big
delay. Reliable estimates of population prevalence are necessary for
understanding the spread of the virus and the effects of mitigation strategies.
We develop a simple Bayesian framework to estimate viral prevalence by
combining the main available data sources. It is based on a discrete-time SIR
model with time-varying reproductive parameter. Our model includes likelihood
components that incorporate data of deaths due to the virus, confirmed cases,
and the number of tests administered on each day. We anchor our inference with
data from random sample testing surveys in Indiana and Ohio. We use the results
from these two states to calibrate the model on positive test counts and
proceed to estimate the infection fatality rate and the number of new
infections on each day in each state in the USA. We estimate the extent to
which reported COVID cases have underestimated true infection counts, which was
large, especially in the first months of the pandemic. We explore the
implications of our results for progress towards herd immunity.
|
2102.10741v1
|
2021-06-03
|
Ultrafast demagnetization of iron induced by optical vs terahertz pulses
|
We study ultrafast magnetization quenching of ferromagnetic iron following
excitation by an optical vs a terahertz pump pulse. While the optical pump
(photon energy of 3.1 eV) induces a strongly nonthermal electron distribution,
terahertz excitation (~4 meV) results in a quasi-thermal perturbation of the
electron population. The pump-induced spin and electron dynamics are
interrogated by the magneto-optic Kerr effect (MOKE). A deconvolution procedure
allows us to push the time resolution down to 130 fs, even though the driving
terahertz pulse is more than 0.5 ps long. Remarkably, the MOKE signals exhibit
an almost identical time evolution for both optical and terahertz pump pulses,
despite the three orders of magnitude different number of excited electrons. We
are able to quantitatively explain our results using a model based on
quasi-elastic spin-flip scattering. It shows that in the small-perturbation
limit, the rate of demagnetization of a metallic ferromagnet is proportional to
the excess energy of the electrons, independent of the precise shape of their
distribution. Our results reveal that the dynamics of ultrafast demagnetization
and of the closely related terahertz spin transport do not depend on the pump
photon energy.
|
2106.01967v1
|
2021-06-14
|
Spin-pure Stochastic-CASSCF via GUGA-FCIQMC applied to Iron Sulfur Clusters
|
In this work we demonstrate how to compute the one- and two-body reduced
density matrices within the spin-adapted full configuration interaction quantum
Monte Carlo (FCIQMC) method, which is based on the graphical unitary group
approach (GUGA). This allows us to use GUGA-FCIQMC as a spin-pure configuration
interaction (CI) eigensolver within the complete active space self-consistent
field (CASSCF) procedure, and hence to stochastically treat active spaces far
larger than conventional CI solvers whilst variationally relaxing orbitals for
specific spin-pure states. We apply the method to investigate the spin-ladder
in iron-sulfur dimer and tetramer model systems. We demonstrate the importance
of the orbital relaxation by comparing the Heisenberg model magnetic coupling
parameters from the CASSCF procedure to those from a CI-only procedure based on
restricted open-shell Hartree-Fock orbitals. We show that orbital relaxation
differentially stabilizes the lower spin states, thus enlarging the coupling
parameters with respect to the values predicted by ignoring orbital relaxation
effects. Moreover, we find that while CI eigenvalues are well fit by a simple
bilinear Heisenberg Hamiltonian, the CASSCF eigenvalues exhibits deviations
that necessitate the inclusion of biquadratic terms in the model Hamiltonian.
|
2106.07775v1
|
2021-06-22
|
A two-stage robust optimization approach for oxygen flexible distribution under uncertainty in iron and steel plants
|
Oxygen optimal distribution is one of the most important energy management
problems in the modern iron and steel industry. Normally, the supply of the
energy generation system is determined by the energy demand of manufacturing
processes. However, the balance between supply and demand fluctuates frequently
due to the uncertainty arising in manufacturing processes. In this paper, we
developed an oxygen optimal distribution model considering uncertain demands
and proposed a two-stage robust optimization (TSRO) with a budget-based
uncertainty set that protects the initial distribution decisions with low
conservatism. The main goal of the TSRO model is to make wait-and-see decisions
maximizing production profits and make here-and-now decisions minimizing
operational stability and surplus/shortage penalty. To represent the
uncertainty set of energy demands, we developed a Gaussian process (GP)-based
time series model to forecast the energy demands of continuous processes and a
capacity-constrained scheduling model to generate multi-scenario energy demands
of discrete processes. We carried out extensive computational studies on TSRO
and its components using well-synthetic instances from historical data. The
results of model validation and analysis are promising and demonstrate our
approach is adapted to solve industrial cases under uncertainty.
|
2106.11635v1
|
2021-06-23
|
Correlation Between Spin and Orbital Dynamics During Laser-Induced Femtosecond Demagnetization
|
Spin and orbital angular momenta are two intrinsic properties of an electron
and are responsible for the physics of a solid. How the spin and orbital evolve
with respect to each other on several hundred femtoseconds is largely unknown,
but it is at the center of laser-induced ultrafast demagnetization. In this
paper, we introduce a concept of the spin-orbital correlation diagram, where
spin angular momentum is plotted against orbital angular momentum, much like
the position-velocity phase diagram in classical mechanics. We use four sets of
highly accurate time-resolved x-ray magnetic circular dichroism (TR-XMCD) data
to construct four correlation diagrams for iron and cobalt. To our surprise, a
pattern emerges. The trace on the correlation diagram for iron is an arc, and
at the end of demagnetization, it has a pronounced cusp. The correlation
diagram for cobalt is different and appears more linear, but with kinks. We
carry out first-principles calculations with two different methods:
time-dependent density functional theory (TDDFT) and time-dependent Liouville
density functional theory (TDLDFT). These two methods agree that the
experimental findings for both Fe and Co are not due t experimental errors. It
is the spin-orbit coupling that correlates the spin dynamics to the orbital
dynamics.Microscopically, Fe and Co have different orbital occupations, which
leads to distinctive correlation diagrams. We believe that this correlation
diagram presents a useful tool to better understand spin and orbital dynamics
on an ultrafast time scale. A brief discussion on the magnetic anisotropy
energy is also provided.
|
2106.12679v1
|
2021-07-09
|
Rapid interrogation of special nuclear materials by combining scattering and transmission nuclear resonance fluorescence spectroscopy
|
The smuggling of special nuclear materials (SNMs) across national borders is
becoming a serious threat to nuclear nonproliferation. This paper presents a
feasibility study on the rapid interrogation of concealed SNMs by combining
scattering and transmission nuclear resonance fluorescence (sNRF and tNRF)
spectroscopy. In sNRF spectroscopy, SNMs such as $^{235, 238}$U are excited by
a wide-band photon beam of appropriate energy and exhibit unique NRF
signatures. Monte Carlo simulations show that one-dimensional scans can realize
isotopic identification of concealed $^{235, 238}$U when the detector array
used for interrogation has sufficiently high energy resolution. The simulated
isotopic ratio $^{235}U/^{238}U$ is in good agreement with the theoretical
value when the SNMs are enclosed in relatively thin iron. This interrogation is
followed by tNRF spectroscopy using a narrow-band photon beam with the goal of
obtaining tomographic images of the concealed SNMs. The reconstructed image
clearly reveals the position of the isotope $^{235}$U inside an iron rod. It is
shown that the interrogation time of sNRF and tNRF spectroscopy is one order of
magnitude lower than that when only tNRF spectroscopy is used and results in a
missed-detection rate of 10$^{-3}$. The proposed method can also be applied for
isotopic imaging of other SNMs such as $^{239, 240}$Pu and $^{237}$Np.
|
2107.04379v1
|
2021-11-01
|
Spectrum evolution and chirping of laser-induced spin wave packets in thin iron films
|
We present the experimental study of ultrafast optical excitation of
magnetostatic surface spin wave (MSSW) packets and their spectral properties in
thin films of pure iron. As the packets leave the excitation area and propagate
in space, their spectra evolve non-trivially. Particularly, low or high
frequency components are suppressed at the border of the excitation area
depending on the orientation of the external magnetic field with respect to the
magnetocrystolline anisotropy axes of the film. The effect is ascribed to the
ultrafast local heating of the film. Further, the time resolution of the
implemented all-optical technique allows us to extract the chirp of the MSSW
packet in the time domain via wavelet analysis. The chirp is a result of the
group velocity dispersion of the MSSW and, thus, is controlled by the film
magnetic parameters, magnetization and anisotropy, and external field
orientation. The demonstrated tunable modulation of MSSW wave packets with
femtosecond laser pulses may find application in future magnonic-photonic
hybrid devices for wave-based data processing.
|
2111.00983v2
|
2021-11-08
|
Microstructure, grain boundary evolution and anisotropic Fe segregation in (0001) textured Ti thin films
|
The structure and chemistry of grain boundaries (GBs) are crucial in
determining polycrystalline materials' properties. Faceting and solute
segregation to minimize the GB energy is a commonly observed phenomenon. In
this paper, a deposition process to obtain pure tilt GBs in titanium (Ti) thin
films is presented. By increasing the power density, a transition from
polycrystalline film growth to a maze bicrystalline Ti film on SrTiO$_3$ (001)
substrate is triggered. All the GBs in the bicrystalline thin film are
characterized to be $\Sigma$13 [0001] coincident site lattice (CSL) boundaries.
The GB planes are seen to distinctly facet into symmetric {$\bar{7}520$} and
asymmetric {$10\bar{1}0$} // {$11\bar{2}0$} segments of 20-50~nm length.
Additionally, EDS reveals preferential segregation of iron (Fe) in every
alternate symmetric {$\bar{7}520$} segment. Both the faceting and the
segregation are explained by a difference in the CSL density between the facet
planes. Furthermore, in the GB plane containing Fe segregation, atom probe
tomography is used to experimentally determine the GB excess solute to be
1.25~atoms/nm$^{2}$. In summary, the study reveals for the first time a
methodology to obtain bicrystalline Ti thin films with strong faceting and
anisotropy in iron (Fe) segregation behaviour within the same family of planes.
|
2111.04606v1
|
2021-11-08
|
The MOBH35 metal-organic barrier heights reconsidered: performance of local-orbital coupled cluster approaches in different static correlation regimes
|
We have revisited the MOBH35 (Metal-Organic Barrier Heights, 35 reactions)
benchmark [Iron, M. A.; Janes, T. J. Phys. Chem. A 2019, 123 (17), 3761-3781;
ibid. 2019, 123, 6379-6380] for realistic organometallic catalytic reactions,
using both canonical CCSD(T) and localized orbital approximations to it. For
low levels of static correlation, all of DLPNO-CCSD(T), PNO-LCCSD(T), and
LNO-CCSD(T) perform well; for moderately strong levels of static correlation,
DLPNO-CCSD(T) and (T1) may break down catastrophically, and PNO-LCCSD(T) is
vulnerable as well. In contrast, LNO-CCSD(T) converges smoothly to the
canonical CCSD(T) answer with increasingly tight convergence settings. The only
two reactions for which our revised MOBH35 reference values differ
substantially from the original ones are reaction 9 and to a lesser extent 8,
both involving iron. For the purpose of evaluating DFT methods for MOBH35, it
would be best to excise reaction 9 entirely as its severe level of static
correlation is just too demanding a test. The magnitude of the difference
between DLPNO-CCSD(T) and DLPNO-CCSD(T1) is a reasonably good predictor for
errors in DLPNO-CCSD(T1) compared to canonical CCSD(T); [...]
|
2111.04777v3
|
2021-11-24
|
Metallicity of the globular cluster NGC 6388 from high resolution spectra of more than 160 giant stars
|
NGC 6388 is one of the most massive Galactic globular clusters (GC) and it is
an old, metal-rich, Galactic bulge cluster. By exploiting previous
spectroscopic observations we were able to bypass the uncertainties in
membership related to the strong field stars contamination. We present the
abundance analysis of 12 new giant stars with UVES spectra and 150 giants with
GIRAFFE spectra acquired at the ESO-VLT. We derived radial velocities,
atmospheric parameters and iron abundances for all stars. When combined to
previous data, we obtain a grand total of 185 stars homogeneously analysed in
NGC 6388 from high-resolution spectroscopy. The average radial velocity of the
185 stars is 81.2+/-0.7, rms = 9.4 km/s. We obtain an average metallicity
[Fe/H]=-0.480 dex, rms = 0.045 dex (35 stars) and [Fe/H]=-0.488 dex, rms =
0.040 dex (150 stars) from the UVES and GIRAFFE samples, respectively.
