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2016-04-07
|
The super-super exchange mechanism in iron-based antiperovskite chalco-halides
|
By using the first-principles electronic structure calculations, we have
systematically studied the magnetism in three recently synthesized iron-based
antiperovskite chalco-halides: Ba$_3$(FeS$_4$)Cl, Ba$_3$(FeS$_4$)Br, and
Ba$_3$(FeSe$_4$)Br. These compounds consist of edge-sharing Ba$Q_6$ ($Q$=Cl or
Br) octahedra intercalated with isolated Fe$X_4$ ($X$=S or Se) tetrahedra. We
find that even though the shortest distances between the nearest-neighboring Fe
atoms in these three compounds already exceed 6 \AA, much larger than the bond
length of a chemical bonding, they all remarkably show antiferromagnetic (AFM)
coupling along $b$ axis with very weak spin-spin correlation along $a$ axis.
Our study shows that the mechanism underlying this novel AFM coupling is such a
new type of exchange interaction between the nearest-neighboring Fe-based
super-moments mediated by Ba cations, which we call the super-super exchange
interaction, in which each magnetic Fe atom partially polarizes its four
nearest-neighboring $X$ atoms to form a super-moment through $p$-$d$ orbital
hybridization and the $X$ atoms in neighboring Fe$X_4$ tetrahedra along $b$
axis antiferromagnetically couple with each others through the intermediate Ba
cations. Different from the conventional superexchange, here it is cations
rather than anions that mediate two neighboring super-moments. According to the
calculated strength of the AFM coupling, we predict that among these compounds
the highest AFM phase transition temperature $T_N$ may reach 110 K in
Ba$_3$(FeSe$_4$)Br, in comparison with the observed $T_N$s of 84 K in
Ba$_3$(FeS$_4$)Br and 95 K in Ba$_3$(FeS$_4$)Cl.
|
1604.02044v1
|
2016-04-08
|
Detection of H2O and evidence for TiO/VO in an ultra hot exoplanet atmosphere
|
We present a primary transit observation for the ultra hot (Teq~2400K) gas
giant expolanet WASP-121b, made using the Hubble Space Telescope Wide Field
Camera 3 in spectroscopic mode across the 1.12-1.64 micron wavelength range.
The 1.4 micron water absorption band is detected at high confidence (5.4 sigma)
in the planetary atmosphere. We also reanalyze ground-based photometric
lightcurves taken in the B, r', and z' filters. Significantly deeper transits
are measured in these optical bandpasses relative to the near-infrared
wavelengths. We conclude that scattering by high-altitude haze alone is
unlikely to account for this difference, and instead interpret it as evidence
for titanium oxide and vanadium oxide absorption. Enhanced opacity is also
inferred across the 1.12-1.3 micron wavelength range, possibly due to iron
hydride absorption. If confirmed, WASP-121b will be the first exoplanet with
titanium oxide, vanadium oxide, and iron hydride detected in transmission. The
latter are important species in M/L dwarfs, and their presence is likely to
have a significant effect on the overall physics and chemistry of the
atmosphere, including the production of a strong thermal inversion.
|
1604.02310v1
|
2016-04-12
|
A Statistical Study of the Average Iron Charge Distributions inside Magnetic Clouds for Solar Cycle 23
|
Magnetic clouds (MCs) are the interplanetary counterpart of coronal magnetic
flux ropes. They can provide valuable information to reveal the flux rope
characteristics at their eruption stage in the corona, which are unable to be
explored in situ at present. In this paper, we make a comprehensive survey of
the average iron charge state (<Q>Fe) distributions inside 96 MCs for solar
cycle 23 using ACE (Advanced Composition Explorer) data. As the <Q>Fe in the
solar wind are typically around 9+ to 11+, the Fe charge state is defined as
high when the <Q>Fe is larger than 12+, which implies the existence of a
considerable amount of Fe ions with high charge states (e.g., \geq 16+). The
statistical results show that the <Q>Fe distributions of 92 (~ 96%) MCs can be
classified into four groups with different characteristics. In group A (11
MCs), the <Q>Fe shows a bimodal distribution with both peaks higher than 12+.
Group B (4 MCs) presents a unimodal distribution of <Q>Fe with its peak higher
than 12+. In groups C (29 MCs) and D (48 MCs), the <Q>Fe remains higher and
lower than 12+ throughout ACE passage through the MC, respectively. Possible
explanations to these distributions are discussed.
|
1604.03205v1
|
2016-04-14
|
Study of Glass and Bakelite properties as electrodes in RPCs
|
India-based Neutrino Observatory (INO) collaboration is planning to build a
magnetized Iron-CALorimeter detector (ICAL) for the study of atmospheric
neutrinos. ICAL detector will be a stack of 151 layers of magnetized iron
plates interleaved with Resistive Plate Chambers (RPCs) as active detector
elements with a total mass of 50 kton. Resistive Plate Chambers are gaseous
detectors made up of two parallel electrodes of high bulk resistivity like
float glass and bakelite. These detectors are extensively used in several high
energy physics experiments since 1980s because of high count rate, excellent
time as well as spatial resolutions, simple to fabricate and operate. Due to
detector aging issue, it is necessary to characterize electrode material so as
to select appropriate electrode material before fabricating the detector. In
the present studies, we measured bulk resistivity and surface current of glass
as well as bakelite. Bulk resistivity of bakelite is ~ 100 times less than that
of glass and surface current of bakelite is higher than that of glass. Also
glass does not need any kind of surface treatment to achieve better surface
uniformity. Therefore, glass electrodes are preferred over bakelite electrodes
in most of the cases. Locally manufactured Asahi glass of ~2 mm thickness and
bakelite sheets were tested during the studies as reported in this paper before
the various stages of detector fabrication.
|
1604.04130v2
|
2016-04-25
|
Equilibrium and Sudden Events in Chemical Evolution
|
We present new analytic solutions for one-zone (fully mixed) chemical
evolution models and explore their implications. In contrast to existing
analytic models, we incorporate a realistic delay time distribution for Type Ia
supernovae (SNIa) and can therefore track the separate evolution of
$\alpha$-elements produced by core collapse supernovae (CCSNe) and iron peak
elements synthesized in both CCSNe and SNIa. In generic cases, $\alpha$ and
iron abundances evolve to an equilibrium at which element production is
balanced by metal consumption and gas dilution, instead of continuing to
increase over time. The equilibrium absolute abundances depend principally on
supernova yields and the outflow mass loading parameter $\eta$, while the
equilibrium abundance ratio [$\alpha$/Fe] depends mainly on yields and
secondarily on star formation history. A stellar population can be metal-poor
either because it has not yet evolved to equilibrium or because high outflow
efficiency makes the equilibrium abundance itself low. Systems with ongoing gas
accretion develop metallicity distribution functions (MDFs) that are sharply
peaked, while "gas starved" systems with rapidly declining star formation have
broadly peaked MDFs. A burst of star formation that consumes a significant
fraction of a system's available gas can temporarily boost [$\alpha$/Fe] by
0.1-0.3 dex, a possible origin for rare, $\alpha$-enhanced stars with
intermediate age or high metallicity. Other sudden transitions in system
properties can produce surprising behavior, including backward evolution of a
stellar population from high metallicity to low metallicity. An Appendix
provides a user's guide for calculating enrichment histories, [$\alpha$/Fe]
tracks, and MDFs for a wide variety of scenarios, including flexible forms of
star formation history.
|
1604.07435v1
|
2016-05-01
|
Development of a method for measuring blood coagulation using superparamagnetic iron oxide nanoparticles and an alternating magnetic field
|
We developed a method for measuring blood coagulation using superparamagnetic
iron oxide nanoparticles (SPIONs) and an alternating magnetic field (AMF). The
3rd and 5th harmonic signals from SPIONs mixed with blood induced by AMF were
detected using a gradiometer coil. Blood coagulation was induced artificially
by adding CaCl2 solution to whole blood of sheep at various temperatures and
hematocrits. We calculated the coagulation rate (k) and normalized signal
intensity at infinite time (Sinf) by fitting the time course of the normalized
3rd harmonic signal to S(t)=(1-Sinf)exp(-kt)+Sinf. The k values increased
significantly with increasing temperature and decreased significantly with
increasing hematocrit. The Sinf values decreased significantly with increasing
temperature and tended to increase with increasing hematocrit. Blood
anticoagulation was induced by adding heparin to the whole blood sampled from
mice. There were significant differences in both the 3rd and 5th harmonic
signals between groups with and without heparin at 25 min or more after adding
heparin. We also calculated the 3rd and 5th harmonic signals for viscosities
ranging from 0.001 to 1 kg/m/s, with an assumption that the magnetization and
particle size distribution of SPIONs obey the Langevin theory of paramagnetism
and log-normal distribution, respectively. The 3rd and 5th harmonic signals
increased slowly with increasing viscosity and had peaks at approximately 0.015
and 0.025 kg/m/s, respectively. After these peaks, they decreased monotonically
with increasing viscosity. These results confirm the rationale of our method.
In conclusion, our method will be useful for measuring blood coagulation and
anticoagulation and for studying their processes.
|
1605.00255v2
|
2016-06-02
|
Orbital selective spin excitations and their impact on superconductivity of LiFe1-xCoxAs
|
We use neutron scattering to study spin excitations in single crystals of
LiFe$_{0.88}$Co$_{0.12}$As, which is located near the boundary of the
superconducting phase of LiFe$_{1-x}$Co$_{x}$As and exhibits non-Fermi-liquid
behavior indicative of a quantum critical point. By comparing spin excitations
of LiFe$_{0.88}$Co$_{0.12}$As with a combined density functional theory (DFT)
and dynamical mean field theory (DMFT) calculation, we conclude that
wave-vector correlated low energy spin excitations are mostly from the $d_{xy}$
orbitals, while high-energy spin excitations arise from the $d_{yz}$ and
$d_{xz}$ orbitals. Unlike most iron pnictides, the strong orbital selective
spin excitations in LiFeAs family cannot be described by anisotropic Heisenberg
Hamiltonian. While the evolution of low-energy spin excitations of
LiFe$_{1-x}$Co$_x$As are consistent with electron-hole Fermi surface nesting
condition for the $d_{xy}$ orbital, the reduced superconductivity in
LiFe$_{0.88}$Co$_{0.12}$As suggests that Fermi surface nesting conditions for
the $d_{yz}$ and $d_{xz}$ orbitals are also important for superconductivity in
iron pnictides.
|
1606.00727v1
|
2016-06-03
|
Swift J174540.7-290015: a new accreting binary in the Galactic Center
|
We report on the identification of the new Galactic Center (GC) transient
Swift J174540.7-290015 as a likely low mass X-ray binary (LMXB) located at only
16 arcsec from Sgr A*. This transient was detected on 2016 February 6th during
the Swift GC monitoring, and it showed long-term spectral variations compatible
with a hard to soft state transition. We observed the field with XMM-Newton on
February 26th for 35 ks, detecting the source in the soft state, characterised
by a low level of variability and a soft X-ray thermal spectrum with a high
energy tail (detected by INTEGRAL up to ~50 keV), typical of either accreting
neutron stars or black holes. We observed: i) a high column density of neutral
absorbing material, suggesting that Swift J174540.7-290015 is located near or
beyond the GC and; ii) a sub-Solar Iron abundance, therefore we argue that Iron
is depleted into dust grains. The lack of detection of FeK absorption lines,
eclipses or dipping suggests that the accretion disc is observed at a low
inclination angle. Radio (VLA) observations did not detect any radio
counterpart to Swift J174540.7-290015. No evidence for X-ray or radio
periodicity is found. The location of the transient was observed also in the
near-IR with GROND at MPG/ESO La Silla 2.2m telescope and VLT/NaCo pre- and
post-outburst. Within the Chandra error region we find multiple objects that
display no significant variations.
|
1606.01138v1
|
2016-06-17
|
Time dependence of Fe/O ratio within a 3D Solar Energetic Particle propagation model including drift
|
Context. The intensity profiles of iron and oxygen in Solar Energetic
Particle (SEP) events often display differences that result in a decreasing
Fe/O ratio over time. The physical mechanisms behind this behaviour are not
fully understood, but these observational signatures provide important tests of
physical modelling efforts. Aims. In this paper we study the propagation of
iron and oxygen SEP ions using a 3D model of propagation which includes the
effect of guiding centre drift in a Parker spiral magnetic field. We derive
time intensity profiles for a variety of observer locations and study the
temporal evolution of the Fe/O ratio. Methods. We use a 3D full orbit test
particle model which includes scattering. The configuration of the
interplanetary magnetic field is a unipolar Parker spiral. Particles are
released instantaneously from a compact region at 2 solar radii and allowed to
propagate in 3D. Results. Both Fe and O experience significant transport across
the magnetic field due to gradient and curvature drifts. We find that Fe ions
drift more than O ions due to their larger mass-to-charge ratio, so that an
observer that is not magnetically well connected to the source region will
observe Fe arriving before O, for particles within the same range in energy per
nucleon. As a result, for the majority of observer locations, the Fe/O ratio
displays a decrease in time. Conclusions. We conclude that propagation effects
associated with drifts produce a decay over time of the Fe/O ratio,
qualitatively reproducing that observed in SEP event profiles.
|
1606.05612v2
|
2016-07-05
|
Heavy Electron doping-induced antiferromagnetic phase as the parent for iron-oxypnictide superconductor LaFeAsO1-xHx
|
We perform transport measurements and band structure calculations of
electron-doped LaFeAsO1-xHx over a wide range of x from 0.01 to 0.66. The T^2
and sqrt(T) dependency of the resistivity are observed at x ~ 0.17 and 0.41,
respectively. The sign change of RH without opening of the spin-density-wave
gap for 0.45 < x < 0.58 and T < TN as well as the calculated non-nested Fermi
surface at x = 0.5 indicate the more localized nature of the AF2 as compared to
spin density wave phase at non-doped sample. Considering the results from band
calculations and the finite size of Hund' s rule coupling, the change of the
normal conducting state with x is reasonably explained by a strong depression
of the coherent scale owing to the increased effective Coulomb repulsion in the
narrow anti-bonding 3dxy band that approaches the half-filled regime. The
following transition from the paramagnet to AF2 is understood as the quenching
or ordering of the less-screened spins with a large entropy at low temperature.
These results suggest that the normal conducting properties over a wide doping
range for the LaFeAsO1-xHx are strongly influenced by the local spins in the
incoherent region. Thus, we conclude that the parent phase in LaFeAsO1-xHx is
not the spin-density wave but the AF2, which may be primarily responsible for
the singular superconducting properties of the 1111 type compared with other
iron pnictides.
|
1607.01238v1
|
2016-07-05
|
Superconductivity-enhanced Nematicity and '$s+d$' Gap Symmetry in Fe(Se$_{1-x}$S$_x$)
|
Superconducting iron chalcogenide FeSe has the simplest crystal structure
among all the Fe-based superconductors. Unlike other iron pnictides, FeSe
exhibits no long range magnetic order accompanying the
tetragonal-to-orthorhombic structural distortion, which raises the fundamental
question about the role of magnetism and its associated spin fluctuations in
mediating both nematicity and superconductivity. The extreme sensitivity of
FeSe to external pressure suggests that chemical pressure, induced by
substitution of Se by the smaller ion S, could also a be good tuning parameter
to further study the coupling between superconductivity and nematicity and to
obtain information on both the Fermi-surface changes and the symmetry of the
superconducting state. Here we study the thermodynamic properties of
Fe(Se$_{1-x}$S$_{x}$) for 3 compositions, $x=0$, 0.08 and 0.15, using
heat-capacity and thermal-expansion measurements. With increasing S content we
observe a significant reduction of the tetragonal-to-orthorhombic transition
temperature T$_{s}$. However, this suppression of T$_{s}$ is counterintuitively
accompanied by an enhancement of the orthorhombic distortion $\delta$ below
T$_c$, which clearly indicates that superconductivity favors the nematic state.
In parallel, the superconducting transition temperature T$_{c}$ is sizeably
enhanced, whereas the increase of the Sommerfeld coefficient $\gamma_{n}$ is
quite moderate. In the T$\to$ 0 limit, an unusually large residual density of
states is found for $x>0$ indicative of significant substitution-induced
disorder. We discuss these observations in the context of $s+d$
superconducting-state symmetry.
|
1607.01288v2
|
2016-07-07
|
Deciphering chemical order/disorder and material properties at the single-atom level
|
Correlating 3D arrangements of atoms and defects with material properties and
functionality forms the core of several scientific disciplines. Here, we
determined the 3D coordinates of 6,569 iron and 16,627 platinum atoms in a
model iron-platinum nanoparticle system to correlate 3D atomic arrangements and
chemical order/disorder with material properties at the single-atom level. We
identified rich structural variety and chemical order/disorder including 3D
atomic composition, grain boundaries, anti-phase boundaries, anti-site point
defects and swap defects. We show for the first time that experimentally
measured 3D atomic coordinates and chemical species with 22 pm precision can be
used as direct input for first-principles calculations of material properties
such as atomic magnetic moments and local magnetocrystalline anisotropy. This
work not only opens the door to determining 3D atomic arrangements and chemical
order/disorder of a wide range of nanostructured materials with high precision,
but also will transform our understanding of structure-property relationships
at the most fundamental level.
|
1607.02051v1
|
2016-07-09
|
Theoretical modelling of the AGN iron-line vs continuum time-lags in the lamp-post geometry
|
Context: Theoretical modelling of time-lags between variations in the Fe
K$\alpha$ emission and the X-ray continuum might shed light on the physics and
geometry of the X-ray emitting region in active galaxies (AGN) and X-ray
binaries. We here present the results from a systematic analysis of time-lags
between variations in two energy bands ($5-7$ vs $2-4\,\mathrm{keV}$) for seven
X-ray bright and variable AGN.
