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2013-08-26
|
Constraints on the source of ultra-high energy cosmic rays using anisotropy vs chemical composition
|
The joint analysis of anisotropy signals and chemical composition of
ultra-high energy cosmic rays offers strong potential for shedding light on the
sources of these particles. Following up on an earlier idea, this paper studies
the anisotropies produced by protons of energy >E/Z, assuming that anisotropies
at energy >E have been produced by nuclei of charge Z, which share the same
magnetic rigidity. We calculate the number of secondary protons produced
through photodisintegration of the primary heavy nuclei. Making the extreme
assumption that the source does not inject any proton, we find that the
source(s) responsible for anisotropies such as reported by the Pierre Auger
Observatory should lie closer than ~20-30, 80-100 and 180-200 Mpc if the
anisotropy signal is mainly composed of oxygen, silicon and iron nuclei
respectively. A violation of this constraint would otherwise result in the
secondary protons forming a more significant anisotropy signal at lower
energies. Even if the source were located closer than this distance, it would
require an extraordinary metallicity >120, 1600, 1100 times solar metallicity
in the acceleration zone of the source, for oxygen, silicon and iron
respectively, to ensure that the concomitantly injected protons not to produce
a more significant low energy anisotropy. This offers interesting prospects for
constraining the nature and the source of ultra-high energy cosmic rays with
the increase in statistics expected from next generation detectors.
|
1308.5699v2
|
2013-09-06
|
Probing the origin of the iron K_alpha line around stellar and supermassive black holes using X-ray polarimetry
|
Asymmetric, broad iron lines are a common feature in the X-ray spectra of
both X-ray binaries (XRBs) and type-1 Active Galactic Nuclei (AGN). It was
suggested that the distortion of the Fe K_alpha emission results from Doppler
and relativistic effects affecting the radiative transfer close to the strong
gravitational well of the central compact object: a stellar mass black hole
(BH) or neutron star (NS) in the case of XRBs, or a super massive black hole
(SMBH) in the case of AGN. However, alternative approaches based on
reprocessing and transmission of radiation through surrounding media also
attempt to explain the line broadening. So far, spectroscopic and timing
analyzes have not yet convinced the whole community to discriminate between the
two scenarios. Here we study to which extent X-ray polarimetric measurements of
black hole X-ray binaries (BHXRBs) and type-1 AGN could help to identify the
possible origin of the line distortion. To do so, we report on recent
simulations obtained for the two BH flavors and show that the proposed
scenarios are found to behave differently in polarization degree and
polarization angle. A relativistic origin for the distortion is found to be
more probable in the context of BHXRBs, supporting the idea that the same
mechanism should lead the way also for AGN. We show that the discriminating
polarization signal could have been detectable by several X-ray polarimetry
missions proposed in the past.
|
1309.1684v1
|
2013-09-08
|
On Iron Monoxide Nanoparticles as a Carrier of the Mysterious 21 Micrometer Emission Feature in Post-Asymptotic Giant Branch Stars
|
A prominent, mysterious emission feature peaking at ~20.1 micrometer ---
historically known as the ``21 micrometer' feature --- is seen in over two
dozen Galactic and Magellanic Cloud carbon-rich post-asymptotic giant branch
(post-AGB) stars. The nature of its carrier remains unknown since the first
detection of the 21 micrometer feature in 1989. Over a dozen materials have
been suggested as possible carrier candidates. However, none of them has been
accepted: they either require too much material (compared to what is available
in the circumstellar shells around these post-AGB stars), or exhibit additional
emission features which are not seen in these 21 micrometer sources.
Recently, iron monoxide (FeO) nanoparticles seem to be a promising carrier
candidate as Fe is an abundant element and FeO emits exclusively at ~21
micrometer. In this work, using the proto-typical protoplanetary nebula HD
56126 as a test case, we examine FeO nanoparticles as a carrier for the 21
micrometer feature by modeling their infrared emission, with FeO being
stochastically heated by single stellar photons. We find that FeO emits too
broad a 21 micrometer feature to explain the observed one and the Fe abundance
required to be locked up in FeO exceeds what is available in HD 56126. We
therefore conclude that FeO nanoparticles are unlikely responsible for the 21
micrometer feature.
|
1309.1995v1
|
2013-09-09
|
Xenon Reacts with Iron at the Conditions of the Earth's Core
|
Studies of the Earth's atmosphere have shown that more than 90% of xenon (Xe)
is depleted compared with its abundance in chondritic meteorites. This
long-standing missing Xe paradox has become the subject of considerable
interest and several models for a Xe reservoir have been proposed. Whether the
missing Xe is hiding in the Earth's core has remained a long unanswered
question. The key to address this issue lies in the reactivity of Xe with iron
(Fe, the main constituent of the Earth's core), which has been denied by
earlier studies. Here we report on the first evidence of the chemical reaction
of Xe and Fe at the conditions of the Earth's core, predicted through
first-principles calculations and unbiased structure searching techniques. We
find that Xe and Fe form a stable, inter-metallic compound of XeFe3, adopting a
Cu3Au-type face-centered cubic structure above 183 GPa and at 4470 K. As the
result of a Xe -> Fe charge transfer, Xe loses its chemical inertness by
opening up the filled 5p electron shell and functioning as a 5p-like element,
whilst Fe is unusually negatively charged, acting as an oxidant rather than a
reductant as usual. Our work establishes that the Earth's core is a natural
reservoir for Xe storage, and possibly provides the key to unlocking the
missing Xe paradox.
|
1309.2169v4
|
2013-09-24
|
Mineralogy, reflectance spectra, and physical properties of the Chelyabinsk LL5 chondrite, insight into shock induced changes in asteroid regoliths
|
The mineralogy and physical properties of Chelyabinsk meteorites (fall,
February 15, 2013) are presented. Three types of meteorite material are
present, described as the light-colored, dark-colored, and impact-melt
lithologies. All are of LL5 composition with the impact-melt lithology being
close to whole-rock melt and the dark-colored lithology being shock-darkened
due to partial melting of iron metal and sulfides. This enables us to study the
effect of increasing shock on material with identical composition and origin.
Based on the magnetic susceptibility, the Chelyabinsk meteorites are richer in
metallic iron as compared to other LL chondrites. The measured bulk and grain
densities and the porosity closely resemble other LL chondrites. Shock
darkening does not have a significant effect on the material physical
properties, but causes a decrease of reflectance and decrease in silicate
absorption bands in the reflectance spectra. This is similar to the space
weathering effects observed on asteroids. However, compared to space weathered
materials, there is a negligible to minor slope change observed in impact-melt
and shock-darkened meteorite spectra. Thus, it is possible that some dark
asteroids with invisible silicate absorption bands may be composed of
relatively fresh shock-darkened chondritic material.
|
1309.6081v6
|
2013-10-14
|
Field-induced nematic-like magnetic transition in an iron pnictide superconductor, Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$
|
We report a high magnetic field study up to 55 T of the nearly optimally
doped iron-pnictide superconductor Ca$_{10}$(Pt$_{3}$As$_{8}$)
((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ (x=0.078(6)) with a Tc 10 K using
magnetic torque, tunnel diode oscillator technique and transport measurements.
We determine the superconducting phase diagram, revealing an anisotropy of the
irreversibility field up to a factor of 10 near Tc and signatures of multiband
superconductivity. Unexpectedly, we find a spin-flop like anomaly in magnetic
torque at 22 T, when the magnetic field is applied perpendicular to the ab
planes, which becomes significantly more pronounced as the temperature is
lowered to 0.33 K. As our superconducting sample lies well outside the
antiferromagnetic region of the phase diagram, the observed field-induced
transition in torque indicates a spin-flop transition not of long-range ordered
moments, but of nematic-like antiferromagnetic fluctuations.
|
1310.3728v1
|
2013-10-14
|
High Pressure Effects on the Superconductivity in Rare-Earth Doped CaFe2As2
|
High-pressure superconductivity in a rare-earth doped Ca0.86Pr0.14Fe2As2
single crystalline sample has been studied up to 12 GPa and temperatures down
to 11 K using designer diamond anvil cell under a quasi-hydrostatic pressure
medium. The electrical resistance measurements were complemented by high
pressure and low temperature x-ray diffraction studies at a synchrotron source.
The electrical resistance measurements show an intriguing observation of
superconductivity under pressure, with Tc as high as ~51 K at 1.9 GPa,
presenting the highest Tc reported in the intermetallic class of 1-2-2
iron-based superconductors. The resistive transition observed suggests a
possible existence of two superconducting phases at low pressures of 0.5 GPa:
one phase starting at Tc1 ~48 K, and the other starting at Tc2~16 K. The two
superconducting transitions show distinct variations with increasing pressure.
High pressure low temperature structural studies indicate that the
superconducting phase is a collapsed tetragonal ThCr2Si2-type (122) crystal
structure. Our high pressure studies indicate that high Tc state attributed to
non-bulk superconductivity in rare-earth doped 1-2-2 iron-based superconductors
is stable under compression over a broad pressure range.
|
1310.3842v2
|
2013-10-17
|
Coexistence of orbital degeneracy lifting and superconductivity in iron-based superconductors
|
In contrast to conventional superconducting (SC) materials, superconductivity
in high-temperature superconductors (HTCs) usually emerges in the presence of
other fluctuating orders with similar or higher energy scales, thus instigating
debates over their relevance for the SC pairing mechanism. In iron-based
superconductors (IBSCs), local orbital fluctuations have been proposed to be
directly responsible for the structural phase transition and closely related to
the observed giant magnetic anisotropy and electronic nematicity. However,
whether superconductivity can emerge from, or even coexist with orbital
fluctuations, remains unclear. Here we report the angle-resolved photoemission
spectroscopy (ARPES) observation of the lifting of symmetry-protected band
degeneracy, and consequently the breakdown of local tetragonal symmetry in the
SC state of Li(Fe1-xCox)As. Supported by theoretical simulations, we analyse
the doping and temperature dependences of this band-splitting and demonstrate
an intimate connection between ferro-orbital correlations and
superconductivity.
|
1310.4601v3
|
2013-10-28
|
Metal-insulator transition and superconductivity in the two-orbital Hubbard-Holstein model for iron-based superconductors
|
We investigate a two-orbital model for iron-based superconductors to
elucidate the effect of interplay between electron correlation and Jahn-Teller
electron-phonon coupling by using the dynamical mean-field theory combined with
the exact diagonalization method. When the intra- and inter-orbital Coulomb
interactions, $U$ and $U'$, increase with $U=U'$, both the local spin and
orbital susceptibilities, $\chi_{s}$ and $\chi_{o}$, increase with
$\chi_{s}=\chi_{o}$ in the absence of the Hund's rule coupling $J$ and the
electron-phonon coupling $g$. In the presence of $J$ and $g$, there are
distinct two regimes: for $J \stackrel{>}{_\sim} 2g^2/\omega_0$ with the phonon
frequency $\omega_0$, $\chi_{s}$ is enhanced relative to $\chi_{o}$ and shows a
divergence at $J=J_c$ above which the system becomes Mott insulator, while for
$J \stackrel{<}{_\sim} 2g^2/\omega_0$, $\chi_{o}$ is enhanced relative to
$\chi_{s}$ and shows a divergence at $g=g_c$ above which the system becomes
bipolaronic insulator. In the former regime, the superconductivity is mediated
by antiferromagnetic fluctuations enhanced due to Fermi-surface nesting and is
found to be largely dependent on carrier doping. On the other hand, in the
latter regime, the superconductivity is mediated by ferro-orbital fluctuations
and is observed for wide doping region including heavily doped case without the
Fermi-surface nesting.
|
1310.7327v3
|
2013-10-29
|
A uniform metal distribution in the intergalactic medium of the Perseus cluster of galaxies
|
Most of the metals (elements heavier than helium) ever produced by stars in
the member galaxies of galaxy clusters currently reside within the hot, X-ray
emitting intra-cluster gas. Observations of X-ray line emission from this
intergalactic medium have suggested a relatively small cluster-to-cluster
scatter outside of the cluster centers and enrichment with iron out to large
radii, leading to the idea that the metal enrichment occurred early in the
history of the Universe. Models with early enrichment predict a uniform metal
distribution at large radii in clusters, while late-time enrichment, favored by
some previous studies, is expected to introduce significant spatial variations
of the metallicity. To discriminate clearly between these competing models, it
is essential to test for potential inhomogeneities by measuring the abundances
out to large radii along multiple directions in clusters, which has not
hitherto been done. Here we report a remarkably uniform measured iron
abundance, as a function of radius and azimuth, that is statistically
consistent with a constant value of 0.306+/-0.012 Solar out to the edge of the
nearby Perseus Cluster. This homogeneous distribution requires that most of the
metal enrichment of the intergalactic medium occurred before the cluster
formed, likely over 10 billion years ago, during the period of maximal star
formation and black hole activity.
|
1310.7948v1
|
2013-10-30
|
Close relationship between superconductivity and the bosonic mode in Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As
|
Since the discovery of high temperature superconductivity in the iron
pnictides and chalcogenides in early 2008, a central issue has been the
microscopic origin of the superconducting pairing. Although previous
experiments suggest that the pairing may be induced by exchanging the
antiferromagnetic spin fluctuations and the superconducting order parameter has
opposite signs in the electron and hole pockets as predicted by the S+- pairing
model, it remains unclear whether there is a bosonic mode from the tunneling
spectrum which has a close and universal relationship with superconductivity as
well as the spin excitation. In this paper, based on the measurements of
scanning tunneling spectroscopy, we show the clear evidence of a bosonic mode
with the energy identical to that of the neutron spin resonance in two
completely different systems Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As with
different superconducting transition temperatures. In both samples, the
superconducting coherence peaks and the mode feature vanish simultaneously
inside the vortex core or above Tc, indicating a close relationship between
superconductivity and the bosonic mode. Our data also demonstrate a universal
ratio between the mode energy and superconducting transition temperature, that
is [mode energy]/kBTc ~ 4.3, which underlines the unconventional mechanism of
superconductivity in the iron pnictide superconductors.
|
1310.8160v1
|
2013-11-04
|
Variability of the High Velocity Outflow in the Quasar PDS 456
|
We present a comparison of two Suzaku X-ray observations of the nearby
(z=0.184), luminous ($L_{bol} \sim 10^{47}$ erg s$^{-1}$) type I quasar,
PDS456. A new 125ks Suzaku observation in 2011 caught the quasar during a
period of low X-ray flux and with a hard X-ray spectrum, in contrast to a
previous 190ks Suzaku observation in 2007 when the quasar appeared brighter and
had a steep ($\Gamma>2$) X-ray spectrum. The 2011 X-ray spectrum contains a
pronounced trough near 9\,keV in the quasar rest frame, which can be modeled
with blue-shifted iron K-shell absorption, most likely from the He and H-like
transitions of iron. The absorption trough is observed at a similar rest-frame
energy as in the earlier 2007 observation, which appears to confirm the
existence of a persistent high velocity wind in PDS 456, at an outflow velocity
of $0.25-0.30$c. The spectral variability between 2007 and 2011 can be
accounted for by variations in a partial covering absorber, increasing in
covering fraction from the brighter 2007 observation to the hard and faint 2011
observation. Overall the low flux 2011 observation can be explained if PDS 456
is observed at relatively low inclination angles through a Compton thick wind,
originating from the accretion disk, which significantly attenuates the X-ray
flux from the quasar.
|
1311.0734v1
|
2013-11-19
|
Suzaku View of the Neutron Star in the Dipping Source 4U 1822$-$37
|
The dipping X-ray source 4U 1822$-$37 was observed by Suzaku on 2006 Octrober
20 for a net exposure of 37 ks. The source was detected with the XIS at a 1-10
keV flux of 5.5$\times10^{-10}$ erg cm$^{-2}$ s$^{-1}$, and with the HXD
(HXD-PIN) at a 10-50 keV flux of 8.9$\times10^{-10}$ erg cm$^{-2}$ s$^{-1}$.
