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2014-08-02 | Synthesis and post-annealing effects of alkaline-metal-ethylenediamine-intercalated superconductors Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) with Tc = 45 K | New iron-based intercalation superconductors Ax(C2H8N2)yFe2-zSe2 (A = Li, Na)
with Tc = 45 K have successfully been synthesized via intercalation of
dissolved alkaline metal in ethylenediamine. The c-axis lengths of
Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) are 20.74(7) {\AA} and 21.9(1) {\AA},
respectively, and are about 50 % larger than that of KxFe2Se2, indicating that
not only alkaline metal but also ethylenediamine is intercalated between the
Se-Se layers of FeSe. It seems that the high-Tc of Ax(C2H8N2)yFe2-zSe2 (A = Li,
Na) is caused by the possible two-dimensional electronic structure due to the
large c-axis length. Through the post-annealing in an evacuated glass tube, it
has been found that Tc decreases with increasing post-annealing temperature and
that deintercalation of EDA from the as-intercalated sample takes place at low
temperatures below 250 {\deg}C. | 1408.0346v1 |
2014-08-06 | Permanent magnet with MgB2 bulk superconductor | Superconductors with persistent zero-resistance currents serve as permanent
magnets for high-field applications requiring a strong and stable magnetic
field, such as magnetic resonance imaging (MRI). The recent global helium
shortage has quickened research into high-temperature superconductors (HTSs)
materials that can be used without conventional liquid-helium cooling to 4.2 K.
Herein, we demonstrate that 40-K-class metallic HTS magnesium diboride (MgB2)
makes an excellent permanent bulk magnet, maintaining 3 T at 20 K for 1 week
with an extremely high stability (<0.1 ppm/h). The magnetic field trapped in
this magnet is uniformly distributed, as for single-crystalline
neodymium-iron-boron. Magnetic hysteresis loop of the MgB2 permanent bulk
magnet was detrmined. Because MgB2 is a simple-binary-line compound that does
not contain rare-earth metals, polycrystalline bulk material can be
industrially fabricated at low cost and with high yield to serve as strong
magnets that are compatible with conventional compact cryocoolers, making MgB2
bulks promising for the next generation of Tesla-class permanent-magnet
applications. | 1408.1277v1 |
2014-08-08 | Orbital-driven nematicity in FeSe | A very fundamental and unconventional characteristic of superconductivity in
iron-based materials is that it occurs in the vicinity of {\it two} other
instabilities. Apart from a tendency towards magnetic order, these Fe-based
systems have a propensity for nematic ordering: a lowering of the rotational
symmetry while time-reversal invariance is preserved. Setting the stage for
superconductivity, it is heavily debated whether the nematic symmetry breaking
is driven by lattice, orbital or spin degrees of freedom. Here we report a very
clear splitting of NMR resonance lines in FeSe at $T_{nem}$ = 91K, far above
superconducting $T_c$ of 9.3 K. The splitting occurs for magnetic fields
perpendicular to the Fe-planes and has the temperature dependence of a
Landau-type order-parameter. Spin-lattice relaxation rates are not affected at
$T_{nem}$, which unequivocally establishes orbital degrees of freedom as
driving the nematic order. We demonstrate that superconductivity competes with
the emerging nematicity. | 1408.1875v3 |
2014-08-09 | Stellar origin of the 182Hf cosmochronometer and the presolar history of solar system matter | Among the short-lived radioactive nuclei inferred to be present in the early
solar system via meteoritic analyses, there are several heavier than iron whose
stellar origin has been poorly understood. In particular, the abundances
inferred for 182Hf (half-life = 8.9 million years) and 129I (half-life = 15.7
million years) are in disagreement with each other if both nuclei are produced
by the rapid neutron-capture process. Here, we demonstrate that contrary to
previous assumption, the slow neutron-capture process in asymptotic giant
branch stars produces 182Hf. This has allowed us to date the last rapid and
slow neutron-capture events that contaminated the solar system material at
roughly 100 million years and 30 million years, respectively, before the
formation of the Sun. | 1408.2050v1 |
2014-08-12 | Nematic spin correlations in the tetragonal state of uniaxial strained BaFe2-xNixAs2 | Understanding the microscopic origins of electronic phases in high-transition
temperature (high-Tc) superconductors is important for elucidating the
mechanism of superconductivity. In the paramagnetic tetragonal phase of
BaFe2-xTxAs2 (where T is Co or Ni) iron pnictides, an in-plane resistivity
anisotropy has been observed. Here we use inelastic neutron scattering to show
that low-energy spin excitations in these materials change from four-fold
symmetric to two-fold symmetric at temperatures corresponding to the onset of
the in-plane resistivity anisotropy. Because resistivity and spin excitation
anisotropies both vanish near optimal superconductivity, we conclude that they
are likely intimately connected. | 1408.2756v1 |
2014-08-13 | Dynamical correlations and screened exchange on the experimental bench: spectral properties of the cobalt pnictide BaCo2As2 | Understanding the Fermi surface and low-energy excitations of iron or cobalt
pnictides is crucial for assessing electronic instabilities such as magnetic or
superconducting states. Here, we propose and implement a new approach to
compute the low-energy properties of correlated electron materials, taking into
account both screened exchange beyond the local density approximation and local
dynamical correlations. The scheme allows us to resolve the puzzle of BaCo2As2,
for which standard electronic structure techniques predict a ferromagnetic
instability not observed in nature. | 1408.3136v2 |
2014-08-14 | Role of magnetic degrees of freedom in a scenario of phase transformations in steel | The diversity of mesostructures formed in steel at cooling from a
high-temperature austenite ("gamma") phase is determined by the interplay of
shear reconstructions of crystal lattice and diffusion of carbon. Combining
first-principles calculations with large-scale phase-field simulations we
demonstrate a decisive role of magnetic degrees of freedom in the formation of
energy relief along the Bain path of "gamma"-"alpha" transformation and, thus,
in this interplay. We show that there is the main factor, namely, the magnetic
state of iron and its evolution with temperature which controls the change in
character of the transformation. Based on the computational results we propose
a simple model which reproduces, in good agreement with experiment, the most
important curves of the phase transformation in Fe-C, namely, the lines
relevant to a start of ferrite, bainite, and martensite transformations.