Aims: We estimate time-lags as accurately as possible and fit them with
theoretical models in the context of the lamp-post geometry. We also constrain
the geometry of the X-ray emitting region in AGN.
Methods: We used all available archival \textit{XMM-Newton} data for the
sources in our sample and extracted light curves in the $5-7$ and
$2-4\,\mathrm{keV}$ energy bands. We used these light curves and applied a
thoroughly tested (through extensive numerical simulations) recipe to estimate
time-lags that have minimal bias, approximately follow a Gaussian distribution,
and have known errors. Using traditional $\chi^2$ minimisation techniques, we
then fitted the observed time-lags with two different models: a
phenomenological model where the time-lags have a power-law dependence on
frequency, and a physical model, using the reverberation time-lags expected in
the lamp-post geometry. The latter were computed assuming a point-like primary
X-ray source above a black hole surrounded by a neutral and prograde accretion
disc with solar iron abundance. We took all relativistic effects into account
for various X-ray source heights, inclination angles, and black hole spin
values.
|
1607.02625v2
|
2016-07-09
|
On the reality of broad iron L lines from the narrow line Seyfert 1 galaxies 1H0707-495 and IRAS 13224-3809
|
We performed time resolved spectroscopy of 1H0707-495 and IRAS 13224-3809
using long XMM-Newton observations. These are strongly variable narrow line
Seyfert 1 galaxies and show broad features around 1 keV that has been
interpreted as relativistically broad Fe L$\alpha$ lines. Such features are not
clearly observed in other AGN despite sometimes having high iron abundance
required by the best fitted blurred reflection models. Given the importance of
these lines, we explore the possibility if rapid variability of spectral
parameters may introduce broad bumps/dips artificially in the time averaged
spectrum, which may then be mistaken as broadened lines. We tested this
hypothesis by performing time resolved spectroscopy using long (> 100 ks)
XMM-Newton observations and by dividing it into segments with typical exposure
of few ks. We extracted spectra from each such segment and modelled using a two
component phenomenological model consisting of a power law to represent hard
component and a black body to represent the soft emission. As expected both the
sources showed variations in the spectral parameters. Using these variation
trends, we simulated model spectra for each segment and then co-added to get a
combined simulated spectrum. In the simulated spectra, we found no broad
features below 1 keV and in particular no deviation near 0.9 keV as seen in the
real average spectra. This implies that the broad Fe L? line that is seen in
the spectra of these sources is not an artifact of the variation of spectral
components and hence providing evidence that the line is indeed genuine.
|
1607.02635v1
|
2016-07-11
|
Magnetotransport of multiple-band nearly-antiferromagnetic metals due to "hot-spot" scattering
|
Multiple-band electronic structure and proximity to antiferromagnetic (AF)
instability are the key properties of iron-based superconductors. We explore
the influence of scattering by the AF spin fluctuations on transport of
multiple-band metals above the magnetic transition. A salient feature of
scattering on the AF fluctuations is that it is strongly enhanced at the Fermi
surface locations where the nesting is perfect ("hot spots" or "hot lines"). We
review derivation of the collision integral for the Boltzmann equation due to
AF-fluctuations scattering. In the paramagnetic state, the enhanced scattering
rate near the hot lines leads to anomalous behavior of electronic transport in
magnetic field. We explore this behavior by analytically solving Boltzmann
transport equation with approximate transition rates. This approach accounts
for return scattering events and is more accurate than the relaxation-time
approximation. The magnetic-field dependences are characterized by two very
different field scales, the lower scale is set by the hot-spot width and the
higher scale is set by the total scattering amplitude. A conventional
magnetotransport behavior is limited to magnetic fields below the lower scale.
In the wide range in between these two scales the longitudinal conductivity has
linear dependence on the magnetic field and the Hall conductivity has quadratic
dependence. The linear dependence of the diagonal component reflects growth of
the Fermi-surface area affected by hot spots proportional to the magnetic
field. We discuss applicability of this theoretical framework for describing of
anomalous magnetotransport properties in different iron pnictides and selenides
in the paramagnetic state.
|
1607.03194v2
|
2016-07-19
|
Sensitivity of the Fe K-alpha Compton shoulder to the geometry and variability of the X-ray illumination of cosmic objects
|
In an X-ray reflection spectrum, a tail-like spectral feature generated via
Compton down-scattering, known as a Compton shoulder (CS), appears at the
low-energy side of the iron K$\alpha$ line. Despite its great diagnostic
potential, its use as a spectral probe of the reflector has been seriously
limited due to observational difficulties and modelling complexities. We
revisit the basic nature of the CS by systematic investigation into its
dependence on spatial and temporal parameters. The calculations are performed
by Monte-Carlo simulations for sphere and slab geometries. The dependence is
obtained in a two-dimensional space of column density and metal abundance,
demonstrating that the CS solves parameter degeneration between them which was
seen in conventional spectral analysis using photoelectric absorption and
fluorescence lines. Unlike the iron line, the CS does not suffer from any
observational dependence on the spectral hardness. The CS profile is highly
dependent on the inclination angle of the slab geometry unless the slab is
Compton-thick, and the time evolution of the CS is shown to be useful to
constrain temporal information on the source if the intrinsic radiation is
variable. We also discuss how atomic binding of the scattering electrons in
cold matter blurs the CS profile, finding that the effect is practically
similar to thermal broadening in a plasma with a moderate temperature of
$\sim$5 eV. Spectral diagnostics using the CS is demonstrated with grating data
of X-ray binary GX 301$-$2, and will be available in future with
high-resolution spectra of active galactic nuclei obtained by
microcalorimeters.
|
1607.05385v1
|
2016-07-19
|
Highly mobile carriers in orthorhombic phases of iron-based superconductors FeSe${}_{1-x}$S${}_{x}$
|
The field and temperature dependencies of the longitudinal and Hall
resistivity have been measured for FeSe${}_{1-x}$S${}_{x}$ (x=0.04, 0.09 and
0.19) single crystals. The sample FeSe${}_{0.81}$S${}_{0.19}$ does not show a
transition to an orthorhombic phase and exhibits at low temperatures the
transport properties quite different from those of orthorhombic samples. The
behavior of FeSe${}_{0.81}$S${}_{0.19}$ is well described by the simple two
band model with comparable values of hole and electron mobility. In particular,
at low temperatures the transverse resistance shows a linear field dependence,
the magnetoresistance follow a quadratic field dependence and obeys to Kohler's
rule. In contrast, Kohler's rule is strongly violated for samples having an
orthorhombic low temperature structure. However, the transport properties of
the orthorhombic samples can be satisfactory described by the three band model
with the pair of almost equivalent to the tetragonal sample hole and electron
bands, supplemented with the highly mobile electron band which has two order
smaller carrier number. Therefore, the peculiarity of the low temperature
transport properties of the orthorhombic Fe(SeS) samples, as probably of many
other orthorhombic iron superconductors, is due to the presence of a small
number of highly mobile carriers which originate from the local regions of the
Fermi surface, presumably, nearby the Van Hove singularity points.
|
1607.05669v2
|
2016-08-02
|
Correlation of Fe-based Superconductivity and Electron-Phonon Coupling in an FeAs/Oxide Heterostructure
|
Interfacial phonons between iron-based superconductors (FeSCs) and perovskite
substrates have received considerable attention due to the possibility of
enhancing preexisting superconductivity. Using scanning tunneling spectroscopy,
we studied the correlation between superconductivity and e-ph interaction with
interfacial-phonons in an iron-based superconductor Sr$_2$VO$_3$FeAs ($T_c
\approx$ 33 K) made of alternating FeSC and oxide layers. The quasiparticle
interference measurement over regions with systematically different average
superconducting gaps due to the e-ph coupling locally modulated by O vacancies
in VO$_2$ layer, and supporting self-consistent momentum-dependent Eliashberg
calculations provide a unique real-space evidence of the forward-scattering
interfacial phonon contribution to the total superconducting pairing.
|
1608.00886v4
|
2016-08-03
|
Structure of spin excitations in heavily electron-doped Li0.8Fe0.2ODFeSe superconductors
|
Heavily electron-doped iron-selenide (HEDIS) high-transition-temperature
(high-$T_{\rm{c}}$) superconductors, which have no hole Fermi pockets, but have
a notably high $T_{\rm{c}}$, have challenged the prevailing $s$$_\pm$ pairing
scenario originally proposed for iron pnictides containing both electron and
hole pockets. The microscopic mechanism underlying the enhanced
superconductivity in HEDIS remains unclear. Here, we used neutron scattering to
study the spin excitations of the HEDIS material Li$_{0.8}$Fe$_{0.2}$ODFeSe
($T_{\rm{c}}$ = 41 K). Our data revealed nearly ring-shaped magnetic resonant
excitations surrounding ($\pi$, $\pi$) at $\sim$ 21 meV. As the energy
increased, the spin excitations assumed a diamond shape, and they dispersed
outward until the energy reached $\sim$ 60 meV and then inward at higher
energies. The observed energy-dependent momentum structure and twisted
dispersion of spin excitations near ($\pi$, $\pi$) are analogous to those of
hole-doped cuprates in several aspects, thus implying that such spin
excitations are essential for the remarkably high $T_{\rm{c}}$ in these
materials.
|
1608.01204v2
|
2016-08-04
|
Isotopic enrichment of forming planetary systems from supernova pollution
|
Heating by short-lived radioisotopes (SLRs) such as aluminum-26 and iron-60
fundamentally shaped the thermal history and interior structure of Solar System
planetesimals during the early stages of planetary formation. The subsequent
thermo-mechanical evolution, such as internal differentiation or rapid volatile
degassing, yields important implications for the final structure, composition
and evolution of terrestrial planets. SLR-driven heating in the Solar System is
sensitive to the absolute abundance and homogeneity of SLRs within the
protoplanetary disk present during the condensation of the first solids. In
order to explain the diverse compositions found for extrasolar planets, it is
important to understand the distribution of SLRs in active planet formation
regions (star clusters) during their first few Myr of evolution. By
constraining the range of possible effects, we show how the imprint of SLRs can
be extrapolated to exoplanetary systems and derive statistical predictions for
the distribution of aluminum-26 and iron-60 based on N-body simulations of
typical to large clusters (1000-10000 stars) with a range of initial
conditions. We quantify the pollution of protoplanetary disks by supernova
ejecta and show that the likelihood of enrichment levels similar to or higher
than the Solar System can vary considerably, depending on the cluster
morphology. Furthermore, many enriched systems show an excess in radiogenic
heating compared to Solar System levels, which implies that the formation and
evolution of planetesimals could vary significantly depending on the birth
environment of their host stars.
|
1608.01435v1
|
2016-08-29
|
Universal Lower Limit on Vortex Creep in Superconductors
|
Superconductors are excellent testbeds for studying vortices, topological
excitations that also appear in superfluids, liquid crystals, and Bose-Einstein
condensates. Vortex motion can be disruptive; it can cause phase transitions,
glitches in pulsars, and losses in superconducting microwave circuits, and it
limits the current carrying capacity of superconductors. Understanding vortex
dynamics is therefore of fundamental and technological importance, and the
competition between the effects of thermal energy and energy barriers defined
by material disorder is not completely understood. In particular, early
measurements of thermally-activated vortex motion (creep) in iron-based
superconductors unveiled fast rates (S) comparable to measurements of YBa2Cu3O7
(YBCO). This was puzzling because S is thought to somehow positively correlate
with the Ginzburg number (Gi), and Gi is orders of magnitude lower in most
iron-based superconductors than in YBCO. Here, we report very slow creep in
BaFe2(As0.67P0.33)2 films, and propose that there is a universal minimum
realizable (where Tc is the superconducting transition temperature) that has
been achieved in our films, few other materials, and violated by none. This
limitation provides new clues on how to design materials with slow creep and
helps elucidate open questions regarding the interplay between system-specific
length scales and vortex dynamics.
|
1608.08092v2
|
2016-09-01
|
Influence of interstitial Fe to the phase diagram of Fe$_{1+y}$Te$_{1-x}$Se$_x$ single crystals
|
Superconductivity (SC) with the suppression of long-range antiferromagnetic
(AFM) order is observed in the parent compounds of both iron-based and cuprate
superconductors. The AFM wave vectors are bicollinear ($\pi$, 0) in the parent
compound FeTe different from the collinear AFM order ($\pi$, $\pi$) in most
iron pnictides. Study of the phase diagram of Fe$_{1+y}$Te$_{1-x}$Se$_x$ is the
most direct way to investigate the competition between bicollinear AFM and SC.
However, presence of interstitial Fe affects both magnetism and SC of
Fe$_{1+y}$Te$_{1-x}$Se$_x$, which hinders the establishment of the real phase
diagram. Here, we report the comparison of doping-temperature ($x$-$T$) phase
diagrams for Fe$_{1+y}$Te$_{1-x}$Se$_x$ (0 $\leq$ $x$ $\leq$ 0.43) single
crystals before and after removing interstitial Fe. Without interstitial Fe,
the AFM state survives only for $x$ $<$ 0.05, and bulk SC emerges from $x$ =
0.05, and does not coexist with the AFM state. The previously reported spin
glass state, and the coexistence of AFM and SC may be originated from the
effect of the interstitial Fe. The phase diagram of Fe$_{1+y}$Te$_{1-x}$Se$_x$
is found to be similar to the case of the "1111" system such as
LaFeAsO$_{1-x}$F$_x$, and is different from that of the "122" system.
|
1609.00105v1
|
2016-09-07
|
Efficiency of Planetesimal Ablation in Giant Planetary Envelopes
|
Observations of exoplanetary spectra are leading to unprecedented constraints
on their atmospheric elemental abundances, particularly O/H, C/H, and C/O
ratios. Recent studies suggest that elemental ratios could provide important
constraints on formation and migration mechanisms of giant exoplanets. A
fundamental assumption in such studies is that the chemical composition of the
planetary envelope represents the sum-total of compositions of the accreted gas
and solids during the formation history of the planet. We investigate the
efficiency with which accreted planetesimals ablate in a giant planetary
envelope thereby contributing to its composition rather than sinking to the
core. From considerations of aerodynamic drag causing `frictional ablation' and
the envelope temperature structure causing `thermal ablation', we compute mass
ablations for impacting planetesimals of radii 30 m to 1 km for different
compositions (ice to iron) and a wide range of velocities and impact angles,
assuming spherical symmetry. Icy impactors are fully ablated in the outer
envelope for a wide range of parameters. Even for Fe impactors substantial
ablation occurs in the envelope for a wide range of sizes and velocities. For
example, iron impactors of sizes below ~0.5 km and velocities above ~30 km/s
are found to ablate by ~60-80% within the outer envelope at pressures below
10^3 bar due to frictional ablation alone. For deeper pressures (~10^7 bar),
substantial ablation happens over a wider range of parameters. Therefore, our
exploratory study suggests that atmospheric abundances of volatile elements in
giant planets reflect their accretion history during formation.
|
1609.02143v1
|
2016-09-18
|
Spintronics Detection of Interfacial Magnetic Switching in a Paramagnetic Tris(8-hydroxyquinoline)iron(III) Thin Film
|
Organic semiconductors find increasing importance in spin transport devices
due to the modulation and control of their properties through chemical
synthetic versatility. The organic materials are used as interlayers between
two ferromagnet (FM) electrodes in organic spin valves (OSV), as well as for
magnetic spin manipulation of metal-organic complexes at the molecular level.
In the latter, specifically, the substrate-induced magnetic switching in a
paramagnetic molecule has been evoked extensively, but studied by delicate
surface spectroscopies. Here we present evidence of the substantial magnetic
switching in a nanosized thin film of the paramagnetic molecule,
tris(8-hydroxyquinoline)iron(III) (Feq3) deposited on a FM substrate, using the
magnetoresistance response of electrical spin-injection in an OSV structure,
and the inverse-spin-Hall effect induced by state-of-art pulsed microwave
spin-pumping. We show that interfacial spin control at the molecular level may
lead to a macroscopic organic spin transport device, thus, bridging the gap
between organic spintronics and molecular spintronics.
|
1609.05421v1
|
2016-10-04
|
Tomographic reflection modelling of quasi-periodic oscillations in the black hole binary H 1743-322
|
Accreting stellar mass black holes (BHs) routinely exhibit Type-C
quasi-periodic oscillations (QPOs). These are often interpreted as
Lense-Thirring precession of the inner accretion flow, a relativistic effect
whereby the spin of the BH distorts the surrounding space-time, inducing nodal
precession. The best evidence for the precession model is the recent discovery,
using a long joint XMM-Newton and NuSTAR observation of H 1743-322, that the
centroid energy of the iron fluorescence line changes systematically with QPO
phase. This was interpreted as the inner flow illuminating different azimuths
of the accretion disc as it precesses, giving rise to a blue/red shifted iron
line when the approaching/receding disc material is illuminated. Here, we
develop a physical model for this interpretation, including a self-consistent
reflection continuum, and fit this to the same H 1743-322 data. We use an
analytic function to parameterise the asymmetric illumination pattern on the
disc surface that would result from inner flow precession, and find that the
data are well described if two bright patches rotate about the disc surface.