With HXD-PIN, the pulsation was detected at a barycentric period of 0.592437 s,
and its change rate was reconfirmed as $-2.43\times$10$^{-12}$ s s$^{-1}$. The
1-50 keV spectra of 4U 1822-37 were found to be very similar to those of Her
X-1 in the slopes, cutoff and iron lines. Three iron lines (Fe K$\alpha$, Fe
XXV, and Fe XXVI) were detected, on top of a 1-50 keV continuum that is
described by an NPEX model plus a soft blackbody. In addition, a cyclotron
resonance scattering feature was detected significantly ($>99\%$ confidence),
at an energy of 33$\pm$2 keV with a depth of 0.4$^{+0.6}_{-0.3}$. Therefore,
the neutron star in this source is concluded to have a strong magnetic field of
2.8$\times10^{12}$ G. Further assuming that the source has a relatively high
intrinsic luminosity of several times 10$^{37}$ erg s$^{-1}$, its spectral and
timing properties are consistently explained.
|
1311.4618v1
|
2013-11-19
|
Metal distributions out to 0.5 r180 in the intracluster medium of four galaxy groups observed with Suzaku
|
We studied the distributions of metal abundances and metal-mass-to-light
ratios in the intracluster medium (ICM) of four galaxy groups, MKW 4, HCG 62,
the NGC 1550 group, and the NGC 5044 group, out to 0.5 r180 observed with
Suzaku. The Fe abundance decreases with radius, and about 0.2-0.4 solar beyond
0.1 r180. At a given radius in units of r180, the Fe abundance in the ICM of
the four galaxy groups were consistent or smaller than those of clusters of
galaxies. The Mg/Fe and Si/Fe ratios in the ICM are nearly constant at the
solar ratio out to 0.5 r180. We also studied systematic uncertainties in the
derived metal abundances comparing the results from two versions of atomic data
for astrophysicists (ATOMDB) and single- and two temperature model fits. Since
the metals have been synthesized in galaxies, we collected K-band luminosities
of galaxies from Two Micron All Sky Survey catalogue (2MASS) and calculated the
integrated iron-mass-to-light-ratios (IMLR), or the ratios of the iron mass in
the ICM to light from stars in galaxies. The groups with smaller gas mass to
light ratios have smaller IMLR values and the IMLR inversely correlated with
the entropy excess. Based on these abundance features, we discussed the past
history of metal enrichment process in groups of galaxies.
|
1311.4636v1
|
2013-11-23
|
Fermi Surface and Band Structure of (Ca,La)FeAs2 Superconductor from Angle-Resolved Photoemission Spectroscopy
|
The (Ca,R)FeAs2 (R=La,Pr and etc.) superconductors with a signature of
superconductivity transition above 40 K possess a new kind of block layers that
consist of zig-zag As chains. In this paper, we report the electronic structure
of the new (Ca,La)FeAs2 superconductor investigated by both band structure
calculations and high resolution angle-resolved photoemission spectroscopy
measurements. Band structure calculations indicate that there are four
hole-like bands around the zone center $\Gamma$(0,0) and two electron-like
bands near the zone corner M(pi,pi) in CaFeAs2. In our angle-resolved
photoemission measurements on (Ca0.9La0.1})FeAs2, we have observed three
hole-like bands around the Gamma point and one electron-like Fermi surface near
the M(pi,pi) point. These results provide important information to compare and
contrast with the electronic structure of other iron-based compounds in
understanding the superconductivity mechanism in the iron-based
superconductors.
|
1311.6042v1
|
2013-12-10
|
Interfacial mode coupling as the origin of the enhancement of Tc in FeSe films on SrTiO3
|
Single unit cell films of iron selenide (1UC FeSe) grown on SrTiO3 (STO)
substrates have recently shown superconducting energy gaps opening at
temperatures close to the boiling point of liquid nitrogen (77 K), a record for
iron-based superconductors. Towards understanding why Cooper pairs form at such
high temperatures, a primary question to address is the role, if any, of the
STO substrate. Here, we report high resolution angle resolved photoemission
spectroscopy (ARPES) results which reveal an unexpected and unique
characteristic of the 1UC FeSe/STO system: shake-off bands suggesting the
presence of bosonic modes, most likely oxygen optical phonons in STO, which
couple to the FeSe electrons with only small momentum transfer. Such coupling
has the unusual benefit of helping superconductivity in most channels,
including those mediated by spin fluctuations. Our calculations suggest such
coupling is responsible for raising the superconducting gap opening temperature
in 1UC FeSe/STO. This discovery suggests a pathway to engineer high temperature
superconductors.
|
1312.2633v6
|
2013-12-17
|
Defect ordering and defect-domain wall interactions in PbTiO$_3$: A first-principles study
|
The properties of ferroelectric materials, such as lead zirconate titanate
(PZT), are heavily influenced by the interaction of defects with domain walls.
These defects are either intrinsic, or are induced by the addition of dopants.
We study here PbTiO$_3$ (the end member of a key family of solid solutions) in
the presence of acceptor (Fe) and donor (Nb) dopants, and the interactions of
the different defects and defect associates with the domain walls. For the case
iron acceptors, the calculations point to the formation of defect associates
involving an iron substitutional defect and a charged oxygen vacancy
(Fe$^{'}_{Ti}$-V$^{^{\textbf{..}}}_O$). This associate exhibits a strong
tendency to align in the direction of the bulk polarization; in fact, ordering
of defects is also observed in pure PbTiO$_3$ in the form of lead-oxygen
divacancies. Conversely, calculations on donor-doped PbTiO$_3$ do not indicate
the formation of polar defect complexes involving donor substitutions. Last, it
is observed that both isolated defects in donor-doped materials and defect
associates in acceptor-doped materials are more stable at 180$^o$ domain walls.
However, polar defect complexes lead to asymmetric potentials at domain walls
due to the interaction of the defect polarization with the bulk polarization.
The relative pinning characteristics of different defects are then compared, to
develop an understanding of defect-domain wall interactions in both doped and
pure PbTiO$_3$. These results may also help understanding hardening and
softening mechanisms in PZT.
|
1312.4701v1
|
2014-02-17
|
Near-substrate composition depth profile of direct current-plated and pulse-plated Fe-Ni alloys
|
Composition depth profiles of d.c.-plated and pulseplated Fe-Ni alloys have
been investigated with the reverse depth profile analysis method. When d.c.
plating is applied, the mole fraction of iron near the substrate is higher than
during steadystate deposition since iron is preferentially deposited beside
nickel and the achievement of the steadystate deposition condition takes time.
The steadystate composition was achieved typically after depositing a
90-nm-thick alloy layer. In the pulseplating mode, samples with nearly uniform
composition could be obtained at a duty cycle of 0.2 or smaller, and a
continuous change in the composition profile could be seen as a function of the
duty cycle above this value. A constant sample composition was achieved with
pulseplating in a wide peak current density interval. The composition depth
profile was also measured for a wide range of Fe2+ concentration. The different
characteristics of the composition depth profile as a function of the
deposition mode can be explained mostly in terms of mass transport effects. The
elucidation of the results is fully in accord with the kinetic models of
anomalous codeposition and with the assumption of the superposition of a
stationary and a pulsating diffusion layer. The results achieved help to
identify the conditions for the deposition of ultrathin magnetic samples with
uniform composition along the growth direction.
|
1402.3943v1
|
2014-02-20
|
Ultrafast observation of critical nematic fluctuations and giant magnetoelastic coupling in iron pnictides
|
Many of the iron pnictides have strongly anisotropic normal-state
characteristics, important for the exotic magnetic and superconducting behavior
these materials exhibit. Yet, the origin of the observed anisotropy is unclear.
Electronically driven nematicity has been suggested, but distinguishing this as
an independent degree of freedom from magnetic and structural orders is
difficult, as these couple together to break the same tetragonal symmetry. Here
we use time-resolved polarimetry to reveal critical nematic fluctuations in
unstrained Ba(Fe_(1-x)Co_x)_2As_2. The femtosecond anisotropic response, which
arises from the two-fold in-plane anisotropy of the complex refractive index,
displays a characteristic two-step recovery absent in the isotropic response.
The fast recovery appears only in the magnetically ordered state, whereas the
slow one persists in the paramagnetic phase with a critical divergence
approaching the structural transition temperature. The dynamics also reveal a
gigantic magnetoelastic coupling that far exceeds electron-spin and
electron-phonon couplings, opposite to conventional magnetic metals.
|
1402.4890v1
|
2014-02-21
|
The form and the origin of the orbital ordering in the electronic nematic phase of the Iron-based superconductors
|
We investigated the form of the orbital ordering in the electronic nematic
phase of the Iron-based superconductors by applying a group theoretical
analysis on a realistic five-band model. We find the orbital order can be
either of the inter-orbital s-wave form or the intra-orbital d-wave form. From
the comparison with existing ARPES measurements of band splitting, we find the
orbital ordering in the 122 system is dominated by an intra-orbital d-wave
component, while that in the 111 system is dominated by an inter-orbital s-wave
component. We find both forms of orbital order are strongly entangled with the
nematicity in the spin correlation of the system. The condensation energy of
the magnetic ordered phase is found to be significantly improved(by more than
20 percents) when the degeneracy between the $(\pi,0)$ and $(0,\pi)$ ordering
pattern is lifted by the orbital order. We argue there should be large
difference in both the scattering rate and the size of the possible pseudogap
on the electron pocket around the $\mathrm{X}=(\pi,0)$ and $\mathrm{Y}=(0,\pi)$
point in the electronic nematic phase. We propose this as a possible origin for
the observed nematicity in resistivity measurements.
|
1402.5209v1
|
2014-02-24
|
Towards a novel laser-driven method of exotic nuclei extraction-acceleration for fundamental physics and technology
|
The measurement of properties of exotic nuclei, essential for fundamental
nuclear physics, now confronts a formidable challenge for contemporary
radiofrequency accelerator technology. A promising option can be found in the
combination of state-of-the-art high-intensity short pulse laser system and
nuclear measurement techniques. We propose a novel Laser-driven Exotic Nuclei
extraction-acceleration method (LENex): a femtosecond petawatt laser,
irradiating a target bombarded by an external ion beam, extracts from the
target and accelerates to few GeV highly-charged nuclear reaction products.
Here a proof-of-principle experiment of LENex is presented: a few
hundred-terawatt laser focused onto an aluminum foil, with a small amount of
iron simulating nuclear reaction products, extracts almost fully stripped iron
nuclei and accelerate them up to 0.9 GeV. Our experiments and numerical
simulations show that short-lived, heavy exotic nuclei, with a much larger
charge-to-mass ratio than in conventional technology, can be obtained in the
form of an energetic, low-emittance, high-current beam.
|
1402.5729v1
|
2014-03-01
|
Sudden reversal in the pressure dependence of Tc in the iron-based superconductor CsFe2As2: A possible link between inelastic scattering and pairing symmetry
|
We report a sudden reversal in the pressure dependence of Tc in the
iron-based superconductor CsFe2As2, similar to that discovered recently in
KFe2As2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no
change in the Hall coefficient at the zero temperature limit, again ruling out
a Lifshitz transition across the critical pressure Pc. We interpret the Tc
reversal in the two materials as a phase transition from one pairing state to
another, tuned by pressure, and investigate what parameters control this
transition. Comparing samples of different residual resistivity, we find that a
6-fold increase in impurity scattering does not shift Pc. From a study of X-ray
diffraction on KFe2As2 under pressure, we report the pressure dependence of
lattice constants and As-Fe-As bond angle. The pressure dependence of these
lattice parameters suggests that Pc should be significantly higher in CsFe2As2
than in KFe2As2, but we find on the contrary that Pc is lower in CsFe2As2.
Resistivity measurements under pressure reveal a change of regime across Pc,
suggesting a possible link between inelastic scattering and pairing symmetry.
|
1403.0110v3
|
2014-03-05
|
Observation of a Robust Zero-energy Bound State in Iron-based Superconductor Fe(Te,Se)
|
A robust zero-energy bound state (ZBS) in a superconductor, such as a
Majorana or Andreev bound state, is often a consequence of non-trivial
topological or symmetry related properties, and can provide indispensable
information about the superconducting state. Here we use scanning tunneling
microscopy/spectroscopy to demonstrate, on the atomic scale, that an isotropic
ZBS emerges at the randomly distributed interstitial excess Fe sites in the
superconducting Fe(Te,Se). This ZBS is localized with a short decay length of ~
10 {\AA}, and surprisingly robust against a magnetic field up to 8 Tesla, as
well as perturbations by neighboring impurities. We find no natural explanation
for the observation of such a robust zero-energy bound state, indicating a
novel mechanism of impurities or an exotic pairing symmetry of the iron-based
superconductivity.
|
1403.1027v1
|
2014-03-10
|
A lithium-ion battery based on a graphene nanoflakes ink anode and a lithium iron phosphate cathode
|
Li-ion rechargeable batteries have enabled the wireless revolution
transforming global communication. Future challenges, however, demands
distributed energy supply at a level that is not feasible with the current
energy-storage technology. New materials, capable of providing higher energy
density are needed. Here we report a new class of lithium-ion batteries based
on a graphene ink anode and a lithium iron phosphate cathode. By carefully
balancing the cell composition and suppressing the initial irreversible
capacity of the anode, we demonstrate an optimal battery performance in terms
of specific capacity, i.e. 165 mAhg-1, estimated energy density of about 190
Whkg-1 and life, with a stable operation for over 80 charge-discharge cycles.
We link these unique properties to the graphene nanoflake anode displaying
crystalline order and high uptake of lithium at the edges, as well as to its
structural and morphological optimization in relation to the overall battery
composition. Our approach, compatible with any printing technologies, is cheap
and scalable and opens up new opportunities for the development of
high-capacity Li-ion batteries.
|
1403.2161v1
|
2014-03-26
|
Realization of practical level current densities in Sr0.6K0.4Fe2As2 tape conductors for high-field applications
|
122 type pnictide superconductors are of particular interest for high-field
applications because of their large upper critical fields Hc2 (> 100 T), low
Hc2 anisotropy r (< 2), and high thin-film critical current densities Jc (> 1
MA/cm2 at 4.2 K). Successful magnet applications require fabrication of
polycrystalline superconducting wires that exhibit large in-field Jc, which is
limited by poor grain coupling and weak-link behavior at grain boundaries. Here
we report our recent achievement in the developing Sr0.6K0.4Fe2As2 tapes with
transport Jc up to 0.1 MA/cm2 at 10 T and 4.2 K. This value is by far the
highest ever recorded for iron based superconducting wires and has surpassed
the threshold for practical application for the first time. The synergy effects
of enhanced grain connectivity, alleviation of the weak-link behavior at grain
boundaries, and the strong intrinsic pinning characteristics led to the
superior Jc performance exhibited in our samples. This advanced Jc result opens
up the possibility for iron-pnictide superconducting wires to win the race in
high-field magnet applications.
|
1403.6704v2
|
2014-04-15
|
Orbital phase resolved spectroscopy of GX 301-2 with MAXI
|
GX 301-2, a bright HMXB with an orbital period of 41.5 days, exhibits stable
periodic orbital intensity modulations with a strong pre-periastron X-ray
flare. Several models have been proposed to explain the accretion at different
orbital phases, invoking accretion via stellar wind, equatorial disk, and
accretion stream from the companion star. We present results from exhaustive
orbital phase resolved spectroscopic measurements of GX 301-2 using data from
the Gas Slit Camera onboard MAXI. Using spectroscopic analysis of the MAXI data
with unprecendented orbital coverage for many orbits continuously, we have
found a strong orbital dependence of the absorption column density and
equivalent width of the iron emission line. A very large equivalent width of
the iron line along with a small value of the column density in the orbital
phase range 0.10-0.30 after the periastron passage indicates presence of high
density absorbing matter behind the neutron star in these orbital phase range.