Comparing these values to internal errors in abundance, we exclude the presence
of a significant intrinsic metallicity spread within the cluster. Since about a
third of giants in NGC 6388 is claimed to belong to the "anomalous red giants"
in the HST pseudo-colour map defining the so-called type-II GCs, we conclude
that either enhanced metallicity is not a necessary requisite to explain this
classification (as also suggested by the null iron spread for NGC 362) or NGC
6388 is not a type-II globular cluster.
|
2111.12721v1
|
2022-06-30
|
SRG/eROSITA discovery of a radio faint X-ray candidate supernova remnant SRGe J003602.3+605421=G121.1-1.9
|
We report the discovery of a candidate X-ray supernova remnant SRGe
J003602.3+605421=G121.1-1.9 in the course of \textit{SRG}/eROSITA all-sky
survey. The object is located at (l,b)=(121.1$^\circ$,-1.9$^\circ$), is
$\approx36$ arcmin in angular size and has a nearly circular shape. Clear
variations in spectral shape of the X-ray emission across the object are
detected, with the emission from the inner (within 9') and outer (9'-18') parts
dominated by iron and oxygen/neon lines, respectively. The non-equilibrium
plasma emission model is capable of describing the spectrum of the outer part
with the initial gas temperature 0.1 keV, final temperature 0.5 keV and the
ionization age $\sim 2\times10^{10}$ cm$^{-3}$ s. The observed spectrum of the
inner region is more complicated (plausibly due to the contribution of the
outer shell) and requires substantial overabundance of iron for all models we
have tried. The derived X-ray absorption equals to $(4-6)\times10^{21}$
cm$^{-2}$, locating the object at the distance beyond 1.5 kpc, and implying its
age $\sim(5-30)\times1000$ yrs. No bright radio, infrared, H$_\alpha$ or
gamma-ray counterpart of this object have been found in the publicly-available
archival data. A model invoking a canonical $10^{51}$ erg explosion (either SN
Ia or core collapse) in the hot and tenuous medium in the outer region of the
Galaxy $\sim$9 kpc away might explain the bulk of the observed features. This
scenario can be tested with future deep X-ray and radio observations.
|
2207.00064v2
|
2022-07-20
|
Dislocation Majorana Bound States in Iron-based Superconductors
|
We show that lattice dislocations of topological iron-based superconductors
such as FeTe$_{1-x}$Se$_x$ will intrinsically trap non-Abelian Majorana
quasiparticles, in the absence of any external magnetic field. Our theory is
motivated by the recent experimental observations of normal-state topology and
surface magnetism that coexist with superconductivity in FeTe$_{1-x}$Se$_x$,
the combination of which naturally evokes an emergent second-order topological
superconductivity in a two-dimensional subsystem spanned by screw or edge
dislocations. This exemplifies a new embedded higher-order topological phase in
class D, where Majorana zero modes appear around the "corners" of a
low-dimensional embedded subsystem, instead of those of the full crystal. A
nested domain wall theory is developed to understand the origin of these defect
Majorana zero modes. When the surface magnetism is absent, we further find that
$s_{\pm}$ pairing symmetry itself is capable of inducing a different type of
class-DIII embedded higher-order topology with defect-bound Majorana Kramers
pairs. We also provide detailed discussions on the real-world material
candidates for our proposals, including FeTe$_{1-x}$Se$_x$, LiFeAs,
$\beta$-PdBi$_2$, and heterostructures of bismuth, etc. Our work establishes
lattice defects as a new venue to achieve high-temperature topological quantum
information processing.
|
2207.10113v2
|
2017-04-18
|
Structure, magnetic susceptibility and specific heat of the spin-orbital-liquid candidate FeSc2S4 : Influence of fe off-stoichiometry
|
We report structural, susceptibility and specific heat studies of
stoichiometric and off-stoichiometric poly- and single crystals of the A-site
spinel compound FeSc2S4. In stoichiometric samples no long-range magnetic order
is found down to 1.8 K. The magnetic susceptibility of these samples is field
independent in the temperature range 10 - 400 K and does not show irreversible
effects at low temperatures. In contrast, the magnetic susceptibility of
samples with iron excess shows substantial field dependence at high
temperatures and manifests a pronounced magnetic irreversibility at low
temperatures with a difference between ZFC and FC susceptibilities and a
maximum at 10 K reminiscent of a magnetic transition. Single crystal x-ray
diffraction of the stoichiometric samples revealed a single phase spinel
structure without site inversion. In single crystalline samples with Fe excess
besides the main spinel phase a second ordered single-crystal phase was
detected with the diffraction pattern of a vacancy-ordered superstructure of
iron sulfide, close to the 5C polytype Fe9S10. Specific heat studies reveal a
broad anomaly, which evolves below 20 K in both stoichiometric and
off-stoichiometric crystals. We show that the low-temperature specific heat can
be well described by considering the low-lying spin-orbital electronic levels
of Fe2+ ions. Our results demonstrate significant influence of excess Fe ions
on intrinsic magnetic behavior of FeSc2S4 and provide support for the
spin-orbital liquid scenario proposed in earlier studies for the stoichiometric
compound.
|
1704.05275v1
|
2017-04-20
|
Experimental Demonstration of the Sign Reversal of the Order Parameter in (Li1-xFex)OHFe1-yZnySe
|
Iron pnictides are the only known family of unconventional high-temperature
superconductors besides cuprates. Until recently, it was widely accepted that
superconductivity is spin-fluctuation driven and intimately related to their
fermiology, specifically, hole and electron pockets separated by the same wave
vector that characterizes the dominant spin fluctuations, and supporting order
parameters (OP) of opposite signs. This picture was questioned after the
discovery of a new family, based on the FeSe layers, either intercalated or in
the monolayer form. The critical temperatures there reach ~40 K, the same as in
optimally doped bulk FeSe - despite the fact that intercalation removes the
hole pockets from the Fermi level and, seemingly, undermines the basis for the
spin-fluctuation theory and the idea of a sign-changing OP. In this paper,
using the recently proposed phase-sensitive quasiparticle interference
technique, we show that in LiOH intercalated FeSe compound the OP does change
sign, albeit within the electronic pockets, and not between the hole and
electron ones. This result unifies the pairing mechanism of iron based
superconductors with or without the hole Fermi pockets and supports the
conclusion that spin fluctuations play the key role in electron pairing.
|
1704.06141v1
|
2017-04-25
|
Role of multiorbital effects in the magnetic phase diagram of iron-pnictides
|
We elucidate the pivotal role of the bandstructure's orbital content in
deciding the type of commensurate magnetic order stabilized within the
itinerant scenario of iron-pnictides. Recent experimental findings in the
tetragonal magnetic phase attest to the existence of the so-called charge and
spin ordered density wave over the spin-vortex crystal phase, the latter of
which tends to be favored in simplified band models of itinerant magnetism.
Here we show that employing a multiorbital itinerant Landau approach based on
realistic bandstructures can account for the experimentally observed magnetic
phase, and thus shed light on the importance of the orbital content in deciding
the magnetic order. In addition, we remark that the presence of a hole pocket
centered at the Brillouin zone's ${\rm M}$-point favors a magnetic stripe
rather than a tetragonal magnetic phase. For inferring the symmetry properties
of the different magnetic phases, we formulate our theory in terms of magnetic
order parameters transforming according to irreducible representations of the
ensuing D$_{\rm 4h}$ point group. The latter method not only provides
transparent understanding of the symmetry breaking schemes but also reveals
that the leading instabilities always belong to the $\{A_{1g},B_{1g}\}$ subset
of irreducible representations, independent of their C$_2$ or C$_4$ nature.
|
1704.07862v2
|
2017-05-18
|
Effects of the Functional Group on the Lithium Ions Across the Port of Carbon Nanotube
|
The mean axial velocity of lithium irons across the entrance of carbon
nanotube VLi is an important factor for the charge-discharge performances of
rechargeable Lithium battery. The molecular dynamics simulation method is
adopted to evaluate the factors and their effects on VLi which include the
diameter of carbon nanotube, functional group type on the port and the number
of a given type of functional group. The statistical analysis of the
calculation results shows that: In the selected carbon nanotubes of four
different diameters, VLi will gradually rise with the increase of CNT diameter
due to lithium irons migration resistance decreasing; as the port of CNT is
successively modified to hydrogen (-H), hydroxyl (-OH), amino (-NH2) and
carboxyl (-COOH), the corresponding migration resistance of lithium ions is
enhanced resulting in the dropping of VLi; in comparison to the effect strength
of four types of functional groups on VLi, -COOH shows strongest, -NH2 and -OH
perform relatively weaker, and the effect difference between -NH2 and -OH is
very small, -H displays weakest; When the number of a given non-hydrogen
functional group on the port sequentially increases, it also shows a trend that
lithium ion migration resistance gradually increases which makes VLi decreases
in turn. The more influential the functional group, the greater the impact of
functional group number changes on VLi. The results of this paper have some
significance on the precise production of lithium-ion battery electrode
materials, enhancing the overall battery cycle efficiency and charging speed.
|
1705.06650v1
|
2017-05-27
|
High transport current superconductivity in powder-in-tube Ba0.6K0.4Fe2As2 tapes at 27 tesla
|
The high upper critical field and low anisotropy of iron-based
superconductors make them being particularly attractive for high-field
applications. However, the current carrying capability needs to be enhanced by
overcoming the weak-link effect between misaligned grains inside wire and tape
conductors. Here we demonstrate a high transport critical current density (Jc)
reaching 1.5x10^5 A/cm^2 (Ic = 437 A) at 4.2 K and 10 T in Ba0.6K0.4Fe2As2
(Ba-122) tapes prepared by a combination of conventional powder-in-tube method
and optimized hot-press technique. The transport Jc measured at 4.2 K under
high magnetic fields of 27 T is still on the level of 5.5x10^4 A/cm^2, which is
much higher than those of low-temperature superconductors. This is the first
report of hot-pressed Ba-122 superconducting tapes and these Jc values are by
far the highest ever reported for iron-based superconducting wires and tapes.