This model is preferred to alternatives considering an oscillating disc
ionisation parameter, disc inner radius and radial emissivity profile. We find
that the reflection fraction varies with QPO phase (3.5 sigma), adding to the
now formidable body of evidence that Type-C QPOs are a geometric effect. This
is the first example of tomographic QPO modelling, initiating a powerful new
technique that utilizes QPOs in order to map the dynamics of accreting material
close to the BH.
|
1610.00948v1
|
2016-09-29
|
Thin layered drawing media probed by THz time-domain spectroscopy
|
Dry and wet drawing materials were investigated by THz time-domain
spectroscopy in transmission mode. Carbon-based and iron-gall inks have been
studied, some prepared following ancient recipes and others using current
synthetic materials; a commercial ink was studied as well. We measured the THz
signals on thin film of liquid inks deposited on polyethylene pellicles,
comparing the results with the thick pellets of dried inks blended with
polyethylene powder. This study required the implementation of a new
experimental method and data analysis procedure able to provide a reliable
extraction of the material transmission parameters from a structured sample
composed of thin layers, down to thickness of few tens of micrometers. THz
measurements on thin ink layers enabled the determination of both the
absorption and the refractive index in an absolute scale in the 0.1 - 3 THz
range, as well as the layer thickness. THz spectroscopic features of a paper
sheet dyed by one of the iron-gall inks were also investigated. Our results
showed that THz time-domain spectroscopy enables to discriminate the various
inks on different supports, including the application on paper, together with a
proper determination of the absorption coefficients and indices of refraction.
|
1610.01025v1
|
2016-10-31
|
Chromium analogues of Iron-based superconductors
|
We theoretically investigate the $d^4$ (Cr$^{2+}$) compound BaCr$_2$As$_2$
and show that, despite non-negligible differences in the electronic structure,
its many-body physics mirrors that of BaFe$_2$As$_2$, which has instead a $d^6$
(Fe$^{2+}$) configuration. This reflects a symmetry of the electron correlation
effects around the half-filled $d^5$ Mott insulating state. The experimentally
known metallic antiferromagnetic phase is correctly modeled by dynamical
mean-field theory, and for realistic values of the interaction it shows a
moderate mass enhancement of order $\sim$2. This value decreases if the ordered
moment grows as a result of a stronger interaction. The antiferromagnetic phase
diagram for this $d^4$ shows similarities with that calculated for the $d^6$
systems. Correspondingly, in the paramagnetic phase the influence of the
half-filled Mott insulator shows up as a crossover from a weakly correlated to
an orbitally differentiated "Hund's metal" phase which reflects an analogous
phenomenon in $d^6$ iron compounds including a strong enhancement of the
compressibility in a zone just inside the frontier between the normal and the
Hund's metal. The experimental evidence and our theoretical description place
BaCr$_2$As$_2$ at interaction strength slightly below the crossover which
implies that negative pressures and/or electron doping (e.g. Cr $\rightarrow$
Mn,Fe or Ba $\rightarrow$ Sc,Y,La) might strongly enhance the compressibility,
thereby possibly inducing a pairing instability in this non-superconducting
compound.
|
1610.10054v1
|
2016-11-17
|
Deformation behaviour of body centered cubic iron nanopillars containing coherent twin boundaries
|
Molecular dynamics simulations were performed to understand the role of twin
boundaries on deformation behaviour of body-centred cubic (BCC) iron (Fe)
nanopillars. The twin boundaries varying from one to five providing twin
boundary spacing in the range 8.5 - 2.8 nm were introduced perpendicular to the
loading direction. The simulation results indicated that the twin boundaries in
BCC Fe play a contrasting role during deformation under tensile and compressive
loadings. During tensile deformation, a large reduction in yield stress was
observed in twinned nanopillars compared to perfect nanopillar. However, the
yield stress exhibited only marginal variation with respect to twin boundary
spacing. On the contrary, a decrease in yield stress with increase in twin
boundary spacing was obtained during compressive deformation. This contrasting
behaviour originates from difference in operating mechanisms during yielding
and subsequent plastic deformation. It has been observed that the deformation
under tensile loading was dominated mainly by twin growth mechanism, due to
which the twin boundaries offers a negligible resistance to slip of twinning
partials. This is reflected in the negligible variation of yield stress as a
function of twin boundary spacing. On the other hand, the deformation was
dominated by nucleation and slip of full dislocations under compressive
loading. The twin boundaries offer a strong repulsive force on full
dislocations resulting in the yield stress dependence on twin boundary spacing.
Further, it has been observed that the curved twin boundary can acts as a
source for full dislocation. The occurrence of twin-twin interaction during
tensile deformation and dislocation-twin interaction during compressive
deformation were presented and discussed.
|
1611.05575v1
|
2016-12-16
|
Pressure-Induced Metallization in Iron-Based Ladder Compounds Ba$_{1-x}$Cs$_x$Fe$_2$Se$_3$
|
Electrical resistivity measurements have been performed on the iron-based
ladder compounds Ba$_{1-x}$Cs$_x$Fe$_2$Se$_3$ ($x$ = 0, 0.25, 0.65, and 1)
under high pressure. A cubic anvil press was used up to 8.0 GPa, whereas
further higher pressure was applied using a diamond anvil cell up to 30.0 GPa.
Metallic behavior of the electrical conductivity was confirmed in the $x$ =
0.25 and 0.65 samples for pressures greater than 11.3 and 14.4 GPa,
respectively, with the low-temperature $\log T$ upturn being consistent with
weak localization of 2D electrons due to random potential. At pressures higher
than 23.8 GPa, three-dimensional Fermi-liquid-like behavior was observed in the
latter sample. No metallic conductivity was observed in the parent compounds
BaFe$_2$Se$_3$ ($x $ = 0) up to 30.0 GPa and CsFe$_2$Se$_3$ ($x$ = 1) up to
17.0 GPa. The present results indicate that the origins of the insulating
ground states in the parent and intermediate compounds are intrinsically
different; the former is a Mott insulator, whereas the latter is an Anderson
insulator owing to the random substitution of Cs for Ba.
|
1612.05394v1
|
2016-12-29
|
Molecular nucleation theory of dust formation in core-collapse supernovae applied to SN 1987A
|
We model dust formation in the core collapse supernova explosion SN 1987A by
treating the gas-phase formation of dust grain nuclei as a chemical process. To
compute the synthesis of fourteen species of grains we integrate a
non-equilibrium network of nucleating and related chemical reactions and follow
the growth of the nuclei into grains via accretion and coagulation. The effects
of the radioactive cobalt, titanium, and sodium on the thermodynamics and
chemistry of the ejecta are taken into account. The grain temperature, which we
allow to differ from the gas temperature, affects the surface-tension-corrected
evaporation rate. We also account for He$^+$, Ne$^+$, Ar$^+$, and O weathering.
We combine our dust synthesis model with a crude prescription for anisotropic
radioactive nickel dredge-up into the core ejecta, the so-called `nickel
bubbles', to compute the total dust mass and molecular-species-specific grain
size distribution. The total mass varies between $0.41\,M_\odot$ and
$0.73\,M_\odot$, depending on the bubble shell density contrast. In the
decreasing order of abundance, the grain species produced are: magnesia,
silicon, forsterite, iron sulfide, carbon, silicon dioxide, alumina, and iron.
The combined grain size distribution is a power law $dN/da\propto a^{-4.39}$.
Early ejecta compaction by expanding radioactive nickel bubbles strongly
enhances dust synthesis. This underscores the need for improved understanding
of hydrodynamic transport and mixing over the entire pre-homologous expansion.
|
1612.09013v2
|
2017-01-11
|
Antiferromagnetic structure and electronic properties of BaCr2As2 and BaCrFeAs2
|
The chromium arsenides BaCr2As2 and BaCrFeAs2 with ThCr2Si2 type structure
(space group I4/mmm; also adopted by '122' iron arsenide superconductors) have
been suggested as mother compounds for possible new superconductors. DFT-based
calculations of the electronic structure evidence metallic antiferromagnetic
ground states for both compounds. By powder neutron diffraction we confirm for
BaCr2As2 a robust ordering in the antiferromagnetic G-type structure at T_N =
580 K with mu_Cr = 1.9 mu_B at T = 2K. Anomalies in the lattice parameters
point to magneto-structural coupling effects. In BaCrFeAs2 the Cr and Fe atoms
randomly occupy the transition-metal site and G-type order is found below 265 K
with mu_Cr/Fe = 1.1 mu_B. 57Fe Moessbauer spectroscopy demonstrates that only a
small ordered moment is associated with the Fe atoms, in agreement with
electronic structure calculations with mu_Fe ~ 0. The temperature dependence of
the hyperfine field does not follow that of the total moments. Both compounds
are metallic but show large enhancements of the linear specific heat
coefficient gamma with respect to the band structure values. The metallic state
and the electrical transport in BaCrFeAs2 is dominated by the atomic disorder
of Cr and Fe and partial magnetic disorder of Fe. Our results indicate that
Neel-type order is unfavorable for the Fe moments and thus it is destabilized
with increasing iron content.
|
1701.03127v1
|
2017-01-16
|
Comprehensive Timing and X-ray Spectral Analysis of GX 1+4
|
We present analysis of RXTE--PCA observations of GX 1+4 between March 3, 2001
and January 31, 2003 together with the CGRO--BATSE X-ray flux and frequency
derivative time series between 1991 and 1999. From the timing analysis of
RXTE-PCA observations, we are able to phase connect pulse arrival times of the
source within two different time intervals and obtain corresponding timing
solutions. Using these pulse arrival times, we contribute to long term pulse
frequency history of the source. We look for episodic correlations and
anti-correlations between torque and X-ray luminosity using CGRO--BATSE X-ray
flux and frequency derivative time series and find that correlation state of GX
1+4 seems to change on $\sim$ 100-200 days long intervals. We estimate torque
noise of the source and observe flickering noise ($f^{-1}$). We achieve to
measure the longest observed timescale for a noise process among accretion
powered X-ray pulsars by extending the noise estimate for a time scale ranging
from 31 days to 44 years. Spectral analysis of individual RXTE-PCA observations
indicates a significant correlation between iron line flux and unabsorbed X-ray
flux. Pulse phase resolved spectra of the source indicate a broadening of iron
line complex at the bin corresponding to the pulse minimum.
|
1701.04328v2
|
2017-01-17
|
Phase diagram and neutron spin resonance of superconducting NaFe$_{1-x}$Cu$_x$As
|
We use transport and neutron scattering to study the electronic phase diagram
and spin excitations of NaFe$_{1-x}$Cu$_x$As single crystals. Similar to Co-
and Ni-doped NaFeAs, a bulk superconducting phase appears near $x\approx2\%$
with the suppression of stripe-type magnetic order in NaFeAs. Upon further
increasing Cu concentration the system becomes insulating, culminating in an
antiferromagnetically ordered insulating phase near $x\approx 50\%$. Using
transport measurements, we demonstrate that the resistivity in
NaFe$_{1-x}$Cu$_x$As exhibits non-Fermi-liquid behavior near $x\approx1.8\%$.
Our inelastic neutron scattering experiments reveal a single neutron spin
resonance mode exhibiting weak dispersion along $c$-axis in
NaFe$_{0.98}$Cu$_{0.02}$As. The resonance is high in energy relative to the
superconducting transition temperature $T_{\rm c}$ but weak in intensity,
likely resulting from impurity effects. These results are similar to other iron
pnictides superconductors despite the superconducting phase in
NaFe$_{1-x}$Cu$_x$As is continuously connected to an antiferromagnetically
ordered insulating phase near $x\approx 50\%$ with significant electronic
correlations. Therefore, electron correlations is an important ingredient of
superconductivity in NaFe$_{1-x}$Cu$_x$As and other iron pnictides.
|
1701.04874v1
|
2017-01-20
|
Tuning sizes, morphologies, and magnetic properties of mono- vs. multi-core iron oxide nanoparticles through control of added water in the polyol synthesis
|
The polyol route is a versatile and up-scalable method to produce large
batches of iron oxide nanoparticles with well-defined structure and magnetic
properties. Controlling parameters such as temperature and duration of
reaction, heating profile, nature of polyol solvent or of organometallic
precursors were reported in previous studies of literature, but none of them
described yet the crucial role of water in the forced hydrolysis pathway, whose
presence is mandatory for nanoparticle production. This communication
investigates the influence of the water amount and temperature at which it is
injected in the reflux system for either pure polyol or mixture with a
poly(hydroxy) amine. Distinct morphologies of nanoparticles were thereby
obtained, from ultra-ultra-small smooth spheres down to 4 nm in diameter to
large ones up to 37 nm in diameter. Nanoflowers were also synthesized, which
are well-defined multi-core assemblies with narrow grain size dispersity. A
diverse and large library of samples was obtained by playing on the nature of
solvents and amount of water traces while keeping all the other parameters
fixed. The varied morphologies lead to magnetic nanoparticles well-fitting to
required applications among magnetic hyperthermia and MRI contrast agent, or
both.
|
1701.05858v3
|
2017-01-28
|
Deep Recurrent Neural Network for Protein Function Prediction from Sequence
|
As high-throughput biological sequencing becomes faster and cheaper, the need
to extract useful information from sequencing becomes ever more paramount,
often limited by low-throughput experimental characterizations. For proteins,
accurate prediction of their functions directly from their primary amino-acid
sequences has been a long standing challenge. Here, machine learning using
artificial recurrent neural networks (RNN) was applied towards classification
of protein function directly from primary sequence without sequence alignment,
heuristic scoring or feature engineering. The RNN models containing
long-short-term-memory (LSTM) units trained on public, annotated datasets from
UniProt achieved high performance for in-class prediction of four important
protein functions tested, particularly compared to other machine learning
algorithms using sequence-derived protein features. RNN models were used also
for out-of-class predictions of phylogenetically distinct protein families with
similar functions, including proteins of the CRISPR-associated nuclease,
ferritin-like iron storage and cytochrome P450 families. Applying the trained
RNN models on the partially unannotated UniRef100 database predicted not only
candidates validated by existing annotations but also currently unannotated
sequences. Some RNN predictions for the ferritin-like iron sequestering
function were experimentally validated, even though their sequences differ
significantly from known, characterized proteins and from each other and cannot
be easily predicted using popular bioinformatics methods. As sequencing and
experimental characterization data increases rapidly, the machine-learning
approach based on RNN could be useful for discovery and prediction of
homologues for a wide range of protein functions.
|
1701.08318v1
|
2017-02-28
|
Neutrino Emission from Supernovae
|
Supernovae are the most powerful cosmic sources of MeV neutrinos. These
elementary particles play a crucial role when the evolution of a massive star
is terminated by the collapse of its core to a neutron star or a black hole and
the star explodes as supernova. The release of electron neutrinos, which are
abundantly produced by electron captures, accelerates the catastrophic infall
and causes a gradual neutronization of the stellar plasma by converting protons
to neutrons as dominant constituents of neutron star matter. The emission of
neutrinos and antineutrinos of all flavors carries away the gravitational
binding energy of the compact remnant and drives its evolution from the hot
initial to the cold final state. The absorption of electron neutrinos and
antineutrinos in the surroundings of the newly formed neutron star can power
the supernova explosion and determines the conditions in the innermost
supernova ejecta, making them an interesting site for the nucleosynthesis of
iron-group elements and trans-iron nuclei. In this Chapter the basic neutrino
physics in supernova cores and nascent neutron stars will be discussed. This
includes the most relevant neutrino production, absorption, and scattering
processes, elementary aspects of neutrino transport in dense environments, the
characteristic neutrino emission phases with their typical signal features, and
the perspectives connected to a measurement of the neutrino signal from a
future galactic supernova.