A low energy excess is also found in the spectrum at orbital phases around the
pre-periastron X-ray flare. The orbital dependence of these parameters are then
used to examine the various models about mode of accretion onto the neutron
star in GX 301-2.
|
1404.3902v1
|
2014-05-01
|
The elemental composition of the Sun II. The iron group elements Sc to Ni
|
We redetermine the abundances of all iron group nuclei in the Sun, based on
neutral and singly-ionised lines of Sc, Ti, V, Mn, Fe, Co and Ni in the solar
spectrum. We employ a realistic 3D hydrodynamic model solar atmosphere,
corrections for departures from local thermodynamic equilibrium (NLTE),
stringent line selection procedures and high quality observational data. We
have scoured the literature for the best quality oscillator strengths,
hyperfine constants and isotopic separations available for our chosen lines. We
find $\log \epsilon_\mathrm{Sc}=3.16\pm0.04$, $\log
\epsilon_\mathrm{Ti}=4.93\pm0.04$, $\log \epsilon_\mathrm{V}=3.89\pm0.08$,
$\log \epsilon_\mathrm{Cr}=5.62\pm0.04$, $\log
\epsilon_\mathrm{Mn}=5.42\pm0.04$, $\log \epsilon_\mathrm{Fe}=7.47\pm0.04$,
$\log \epsilon_\mathrm{Co}=4.93\pm0.05$ and $\log
\epsilon_\mathrm{Ni}=6.20\pm0.04$. Our uncertainties factor in both statistical
and systematic errors (the latter estimated for possible errors in the model
atmospheres and NLTE line formation). The new abundances are generally in good
agreement with the CI meteoritic abundances but with some notable exceptions.
This analysis constitutes both a full exposition and a slight update of the
preliminary results we presented in Asplund, Grevesse, Sauval & Scott
(arXiv:0909.0948), including full line lists and details of all input data we
employed.
|
1405.0287v2
|
2014-05-13
|
Intra-unit-cell nematic charge order in the titanium-oxypnictide family of superconductors
|
Understanding the role played by broken symmetry states such as charge, spin,
and orbital orders in the mechanism of emergent properties such as
high-temperature superconductivity (HTSC) is a major current topic in materials
research. That the order may be within one unit cell, such as nematic, was only
recently considered theoretically, but its observation in the iron-pnictide and
doped cuprate superconductors places it at the forefront of current research.
Here we show that the recently discovered BaTi$_2$Sb$_2$O superconductor and
its "parent" compound BaTi$_2$As$_2$O form a symmetry-breaking nematic ground
state that can be naturally explained as an intra-unit-cell charge order with
$d$-wave symmetry, pointing to the ubiquity of the phenomenon. These findings,
together with the key structural features in these materials being intermediate
between the cuprate and iron-pnictide HTSC materials, render the titanium
oxypnictides an important new material system to understand the nature of
nematic order and its relationship to superconductivity.
|
1405.3236v2
|
2014-05-30
|
Evolution of quasiparticle states with and without a Zn-impurity in doped 122 iron pnictides
|
Based on a minimal two-orbital model [Tai {\it et al.}, Europhys. Lett.
\textbf{103}, 67001 (2013)], which captures the canonical electron-hole-doping
phase diagram of the iron-pnictide BaFe$_{2}$As$_{2}$, we study the evolution
of quasiparticle states as a function of doping using the Bogoliubov-de Gennes
equations with and without a single impurity. Analyzing the density of states
of uniformly doped samples, we are able to identify the origin of the two
superconducting gaps observed in optimally hole- or electron-doped systems. The
local density of states (LDOS) is then examined near a single impurity in
samples without antiferromagnetic order. The qualitative features of our
results near the single impurity are consistent with a work based on a
five-orbital model[K. Toshikaze {\it et al.}, J. Phys. Soc. Jpn. \textbf{79},
083704 (2010)]. This further supports the validity of our two-orbital model in
dealing with LDOS in the single-impurity problem. Finally, we investigate the
evolution of the LDOS with doping near a single impurity in the unitary or
strong scattering limit, such as Zn replacing Fe. The positions of the ingap
resonance peaks exhibited in our LDOS may indirectly reflect the evolution of
the Fermi surface topology according to the phase diagram. Our prediction of
ingap states and the evolution of the LDOS near a strong scattering single
impurity can be validated by experiments probing the local quasiparticle
spectrum.
|
1406.0027v1
|
2014-06-02
|
Superconductivity in titanium-based pnictide oxide compounds
|
Superconductivity in a novel class of layered materials, Ti-based pnictide
oxides, was recently discovered. These compounds have attracted interest since
they combine features of copper oxide and iron pnictide superconductors. Here
the transition metal (titanium) forms two-dimensional Ti$_2$O layers (anti
structure to the CuO$_2$ planes), capped by pnictogen ions (similar to
Fe$_2$As$_2$ layers). The pnictide oxide compounds show a spin or charge
density wave phase which coexists with superconductivity in some members of the
family. Unlike the cuprates, but similar to iron pnictides, the parent
compounds of pnictide oxides are metals with specific nesting properties of the
Fermi surface which leads to the density wave instability. The nature of the
superconductivity, coexisting with the density wave order, and the possible
competition or mutual interaction between both states is one of the central
questions of recent studies. This short review summarizes the current knowledge
from an experimental as well as theoretical point of view and discusses some of
the open questions and possible future developments.
|
1406.0442v1
|
2014-06-02
|
The Distribution of Alpha Elements in Andromeda Dwarf Galaxies
|
We present alpha to iron abundance ratios for 226 individual red giant branch
stars in nine dwarf galaxies of the Andromeda (M 31) satellite system. The
abundances are measured from the combined signal of Mg, Si, Ca, and Ti lines in
Keck/DEIMOS medium-resolution spectra. This constitutes the first large sample
of alpha abundance ratios measured in the M 31 satellite system. The dwarf
galaxies in our sample exhibit a variety of alpha abundance ratios, with the
average values in each galaxy ranging from approximately solar ([alpha/Fe] ~
+0.0) to alpha-enhanced ([alpha/Fe] ~ +0.5). These variations do not show a
correlation with internal kinematics, environment, or stellar density. We
confirm radial gradients in the iron abundance of two galaxies out of the five
with sufficient data (NGC 185 and And II). There is only tentative evidence for
an alpha abundance radial gradient in NGC 185. We homogeneously compare our
results to the Milky Way classical dwarf spheroidals, finding evidence for
wider variation in average alpha abundance. In the absence of chemical
abundances for the M 31 stellar halo, we compare to the Milky Way stellar halo.
A stellar halo comprised of disrupted M31 satellites is too metal-rich and
inconsistent with the Milky Way halo alpha abundance distribution even if
considering only satellites with predominantly old stellar populations. The M
31 satellite population provides a second system in which to study chemical
abundances of dwarf galaxies and reveals a wider variety of abundance patterns
than the Milky Way.
|
1406.0510v1
|
2014-07-11
|
Band-edge BCS-BEC crossover in a two-band superconductor: physical properties and detection parameters
|
Superconductivity in iron-based, magnesium diborides, and other novel
superconducting materials has a strong multi-band and multi-gap character.
Recent experiments support the possibillity for a BCS-BEC crossover induced by
strong-coupling and proximity of the chemical potential to the band edge of one
of the bands. Here we study the simplest theoretical model which accounts for
the BCS-BEC crossover in a two-band superconductor, considering tunable
interactions and tunable energy separations between the bands. Mean-field
results for condensate fraction, correlation length, and superconducting gap
are reported in typical crossover diagrams to locate the boundaries of the BCS,
crossover, and BEC regimes. When the superconducting gap is of the order of the
local chemical potential, superconductivity is in the crossover regime of the
BCS-BEC crossover and the Fermi surface of the small band is smeared by the gap
opening. In this situation, small and large Cooper pairs coexist in the total
condensate, which is the optimal condition for high-Tc superconductivity. The
ratio between the gap and the Fermi energy in a given band results to be the
best detection parameter for experiments to locate the system in the BCS-BEC
crossover. Using available experimental data, our analysis shows that
iron-based superconductors have the partial condensate of the small Fermi
surface in the crossover regime of the BCS-BEC crossover, supporting the recent
ARPES findings.
|
1407.3109v1
|
2014-07-25
|
Spectroscopic parameters for solar-type stars with moderate/high rotation. New parameters for 10 planet-hosts
|
Planetary studies demand precise and accurate stellar parameters as input to
infer the planetary properties. Different methods often provide different
results that could lead to biases in the planetary parameters. In this work, we
present a refinement of the spectral synthesis technique designed to treat
better more rapidly rotating FGK stars. This method is used to derive precise
stellar parameters, namely effective temperature, surface gravity,
metallicitity and rotational velocity. This procedure is tested for samples of
low and moderate/fast rotating FGK stars. The spectroscopic analysis is based
on the spectral synthesis package Spectroscopy Made Easy (SME), assuming Kurucz
model atmospheres in LTE. The line list where the synthesis is conducted, is
comprised of iron lines and the atomic data are derived after solar
calibration. The comparison of our stellar parameters shows good agreement with
literature values, both for low and for higher rotating stars. In addition, our
results are on the same scale with the parameters derived from the iron
ionization and excitation method presented in our previous works. We present
new atmospheric parameters for 10 transiting planet-hosts as an update to the
SWEET-Cat catalogue. We also re-analyse their transit light curves to derive
new updated planetary properties.
|
1407.6765v1
|
2014-08-14
|
Iron based superconductors: A brief overview
|
Fe-based superconductors were discovered in 2008. This discovery with T$_c$
values up to 56 K, generated a new belief in the field of superconductivity.
Till its discovery, high temperature superconductivity in cuprates, created a
prejudice that Cu-oxides are essential building blocks for a high temperature
superconducting material. These Fe based superconductors do not contain Cu-O
planes (some of the materials are even O free). It will be argued in this
review, that these iron pnictide and chalcogenide (FePn/Ch) superconductors
have Fe electrons at the Fermi surface together with an unusual Fermiology that
can change rapidly with doping. This may lead to very different normal and
superconducting state properties compared to those in standard electron-phonon
coupled {\it conventional} superconductors. There are a large number of
evidences showing that superconductivity, magnetism, orbital fluctuations are
intimately related and coexist in these materials although the mechanism of
superconductivity in these compounds is still unknown. The electronic specific
heat, $\frac{2\Delta}{k_B T_c}$ ratio, phase diagrams, isotope effect, crystal
structures and there correlation to T$_c$ from various available experimental
data are main inputs of this review to show the above.
|
1408.3244v1
|
2014-08-21
|
The changing X-ray time lag in MCG-6-30-15
|
MCG-6-30-15 is one of the most observed Narrow Line Seyfert 1 galaxies in the
X-ray band. In this paper we examine the X-ray time lags in this source using a
total of 600 ks in observations (440 ks exposure) taken with the XMM-Newton
telescope (300 ks in 2001 and 300 ks in 2013). Both the old and new
observations show the usual hard lag that increases with energy, however, the
hard lag turns over to a soft lag at frequencies below ~1e-4 Hz. The highest
frequencies (~1e-3 Hz) in this source show a clear soft lag, as previously
presented for the first 300 ks observation, but no clear iron K lag is detected
in either the old or new observation. The soft lag is more significant in the
old observation than the new. The observations are consistent with a
reverberation interpretation, where the soft, reflected emission is delayed
with respect to the hard powerlaw component. These spectral timing results
suggest that two distinct variability mechanisms are important in this source:
intrinsic coronal variations (which lead to correlated variability in the
reprocessed emission), and geometrical changes in the corona. Variability due
to geometrical changes does not result in correlated variability in the
reflection, and therefore inhibits the clear detection of an iron K lag.
|
1408.5051v1
|
2014-09-01
|
Chemical Evolution on the Scale of Clusters of Galaxies: A Conundrum?
|
The metal content of clusters of galaxies and its relation to their stellar
content is revisited making use of a cluster sample for which all four basic
parameters are homogeneously measured within consistent radii, namely
core-excised mass-weighted metallicity plus total, stellar and ICM masses. For
clusters of total mass $M_{500} >$ $\sim 10^{14}$ $M_{\odot}$ nice agreement is
found between their iron content and what expected from empirical supernova
yields. For the same clusters, there also appears to be at least as much iron
in the intracluster medium (ICM) as there is still locked into stars (i.e., the
ICM/stars metal share is about unity). However, for more massive clusters the
stellar mass fraction appears to drop substantially without being accompanied
by a drop in the ICM metallicity, thus generating a major tension with the
nucleosynthesis expectation and inflating the metal share to extremely high
values (up to $\sim 6$). Various possible solutions of this conundrum are
discussed, but are all considered either astrophysically implausible, or
lacking an independent observational support. For this reason we still
entertain the possibility that even some of the best cluster data may be
faulty, though we are not able to identify any obvious bias. Finally, based on
the stellar mass-metallicity relation for local galaxies we estimate the
contribution of galaxies to the ICM enrichment as a function of their mass,
concluding that even the most massive galaxies must have lost a major fraction
of the metals they have produced.
|
1409.0307v1
|
2014-09-05
|
Ceci n'est pas a globular cluster: the metallicity distribution of the stellar system Terzan 5
|
We present new determinations of the iron abundance for 220 stars belonging
to the stellar system Terzan 5 in the Galactic bulge. The spectra have been
acquired with FLAMES at the Very Large Telescope of the European Southern
Observatory and DEIMOS at the Keck II Telescope. This is by far the largest
spectroscopic sample of stars ever observed in this stellar system. From this
dataset, a subsample of targets with spectra unaffected by TiO bands was
extracted and statistically decontaminated from field stars. Once combined with
34 additional stars previously published by our group, a total sample of 135
member stars covering the entire radial extent of the system has been used to
determine the metallicity distribution function of Terzan 5. The iron
distribution clearly shows three peaks: a super-solar component at
[Fe/H]$\simeq0.25$ dex, accounting for 29% of the sample, a dominant sub-solar
population at [Fe/H]$\simeq-0.30$ dex, corresponding to 62% of the total, and a
minor (6%) metal-poor component at [Fe/H]$\simeq-0.8$ dex. Such a broad,
multi-modal metallicity distribution demonstrates that Terzan 5 is not a
genuine globular cluster but the remnant of a much more complex stellar system.
|
1409.1682v1
|
2014-09-15
|
Do cluster properties affect the quenching rate?
|
The quenching rate is known to depend on galaxy stellar mass and environment,
however, possible dependences on the hosting halo properties, such as mass,
richness, and dynamical status, are still debated. The determination of these
dependences is hampered by systematics, induced by noisy estimates of cluster
mass or by the lack of control on galaxy stellar mass, which may mask existing
trends or introduce fake trends. We studied a sample of local clusters (20 with
0.02<z<0.1 and log(M200/Msun)>14), selected independent of the galaxy
properties under study, having homogeneous optical photometry and X-ray
estimated properties. Using those top quality measurements of cluster mass,
hence of cluster scale, richness, iron abundance, and cooling time/presence of
a cool-core, we study the simultaneous dependence of quenching on these cluster
properties on galaxy stellar mass M and normalised cluster-centric distance
r/r200. We found that the quenching rate can be completely described by two
variables only, galaxy stellar mass and normalised cluster-centric distance,
and is independent of halo properties (mass, richness, iron abundance, presence
of a cool-core, and central cooling time). These halo properties change, in
most cases, by less than 3% the probability that a galaxy is quenched, once the
mass-size (M200-r200) scaling relation is accounted for through cluster-centric
distance normalisation.
|
1409.4416v1
|
2014-09-23
|
Structural and Magnetic Phase Diagram of CrAs and its Relationship with Pressure-induced Superconductivity
|
Most unconventional superconductors, including cuprates and iron-based
superconductors, are derived from chemical doping or application of pressure on
their collinearly magnetic-ordered parent compounds[1-5]. The recently
discovered pressure-induced superconductor CrAs, as a rare example of a
non-collinear helimagnetic superconductor, has therefore generated great
interest in understanding microscopic magnetic properties and their interplay
with superconductivity [6-8]. Unlike cuprates and iron based superconductors
where the magnetic moment direction barely changes upon doping, here we show
that CrAs exhibits a spin reorientation from the ab plane to the ac plane,
along with an abrupt drop of the magnetic propagation vector at a critical
pressure (Pc~0.6 GPa). This magnetic phase transition coincides with the
emergence of bulk superconductivity, indicating a direct connection between
magnetism and superconductivity. With further increasing pressure, the magnetic
order completely disappears near the optimal Tc regime (P~0.94 GPa). Moreover,
the Cr magnetic moments between nearest neighbors tend to be aligned
antiparallel with increasing pressure toward the optimal superconductivity
regime. Our findings suggest that the non-collinear helimagnetic order is
strongly coupled to structural and electronic degrees of freedom, and that
antiferromagnetic correlations associated with the low magnetic vector phase
are crucial for superconductivity.
|
1409.6615v3
|
2014-09-29
|
Disk emission and absorption lines in LMXB. Note on the physical conditions of an absorbing material
|
We show that the continuum X-ray spectrum of 4U 1630-472 with iron absorption
lines can be satisfactorily modeled by the spectrum from an accretion disk
atmosphere. We performed full radiative transfer calculations using our code
ATM21 to model the emission from an accretion disk surface that is seen at
different viewing angles. Computed models are then fitted to the
high-resolution X-ray spectra of 4U 1630-472 obtained by {\it Suzaku}
satellite. Absorption lines of highly ionized iron originating in a hot
accretion-disk atmospheres are important part of the observed line profile, and
can be an alternative or complementary explanation to the wind model usually
favored for this type of sources. Next, assuming that absorption lines
originate from the wind illuminated by X-ray central source in LMXBs, we can
put constrains on the wind location only if we know the volume density number
of the absorbing material. There are a few derivations of the distance to the
wind in X-ray binaries. We show here, that the density number and the wind
location agree with the density of an upper disk atmosphere at optical depth of
two-thirds, at the same distance from the black hole. This comparison is done
assuming optically thick, geometrically thin standard accretion disk model.