Phase-field simulations within the model describe qualitatively typical
patterns at these transformations. | 1408.3275v2 |
2014-08-17 | Interaction between multi components vortices at arbitrary distances using a variational method in the Ginzburg-Landau theory | We study the interaction between the vortices in multi components
superconductors based on the Jacobs and Rebbi variation method using
Ginzburg-Landau theory. With one condensation, we get attraction interaction
between the vortices for type I and repulsion for type II superconductors. With
two condensation states such as Mg B_{2} superconductors the behavior is quite
different. There is attraction at large distances and repulsion when the
vortices are close to each other. A stability point at distance
2.7/{\lambda}_{1} is obtained. In the case of three condensation states such as
iron based superconductors,we see different behavior depending on penetration
depth and correlation length. The formation energy of a vortex with three
condensation states is larger than the one with one condensation state with
comparable penetration and correlation length. We obtain two stability points
for the superconductors with three condensation states. | 1408.3834v2 |
2014-08-19 | Anomalous Superconducting-Gap Structure of Slightly Overdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ | We observed the anisotropic superconducting-gap (SC-gap) structure of a
slightly overdoped superconductor, Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$
($x=0.1$), using three-dimensional (3D) angle-resolved photoemission
spectroscopy. Two hole Fermi surfaces (FSs) observed at the Brillouin zone
center and an inner electron FS at the zone corner showed a nearly isotropic SC
gap in 3D momentum space. However, the outer electron FS showed an anisotropic
SC gap with nodes or gap minima around the M and A points. The different
anisotropies obtained the SC gap between the outer and inner electron FSs
cannot be expected from all theoretical predictions with spin fluctuation,
orbital fluctuation, and both competition. Our results provide a new insight
into the SC mechanisms of iron pnictide superconductors. | 1408.4260v1 |
2014-08-25 | Ground state, collective mode, phase soliton and vortex in multiband superconductors | This article reviews theoretical and experimental work on the novel physics
in multiband superconductors. Multiband superconductors are characterized by
multiple superconducting energy gaps in different bands with interaction
between Cooper pairs in these bands. The discovery of prominent multiband
superconductors $\mathrm{MgB_2}$ and later iron-based superconductors has
triggered enormous interests in multiband superconductors. Most recently
discovered superconductors exhibit multiband features. The multiband
superconductors possess novel properties that are not shared by their
single-band counterpart. Examples include the time-reversal symmetry broken
state in multiband superconductors with frustrated interband couplings, the
collective oscillation of number of Cooper pairs between different bands, known
as the Leggett mode, the phase soliton and fractional vortex, which are the
main focus of this review. This review presents a survey of a wide range of
theoretical exploration and experimental investigations of novel physics in
multiband superconductors. Vast information derived from these studies is shown
to highlight unusual and unique properties of multiband superconductors, and to
reveal the challenges and opportunities in the research on the multiband
superconductivity. | 1408.5938v2 |
2014-08-26 | Measurement of the Velocity of the Neutrino with MINOS | The MINOS experiment uses a beam of predominantly muon-type neutrinos
generated using protons from the Main Injector at Fermilab in Batavia, IL, and
travelling 735 km through the Earth to a disused iron mine in Soudan, MN. The
10{\mu}s-long beam pulse contains fine time structure which allows a precise
measurement of the neutrino time of flight to be made. The time structure of
the parent proton pulse is measured in the beamline after extraction from the
Main Injector, and neutrino interactions are timestamped at the Fermilab site
in the Near Detector (ND), and at the Soudan site in the Far Detector (FD).
Small, transportable auxiliary detectors, consisting of scintillator planes and
associated readout electronics, are used to measure the relative latency
between the two large detectors. Time at each location is measured with respect
to HP5071A Cesium clocks, and time is transferred using GPS Precise Point
Positioning (PPP) solutions for the clock offset at each location. We describe
the timing calibration of the detectors and derive a measurement of the
neutrino velocity, based on data from March and April 2012. We discuss the
prospects for further improvements that would yield a still more accurate
result. | 1408.6267v1 |
2014-08-29 | The spin of the black hole 4U 1543-47 | We present a new analysis of Rossi X-ray Timing Explorer observations of the
2002 outburst of the transient X-ray nova 4U 1543-47. We focus on observations
in the High/Soft state, and attempt to measure the spin of the black hole by
simultaneously fitting the thermal disk continuum and by modeling the broadened
iron k-shell emission lines and additional blurred reflection features.
Previous works have found that use of these methods individually returns
contradictory values for the dimensionless spin parameter a* =cJ/GM^2. We find
that when used in conjunction with each other, a moderate spin is obtained
(a*=0.43 +0.22 -0.31) that is actually consistent with both other values within
errors. We discuss limitations of our analysis, systematic uncertainties, and
implications of this measurement, and compare our result to those previously
claimed for 4U 1543-47. | 1408.7028v1 |
2014-09-01 | Magnetic structure of the Eu2+ moments in superconducting EuFe2(As1-xPx)2 with x = 0.19 | The magnetic structure of the Eu2+ moments in the superconducting
EuFe2(As1-xPx)2 sample with x = 0.19 has been determined using neutron
scattering. We conclude that the Eu2+ moments are aligned along the c direction
below T_C = 19.0(1) K with an ordered moment of 6.6(2) mu_B in the
superconducting state. An impurity phase similar to the underdoped phase exists
within the bulk sample which orders antiferromagnetically below T_N = 17.0(2)
K. We found no indication of iron magnetic order, nor any incommensurate
magnetic order of the Eu2+ moments in the sample. | 1409.0432v1 |
2014-09-07 | Quantum Monte Carlo study of the $S_4$ symmetric microscopic model for iron-based superconductors | The $S_4$ symmetric microscopic model with two iso-spin components has been
studied via constrained-path quantum Monte Carlo simulation. Our results
demonstrate a stable $(\pi,0)$ or $(0,\pi)$ magnetic order which is
significantly enhanced on increasing both the Coulomb repulsion $U$ and Hund's
coupling strength $J$. Also, our simulation indicates that the magnetic order
tends to be in an orthomagnetic one, in which the nearest-neighbour magnetic
moment are orthogonal to each other, rather than in a collinear
antiferromagnetic state. Interestingly, when the system is doped away from half
filling, the magnetic order is obviously elevated in the low doping density,
and then significantly suppressed when more electrons are introduced.
Meanwhile, we find that an $A_{1g}$ $s_{\pm}$-wave pairing dominates all the
singlet nearest-neighbour pairings, and is significantly enhanced via electron
doping. | 1409.2075v1 |
2014-09-08 | The sensitivity of the ICAL detector at India-based Neutrino Observatory to neutrino oscillation parameters | The India-based Neutrino Observatory (INO) will host a 50 kt magnetized iron
calorimeter (ICAL) detector that will be able to detect muon tracks and hadron
showers produced by Charged-Current muon neutrino interactions in the detector.
The ICAL experiment will be able to determine the precision of atmospheric
neutrino mixing parameters and neutrino mass hierarchy using atmospheric muon
neutrinos through earth matter effect. In this paper, we report on the
sensitivity for the atmospheric neutrino mixing parameters
($\sin^{2}\theta_{23}$ and $|\Delta m^{2}_{32}|$) for the ICAL detector using
the reconstructed neutrino energy and muon direction as observables. We apply
realistic resolutions and efficiencies obtained by the ICAL collaboration with
a GEANT4-based simulation to reconstruct neutrino energy and muon direction.