|
1702.08713v1
|
2017-03-24
|
Role of the orbital degree of freedom in iron-based superconductors
|
Almost a decade has passed since the serendipitous discovery of the
iron-based high temperature superconductors (FeSCs) in 2008. The question of
how much similarity the FeSCs have with the copper oxide high temperature
superconductors emerged since the initial discovery of long-range
antiferromagnetism in the FeSCs in proximity to superconductivity. Despite the
great resemblance in their phase diagrams, there exist important disparities
between FeSCs and cuprates that need to be considered in order to paint a full
picture of these two families of high temperature superconductors. One of the
key differences lies in the multi-orbital multi-band nature of FeSCs, in
contrast to the effective single-band model for cuprates. Due to the complexity
of multi-orbital band structures, the orbital degree of freedom is often
neglected in formulating the theoretical models for FeSCs. On the experimental
side, systematic studies of the orbital related phenomena in FeSCs have been
largely lacking. In this review, we summarize angle-resolved photoemission
spectroscopy (ARPES) measurements across various FeSC families in literature,
focusing on the systematic trend of orbital dependent electron correlations and
the role of different Fe 3d orbitals in driving the nematic transition, the
spin-density-wave transition, and implications for superconductivity.
|
1703.08622v2
|
2017-07-04
|
Blocking metal accretion onto population III stars by stellar wind
|
Low-mass population III (PopIII) stars of $\lesssim 0.8 M_{\odot}$ could
survive up until the present. Non-detection of low-mass PopIII stars in our
Galaxy has already put a stringent constraint on the initial mass function
(IMF) of PopIII stars, suggesting that PopIII stars have a top-heavy IMF. On
the other hand, some claims that the lack of such stars stems from metal
enrichment of their surface by accretion of heavy elements from interstellar
medium (ISM). We investigate effects of the stellar wind on the metal accretion
onto low-mass PopIII stars because accretion of the local ISM onto the Sun is
prevented by the solar wind even for neutrals. The stellar wind and radiation
of low-mass PopIII stars are modeled based on knowledge of nearby low-mass
stellar systems including our Sun. We find that low-mass PopIII stars traveling
across the Galaxy forms the stellar magnetosphere in most of their life. Once
the magnetosphere is formed, most of neutral interstellar particles are
photoionized before reaching to the stellar surface and are blown away by the
wind. Especially, the accretion abundance of iron will be reduced by a factor
of $< 10^{-12}$ compared with Bondi-Hoyle-Lyttleton accretion. The metal
accretion can enhance iron abundance [Fe/H] only up to $\sim -14$. This
demonstrates that low-mass PopIII stars remain pristine and will be found as
metal free stars and that further searches for them are valuable to constrain
the IMF of PopIII stars.
|
1707.00989v1
|
2017-07-05
|
Interactions between Coherent Twin Boundaries and Phase Transition of Iron under Dynamic Loading and Unloading
|
Under high pressures, phase transition, as well as deformation twins, are
constantly reported in many BCC metals, whose interactions are of fundamental
importance to understand strengthen mechanism of these metals under extreme
conditions. However, the interactions between twins and phase transition in BCC
metals are remain largely unexplored. In this work, interactions between
coherent twin boundaries and $\alpha \to \epsilon$ phase transition of iron are
investigated using both non- equilibrium molecular dynamics simulations and
nudge elastic band method. Mechanisms of both twin-assisted phase transition
and reverse phase transition are studied and orientation relationships between
BCC and HCP phase are found to be <11-1>BCC || <-12-10>HCP and <1-10>BCC ||
<0001>HCP for both cases. The twin boundary corresponds to after the phase
transition. It is amazing that the reverse transition seems to be able to
"memory" and recover the initial BCC twins. The memory would partly loss when
plastic slips take place in the HCP phase before the reverse transition. In the
recovered initial BCC twins, three major twin spacing are observed, which are
well explained in terms of energy barriers of the transition from HCP phase to
BCC twin. Besides, variant selection rule of the twin assisted phase transition
is also discussed. The results of present work could be expected to give some
clues for producing ultra-fine grain structure in materials exhibiting
martensitic phase transition.
|
1707.01276v1
|
2017-07-06
|
Phonon Spectra in the Parent Superconducting Iron-tuned Telluride Fe$_{1+x}$Te from Inelastic Neutron Scattering and Ab Initio Calculations
|
We report inelastic neutron scattering measurements of phonon spectra in the
parent superconductor iron-tuned chalcogenide Fe$_{1+x}$Te, for two different x
contents (x $\leq$ 0.11), using neutron time-of-flight technique. Thermal
neutron spectroscopy allowed to collect the low-temperature Stokes spectra over
an extended Q-range, at 2, 40 and 120K - hence covering both the magnetic
monoclinic and the paramagnetic tetragonal phases. Whereas cold-neutrons
allowed to measure high-resolution anti-Stokes spectra at 140, 220 and 300K,
thus covering the tetragonal phase. Our results evidence a spin-phonon coupling
behaviour towards the observed noticeable temperature-dependent change of the
Stokes spectra across the transition temperatures. On the other hand, the
anti-Stokes spectra reveal a pronounced hardening of the low-energy, acoustic
region, of the phonon spectrum, upon heating, indicating a strong anharmonicity
and a subtle dependence of phonons on structural evolution within the
tetragonal phase. Experimental results are accompanied by ab initio
calculations of phonon spectra of the tetragonal stoichiometric phase for a
comparison with the high-resolution anti-Stokes spectra. Calculations included
different density functional methods. Spin polarization and van der Waals
interaction, were either considered or neglected, individually or
concomitantly, in order to study their respective effect on lattice dynamics
description. Our results suggest that including van der Waals interaction has
only a slight effect on phonon dynamics, however, phonon spectra are better
described when spin polarization is included, in a cooperative way with van der
Waals interactions.
|
1707.01970v2
|
2017-07-17
|
Coupling of structure to magnetic and superconducting orders in quasi-one-dimensional $\text{K}_2\text{Cr}_3\text{As}_3$
|
Quasi-one-dimensional $A_2\text{Cr}_3\text{As}_3$ (with $A = \text{K, Cs,
Rb}$) is an intriguing new family of superconductors which exhibit many similar
features to the cuprate and iron-based unconventional superconductor families.
Yet in contrast to these systems, no charge or magnetic ordering has been
observed which could provide the electronic correlations presumed necessary for
an unconventional superconducting pairing mechanism - an absence which defies
predictions of first principles models. We report the results of neutron
scattering experiments on polycrystalline $\text{K}_2\text{Cr}_3\text{As}_3$
$(T_c \sim 7\text{K})$ which probed the low temperature dynamics near $T_c$ .
Neutron diffraction data evidence a strong response of the nuclear lattice to
the onset of superconductivity while inelastic scattering reveals a highly
dispersive column of intensity at the commensurate wavevector $q =
(00\frac{1}{2})$ which loses intensity beneath $T_c$ - indicative of
short-range magnetic fluctuations. Using linear spin-wave theory we model the
observed scattering and suggest a possible structure to the short-range
magnetic order. These observations suggest that
$\text{K}_2\text{Cr}_3\text{As}_3$ is in close proximity to a magnetic
instability and that the incipient magnetic order both couples strongly to the
lattice and competes with superconductivity - in direct analogy with the
iron-based superconductors.
|
1707.05286v1
|
2017-07-20
|
Nuclear absorption and emission in the AGN merger NGC 6240: the hard X-ray view
|
We present the analysis of four NuSTAR observations of the luminous infrared
galaxy merger NGC 6240, hosting a close pair of highly obscured active galactic
nuclei (AGN). Over a period of about two years, the source exhibits hard X-ray
variability of the order of 20 per cent, peaking around 20 keV. When the two
AGN are resolved with Chandra, column densities in the range $N_\textrm{H} \sim
1-2 \times 10^{24}$ cm$^{-2}$ are estimated for both of them. The exact values
are hard to determine, as they appear to depend on aspects that are sometimes
overlooked in Compton-thick objects, such as the covering factor of the
absorber, iron abundance, and the contamination in the Fe-K band from
foreground hot-gas emission. Nearly spherical covering and slightly subsolar
iron abundance are preferred in this case. While the southern nucleus is
suggested to be intrinsically more powerful, as also implied by the mid-IR and
2-10 keV brightness ratios, solutions involving a similar X-ray luminosity of
the two AGN cannot be ruled out. The observed variability is rather limited
compared to the one revealed by the Swift/BAT light curve, and it can be fully
explained by changes in the continuum flux from the two AGN, without requiring
significant column density variations. NGC 6240 is hereby confirmed to
represent a unique opportunity to investigate the X-ray (and broad-band)
properties of massive galaxy mergers, which were much more frequent in the
early Universe.
|
1707.07613v1
|
2017-08-18
|
Superconductivity across Lifshitz transition and anomalous insulating state in surface K-dosed (Li0.8Fe0.2OH)FeSe
|
In the iron-based superconductors, understanding the relation between
superconductivity and electronic structure upon doping is crucial for exploring
the pairing mechanism. Recently it was found that in iron selenide (FeSe),
enhanced superconductivity (Tc over 40K) can be achieved via electron doping,
with the Fermi surface only comprising M-centered electron pockets. Here by
utilizing surface potassium dosing, scanning tunneling microscopy/spectroscopy
(STM/STS) and angle-resolved photoemission spectroscopy (ARPES), we studied the
electronic structure and superconductivity of (Li0.8Fe0.2OH)FeSe in the deep
electron-doped regime. We find that a {\Gamma}-centered electron band, which
originally lies above the Fermi level (EF), can be continuously tuned to cross
EF and contribute a new electron pocket at {\Gamma}. When this Lifshitz
transition occurs, the superconductivity in the M-centered electron pocket is
slightly suppressed; while a possible superconducting gap with small size (up
to ~5 meV) and a dome-like doping dependence is observed on the new {\Gamma}
electron pocket. Upon further K dosing, the system eventually evolves into an
insulating state. Our findings provide new clues to understand
superconductivity versus Fermi surface topology and the correlation effect in
FeSe-based superconductors.
|
1708.05635v1
|
2017-09-01
|
Two-dimensional Massless Dirac Fermions in Antiferromagnetic AFe2As2 (A = Ba, Sr)
|
We report infrared studies of AFe$_{2}$As$_{2}$ (A = Ba, Sr), two
representative parent compounds of iron-arsenide superconductors, at magnetic
fields (B) up to 17.5 T. Optical transitions between Landau levels (LLs) were
observed in the antiferromagnetic states of these two parent compounds. Our
observation of a $\sqrt{B}$ dependence of the LL transition energies, the
zero-energy intercepts at B = 0 T under the linear extrapolations of the
transition energies and the energy ratio ($\sim$ 2.4) between the observed LL
transitions, combined with the linear band dispersions in two-dimensional (2D)
momentum space obtained by theoretical calculations, demonstrates the existence
of massless Dirac fermions in antiferromagnetic BaFe$_{2}$As$_{2}$. More
importantly, the observed dominance of the zeroth-LL-related absorption
features and the calculated bands with extremely weak dispersions along the
momentum direction $k_{z}$ indicate that massless Dirac fermions in
BaFe$_{2}$As$_{2}$ are 2D. Furthermore, we find that the total substitution of
the barium atoms in BaFe$_{2}$As$_{2}$ by strontium atoms not only maintains 2D
massless Dirac fermions in this system, but also enhances their Fermi velocity,
which supports that the Dirac points in iron-arsenide parent compounds are
topologically protected.
|
1709.00203v1
|
2017-09-08
|
Machine learning modeling of superconducting critical temperature
|
Superconductivity has been the focus of enormous research effort since its
discovery more than a century ago. Yet, some features of this unique phenomenon
remain poorly understood; prime among these is the connection between
superconductivity and chemical/structural properties of materials. To bridge
the gap, several machine learning schemes are developed herein to model the
critical temperatures ($T_{\mathrm{c}}$) of the 12,000+ known superconductors
available via the SuperCon database. Materials are first divided into two
classes based on their $T_{\mathrm{c}}$ values, above and below 10 K, and a
classification model predicting this label is trained. The model uses
coarse-grained features based only on the chemical compositions. It shows
strong predictive power, with out-of-sample accuracy of about 92%. Separate
regression models are developed to predict the values of $T_{\mathrm{c}}$ for
cuprate, iron-based, and "low-$T_{\mathrm{c}}$" compounds. These models also
demonstrate good performance, with learned predictors offering potential
insights into the mechanisms behind superconductivity in different families of
materials. To improve the accuracy and interpretability of these models, new
features are incorporated using materials data from the AFLOW Online
Repositories. Finally, the classification and regression models are combined
into a single integrated pipeline and employed to search the entire Inorganic
Crystallographic Structure Database (ICSD) for potential new superconductors.
We identify more than 30 non-cuprate and non-iron-based oxides as candidate
materials.
|
1709.02727v2
|
2017-09-15
|
Angular dependence of vortex instability in a layered superconductor: the case study of Fe(Se,Te) material
|
Anisotropy effects on flux pinning and flux flow are strongly effective in
cuprate as well as iron-based superconductors due to their intrinsically
layered crystallographic structure. However $\textrm{Fe(Se,Te)}$ thin films
grown on $\textrm{CaF}_2$ substrate result less anisotropic with respect to all
the other iron based superconductors. We present the first study on the angular
dependence of the flux flow instability, which occurs in the flux flow regime
as a current driven transition to the normal state at the instability point
($I^*$,$V^*$) in the current-voltage characteristics. The voltage jumps are
systematically investigated as a function of the temperature, the external
magnetic field, and the angle between the field and the $\textrm{Fe(Se,Te)}$
film. The scaling procedure based on the anisotropic Ginzburg-Landau approach
is successfully applied to the observed angular dependence of the critical
voltage $V^*$. Anyway, we find out that $\textrm{Fe(Se,Te)}$ represents the
case study of a layered material characterized by a weak anisotropy of its
static superconducting properties, but with an increased anisotropy in its
vortex dynamics due to the predominant perpendicular component of the external
applied magnetic field. Indeed, $I^*$ shows less sensitivity to angle
variations, thus being promising for high field applications.
|
1709.05089v3
|
2017-10-04
|
Surface abundance and the hunt for stratification in chemically peculiar hot subdwarfs: PG 0909+276 and UVO 0512-08
|
Edelmann (2003) identified two chemically peculiar hot subdwarfs, PG 0909+276
and UVO 0512-08, as having very high overabundances of iron-group elements. We
obtained high-resolution ultraviolet spectroscopy in order to measure
abundances of species not observable in the optical, and to seek evidence for
chemical stratification in the photosphere. Abundances were measured in three
wavelength regions; the optical 3900\AA-6900\AA\ range was re-analysed to
confirm consistency with Edelmann. Ultraviolet spectra were obtained with the
Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope
(HST), covering the far-UV (1140\AA-1740\AA) and the near-UV (1740\AA-2500\AA).
We computed a grid of theoretical LTE spectra to find basic parameters
(effective temperatures, surface gravity, surface hydrogen and helium
fractions). We measured abundances using a spectral-synthesis approach in each
wavelength range. We confirm that several iron-group metals are highly
enriched, including cobalt, copper and zinc, relative to typical sdB stars. We
detect gallium, germanium, tin, and lead, similar to analysis of ultraviolet
spectra of some other sdB stars. Our results confirm that PG 0909+276 and UVO
0512-08 exhibit peculiarities which make them distinct from both the normal
H-rich sdB and intermediate He-rich sdB stars. The process which leads to this
particular composition has still to be identified.
|
1710.01663v1
|
2017-10-18
|
Using MRI Cell Tracking to Monitor Immune Cell Recruitment in Response to a Peptide-Based Cancer Vaccine
|
Purpose: MRI cell tracking can be used to monitor immune cells involved in
the immunotherapy response, providing insight into the mechanism of action,
temporal progression of tumour growth and individual potency of therapies. To
evaluate whether MRI could be used to track immune cell populations in response
to immunotherapy, CD8+ cytotoxic T cells (CTLs), CD4+CD25+FoxP3+ regulatory T
cells (Tregs) and myeloid derived suppressor cells (MDSCs) were labelled with
superparamagnetic iron oxide (SPIO) particles.
Methods: SPIO-labelled cells were injected into mice (one cell type/mouse)
implanted with an HPV-based cervical cancer model. Half of these mice were also
vaccinated with DepoVaxTM, a lipid-based vaccine platform that was developed to
enhance the potency of peptide-based vaccines.
Results: MRI visualization of CTLs, Tregs and MDSCs was apparent 24 hours
post-injection, with hypointensities due to iron labelled cells clearing
approximately 72 hours post-injection. Vaccination resulted in increased
recruitment of CTLs and decreased recruitment of MDSCs and Tregs to the tumour.
We also found that MDSC and Treg recruitment was positively correlated with
final tumour volume.