Nevertheless, it shows that the wind physical conditions are the same as in
thermalized disk gas, and we only have to figure out how the wind is blowing?
|
1409.8052v1
|
2014-11-03
|
Discovery of a ~2 hr high frequency X-ray QPO and iron K alpha reverberation in the active galaxy MS 2254.9-3712
|
We report the discovery of a $\sim 1.5 \times 10^{-4}$ Hz ($\sim 2$ hr) X-ray
quasi-periodic oscillation (QPO) in the active galaxy MS 2254.9-3712, using a
$\sim 70$ ks XMM-Newton observation. The QPO is significantly detected ($\sim
3.3 \sigma$) in the $1.2 - 5.0$ keV band only, connecting its origin with the
primary X-ray power-law continuum. We detect a highly coherent soft lag between
the $0.3 - 0.7$ keV and $1.2 - 5.0$ keV energy bands at the QPO frequency and
at a frequency band in a 3:2 ratio, strongly suggesting the presence of a QPO
harmonic. An iron K$\alpha$ reverberation lag is found at the harmonic
frequency, indicating the reflecting material subtends some angle to the
primary continuum, which is modulated by the QPO mechanism. Frequency resolved
spectroscopy reveals the QPO and harmonic to have a hard energy dependence.
These properties of the QPO variability, together with the current black hole
mass estimate, $M_{\rm bh} \sim 4 \times 10^{6} M_{\rm sun}$, are consistent
with the QPO originating from the same process as the high frequency QPO
phenomenon observed in black hole X-ray binaries. Principle component analysis
reveals the spectral variability in MS 2254.9-3712 is similar to that of the
active galaxy RE J1034+396, a source which also displays an X-ray QPO. This
suggests a distinct spectral variability pattern for accreting black holes when
in a state where QPOs are present.
|
1411.0684v2
|
2014-11-06
|
Ising-nematic order in the bilinear-biquadratic model for the iron pnictides
|
Motivated by the recent inelastic neutron scattering (INS) measurements in
the iron pnictides which show a strong anisotropy of spin excitations in
directions perpendicular and parallel to the ordering wave-vector even above
the magnetic transition temperature $T_N$, we study the frustrated Heisenberg
model with a biquadratic spin-spin exchange interaction. Using the Dyson-Maleev
(DM) representation, which proves appropriate for all temperature regimes, we
find that the spin-spin dynamical structure factors are in excellent agreement
with experiment, exhibiting breaking of the $C_4$ symmetry even into the
paramagnetic region $T_N<T<T_{\sigma}$ which we refer to as the Ising-nematic
phase. In addition to the Heisenberg spin interaction, we include the
biquadratic coupling $K (\mathbf{S}_i\cdot \mathbf{S}_j)^2$ and study its
effect on the dynamical temperature range $T_{\sigma}-T_N$ of the Ising-nematic
phase. We find that this range reduces dramatically when even small values of
the interlayer exchange $J_c$ and biquadratic coupling $K$ are included. To
supplement our analysis, we benchmark the results obtained using the DM method
against those from different non-linear spin-wave theories, including the
recently developed generalized spin-wave theory (GSWT), and find good
qualitative agreement among the different theoretical approaches as well as
experiment for both the spin-wave dispersions and the dynamical structure
factors.
|
1411.1462v2
|
2014-11-07
|
Phase-resolved spectroscopy of low frequency quasi-periodic oscillations in GRS 1915+105
|
X-ray radiation from black hole binary (BHB) systems regularly displays
quasi-periodic oscillations (QPOs). In principle, a number of suggested
physical mechanisms can reproduce their power spectral properties, thus more
powerful diagnostics which preserve phase are required to discern between
different models. In this paper, we first find for two Rossi X-ray Timing
Explorer (RXTE) observations of the BHB GRS 1915+105 that the QPO has a well
defined average waveform. That is, the phase difference and amplitude ratios
between the first two harmonics vary tightly around a well defined mean. This
enables us to reconstruct QPO waveforms in each energy channel, in order to
constrain QPO phase-resolved spectra. We fit these phase resolved spectra
across 16 phases with a model including Comptonisation and reflection (Gaussian
and smeared edge components) to find strong spectral pivoting and a modulation
in the iron line equivalent width. The latter indicates the observed reflection
fraction is changing throughout the QPO cycle. This points to a geometric QPO
origin, although we note that the data presented here do not entirely rule out
an alternative interpretation of variable disc ionisation state. We also see
tentative hints of modulations in the iron line centroid and width which,
although not statistically significant, could result from a non-azimuthally
symmetric QPO mechanism.
|
1411.1967v1
|
2014-11-19
|
The Seyfert 2 galaxy NGC 2110: hard X-ray emission observed by NuSTAR and variability of the iron K$α$ line
|
We present NuSTAR observations of the bright Seyfert 2 galaxy NGC 2110
obtained in 2012, when the source was at the highest flux level ever observed,
and in 2013, when the source was at a more typical flux level. We include
archival observations from other X-ray satellites, namely XMM-Newton, Suzaku,
BeppoSAX, Chandra and Swift. Simultaneous NuSTAR and Swift broad band spectra
(in the 3-80 keV range) indicate a cutoff energy $E_{\rm c}>210$ keV, with no
detectable contribution from Compton reflection. NGC 2110 is one of the very
few sources where no evidence for distant Compton thick scattering is found
and, by using temporal information collected over more than a decade, we
investigate variations of the iron K$\alpha$ line on time scales of years. The
Fe K$\alpha$ line is likely the sum of two components: one constant
(originating from distant Compton-thick material) and the other one variable
and linearly correlated with the source flux (possibly arising from
Compton-thin material much closer to the black hole).
|
1411.5151v1
|
2014-11-20
|
The Role of the Magnetorotational Instability in Massive Stars
|
The magnetorotational instability (MRI) is key physics in accretion disks and
is widely considered to play some role in massive-star core collapse. Models of
rotating massive stars naturally develop very strong shear at composition
boundaries, a necessary condition for MRI instability, and the MRI is subject
to triply-diffusive destabilizing effects in radiative regions. We have used
the MESA stellar evolution code to compute magnetic effects due to the
Spruit-Taylor mechanism and the MRI, separately and together, in a sample of
massive star models. We find that the MRI can be active in the later stages of
massive star evolution, leading to mixing effects that are not captured in
models that neglect the MRI. The MRI and related magneto-rotational effects can
move models of given ZAMS mass across "boundaries" from degenerate CO cores to
degenerate O/Ne/Mg cores and from degenerate O/Ne/Mg cores to iron cores, thus
affecting the final evolution and the physics of core collapse. The MRI acting
alone can slow the rotation of the inner core in general agreement with the
observed "initial" rotation rates of pulsars. The MRI analysis suggests that
localized fields ~10^{12} G may exist at the boundary of the iron core. With
both the ST and MRI mechanisms active in the 20 solar mass model, we find that
the helium shell mixes entirely out into the envelope. Enhanced mixing could
yield a population of yellow or even blue supergiant supernova progenitors that
would not be standard SN IIP.
|
1411.5714v1
|
2014-12-05
|
A NuSTAR observation of the fast symbiotic nova V745 Sco in outburst
|
The fast recurrent nova V745 Sco was observed in the 3-79 keV X-rays band
with NuSTAR 10 days after the optical discovery. The measured X-ray emission is
consistent with a collisionally ionized optically thin plasma at temperature of
about 2.7 keV. A prominent iron line observed at 6.7 keV does not require
enhanced iron in the ejecta. We attribute the X-ray flux to shocked
circumstellar material. No X-ray emission was observed at energies above 20
keV, and the flux in the 3-20 keV range was about 1.6 $\times$ 10$^{-11}$ erg
cm$^{-2}$ s$^{-1}$. The emission measure indicates an average electron density
of order of 10$^7$ cm$^{-3}$.
The X-ray flux in the 0.3-10 keV band almost simultaneously measured with
Swift was about 40 times larger, mainly due to the luminous central supersoft
source emitting at energy below 1 keV. The fact that the NuSTAR spectrum cannot
be fitted with a power law, and the lack of hard X-ray emission, allow us to
rule out Comptonized gamma rays, and to place an upper limit of the order of
10$^{-11}$ erg cm$^{-2}$ s$^{-1}$ on the gamma-ray flux of the nova on the
tenth day of the outburst.
|
1412.2088v2
|
2014-12-09
|
The final fate of stars that ignite neon and oxygen off-center: electron capture or iron core-collapse supernova?
|
In the ONeMg cores of $8.8-9.5~{\rm M}_\odot$ stars, neon and oxygen burning
is ignited off-center. Whether the neon-oxygen flame propagates to the center
is critical to determine whether these stars undergo Fe core collapse or
electron capture induced ONeMg core collapse. We present more details of stars
that ignite neon and oxygen burning off-center. The neon flame is established
in a similar manner to the carbon flame of super-AGB stars, albeit with a
narrower flame width. The criteria for establishing a flame are able to be met
if the strict Schwarzschild criterion for convective instability is adopted.
Mixing across the interface of the convective shell disrupts the conditions for
the propagation of the burning front and instead the shell burns as a series of
inward-moving flashes. While this may not directly affect whether the burning
will reach the center (as in super-AGB stars), the core is allowed to contract
between each shell flash. Reduction of the electron fraction in the shell
reduces the Chandrasekhar mass and the center reaches the threshold density for
the URCA process to activate and steer the remaining evolution of the core.
This highlights the importance of a more accurate treatment of mixing in the
stellar interior for yet another important question in stellar astrophysics -
determining the properties of stellar evolution and supernova progenitors at
the boundary between electron capture supernova and iron core-collapse
supernova.
|
1412.2878v2
|
2014-12-15
|
Synthesis, Structural, and Transport Properties of Cr-doped BaTi_2As_2O
|
The interplay between unconventional superconductivity and the ordering of
charge/spin density wave is one of the most vital issues in both condensed
matter physics and material science. The Ti-based compound BaTi_2As_2O, which
can be seen as the parent phase of superconducting BaTi_2Sb_2O, has a layered
structure with a space group P4/mmm, similar to that of cuprate and iron-based
superconductors. This material exhibits a charge density wave (CDW) ordering
transition revealed by an anomaly at around 200 K in transport measurements.
Here, we report the synthesis and systematical study of the physical properties
in Cr-doped BaTi_{2-x}Cr_xAs_2O (x = 0 - 0.154), and demonstrate that the
transition temperature of the CDW ordering is suppressed gradually by the doped
Cr element. The magnetization measurements confirm the evolution of the CDW
ordering transition. These observed behaviors are similar to that observed in
iron-based superconductors, but no superconductivity emerges down to 2 K. In
addition, the first-principles calculations are also carried out for
well-understanding the nature of experimental observations.
|
1412.4469v1
|
2014-12-16
|
Characterization of 3 mm Glass Electrodes and Development of RPC Detectors for $INO-ICAL$ Experiment
|
India-based Neutrino Observatory (INO) is a multi-institutional facility,
planned to be built up in South India. The INO facility will host a 51 kton
magnetized Iron CALorimeter (ICAL) detector to study atmospheric muon
neutrinos. Iron plates have been chosen as the target material whereas
Resistive Plate Chambers (RPCs) have been chosen as the active detector element
for the ICAL experiment. Due to the large number of RPCs needed ($\sim$ 28,000
of $2~m \times 2~m$ in size) for ICAL experiment and for the long lifetime of
the experiment, it is necessary to perform a detailed $R\&D$ such that each and
every parameter of the detector performance can be optimized to improve the
physics output. In this paper, we report on the detailed material and
electrical properties studies for various types of glass electrodes available
locally. We also report on the performance studies carried out on the RPCs made
with these electrodes as well as the effect of gas composition and
environmental temperature on the detector performance. We also lay emphasis on
the usage of materials for RPC electrodes and the suitable enviormental
conditions applicable for operating the RPC detector for optimal physics output
at INO-ICAL experiment.
|
1412.4998v1
|
2014-12-16
|
Thermally Induced Chemistry of Meteoritic Complex Organic Molecules: A New Heat-Diffusion Model for the Atmospheric Entry of Meteorites
|
Research over the past four decades has shown a rich variety of complex
organic molecular content in some meteorites. This current study is an attempt
to gain a better insight into the thermal conditions experienced by these
molecules inside meteorites during atmospheric entry. In particular, we wish to
understand possible chemical processes that can occur during entry and that
might have had an effect on complex organic or prebiotic species that were
delivered in this way to the early Earth. A simulation was written in Fortran
to model heating by the shock generated during entry and the subsequent thermal
diffusion inside the body of a meteorite. Experimental data was used for the
thermal parameters of several types of meteorites, including iron-nickel and
several classes of chondrites. A Sutton-Graves model of stagnation-point
heating was used to calculate peak surface temperatures and an explicit
difference formula was used to generate thermal diffusion profiles for both
chondrites and iron-nickel type meteorites. Results from the simulation show
pyrolytic temperature penetration to a depth of ca. 0.5 to 1 cm.
Non-dissociative warming of meteorite interiors penetrates further to ca. 4 cm.
These results support the findings that extraterrestrial delivery is a viable
option for prebiotic molecular "seeding" of a planet.
|
1412.5134v3
|
2014-12-19
|
Universal V-shaped temperature-pressure phase diagram in the iron-based superconductors KFe2As2, RbFe2As2, and CsFe2As2
|
We report a sudden reversal in the pressure dependence of Tc in the
iron-based superconductor RbFe2As2, at a critical pressure Pc = 11 kbar.