Our study shows that using neutrino energy and muon direction as observables
for a $\chi^{2}$ analysis, ICAL detector can measure $\sin^{2}\theta_{23}$ and
$|\Delta m^{2}_{32}|$ with 13% and 4% uncertainties at 1$\sigma$ confidence
level for 10 years of exposure. | 1409.2231v1 |
2014-09-09 | Anomalous density of states in multiband superconductors near Lifshitz transition | We consider a multiband metal with deep primary bands and a shallow secondary
one. In the normal state the system undergoes Lifshitz transition when the
bottom of the shallow band crosses the Fermi level. In the superconducting
state Cooper pairing in the shallow band is induced by the deep ones. As a
result, the density of electrons in the shallow band remains finite even when
the bottom of the band is above the Fermi level. We study the density of states
in the system and find qualitatively different behaviors on the two sides of
the Lifshitz transition. On one side of the transition the density of states
diverges at the energy equal to the induced gap, whereas on the other side it
vanishes. We argue that this physical picture describes the recently measured
gap structure in shallow bands of iron pnictides and selenides. | 1409.2807v2 |
2014-09-09 | The diversity of quasars unified by accretion and orientation | Quasars are rapidly accreting supermassive black holes at the center of
massive galaxies. They display a broad range of properties across all
wavelengths, reflecting the diversity in the physical conditions of the regions
close to the central engine. These properties, however, are not random, but
form well-defined trends. The dominant trend is known as Eigenvector 1, where
many properties correlate with the strength of optical iron and [OIII]
emission. The main physical driver of Eigenvector 1 has long been suspected to
be the quasar luminosity normalized by the mass of the hole (the Eddington
ratio), an important quantity of the black hole accretion process. But a
definitive proof has been missing. Here we report an analysis of archival data
that reveals that Eddington ratio indeed drives Eigenvector 1. We also find
that orientation plays a significant role in determining the observed
kinematics of the gas, implying a flattened, disklike geometry for the
fast-moving clouds close to the hole. Our results show that most of the
diversity of quasar phenomenology can be unified with two simple quantities,
Eddington ratio and orientation. | 1409.2887v1 |
2014-09-10 | Theoretical investigation of the electronic and magnetic properties of the orthorhombic phase of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ | We present a comprehensive study on the low-temperature orthorhombic phase of
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ based on the Korringa-Kohn-Rostoker-Green
function approach. Using this bandstructure method in combination with the
coherent potential approximation alloy theory we are able to investigate the
evolution of the magnetic and electronic properties of this prototype iron
pnictide for arbitrary concentrations $x$, while dealing with the chemical
disorder without uncontrolled simplifications by using solely a rigid band
shift or the virtual crystal approximation. We discuss the development of the
site resolved magnetic moments for the experimentally observed stripe
antiferromagnetic order together with the strong electronic anisotropy of the
Fermi surface and compare it with angle-resolved photoemission spectroscopy
measurements of detwinned crystals. We furthermore calculate magnetic exchange
coupling parameters $J_{ij}$ and use them for Monte-Carlo simulations on the
basis of the classical Heisenberg model to get an insight on the temperature
dependence of the magnetic ordering on the cobalt concentration. | 1409.3099v1 |
2014-09-10 | On the Raman O VI and related lines in classical novae | We critically examine the recent claimed detection of Raman scattered O VI at
around 6830\AA\ in the iron curtain stage spectra of the classical CO nova V339
Del. The observed line variations are compatible in profile and timing of
emission line strength with an excited state transition of neutral carbon. Line
formation in classical nova ejecta is physically very different from that in
symbiotic binaries, in which the O VI emission line is formed within the wind
of the companion red giant at low differential velocity. The ejecta velocity
and density structure prevent the scattering from producing analogous features.
There might , however, be a broadband spectropolarimetric signature of the
Raman process and also Rayleigh scattering at some stage in the expansion. We
show that the neutral carbon spectrum, hitherto under-exploited for novae, is
especially useful as a probe of the structure of the ejecta during the early,
optically thick stages of the expansion | 1409.3240v1 |
2014-09-11 | Effects of applied strain on radiation damage generation in body-centered cubic iron | Radiation damage in body-centered cubic (BCC) Fe has been extensively studied
by computer simulations to quantify effects of temperature, impinging particle
energy, and the presence of extrinsic particles. However, limited investigation
has been conducted into the effects of mechanical stresses and strain. In a
reactor environment, structural materials are often mechanically strained, and
an expanded understanding of how this strain affects the generation of defects
may be important for predicting microstructural evolution and damage
accumulation under such conditions. In this study, we have performed molecular
dynamics simulations in which various types of homogeneous strains are applied
to BCC Fe and the effect on defect generation is examined. It is found that
volume-conserving shear strains yield no statistically significant variations
in the stable number of defects created via cascades in BCC Fe. However,
strains that result in volume changes are found to produce significant effects
on defect generation. | 1409.3519v2 |
2014-09-13 | Coexistence of 3d-ferromagnetism and superconductivity in [(Li(1-x)Fex)OH](Fe(1-y)Liy)Se | Superconducting [(Li(1-x)Fex)OH](Fe(1-y)Liy)Se (x ~ 0.2, y ~ 0.08) was
synthesized by hydrothermal methods and structurally characterized by single
crystal X-ray diffraction. The crystal structure contains anti-PbO type
(Fe(1-y)Liy)Se layers separated by layers of (Li(1-x)Fex)OH. Electrical
resistivity and magnetic susceptibility measurements reveal superconductivity
at 43 K. An anomaly in the diamagnetic shielding indicates ferromagnetic
ordering near 10 K while superconductivity is retained. The ferromagnetism
emerges from the iron atoms in the (Li(1-x)Fex)OH layer. Isothermal
magnetization measurements confirm the superposition of ferromagnetic with
superconducting hysteresis. The internal ferromagnetic field is larger than the
lower, but smaller than the upper critical field of the superconductor, which
gives evidence for a spontaneous vortex phase where both orders coexist.
57Fe-M\"ossbauer spectra, 7Li-NMR spectra, and muSR experiments consistently
support this rare situation, especially in a bulk material where magnetism
emerges from a 3d-element. | 1409.3982v1 |
2014-09-14 | Soft striped magnetic fluctuations competing with superconductivity in Fe_{1+x}Te | Neutron spectroscopy is used to investigate the magnetic fluctuations in
Fe_{1+x}Te - a parent compound of chalcogenide superconductors. Incommensurate
"stripe-like" excitations soften with increased interstitial iron
concentration. The energy crossover from incommensurate to stripy fluctuations
defines an apparent hour-glass dispersion. Application of sum rules of neutron
scattering find that the integrated intensity is inconsistent with an S=1
Fe^{2+} ground state and significantly less than S=2 predicted from weak
crystal field arguments pointing towards the Fe^{2+} being in a superposition
of orbital states. The results suggest that a highly anisotropic order competes
with superconductivity in chalcogenide systems. | 1409.4051v1 |
2014-09-16 | Inverse problem for extragalactic transport of ultra-high energy cosmic rays | The energy spectra and composition of ultra-high energy cosmic rays are
changing in a course of propagation in the expanding Universe filled with
background radiation. We developed a numerical code for solution of inverse
problem for cosmic-ray transport equations that allows the determination of
average source spectra of different nuclei from the cosmic ray spectra observed
at the Earth. Employing this approach, the injection spectra of protons and
Iron nuclei in extragalactic sources are found assuming that only these species
are accelerated at the source. The data from the Auger experiment and the
combined data from the Telescope Array + HiRes experiments are used to
illustrate the method. | 1409.4654v4 |
2014-09-17 | Increased magnetocrystalline anisotropy in epitaxial Fe-Co-C thin films with spontaneous strain | Rare earth free alloys are in focus of permanent magnet research since the
accessibility of the elements needed for nowadays conventional magnets is
limited. Tetragonally strained iron-cobalt (Fe-Co) has attracted large interest