Conclusion: This type of analysis can be used to non-invasively study changes
in immune cell recruitment in individual mice over time, potentially allowing
improved application and combination of immunotherapies.
|
1710.06817v1
|
2017-11-02
|
Constraining planet structure and composition from stellar chemistry: trends in different stellar populations
|
The chemical composition of stars that have orbiting planets provides
important clues about the frequency, architecture, and composition of exoplanet
systems. We explore the possibility that stars from different galactic
populations that have different intrinsic abundance ratios may produce planets
with a different overall composition. We compiled abundances for Fe, O, C, Mg,
and Si in a large sample of solar neighbourhood stars that belong to different
galactic populations. We then used a simple stoichiometric model to predict the
expected iron-to-silicate mass fraction and water mass fraction of the planet
building blocks, as well as the summed mass percentage of all heavy elements in
the disc. Assuming that overall the chemical composition of the planet building
blocks will be reflected in the composition of the formed planets, we show that
according to our model, discs around stars from different galactic populations,
as well as around stars from different regions in the Galaxy, are expected to
form rocky planets with significantly different iron-to-silicate mass
fractions. The available water mass fraction also changes significantly from
one galactic population to another. The results may be used to set constraints
for models of planet formation and chemical composition. Furthermore, the
results may have impact on our understanding of the frequency of planets in the
Galaxy, as well as on the existence of conditions for habitability.
|
1711.00777v1
|
2017-11-08
|
Matrix-assisted fabrication and exotic charge mobility of (Li,Fe)OHFeSe superconductor films
|
Superconducting (Li1-xFex)OHFe1-ySe films are attractive for both the basic
research and practical application. However, the conventional vapor deposition
techniques are not applicable in synthesizing the films of such a complex
system. So no intrinsic charge transport measurements on the films are
available so far to reveal the nature of charge carriers, which is fundamental
to understanding the iron-based superconductivity mechanism. Herein we report a
soft chemical film technique (matrix-assisted hydrothermal epitaxial growth),
by which we have succeeded in growing a series of (Li1-xFex)OHFe1-ySe films
covering the whole superconducting regime, with the superconducting transition
temperature (Tc) from 4 K up to 42 K. This film technique opens up a new way
for fabricating other complex functional materials as well. Furthermore, our
systematic transport investigation on the film samples indicates that both the
electron and hole carriers contribute to the charge transport, with the
scattering rates deviating from the Fermi liquid. We find that the
superconductivity occurs upon the electron and hole mobility becoming
divergent. And in the high Tc samples, the electron carriers are found much
more mobile than the holes, a feature distinct from the low Tc samples. Hence,
our transport results provide key insights into the underlying physics for
iron-based high-Tc superconductivity.
|
1711.02920v1
|
2017-11-17
|
Nematic superconducting state in iron pnictide superconductors
|
Nematic order often breaks the tetragonal symmetry of iron-based
superconductors. It arises from regular structural transition or electronic
instability in the normal phase. Here, we report the observation of a nematic
superconducting state, by measuring the angular dependence of the in-plane and
out-of-plane magnetoresistivity of Ba0.5K0.5Fe2As2 single crystals. We find
large twofold oscillations in the vicinity of the superconducting transition,
when the direction of applied magnetic field is rotated within the basal plane.
To avoid the influences from sample geometry or current flow direction, the
sample was designed as Corbino-shape for in-plane and mesa-shape for
out-of-plane measurements. Theoretical analysis shows that the nematic
superconductivity arises from the weak mixture of the quasi-degenerate s-wave
and d-wave components of the superconducting condensate, most probably induced
by a weak anisotropy of stresses inherent to single crystals.
|
1711.06383v1
|
2017-11-17
|
Frustrated magnetism in tetragonal CoSe, analogue to superconducting FeSe
|
Recently synthesized metastable tetragonal CoSe, isostructural to the FeSe
superconductor, offers a new avenue for investigating systems in close
proximity to the iron-based superconductors. We present magnetic and transport
property measurements on powders and single crystals of CoSe. High field
magnetic susceptibility measurements indicate a suppression of the previously
reported 10 K ferromagnetic transition with the magnetic susceptibility
exhibiting time-dependence below the proposed transition. Dynamic scaling
analysis of the time-dependence yields a critical relaxation time of $\tau^{*}
= 0.064 \pm 0.008 $ s which in turn yields an activation energy of $E_{a}^{*}$
= 14.84 $\pm$ 0.59 K and an ideal glass temperature $T_{0}^{*}$ = 8.91 $\pm$
0.09 K from Vogel-Fulcher analysis. No transition is observed in resistivity
and specific heat measurements, but both measurements indicate that CoSe is
metallic. These results are interpreted on the basis of CoSe exhibiting
frustrated magnetic ordering arising from competing magnetic interactions.
Arrott analysis of single crystal magnetic susceptibility has indicated that
the magnetic moments lie in the $ab$-plane so frustration may arise from
intralayer magnetic fluctuations as well as interlayer coupling. The results
have implications for understanding the superconductivity in the iron
chalcogenide systems as well as utilizing CoSe as a host for chemical and
physical manipulation to tune and explore emergent phenomena within an
expanding new class of transition metal chalcogenides.
|
1711.06725v2
|
2017-11-21
|
The X-ray footprint of the CircumNuclear Disk
|
We studied the central regions of the Galactic Centre to determine if the
CircumNuclear Disk (CND) acts as an absorber or a barrier for the central
X-rays diffuse emission. After reprocessing 4.6Ms of Chandra observations, we
were able to detect, for the first time, a depression in the X-ray luminosity
of the diffuse emission whose size and location correspond to those of the CND.
We extracted the X-ray spectra for various regions inside the CND footprint as
well as for the region where the footprint is observed and for a region located
outside the footprint. We simultaneously fitted these spectra as an optically
thin plasma whose absorption by the interstellar medium and by the local plasma
were fitted independently using the MCMC method. The hydrogen column density of
the ISM is 7.5x10^22 cm^-2. The X-ray diffuse emission inside the CND footprint
is formed by a 2T plasma of 1 and 4keV with slightly super-solar abundances
except for the iron and carbon which are sub-solar. The plasma from the CND, in
turn, is better described by a 1T model with abundances and local hydrogen
column density which are very different to those of the innermost regions. The
large iron abundance in this region confirms that the CND is dominated by the
shock-heated ejecta of the Sgr A East supernova remnant. We deduced that the
CND rather acts as a barrier for the Galactic Centre plasma and that the plasma
located outside the CND may correspond to the collimated outflow possibly
created by Sgr A* or the interaction between the wind of massive stars and the
mini-spiral material.
|
1711.07841v1
|
2017-11-28
|
SFXTs versus classical SgXBs: Does the difference lie in the companion wind?
|
We present a comparative study of stellar winds in classical supergiant high
mass X-ray binaries (SgXBs) and supergiant fast X-ray transients (SFXTs) based
on the analysis of publicly available out-of-eclipse observations performed
with Suzaku and XMM-Newton. Our data-set includes 55 observations of classical
SgXBs and 21 observations of SFXTs. We found that classical SgXBs are
characterized by a systematically higher absorption and luminosity compared to
the SFXTs, confirming the results of previous works in the literature.
Additionally, we show that the equivalent width of the fluorescence K{\alpha}
iron line in the classical SgXBs is significantly larger than that of the SFXTs
(outside X-ray eclipses). Based on our current understanding of the physics of
accretion in these systems, we conclude that the most likely explanation of
these differences is to be ascribed to the presence of mechanisms inhibiting
accretion for most of the time in the SFXTs and leading to a much less
efficient photoionization of the stellar wind compared to classical SgXBs.We do
not find evidence for the previously reported anti-correlation between the
equivalent width of the fluorescence iron line and the luminosity of SgXBs.
|
1711.10510v3
|
2017-12-04
|
Chemical fingerprints of hot Jupiter planet formation
|
The current paradigm to explain the presence of Jupiters with small orbital
periods (P $<$ 10 days; hot Jupiters) that involves their formation beyond the
snow line following inward migration, has been challenged by recent works that
explored the possibility of in situ formation. We aim to test whether stars
harbouring hot Jupiters and stars with more distant gas-giant planets show any
chemical peculiarity that could be related to different formation processes.
Our results show that stars with hot Jupiters have higher metallicities than
stars with cool distant gas-giant planets in the metallicity range +0.00/+0.20
dex. The data also shows a tendency of stars with cool Jupiters to show larger
abundances of $\alpha$ elements. No abundance differences between stars with
cool and hot Jupiters are found when considering iron peak, volatile elements
or the C/O, and Mg/Si ratios. The corresponding $p$-values from the statistical
tests comparing the cumulative distributions of cool and hot planet hosts are
0.20, $<$ 0.01, 0.81, and 0.16 for metallicity, $\alpha$, iron-peak, and
volatile elements, respectively. We confirm previous works suggesting that more
distant planets show higher planetary masses as well as larger eccentricities.
We note differences in age and spectral type between the hot and cool planet
hosts samples that might affect the abundance comparison. The differences in
the distribution of planetary mass, period, eccentricity, and stellar host
metallicity suggest a different formation mechanism for hot and cool Jupiters.
The slightly larger $\alpha$ abundances found in stars harbouring cool Jupiters
might compensate their lower metallicities allowing the formation of gas-giant
planets.
|
1712.01035v2
|
2017-12-04
|
Protonation induced high-Tc phases in iron-based superconductors evidenced by NMR and magnetization measurements
|
Chemical substitution during growth is a well-established method to
manipulate electronic states of quantum materials, and leads to rich spectra of
phase diagrams in cuprate and iron-based superconductors. Here we report a
novel and generic strategy to achieve nonvolatile electron doping in series of
(i.e. 11 and 122 structures) Fe-based superconductors by ionic liquid gating
induced protonation at room temperature. Accumulation of protons in bulk
compounds induces superconductivity in the parent compounds, and enhances the
Tc largely in some superconducting ones. Furthermore, the existence of proton
in the lattice enables the first proton nuclear magnetic resonance (NMR) study
to probe directly superconductivity. Using FeS as a model system, our NMR study
reveals an emergent high-Tc phase with no coherence peak which is hard to
measure by NMR with other isotopes. This novel electric-field-induced proton
evolution opens up an avenue for manipulation of competing electronic states
(e.g. Mott insulators), and may provide an innovative way for a broad
perspective of NMR measurements with greatly enhanced detecting resolution.
|
1712.01191v4
|
2017-12-06
|
Constraining the final fates of massive stars by oxygen and iron enrichment history in the Galaxy
|
Recent observational studies of core-collapse supernovae suggest only stars
with zero-age main sequence masses smaller than $16$-$18\ M_\odot$ explode when
they are red supergiants, producing type IIP supernovae. This may imply that
more massive stars produce other types of supernovae or they simply collapse to
black holes without giving rise to bright supernovae. This failed supernova
hypothesis can lead to significantly inefficient oxygen production because
oxygen abundantly produced in inner layers of massive stars with zero-age main
sequence masses around $20$-$30\ M_\odot$ might not be ejected into the
surrounding interstellar space. We first assume an unspecified population of
oxygen injection events related to massive stars and obtain a model-independent
constraint on how much oxygen should be released in a single event and how
frequently such events should happen. We further carry out one-box galactic
chemical enrichment calculations with different mass ranges of massive stars
exploding as core-collapse supernovae. Our results suggest that the model
assuming that all massive stars with $9$-$100\ M_\odot$ explode as
core-collapse supernovae is still most appropriate in explaining the solar
abundances of oxygen and iron and their enrichment history in the Galaxy. The
oxygen mass in the Galaxy is not explained when assuming that only massive
stars with zero-age main sequence masses in the range of 9-17 $M_\odot$,
contribute to the galactic oxygen enrichment. This finding implies that a good
fraction of stars more massive than $17M_\odot$ should eject their oxygen
layers in either supernova explosions or some other mass loss processes.
|
1712.02013v1
|
2017-12-06
|
Interaction of the accretion flows in corona and disk near the black hole in AGN
|
Accretion flows toward black holes can be of a quite different nature,
described as an optically thick cool gas flow in a disk for high accretion
rates or as a hot coronal optically thin gas flow for low accretion rates,
possibly affected by outflowing gas. The detection of broad iron emission lines
in active galactic nuclei (AGN) indicates the coexistence of corona and disk.
The appearance and relative strength of such flows essentially depends on their
interaction. Liu et al. suggested that condensation of gas from the corona to
the disk allows to understand accretion flows of comparable strength of
emission. Matter inflow due to gravitational capture of gas is important for
the condensation process. We discuss observational features predicted by the
model. Data from simultaneous observations of AGN with {\it {Swift's}} X-ray
and UV-optical telescopes are compared with the theoretical predictions. The
frequent detection of broad iron K$\alpha$ emission lines and the dependence of
the emitted spectra on the Eddington ratio, described by the values of the
photon index $\Gamma$ and the two-point spectral index $\alpha_{\rm{ox}}$ are
in approximate agreement with the predictions of the condensation model; the
latter, however, with a large scatter. The model further yields a coronal
emission concentrated in a narrow inner region as is also deduced from the
analysis of emissivity profiles. The accretion flows in bright AGN could be
described by the accretion of stellar wind or interstellar medium and its
condensation into a thin disk.
|
1712.02031v1
|
2017-12-24
|
Search For Gravitational Redshifted Absorption Lines In LMXB Serpens X-1
|
The equation of state for ultra-dense matter can be tested from observations
of the ratio of mass to radius of neutron stars. This could be measured
precisely from the redshift of a narrow line produced on the surface. X-rays
bursts have been intensively searched for such features, but so far without
detection. Here instead we search for redshifted lines in the persistent
emission, where the accretion flow dominates over the surface emission. We
discuss the requirements for narrow lines to be produced, and show that narrow
absorption lines from highly ionized iron can potentially be observable in
accreting low mass X-ray binaries (low B field) which have either low spin or
low inclination so that Doppler broadening is small. This selects Serpens X-1
as the only potential candidate persistent LMXB due to its low inclination.
Including surface models in the broad band accretion flow model predicts that
the absorption line from He-like iron at 6.7 keV should be redshifted to $\sim$
5.1 - 5.7 keV (10 - 15 km for $1.4\mathrm{M_\odot}$) and have an equivalent
width of 0.8 - 8.0 eV for surface temperatures of 7 - 10 $\times$ 10$^6$ K. We
use the high resolution Chandra grating data to give a firm upper limit of 2 -
3 eV for an absorption line at $\sim 5$ keV. We discuss possible reasons for
this lack of detection (the surface temperature and the geometry of the
boundary layer etc.). Future instruments with better sensitivity are required
in order to explore the existence of such features.
|
1712.08918v1
|
2018-01-09
|
Diverse fluctuations and anisotropic Gr{\" u}neisen parameter behavior in iron-based superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and their correlation with superconductivity
|
In this study, the temperature dependence of elastic constants $C_{11}$,
$C_{33}$, $C_{\rm E} = (C_{11}-C_{12})/2$, $C_{66}$ and $C_{44}$ of the
iron-based superconductor Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (0 $\leqq x
\leqq$ 0.245) have been measured. This system shows a large elastic softening
in $C_{66}$ towards low temperatures. In addition to $C_{66}$, which originates
from orthorhombic structural fluctuation, the samples near the optimal
concentration show remarkable structural fluctuation in $C_{11}$ and $C_{33}$
elastic modes, which correspond to $\Gamma_{1}$ (C4) symmetry. It suggests the
existence of diverse fluctuations in this system. Gr{\" u}neisen parameters
were analyzed under some assumptions for structural and magnetic transition
temperatures. Results showed that the Gr{\" u}neisen parameters for the
inter-plane strain are remarkably enhanced toward the QCP, while those for the
in-plane stress tend to turn down near the QCP. Gr{\" u}neisen parameters for
the superconducting transition are anisotropic and shows remarkable
Co-concentration dependence, suggesting that the in-plane isotropic compression
and inter-layer elongation enhance the superconductivity. The correlation of
Gr{\" u}neisen parameters between $T_{\rm S}$, $T_{\rm N}$ and $T_{\rm sc}$
shows $c$-axis elongation and its relevant role in the emergence of
superconductivity in this system.
|
1801.02791v1
|
2018-01-13
|
Advanced first-principles theory of superconductivity including both lattice vibrations and spin fluctuations: The case of FeB$_4$
|
We present an advanced method to study spin fluctuations in superconductors
quantitatively, and entirely from first principles. This method can be
generally applied to materials where electron-phonon coupling and spin
fluctuations coexist. We employ it here to examine the recently synthesized
superconductor iron tetraboride (FeB$_4$) with experimental $T_{\mathrm{c}}\sim
2.4$ K [H. Gou \textit{et al.}, Phys. Rev. Lett. \textbf{111}, 157002 (2013)].