Combined with our prior results on KFe2As2 and CsFe2As2, we find a universal
V-shaped phase diagram for Tc vs P in these fully hole-doped 122 materials,
when measured relative to the critical point (Pc, Tc). From measurements of the
upper critical field Hc2(T) under pressure in KFe2As2 and RbFe2As2, we observe
the same two-fold jump in (1/Tc)(-dHc2/dT) across Pc, compelling evidence for a
sudden change in the structure of the superconducting gap. We argue that this
change is due to a transition from one pairing state to another, with different
symmetries on either side of Pc. We discuss a possible link between scattering
and pairing, and a scenario where a d-wave state favored by high-Q scattering
at low pressure changes to a state with s+- symmetry favored by low-Q
scattering at high pressure.
|
1412.6196v2
|
2014-12-19
|
Resonant inelastic x-ray scattering study of electronic excitations in insulating K$_{0.83}$Fe$_{1.53}$Se$_2$
|
We report an Fe $K$-edge resonant inelastic X-ray scattering (RIXS) study of
K$_{0.83}$Fe$_{1.53}$Se$_2$. This material is an insulator, unlike many parent
compounds of iron-based superconductors. We found a sharp excitation around 1
eV, which is resonantly enhanced when the incident photon energy is tuned near
the pre-edge region of the absorption spectrum. The spectral weight and line
shape of this excitation exhibit clear momentum dependence. In addition, we
observe momentum-independent broad interband transitions at higher excitation
energy of 3-7 eV. Calculations based on a 70 band $dp$ orbital model, using a
moderate $U_{\rm eff}\approx 2.5$ eV, indicate that the $\sim$1 eV feature
originates from the correlated Fe 3$d$ electrons, with a dominant $d_{xz}$ and
$d_{yz}$ orbital character. We find that a moderate $U_{\rm eff}$ yields a
satisfying agreement with the experimental spectra, suggesting that the
electron correlations in the insulating and metallic iron based superconductors
are comparable.
|
1412.6429v2
|
2014-12-22
|
Inverse magneto-refraction as a mechanism for laser modification of spin-spin exchange parameters and subsequent terahertz emission from iron oxides
|
Ultrafast non-thermal manipulation of magnetization by light relies on either
indirect coupling of the electric field component of the light with spins via
spin-orbit interaction or direct coupling between the magnetic field component
and spins. Here we propose a novel scenario for coupling between the electric
field of light and spins via optical modification of the exchange interaction,
one of the strongest quantum effects, the strength of which can reach 1000
Tesla. We demonstrate that this isotropic opto-magnetic effect, which can be
called the inverse magneto-refraction, is allowed in a material of any
symmetry. Its existence is corroborated by the experimental observation of THz
emission by magnetic-dipole active spin resonances optically excited in a broad
class of iron oxides with a canted spin configuration. From its strength we
estimate that a sub-picosecond laser pulse with a moderate fluence of ~ 1
mJ/cm^2 acts as a pulsed effective magnetic field of 0.01 Tesla, arising from
the optically perturbed balance between the exchange parameters. Our findings
are supported by a low-energy theory for the microscopic magnetic interactions
between non-equilibrium electrons subjected to an optical field which suggests
a possibility to modify the exchange interactions by light over 1 %.
|
1412.7094v1
|
2014-12-24
|
Tests of the nuclear equation of state and superfluid and superconducting gaps using the Cassiopeia A neutron star
|
The observed rapid cooling of the Cassiopeia A neutron star can be
interpreted as being caused by neutron and proton transitions from normal to
superfluid and superconducting states in the stellar core. Here we present two
new Chandra ACIS-S Graded observations of this neutron star and measurements of
the neutron star mass M and radius R found from consistent fitting of both the
X-ray spectra and cooling behavior. This comparison is only possible for
individual nuclear equations of state. We test phenomenological superfluid and
superconducting gap models which mimic many of the known theoretical models
against the cooling behavior. Our best-fit solution to the Cassiopeia A data is
one in which the (M,R) = (1.44 Msun,12.6 km) neutron star is built with the
BSk21 equation of state, strong proton superconductor and moderate neutron
triplet superfluid gap models, and a pure iron envelope or a thin carbon layer
on top of an iron envelope, although there are still large observational and
theoretical uncertainties.
|
1412.7759v2
|
2015-01-07
|
Detoxification of superoxide without production of H2O2: antioxidant activity of superoxide reductase complexed with ferrocyanide
|
The superoxide radical O(2)(-.) is a toxic by-product of oxygen metabolism.
Two O(2)(-.) detoxifying enzymes have been described so far, superoxide
dismutase and superoxide reductase (SOR), both forming H2O2 as a reaction
product. Recently, the SOR active site, a ferrous iron in a [Fe(2+) (N-His)(4)
(S-Cys)] pentacoordination, was shown to have the ability to form a complex
with the organometallic compound ferrocyanide. Here, we have investigated in
detail the reactivity of the SOR-ferrocyanide complex with O(2)(-.) by pulse
and gamma-ray radiolysis, infrared, and UV-visible spectroscopies. The complex
reacts very efficiently with O(2)(-.). However, the presence of the
ferrocyanide adduct markedly modifies the reaction mechanism of SOR, with the
formation of transient intermediates different from those observed for SOR
alone. A one-electron redox chemistry appears to be carried out by the
ferrocyanide moiety of the complex, whereas the SOR iron site remains in the
reduced state. Surprisingly, the toxic H2O2 species is no longer the reaction
product. Accordingly, in vivo experiments showed that formation of the
SOR-ferrocyanide complex increased the antioxidant capabilities of SOR
expressed in an Escherichia coli sodA sodB recA mutant strain. Altogether,
these data describe an unprecedented O(2)(-.) detoxification activity,
catalyzed by the SOR-ferrocyanide complex, which does not conduct to the
production of the toxic H2O2 species.
|
1501.01439v1
|
2015-01-12
|
X-ray Probes of Black Hole Accretion Disks for Testing the No-Hair Theorem
|
The spins of a number of supermassive and stellar-mass black holes have been
measured based on detections of thermal continuum emission and relativistically
broadened iron lines in their x-ray spectra. Likewise, quasiperiodic
variability has been observed in several sources. Such measurements commonly
make the assumption that black holes are described by the Kerr metric, which
according to the no-hair theorem characterizes black holes uniquely in terms of
their masses and spins. This fundamental property of black holes can be tested
observationally by measuring potential deviations from the Kerr metric
introduced by a parametrically deformed Kerr-like spacetime. Thermal spectra,
iron lines, and variability have already been studied extensively in several
such metrics, which usually depend on only one particular type of deviation or
contain unphysical regions outside of the compact object. In this paper, I
study these x-ray probes in the background of a new Kerr-like metric which
depends on four independent deviation functions and is free of pathological
regions outside of the event horizon. I show that the observed signals depend
significantly on primarily two types of deviations and that the strong
correlation between the spin and the deviation parameters found previously in
other Kerr-like metrics is partially broken for rapidly spinning black holes.
This suggests that high-spin sources are the best candidates for tests of the
no-hair theorem with x-rays and I obtain first constraints on such deviations
from the stellar-mass black hole Cygnus X-1.
|
1501.02815v1
|
2015-01-22
|
Mathematical analysis of the $PO_4$-$DOP$-$Fe$ marine ecosystem model driven by 3-D ocean transport
|
Marine ecosystem models are developed to understand and simulate the
biogeochemical processes involved in marine ecosystems. Parekh, Follows and
Boyle introduced the $PO_4$-$DOP$-$Fe$ model of the coupled phosphorus and iron
cycles in 2005. Especially the part describing the phosphorus cycle
($PO_4$-$DOP$ model) is often applied in the context of parameter
identification. The mathematical analysis presented in this study is concerned
with the existence of solutions and the reconstruction of parameters from given
data. Both are important questions in the numerical model's assessment and
validation not answered so far. In this study, we obtain transient, stationary
and periodic solutions (steady annual cycles) of the $PO_4$-$DOP$-$Fe$ model
equations after a slight change in the equation modeling iron. This result
confirms the validity of the solutions computed numerically. Furthermore, we
present a calculation showing that four of the $PO_4$-$DOP$ model's parameters
are possibly dependent, i.e. different parameter values might be associated
with the same model output. Thereby, we identify a relevant source of
uncertainty in parameter identification. On the basis of the results, possible
ways to overcome this deficit can be proposed. In addition, the stated
mathematical conditions for solvability are universal and thus applicable to
the analysis of other ecosystem models as well.
|
1501.05428v1
|
2015-01-26
|
Origin of the non-monotonic variance of Tc in the 1111 iron based superconductors with isovalent doping
|
Motivated by recent experimental investigations of the isovalent doping
iron-based superconductors LaFe(AsxP1-x)O1-yFy and NdFe(AsxP1-x)O1-yFy we
theoretically study the correlation between the local lattice structure, the
Fermi surface, the spin fluctuation-mediated superconductivity, and the
composition ratio. In the phosphides, the dXZ and dYZ orbitals barely hybridize
around the Gamma point to give rise to two intersecting ellipse shape Fermi
surfaces. As the arsenic content increases and the Fe-As-Fe bond angle is
reduced, the hybridization increases, so that the two bands are mixed to result
in concentric inner and outer Fermi surfaces, and the orbital character
gradually changes to dxz and dyz, where x-y axes are rotated by 45 degrees from
X-Y. This makes the orbital matching between the electron and hole Fermi
surfaces better and enhances the spin fluctuation within the dxz/yz orbitals.
On the other hand, the hybridization splits the two bands, resulting in a more
dispersive inner band. Hence, there is a trade-off between the density of
states and the orbital matching, thereby locally maximizing the dxz/yz spin
fluctuation and superconductivity in the intermediate regime of As/P ratio. The
consistency with the experiment strongly indicate the importance of the spin
fluctuation played in this series of superconductors.
|
1501.06303v1
|
2015-03-03
|
Tunable room-temperature ferromagnet using an iron-oxide and graphene oxide nanocomposite
|
Magnetic materials have found wide application ranging from electronics and
memories to medicine. Essential to these advances is the control of the
magnetic order. To date, most room-temperature applications have a fixed
magnetic moment whose orientation is manipulated for functionality. Here we
demonstrate an iron-oxide and graphene oxide nanocomposite based device that
acts as a tunable ferromagnet at room temperature. Not only can we tune its
transition temperature in a wide range of temperatures around room temperature,
but the magnetization can also be tuned from zero to 0.011 A.m$^2$/kg through
an initialization process with two readily accessible knobs (magnetic field and
electric current), after which the system retains its magnetic properties
semi-permanently until the next initialization process. We construct a
theoretical model to illustrate that this tunability originates from an
indirect exchange interaction mediated by spin-imbalanced electrons inside the
nanocomposite.
|
1503.00960v1
|
2015-03-03
|
Dark Matter Balls Help Supernovae to Explode
|
As a solution to the well-known problem that the shock wave potentially
responsible for the explosion of a supernova actually tends to stall, we
propose a new energy source arising from our model for dark matter. Our earlier
model proposed that dark matter should consist of cm-large white dwarf-like
objects kept together by a skin separating two different sorts of vacua. These
dark matter balls or pearls will collect in the middle of any star throughout
its lifetime. At some stage during the development of a supernova the balls
will begin to take in neutrons and then other surrounding material. By passing
into a ball nucleons fall through a potential of order 10 MeV, causing a severe
production of heat - of order 10 foe for a solar mass of material eaten by the
balls. The temperature in the iron core will thereby be raised, splitting up
the iron into smaller nuclei. This provides a mechanism for reviving the shock
wave when it arrives and making the supernova explosion really occur. The onset
of the heating due to the dark matter balls would at first stop the collapse of
the supernova progenitor. This opens up the possibility of there being {\em
two} collapses giving two neutrino outbursts, as apparently seen in the
supernova SN1987A - one in Mont Blanc, and one 4 hours 43 minutes later in both
IMB and Kamiokande.
|
1503.01089v2
|
2015-03-05
|
Distinctive momentum dependence of the band reconstruction in the nematic state of FeSe thin film
|
Nematic state, where the system is translationally invariant but breaks the
rotational symmetry, has drawn great attentions recently due to experimental
observations of such a state in both cuprates and iron-based superconductors.
The mechanism of nematicity that is likely tied to the pairing mechanism of
high-Tc, however, still remains controversial. Here, we studied the electronic
structure of multilayer FeSe film by angle-resolved photoemission spectroscopy
(ARPES). We found that the FeSe film enters the nematic state around 125 K,
while the electronic signature of long range magnetic order has not been
observed down to 20K indicating the non-magnetic origin of the nematicity. The
band reconstruction in the nematic state is characterized by the splitting of
the dxz and dyz bands. More intriguingly, such energy splitting is strong
momentum dependent with the largest band splitting of ~80meV at the zone
corner. The simple on-site ferro-orbital ordering is insufficient to reproduce
the nontrivial momentum dependence of the band reconstruction. Instead, our
results suggest that the nearest-neighbor hopping of dxz and dyz is highly
anisotropic in the nematic state, the origin of which holds the key in
understanding the nematicity in iron-based superconductors.
|
1503.01556v1
|
2015-03-09
|
Determination of the spectroscopic stellar parameters for 257 field giant stars
|
The study of stellar parameters of planet-hosting stars, such as metallicity
and chemical abundances, help us to understand the theory of planet formation
and stellar evolution. Here, we present a catalogue of accurate stellar
atmospheric parameters and iron abundances for a sample of 257 K and G field
evolved stars that are being surveyed for planets using precise
radial--velocity measurements as part of the CORALIE programme to search for
planets around giants. The analysis was done using a set of high--resolution
and high--signal-to-noise Ultraviolet and Visible Echelle Spectrograph spectra.
The stellar parameters were derived using Fe I and II ionization and excitation
equilibrium methods. To take into account possible effects related to the
choice of the lines on the derived parameters, we used three different iron
line-list sets in our analysis, and the results differ among themselves by a
small factor for most of stars. {For those stars with previous literature
parameter estimates, we found very good agreement with our own values.} In the
present catalogue we are providing new precise spectroscopic measurements of
effective temperature, surface gravity, microturbulence, and metallicity for
190 stars for which it has not been found or published in previous articles.
|
1503.02556v1
|
2015-03-10
|
Detailed Abundances of 15 Stars in the Metal-Poor Globular Cluster NGC 4833
|
We have observed 15 red giant stars in the relatively massive, metal-poor
globular cluster NGC 4833 using the Magellan Inamori Kyocera Echelle
spectrograph at Magellan. We calculate stellar parameters for each star and
perform a standard abundance analysis to derive abundances of 43 species of 39
elements, including 20 elements heavier than the iron group. We derive <[Fe/H]>
= -2.25 +/- 0.02 from Fe I lines and <[Fe/H> = -2.19 +/- 0.013 from Fe II
lines. We confirm earlier results that found no internal metallicity spread in
NGC 4833, and there are no significant star-to-star abundance dispersions among
any elements in the iron group (19 <= Z <= 30). We recover the usual abundance
variations among the light elements C, N, O, Na, Mg, Al, and possibly Si. The
heavy-element distribution reflects enrichment by r-process nucleosynthesis
([Eu/Fe] = +0.36 +/- 0.03), as found in many other metal-poor globular
clusters. We investigate small star-to-star variations found among the
neutron-capture elements, and we conclude that these are probably not real
variations. Upper limits on the Th abundance, log epsilon (Th/Eu) < -0.47 +/-
0.09, indicate that NGC 4833, like other globular clusters where Th has been
studied, did not experience a so-called "actinide boost."
|
1503.03079v2
|
2015-03-11
|
Molecular Beam Epitaxy Growth of Superconducting LiFeAs Film on SrTiO3(001) Substrate
|
The stoichiometric "111" iron-based superconductor, LiFeAs, has attacted
great research interest in recent years. For the first time, we have
successfully grown LiFeAs thin film by molecular beam epitaxy (MBE) on
SrTiO3(001) substrate, and studied the interfacial growth behavior by
reflection high energy electron diffraction (RHEED) and low-temperature
scanning tunneling microscope (LT-STM). The effects of substrate temperature
and Li/Fe flux ratio were investigated. Uniform LiFeAs film as thin as 3
quintuple-layer (QL) is formed. Superconducting gap appears in LiFeAs films
thicker than 4 QL at 4.7 K. When the film is thicker than 13 QL, the
superconducting gap determined by the distance between coherence peaks is about
7 meV, close to the value of bulk material. The ex situ transport measurement
of thick LiFeAs film shows a sharp superconducting transition around 16 K. The
upper critical field, Hc2(0)=13.0 T, is estimated from the temperature
dependent magnetoresistance. The precise thickness and quality control of
LiFeAs film paves the road of growing similar ultrathin iron arsenide films.
|
1503.03216v1
|
2015-03-11
|
Simulations Study of Muon Response in the Peripheral Regions of the Iron Calorimeter Detector at the India-based Neutrino Observatory
|
The magnetized Iron CALorimeter detector (ICAL) which is proposed to be built
in the India-based Neutrino Observatory (INO) laboratory, aims to study
atmospheric neutrino oscillations primarily through charged current
interactions of muon neutrinos and anti-neutrinos with the detector. The
response of muons and charge identification efficiency, angle and energy
resolution as a function of muon momentum and direction are studied from
GEANT4-based simulations in the peripheral regions of the detector. This
completes the characterisation of ICAL with respect to muons over the entire
detector and has implications for the sensitivity of ICAL to the oscillation
parameters and mass hierarchy compared to the studies where only the
resolutions and efficiencies of the central region of ICAL were assumed for the
entire detector.