as promising candidate due to theoretical calculations. In experiments,
however, the applied strain quickly relaxes with increasing film thickness and
hampers stabilization of a strong magnetocrystalline anisotropy. In our study
we show that already 2 at% of carbon substantially reduce the lattice
relaxation leading to the formation of a spontaneously strained phase with 3 %
tetragonal distortion. In these strained
(Fe$_{0.4}$Co$_{0.6}$)$_{0.98}$C$_{0.02}$ films, a magnetocrystalline
anisotropy above 0.4 MJ/m$^3$ is observed while the large polarization of 2.1 T
is maintained. Compared to binary Fe-Co this is a remarkable improvement of the
intrinsic magnetic properties. In this paper, we relate our experimental work
to theoretical studies of strained Fe-Co-C and find a very good agreement. | 1409.4952v1 |
2014-09-25 | Characterisation of Glass Electrodes and RPC Detectors for $INO-ICAL$ Experiment | India-based Neutrino Observatory (INO) is a planned neutrino experiment to be
build up in southern part of India.The INO observatory will host a 51 kton Iron
Calorimeter (ICAL) detector to detect atmospheric neutrinos. Resistive Plate
Chamber (RPC) has been chosen as the active detector element for the ICAL
experiment. The ICAL experiment will consist of about 28,000 RPC's of dimension
$2~m\times 2~m$, divided into three modules. The experiment is planned to take
data at least for 20 years from its start date. Due to the large number of RPC
needed for ICAL experiment and the long lifetime of the experiment, it is
necessary to carry out detailed $R\&D$ to optimise each and every parameter of
the detector performance. We report on the performance studies carried out on
the RPC's made with these electrodes, and finally compare the detector
performance with that of the material properties to optimise the detector
parameters. | 1409.7184v1 |
2014-09-26 | Superconducting gap structure of BaFe_2(As_{1-x}P_{x})_2 | We present a study of the superconducting gap structure in the iron-pnictide
series BaFe2(As1-xPx)2. By measuring the variation of the specific heat as a
function of temperature and magnetic field we are able to determine the number
and Fermi surface location of the nodes in the superconducting gap. In
particular, from measurements of the variation of the specific heat as the
magnetic field is rotated in the ab plane of the sample we conclude that the
nodes are in the [110] directions. Then from a quantitative analysis of the
temperature and field dependence of the specific heat we further conclude that
nodes exists on all Fermi surface sheets. | 1409.7523v1 |
2014-09-30 | Observation of an electron band above the Fermi level in FeTe$_{0.55}$Se$_{0.45}$ from \emph{in-situ} surface doping | We used \emph{in-situ} potassium (K) evaporation to dope the surface of the
iron-based superconductor FeTe$_{0.55}$Se$_{0.45}$. The systematic study of the
bands near the Fermi level confirms that electrons are doped into the system,
allowing us to tune the Fermi level of this material and to access otherwise
unoccupied electronic states. In particular, we observe an electron band
located above the Fermi level before doping that shares similarities with a
small three-dimensional pocket observed in the cousin, heavily-electron-doped
KFe$_{2-x}$Se$_2$ compound. | 1409.8383v2 |
2014-09-30 | Testing non-standard neutrino matter interactions in atmospheric neutrino propagation | We study the effects of non-standard interactions on the oscillation pattern
of atmospheric neutrinos. We use neutrino oscillograms as our main tool to
infer the role of non-standard interactions (NSI) parameters at the probability
level in the energy range, $E \in [1,20]$ GeV and zenith angle range, $\cos
\theta \in [-1,0]$. We compute the event rates for atmospheric neutrino events
in presence of NSI parameters in the energy range $E \in [1,10]$ GeV for two
different detector configurations - a magnetized iron calorimeter and an
unmagnetized liquid Argon time projection chamber which have different
sensitivities to NSI parameters due to their complementary characteristics. As
an application, we discuss how NSI parameter, $\epsilon_{\mu\tau}$ impacts the
determination of the correct octant of $\theta_{23}$. | 1409.8472v2 |
2014-10-02 | Observation of Majorana Fermions in Ferromagnetic Atomic Chains on a Superconductor | Majorana fermions are predicted to localize at the edge of a topological
superconductor, a state of matter that can form when a ferromagnetic system is
placed in proximity to a conventional superconductor with strong spin-orbit
interaction. With the goal of realizing a one-dimensional topological
superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the
surface of superconducting lead (Pb). Using high-resolution spectroscopic
imaging techniques, we show that the onset of superconductivity, which gaps the
electronic density of states in the bulk of the Fe chains, is accompanied by
the appearance of zero energy end states. This spatially resolved signature
provides strong evidence, corroborated by other observations, for the formation
of a topological phase and edge-bound Majorana fermions in our atomic chains. | 1410.0682v1 |
2014-10-03 | Unexpected impact of magnetic disorder on multiband superconductivity | We analyze how the magnetic disorder affects the properties of the two-band
$s_\pm$ and $s_{++}$ models, which are subject of hot discussions regarding
iron-based superconductors and other multiband systems like MgB$_2$. We show
that there are several cases when the transition temperature $T_c$ is not fully
suppressed by magnetic impurities in contrast to the Abrikosov-Gor'kov theory,
but a saturation of $T_c$ takes place in the regime of strong disorder. These
cases are: (1) the purely interband impurity scattering, (2) the unitary
scattering limit. We show that in the former case the $s_\pm$ gap is preserved,
while the $s_{++}$ state transforms into the $s_\pm$ state with increasing
magnetic disorder. For the case (2), the gap structure remains intact. | 1410.0832v2 |
2014-10-13 | [$α$/Fe] Abundances of Four Outer M 31 Halo Stars | We present alpha element to iron abundance ratios, [$\alpha$/Fe], for four
stars in the outer stellar halo of the Andromeda Galaxy (M 31). The stars were
identified as high-likelihood field halo stars by Gilbert et al. (2012) and lie
at projected distances between 70 and 140 kpc from M 31's center. These are the
first alpha abundances measured for a halo star in a galaxy beyond the Milky
Way. The stars range in metallicity between [Fe/H]= -2.2 and [Fe/H]= -1.4. The
sample's average [$\alpha$/Fe] ratio is +0.20+/-0.20. The best-fit average
value is elevated above solar which is consistent with rapid chemical
enrichment from Type II supernovae. The mean [$\alpha$/Fe] ratio of our M31
outer halo sample agrees (within the uncertainties) with that of Milky Way
inner/outer halo stars that have a comparable range of [Fe/H]. | 1410.3475v1 |
2014-10-17 | Influence of Electron-Impact Multiple Ionization on Equilibrium and Dynamic Charge State Distributions: A Case Study Using Iron | We describe the influence of electron-impact multiple ionization (EIMI) on
the ionization balance of collisionally ionized plasmas. We are unaware of any
previous ionization balance calculations that have included EIMI, which is
usually assumed to be unimportant. Here, we incorporate EIMI cross-section data
into calculations of both equilibrium and non-equilibrium charge-state
distributions (CSDs). For equilibrium CSDs, we find that EIMI has only a small
effect and can usually be ignored. However, for non-equilibrium plasmas the
influence of EIMI can be important. In particular, we find that for plasmas in
which the temperature oscillates there are significant differences in the CSD
when including versus neglecting EIMI. These results have implications for
modeling and spectroscopy of impulsively heated plasmas, such as nanoflare
heating of the solar corona. | 1410.4850v1 |
2014-10-20 | Sensitivity for detection of decay of dark matter particle using ICAL at INO | We report on the simulation studies on the possibility of dark matter
particle (DMP) decaying into leptonic modes. While not much is known about the
properties of dark matter particles except through their gravitational effect,
it has been recently conjectured that the so called "anomalous Kolar Events"
observed some decades ago may be due to the decay of unstable dark matter
particles (M.V.N. Murthy and G.Rajasekaran, Pramana, {\bf 82}, 609 (2014)). The
aim of this study is to see if this conjecture can be verified at the proposed
Iron Calorimeter (ICAL) detector at INO. We study the possible decay to
leptonic modes which may be seen in this detector with some modifications. For
the purposes of simulation we assume that each channel saturates the decay
width for the mass ranging from $1-50 \rm{GeV/c^2}$. The aim is not only to
investigate the decay signatures, but also, more generally, to establish lower
bounds on the life time of DMP even if no such decay takes place. | 1410.5182v1 |
2014-10-21 | Possible superconductivity in Sr$_{2}$IrO$_{4}$ probed by quasiparticle interference | Based on the possible superconducting (SC) pairing symmetries recently