These high-performance tapes exhibit high degree of c-axis texture of Ba-122
grains and low anisotropy of transport Jc, showing great potential for
construction of high-field superconducting magnets.
|
1705.09788v1
|
2017-05-29
|
Critical Slowing Down of Quadrupole and Hexadecapole Orderings in Iron Pnictide Superconductor
|
Ultrasonic measurements have been carried out to investigate the critical
dynamics of structural and superconducting transitions due to degenerate
orbital bands in iron pnictide compounds with the formula
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. The attenuation coefficient
$\alpha_{\mathrm{L}[110]}$ of the longitudinal ultrasonic wave for
$(C_{11}+C_{12}+2C_{66})/2$ for $x = 0.036$ reveals the critical slowing down
of the relaxation time around the structural transition at $T_\mathrm{s} = 65$
K, which is caused by ferro-type ordering of the quadrupole $O_{x'^2-y'^2}$
coupled to the strain $\varepsilon_{xy}$. The attenuation coefficient
$\alpha_{66}$ of the transverse ultrasonic wave for $C_{66}$ for $x = 0.071$
also exhibits the critical slowing down around the superconducting transition
at $T_\mathrm{SC} = 23$ K, which is caused by ferro-type ordering of the
hexadecapole $H_z^\alpha \bigl( \boldsymbol{r}_i, \boldsymbol{r}_j \bigr) =
O_{x'y'}\bigl( \boldsymbol{r}_i \bigr) O_{x'^2 - y'^2}\bigl( \boldsymbol{r}_j
\bigr)
+ O_{x'^2 - y'^2}\bigl( \boldsymbol{r}_i \bigr) O_{x'y'}\bigl(
\boldsymbol{r}_j \bigr)$ of the bound two-electron state coupled to the
rotation $\omega_{xy}$. It is proposed that the hexadecapole ordering
associated with the superconductivity brings about spontaneous rotation of the
macroscopic superconducting state with respect to the host tetragonal lattice.
|
1705.10057v1
|
2017-05-31
|
Terahertz spin currents and inverse spin Hall effect in thin-film heterostructures containing complex magnetic compounds
|
Terahertz emission spectroscopy of ultrathin multilayers of magnetic and
heavy metals has recently attracted much interest. This method not only
provides fundamental insights into photoinduced spin transport and spin-orbit
interaction at highest frequencies but has also paved the way to applications
such as efficient and ultrabroadband emitters of terahertz electromagnetic
radiation. So far, predominantly standard ferromagnetic materials have been
exploited. Here, by introducing a suitable figure of merit, we systematically
compare the strength of terahertz emission from X/Pt bilayers with X being a
complex ferro-, ferri- and antiferromagnetic metal, that is, dysprosium cobalt
(DyCo$_5$), gadolinium iron (Gd$_{24}$Fe$_{76}$), Magnetite (Fe$_3$O$_4$) and
iron rhodium (FeRh). We find that the performance in terms of spin-current
generation not only depends on the spin polarization of the magnet's conduction
electrons but also on the specific interface conditions, thereby suggesting
terahertz emission spectroscopy to be a highly surface-sensitive technique. In
general, our results are relevant for all applications that rely on the optical
generation of ultrafast spin currents in spintronic metallic multilayers.
|
1705.11069v2
|
2017-06-02
|
Quenching current by flux-flow instability in iron-chalcogenides thin films
|
The stability against quench is one of the main issue to be pursued in a
superconducting material which should be able to perform at very high levels of
current densities. Here we focus on the connection between the critical current
$I_c$ and the quenching current $I^*$ associated to the so-called flux-flow
instability phenomenon, which sets in as an abrupt transition from the flux
flow state to the normal state. To this purpose, we analyze several
current-voltage characteristics of three types of iron-based thin films,
acquired at different temperature and applied magnetic field values. For these
samples, we discuss the impact of a possible coexistence of intrinsic
electronic mechanisms and extrinsic thermal effects on the quenching current
dependence upon the applied magnetic field. The differences between the
quenching current and the critical current are reported also in the case of
predominant intrinsic mechanisms. Carrying out a comparison with
high-temperature cuprate superconductors, we suggest which material can be the
best trade-off between maximum operating temperature, higher upper critical
field and stability under high current bias.
|
1706.00772v1
|
2017-06-19
|
Evidence for Majorana bound state in an iron-based superconductor
|
The search for Majorana bound state (MBS) has recently emerged as one of the
most active research areas in condensed matter physics, fueled by the prospect
of using its non-Abelian statistics for robust quantum computation. A highly
sought-after platform for MBS is two-dimensional topological superconductors,
where MBS is predicted to exist as a zero-energy mode in the core of a vortex.
A clear observation of MBS, however, is often hindered by the presence of
additional low-lying bound states inside the vortex core. By using scanning
tunneling microscope on the newly discovered superconducting Dirac surface
state of iron-based superconductor FeTe1-xSex (x = 0.45, superconducting
transition temperature Tc = 14.5 K), we clearly observe a sharp and non-split
zero-bias peak inside a vortex core. Systematic studies of its evolution under
different magnetic fields, temperatures, and tunneling barriers strongly
suggest that this is the case of tunneling to a nearly pure MBS, separated from
non-topological bound states which is moved away from the zero energy due to
the high ratio between the superconducting gap and the Fermi energy in this
material. This observation offers a new, robust platform for realizing and
manipulating MBSs at a relatively high temperature.
|
1706.06074v3
|
2017-06-25
|
Frustration-driven C4 symmetric orders in a hetero-structured iron-based superconductor
|
A subtle balance between competing interactions in strongly correlated
systems can be easily tipped by additional interfacial interactions in a
heterostructure. This often induces exotic phases with unprecedented
properties, as recently exemplified by high-Tc superconductivity in FeSe
monolayer on the nonmagnetic SrTiO3. When the proximity-coupled layer is
magnetically active, even richer phase diagrams are expected in iron-based
superconductors (FeSCs), which however has not been explored due to the lack of
a proper material system. One promising candidate is Sr2VO3FeAs, a
naturally-assembled heterostructure of a FeSC and a Mott-insulating vanadium
oxide. Here, using high-quality single crystals and high-accuracy 75As and 51V
nuclear magnetic resonance (NMR) measurements, we show that a novel electronic
phase is emerging in the FeAs layer below T0 ~ 155 K without either static
magnetism or a crystal symmetry change, which has never been observed in other
FeSCs. We find that frustration of the otherwise dominant Fe stripe and V Neel
fluctuations via interfacial coupling induces a charge/orbital order with
C4-symmetry in the FeAs layers, while suppressing the Neel antiferromagnetism
in the SrVO3 layers. These findings demonstrate that the magnetic proximity
coupling is effective to stabilize a hidden order in FeSCs and, more generally,
in strongly correlated heterostructures.
|
1706.08157v1
|
2017-06-27
|
Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by 75As NMR
|
We report $^{75}$As nuclear magnetic resonance (NMR) studies on a new
iron-based superconductor CaKFe$_4$As$_4$ with $T_{\rm c}$ = 35 K. $^{75}$As
NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites
close to the K and Ca layers, respectively, revealing that K and Ca layers are
well ordered without site inversions. We found that nuclear quadrupole
frequencies $\nu_{\rm Q}$ of the As(1) and As(2) sites show an opposite
temperature ($T$) dependence. Nearly $T$ independent behavior of the Knight
shifts $K$ are observed in the normal state, and a sudden decrease in $K$ in
the superconducting (SC) state clearly evidences spin-singlet Cooper pairs.
$^{75}$As spin-lattice relaxation rates 1/$T_1$ show a power law $T$ dependence
with different exponents for the two As sites. The isotropic antiferromagnetic
spin fluctuations characterized by the wavevector ${\bf q}$ = ($\pi$, 0) or (0,
$\pi$) in the single-iron Brillouin zone notation are revealed by 1/$T_1T$ and
$K$ measurements. Such magnetic fluctuations are necessary to explain the
observed temperature dependence of the $^{75}$As quadrupole frequencies, as
evidenced by our first-principles calculations. In the SC state, 1/$T_1$ shows
a rapid decrease below $T_{\rm c}$ without a Hebel-Slichter peak and decreases
exponentially at low $T$, consistent with an $s^{\pm}$ nodeless two-gap
superconductor.
|
1706.09075v2
|
2018-02-05
|
Discrete superconducting phases in FeSe-derived superconductors
|
A general feature of unconventional superconductors is the existence of a
superconducting dome in the phase diagram as a function of carrier
concentration. For the simplest iron-based superconductor FeSe (with transition
temperature Tc ~ 8 K), its Tc can be greatly enhanced by doping electrons via
many routes, even up to 65 K in monolayer FeSe/SiTiO3. However, a clear phase
diagram with carrier concentration for FeSe-derived superconductors is still
lacking. Here, we report the observation of a series of discrete
superconducting phases in FeSe thin flakes by continuously tuning carrier
concentration through the intercalation of Li and Na ions with a solid ionic
gating technique. Such discrete superconducting phases are robust against the
substitution of Se by 20% S, but are vulnerable to the substitution of Fe by 2%
Cu, highlighting the importance of the iron site being intact. A complete
superconducting phase diagram for FeSe-derivatives is given, which is distinct
from other unconventional superconductors.
|
1802.01484v1
|
2018-02-06
|
Imaging Orbital-selective Quasiparticles in the Hund's Metal State of FeSe
|
Strong electronic correlations, emerging from the parent Mott insulator
phase, are key to copper-based high temperature superconductivity (HTS). By
contrast, the parent phase of iron-based HTS is never a correlated insulator.
But this distinction may be deceptive because Fe has five active d-orbitals
while Cu has only one. In theory, such orbital multiplicity can generate a
Hund's Metal state, in which alignment of the Fe spins suppresses inter-orbital
fluctuations producing orbitally selective strong correlations. The spectral
weights $Z_m$ of quasiparticles associated with different Fe orbitals m should
then be radically different. Here we use quasiparticle scattering interference
resolved by orbital content to explore these predictions in FeSe. Signatures of
strong, orbitally selective differences of quasiparticle $Z_m$ appear on all
detectable bands over a wide energy range. Further, the quasiparticle
interference amplitudes reveal that $Z_{xy}<Z_{xz}<<Z_{yz}$, consistent with
earlier orbital-selective Cooper pairing studies. Thus, orbital-selective
strong correlations dominate the parent state of iron-based HTS in FeSe.
|
1802.02266v2
|
2018-02-20
|
Monte-Carlo simulations of the detailed iron absorption line profiles from thermal winds in X-ray binaries
|
Blue shifted absorption lines from highly ionised iron are seen in some high
inclination X-ray binary systems, indicating the presence of an equatorial disc
wind. This launch mechanism is under debate, but thermal driving should be
ubiquitous. X-ray irradiation from the central source heats disc surface,
forming a wind from the outer disc where the local escape velocity is lower
than the sound speed. The mass loss rate from each part of the disc is
determined by the luminosity and spectral shape of the central source. We use
these together with an assumed density and velocity structure of the wind to
predict the column density and ionisation state, then combine this with a Monte
Carlo radiation transfer to predict the detailed shape of the absorption (and
emission) line profiles. We test this on the persistent wind seen in the bright
neutron star binary GX 13+1, with luminosity L/LEdd ~ 0.5. We approximately
include the effect of radiation pressure because of high luminosity, and
compute line features. We compare these to the highest resolution data, the
Chandra third order grating spectra, which we show here for the first time.