We prove that FeB$_4$ is particularly prone to ferromagnetic spin fluctuations
due to the presence of iron, resulting in a large Stoner interaction strength,
$I=1.5$ eV, as calculated from first principles. The other important factor is
its Fermi surface that consists of three separate sheets, among which two
nested ellipsoids. The resulting susceptibility has a ferromagnetic peak around
$\textbf{q}=0$, from which we calculated the repulsive interaction between
Cooper pair electrons using the random phase approximation. Subsequently, we
combined the electron-phonon interaction calculated from first principles with
the spin fluctuation interaction in fully anisotropic Eliashberg theory
calculations. We show that the resulting superconducting gap spectrum is
conventional, yet very strongly depleted due to coupling to the spin
fluctuations. The critical temperature decreases from $T_{\mathrm{c}}= 41$ K,
if they are not taken into account, to $T_{\mathrm{c}}= 1.7$ K, in good
agreement with the experimental value.
|
1801.04421v1
|
2018-01-15
|
Unconventional Charge Density Wave Order in the Pnictide Superconductor Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$
|
Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ is a structural homologue of the pnictide high
temperature superconductor, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, in which the Fe
atoms are replaced by Ni. Superconductivity is highly suppressed in this
system, reaching a maximum $T_c$ = 2.3 K, compared to 24 K in its iron-based
cousin, and the origin of this $T_c$ suppression is not known. Using x-ray
scattering, we show that Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ exhibits a
unidirectional charge density wave (CDW) at its triclinic phase transition. The
CDW is incommensurate, exhibits a sizable lattice distortion, and is
accompanied by the appearance of $\alpha$ Fermi surface pockets in
photoemission [B. Zhou et al., Phys. Rev. B 83, 035110 (2011)], suggesting it
forms by an unconventional mechanism. Co doping suppresses the CDW, paralleling
the behavior of antiferromagnetism in iron-based superconductors. Our study
demonstrates that pnictide superconductors can exhibit competing CDW order,
which may be the origin of $T_c$ suppression in this system.
|
1801.04874v2
|
2018-01-26
|
Models for the Unusual Supernova iPTF14hls
|
Supernova iPTF14hls maintained a bright, variable luminosity for more than
600 days, while lines of hydrogen and iron in its spectrum had different
speeds, but showed little evolution. Here several varieties of models are
explored for iPTF14hls-like events. They are based upon circumstellar medium
(CSM) interaction in an ordinary supernova, pulsational pair-instability
supernovae (PPISN), and magnetar formation. Each is able to explain the
enduring emission and brightness of iPTF14hls, but has shortcomings when
confronted with other observed characteristics. The PPISN model can, in some
cases, produce a presupernova transient like the one observed at the site of
iPTF14hls in 1954. It also offers a clear path to providing the necessary half
solar mass of material at $\sim 5 \times 10^{16}$ cm for CSM interaction to
work, and can give an irregular light curve without invoking additional
assumptions. It explains the 4000 km s$^{-1}$ seen in the iron lines, but
without additional energy input, strains to explain the nearly constant 8000 km
s$^{-1}$ velocity seen in H$_{\alpha}$. Magnetar models can also explain many
of the observed features, but give a smooth light curve and may require an
evolving magnetic field strength. Their dynamics may be difficult to reconcile
with the observation of slow-moving hydrogen at late times. The various models
predict different spectral characteristics and a remnant that, today, could be
a black hole, magnetar, or even a star. Further observations and calculations
of radiation transport will narrow the range of possibilities.
|
1801.08666v1
|
2018-03-01
|
Hydrogen Diffusion and Trapping in α-Iron: The Role of Quantum and Anharmonic Fluctuations
|
We investigate the thermodynamics and kinetics of a hydrogen interstitial in
magnetic {\alpha}-iron, taking account of the quantum fluctuations of the
proton as well as the anharmonicities of lattice vibrations and hydrogen
hopping. We show that the diffusivity of hydrogen in the lattice of BCC iron
deviates strongly from an Arrhenius behavior at and below room temperature. We
compare a quantum transition state theory to explicit ring polymer molecular
dynamics in the calculation of diffusivity and we find that the role of phonons
is to inhibit, not to enhance, diffusivity at intermediate temperatures in
constrast to the usual polaron picture of hopping. We then address the trapping
of hydrogen by a vacancy as a prototype lattice defect. By a sequence of steps
in a thought experiment, each involving a thermodynamic integration, we are
able to separate out the binding free energy of a proton to a defect into
harmonic and anharmonic, and classical and quantum contributions. We find that
about 30% of a typical binding free energy of hydrogen to a lattice defect in
iron is accounted for by finite temperature effects and about half of these
arise from quantum proton fluctuations. This has huge implications for the
comparison between thermal desorption and permeation experiments and standard
electronic structure theory. The implications are even greater for the
interpretation of muon spin resonance experiments.
|
1803.00600v1
|
2018-03-02
|
Topological Dirac semimetal phase in the iron-based superconductor Fe(Te,Se)
|
Topological Dirac semimetals (TDSs) exhibit bulk Dirac cones protected by
time reversal and crystal symmetry, as well as surface states originating from
non-trivial topology. While there is a manifold possible onset of
superconducting order in such systems, few observations of intrinsic
superconductivity have so far been reported for TDSs. We observe evidence for a
TDS phase in FeTe$_{1-x}$Se$_x$ ($x$ = 0.45), one of the high transition
temperature ($T_c$) iron-based superconductors. In angle-resolved photoelectron
spectroscopy (ARPES) and transport experiments, we find spin-polarized states
overlapping with the bulk states on the (001) surface, and linear
magnetoresistance (MR) starting from 6 T. Combined, this strongly suggests the
existence of a TDS phase, which is confirmed by theoretical calculations. In
total, the topological electronic states in Fe(Te,Se) provide a promising high
$T_c$ platform to realize multiple topological superconducting phases.
|
1803.00845v1
|
2018-03-20
|
Further insight on the hypervelocity white dwarf, LP 40-365 (GD 492): a nearby emissary from a single-degenerate Type Ia supernova
|
The recently discovered hypervelocity white dwarf LP 40-65 (aka GD 492) has
been suggested as the outcome of the failed disruption of a white dwarf in a
sub-luminous Type Ia supernova (SN Ia). We present new observations confirming
GD 492 as a single star with unique spectral features. Our spectroscopic
analysis suggests that a helium-dominated atmosphere, with ~ 33 percent neon
and 2 percent oxygen by mass, can reproduce most of the observed properties of
this highly unusual star. Although our atmospheric model contrasts with the
previous analysis in terms of dominant atmospheric species, we confirm that the
atmosphere of GD 492 is strongly hydrogen deficient, log(H/He) < -5, and
displays traces of eleven other alpha- and iron-group elements (with sulfur,
chromium, manganese, and titanium as new detections), indicating nuclear
processing of carbon and silicon. We measure a manganese-to-iron ratio seven
times larger than Solar. While the observed abundances of GD 492 do not fully
match any predicted nuclear yields of a partially-burned supernova remnant, the
manganese excess strongly favors a link with a single-degenerate SN Ia event
over alternative scenarios.
|
1803.07564v1
|
2018-04-07
|
Bare and Polymer Coated Iron Oxide Superparamagnetic Nanoparticles for Effective Removal of U (VI) from Acidic and Neutral Aqueous Medium
|
Superparamagnetic {\gamma}-Fe2O3 nanoparticles (5 nm diameter) were
synthesized in water. The bare particles exhibit good colloidal stability at ~
pH 2 because of the strong electrostatic repulsion with a surface charge of +25
mV. The polyacrylic acid (PAA)-coated particles exhibit remarkable colloidal
stability at ~ pH 7 with abundant free carboxyl groups as reactive sites for
subsequent functionalization. In this work, we used zeta potential analysis,
transmission electron microscopy, small angle X-ray scattering, and Inductively
coupled plasma mass spectrometry to investigate the adsorption behavior of U
(VI) on bare and coated colloidal superparamagnetic nanoparticles at pH 2 and
pH 7. At pH 2, uranyl ion (UO22+) absorbed on the surface of the bare particles
with decreasing particle surface charge. This induced particle agglomeration.
At pH 7, uranyl ion (UO22+) hydrolyzed and formed plate-like particles of
uranium hydroxide that were ~ 50 nm in diameter. The PAA-coated iron oxide
nanoparticles absorbed on the surface of these U (VI) hydroxide plates to form
large aggregates that precipitate to the bottom of the dispersion. At both pH 2
and pH 7, the resulting U (VI)/nanoparticle complex can be easily collected and
extracted from the aqueous environment via an external magnetic field. The
results show that both bare and polymer-coated superparamagnetic {\gamma}-Fe2O3
nanoparticles are potential absorbents for removing U (VI) from water.
|
1804.02522v1
|
2018-04-10
|
Absence of superconductivity in iron polyhydrides at high pressures
|
Recently, C. M. P\'epin \textit{et al.} [Science \textbf{357}, 382 (2017)]
reported the formation of several new iron polyhydrides FeH$_x$ at pressures in
the megabar range, and spotted FeH$_5$, which forms above 130 GPa, as a
potential high-\tc \ superconductor, because of an alleged layer of dense
metallic hydrogen. Shortly after, two studies by A.~Majumdar \textit{et al.}
[Phys. Rev. B \textbf{96}, 201107 (2017)] and A.~G.~Kvashnin \textit{et al.}
[J. Phys. Chem. C \textbf{122}, 4731 (2018)] based on {\em ab initio}
Migdal-Eliashberg theory seemed to independently confirm such a conjecture. We
conversely find, on the same theoretical-numerical basis, that neither FeH$_5$
nor its precursor, FeH$_3$, shows any conventional superconductivity and
explain why this is the case. We also show that superconductivity may be
attained by transition-metal polyhydrides in the FeH$_3$ structure type by
adding more electrons to partially fill one of the Fe--H hybrid bands (as,
e.g., in NiH$_3$). Critical temperatures, however, will remain low because the
$d$--metal bonding, and not the metallic hydrogen, dominates the behavior of
electrons and phonons involved in the superconducting pairing in these
compounds.
|
1804.03572v2
|
2018-04-13
|
A Long Serendipitous XMM-Newton Observation of the Intermediate Polar XY Ari
|
XY Ari is one of the few known eclipsing intermediate polars. We present
results from a detailed analysis of an unpublished archival observation using
XMM-Newton EPIC pn and MOS data in a quiescent state of XY Ari. The X-ray
orbital modulation and spin pulse variations were investigated for energy
dependent modulations in different energy bands. The broad orbital modulation
observed with various observations was confirmed with XMM-Newton at hard X-ray
(>1.6 keV). The EPIC light curves folded at the spin phases show a double peak
profile as expected from two pole accretion. The pulse profile is found to be
energy dependent. Hardness ratio variations and energy modulation depth during
spin modulation can be explained by photoelectric absorption. The
simultaneously fitted EPIC spectra with CEVMKL model yield maximum plasma
temperature of $28^{+3.1}_{-2.9}$ keV with an iron abundance
$\mathrm{Fe}/\mathrm{Fe}_\odot=0.37^{+0.06}_{-0.05}$. We find two intrinsic
partial covering absorption columns of $6.2^{+1.0}_{-0.9} \times 10^{22}$ and
$105.3^{+35.4}_{-30.4} \times 10^{22} \,\mathrm{cm^{-2}}$ with covering
fractions of $0.53^{+0.05}_{-0.04}$, $0.41^{+0.14}_{-0.13}$ respectively. In
addition, a Gaussian emission line at $6.43^{+0.01}_{-0.02}$ keV with an
equivalent width of $51^{+12}_{-10}$ eV is required to account for fluorescent
emission from neutral iron. The X-ray luminosity of the source is $4.2 \times
10^{32} \,\mathrm{erg \,s^{-1}}$ in the 0.2-10.0 keV energy band.
|
1804.04825v1
|
2018-04-23
|
Spectral and Timing Properties of Atoll Source 4U 1705-44 : LAXPC/AstroSat Results
|
In this paper, we present the first results of spectral and timing properties
of the atoll source 4U 1705-44 using $\sim$ 100 ks data obtained with Large
Area X-ray Proportional Counter (LAXPC) onboard {\it AstroSat}. The source was
in the high-soft state during our observations and traced out a {\it banana
track} in the Hardness Intensity Diagram (HID). We study {\bf the} evolution of
the Power Density Spectra (PDS) and the energy spectra along the HID. PDS show
presence of a broad Lorentzian feature (Peaked Noise or PN) centered at $1-13$
Hz and a very low frequency noise (VLFN). The energy spectra can be described
by sum of a thermal Comptonized component, a power-law and a broad iron line.
The hard tail seen in the energy spectra is variable and contribute $4-30$\% of
the total flux. The iron line seen in this source is broad (FWHM $\sim$ 2 keV)
and strong (EW $\sim$ $369-512$ eV). Only relativistic smearing in the
accretion disc can not explain the origin of this feature and requires other
mechanism such as broadening by Comptonization process in the external part of
the `Comptonized Corona'. A subtle and systematic evolution of the spectral
parameters (optical depth, electron temperature etc.) is seen as the source
moves along the HID. We study the correlation between frequency of the PN and
the spectral parameters. PN frequency seems to be correlated with the strength
of the corona. We discuss the implication of the results in the paper.
|
1804.08371v1
|
2018-05-10
|
A Tale of Two Metals: contrasting criticalities in the pnictides and hole-doped cuprates
|
The iron-based high temperature superconductors share a number of
similarities with their copper-based counterparts, such as reduced
dimensionality, proximity to states of competing order, and a critical role for
3d electron orbitals. Their respective temperature-doping phase diagrams also
contain certain commonalities that have led to claims that the metallic and
superconducting properties of both families are governed by their proximity to
a quantum critical point (QCP) located inside the superconducting dome. In this
review, we critically examine these claims and highlight significant
differences in the bulk physical properties of both systems. While there is now
a large body of evidence supporting the presence of a (magnetic) QCP in the
iron pnictides, the situation in the cuprates is much less apparent, at least
for the end point of the pseudogap phase. We argue that the opening of the
normal state pseudogap in cuprates, so often tied to a putative QCP, arises
from a momentum-dependent breakdown of quasiparticle coherence that sets in at
much higher doping levels but which is driven by the proximity to the Mott
insulating state at half filling. Finally, we present a new scenario for the
cuprates in which this loss of quasiparticle integrity and its evolution with
momentum, temperature and doping plays a key role in shaping the resultant
phase diagram.
|
1805.03866v1
|
2018-05-10
|
Stabilization of $s$-wave superconductivity through arsenic $p$-orbital hybridization in electron-doped BaFe$_2$As$_2$
|
Using random-phase approximation spin-fluctuation theory, we study the
influence of the hybridization between iron $d$-orbitals and pnictide
$p$-orbitals on the superconducting pairing state in iron-based
superconductors. The calculations are performed for a 16-orbital Hubbard-Hund
tight-binding model of BaFe$_2$As$_2$ that includes the As-$p$ orbital degrees
of freedom in addition to the Fe-$d$ orbitals and compared to calculations for
a 10-orbital Fe-$d$ only model. In both models we find a leading $s^\pm$
pairing state and a subleading $d_ {x^2-y^2}$-wave state in the parent
compound. Upon doping, we find that the $s^\pm$ state remains the leading state
in the 16-orbital model up to a doping level of 0.475 electrons per unit cell,
at which the hole Fermi surface pockets at the zone center start to disappear.
This is in contrast to the 10-orbital model, where the $d$-wave state becomes
the leading state at a doping of less than 0.2 electrons. This improved
stability of $s^\pm$ pairing is found to arise from a decrease of $d_{xy}$
orbital weight on the electron pockets due to hybridization with the As-$p$
orbitals and the resulting reduction of near $(\pi,\pi)$ spin-fluctuation
scattering which favors the competing $d$-wave state. These results show that
the orbital dependent hybridization of Fermi surface Bloch states with the
usually neglected $p$-orbital states is an important ingredient in an improved
itinerant pairing theory.
|
1805.04181v2
|
2018-05-30
|
Tracking the Iron K$α$ line and the Ultra Fast Outflow in NGC 2992 at different accretion states
|
The Seyfert 2 galaxy NGC 2992 has been monitored eight times by XMM-Newton in
2010 and then observed again in 2013, while in 2015 it was simultaneously
targeted by Swift and NuSTAR. XMM-Newton always caught the source in a faint
state (2-10 keV fluxes ranging from 0.3 to 1.6$\times10^{-11}$ erg cm$^{-2}$
s$^{-1}$) but NuSTAR showed an increase in the 2-10 keV flux up to
6$\times10^{-11}$ erg cm$^{-2}$ s$^{-1}$. We find possible evidence of an Ultra
Fast Outflow with velocity $v_1=0.21\pm0.01c$ (detected at about 99% confidence
level) in such a flux state. The UFO in NGC 2992 is consistent with being
ejected at a few tens of gravitational radii only at accretion rates greater
than 2% of the Eddington luminosity. The analysis of the low flux 2010/2013 XMM
data allowed us to determine that the Iron K$\alpha$ emission line complex in
this object is likely the sum of three distinct components: a constant, narrow
one due to reflection from cold, distant material (likely the molecular torus);
a narrow, but variable one which is more intense in brighter observations and a
broad relativistic one emitted in the innermost regions of the accretion disk,
which has been detected only in the 2003 XMM observation.
|
1805.12149v1
|
2018-06-04
|
Towards a systematically improvable many-body description of antiferromagnetic iron oxide
|
We report variational and fixed-node diffusion quantum Monte Carlo (QMC)
calculations of anti-ferromagnetic iron oxide (FeO) in the ground state B1
crystal structure. The goal of this study was a systematic investigation of the
sensitivity of several ground state properties to a variety of QMC wave
function generation techniques including advanced wave functions such as
multi-determinant expansions and backflow transformations. We found that the
predicted lattice distortion was largely controlled by the choice of single
particle orbitals used to construct the wave function, rather than by
subsequent wave function optimization techniques within QMC. However, the
absolute magnetic moment was remarkably insensitive to the method of wave
function construction. QMC estimates of total spin density indicate that in
addition to strong electronic correlation of the Fe $3d$ states, charge
transfer may be an important but challenging piece of physics to accurately
capture within existing QMC methods. Finally, we highlight the need for
advanced and systematically improvable many-body wave functions suitable for
accurately describing challenging real systems.
|
1806.01383v2
|
2018-09-04
|
Anomalous peak effect in iron-based superconductors Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($x \approx$ 0.69 and 0.76) for magnetic-field directions close to the $ab$ plane and its possible relation to the spin paramagnetic effect
|
We report magnetic torque measurements on iron-pnictide superconductors
Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($x \approx$ 0.69 and 0.76) up to an applied field
of $B_a$ = 45 T. The peak effect is observed in torque-vs-field curves below
the irreversibility field. It is enhanced and becomes asymmetric as the field
is tilted from the $c$ axis. For field directions close to the $ab$ plane,
increasing- and decreasing-field curves peak at markedly different fields, and
exhibit a sharp jump, suggestive of a first-order phase transition, on the
high- and the low-field side of the peak, respectively. Complicated history
dependence of the torque is observed in the peak-effect region. We construct
and discuss the temperature ($T$)--applied-magnetic-field ($B_a$) phase
diagram. Since the upper critical field for the $ab$-plane direction is
comparable to the Pauli limit, we also consider possible influence of the spin
paramagnetic effect on the anomalous peak effect.
|
1809.00762v3
|
2018-09-07
|
Electron-magnon scattering in elementary ferromagnets from first principles: lifetime broadening and band anomalies
|
We study the electron-magnon scattering in bulk Fe, Co, and Ni within the
framework of many-body perturbation theory implemented in the full-potential
linearized augmented-plane-wave method. To this end, a $\mathbf{k}$-dependent
self-energy ($GT$ self-energy) describing the scattering of electrons and
magnons is constructed from the solution of a Bethe-Salpeter equation for the
two-particle (electron-hole) Green function, in which single-particle Stoner
and collective spin-wave excitations (magnons) are treated on the same footing.