proposed, the quasiparticle interference (QPI) patterns in electron- and
hole-doped Sr$_{2}$IrO$_{4}$ are theoretically investigated. In the
electron-doped case, the QPI spectra can be explained based on a model similar
to the octet model of the cuprates while in the hole-doped case, both the Fermi
surface topology and the sign of the SC order parameter resemble those of the
iron pnictides and there exists a QPI vector resulting from the interpocket
scattering between the electron and hole pockets. In both cases, the evolution
of the QPI vectors with energy and their behaviors in the nonmagnetic and
magnetic impurity scattering cases can well be explained based on the evolution
of the constant-energy contours and the sign structure of the SC order
parameter. The QPI spectra presented in this paper can be compared with future
scanning tunneling microscopy experiments to test whether there are SC phases
in electron- and hole-doped Sr$_{2}$IrO$_{4}$ and what the pairing symmetry is. | 1410.5512v2 |
2014-10-23 | Superconductivity and phase instability of NH3-free Na-intercalated FeSe1-zSz | The discovery of ThCr2Si2-type AxFe2-ySe2 (A = K, Rb, Cs and Tl) with Tc ~
30K make much progress in iron-based superconducting field, but their
multiple-phase separations are disadvantageous for understanding the origin. On
the other hand, for small alkali metals, studies on (Li,Na)FeCu(S,Se)2 and
NaFe2-{\delta}S2 shows that these compounds possess CaAl2Si2-type structure,
implying that ThCr2Si2-type structure is unstable for small alkali-metal
intercalated FeSe under high-temperature. Here we report a new intercalate
Na0.65(1)Fe1.93(1)Se2 with Tc ~ 37 K, synthesized by low-temperature
ammonothermal method. The notable finding is that the Na0.65(1)Fe1.93(1)Se2
shows a ThCr2Si2-type structure, which is the first instance of small-sized
alkali metal intercalates without NH3 co-intercalation. Besides, the NH3-poor
Na0.80(4)(NH3)0.60Fe1.86(1)Se2 and NH3-rich phase with Tcs at 45 K and 42 K are
identified by tuning the concentration of Na-NH3 solutions. The modulation of
interlayer spacing reveals the versatile evolution of structural stability and
superconductivity in these intercalates. | 1410.6385v1 |
2014-10-27 | Charge Dynamics in a Correlated Fermion System on a Geometrically Frustrated Lattice | Charge dynamics in an interacting fermionic model on a geometrically
frustrated lattice are examined. We analyze a spinless fermion model on a
paired triangular lattice, an electronic model for layered iron oxides, in zero
and finite temperatures by the exact diagonalization methods. We focus on the
two charge ordered (CO) phases, termed CO_1/2 and CO_1/3, which, respectively,
are realized by the inter-site Coulomb interaction and the quantum/thermal
fluctuations. The optical spectra in CO_1/3 show multiple components and their
low-energy weights are survived even below the ordering temperature (T_CO).
Changes of the dynamical charge correlation below T_CO in CO_1/3 are weakly
momentum dependent, in sharply contrast to CO_1/2. These characteristic
dynamics in CO_1/3 are attributable to the charge frustration effects, and
reproduce some aspects of the recent experimental optical and x-ray scattering
spectra. | 1410.7218v1 |
2014-10-27 | Eight billion asteroids in the Oort cloud | The Oort cloud is usually thought of as a collection of icy comets inhabiting
the outer reaches of the Solar system, but this picture is incomplete. We use
simulations of the formation of the Oort cloud to show that ~4% of the small
bodies in the Oort cloud should have formed within 2.5 au of the Sun, and hence
be ice-free rock-iron bodies. If we assume these Oort cloud asteroids have the
same size distribution as their cometary counterparts, the Large Synoptic
Survey Telescope should find roughly a dozen Oort cloud asteroids during ten
years of operations. Measurement of the asteroid fraction within the Oort cloud
can serve as an excellent test of the Solar system's formation and dynamical
history. Oort cloud asteroids could be of particular concern as impact hazards
as their high mass density, high impact velocity, and low visibility make them
both hard to detect and hard to divert or destroy. However, they should be a
rare class of object, and we estimate globally catastrophic collisions should
only occur about once per billion years. | 1410.7403v1 |
2014-10-28 | Unified picture of the doping dependence of superconducting transition temperatures in alkali metal/ammonia intercalated FeSe | In the recently synthesized Li$_x$(NH$_2$)$_y$(NH$_3$)$_z$Fe$_2$Se$_2$ family
of iron chalcogenides a molecular spacer consisting of lithium ions, lithium
amide and ammonia separates layers of FeSe. It has been shown that upon
variation of the chemical composition of the spacer layer, superconducting
transition temperatures can reach $T_c\sim 44 \mathrm{K}$, but the relative
importance of the layer separation and effective doping to the $T_c$
enhancement is currently unclear. Using state of the art band structure
unfolding techniques, we construct eight-orbital models from ab-initio density
functional theory calculations for these materials. Within an RPA
spin-fluctuation approach, we show that the electron doping enhances the
superconducting pairing, which is of $s_\pm$-symmetry and explain the
experimentally observed limit to $T_c$ in the molecular spacer intercalated
FeSe class of materials. | 1410.7565v3 |
2014-10-30 | Detection of Neutral Phosphorus in the Near Ultraviolet Spectra of Late-Type Stars | We report the detection of several absorption lines of neutral phosphorus (P,
Z=15) in archival near ultraviolet spectra obtained with the Space Telescope
Imaging Spectrograph on board the Hubble Space Telescope. We derive phosphorus
abundances or interesting upper limits in 14 late-type stars with metallicities
spanning -3.8<[Fe/H]<-0.1. Previously, phosphorus had only been studied in
Galactic stars with -1.0<[Fe/H]<+0.3. Iron lines reveal abundance offsets
between the optical and ultraviolet regions, and we discuss and apply a
correction factor to account for this offset. In stars with [Fe/H]>-1.0, the
[P/Fe] ratio decreases toward the solar value with increasing metallicity, in
agreement with previous observational studies. In stars with [Fe/H]<-1.0,
<[P/Fe]>=+0.04+/-0.10, which overlaps with the [P/Fe] ratios found in several
high-redshift damped Lyman-alpha systems. This behavior hints at a primary
origin in massive stars. | 1410.8539v1 |
2014-11-01 | Glide-Plane Symmetry and Superconducting Gap Structure of Iron-Based Superconductors | We consider the effect of glide-plane symmetry of the Fe-pnictogen/chalcogen
layer in Fe-based superconductors on pairing in spin fluctuation models. Recent
theories have proposed that so-called $\eta$-pairing states with nonzero total
momentum can be realized and possess exotic properties such as odd parity spin
singlet symmetry and time-reversal symmetry breaking. Here we show that $\eta$
pairing is inevitable when there is orbital weight at the Fermi level from
orbitals with even and odd mirror reflection symmetry in $z$; however, by
explicit calculation, we conclude that the gap function that appears in
observable quantities is identical to that found in earlier, 1 Fe per unit cell
pseudocrystal momentum calculations. | 1411.0070v2 |
2014-11-04 | In-plane magnetic field vs. temperature phase diagram of a quasi-2D frustrated multiband superconductor | Motivated by the recent discovery of iron-based superconductors, with high
critical temperatures and multiple bands crossing the Fermi level, we address
the conditions for the presence of chiral superconducting phases configurations
in the in-plane magnetic field vs. temperature phase diagram of a quasi-2D
frustrated three-band superconductor. Due to Zeeman splitting, the coupled
superconducting gap equations present a complex set of solutions. For weak
interband couplings, chiral configurations are only attained in a narrow strip
of the in-plane magnetic field vs. temperature phase diagram. This strip of
chiral states becomes narrower and disappears at low temperatures, giving way
to a first-order transition between non-chiral superconducting states. For
stronger interband couplings, the chiral strip is much broader, if the
interband couplings are approximately equal; otherwise, the chiral region is
expected to be completely absent of the phase diagram. | 1411.0937v2 |
2014-11-12 | Spontaneous Breakdown of Time-Reversal Symmetry Induced by Thermal Fluctuations | In systems with broken $U(1)$ symmetry, such as superfluids, superconductors
or magnets, the symmetry restoration is driven by proliferation of topological
defects in the form of vortex loops. Here we discuss that in certain systems
the proliferation of topological defects can, by contrast, lead to the
breakdown of an additional symmetry. As a particular example we demonstrate
that this effect should take place in $s+is$ superconductors, which are widely
discussed in connection with the Iron-based materials. In these systems a
vortex excitation can create a "bubble" of fluctuating $Z_2$ order parameter.