This is the first physical model for the wind in this system, and it succeeds
in reproducing many of the features seen in the data, showing that the wind in
GX13+1 is most likely a thermal-radiation driven wind. This approach, combined
with better streamline structures derived from full radiation hydrodynamic
simulations, will allow future calorimeter data to explore the detail wind
structure.
|
1802.07019v1
|
2018-07-04
|
Doping effects of Cr on the physical properties of BaFe$_{1.9-x}$Ni$_{0.1}$Cr$_{x}$As$_{2}$
|
We present a systematic study on the heavily Cr doped iron pnictides
BaFe$_{1.9-x}$Ni$_{0.1}$Cr$_{x}$As$_{2}$ by using elastic neutron scattering,
high-resolution synchrotron X-ray diffraction (XRD), resistivity and Hall
transport measurements. When the Cr concentration increases from $x=$ 0 to 0.8,
neutron diffraction experiments suggest that the collinear antiferromagnetism
persists in the whole doping range, where the N\'{e}el temperature $T_N$
coincides with the tetragonal-to-orthorhombic structural transition temperature
$T_s$, and both of them keeps around 35 K. The magnetic ordered moment, on the
other hand, increases within increasing $x$ until $x=$ 0.5, and then decreases
with further increasing $x$. Detailed refinement of the powder XRD patterns
reveals that the Cr substitutions actually stretch the FeAs$_4$ tetrahedron
along the $c-$axis and lift the arsenic height away Fe-Fe plane. Transport
results indicate that the charge carriers become more localized upon Cr doping,
then changes from electron-type to hole-type around $x=$ 0.5. Our results
suggest that the ordered moment and the ordered temperature of static magnetism
in iron pnictides can be decoupled and tuned separately by chemical doping.
|
1807.01612v1
|
2018-07-17
|
Chemical Compositions of Field and Globular Cluster RR Lyrae Stars: I. NGC 3201
|
We present a detailed spectroscopic analysis of horizontal branch stars in
the globular cluster NGC 3201. We collected optical (4580-5330 A), high
resolution (~34,000), high signal-to-noise ratio (~200) spectra for eleven RR
Lyrae stars and one red horizontal branch star with the multifiber spectrograph
M2FS at the 6.5m Magellan telescope at the Las Campanas Observatory. From
measured equivalent widths we derived atmospheric parameters and abundance
ratios for {\alpha} (Mg, Ca, Ti), iron peak (Sc, Cr, Ni, Zn) and s-process (Y)
elements. We found that NGC 3201 is a homogeneous, mono-metallic ([Fe/H]=-1.47
+- 0.04), {\alpha}-enhanced ([{\alpha}/Fe]=0.37 +- 0.04) cluster. The relative
abundances of the iron peak and s-process elements were found to be consistent
with solar values. In comparison with other large stellar samples, NGC 3201 RR
Lyraes have similar chemical enrichment histories as do those of other old
(t>10 Gyr) Halo components (globular clusters, red giants, blue and red
horizontal branch stars, RR Lyraes). We also provided a new average radial
velocity estimate for NGC 3201 by using a template velocity curve to overcome
the limit of single epoch measurements of variable stars: Vrad=494 +- 2 km
s-1({\sigma}=8 km s-1).
|
1807.06681v1
|
2018-07-27
|
Relativistic reflection from accretion disks in the population of Active Galactic Nuclei at z=0.5-4
|
We report the detection of relativistically broadened iron K alpha emission
in the X-ray spectra of AGN detected in the 4Ms CDF-S. Using the Bayesian X-ray
analysis (BXA) package, we fit 199 hard band (2-7 keV) selected sources in the
redshift range z=0.5--4 with three models: (i) an absorbed power-law, (ii) the
first model plus a narrow reflection component, and (iii) the second model with
an additional relativistic broadened reflection. The Bayesian evidence for the
full sample of sources selects the model with the additional broad component as
being 10^5 times more probable to describe the data better than the second
model. For the two brightest sources in our sample, CID 190 (z=0.734) and CID
104 (z=0.543), BXA reveals the relativistic signatures in the individual
spectra. We estimate the fraction of sources containing a broad component to be
54^{+35}_{-37}% (107/199 sources). Considering that the low signal-to-noise
ratio of some spectra prevents the detection of the broad iron K alpha line, we
infer an intrinsic fraction with broad emission of around two thirds. The
detection of relativistic signatures in the X-ray spectra of these sources
suggests that they are powered by a radiatively efficient accretion disk.
Preliminary evidence is found that the spin of the black hole is high, with a
maximally spinning Kerr BH model (a=1) providing a significantly better fit
than a Schwarzschild model (a=0). Our analysis demonstrate the potential of
X-ray spectroscopy to measure this key parameter in typical SMBH systems at the
peak of BH growth.
|
1807.10677v1
|
2018-08-10
|
Majority carrier type inversion in FeSe family and "doped semimetal" scheme in iron-based superconductors
|
The field and temperature dependencies of the longitudinal and Hall
resistivity have been studied for high-quality FeSe${}_{1-x}$S${}_{x}$ (x up to
0.14) single crystals. Quasiclassical analysis of the obtained data indicates a
strong variation of the electron and hole concentrations under the studied
isovalent substitution and proximity of FeSe to the point of the majority
carrier-type inversion. On this basis, we propose a `doped semimetal' scheme
for the superconducting phase diagram of the FeSe family, which can be applied
to other iron-based superconductors. In this scheme, the two local maxima of
the superconducting temperature can be associated with the Van Hove
singularities of a simplified semi-metallic electronic structure. The
multicarrier analysis of the experimental data also reveals the presence of a
tiny and highly mobile electron band for all the samples studied. Sulfur
substitution in the studied range leads to a decrease in the number of mobile
electrons by more than ten times, from about 3\% to about 0.2\%. This behavior
may indicate a successive change of the Fermi level position relative to
singular points of the electronic structure which is consistent with the `doped
semimetal' scheme. The scattering time for mobile carriers does not depend on
impurities, which allows us to consider this group as a possible source of
unusual acoustic properties of FeSe.
|
1808.03551v2
|
2018-08-11
|
Physical conditions for the r-process I. radioactive energy sources of kilonovae
|
Radioactive energies from unstable nuclei made in the ejecta of neutron star
mergers play principal roles in powering kilonovae. In previous studies
power-law-type heating rates (e.g., ~ t^-1.3) have frequently been used, which
may be inadequate if the ejecta are dominated by nuclei other than the A ~ 130
region. We consider, therefore, two reference abundance distributions that
match the r-process residuals to the solar abundances for A >= 69 (light
trans-iron plus r-process elements) and A >= 90 (r-process elements).
Nucleosynthetic abundances are obtained by using free-expansion models with
three parameters: expansion velocity, entropy, and electron fraction.
Radioactive energies are calculated as an ensemble of weighted free-expansion
models that reproduce the reference abundance patterns. The results are
compared with the bolometric luminosity (> a few days since merger) of the
kilonova associated with GW170817. We find that the former case (fitted for A
>= 69) with an ejecta mass 0.06 M_sun reproduces the light curve remarkably
well including its steepening at > 7 days, in which the mass of r-process
elements is ~ 0.01 M_sun. Two beta-decay chains are identified: 66Ni -> 66Cu ->
66Zn and 72Zn -> 72Ga -> 72Ge with similar halflives of parent isotopes (~ 2
days), which leads to an exponential-like evolution of heating rates during
1-15 days. The light curve at late times (> 40 days) is consistent with
additional contributions from the spontaneous fission of 254Cf and a few Fm
isotopes. If this is the case, the event GW170817 is best explained by the
production of both light trans-iron and r-process elements that originate from
dynamical ejecta and subsequent disk outflows from the neutron star merger.
|
1808.03763v2
|
2018-08-15
|
c-axis pressure induced antiferromagnetic order in optimally P-doped BaFe2(As0.70P0.30)2 superconductor
|
Superconductivity in BaFe2(As1-xPx)2 iron pnictides emerges when its in-plane
two-dimensional (2D) orthorhombic lattice distortion associated with nematic
phase at Ts and three-dimensional (3D) collinear antiferromagnetic (AF) order
at TN (Ts = TN) are gradually suppressed with increasing x, reaching optimal
superconductivity around x = 0.30 with Tc $\approx$ 30 K. Here we show that a
moderate uniaxial pressure along the c-axis in BaFe2(As0.70P0.30)2
spontaneously induces a 3D collinear AF order with TN = Ts > 30 K, while only
slightly suppresses Tc. Although a ~ 400 MPa pressure compresses the c-axis
lattice while expanding the in-plane lattice and increasing the
nearest-neighbor Fe-Fe distance, it barely changes the average iron-pnictogen
height in BaFe2(As0.70P0.30)2. Therefore, the pressure- induced AF order must
arise from a strong in-plane magnetoelastic coupling, suggesting that the 2D
nematic phase is a competing state with superconductivity.
|
1808.05153v1
|
2018-08-16
|
Relativistic Tidal Disruption and Nuclear Ignition of White Dwarf Stars by Intermediate Mass Black Holes
|
We present results from general relativistic calculations of the tidal
disruption of white dwarf stars from near encounters with intermediate mass
black holes. We follow the evolution of 0.2 and $0.6 M_\odot$ stars on
parabolic trajectories that approach $10^3$ - $10^4 M_\odot$ black holes as
close as a few Schwarzschild radii at periapsis, paying particular attention to
the effect tidal disruption has on thermonuclear reactions and the synthesis of
intermediate to heavy ion elements. These encounters create diverse
thermonuclear environments characteristic of Type I supernovae and capable of
producing both intermediate and heavy mass elements in arbitrary ratios,
depending on the strength (or proximity) of the interaction. Nuclear ignition
is triggered in all of our calculations, even at weak tidal strengths $\beta
\sim 2.6$ and large periapsis radius $R_P \sim 28$ Schwarzschild radii. A
strong inverse correlation exists between the mass ratio of calcium to iron
group elements and tidal strength, with $\beta \lesssim 5$ producing
predominately calcium-rich debris. At these moderate to weak interactions,
nucleosynthesis is not especially efficient, limiting the total mass and
outflows of calcium group elements to $< 15$\% of available nuclear fuel. Iron
group elements however continue to be produced in greater quantity and ratio
with increasing tidal strength, peaking at $\sim 60$\% mass conversion
efficiency in our closest encounter cases. These events generate short bursts
of gravitational waves with characteristic frequencies 0.1-0.7 Hz and strain
amplitudes $0.5\times10^{-22}$ - $3.5\times10^{-22}$ at 10 Mpc source distance.
|
1808.05664v1
|
2018-08-18
|
Ultra-high critical current densities, the vortex phase diagram and the effect of granularity of the stoichiometric high-Tc superconductor, CaKFe$_4$As$_4$
|
We present a comprehensive study of the critical current densities and the
superconducting vortex phase diagram in the stoichiometric superconductor
CaKFe$_4$As$_4$ which has a critical temperature of 35 K. We performed detailed
magnetization measurements both of high quality single crystals for different
orientations in an applied magnetic field up to 16 T and for a powder sample.