Partial self-consistency is achieved by the alignment of the chemical
potentials. The resulting renormalized electronic band structures exhibit
strong spin-dependent lifetime effects close to the Fermi energy, which are
strongest in Fe. The renormalization can give rise to a loss of quasiparticle
character close to the Fermi energy, which we attribute to electron scattering
with spatially extended spin waves. This scattering is also responsible for
dispersion anomalies in conduction bands of iron and for the formation of
satellite bands in nickel. Furthermore, we find a band anomaly at a binding
energy of 1.5~eV in iron, which results from a coupling of the quasihole with
single-particle excitations that form a peak in the Stoner continuum. This band
anomaly was recently observed in photoemission experiments. On the theory side,
we show that the contribution of the Goldstone mode to the $GT$ self-energy is
expected to (nearly) vanish in the long-wavelength limit. We also present an
in-depth discussion about the possible violation of causality when an
incomplete subset of self-energy diagrams is chosen.
|
1809.02395v2
|
2018-09-21
|
Electronic Spin transition in FeO$_{2}$: evidence for Fe(II) with peroxide O$_{2}^{2-}$
|
The discovery of FeO$_{2}$ containing more oxygen than hematite
(Fe$_{2}$O$_{3}$) that was previously believed to be the most oxygen rich iron
compounds, has important implications on the study of the deep lower mantle
compositions. Compared to other iron compounds, there are limited reports on
FeO$_{2}$ making studies of its physical properties of great interest in
fundamental condensed matter physics and geoscience. Even the oxidation state
of Fe in FeO$_{2}$ is the subject of debate in theoretical works and there have
not been reports from experimental electronic and magnetic properties
measurements. Here, we report the pressure-induced spin state transition from
synchrotron experiments and our computational results explain the underlying
mechanism. Using density functional theory and dynamical mean field theory, we
calculated spin states of Fe with volume and Hubbard interaction $U$ change,
which clearly demonstrate that Fe in FeO$_{2}$ consists of Fe(II) and peroxide
O$_{2}^{2-}$. Our study suggests that localized nature of both Fe 3$d$ orbitals
and O$_{2}$ molecular orbitals should be correctly treated for unveiling the
structural and electronic properties of FeO$_{2}$.
|
1809.07969v1
|
2018-09-24
|
The Variable Relativistic Outflow of IRAS 13224-3809
|
The discovery of an ultrafast outflow has been reported in the z=0.0658
narrow line Seyfert galaxy IRAS 13224-3809 (Parker et al. 2017a). The ultrafast
outflow was first inferred through the detection of highly blueshifted
absorption lines (Parker et al. 2017a) and then confirmed with a principal
component analysis (PCA) (Parker et al. 2017b). Two of the reported properties
of this outflow differed from those typically detected in other AGN with
ultrafast outflows. First, the outflow velocity was found not to vary with
v=0.236c +/- 0.006c. Second, the equivalent width of the highly blueshifted
absorption line was reported to be anti-correlated with the 3-10 keV flux of
this source. We present a re-analysis of the XMM-Newton observations of IRAS
13224-3809 considering the influence of background. We also undertook a
different analysis approach in combining the spectra and investigated the
change of the properties of the outflow as a function of 3-10 keV flux and
time. We confirm the presence of an ultrafast outflow in IRAS 13224-3809,
however, we find that the background spectra used in the Parker et al. analyses
dominate the source spectra for energies near the blueshifted iron lines. By
reducing the source extraction regions to improve the signal-to-noise ratio we
discover larger than previously reported outflow velocities and find that the
outflow velocity varies from ~0.2c to ~0.3c and increases with 3-10~keV flux.
The previously reported anti-correlation between equivalent width of the iron
line and 3-10 keV flux disappears when the background spectra are reduced by
optimizing the source extraction regions.
|
1809.09138v1
|
2018-11-06
|
Energy scale of nematic ordering in the parent iron-based superconductor:BaFe2As2
|
Nematicity plays an important role in the physics of iron-based
superconductors (IBS). Its microscopic origin and in particular its importance
for the mechanism of high-temperature superconductivity itself are highly
debated. A crucial knowledge in this regard is the degree to which the nematic
order influences the electronic structure of these materials. Earlier
angle-resolved photoemission spectroscopy (ARPES) studies found that the effect
is dramatic in three families of IBS including 11, 111 and 122 compounds:
energy splitting reaches 70 meV and Fermi surface becomes noticeably distorted.
More recent experiments, however, reported significantly lower energy scale in
11 and 111 families, thus questioning the degree and universality of the impact
of nematicity on the electronic structure of IBS. Here we revisit the
electronic structure of undoped parent BaFe2As2 (122 family). Our systematic
ARPES study including the detailed temperature and photon energy dependencies
points to the significantly smaller energy scale also in this family of
materials, thus establishing the universal scale of this phenomenon in IBS. Our
results form a necessary quantitative basis for theories of high-temperature
superconductivity focused on the nematicity.
|
1811.02430v2
|
2018-11-14
|
Giant anisotropy in superconducting single crystals of CsCa$_2$Fe$_4$As$_4$F$_2$
|
CsCa$_2$Fe$_4$As$_4$F$_2$ is a newly discovered iron-based superconductor
with $T_\mathrm{c}\sim$ 30 K containing double Fe$_2$As$_2$ layers that are
separated by insulating Ca$_2$F$_2$ spacer layers. Here we report the transport
and magnetization measurements on CsCa$_2$Fe$_4$As$_4$F$_2$ single crystals
grown for the first time using the self flux of CsAs. We observed a huge
resistivity anisotropy $\rho_c(T)/\rho_{ab}(T)$, which increases with
decreasing temperature, from 750 at 300 K to 3150 at 32 K. The $\rho_c(T)$ data
exhibit a non-metallic behavior above $\sim$140 K, suggesting an incoherent
electronic state at high temperatures due to the dimension crossover. The
superconducting onset transition temperature in $\rho_{ab}$ is 0.7 K higher
than that in $\rho_c$, suggesting two-dimensional (2D) superconducting
fluctuations. The lower and upper critical fields also show an exceptional
anisotropy among iron-based superconductors. The $H_{c1}^\bot(T)$ data are well
fitted using the model with two $s$-wave-like superconducting gaps,
$\Delta_1(0)=6.75$ meV and $\Delta_2(0)=2.32$ meV. The inter-plane coherence
length $\xi_c(0)$ is $3.6$ \AA, remarkably smaller than the distance between
conducting layers (8.6 \AA), consolidating the 2D nature in the title material.
|
1811.05706v1
|
2018-12-04
|
Adsorption and dissociation of iron phthalocyanine on H/Si(111): Impact of van-der-Waals interactions and perspectives for subsurface doping
|
The adsorption of iron phthalocyanine (FePc) on the passivated H/Si(111)
surface is explored from first principles. We find that the organic molecule is
predominantly physisorbed with a distance to the surface of $2.6 \pm 0.1$
Angstrom, but also exhibits sizable resonance with the underlying substrate.
This establishes the present system as interesting mixed
covalent-van-der-Waals-bound test case, which we use to compare the impact of
different approaches to van-der-Waals interactions. (Spin-polarized) scanning
tunneling microscopy (SP STM) images are simulated, selectively accessing
different molecular orbitals via the applied bias voltage in the spirit of
scanning tunneling spectroscopy. Comparison with experimental STM images
reveals very good agreement. We find a significant magnetic contrast exceeding
$\pm 1$ Angstrom in the SP STM images for $-2$ and $+1.5$ V. Binding energies
of different (transition metal) atoms in the center of the Pc ring are
presented, which particularly show that Fe is strongly bound in the molecule
(about $9.6$ eV). Finally, we discuss different reactions for subsurface doping
by room-temperature FePc deposition and point out two feasible reactions.
|
1812.01639v3
|
2018-12-06
|
Artificially Engineered Nanostrain in Iron Chalcogenide Superconductor Thin Film for Enhancing Supercurrent
|
Although nanoscale deformation, such as nanostrain in iron chalcogenide
(FeSexTe1-x, FST) thin films, has attracted attention owing to the enhancement
of general superconducting properties, including critical current density (Jc)
and critical transition temperature, its formation has proven to be an
extremely challenging and complex process thus far. Herein, we successfully
fabricated an epitaxial FST thin film with uniformly distributed nanostrain by
injection of a trace amount of CeO2 inside FST matrix using sequential pulsed
laser deposition. Using transmission electron microscopy and geometrical phase
analysis, we verified that a trace amount of CeO2 injection forms nanoscale
fine defects with a nanostrained region, which has a tensile strain (ezz ~
0.02) along the c-axis of the FST matrix. The nanostrained FST thin film
achieves a remarkable Jc of 3.5 MA/cm2 for a self-field at 6 K and a highly
enhanced Jc under the entire magnetic field with respect to a pristine FST thin
film.
|
1812.02380v3
|
2018-12-13
|
Emergent XY electronic nematicity in iron-based superconductors
|
Electronic nematicity, a correlated state that spontaneously breaks
rotational symmetry, is observed in several layered quantum materials. In
contrast to their liquid-crystal counterparts, the nematic director cannot
usually point in an arbitrary direction (XY nematics), but is locked by the
crystal to discrete directions (Ising nematics),resulting in strongly
anisotropic fluctuations above the transition. Here, we report on the
observation of nearly isotropic XY-nematic fluctuations, via elastoresistance
measurements, in hole-doped Ba$_{1-x}$Rb$_{x}$Fe$_{2}$As$_{2}$ iron-based
superconductors. While for $x=0$ the nematic director points along the in-plane
diagonals of the tetragonal lattice, for $x=1$ it points along the horizontal
and vertical axes. Remarkably, for intermediate doping, the susceptibilities of
these two symmetry-irreducible nematic channels display comparable Curie-Weiss
behavior, thus revealing a nearly XY-nematic state. This opens a new route to
assess this elusive electronic quantum liquid-crystalline state, which is a
candidate to host unique phenomena not present in the Ising-nematic case.
|
1812.05267v2
|
2018-12-13
|
Visualizing the Nonlinear Coupling between Strain and Electronic Nematicity in the Iron Pnictides by Elasto-Scanning Tunneling Spectroscopy
|
Mechanical strain is a powerful technique for tuning electronic structure and
interactions in quantum materials. In a system with tetragonal symmetry, a
tunable uniaxial in-plane strain can be used to probe nematic correlations in
the same way that a tunable magnetic field is used to probe magnetic
correlations. Here, we present a new spectroscopic scanned probe technique that
provides atomic-resolution insight into the effect of anisotropic strain on the
electronic structure. We use this technique to study nematic fluctuations and
nematic order across the phase diagram of a prototypical iron-based
superconductor. By extracting quantitatively the electronic anisotropy as
function of applied strain, we show that while true long range nematic order is
established at the tetragonal to orthorhombic structural transition
temperature, sizable nematic fluctuations persist to high temperatures and also
to the overdoped end of the superconducting dome. Remarkably, we find that
uniaxial strain in the pnictides significantly enhances the amplitude of the
nematic fluctuations, indicating a strong nonlinear coupling between structure
and electronic nematicity.
|
1812.05287v1
|
2018-12-20
|
The long-term evolution and appearance of Type Iax postgenitor stars
|
Type Iax supernovae may arise from failed explosions of white dwarfs that
leave behind a bound remnant (i.e., a "postgenitor" star) that could be
identified in wide field surveys. To understand their observational signatures,
we simulate these white dwarf (WD) postgenitors from shortly after explosion
until they move back down the WD cooling track, and we consider several
possible WD masses and explosion energies. To predict the peculiar surface
abundances of the WD postgenitors, our models take into account gravitational
settling and radiative levitation. We find that radiative levitation is
significant at temperatures above a mass-dependent critical temperature,
typically in the range Teff ~ 50-100 * 10^3 K, significantly increasing surface
abundances of iron-group elements. Due to enhanced iron group opacity compared
to normal WDs, the postgenitor peak luminosity and cooling timescale depend
sensitively on mass, with more massive WDs becoming brighter but cooling much
faster. We discuss our results in light of recently discovered hypervelocity
white dwarfs with peculiar surface compositions, finding that our low-mass
postgenitor models match many of their observational characteristics. Finally,
we explore the effects of thermohaline diffusion, tentatively finding that it
strongly suppresses abundance enhancements created by radiative levitation, but
more realistic modeling is required to reach a firm conclusion.
|
1812.08793v2
|
2019-02-14
|
Photoionization Emission Models for the Cyg X-3 X-ray Spectrum
|
We present model fits to the X-ray line spectrum of the well known High Mass
X-ray binary Cyg X-3. The primary observational dataset is a spectrum taken
with the $Chandra$ X-ray Observatory High Energy Transmission Grating (HETG) in
2006, though we compare it to all the other observations of this source taken
so far by this instrument. We show that the density must be $\geq 10^{12}$
cm$^{-3}$ in the region responsible for most of the emission. We discuss the
influence of the dust scattering halo on the broad band spectrum and we argue
that dust scattering and extinction is not the most likely origin for the
narrow featureseen near the Si K edge. We identify the features of a wind in
the profiles of the strong resonance lines and we show that the wind is more
apparent in the lines from the lighter elements. We argue that this wind is
most likely associated with the companion star. We show that the intensities of
most lines can be fitted, crudely, by a single component photoionized model.