Thermal excitation of vortices then leads to breakdown of $Z_2$ time-reversal
symmetry when the temperature is increased. | 1411.3202v2 |
2014-11-14 | The impact of global nuclear mass model uncertainties on $r$-process abundance predictions | Rapid neutron capture or `$r$-process' nucleosynthesis may be responsible for
half the production of heavy elements above iron on the periodic table. Masses
are one of the most important nuclear physics ingredients that go into
calculations of $r$-process nucleosynthesis as they enter into the calculations
of reaction rates, decay rates, branching ratios and Q-values. We explore the
impact of uncertainties in three nuclear mass models on $r$-process abundances
by performing global monte carlo simulations. We show that root-mean-square
(rms) errors of current mass models are large so that current $r$-process
predictions are insufficient in predicting features found in solar residuals
and in $r$-process enhanced metal poor stars. We conclude that the reduction of
global rms errors below $100$ keV will allow for more robust $r$-process
predictions. | 1411.3973v1 |
2014-11-14 | A mini review on NiFe-based materials as highly active oxygen evolution reaction electrocatalysts | Oxygen evolution reaction (OER) electrolysis, as an important reaction
involved in water splitting and rechargeable metal-air battery, has attracted
increasing attention for clean energy generation and efficient energy storage.
Nickel/iron (NiFe)-based compounds have been known as active OER catalysts
since the last century, and renewed interest has been witnessed in recent years
on developing advanced NiFe-based materials for better activity and stability.
In this review, we present the early discovery and recent progress on
NiFe-based OER electrocatalysts in terms of chemical properties, synthetic
methodologies and catalytic performances. The advantages and disadvantages of
each class of NiFe-based compounds are summarized, including NiFe alloys,
electro-deposited films and layered-double hydroxide nanoplates. Some
mechanistic studies of the active phase of NiFe-based compounds are introduced
and discussed to give insight into the nature of active catalytic site, which
could facilitate further improving NiFe based OER electrocatalysts. Finally,
some applications of NiFe-based compounds for OER are described, including the
development of electrolyzer operating with a single AAA battery with voltage
below 1.5V and high performance rechargeable Zn-air batteries. | 1411.4677v1 |
2014-11-19 | Stability and magnetization of free-standing and graphene-embedded iron membranes | Inspired by recent experimental realizations of monolayer Fe membranes in
graphene perforations, we perform ab initio calculations of Fe monolayers and
membranes embedded in graphene in order to assess their structural stability
and magnetization. We demonstrate that monolayer Fe has a larger spin
magnetization per atom than bulk Fe and that Fe membranes embedded in graphene
exhibit spin magnetization comparable to monolayer Fe. We find that
free-standing monolayer Fe is structurally more stable in a triangular lattice
compared to both square and honeycomb lattices. This is contradictory to the
experimental observation that the embedded Fe membranes form a square lattice.
However, we find that embedded Fe membranes in graphene perforations can be
more stable in the square lattice configuration compared to the triangular. In
addition, we find that the square lattice has a lower edge formation energy,
which means that the square Fe lattice may be favored during formation of the
membrane. | 1411.5193v1 |
2014-11-19 | Structural ${γ\textrm{-}\varepsilon}$ phase transition in Fe-Mn alloys from CPA+DMFT approach | We present a computational scheme for total energy calculations of disordered
alloys with strong electronic correlations. It employs the coherent potential
approximation combined with the dynamical mean-field theory and allows one to
study the structural transformations. The material-specific Hamiltonians in the
Wannier function basis are obtained by density functional theory. The proposed
computational scheme is applied to study the ${\gamma\textrm{-}\varepsilon}$
structural transition in paramagnetic Fe-Mn alloys for Mn content from 10 to 20
at. %. The electronic correlations are found to play a crucial role in this
transition. The calculated transition temperature decreases with increasing Mn
content and is in a good agreement with experiment. We demonstrate that in
contrast to the ${\alpha\textrm{-}\gamma}$ transition in pure iron, the
${\gamma\textrm{-}\varepsilon}$ transition in Fe-Mn alloys is driven by a
combination of kinetic and Coulomb energies. The latter is found to be
responsible for the decrease of the ${\gamma\textrm{-}\varepsilon}$ transition
temperature with Mn content. | 1411.5356v3 |
2014-11-19 | Interplay between Kondo effect and Ruderman-Kittel-Kasuya-Yosida interaction | The interplay between the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction
and the Kondo effect is expected to provide the driving force for the emergence
of many phenomena in strongly correlated electron materials. Two magnetic
impurities in a metal are the smallest possible system containing all these
ingredients and define a bottom up approach towards a long term understanding
of concentrated / dense systems. Here we report on the experimental and
theoretical investigation of iron dimers buried below a Cu(100) surface by
means of low temperature scanning tunnelling spectroscopy (STS) combined with
density functional theory (DFT) and numerical renormalization group (NRG)
calculations. The Kondo effect, in particular the width of the Abrikosov-Suhl
resonance, is strongly altered or even suppressed due to magnetic coupling
between the impurities. It oscillates as function of dimer separation revealing
that it is related to the RKKY interaction mediated by the conduction
electrons. Simulations based on density functional theory support this concept
showing the same oscillation period and trends in the coupling strength as
found in the experiment. | 1411.5388v1 |
2014-11-25 | Quantum critical elasticity | We discuss elastic instabilities of the atomic crystal lattice at zero
temperature. Due to long-range shear forces of the solid, at such transitions
the phonon velocities vanish, if at all, only along certain crystallographic
directions, and, consequently, the critical phonon fluctuations are suppressed
to a lower dimensional manifold and governed by a Gaussian fixed-point. In case
of symmetry-breaking elastic transitions, a characteristic critical phonon
thermodynamics arises that is found, e.g., to violate Debye's $T^3$-law for the
specific heat. We point out that quantum critical elasticity is triggered
whenever a critical soft mode couples linearly to the strain tensor. In
particular, this is relevant for the electronic Ising-nematic quantum phase
transition in a tetragonal crystal as discussed in the context of certain
cuprates, ruthenates and iron-based superconductors. | 1411.6925v3 |
2014-11-26 | Effect of bombarding steel with Xe$^+$ ions on the surface nanostructure and on pulsed plasma nitriding process | The modification of steel (AISI 316L and AISI 4140) surface morphology and
underlying inter-crystalline grains strain due to Xe$^+$ ion bombardment are
reported to affect nitrogen diffusion after a pulsed plasma nitriding process.