We find an extremely large critical current density, Jc, up to 10$^8$ A/cm2 for
single crystals when H||(ab) at 5 K, which remains robust in fields up to 16 T,
being the largest of any other iron-based superconductor. The critical current
density is reduced by a factor 10 in single crystals when H||c at 5 K and
significantly suppressed by the presence of grain boundaries in the powder
sample. We also observe the presence of the fishtail effect in the magnetic
hysteresis loops of single crystals when H||c. The flux pinning force density
and the pinning parameters suggest that the large critical current could be
linked to the existence of point core and surface pinning. Based on the vortex
phase diagram and the large critical current densities, CaKFe$_4$As$_4$ is now
established as a potential iron-based superconductor candidate for practical
applications.
|
1808.06072v1
|
2018-08-19
|
Local Electronic Structure of Interstitial Hydrogen in Iron Disulfide
|
The electronic structure of interstitial hydrogen in a compound semiconductor
FeS$_2$ (naturally $n$-type) is inferred from a muon study. An implanted muon
(Mu, a pseudo-hydrogen) forms electronically different defect centers discerned
by the hyperfine parameter ($\omega_{\rm hf}$). A body of evidence indicates
that one muon is situated at the center of an iron-cornered tetrahedron with
nearly isotropic $\omega_{\rm hf}$ (Mu$_{\rm p}$), and that the other exists as
a diamagnetic state (Mu$_{\rm d}$, $\omega_{\rm hf}\simeq 0$). Their response
to thermal agitation indicates that the Mu$_{\rm d}$ center accompanies a
shallow level (donor or acceptor) understood by effective mass model while the
electronic structure of Mu$_{\rm p}$ center is more isolated from host than
Mu$_{\rm d}$ to form a deeper donor level. These observations suggest that
interstitial hydrogen also serves as an electronically active impurity in
FeS$_2$. Based on earlier reports on the hydrogen diffusion in FeS$_2$,
possibility of fast diffusion for Mu$_{\rm p}$ leading to formation of a
complex defect state (Mu$^*_{\rm d}$, $T\le 100$ K) or to motional narrowing
state (Mu$^*_{\rm p}$, $T\ge 150$ K) is also discussed.
|
1808.06153v1
|
2018-08-23
|
Correlation between non-Fermi-liquid behavior and superconductivity in (Ca, La)(Fe,Co)As2 iron arsenides: A high-pressure study
|
Non-Fermi-liquid (NFL) phenomena associated with correlation effects have
been widely observed in the phase diagrams of unconventional superconducting
families. Exploration of the correlation between the normal state NFL,
regardless of its microscopic origins, and the superconductivity has been
argued as a key to unveiling the mystery of the high-Tc pairing mechanism. Here
we systematically investigate the pressure-dependent in-plane resistivity and
Hall coefficient (RH ) of a high-quality 112-type Fe-based superconductor
Ca1-xLaxFe1-yCoyAs2 (x = 0.2,y = 0.02). With increasing pressure, the
normal-state resistivity of the studied sample exhibits a pronounced crossover
from non-Fermi-liquid to Fermi-liquid behaviors. Accompanied with this
crossover, Tc is gradually suppressed. In parallel, the extremum in the Hall
coefficient RH (T ) curve, possibly due to anisotropic scattering induced by
spin fluctuations, is also gradually suppressed. The symbiosis of NFL and
superconductivity implies that these two phenomena are intimately related.
Further study on the pressure-dependent upper critical field reveals that the
two-band effects are also gradually weakened with increasing pressure and
reduced to the one-band Werthamer-Helfand-Hohenberg limit in the low-Tc regime.
Overall, our paper supports the picture that NFL, multigap, and extreme RH (T )
are all of the same magnetic origin, i.e., the spin fluctuations in the 112
iron arsenide superconductors.
|
1808.07609v1
|
2018-08-26
|
Intertwined Spin and Orbital Density Waves in MnP Uncovered by Resonant Soft X-ray Scattering
|
Unconventional superconductors are often characterized by numerous competing
and even intertwined orders in their phase diagrams. In particular, the
electronic nematic phases, which spontaneously break rotational symmetry and
often simultaneously involve spin, charge and/or orbital orders, appear
conspicuously in both the cuprate and iron-based superconductors. The
fluctuations associated with these phases may provide the exotic pairing glue
that underlies their high-temperature superconductivity. Helimagnet MnP, the
first Mn-based superconductor under pressure, lacks high rotational symmetry.
However our resonant soft X-ray scattering (RSXS) experiment discovers novel
helical orbital density wave (ODW) orders in this three-dimensional,
low-symmetry system, and reveals intertwined ordering phenomena in
unprecedented detail. In particular, a ODW forms with half the period of the
spin order and fully develops slightly above the spin ordering temperature,
their domains develop simultaneously, yet the spin order domains are larger
than those of the ODW, and they cooperatively produce another ODW with 1/3 the
period of the spin order. These observations provide a comprehensive picture of
the intricate interplay between spin and orbital orders in correlated
materials, and they suggest that nematic-like physics ubiquitously exists
beyond two-dimensional and high-symmetry systems, and the superconducting
mechanism of MnP is likely analogous to those of cuprate and iron-based
superconductors.
|
1808.08562v1
|
2018-10-18
|
Human-sized Magnetic Particle Imaging for Brain Applications
|
Determining the brain perfusion is an important task for the diagnosis and
treatment of vascular diseases such as occlusions and intracerebral
haemorrhage. Even after successful diagnosis and treatment, there is a high
risk of restenosis or rebleeding such that patients need intense and frequent
attention in the days after treatment. Within this work, we will present a
diagnostic tomographic imager that allows access to brain perfusion information
quantitatively in short intervals. The imager is the first functional magnetic
particle imaging device for brain imaging on a human-scale. It is highly
sensitive and allows the detection of an iron concentration of 14.7 ng /ml (263
pmol\ml), which is the lowest iron concentration imaged by MPI so far. The
imager is self-shielded and can be used in unshielded environments such as
intensive care units. In combination with the low technical requirements this
opens up a whole variety of possible medical applications and would allow
monitoring possibilities on the stroke and intensive care units.
|
1810.07987v1
|
2019-01-08
|
Observation of half-integer level shift of vortex bound states in an iron-based superconductor
|
Vortices in topological superconductors host Majorana zero modes (MZMs),
which are proposed to be building blocks of fault-tolerant topological quantum
computers. Recently, a new single-material platform for realizing MZM has been
discovered in iron-based superconductors, without involving hybrid
semiconductor-superconductor structures. Here we report on a detailed scanning
tunneling spectroscopy study of a FeTe0.55Se0.45 single crystal, revealing two
distinct classes of vortices present in this system which differ by a
half-integer level shift in the energy spectra of the vortex bound states. This
level shift is directly tied with the presence or absence of zero-bias peak and
also alters the ratios of higher energy levels from integer to
half-odd-integer. Our model calculations fully reproduce the spectra of these
two types of vortex bound states, suggesting the presence of topological and
conventional superconducting regions that coexist within the same crystal. Our
findings provide strong evidence for the topological nature of
superconductivity in FeTe0.55Se0.45 and establish it as an excellent platform
for further studies on MZMs.
|
1901.02293v1
|
2019-01-19
|
Study of the X-ray properties of the neutron-star binary 4U 1728$-$34 from the soft to hard state
|
We studied five XMM-Newton observations of the neutron-star binary 4U
1728$-$34 covering the hard, intermediate and soft spectral states. By jointly
fitting the spectra with several reflection models, we obtained an inclination
angle of 25$-$53$\deg$ and an iron abundance up to 10 times the solar. From the
fits with reflection models, we found that the fluxes of the reflection and the
Comptonised components vary inconsistently; since the latter is assumed to be
the illuminating source, this result possibly indicates the contribution of the
neutron star surface/boundary layer to the disc reflection. As the source
evolved from the relatively soft to the intermediate state, the disc inner
radius decreased, opposite to the prediction of the standard accretion disc
model. We also explore the possible reasons why the supersolar iron abundance
is required by the data and found that this high value is probably caused by
the absence of the hard photons in the XMM-Newton data.
|
1901.06473v1
|
2019-01-28
|
Super-Eddington accretion onto the Galactic ultraluminous X-ray pulsar Swift J0243.6+6124
|
We report on the spectral behavior of the first Galactic ultraluminous X-ray
pulsar Swift J0243.6+6124 with NuSTAR observations during its 2017-2018
outburst. At sub-Eddington levels, the source spectrum is characterized by
three emission components, respectively from the accretion column, the hot
spot, and a broad iron line emission region. When the source is above the
Eddington limit, the hot spot temperature increases and the spectrum features
two more blackbody components. One blackbody component has a radius of 10-20 km
and is likely originated from the top of the accretion column. The other one
saturates at a blackbody luminosity of (1 - 2)*10^38 erg/s, coincident with the
Eddington limit of a neutron star. This is well consistent with the scenario
that super-Eddington accretion onto compact objects will power optically-thick
outflows and indicates an accretion rate 60-80 times the critical value. This
suggests that super-Eddington accretion onto magnetized systems can also power
massive winds. At super-Eddington levels, the iron line becomes more
significant and blueshifted, and is argued to be associated with the ultrafast
wind in the central funnel or jets. This source, if located in external
galaxies, will appear like other ultraluminous pulsars.
|
1901.09485v1
|
2019-04-01
|
A Chandra and ALMA Study of X-ray-irradiated Gas in the Central ~100 pc of the Circinus Galaxy
|
We report a study of X-ray-irradiated gas in the central ~100 pc of the
Circinus galaxy, hosting a Compton-thick active galactic nucleus (AGN), at
10-pc resolution using Chandra and ALMA. Based on ~200 ksec Chandra/ACIS-S
data, we created an image of the Fe Kalpha line at 6.4 keV, tracing
X-ray-irradiated dense gas. The ALMA data in Bands 6 (~270 GHz) and 7 (~350
GHz) cover five molecular lines: CO(J=3--2), HCN(J=3--2), HCN(J=4--3),
HCO^+(J=3--2), and HCO^+(J=4--3). The detailed spatial distribution of dense
molecular gas was revealed, and compared to the iron line image. The molecular
gas emission appeared faint in regions with bright iron emission. Motivated by
this, we quantitatively discuss the possibility that the molecular gas is
efficiently dissociated by AGN X-ray irradiation (i.e., creating an
X-ray-dominated region). Based on a non-local thermodynamic equilibrium model,
we constrained the molecular gas densities and found that they are as low as
interpreted by X-ray dissociation. Furthermore, judging from inactive star
formation (SF) reported in the literature, we suggest that the X-ray emission
has potential to suppress SF, particularly in the proximity of the AGN.
|
1904.01144v1
|
2019-04-12
|
Nearly quantized conductance plateau of vortex zero mode in an iron-based superconductor
|
Majorana zero-modes (MZMs) are spatially-localized zero-energy fractional
quasiparticles with non-Abelian braiding statistics that hold a great promise
for topological quantum computing. Due to its particle-antiparticle
equivalence, an MZM exhibits robust resonant Andreev reflection and 2e2/h
quantized conductance at low temperature. By utilizing variable-tunnel-coupled
scanning tunneling spectroscopy, we study tunneling conductance of vortex bound
states on FeTe0.55Se0.45 superconductors. We report observations of conductance
plateaus as a function of tunnel coupling for zero-energy vortex bound states
with values close to or even reaching the 2e2/h quantum conductance. In
contrast, no such plateau behaviors were observed on either finite energy
Caroli-de Genne-Matricon bound states or in the continuum of electronic states
outside the superconducting gap. This unique behavior of the zero-mode
conductance reaching a plateau strongly supports the existence of MZMs in this
iron-based superconductor, which serves as a promising single-material platform
for Majorana braiding at a relatively high temperature.