However, the iron K lines do not fit with this model. We show that the iron K
line variability as a function of orbital phase is different from the lower
energy lines, which indicates that the lines arise in physically distinct
regions. We discuss the interpretation of these results in the context of what
is known about the system and similar sys
|
1902.05589v3
|
2019-02-28
|
Quantum phase transition inside the superconducting dome of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ probed by optical magneto-sensing using NV-centers in diamond
|
While unusual normal state properties, such as non-Fermi liquid behavior of
the resistivity, are commonly associated with strong quantum fluctuations,
evidence for its presence inside the superconducting dome are much scarcer. In
this paper, we use sensitive and minimally invasive optical magnetometry based
on NV-centers in diamond to probe the doping evolution of the $T=0$ penetration
depth in the electron-doped iron-based superconductor
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. A non-monotonic evolution with a pronounced
peak in the vicinity of the putative magnetic QPT is found. This behavior is
reminiscent to that previously seen in isovalently-substituted
BaFe$_2$(As$_{1-x}$P$_x$)$_2$ compounds, despite the notable differences
between these two systems. Whereas the latter is a very clean system that
displays nodal superconductivity and a single simultaneous first-order
nematic-magnetic transition above, and even somewhat below, $T_c$, the former
is a significantly dirtier system with fully gapped superconductivity and split
second-order nematic and magnetic transition above $T_c$. Thus our observation
that such distinct systems display remarkably similar penetration depth peaks,
combined with the theoretical result that a QPT alone does not ensure the
existence of a peak, unveils a puzzling and seemingly universal manifestation
of quantum fluctuations in the iron pnictides.
|
1903.00053v2
|
2019-03-01
|
Observations of the Ultraviolet-Bright Star Barnard 29 in the Globular Cluster M13 (NGC 6205)
|
We have analyzed FUSE, COS, GHRS, and Keck HIRES spectra of the UV-bright
star Barnard 29 in M13 (NGC 6205). By comparing the photospheric abundances
derived from multiple ionization states of C, N, O, Si, and S, we infer an
effective temperature T_eff = 21,400 +/- 400 K. Balmer-line fits yield a
surface gravity log g = 3.10 +/- 0.03. We derive photospheric abundances of He,
C, N, O, Mg, Al, Si, P, S, Cl, Ar, Ti, Cr, Fe, Ni, and Ge. Barnard 29 exhibits
an abundance pattern typical of the first-generation stars in M13, enhanced in
oxygen and depleted in aluminum. An underabundance of C and an overabundance of
N suggest that the star experienced nonconvective mixing on the RGB. We see no
evidence of significant chemical evolution since the star left the RGB; in
particular, it did not undergo third dredge-up. Previous workers found that the
star's FUV spectra yield an iron abundance about 0.5 dex lower than its optical
spectrum, but the iron abundances derived from all of our spectra are
consistent with the cluster value. We attribute this difference to our use of
model atmospheres without microturbulence, which is ruled out by careful fits
to optical absorption features. We derive a mass M_*/M_sun = 0.45 - 0.55 and
luminosity log (L_*/L_sun) = 3.26 - 3.35. Comparison with stellar-evolution
models suggests that Barnard 29 evolved from a ZAHB star of mass M_*/M_sun
between 0.50 and 0.55, near the boundary between the extreme and blue
horizontal branches.
|
1903.00350v1
|
2019-03-04
|
Structural and magnetic properties in sputtered iron oxide epitaxial thin films -- Magnetite Fe$_3$O$_4$ and epsilon ferrite e-Fe$_2$O$_3$
|
Epitaxial thin film fabrication of iron oxides including magnetite Fe3O4 and
epsilon-ferrite epsilon-Fe2O3 with the potential for advancing electromagnetic
devices has been investigated, which led to the first ever epsilon-ferrite
epitaxial layer being synthesized in the conventional sputtering process.
Concerning Fe3O4 (100) / MgO (100) films, a cube-on-cube epitaxial relationship
and sharp rocking curves with FWHM of 50 - 350 arcsec were confirmed regardless
of the small amount of Ge additions. Sputtering Ar gas pressure PAr heavily
influenced their magnetic and transport properties. High PAr = 15 mTorr caused
a high magnetization of 6.52 kG for the Ge added sample and the clear Verwey
transition at 122 K for the non Ge addition case. Conversion electron Mossbauer
spectroscopy (CEMS) measurements revealed that low PAr < 10 mTorr causes Fe/O
off-stoichiometry on the oxidizing side for the non Ge addition case and the
reductive side for the Ge addition case, respectively. Regarding the
epsilon-Fe2O3 (001) / SrTiO3(111) epilayer synthesis, bilayer microstructure
composed of an approximately 5nm thick initially grown epsilon-Fe2O3 (001)
epilayer and subsequently grown e-Fe2O3 (001) epilayer was confirmed from
cross-sectional TEM observations. The coexistence of magnetically hard and soft
phases was confirmed from the magnetization measurements. As a possible
application of the single nm thick epsilon-Fe2O3 layer, 4-resistive-state
multiferroic tunnel junction (MFTJ) is considered.
|
1903.01022v1
|
2019-03-11
|
The Nematic Energy Scale and the Missing Electron Pocket in FeSe
|
Superconductivity emerges in proximity to a nematic phase in most iron-based
superconductors. It is therefore important to understand the impact of
nematicity on the electronic structure. Orbital assignment and tracking across
the nematic phase transition prove to be challenging due to the multiband
nature of iron-based superconductors and twinning effects. Here, we report a
detailed study of the electronic structure of fully detwinned FeSe across the
nematic phase transition using angle-resolved photoemission spectroscopy. We
clearly observe a nematicity-driven band reconstruction involving dxz, dyz, and
dxy orbitals. The nematic energy scale between dxz and dyz bands reaches a
maximum of 50 meV at the Brillouin zone corner. We are also able to track the
dxz electron pocket across the nematic transition and explain its absence in
the nematic state. Our comprehensive data of the electronic structure provide
an accurate basis for theoretical models of the superconducting pairing in
FeSe.
|
1903.04557v2
|
2019-03-14
|
3D visualizations of nano-scale phase separation and ultrafast dynamic correlation between phases in (Na0.32K0.68)0.95Fe1.75Se2
|
Phase separation of metallic and antiferromagnetic (AFM) insulating phases in
alkaline iron selenides (AxFe2-ySe2) continues to attract intense interest
because the relationship between two peculiar features probably is a key to
clarifying the pairing mechanism of AxFe2-ySe2 superconductors. Here we report
that the 3D visualizations of nano-scale phase separation in
(Na0.32K0.68)0.95Fe1.75Se2 single crystals are revealed by hybrid
focused-ion-beam scanning electron microscopy and the superconducting paths are
fully percolative in 3D. Moreover, the phase-related ultrafast dynamics in
(Na0.32K0.68)0.95Fe1.75Se2 is studied by dual-color pump-probe spectroscopy.
The anomalous changes in the electron and acoustic phonon components of
transient reflectivity change ({\Delta}R/R) identify two characteristic
temperatures T*~100 K (the onset temperature of coupling between nano-metallic
and AFM phases) and TH~230 K (the onset temperature of
metallic-interface-phase). An energy-transfer channel between the nano-metallic
and AFM phases is inferred. This proposed channel provides a new insight into
the pairing mechanism of alkaline iron selenide superconductors.
|
1903.05797v1
|
2019-03-15
|
Band-selective clean- and dirty-limit superconductivity with nodeless gaps in the bilayer iron-based superconductor CsCa$_2$Fe$_4$As$_4$F$_2$
|
The optical properties of the new iron-based superconductor
CsCa$_2$Fe$_4$As$_4$F$_2$ with $T_c \sim 29$~K have been determined. In the
normal state a good description of the low-frequency response is obtained with
a superposition of two Drude components of which one has a very low scattering
rate (narrow Drude-peak) and the other a rather large one (broad Drude-peak).
Well below $T_c \sim 29$~K, a pronounced gap feature is observed which involves
a complete suppression of the optical conductivity below $\sim$ 110~cm$^{-1}$
and thus is characteristic of a nodeless superconducting state. The optical
response of the broad Drude-component can be described with a dirty-limit
Mattis-Bardeen-type response with a single isotropic gap of $2\Delta \simeq
14$~meV. To the contrary, the response of the narrow Drude-component is in the
ultra-clean-limit and its entire spectral weight is transferred to the
zero-frequency $\delta(\omega)$ function that accounts for the loss-free
response of the condensate. These observations provide clear evidence for a
band-selective coexistence of clean- and dirty-limit superconductivity with
nodeless gaps in CsCa$_2$Fe$_4$As$_4$F$_2$.
|
1903.06466v1
|
2019-05-07
|
Single-parameter scaling in the magnetoresistance of optimally doped La$_{2-x}$Sr$_{x}$CuO$_4$
|
We show that the recent magnetoresistance data on thin-film
La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO) in strong magnetic fields ($B$) obeys a
single-parameter scaling of the form MR$(B,T)=f(\mu_H(T)B)$, where
$\mu_H^{-1}(T)\sim T^{\alpha}$ ($1\le\alpha\le2$), from $T=180$K until
$T\sim20$K, at which point the single-parameter scaling breaks down. The
functional form of the MR is distinct from the simple quadratic-to-linear
quadrature combination of temperature and magnetic field found in the optimally
doped iron superconductor BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$. Further,
low-temperature departure of the MR in LSCO from its high-temperature scaling
law leads us to conclude that the MR curve collapse is not the result of
quantum critical scaling. We examine the classical effective medium theory
(EMT) previously used to obtain the quadrature resistivity dependence on field
and temperature for metals with a $T$-linear zero-field resistivity. It appears
that this scaling form results only for a binary, random distribution of
metallic components. More generally, we find a low-temperature, high-field
region where the resistivity is simultaneously $T$ and $B$ linear when multiple
metallic components are present. Our findings indicate that if mesoscopic
disorder is relevant to the magnetoresistance in strange metal materials, the
binary-distribution model which seems to be relevant to the iron pnictides is
distinct from the more broad-continuous distributions relevant to the cuprates.
Using the latter, we examine the applicability of classical effective medium
theory to the MR in LSCO and compare calculated MR curves with the experimental
data.
|
1905.02737v1
|
2019-05-13
|
Kinetics of phase separation, border of miscibility gap in Fe-Cr and limit of Cr solubility in iron at 832 K
|
Kinetics of phase decomposition accompanied by precipitation of sigma-phase
in a Fe73.7Cr26.3 alloy isothermally annealed at 832 K was studied by means of
M\"ossbauer spectroscopy. Two stage decomposition process has been revealed by
three different quantities viz. the average hyperfine field, <H>, the
short-range parameter, alpha1, and the probability of atomic configuration with
no Cr atoms within the first two coordination shells around the probe Fe atoms,
P(0,0). The first stage, that has terminated after ca.300 h of annealing, has
been associated with the decomposition into Fe-rich phase in which the
concentration of Cr, determined as 20.9 at.%, can be interpreted as the border
of the metastable miscibility gap at 832 K. The second stage can be regarded as
a continuation of the phase decomposition process combined with a precipitation
of sigma. The three relevant parameters for this stage have also
saturation-like behavior vs. annealing time and the saturation can be
interpreted as termination of the two processes. The concentration of Cr in the
Fe-rich phase has been determined as 19.8 at.% and this value can be regarded
as the limit of Cr solubility in iron at 832 K. Both stages of the kinetics
were found to be in line with the Johnson-Mehl-Avrami-Kolgomorov equation
yielding values of the rate constant and the Avrami exponent. The activation
energy of the second-stage process was determined to be by ca.12 kJ/mol higher.
|
1905.05156v1
|
2019-05-24
|
The empirical aspects of Eliashberg formalism for the superconducting mechanism in Iron-based superconductors
|
We have investigated experimentally how properties of NdFeAsO0.8F0.2
superconductor affected due to the substitution of the Ca2+/Nd3+ doping. Based
on the XRD data refinement, various structural parameters such as lattice
parameters, bond angles, bond length, and etc. were studied. We have determined
the upper limit of the calcium solubility in the NdFeAsO0.8F0.2 phase and it is
restricted to x=0.05. Also, we have found that the lattice parameters and the
cell volume decreased by increasing the calcium content. According to the XRD
data analysis, we have argued that these reductions are due to the variations
in the bond lengths and the bond angles of (O/F)-Nd-(O/F) and As-Fe-As i.e.
alpha, beta upon increasing the calcium dopant. So, we have expected that the
superconducting transition temperature (TC) will be sensitive to the calcium
doping values. Experimentally, the TC of our samples was reduced from 53 K (for
x = 0) to 48 K (for x = 0.01) and 27 K (for x=0.025) and disappeared for our
other sample. Then we have studied the dependence of TC and bond angles, bond
length, the pnictogen height, and the lattice parameter to examine the
available theories from an empirical point of view. The consistency of our
experimental results and the theoretical reports based on the spin- and the
orbital-fluctuation theories shows that these models play an important role in
the pairing mechanism of the iron-based superconductors.
|
1905.11378v1
|
2019-05-28
|
Photo-induced Superconducting State with Long-lived Disproportionate Band Filling in FeSe
|
Photo-excitation is a very powerful way to instantaneously drive a material
into a novel quantum state without any fabrication, and variable ultrafast
techniques have been developed to observe how electron-, lattice-, and
spin-degrees of freedom change. One of the most spectacular phenomena is
photo-induced superconductivity, and it has been suggested in cuprates that the
transition temperature Tc can be enhanced from original Tc with significant
lattice modulations. Here we show another photo-induced high-Tc superconducting
state in the iron-based superconductor FeSe with semi-metallic hole and
electron bands. The transient electronic state in the entire Brillouin zone is
directly observed by the time- and angle-resolved photoemission spectroscopy
using extreme ultraviolet pulses obtained from high harmonic generation. Our
results of dynamical behaviors on timescales from 50 fs to 800 ps consistently
support the favorable superconducting state after photo-excitation well above
Tc. This finding demonstrates that multiband iron-based superconductors emerge
as an alternative candidate for photo-induced superconductors.
|
1905.12138v1
|
2019-05-29
|
Photoinduced nematic state in FeSe$_{0.4}$Te$_{0.6}$
|
FeSe$_{x}$Te$_{1-x}$ compounds present a complex phase diagram, ranging from
the nematicity of FeSe to the $(\pi, \pi)$ magnetism of FeTe. We focus on
FeSe$_{0.4}$Te$_{0.6}$, where the nematic ordering is absent at equilibrium. We
use a time-resolved approach based on femtosecond light pulses to study the
dynamics following photoexcitation in this system. The use of
polarization-dependent time- and angle-resolved photoelectron spectroscopy
allows us to reveal a photoinduced nematic metastable state, whose
stabilization cannot be interpreted in terms of an effective photodoping. We
argue that the 1.55 eV photon-energy-pump-pulse perturbs the $C_4$ symmetry of
the system triggering the realization of the nematic state. The possibility to
induce nematicity using an ultra-short pulse sheds a new light on the driving
force behind the nematic symmetry breaking in iron-based superconductors. Our
results weaken the idea that a low-energy coupling with fluctuations is a
necessary condition to stabilize the nematic order and ascribe the origin of
the nematic order in iron-based superconductors to a clear tendency of those
systems towards orbital differentiation due to strong electronic correlations
induced by the Hund's coupling.
|
1905.12448v1
|
2019-05-30
|
Quantum dynamics of hydrogen in iron-based superconductor LaFeAsO0.9D0.1 measured with inelastic neutron spectroscopy
|
Inelastic neutron scattering was performed for an iron-based superconductor
LaFeAsO0.9D0.1, where most of D (deuterium) replaces oxygen, while a tiny
amount goes into interstitial sites. By first-principle calculation, we
characterize the interstitial sites for D (and for H slightly mixed) with four
equivalent potential minima. Below the superconducting transition temperature
Tc = 26 K,new excitations emerge in the range 5-15 meV, while they are absent
in the reference system LaFeAsO0.9F0.1. The strong excitations at 14.5 meV and
11.1 meV broaden rapidly around 15 K and 20 K, respectively, where each energy
becomes comparable to twice of the superconducting gap. The strong excitations
are ascribed to a quantum rattling, or a band motion of hydrogen, which arises
only if the number of potential minima is larger than two.
|
1905.12826v2
|
2019-05-30
|
Changes in long-term properties of the Danube river level and flow induced by damming
|
In this paper we assessed changes in scaling properties of the river Danube
level and flow data, associated with building of Djerdap/Iron Gates
hydrological power plants positioned on the border of Romania and Serbia. We
used detrended fluctuation analysis (DFA), wavelet transform spectral analysis
(WTS) and wavelet-based modulus maxima method (WTMM) to investigate time series
of river levels and river flows recorded at hydrological stations in the
vicinity of dams and in the area of up to 480 km upstream from dams, and time
series of simulated NOAA-CIRES 20th Century Global Reanalysis precipitation
records for the Djerdap/Iron Gates region. By comparing river dynamics during
the periods before and after construction of dams, we were able to register
changes in scaling that are different for recordings from upstream and from
downstream (from dams) areas. We found that damming caused appearance of
human-made or enhancement of natural cycles in the small time scales region,
which largely influenced the change in temporal scaling in downstream recording
stations. We additionally found disappearance or decline in the amplitude of
large-time-scale cycles as a result of damming, that changed the dynamics of
upstream data. The most prominent finding of our paper is a demonstration of a
complete or partial loss of annual cycles in the upstream stations' data that
we found to extend as far as 220 km from dams. We discussed probable sources of
such found changes in scaling, aiming to provide explanations that could be of
use in future environmental assessments.
|
1905.13144v2
|
2019-08-06
|
FeOOH instability at the lower mantle conditions
|
Goethite, {\alpha}-FeOOH, is a major component among oxidized iron species,
called rust, which formed as a product of metabolism of anoxygenic prokaryotes
(1, 2) inhabiting the Earth from about 3.8 billion years (Gy) ago until the
Great Oxidation Event (GOE) of about 2.5 Gy ago. The rust was buried on the
ocean floor (1, 2) and had to submerge into the Earth mantle with subducting
slabs due to the plate tectonics started about 2.8 Gy ago (3). The fate and the
geological role of the rust at the lower mantle high-pressure and
high-temperature(HPHT) conditions is unknown. We studied the behavior of
goethite up to 82(2) GPa and 2300(100) K using in situ synchrotron
single-crystal X-ray diffraction. At these conditions, corresponding to the
coldest slabs at the depth of about 1000 km, {\alpha}-FeOOH decomposes to
various iron oxides (Fe2O3, Fe5O7, Fe7O10, Fe6.32O9) and an oxygen-rich fluid.
Our results suggest that recycling of the rust in the Earth mantle could
contribute to oxygen release to the atmosphere and explain the sporadic
increase of the oxygen level before the GOE linked to the formation of Large
Igneous Provinces(4).
|
1908.02114v1
|
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