The ion bombardment induces regular nanometric patterns and increases the
roughness of the material surface. The strain induced by the noble gas
bombardment is observed in depths which are orders of magnitude larger than the
projectiles' stopping distance. The pre-bombarded samples show peculiar
microstructures formed in the nitrided layers, modifying the in-depth hardness
profile. Unlike the double nitrided layer normally obtained in austenitic
stainless steel by pulsed plasma nitriding process, the Xe$^+$ pre-bombardment
treatment leads to a single thick compact layer. In nitrided pre-bombarded AISI
4140 steel, the diffusion zone shows long iron nitride needle-shaped
precipitates, while in non-pre-bombarded samples finer precipitates are
distributed in the material. | 1411.7281v1 |
2014-11-27 | Competing superconducting and magnetic order parameters and field-induced magnetism in electron doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ | We have studied the magnetic and superconducting properties of
Ba(Fe$_{0.95}$Co$_{0.05}$)$_{2}$As$_{2}$ as a function of temperature and
external magnetic field using neutron scattering and muon spin rotation. Below
the superconducting transition temperature the magnetic and superconducting
order parameters coexist and compete. A magnetic field can significantly
enhance the magnetic scattering in the superconducting state, roughly doubling
the Bragg intensity at 13.5 T. We perform a microscopic modelling of the data
by use of a five-band Hamiltonian relevant to iron pnictides. In the
superconducting state, vortices can slow down and freeze spin fluctuations
locally. When such regions couple they result in a long-range ordered
antiferromagnetic phase producing the enhanced magnetic elastic scattering in
agreement with experiments. | 1411.7506v1 |
2014-12-09 | An extreme gravitationally redshifted iron line at 4.8 keV in Mrk 876 | X-ray spectral lines at unforeseen energies are important because they can
shed light on the extreme physical conditions of the environment around the
supermassive black holes of active galactic nuclei (AGN). Mrk 876 displays such
a line at 4.8 keV rest-frame energy. A possible interpretation of its origin
can be found in the hotspot scenario. In this scenario the primary radiation
from a flare in the hot corona of an AGN illuminates a limited portion of the
accretion disk that emits by fluorescence. In this context the line can
represent an extreme gravitationally redshifted Fe line originating on the
accretion disk below 6 gravitational radii from a rotating supermassive black
hole. The correct estimate of the line significance requires a dedicated
approach. Based on an existing rigorous approach, we have performed extensive
Monte Carlo simulations. We determine that the line is a real feature at ~99
confidence level. | 1412.3112v1 |
2014-12-10 | M{ö}ssbauer characterization of an unusual high-spin side-on peroxo-Fe3+ species in the active site of superoxide reductase from Desulfoarculus Baarsii. Density functional calculations on related models | Superoxide reductase (SOR) is an Fe protein that catalyzes the reduction of
superoxide to give H(2)O(2). Recently, the mutation of the Glu47 residue into
alanine (E47A) in the active site of SOR from Desulfoarculus baarsii has
allowed the stabilization of an iron-peroxo species when quickly reacted with
H(2)O(2) [Math{\'e} et al. (2002) J. Am. Chem. Soc. 124, 4966-4967]. To further
investigate this non-heme peroxo-iron species, we have carried out a
M{\"o}ssbauer study of the (57)Fe-enriched E47A SOR from D. baarsii reacted
quickly with H(2)O(2). Considering the M{\"o}ssbauer data, we conclude, in
conjunction with the other spectroscopic data available and with the results of
density functional calculations on related models, that this species
corresponds to a high-spin side-on peroxo-Fe(3+) complex. This is one of the
first examples of such a species in a biological system for which M{\"o}ssbauer
parameters are now available: delta(/Fe) = 0.54 (1) mm/s, DeltaE(Q) = -0.80 (5)
mm/s, and the asymmetry parameter eta = 0.60 (5) mm/s. The M{\"o}ssbauer and
spin Hamiltonian parameters have been evaluated on a model from the side-on
peroxo complex (model 2) issued from the oxidized iron center in SOR from
Pyrococcus furiosus, for which structural data are available in the literature
[Yeh et al. (2000) Biochemistry 39, 2499-2508]. For comparison, similar
calculations have been carried out on a model derived from 2 (model 3), where
the [CH(3)-S](1)(-) group has been replaced by the neutral [NH(3)](0) group
[Neese and Solomon (1998) J. Am. Chem. Soc. 120, 12829-12848]. Both models 2
and 3 contain a formally high-spin Fe(3+) ion (i.e., with empty minority spin
orbitals). We found, however, a significant fraction (approximately 0.6 for 2,
approximately 0.8 for 3) of spin (equivalently charge) spread over two occupied
(minority spin) orbitals. The quadrupole splitting value for 2 is found to be
negative and matches quite well the experimental value. The computed quadrupole
tensors are rhombic in the case of 2 and axial in the case of 3. This
difference originates directly from the presence of the thiolate ligand in 2. A
correlation between experimental isomer shifts for Fe(3+) mononuclear complexes
with computed electron densities at the iron nucleus has been built and used to
evaluate the isomer shift values for 2 and 3 (0.56 and 0.63 mm/s,
respectively). A significant increase of isomer shift value is found upon going
from a methylthiolate to a nitrogen ligand for the Fe(3+) ion, consistent with
covalency effects due to the presence of the axial thiolate ligand. Considering
that the isomer shift value for 3 is likely to be in the 0.61-0.65 mm/s range
[Horner et al. (2002) Eur. J. Inorg. Chem., 3278-3283], the isomer shift value
for a high-spin eta(2)-O(2) Fe(3+) complex with an axial thiolate group can be
estimated to be in the 0.54-0.58 mm/s range. The occurrence of a side-on peroxo
intermediate in SOR is discussed in relation to the recent data published for a
side-on peroxo-Fe(3+) species in another biological system [Karlsson et al.
(2003) Science 299, 1039-1042]. | 1412.3293v1 |
2014-12-10 | Topological Characters in Fe(Te$_{1-x}$Se$_x$) thin films | We investigate topological properties in the Fe(Te,Se) thin films. We find
that the single layer FeTe$_{1-x}$Se$_x$ has nontrivial $Z_2$ topological
invariance which originates from the parity exchange at $\Gamma$ point of
Brillouin zone. The nontrivial topology is mainly controlled by the Te(Se)
height. Adjusting the height, which can be realized as function of $x$ in
FeTe$_{1-x}$Se$_x$, can drive a topological phase transition. In a bulk
material, the two dimensional $Z_2$ topology invariance is extended to a strong
three-dimensional one. In a thin film, we predict that the topological
invariance oscillates with the number of layers. The results can also be
applied to iron-pnictides. Our research establishes FeTe$_{1-x}$Se$_x$ as a
unique system to integrate high T$_c$ superconductivity and topological
properties in a single electronic structure. | 1412.3375v1 |
2014-12-11 | Transition-Metal Substitutions in Iron Chalcogenides | The $ab$-plane resistivity and Hall effect are studied in
Fe$_{1-y}$M$_y$Te$_{0.65}$Se$_{0.35}$ single crystals doped with two transition
metal elements, M = Co or Ni, over a wide doping range, $0 \leq y \leq 0.2$.