|
1904.06124v2
|
2019-04-14
|
Impact of the first order antiferromagnetic phase transition on the paramagnetic spin excitations and nematic phase of SrFe$_2$As$_2$
|
Understanding the nature of the electronic nematic phase in iron pnictide
superconductors is important for elucidating its impact on high-temperature
superconductivity. Here we use transport and inelastic neutron scattering to
study spin excitations and in-plane resistivity anisotropy in uniaxial pressure
detwinned BaFe$_2$As$_2$ and SrFe$_2$As$_2$, the parent compounds of iron
pnictide superconductors. While BaFe$_2$As$_2$ exhibits weakly first order
tetragonal-to-orthorhombic structural and antiferromagnetic (AF) phase
transitions below $T_s > T_N\approx 138$ K, SrFe$_2$As$_2$ has strongly coupled
first order structural and AF transitions below $T_s= T_N\approx 210$ K. We
find that the direct signatures of the nematic phase persist to lower
temperatures above the phase transition in the case of SrFe$_2$As$_2$ compared
to BaFe$_2$As$_2$. Our findings support the conclusion that the strongly
first-order nature of the magnetic transition in SrFe$_2$As$_2$ weakens the
nematic phase and resistivity anisotropy in the system.
|
1904.06743v1
|
2019-06-13
|
The influence of antiferromagnetism, soft out-of plane phonons and heavy electrons on the superconducting pairing mechanism of Ba1-xKxFe2As2
|
Based on ab-initio calculated parameters, we apply a theoretical model on the
iron-based BaFe2As2 superconductor that takes into account dramatic
enhancements of the electron-phonon coupling of soft transverse phonons in the
FeAs layers and antiferromagnetism. Our model is able to reproduce the Tc
values of BaFe2As2 found under pressure in experiments. To calculate the Tc of
the K-doped Ba1-xKxFe2As2 system as a function of the K content, we
additionally consider the experimentally observed effective mass enhancements
and Kondo temperatures in the strongly over-doped region (0.8 < x < 1), which
decouple the antiferromagnetism and electron-phonon scattering. The highest
theoretical Tc at the optimal doping concentration is reproduced after
optimization of antiferromagnetic fluctuations and electron-phonon coupling.
Our model is also able to reproduce the dip-like structure in Tc in the region
where a re-entrant tetragonal phase of C4 symmetry is found (0.24 < x < 0.28)
and indicates the weakening effect of local exchange correlation energy as
responsible for Tc reduction. Our model thus demonstrates that the high
transition temperatures and the exact doping and pressure dependence of this
iron-based superconductor can be explained within an extended electron-phonon
coupling model in which the structural, magnetic and electronic degrees of
freedom are strongly intertwined.
|
1906.05789v1
|
2019-06-24
|
Melting of vortex lattice in magnetic superconductor $\mathrm{Rb}\mathrm{Eu}\mathrm{Fe}_{4}\mathrm{As}_{4}$
|
The iron-based superconductors are characterized by strong fluctuations due
to high transition temperatures and small coherence lengths. We investigate
fluctuation behavior in the magnetic iron-pnictide superconductor
$\mathrm{Rb}\mathrm{Eu}\mathrm{Fe}_{4}\mathrm{As}_{4}$ by calorimetry and
transport. We find that the broadening of the specific-heat transition in
magnetic fields is very well described by the lowest-Landau-level scaling. We
report calorimetric and transport observations for vortex-lattice melting,
which is seen as a sharp drop of the resistivity and a step of the specific
heat at the magnetic-field-dependent temperature. The melting line in the
temperature/magnetic-field plane lies noticeably below the upper-critical-field
line and its location is in quantitative agreement with theoretical predictions
without fitting parameters. Finally, we compare the melting behavior of
$\mathrm{Rb}\mathrm{Eu}\mathrm{Fe}_{4}\mathrm{As}_{4}$ with other
superconducting materials showing that thermal fluctuations of vortices are not
as prevalent as in the high-temperature superconducting cuprates, yet they
still noticeably influence the properties of the vortex matter.
|
1906.10236v2
|
2019-06-26
|
Near 100% CO Selectivity in Nanoscaled Iron-Based Oxygen Carriers for Chemical Looping Methane Partial Oxidation
|
Chemical looping methane partial oxidation provides an energy and cost
effective route for methane utilization. However, there is considerable CO2
co-production in state-of-the-art chemical looping systems, rendering a
decreased productivity in value-added fuels or chemicals. In this work, we show
that the co-production of CO2 can be dramatically suppressed in methane partial
oxidation reactions using iron oxide nanoparticles, with a size of 2~8 nm, as
the oxygen carrier. To stabilize these nanoparticles at high temperatures, they
are embedded in an ordered, gas-permeable mesoporous silica matrix. We
experimentally obtained near 100% CO selectivity in a cyclic redox system at
750{\deg}C to 935{\deg}C, which is a significantly lower temperature range than
in conventional oxygen carrier systems. Density functional theory calculations
elucidate the origins for such selectivity and reveal that CH4 adsorption
energies decrease with increasing nanoparticle size. These calculations also
show that low-coordinated lattice oxygen atoms on the surface of nanoparticles
significantly promote Fe-O bond cleavage and CO formation. We envision that
embedded nanostructured oxygen carriers have the potential to serve as a
general materials platform for achieving 100% selectivity in redox reactions at
high temperatures.
|
1906.11160v1
|
2019-07-15
|
Stellar atmospheric parameters for 754 spectra from the X-shooter Spectral Library
|
The X-shooter Spectral Library (XSL) is an empirical stellar library at
medium spectral resolution covering the wavelength range from 3000 \AA to 24
800 \AA. This library aims to provide a benchmark for stellar population
studies. In this work, we present a uniform set of stellar atmospheric
parameters, effective temperatures, surface gravities, and iron abundances for
754 spectra of 616 XSL stars. We used the full-spectrum fitting package ULySS
with the empirical MILES library as reference to fit the ultraviolet-blue (UVB)
and visible (VIS) spectra. We tested the internal consistency and we compared
our results with compilations from the literature. The stars cover a range of
effective temperature 2900 < Teff < 38 000 K, surface gravity 0 < log g < 5.7,
and iron abundance -2.5 < [Fe/H] < +1.0, with a couple of stars extending down
to [Fe/H] = -3.9. The precisions of the measurements for the G- and K-type
stars are 0.9%, 0.14, and 0.06 in Teff, log g, and [Fe/H], respectively. For
the cool giants with log g < 1, the precisions are 2.1%, 0.21, and 0.22, and
for the other cool stars these values are 1%, 0.14, and 0.10. For the hotter
stars (Teff > 6500 K), these values are 2.6%, 0.20, and 0.10 for the three
parameters.
|
1907.06391v1
|
2019-07-15
|
Neutron powder diffraction study on the non-superconducting phases of ThFeAsN$_{1-x}$O$_x$ ($x=0.15, 0.6$) iron pnictide
|
We use neutron powder diffraction to study on the non-superconducting phases
of ThFeAsN$_{1-x}$O$_x$ with $x=0.15, 0.6$. In our previous results on the
superconducting phase ThFeAsN with $T_c=$ 30 K, no magnetic transition is
observed by cooling down to 6 K, and possible oxygen occupancy at the nitrogen
site is shown in the refinement(H. C. Mao \emph{et al.}, EPL, 117, 57005
(2017)). Here, in the oxygen doped system ThFeAsN$_{1-x}$O$_x$, two
superconducting region ($0\leqslant x \leqslant 0.1$ and $0.25\leqslant x
\leqslant 0.55$) have been identified by transport experiments (B. Z. Li
\emph{et al.}, J. Phys.: Condens. Matter 30, 255602 (2018)). However, within
the resolution of our neutron powder diffraction experiment, neither the
intermediate doping $x=0.15$ nor the heavily overdoped compound $x= 0.6$ shows
any magnetic order from 300 K to 4 K. Therefore, while it shares the common
phenomenon of two superconducting domes as most of 1111-type iron-based
superconductors, the magnetically ordered parent compound may not exist in this
nitride family.
|
1907.06531v1
|
2019-07-17
|
Detailed spectroscopic and photometric study of three detached eclipsing binaries
|
Detached eclipsing binaries are remarkable systems to provide accurate
fundamental stellar parameters. The fundamental stellar parameters and the
metallicity values of stellar systems are needed to deeply understand the
stellar evolution and formation. In this study, we focus on the detailed
spectroscopic and photometric studies of three detached eclipsing binary
systems, V372\,And, V2080\,Cyg, and CF\,Lyn to obtain their accurate stellar,
atmospheric parameters,and chemical compositions. An analysis of light and
radial velocity curves was carried out to derive the orbital and stellar
parameters. The disentangled spectra of component stars were obtained for the
spectroscopic analysis. Final \teff, \logg, $\xi$, \vsini\, parameters and the
element abundances of component stars were derived by using the spectrum
synthesis method. The fundamental stellar parameters were determined with a
high certainty for V372\,And, V2080\,Cyg ($\sim$$1-2$\%) and with an accuracy
for CF\,Lyn ($\sim$$2-6$\%). The evolutionary status of the systems was
examined and their ages were obtained. It was found that the component stars of
V2080\,Cyg have similar iron abundance which is slightly lower than solar iron
abundance. Additionally, we showed that the primary component of CF\,Lyn
exhibits a non-spherical shape with its 80\% Roche lobe filling factor. It
could be estimated that CF\,Lyn will start its first Roche overflow in the next
0.02\,Gyr.
|
1907.07560v1
|
2019-07-29
|
Near-linear Scaling in DMRG-based Tailored Coupled Clusters: An Implementation of DLPNO-TCCSD and DLPNO-TCCSD(T)
|
We present a new implementation of DMRG-based tailored coupled clusters
method (TCCSD), which employs the domain-based local pair natural orbital
approach (DLPNO-TCCSD). Compared to the previous LPNO version of the method,
the new implementation is more accurate, offers more favorable scaling and
provides more consistent behavior across the variety of systems. On top of the
singles and doubles, we include the perturbative triples correction (T), which
is able to retrieve even more dynamic correlation. The methods were tested on
three systems: tetramethyleneethane, oxo-Mn(Salen) and Iron(II)-porphyrin
model. The first two were revisited to assess the performance with respect to
LPNO-TCCSD. For oxo-Mn(Salen), we retrieved between 99.8-99.9% of the total
canonical correlation energy which is the improvement of 0.2% over the LPNO
version in less than 63% of the total LPNO runtime. Similar results were
obtained for Iron(II)-porphyrin. When the perturbative triples correction was
employed, irrespective of the active space size or system, the obtained energy
differences between two spin states were within the chemical accuracy of 1
kcal/mol using the default DLPNO settings.
|
1907.13466v3
|
2019-10-02
|
Nuclear Ignition of White Dwarf Stars by Relativistic Encounters with Rotating Intermediate Mass Black Holes
|
We present results from general relativistic calculations of nuclear ignition
in white dwarf stars triggered by near encounters with rotating intermediate
mass black holes with different spin and alignment parameters. These encounters
create thermonuclear environments characteristic of Type Ia supernovae capable
of producing both calcium and iron group elements in arbitrary ratios,
depending primarily on the proximity of the interaction which acts as a strong
moderator of nucleosynthesis. We explore the effects of black hole spin and
spin-orbital alignment on burn product synthesis to determine whether they
might also be capable of moderating reactive flows. When normalized to
equivalent impact penetration, accounting for frame dragging corrections, the
influence of spin is weak, no more than 25% as measured by nuclear energy
release and mass of burn products, even for near maximally rotating black
holes. Stars on prograde trajectories approach closer to the black hole and
produce significantly more unbound debris and iron group elements than is
possible by encounters with nonrotating black holes or by retrograde orbits, at
more than 50% mass conversion efficiency. The debris contains several
radioisotopes, most notably Ni56, made in amounts that produce sub-luminous
(but still observable) light curves compared to branch-normal SNe Ia.