Selection criteria for track reconstruction in the peripheral region of the
detector were determined from the detector response. On applying these, for the
1--20 GeV energy region of interest for mass hierarchy studies, an average
angle-dependent momentum resolution of 15--24%, reconstruction efficiency of
about 60--70% and a correct charge identification of about 97% of the
reconstructed muons were obtained. In addition, muon response at higher
energies upto 50 GeV was studied as relevant for understanding the response to
so-called rock muons and cosmic ray muons. An angular resolution of better than
a degree for muon energies greater than 4 GeV was obtained in the peripheral
regions, which is the same as that in the central region.
|
1503.03369v1
|
2015-03-16
|
Spectral properties of the soft excess pulsar RX J0059.2-7138 during its 2013 outburst
|
We report on an X-ray observation of the Be X-ray Binary Pulsar RX
J0059.2-7138, performed by XMM-Newton in March 2014. The 19 ks long observation
was carried out about three months after the discovery of the latest outburst
from this Small Magellanic Cloud transient, when the source luminosity was Lx ~
10$^{38}$ erg/s. A spin period of P=2.762383(5) s was derived, corresponding to
an average spin-up of $\dot{P}_{\mathrm{spin}} = -(1.27\pm0.01)\times10^{-12}$
s $s^{-1}$ from the only previous period measurement, obtained more than 20
years earlier. The time-averaged continuum spectrum (0.2-12 keV) consisted of a
hard power-law (photon index ~0.44) with an exponential cut-off at a
phase-dependent energy (20-50 keV) plus a significant soft excess below about
0.5 keV. In addition, several features were observed in the spectrum: an
emission line at 6.6 keV from highly ionized iron, a broad feature at 0.9-1 keV
likely due to a blend of Fe L-shell lines, and narrow emission and absorption
lines consistent with transitions in highly ionized oxygen, nitrogen and iron
visible in the high resolution RGS data (0.4-2.1 keV). Given the different
ionization stages of the narrow line components, indicative of photoionization
from the luminous X-ray pulsar, we argue that the soft excess in RX
J0059.2-7138 is produced by reprocessing of the pulsar emission in the inner
regions of the accretion disc.
|
1503.04574v1
|
2015-03-20
|
Relative Abundance Measurements in Plumes and Interplumes
|
We present measurements of relative elemental abundances in plumes and
interplumes. Plumes are bright, narrow structures in coronal holes that extend
along open magnetic field lines far out into the corona. Previous work has
found that in some coronal structures the abundances of elements with a low
first ionization potential (FIP) < 10 eV are enhanced relative to their
photospheric abundances. This coronal-to-photospheric abundance ratio, commonly
called the FIP bias, is typically 1 for element with a high-FIP (> 10 eV). We
have used EIS spectroscopic observations made on 2007 March 13 and 14 over an
~24 hour period to characterize abundance variations in plumes and interplumes.
To assess their elemental composition, we have used a differential emission
measure (DEM) analysis, which accounts for the thermal structure of the
observed plasma. We have used lines from ions of iron, silicon, and sulfur.
From these we have estimated the ratio of the iron and silicon FIP bias
relative to that for sulfur. From the results, we have created FIP-bias-ratio
maps. We find that the FIP-bias ratio is sometimes higher in plumes than in
interplumes and that this enhancement can be time dependent. These results may
help to identify whether plumes or interplumes contribute to the fast solar
wind observed in situ and may also provides constraints on the formation and
heating mechanisms of plumes.
|
1503.06044v1
|
2015-04-01
|
The Case for a Hot Archean Climate and its Implications to the History of the Biosphere
|
The case for a much warmer climate on the early Earth than now is presented.
The oxygen isotope record in sedimentary chert and the compelling case for a
near constant isotopic oxygen composition of seawater over geologic time
support thermophilic surface temperatures prevailing in the Archean, with some
support for hot conditions lasting until about 1.5 billion years ago, aside
from lower temperatures including glacial episodes at 2.1-2.4 Ga and possibly
an earlier one at 2.9 Ga. Other evidence includes the following: 1) Melting
temperatures of proteins resurrected from sequences inferred from robust
molecular phylogenies give paleotemperatures at emergence consistent with a
very warm early climate. 2) High atmospheric pCO2 levels in the Archean are
consistent with high climatic temperatures near the triple point of primary
iron minerals in banded iron formations, the formation of Mn-bicarbonate
clusters leading to oxygenic photosynthesis and generally higher weathering
intensities on land. These higher weathering intensities would not have
occurred if seafloor weathering dominated the carbon sink, pulling down the
temperature, hence this empirical evidence supports a hot climate and high
carbon dioxide levels. 3) The inferred viscosity of seawater at 2.7 Ga is
consistent with a hot Archean climate. 5) A cold Archean is hard to explain
taking into account the higher outgassing rates of carbon dioxide,
significantly smaller land areas and weaker biotic enhancement of weathering
than present in the context of the long-term carbon cycle, taking into account
the fainter Archean sun in climate modeling. This evidence points to an
important conclusion regarding biological evolution, namely to the critical
role of a temperature constraint holding back the emergence of major organismal
groups, starting with phototrophs, culminating with metazoans in the latest
Precambrian.
|
1504.00401v1
|
2015-05-05
|
Direct evidence for the emergence of a pressure induced nodal superconducting gap in the iron-based superconductor Ba_0.65Rb_0.35Fe_2As_2
|
Identifying the superconducting (SC) gap structure of the iron-based
high-temperature superconductors (Fe-HTS's) remains a key issue for the
understanding of superconductivity in these materials. In contrast to other
unconventional superconductors, in the Fe-HTS's both $d$-wave and extended
s-wave pairing symmetries are close in energy, with the latter believed to be
generally favored over the former. Probing the proximity between these very
different SC states and identifying experimental parameters that can tune them,
are of central interest. Here we report high-pressure muon spin rotation
experiments on the temperature-dependent magnetic penetration depth (lambda) in
the optimally doped Fe-HTS Ba_0.65Rb_0.35Fe_2As_2. At ambient pressure this
material is known to be a nodeless s-wave superconductor. Upon pressure a
strong decrease of (lambda) is observed, while the SC transition temperature
remains nearly constant. More importantly, the low-temperature behavior of
(1/lambda^{2}) changes from exponential saturation at zero pressure to a
power-law with increasing pressure, providing unambiguous evidence that
hydrostatic pressure promotes nodal SC gaps. Comparison to microscopic models
favors a d-wave over a nodal s^{+-}-wave pairing as the origin of the nodes.
Our results provide a new route of understanding the complex topology of the SC
gap in Fe-HTS's.
|
1505.01223v1
|
2015-05-08
|
Magnetic spin moment reduction in photoexcited ferromagnets through exchange interaction quenching: Beyond the rigid band approximation
|
The exchange interaction among electrons is one of the most fundamental
quantum mechanical interactions in nature and underlies any magnetic phenomena
from ferromagnetic ordering to magnetic storage. The current technology is
built upon a thermal or magnetic field, but a frontier is emerging to directly
control magnetism using ultrashort laser pulses. However, little is known about
the fate of the exchange interaction. Here we report unambiguously that
photoexcitation is capable of quenching the exchange interaction in all three
$3d$ ferromagnetic metals. The entire process starts with a small number of
photoexcited electrons which build up a new and self-destructive potential that
collapses the system into a new state with a reduced exchange splitting. The
spin moment reduction follows a Bloch-like law as $M_z(\Delta
E)=M_z(0)(1-{\Delta E}/{\Delta E_0})^{\frac{1}{\beta}}$, where $\Delta E$ is
the absorbed photon energy and $\beta$ is a scaling exponent. A good agreement
is found between the experimental and our theoretical results. Our findings may
have a broader implication for dynamic electron correlation effects in
laser-excited iron-based superconductors, iron borate, rare-earth
orthoferrites, hematites and rare-earth transition metal alloys.
|
1505.01904v1
|
2015-05-23
|
Common Electronic Origin of Superconductivity in (Li,Fe)OHFeSe Bulk Superconductor and Single-Layer FeSe/SrTiO3 Films
|
The mechanism of high temperature superconductivity in the iron-based
superconductors remains an outstanding issue in condensed matter physics. The
electronic structure, in particular the Fermi surface topology, is considered
to play an essential role in dictating the superconductivity. Recent revelation
of distinct electronic structure and possible high temperature
superconductivity with a transition temperature Tc above 65 K in the
single-layer FeSe films grown on the SrTiO3 substrate provides key information
on the roles of Fermi surface topology and interface in inducing or enhancing
superconductivity. Here we report high resolution angle-resolved photoemission
measurement on the electronic structure and superconducting gap of a novel
FeSe-based superconductor, (Li0.84Fe0.16)OHFe0.98Se, with a Tc at 41 K. We find
that this single-phase bulk superconductor shows remarkably similar electronic
behaviors to that of the superconducting single-layer FeSe/SrTiO3 film in terms
of Fermi surface topology, band structure and nearly isotropic superconducting
gap without nodes. These observations provide significant insights in
understanding high temperature superconductivity in the single-layer
FeSe/SrTiO3 film in particular, and the mechanism of superconductivity in the
iron-based superconductors in general.
|
1505.06361v1
|
2015-05-29
|
Impact of oxygen doping and oxidation state of iron on the electronic and magnetic properties of BaFeO$_{3-δ}
|
We studied structural, electronic and magnetic properties of a cubic
perovskite BaFeO$_{3-\delta}$ ($0 \le \delta \le 0.5$) within the density
functional theory using a generalized gradient approximation and a GGA+U
method. According to our calculations, BaFeO$_3$ in its stoichiometric cubic
structure should be half-metallic and strongly ferromagnetic, with extremely
high Curie temperature ($T_C$) of 700 - 900 K. However, a such estimate of
$T_C$ disagrees with all available experiments, which report that $T_C$ of the
BaFeO$_3$ and undoped BaFeO$_{3-\delta}$ films varies between 111 K and 235 K
or, alternatively, that no ferromagnetic order was detected there. Fitting the
calculated x-ray magnetic circular dichroism spectra to the experimental
features seen for BaFeO$_3$, we concluded that the presence of oxygen vacancies
in our model enables a good agreement. Thus, the relatively low $T_C$ measured
in BaFeO$_3$ can be explained by oxygen vacancies intrinsically presented in
the material. Since iron species near the O vacancy change their oxidation
state from $4+$ to $3+$, the interaction between Fe$^{4+}$ and Fe$^{3+}$, which
is antiferromagnetic, weakens the effective magnetic interaction in the system,
which is predominantly ferromagnetic. With increasing $\delta$ in
BaFeO$_{3-\delta}$, its $T_C$ decreases down to the critical value when the
magnetic order becomes antiferromagnetic. Our calculations of the electronic
structure of BaFeO$_{3-\delta}$ illustrate how the ferromagnetism originates
and also how one can keep this cubic perovskite robustly ferromagnetic far
above the room temperature.
|
1505.07942v1
|
2015-06-04
|
Weak and strong electronic correlations in Fe superconductors
|
In this chapter the strength of electronic correlations in the normal phase
of Fe-superconductors is discussed. It will be shown that the agreement between
a wealth of experiments and DFT+DMFT or similar approaches supports a scenario
in which strongly-correlated and weakly-correlated electrons coexist in the
conduction bands of these materials. I will then reverse-engineer the realistic
calculations and justify this scenario in terms of simpler behaviors easily
interpreted through model results. All pieces come together to show that Hund's
coupling, besides being responsible for the electronic correlations even in
absence of a strong Coulomb repulsion is also the origin of a subtle emergent
behavior: orbital decoupling. Indeed Hund's exchange decouples the charge
excitations in the different Iron orbitals involved in the conduction bands
thus causing an independent tuning of the degree of electronic correlation in
each one of them. The latter becomes sensitive almost only to the offset of the
orbital population from half-filling, where a Mott insulating state is
invariably realized at these interaction strengths. Depending on the difference
in orbital population a different 'Mottness' affects each orbital, and thus
reflects in the conduction bands and in the Fermi surfaces depending on the
orbital content.
|
1506.01678v1
|
2015-06-15
|
Simultaneous spectral and reverberation modelling of relativistic reflection in Mrk 335
|
We present an X-ray spectral and timing model to investigate the broad and
variable iron line seen in the high flux state of Mrk 335. The model consists
of a variable X-ray source positioned along the rotation axis of the black hole
that illuminates the accretion disc producing a back-scattered, ionized
reflection spectrum. We compute time lags including full dilution effects and
perform simultaneous fitting of the 2-10 keV spectrum and the
frequency-dependent time lags of 2.5-4 vs. 4-6.5 keV bands. The best-fitting
parameters are consistent with a black hole mass of approximately 1.3 x 10^7
M_sun, disc inclination of 45 degrees and the photon index of the direct
continuum of 2.4. The iron abundance is 0.5 and the ionization parameter is
10^3 erg cm / s at the innermost part of the disc and decreases further out.
The X-ray source height is very small, approximately 2 r_g. Furthermore, we fit
the Fe L lags simultaneously with the 0.3-10 keV spectrum. The key parameters
are comparable to those previously obtained. We also report the differences
below 2 keV using the xillver and reflionx models which could affect the
interpretation of the soft excess. While simultaneously fitting spectroscopic
and timing data can break the degeneracy between the source height and the
black hole mass, we find that the measurements of the source height and the
central mass significantly depend on the ionization state of the disc and are
possibly model-dependent.
|
1506.04524v1
|
2015-07-07
|
The NuSTAR X-ray spectrum of the low-luminosity AGN in NGC 7213
|
We present an analysis of the 3-79 keV NuSTAR spectrum of the low-luminosity
active galactic nucleus NGC 7213. In agreement with past observations, we find
a lower limit to the high-energy cut-off of Ec > 140 keV, no evidence for a
Compton-reflected continuum, and the presence of an iron Kalpha complex,
possibly produced in the broad-line region. From the application of the MYTorus
model, we find that the line-emitting material is consistent with the absence
of a significant Compton reflection if arising from a Compton-thin torus of gas
with a column density of 5.0(+2.0,-1.6) cm^-2. We report variability of the
equivalent width of the iron lines on the time-scale of years using archival
observations from XMM-Newton, Chandra and Suzaku. This analysis suggests a
possible contribution from dusty gas. A fit with a Comptonization model
indicates the presence of a hot corona with a temperature kTe > 40 keV and an
optical depth tau < 1, assuming a spherical geometry.
|
1507.01775v1
|
2015-07-09
|
The phase diagrams of iron-based superconductors: theory and experiments
|
Phase diagrams play a primary role in the understanding of materials
properties. For iron-based superconductors (Fe-SC), the correct definition of
their phase diagrams is crucial because of the close interplay between their
crystallo-chemical and magnetic properties, on one side, and the possible
coexistence of magnetism and superconductivity, on the other. The two most
difficult issues for understanding the Fe-SC phase diagrams are: 1) the origin
of the structural transformation taking place during cooling and its
relationship with magnetism; 2) the correct description of the region where a
crossover between the magnetic and superconducting electronic ground states
takes place. Hence a proper and accurate definition of the structural, magnetic
and electronic phase boundaries provides an extremely powerful tool for
material scientists. For this reason, an exact definition of the thermodynamic
phase fields characterizing the different structural and physical properties
involved is needed, although it is not easy to obtain in many cases. Moreover,
physical properties can often be strongly dependent on the occurrence of
micro-structural and other local-scale features (lattice micro-strain, chemical
fluctuations, domain walls, grain boundaries, defects), which, as a rule, are
not described in a structural phase diagram. In this review, we critically
summarize the results for the most studied 11-, 122- and 1111-type compound
systems, providing a correlation between experimental evidence and theory.
|
1507.02406v1
|
2015-07-09
|
Stability of Two-Dimensional Iron-Carbides Suspended across Graphene Pores: First-principles Particle Swarm Optimization
|
Inspired by recent experimental realizations of two-dimensional (2D) metals
and alloys, we theoretically investigate the stability and electronic
properties of monolayer (ML) Fe-C compounds and pure Fe. According to our and
others theoretical results, the experiment [Science 343, 1228 (2014)] proposed
ML pure Fe square-lattices embedded in graphene (Gr) pores are energetically
unstable compared to that of the Fe triangular-lattices in Gr. To solve the
above contradiction, we search for the stable structures of ML Fe-C with
various Fe to C ratios (as a generalization of ML Fe in Gr) using ab initio
particle swarm optimization technique. A Fe1C1 square-lattice embedded in Gr is
found. We propose and demonstrate that the square-lattices observed in the
experiment were iron-carbides (Fe-C) but not pure Fe from the square-lattice
shape, Fe-Fe lattice constant and energetic considerations. Note that the
coexistence of C with Fe cannot be excluded from the experiment. More
importantly, we find a lowest energy and dynamically stable structure, ML Fe2C2
with Fe atoms form distorted square lattices. High spin polarization around the
Fermi level is predicted for different 2D Fe-C structures due to significant
orbital hybridization between C and Fe.
|
1507.02418v2
|
2015-07-09
|
Chemical Abundances in the PN Wray16-423 in the Sagittarius Dwarf Spheroidal Galaxy: Constraining the Dust Composition
|
We performed a detailed analysis of elemental abundances, dust features, and
polycyclic aromatic hydrocarbons (PAHs) in the C-rich planetary nebula (PN)
Wray16-423 in the Sagittarius dwarf spheroidal galaxy, based on a unique
dataset taken from the Subaru/HDS, MPG/ESO FEROS, HST/WFPC2, and Spitzer/IRS.