|
1910.01054v1
|
2019-10-16
|
Controlling Ferromagnetic Ground States and Solitons in Thin Films and Nanowires built from Iron Phthalocyanine Chains
|
Iron phthalocyanine (FePc) is a molecular semiconductor whose building blocks
are one-dimensional ferromagnetic chains. We show that its optical and magnetic
properties are controlled by the growth strategy, obtaining extremely high
coercivities of over 1 T and modulating the exchange constant between 15 and 29
K through tuning the crystal phase by switching from organic molecular beam
deposition, producing continuous thin films of nanocrystals with controlled
orientations, to organic vapour phase deposition, producing ultralong
nanowires. Magnetisation measurements are analysed using a suite of concepts
and simply stated formulas with broad applicability to all one-dimensional
ferromagnetic chains. They show that FePc is best described by a Heisenberg
model with a preference for the moments to lie in the molecular planes, where
the chain Hamiltonian is very similar to that for the classic inorganic magnet
CsNiF3, but with ferromagnetic rather than antiferromagnetic interchain
interactions. The data at large magnetic fields are well-described by the
soliton picture, where the dominant (and topologically non-trivial) degrees of
freedom are moving one-dimensional magnetic domain walls, which was successful
for CsNiF3, and at low temperatures and fields by the super-Curie-Weiss law of
1/(T^2+theta^2) characteristic of nearly one-dimensional xy and Heisenberg
ferromagnets. The ability to control the molecular orientation and
ferromagnetism of FePc systems, and produce them on flexible substrates as thin
films or nanowires, taken together with excellent transistor characteristics
reported previously for nanowires of copper and cobalt analogues, makes them
potentially useful for magneto-optical and spintronic devices.
|
1910.07229v1
|
2019-10-18
|
An ionised accretion disc wind in Hercules X-1
|
Hercules X-1 is one of the best studied highly magnetised neutron star X-ray
binaries with a wealth of archival data. We present the discovery of an ionised
wind in its X-ray spectrum when the source is in the high state. The wind
detection is statistically significant in most of the XMM-Newton observations,
with velocities ranging from 200 to 1000 km/s. Observed features in the iron K
band can be explained by both wind absorption or by a forest of iron emission
lines. However, we also detect nitrogen, oxygen and neon absorption lines at
the same systematic velocity in the high-resolution RGS grating spectra. The
wind must be launched from the accretion disc, and could be the progenitor of
the UV absorption features observed at comparable velocities, but the latter
likely originate at significantly larger distances from the compact object. We
find strong correlations between the ionisation level of the outflowing
material and the ionising luminosity as well as the super-orbital phase. If the
luminosity is driving the correlation, the wind could be launched by a
combination of Compton heating and radiation pressure. If instead the
super-orbital phase is the driver for the variations, the observations are
likely scanning the wind at different heights above the warped accretion disc.
If this is the case, we can estimate the wind mass outflow rate, corrected for
the limited launching solid angle, to be roughly 70% of the mass accretion
rate.
|
1910.08337v2
|
2019-10-20
|
PUSHing core-collapse supernovae to explosions in spherical symmetry IV: Explodability, remnant properties and nucleosynthesis yields of low metallicity stars
|
In this fourth paper of the series, we use the parametrized, spherically
symmetric explosion method PUSH to perform a systematic study of two sets of
non-rotating stellar progenitor models. Our study includes pre-explosion models
with metallicities Z=0 and Z=Z$_{\odot}\times 10^{-4}$ and covers a progenitor
mass range from 11 up to 75 M$_\odot$. We present and discuss the explosion
properties of all models and predict remnant (neutron star or black hole) mass
distributions within this approach. We also perform systematic nucleosynthesis
studies and predict detailed isotopic yields as function of the progenitor mass
and metallicity. We present a comparison of our nucleosynthesis results with
observationally derived $^{56}$Ni ejecta from normal core-collapse supernovae
and with iron-group abundances for metal-poor star HD~84937. Overall, our
results for explosion energies, remnant mass distribution, $^{56}$Ni mass, and
iron group yields are consistent with observations of normal CCSNe. We find
that stellar progenitors at low and zero metallicity are more prone to BH
formation than those at solar metallicity, which allows for the formation of
BHs in the mass range observed by LIGO/VIRGO.
|
1910.08958v2
|
2019-12-08
|
The vertical Na-O relation in the bulge globular cluster NGC 6553
|
In this article, we present a detailed chemical analysis of seven red giant
members of NGC 6553 using high-resolution spectroscopy from VLT FLAMES. We
obtained the stellar parameters (Teff, Log(g), vt, [Fe/H]) of these stars from
the spectra, and we measured the chemical abundance for 20 elements, including
light elements, iron-peak elements, alpha-elements and neutron-capture
elements. The metallicities in our sample stars are consistent with a
homogeneous distribution. We found a mean of [Fe/H]=-0.14+/-0.07 dex, in
agreement with other studies. Using the alpha-elements Mg, Si, Ca and Ti we
obtain the mean of [alpha/Fe]=0.11+/-0.05. We found a vertical relation between
Na and O, characterized by a significant spread in Na and an almost
non-existent spread in O. In fact, Na and Al are the only two light elements
with a large intrinsic spread, which demonstrates the presence of Multiple
Populations (MPs). An intrinsic spread in Mg is not detected in this study. The
alpha, iron-peak and neutron capture elements show good agreement with the
trend of the bulge field stars, indicating similar origin and evolution, in
concordance with our previous studies for two other bulge GCs (NGC 6440 and NGC
6528).
|
1912.03742v2
|
2019-12-10
|
Magnetic properties of thin epitaxial Pd$_{1-x}$Fe$_x$ alloy films
|
In the paper we present the results of extensive studies of palladium-rich
Pd1-xFex alloy films epitaxially grown on MgO single-crystal substrate. In a
composition range of x = 0.01-0.07 these materials are soft ferromagnets, the
saturation magnetization and magnetic anisotropy of which can be tuned by its
composition. Vibrating sample magnetometry was used to study temperature
dependences of spontaneous magnetic moment and to establish the temperature of
magnetic ordering (Curie temperature). Ferromagnetic resonance (FMR)
measurements at low temperatures in the in-plane and out-of-plane geometries
revealed the four-fold in-plane magnetic anisotropy with the easy directions
along the <110> axes of the substrate. The modelling of the angular dependence
of the field for resonance allowed to extract the cubic and tetragonal
contributions to the magnetic anisotropy of the films and establish their
dependence on the concentration of iron in the alloy. Experimental data are
discussed in the framework of existing theories of dilute magnetic alloys.
Using the anisotropy constants established from FMR, the magnetic hysteresis
loops are reproduced utilizing the Stoner-Wohlfarth model thus indicating the
predominant coherent magnetic moment rotation at low temperatures. The obtained
results compile a database of magnetic properties of a palladium-iron alloy
considered as a material for superconducting spintronics.
|
1912.04852v1
|
2019-12-16
|
High-Temperature Quantum Anomalous Hall Insulators in Lithium-Decorated Iron-Based Superconductor Materials
|
Quantum anomalous Hall (QAH) insulator is the key material to study emergent
topological quantum effects, but its ultralow working temperature limits
experiments. Here, by first-principles calculations, we find a family of stable
two-dimensional (2D) structures generated by lithium decoration of layered
iron-based superconductor materials FeX (X = S, Se, Te), and predict
room-temperature ferromagnetic semiconductors together with large-gap
high-Chern-number QAH insulators in the 2D materials. The extremely robust
ferromagnetic order is induced by the electron injection from Li to Fe and
stabilized by strong ferromagnetic kinetic exchange in the 2D Fe layer. While
in the absence of spin-orbit coupling (SOC), the ferromagnetism polarizes the
system into a half Dirac semimetal state protected by mirror symmetry, the SOC
effect results in a spontaneous breaking of mirror symmetry and introduces a
Dirac mass term, which creates QAH states with sizable gaps (several tens of
meV) and multiple chiral edge modes. We also find a 3D QAH insulator phase
featured by macroscopic number of chiral conduction channels in bulk
LiOH-LiFeX. The findings open new opportunities to realize novel QAH physics
and applications at high temperatures.
|
1912.07461v4
|
2020-03-06
|
Ab initio theory of graphene-iron(II) phthalocyanine hybrid systems as scalable molecular spintronics
|
Graphene - transition metal phthalocyanine (G-MPc) hybrid systems constitute
promising platforms for densely-packed single-molecule magnets (SMMs). Here, we
selected iron(II) phthalocyanine (FePc) and investigated its interaction with
pristine and defective graphene layers employing density functional theory. Our
calculations indicate that thorough proper dehydrogenation of the benzol rings
in the FePc molecule its adsorption to graphene is thermodynamically favorable.
In general, the presence of anchoring sites on the graphene layer, i.e. point
defects, additionally facilitates the adsorption of FePc, allowing one to
achieve high density of SMMs per unit area. Using the combination of group
theory, ligand field splitting, and the calculated PBE0 Kohn-Sham eigenvalue
spectrum, we resolved the electronic structure and predicted the spin states of
both, the isolated FePc and G-FePc hybrid systems. Regardless of adsorption
site and the number of removed hydrogen atoms from the benzol rings of FePc,
the magnetic moment of the SMM remains unchanged with respect to free FePc.
These results should mediate a successful synthesis of densely-packed G-MPc
systems and may open up new avenue in designing scalable graphene - SMMs
systems for spintronics applications.
|
2003.03189v1
|
2020-03-22
|
A statistical study of the plasma and composition distribution inside magnetic clouds: 1998-2011
|
A comprehensive analysis of plasma and composition characteristics inside
magnetic clouds (MCs) observed by the Advanced Composition Explorer (ACE)
spacecraft from 1998 February to 2011 August is presented. The results show
that MCs have specific interior structures, and MCs of different speeds show
differences in composition and structure. Compared with the slow MCs, fast MCs
have enhanced mean charge states of iron, oxygen, silicon, magnesium,
$\mathrm{O^{7+}/O^{6+}}$, $\mathrm{C^{6+}/C^{5+}}$, $\mathrm{C^{6+}/C^{4+}}$
and $\mathrm{Fe^{\geq16+}/Fe_{total}}$ values. For ionic species in fast MCs, a
higher atomic number represents a greater enhancement of mean charge state than
slow MCs. We also find that both the fast and slow MCs display bimodal
structure distribution in the mean iron charge state ($\mathrm{\langle Q\rangle
Fe}$), which suggests that the existence of flux rope prior to the eruption is
common. Furthermore, the $\mathrm{\langle Q\rangle Fe} $,
$\mathrm{Fe^{\geq16+}/Fe_{total}}$, and $\mathrm{O^{7+}/O^{6+}}$ ratio
distribution inside fast MCs have the feature that the posterior peak is higher
than the anterior one. This result agrees with the "standard model" for
CME/flares, by which magnetic reconnection occurs beneath the flux rope,
thereby ionizing the ions of the posterior part of flux rope sufficiently by
high-energy electron collisions or by direct heating in the reconnection
region.
|
2003.09965v1
|
2020-08-03
|
When superconductivity does not fear magnetism: Insight into electronic structure of RbEuFe$_{4}$As$_{4}$
|
In the novel stoichiometric iron-based material RbEuFe$_{4}$As$_{4}$
superconductivity coexists with a peculiar long-range magnetic order of Eu 4f
states. Using angle-resolved photoemission spectroscopy, we reveal a complex
three dimensional electronic structure and compare it with density functional
theory calculations. Multiple superconducting gaps were measured on various
sheets of the Fermi surface. High resolution resonant photoemission
spectroscopy reveals magnetic order of the Eu 4f states deep into the
superconducting phase. Both the absolute values and the anisotropy of the
superconducting gaps are remarkably similar to the sibling compound without Eu,
indicating that Eu magnetism does not affect the pairing of electrons. A
complete decoupling between Fe- and Eu-derived states was established from
their evolution with temperature, thus unambiguously demonstrating that
superconducting and a long range magnetic orders exist independently from each
other. The established electronic structure of RbEuFe$_{4}$As$_{4}$ opens
opportunities for the future studies of the highly unorthodox electron pairing
and phase competition in this family of iron-based superconductors with doping.
|
2008.00736v1
|
2020-08-10
|
On The Development of Multidimensional Progenitor Models For Core-collapse Supernovae
|
Multidimensional hydrodynamic simulations of shell convection in massive
stars suggest the development of aspherical perturbations that may be amplified
during iron core-collapse. These perturbations have a crucial and qualitative
impact on the delayed neutrino-driven core-collapse supernova explosion
mechanism by increasing the total stress behind the stalled shock. In this
paper, we investigate the properties of a 15 \msun model evolved in 1-,2-, and
3-dimensions (3D) for the final $\sim$424 seconds before gravitational
instability and iron core-collapse using MESA and the FLASH simulation
framework. We find that just before collapse, our initially perturbed fully 3D
model reaches angle-averaged convective velocity magnitudes of $\approx$
240-260 km s$^{-1}$ in the Si- and O-shell regions with a Mach number $\approx$
0.06. We find the bulk of the power in the O-shell resides at large scales,
characterized by spherical harmonic orders ($\ell$) of 2-4, while the Si-shell
shows broad spectra on smaller scales of $\ell\approx30-40$. Both convective
regions show an increase in power at $\ell=5$ near collapse. We show that the
1D \texttt{MESA} model agrees with the convective velocity profile and speeds
of the Si-shell when compared to our highest resolution 3D model. However, in
the O-shell region, we find that \texttt{MESA} predicts speeds approximately
\emph{four} times slower than all of our 3D models suggest. All eight of the
multi-dimensional stellar models considered in this work are publicly
available.
|
2008.04266v1
|
2020-08-31
|
Creep deformation of WC hardmetals with iron-based binders
|
Iron is a candidate to replace cobalt in WC hardmetals, due to its lower cost
and toxicity. A WC-FeCr hardmetal was compression tested at 900-1200 {\deg}C.
Particular attention is paid to the steady-state creep rates and
stress-exponents (n) during isostress treatments. Three regimes of stress
dependence are observed. Two of these were previously reported for WC-Co: power
law creep (n~3) at medium stresses; and grain boundary sliding (n~1) at higher
stresses, generally >100MPa. A previously unreported low stress (<10MPa) regime
with an exponent of n~2 is also observed. By combining electron microscopy with
X-ray diffraction texture measurements, the low stress regime is attributed to
viscous flow of the binder, which is accommodated by diffusional creep in the
WC skeleton. The mechanism may be applicable to other hardmetals. Compared to
analogous WC-Co materials, WC-FeCr shows improved creep resistance below 1000
{\deg}C, which can be explained by its lower self-diffusivity, and a lower
solubility for WC than Co. However, at temperatures corresponding to liquid
eutectic formation (~1140 {\deg}C), its creep resistance becomes inferior.
These results indicate FeCr may be a suitable replacement for Co provided the
eutectic temperature is not exceeded.
|
2008.13565v1
|
2020-09-02
|
Multimodal assessment of nigrosomal degeneration in Parkinson's disease
|
Background: Approximately forty percent of all dopaminergic neurons in SNpc
are located in five dense neuronal clusters, named nigrosomes. T2- or
T2*-weighted images are used to delineate the largest nigrosome, named
nigrosome-1. In these images, nigrosome-1 is a hyperintense region in the
caudal and dorsal portion of the T2- or T2*-weighted substantia nigra. In PD,
nigrosome-1 experiences iron accumulation, which leads to a reduction in
T2-weighted hyperintensity. Here, we examine neuromelanin-depletion and iron
deposition in regions of interest (ROIs) derived from quantitative-voxel based
morphometry (qVBM) on neuromelanin-sensitive images and compare the ROIs with
nigrosome-1 identified in T2*-weighted images.
Methods: Neuromelanin-sensitive and multi-echo gradient echo imaging data
were obtained. R2* was calculated from multi-echo gradient echo imaging data.
qVBM analysis was performed on neuromelanin-sensitive images and restricted to
SNpc. Mean neuromelanin-sensitive contrast and R2* was measured from the
resulting qVBM clusters. Nigrosome-1 was segmented in T2*-weighted images of
control subjects and its location was compared to the spatial location of the
qVBM clusters.
Results: Two bilateral clusters emerged from the qVBM analysis. These
clusters showed reduced neuromelanin-sensitive contrast and increased mean R2*
in PD as compared to controls. Cluster-1 from the qVBM analysis was in a
similar spatial location as nigrosome-1, as seen in T2*-weighted images.
Conclusion: qVBM cluster-1 shows reduced neuromelanin-sensitive contrast and
is in a similar spatial position as nigrosome-1. This region likely corresponds
to nigrosome-1 while the second cluster may correspond to nigrosome-2.
|
2009.01378v1
|
2020-09-09
|
Accretion disk's magnetic field controlled the composition of the terrestrial planets
|
Chondrites, the building blocks of the terrestrial planets, have mass and
atomic proportions of oxygen, iron, magnesium, and silicon totaling $\geq$90\%
and variable Mg/Si ($\sim$25\%), Fe/Si (factor of $\geq$2), and Fe/O (factor of
$\geq$3). The Earth and terrestrial planets (Mercury, Venus, and Mars) are
differentiated into three layers: a metallic core, a silicate shell (mantle and
crust), and a volatile envelope of gases, ices, and, for the Earth, liquid
water. Each layer has different dominant elements (e.g., increasing Fe content
with depth and increasing oxygen content to the surface). What remains an
unknown is to what degree did physical processes during nebular disk accretion
versus those during post-nebular disk accretion (e.g., impact erosion)
influence these final bulk compositions. Here we predict terrestrial planet
compositions and show that their core mass fractions and uncompressed densities
correlate with their heliocentric distance, and follow a simple model of the
magnetic field strength in the protoplanetary disk. Our model assesses the
distribution of iron in terms of increasing oxidation state, aerodynamics, and
a decreasing magnetic field strength outward from the Sun, leading to
decreasing core size of the terrestrial planets with radial distance. This
distribution would enhance habitability in our solar system, and would be
equally applicable to exo-planetary systems.
|
2009.04311v1
|
2020-09-11
|
Electronic nematic states tuned by isoelectronic substitution in bulk FeSe1-xSx
|
Isoelectronic substitution is an ideal tuning parameter to alter electronic
states and correlations in iron-based superconductors. As this substitution
takes place outside the conducting Fe planes, the electronic behaviour is less
affected by the impurity scattering experimentally and relevant key electronic
parameters can be accessed. In this short review, I present the experimental
progress made in understanding the electronic behaviour of the nematic
electronic superconductors, FeSe1-xSx. A direct signature of the nematic
electronic state is in-plane anisotropic distortion of the Fermi surface
triggered by orbital ordering effects and electronic interactions that result
in multi-band shifts detected by ARPES. Upon sulphur substitution, the
electronic correlations and the Fermi velocities decrease in the tetragonal
phase. Quantum oscillations are observed for the whole series in ultra-high
magnetic fields and show a complex spectra due to the presence of many small
orbits. Effective masses associated to the largest orbit display non-divergent
behaviour at the nematic end point (x~0.175(5)), as opposed to critical
spin-fluctuations in other iron pnictides. Magnetotransport behaviour has a
strong deviation from the Fermi liquid behaviour and linear T resistivity is
detected at low temperatures inside the nematic phase, where scattering from
low energy spin-fluctuations are likely to be present. The superconductivity is
not enhanced in FeSe1-xSx and there are no divergent electronic correlations at
the nematic end point. These manifestations indicate a strong coupling with the
lattice in FeSe1-xSx and a pairing mechanism likely promoted by spin
fluctuations.
|
2009.05523v1
|
2020-09-21
|
The Impact of Outflows driven by Active Galactic Nuclei on Metals in and around Galaxies
|
Metals in the hot gaseous halos of galaxies encode the history of star
formation as well as the feedback processes that eject metals from the
galaxies. X-ray observations suggest that massive galaxies have extended
distributions of metals in their gas halos. We present predictions for the
metal properties of massive galaxies and their gaseous halos from recent high
resolution zoom-in simulations that include mechanical and radiation driven
feedback from Active Galactic Nuclei (AGN). In these simulations, AGN launch
high-velocity outflows, mimicking observed broad absorption line winds. By
comparing two sets of simulations with and without AGN feedback, we show that
our prescription for AGN feedback is capable of driving winds and enriching
halo gas `inside-out' by spreading centrally enriched metals to the outskirts
of galaxies, into the halo and beyond. The metal (iron) profiles of halos
simulated with AGN feedback have a flatter slope than those without AGN
feedback, consistent with recent X-ray observations. The predicted gas iron
abundance of group scale galaxies simulated with AGN feedback is $Z_{\rm Fe} =
0.23$ $Z_{\rm Fe,\odot}$ at $0.5 r_{500}$, which is 2.5 times higher than that
in simulations without AGN feedback. In these simulations, AGN winds are also
important for the metal enrichment of the intergalactic medium, as the vast
majority of metals ejected from the galaxy by AGN-driven winds end up beyond
the halo virial radius.
|
2009.10184v1
|
2020-09-24
|
Large magnetoresistance in the iron-free pnictide superconductor LaRu$_2$P$_2$
|
The magnetoresistance of iron pnictide superconductors is often dominated by
electron-electron correlations and deviates from the H$^2$ or saturating
behaviors expected for uncorrelated metals. Contrary to similar Fe-based
pnictide systems, the superconductor LaRu$_2$P$_2$ (T$_c$ = 4 K) shows no
enhancement of electron-electron correlations. Here we report a non-saturating
magnetoresistance deviating from the H$^2$ or saturating behaviors in
LaRu$_2$P$_2$. We have grown and characterized high quality single crystals of
LaRu$_2$P$_2$ and measured a magnetoresistance following H$^{1.3}$ up to 22 T.
We discuss our result by comparing the bandstructure of LaRu$_2$P$_2$ with Fe
based pnictide superconductors. The different orbital structures of Fe and Ru
leads to a 3D Fermi surface with negligible bandwidth renormalization in
LaRu$_2$P$_2$, that contains a large open sheet over the whole Brillouin zone.
We show that the large magnetoresistance in LaRu$_2$P$_2$ is unrelated to the
one obtained in materials with strong electron-electron correlations and that
it is compatible instead with conduction due to open orbits on the rather
complex Fermi surface structure of LaRu$_2$P$_2$.
|
2009.11585v1
|
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