We performed the first measurements of Kr, Fe, and recombination O abundance in
this PN. The extremely small [Fe/H] implies that most Fe atoms are in the solid
phase, considering into account the abundance of [Ar/H]. The Spitzer/IRS
spectrum displays broad 16-24 um and 30 um features, as well as PAH bands at
6-9 um and 10-14 um. The unidentified broad 16-24 um feature may not be related
to iron sulfide (FeS), amorphous silicate, or PAHs. Using the spectral energy
distribution model, we derived the luminosity and effective temperature of the
central star, and the gas and dust masses. The observed elemental abundances
and derived gas mass are in good agreement with asymptotic giant branch
nucleosynthesis models for an initial mass of 1.90 Msun and a metallicity of
Z=0.004. We infer that respectively about 80 %, 50 %, and 90 % of the Mg, S,
and Fe atoms are in the solid phase. We also assessed the maximum possible
magnesium sulfide (MgS) and iron-rich sulfide (Fe50S) masses and tested whether
these species can produce the band flux of the observed 30 um feature.
Depending on what fraction of the sulfur is in sulfide molecules such as CS, we
conclude that MgS and Fe50S could be possible carriers of the 30 um feature in
this PN.
|
1507.02667v1
|
2015-08-07
|
Correlation-enhanced odd-parity inter-orbital singlet pairing in the iron-pnictide superconductor LiFeAs
|
The rich variety of iron-based superconductors and their complex electronic
structure lead to a wide range of possibilities for gap symmetry and pairing
components. Here we solve in the 2-Fe Brillouin zone the full
frequency-dependent linearized Eliashberg equations for LiFeAs with
spin-fluctuation mediated pairing interactions. The magnetic excitations are
calculated with the random phase approximation on a correlated electronic
structure obtained with density functional theory and dynamical mean field
theory. Correlations induce long-lived local moments with orbital-dependent
dynamics. The interaction between electrons through Hund's coupling promotes
inter-orbital magnetic susceptibility. As a consequence, the leading pairing
channel, conventional $s^{+-}$, acquires sizeable inter-orbital
$d_{xy}-d_{xz(yz)}$ singlet pairing with odd parity under glide-plane symmetry.
These components reduce the superconducting gap magnitude induced by the
intra-orbital components of the gap function at the electron pockets
intersection where the Fe-d $t_{2g}$ orbitals strongly mix. This in turn makes
the results consistent with available experiments on the angular dependence of
the gaps observed on the different Fermi surfaces.
|
1508.01789v4
|
2015-08-21
|
Computational searches for iron oxides at high pressures
|
We have used density-functional-theory methods and the ab initio random
structure searching (AIRSS) approach to predict stable structures and
stoichiometries of mixtures of iron and oxygen at high pressures. Searching was
performed for 12 different stoichiometries at pressures of 100, 350 and 500
GPa, which involved relaxing more than 32,000 structures. We find that
Fe$_2$O$_3$ and FeO$_2$ are the only phases stable to decomposition at 100 GPa,
while at 350 and 500 GPa several stoichiometries are found to be stable or very
nearly stable. We report a new structure of Fe$_2$O$_3$ with $P2_12_12_1$
symmetry which is found to be more stable than the known Rh$_2$O$_3$(II) phase
at pressures above $\sim$233 GPa. We also report two new structures of FeO,
with $Pnma$ and $R\bar{3}m$ symmetries, which are found to be stable within the
ranges 195-285 GPa and 285-500 GPa, respectively, and two new structures of
Fe$_3$O$_4$ with $Pca2_1$ and $P2_1/c$ symmetries, which are found to be stable
within the ranges 100-340 GPa and 340-500 GPa, respectively. Finally, we report
two new structures of Fe$_4$O$_5$ with $P4_2/n$ and $P\bar{3}m1$ symmetries,
which are found to be stable within the ranges 100-231 GPa and 231-500 GPa,
respectively. Our new structures of Fe$_3$O$_4$ and Fe$_4$O$_5$ are found to
have lower enthalpies than their known structures within their respective
stable pressure ranges.
|
1508.05247v1
|
2015-09-03
|
The Gaia-ESO Survey: chemical signatures of rocky accretion in a young solar-type star
|
It is well known that newly formed planetary systems undergo processes of
orbital reconfiguration and planetary migration. As a result, planets or
protoplanetary objects may accrete onto the central star, being fused and mixed
into its external layers. If the accreted mass is sufficiently high and the
star has a sufficiently thin convective envelope, such events may result in a
modification of the chemical composition of the stellar photosphere in an
observable way, enhancing it with elements that were abundant in the accreted
mass. The recent Gaia-ESO Survey observations of the 10-20 Myr old Gamma
Velorum cluster have enabled identifying a star that is significantly enriched
in iron with respect to other cluster members. In this Letter we further
investigate the abundance pattern of this star, showing that its abundance
anomaly is not limited to iron, but is also present in the refractory elements,
whose overabundances are correlated with the condensation temperature. This
finding strongly supports the hypothesis of a recent accretion of rocky
material.
|
1509.00933v2
|
2015-09-05
|
Two-Dimensional Oxide Topological Insulator With Iron-Pnictide Superconductor LiFeAs Structure
|
By using first-principles calculations, we propose that ZrSiO can be looked
as a three-dimensional (3D) oxide weak topological insulator (TI) and its
single layer is a long-sought-after 2D oxide TI with a band gap up to 30 meV.
Calculated phonon spectrum of the single layer ZrSiO indicates it is
dynamically stable and the experimental achievements in growing oxides with
atomic precision ensure that it can be readily synthesized. This will lead to
novel devices based on TIs, the so called "topotronic" devices, operating under
room-temperature and stable when exposed in the air. Thus, a new field of
"topotronics" will arise. Another intriguing thing is this oxide 2D TI has the
similar crystal structure as the well-known iron-pnictide superconductor
LiFeAs. This brings great promise in realizing the combination of
superconductor and TI, paving the way to various extraordinary quantum
phenomena, such as topological superconductor and Majorana modes. We further
find that there are many other isostructural compounds hosting the similar
electronic structure and forming a $WHM$-family with $W$ being Zr, Hf or La,
$H$ being group IV or group V element, and $M$ being group VI one.
|
1509.01686v2
|
2015-09-11
|
New opacity measurement principle for LMJ-PETAL laser facility
|
Stellar seismology reveals some interior properties of thousands of
solar-type stars but the solar seismic sound speed stays puzzling since a
decade as it disagrees with the Standard Solar Model (SSM) prediction. One of
the explanations of this disagreement may be found in the treatment of the
transport of radiation from the solar core to the surface. As the same
framework is used for other stars, it is important to check precisely the
reliability of the interacting cross sections of photons with each species in
order to ensure the energy transport for temperature T > 2 - 10$^6$ K and
density $\rho$ > 0.2 g/cm$^3$. In this paper, we propose a new technique to
reach the domain of temperature and density found in the solar radiative
interior. This technique called the Double Ablation Front (DAF) is based on a
high conversion of the laser energy into X-rays thanks to moderated Z material
irradiated by laser intensity between 1.5 $\times$ 10$^{15}$ W/cm$^2$ and 4
$\times$ 10$^{15}$ W/cm$^2$. This high conversion creates, in addition to the
electronic front a second ablation front in the moderated Z material. Between
the two fronts there is a plateau of density and temperature that we exploit to
heat a sample of iron or of oxide. The first simulations realized with the
hydrodynamic code CHIC show that this technique allows to reach conditions
equivalent to half the radiative zone of the Sun with high stability both in
time and space. We examine the possibility to measure both iron and oxygen
absorption spectra.
|
1509.03563v1
|
2015-09-17
|
Local Radiation Hydrodynamic Simulations of Massive Star Envelopes at the Iron Opacity Peak
|
We perform three dimensional radiation hydrodynamic simulations of the
structure and dynamics of radiation dominated envelopes of massive stars at the
location of the iron opacity peak. One dimensional hydrostatic calculations
predict an unstable density inversion at this location, whereas our simulations
reveal a complex interplay of convective and radiative transport whose behavior
depends on the ratio of the photon diffusion time to the dynamical time. The
latter is set by the ratio of the optical depth per pressure scale height,
$\tau_0$, to $\tau_c=c/c_g$, where $c_g \approx$ 50 km/s is the isothermal
sound speed in the gas alone. When $\tau_0 \gg \tau_c$, convection reduces the
radiation acceleration and removes the density inversion. The turbulent energy
transport in the simulations agrees with mixing length theory and provides its
first numerical calibration in the radiation dominated regime. When $\tau_0 \ll
\tau_c$, convection becomes inefficient and the turbulent energy transport is
negligible. The turbulent velocities exceed $c_g$, driving shocks and large
density fluctuations that allow photons to preferentially diffuse out through
low-density regions. However, the effective radiation acceleration is still
larger than the gravitational acceleration so that the time average density
profile contains a modest density inversion. In addition, the simulated
envelope undergoes large-scale oscillations with periods of a few hours. The
turbulent velocity field may affect the broadening of spectral lines and
therefore stellar rotation measurements in massive stars, while the time
variable outer atmosphere could lead to variations in their mass loss and
stellar radius.
|
1509.05417v1
|
2015-09-18
|
Origin of the spectral upturn in the cosmic-ray C/Fe and O/Fe ratios
|
The observed spectrum of Galactic cosmic rays has several exciting features
such as the rise in the positron fraction above ~10 GeV of energy and the
spectral hardening of protons and helium at ~300 GeV/nucleon of energy. The
ATIC-2 experiment has recently reported an unexpected spectral upturn in the
elemental ratios involving iron, such as the C/Fe or O/Fe ratios, at energy
above 50 GeV per nucleon. It is recognized that the observed positron excess
can be explained by pion production processes during diffusive shock
acceleration of cosmic-ray hadrons in nearby sources. Recently, it was
suggested that a scenario with nearby source dominating the GeV-TeV spectrum
may be connected with the change of slope observed in protons and nuclei, which
would be interpreted as a flux transition between the local component and the
large-scale distribution of Galactic sources. Here I show that, under a
two-component scenario with nearby source, the shape of the spectral transition
is expected to be slightly different for heavy nuclei, such as iron, because
their propagation range is spatially limited by inelastic collisions with the
interstellar matter. This enables a prediction for the primary/primary ratios
between light and heavy nuclei. From this effect, a spectral upturn is
predicted in the C/Fe and O/Fe ratios in good accordance with the ATIC-2 data.
|
1509.05774v1
|
2015-09-23
|
Anisotropic hyperfine interactions in FeP studied by 57Fe Mossbauer spectroscopy and 31P NMR
|
We report results of 57Fe Mossbauer and 31P NMR studies of a phosphide FeP
powder sample performed in a wide temperature range including the point (TN ~
120 K) of magnetic phase transitions. The 57Fe Mossbauer spectra at low
temperatures T < TN consist of very diffuse Zeeman pattern with line
broadenings and sizeable spectral asymmetry. It was shown that the change of
the observed spectral shape is consistent with the transition into the space
modulated helicoidal magnetic structure. Analysis of the experimental spectra
was carried out assuming the anisotropy of the magnetic hyperfine field Hhf at
the 57Fe nuclei when the Fe3+ magnetic moment rotates with respect to the
principal axis of the electric field gradient (EFG) tensor. The obtained large
temperature independent anharmonicity parameter m ~ 0.96 of the helicoidal spin
structure results from easy-axis anisotropy in the plane of the iron spin
rotation. It was assumed that the very low maximal value of Hhf(11K) ~ 36 kOe
and its high anisotropy delta_Hanis(11K) ~ 30 kOe can be attributed to the
stabilization of iron cations in the low-spin state (SFe = 1/2). The 31P NMR
measurements demonstrate an extremely broad linewidth reflecting the spatial
distribution of the transferred internal magnetic fields of the Fe3+ ions onto
P sites in the magnetically ordered state.
|
1509.06953v2
|
2015-09-23
|
The 1999aa-like Type Ia Supernova iPTF14bdn in the Ultraviolet and Optical
|
We present ultraviolet (UV) and optical photometry and spectra of the
1999aa-like supernova (SN) iPTF14bdn. The UV data were observed using the Swift
Ultraviolet/Optical Telescope (UVOT) and constitute the first UV spectral
series of a 1999aa-like SN. From the photometry we measure $\Delta m_{15}({\it
B})\,=\,0.84 \pm0.05$ mag and blue UV colors at epochs earlier than $-5$ days.
The spectra show that the early-time blue colors are the result of less
absorption between $2800 - 3200 \,\AA~$ than is present in normal SNe Ia. Using
model spectra fits of the data at $-10 $ and $+10 $ days, we identify the
origin of this spectral feature to be a temperature effect in which doubly
ionized iron group elements create an opacity 'window'. We determine that the
detection of high temperatures and large quantities of iron group elements at
early epochs imply the mixing of a high Ni mass into the outer layers of the SN
ejecta. We also identify the source of the I-band secondary maximum in
iPTF14bdn to be the decay of Fe III to Fe II, as is seen in normal SNe Ia.
|
1509.07129v1
|
2015-11-11
|
Discovery of Extremely Broad Balmer Absorption Lines in SDSS J152350.42+391405.2
|
We present the discovery of Balmer line absorption from H$\alpha$ to
H$\gamma$ in an iron low-ionization broad absorption line (FeLoBAL) quasar SDSS
J152350.42+391405.2 (hereafter J1523), by the quasi-simultaneous optical and
near-infrared spectroscopy. The Balmer line absorption is at $z_{absor}$ =
0.6039 +/-0.0021 and blueshifted by v=10,353 km/s with respect to the Balmer
emission lines. All Balmer BALs have uniform absorption profile with the widths
of $\Delta$ v ~12,000 km/s. We also found the absorption trough in He 1*
$\lambda$10830 with the same velocity and width in the H-band TripleSpec
spectrum of J1523. This object is only the tenth active galactic nucleus known
to exhibit non-stellar Balmer absorption, and also the case with the highest
velocity and broadest Balmer absorption lines which have ever been found. A
CLOUDY analysis shows that the absorbers require an gas density of $log_{10} n_
e (cm^{-3})=9$ and an ionization parameter of $log_{10} U=-1.0$. They locate at
a distance of ~0.2 pc from the central ionizing source which is slightly
farther than that of BELRs. Furthermore, J1523 is one of the brightest
Balmer-BAL quasar ever reported, with unique iron absorption variations, making
it as the most promising candidate for follow up high-resolution spectroscopy,
multi-band observations, and long-term monitoring.
|
1511.03422v1
|
2015-12-17
|
The Fe/Ni ratio in ionized nebulae: clues on dust depletion patterns
|
We perform a homogeneous analysis of the Fe/Ni abundance ratio in eight
Galactic planetary nebulae (PNe) and three Galactic H II regions that include
the Orion nebula, where we study four nebular zones and one shocked region. We
use [Fe ii], [Fe iii], and [Ni iii] lines, and ionization correction factors
(ICFs) that account for the unobserved ions. We derive an ICF for nickel from
an extensive grid of photoionization models. We compare our results with those
derived by other authors for 16 neutral clouds in the solar neighbourhood with
available Fe/Ni ratios in the literature. We find an excellent agreement
between the ionized nebulae and the diffuse clouds, with both types of regions
showing a clear correlation between the Fe/Ni ratios and the iron and nickel
depletion factors. The trend shows that the objects with a relatively low
depletion have near solar Fe/Ni ratios whereas at higher depletions the Fe/Ni
ratio increases with the depletion. Our results confirm that, compared to iron
atoms, nickel ones are more efficiently stuck to the dust grains in ambients
where dust formation or growth have been more efficient.
|
1512.05664v1
|
2015-12-18
|
Expansion of the tetragonal magnetic phase with pressure in the iron-arsenide superconductor Ba{1-x}KxFe2As2
|
In the temperature-concentration phase diagram of most iron-based
superconductors, antiferromagnetic order is gradually suppressed to zero at a
critical point, and a dome of superconductivity forms around that point. The
nature of the magnetic phase and its fluctuations is of fundamental importance
for elucidating the pairing mechanism. In Ba{1-x}KxFe2As2 and Ba{1-x}NaxFe2As2,
it has recently become clear that the usual stripe-like magnetic phase, of
orthorhombic symmetry, gives way to a second magnetic phase, of tetragonal
symmetry, near the critical point, between x = 0.24 and x = 0.28. Here we
report measurements of the electrical resistivity of Ba{1-x}KxFe2As2 under
applied hydrostatic pressures up to 2.75 GPa, for x = 0.22, 0.24 and 0.28. We
track the onset of the tetragonal magnetic phase using the sharp anomaly it
produces in the resistivity. In the temperature-concentration phase diagram of
Ba{1-x}KxFe2As2, we find that pressure greatly expands the tetragonal magnetic
phase, while the stripe-like phase shrinks. This raises the interesting
possibility that the fluctuations of the former phase might be involved in the
pairing mechanism responsible for the superconductivity.
|
1512.05912v1
|
2015-12-22
|
Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe
|
The coexistence and competition between superconductivity and electronic
orders, such as spin or charge density waves, have been a central issue in high
transition-temperature (${T_{\rm c}}$) superconductors. Unlike other iron-based
superconductors, FeSe exhibits nematic ordering without magnetism whose
relationship with its superconductivity remains unclear. More importantly, a
pressure-induced fourfold increase of ${T_{\rm c}}$ has been reported, which
poses a profound mystery. Here we report high-pressure magnetotransport
measurements in FeSe up to $\sim9$ GPa, which uncover a hidden magnetic dome
superseding the nematic order. Above ${\sim6}$ GPa the sudden enhancement of
superconductivity (${T_{\rm c}\le38.3}$ K) accompanies a suppression of
magnetic order, demonstrating their competing nature with very similar energy
scales. Above the magnetic dome we find anomalous transport properties
suggesting a possible pseudogap formation, whereas linear-in-temperature
resistivity is observed above the high-${T_{\rm c}}$ phase. The obtained phase
diagram highlights unique features among iron-based superconductors, but bears
some resemblance to that of high-${T_{\rm c}}$ cuprates.
|
1512.06951v1
|
2015-12-27
|
Two types of superconducting domes in unconventional superconductors
|
Uncovering the origin of unconventional superconductivity is often plagued by
the overwhelming material diversity with varying normal and superconducting
(SC) properties. In this article, we deliver a comprehensive study of the SC
properties and phase diagrams using multiple tunings (such as disorder,
pressure or magnetic field in addition to doping and vice versa) across several
families of unconventional superconductors, including the copper-oxides,
heavy-fermions, organics and the recently discovered iron-pnictides,
iron-chalcogenides, and oxybismuthides. We discover that all these families
often possess two types of SC domes, with lower and higher superconducting
transition temperatures Tc, both unconventional but with distinct SC and normal
states properties. The lower Tc dome arises with or without a quantum critical
point (QCP), and not always associated with a non-Fermi liquid (NFL)
background. On the contrary, the higher-Tc dome clearly stems from a NFL or
strange metal phase, without an apparent intervening phase transition or a QCP.
The two domes appear either fully separated in the phase diagram, or merged
into one, or arise independently owing to their respective normal state
characteristics. Our findings suggest that a QCP-related mechanism is an
unlikely scenario for the NFL phase in these materials, and thereby narrows the
possibility towards short-range fluctuations of various degrees of freedom in
the momentum and frequency space. We also find that NFL physics may be a
generic route to higher-Tc superconductivity.
|
1512.08186v2
|
2016-01-04
|
An Intermediate Type Ia Supernova Between Normal And Super-Chandrasekhar
|
In this paper, we report observations of a peculiar Type Ia supernova
iPTF13asv (a.k.a., SN2013cv) from the onset of the explosion to months after
its peak. The early-phase spectra of iPTF13asv show absence of iron absorption,
indicating that synthesized iron elements are confined to low-velocity regions
of the ejecta, which, in turn, implies a stratified ejecta structure along the
line of sight. Our analysis of iPTF13asv's light curves and spectra shows that
it is an intermediate case between normal and super-Chandrasekhar events. On
the one hand, its light curve shape (B-band $\Delta m_{15}=1.03\pm0.01$) and
overall spectral features resemble those of normal Type Ia supernovae. On the
other hand, similar to super-Chandrasekhar events, it shows large peak optical
and UV luminosity ($M_B=-19.84\,\rm{mag}$, $M_{uvm2}=-15.5\,\rm{mag}$) a
relatively low but almost constant \ion{Si}{2} velocities of about
$10,000\,\rm{km}\,\rm{s}^{-1}$, and persistent carbon absorption in the
spectra. We estimate a $^{56}$Ni mass of $0.81^{+0.10}_{-0.18}M_\odot$ and a
total ejecta mass of $1.59^{+0.45}_{-0.12}M_\odot$. The large ejecta mass of
iPTF13asv and its stratified ejecta structure together seemingly favor a
double-degenerate origin.
|
1601.00686v2
|
2016-01-21
|
The temperature and chronology of heavy-element synthesis in low-mass stars
|
Roughly half of the heavy elements (atomic mass greater than that of iron)
are believed to be synthesized in the late evolutionary stages of stars with
masses between 0.8 and 8 solar masses. Deep inside the star, nuclei (mainly
iron) capture neutrons and progressively build up (through the
slow-neutron-capture process, or s-process) heavier elements that are
subsequently brought to the stellar surface by convection. Two neutron sources,
activated at distinct temperatures, have been proposed: 13C and 22Ne, each
releasing one neutron per alpha-particle (4He) captured. To explain the
measured stellar abundances, stellar evolution models invoking the 13C neutron
source (which operates at temperatures of about one hundred million kelvin) are
favoured. Isotopic ratios in primitive meteorites, however, reflecting
nucleosynthesis in the previous generations of stars that contributed material
to the Solar System, point to higher temperatures (more than three hundred
million kelvin), requiring at least a late activation of 22Ne. Here we report a
determination of the s-process temperature directly in evolved low-mass giant
stars, using zirconium and niobium abundances, independently of stellar
evolution models. The derived temperature supports 13C as the s-process neutron
source. The radioactive pair 93Zr-93Nb used to estimate the s-process
temperature also provides, together with the pair 99Tc-99Ru, chronometric
information on the time elapsed since the start of the s-process, which we
determine to be one million to three million years.
|
1601.05640v1
|
2016-01-21
|
Detailed magnetic and structural analysis mapping a robust magnetic C4 dome in Sr1-xNaxFe2As2
|
The recently discovered $C_4$ tetragonal magnetic phase in hole-doped members
of the iron-based superconductors provides new insights into the origin of
unconventional superconductivity. Previously observed in
Ba$_{1-x}A_x$Fe$_2$As$_2$ (with $A =$ K, Na), the $C_4$ magnetic phase exists
within the well studied $C_2$ spin-density wave (SDW) dome, arising just before
the complete suppression of antiferromagnetic (AFM) order but after the onset
of superconductivity. Here, we present detailed x-ray and neutron diffraction
studies of Sr$_{1-x}$Na$_x$Fe$_2$As$_2$ ($0.10 \leq\ x \leq\ 0.60$) to
determine their structural evolution and the extent of the $C_4$ phase.
Spanning $\Delta x\sim 0.14$ in composition, the $C_4$ phase is found to extend
over a larger range of compositions, and to exhibit a significantly higher
transition temperature, $T_r \sim 65$K, than in either of the other systems in
which it has been observed. The onset of this phase is seen near a composition
($x \sim 0.30$) where the bonding angles of the Fe$_2$As$_2$ layers approach
the perfect $109.46^\circ$ tetrahedral angle. We discuss the possible role of
this return to a higher symmetry environment for the magnetic iron site in
triggering the magnetic reorientation and the coupled re-entrance to the
tetragonal structure. Finally, we present a new phase diagram, complete with
the $C_4$ phase, and use its observation in a third hole-doped 122 system to
suggest the universality of this phase.
|
1601.05693v1
|
2016-02-11
|
Doping evolution of the superconducting gap structure in the underdoped iron arsenide Ba1-xKxFe2As2 revealed by thermal conductivity
|
The thermal conductivity kappa of the iron-arsenide superconductor
Ba1-xKxFe2As2 was measured for heat currents parallel and perpendicular to the
tetragonal c axis at temperatures down to 50 mK and in magnetic fields up to 15
T. Measurements were performed on samples with compositions ranging from
optimal doping (x = 0.34; Tc = 39 K) down to dopings deep into the region where
antiferromagnetic order coexists with superconductivity (x = 0.16; Tc = 7 K).
In zero field, there is no residual linear term in kappa(T) as T goes to 0 at
any doping, whether for in-plane or inter-plane transport. This shows that
there are no nodes in the superconducting gap. However, as x decreases into the
range of coexistence with antiferromagnetism, the residual linear term grows
more and more rapidly with applied magnetic field. This shows that the
superconducting energy gap develops minima at certain locations on the Fermi
surface and these minima deepen with decreasing x. We propose that the minima
in the gap structure arise when the Fermi surface of Ba1-xKxFe2As2 is
reconstructed by the antiferromagnetic order.
|
1602.03914v1
|
2016-03-03
|
Fe dopant in ZnO: 2+ vs 3+ valency and ion-carrier s,p-d exchange interaction
|
Dopants of transition metal ions in II-VI semiconductors exhibit native 2+
valency. Despite this, 3+ or mixed 3+/2+ valency of iron ions in ZnO was
reported previously. Several contradictory mechanisms have been put forward for
explanation of this fact so far. Here, we analyze Fe valency in ZnO by
complementary theoretical and experimental studies. Our calculations within the
generalized gradient approximation (GGA+U) indicate that the Fe ion is a
relatively shallow donor. Its stable charge state is Fe2+ in ideal ZnO,
however, the high energy of the (+/0) transition level enhances the
compensation of Fe2+ to Fe3+ by non-intentional acceptors in real samples.
Using several experimental methods like electron paramagnetic resonance,
magnetometry, conductivity, excitonic magnetic circular dichroism and
magneto-photoluminescence we confirm the 3+ valency of the iron ions in
polycrystalline (Zn,Fe)O films with the Fe content attaining 0.2%.We find a
predicted increase of n-type conductivity upon the Fe doping with the Fe donor
ionization energy of 0.25 +/- 0.02 eV consistent with the results of
theoretical considerations. Moreover, our magnetooptical measurements confirm
the calculated non-vanishing s,p-d exchange interaction between band carriers
and localized magnetic moments of the Fe3+ ions in the ZnO, being so far an
unsettled issue.
|
1603.01123v2
|
2016-03-05
|
Development and characterization of single gap glass RPC
|
India-based Neutrino Observatory (INO) facility is going to have a 50 kton
magnetized Iron CALorimeter (ICAL) detector for precision measurements of
neutrino oscillations using atmospheric neutrinos. The proposed ICAL detector
will be a stack of magnetized iron plates (acting as target material)
interleaved with glass Resistive Plate Chambers (RPCs) as the active detector
elements. An RPC is a gaseous detector made up of two parallel electrode plates
having high bulk resistivity like that of float glass and bakelite. For the
ICAL detector, glass is preferred over bakelite as it does not need any kind of
surface treatment to achieve better surface uniformity and also the cost of
associated electronics is reduced. Under the detector R&D efforts for the
proposed glass RPC detector, a few glass RPCs of 1m X 1m dimension are
fabricated procuring glass of ~ 2 mm thickness from one of the Indian glass
manufacturers (Asahi). In the present paper, we report the characterization of
RPC based on leakage current study, muon detection efficiency and noise rate
studies with varying gas compositions.
|
1603.01719v5
|
2016-03-09
|
Suppression of spin-exciton state in hole overdoped iron-based superconductors
|
The mechanism of Cooper pair formation in iron-based superconductors remains
a controversial topic. The main question is whether spin or orbital
fluctuations are responsible for the pairing mechanism. To solve this problem,
a crucial clue can be obtained by examining the remarkable enhancement of
magnetic neutron scattering signals appearing in a superconducting phase. The
enhancement is called spin resonance for a spin fluctuation model, in which
their energy is restricted below twice the superconducting gap value (2Ds),
whereas larger energies are possible in other models such as an orbital
fluctuation model. Here we report the doping dependence of low-energy magnetic
excitation spectra in Ba1-xKxFe2As2 for 0.5<x<0.84 studied by inelastic neutron
scattering. We find that the behavior of the spin resonance dramatically
changes from optimum to overdoped regions. Strong resonance peaks are observed
clearly below 2Ds in the optimum doping region, while they are absent in the
overdoped region. Instead, there is a transfer of spectral weight from energies
below 2Ds to higher energies, peaking at values of 3Ds for x = 0.84. These
results suggest a reduced impact of magnetism on Cooper pair formation in the
overdoped region.
|
1603.02762v1
|
2016-03-17
|
The origin of the $α$-enhancement of massive galaxies
|
We study the origin of the stellar $\alpha$-element-to-iron abundance ratio,
$[\alpha/\mathrm{Fe}]_{\ast}$, of present-day central galaxies, using
cosmological, hydrodynamical simulations from the Evolution and Assembly of
GaLaxies and their Environments (EAGLE) project. For galaxies with stellar
masses of $M_{\ast} > 10^{10.5}$ M$_{\odot}$, $[\alpha/\mathrm{Fe}]_{\ast}$
increases with increasing galaxy stellar mass and age. These trends are in good
agreement with observations of early-type galaxies, and are consistent with a
`downsizing' galaxy formation scenario: more massive galaxies have formed the
bulk of their stars earlier and more rapidly, hence from an interstellar medium
that was mostly $\alpha$-enriched by massive stars. In the absence of feedback
from active galactic nuclei (AGN), however, $[\alpha/\mathrm{Fe}]_{\ast}$ in
$M_{\ast} > 10^{10.5}$ M$_{\odot}$ galaxies is roughly constant with stellar
mass and decreases with mean stellar age, extending the trends found for
lower-mass galaxies in both simulations with and without AGN. We conclude that
AGN feedback can account for the $\alpha$-enhancement of massive galaxies, as
it suppresses their star formation, quenching more massive galaxies at earlier
times, thereby preventing the iron from longer-lived intermediate-mass stars
(supernova Type Ia) from being incorporated into younger stars.
|
1603.05653v3
|
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