The superconducting transition temperature, $T_{c}$, reaches zero for Co at $y
\simeq 0.14$ and for Ni at $y \simeq 0.032$, while the resistivity at the
$T_{c}$ onset increases weakly with Co doping, and strongly with Ni doping. The
Hall coefficient $R_H$, positive for $y$ = 0, remains so at high temperatures
for all $y$, while it changes sign to negative at low $T$ for $y > 0.135$ (Co)
and $y > 0.06$ (Ni). The analysis based on a two band model suggests that at
high $T$ residual hole pockets survive the doping, but holes get localized upon
the lowering of $T$, so that the effect of the electron doping on the transport
becomes evident. The suppression of the $T_c$ by Co impurity is related to
electron doping, while in case of the Ni impurity strong electron localization
most likely contributes to fast decrease of the $T_c$. | 1412.3599v1 |
2014-12-15 | The hard X-ray spectrum of NGC 5506 as seen by NuSTAR | NuSTAR observed the bright Compton-thin, narrow line Seyfert 1 galaxy, NGC
5506, for about 56 ks. In agreement with past observations, the spectrum is
well fit by a power law with Gamma~1.9, a distant reflection component and
narrow ionized iron lines. A relativistically blurred reflection component is
not required by the data. When an exponential high energy cutoff is added to
the power law, a value of 720(+130,-190) keV (90% confidence level) is found.
Even allowing for systematic uncertainties, we find a 3 sigma lower limit to
the high-energy cutoff of 350 keV, the highest lower limit to the cutoff energy
found so far in an AGN by NuSTAR. | 1412.4541v1 |
2014-12-19 | [(Li0.8Fe0.2)OH]FeS and the ferromagnetic superconductors [(Li0.8Fe0.2)OH]Fe(S1-xSex) (0<x<1) | [(Li0.8Fe0.2)OH]FeS and the series [(Li0.8Fe0.2)OH]Fe(S1-xSex) (0<x<1) were
synthesized by hydrothermal methods and characterized by X-ray single crystal
and powder diffraction, EDX and chemical analysis. Selenium-rich compounds show
the coexistence of magnetic ordering with superconductivity known from the pure
selenium compound. Sulphur doping decreases the critical temperature through
chemical pressure until superconductivity is completely absent in
[(Li0.8Fe0.2)OH]FeS, while the ferromagnetism in the [(Li0.8Fe0.2)OH] layers
persists. The Li:Fe ratio in the hydroxide layer, and thus the charge transfer
of 0.2 electrons from the hydroxide to the iron chalcogenide layers remains
unchanged in [(Li0.8Fe0.2)OH]Fe(S1-xSex), which indicates that the chemical
pressure effect of the smaller sulphide ions impedes superconductivity in
[(Li0.8Fe0.2)OH]FeS | 1412.6475v1 |
2014-12-19 | Red Supergiant Stars as Cosmic Abundance Probes. III. NLTE effects in J-band Magnesium lines | Non-LTE calculations for Mg I in red supergiant stellar atmospheres are
presented to investigate the importance of non-LTE for the formation of Mg I
lines in the NIR J-band. Recent work using medium resolution spectroscopy of
atomic lines in the J-band of individual red supergiant stars has demonstrated
that technique is a very promising tool to investigate the chemical composition
of the young stellar population in star forming galaxies. As in previous work,
where non-LTE effects were studied for iron, titanium and silicon, substantial
effects are found resulting in significantly stronger Mg I absorption lines.
For the quantitative spectral analysis the non-LTE effects lead to magnesium
abundances significantly smaller than in LTE with the non-LTE abundance
corrections varying smoothly between -0.4 dex and -0.1 dex for effective
temperatures between 3400 K and 4400 K. We discuss the physical reasons of the
non-LTE effects and the consequences for extragalactic J-band abundance studies
using individual red supergiants in the young massive galactic double cluster h
and chi Persei. | 1412.6527v2 |
2014-12-22 | Superconductivity-induced reentrance of orthorhombic distortion in Ba1-xKxFe2As2 | Detailed knowledge of the phase diagram and the nature of the competing
magnetic and superconducting phases is imperative for an understanding of the
physics of iron-based superconductivity. Here, we show using thermodynamic
probes that the phase diagram of the first discovered, and highest Tc, 122-type
material, Ba1-xKxFe2As2 is in fact much richer than previously reported. Inside
the usual stripe-type magnetic order with C2-symmetry, we find a small pocket
of a tetragonal, C4-symmetric phase, which surprisingly reverts back to the
C2-phase at or slightly below the superconducting transition. This re-entrance
to a low-temperature orthorhombic state induced by superconductivity is
discussed in terms of competition of the two magnetic phases with
superconductivity and is illustrated by the measured changes in the electronic
entropy of the system. Using our thermodynamic data, we make predictions about
how the phase diagram of these competing orders will change under pressure. | 1412.7038v2 |
2014-12-23 | Tuning the magnetic anisotropy of single molecules | The magnetism of single atoms and molecules is governed by the atomic scale
environment. In general, the reduced symmetry of the surrounding splits the $d$
states and aligns the magnetic moment along certain favorable directions. Here,
we show that we can reversibly modify the magnetocrystalline anisotropy by
manipulating the environment of single iron(II) porphyrin molecules adsorbed on
Pb(111) with the tip of a scanning tunneling microscope. When we decrease the
tip--molecule distance, we first observe a small increase followed by an
exponential decrease of the axial anisotropy on the molecules. This is in
contrast to the monotonous increase observed earlier for the same molecule with
an additional axial Cl ligand. We ascribe the changes in the anisotropy of both
species to a deformation of the molecules in the presence of the attractive
force of the tip, which leads to a change in the $d$ level alignment. These
experiments demonstrate the feasibility of a precise tuning of the magnetic
anisotropy of an individual molecule by mechanical control. | 1412.7454v2 |
2014-12-30 | GPU-based acceleration of free energy calculations in solid state physics | Obtaining a thermodynamically accurate phase diagram through numerical
calculations is a computationally expensive problem that is crucially important
to understanding the complex phenomena of solid state physics, such as
superconductivity. In this work we show how this type of analysis can be
significantly accelerated through the use of modern GPUs. We illustrate this
with a concrete example of free energy calculation in multi-band iron-based
superconductors, known to exhibit a superconducting state with oscillating
order parameter. Our approach can also be used for classical BCS-type
superconductors. With a customized algorithm and compiler tuning we are able to
achieve a 19x speedup compared to the CPU (119x compared to a single CPU core),
reducing calculation time from minutes to mere seconds, enabling the analysis
of larger systems and the elimination of finite size effects. | 1412.8754v2 |
2014-12-31 | La$_2$O$_3$Fe$_2$Se$_2$, a Mott insulator on the brink of orbital-selective metalization | La$_2$O$_3$Fe$_2$Se$_2$ can be explained in terms of Mott localization in
sharp contrast with the metallic behavior of FeSe and other parent parent
compounds of iron superconductors. We demonstrate that the key ingredient that
makes La$_2$O$_3$Fe$_2$Se$_2$ a Mott insulator, rather than a correlated metal
dominated by the Hund's coupling is the enhanced crystal-field splitting,
accompanied by a smaller orbital-resolved kinetic energy. The strong deviation
from orbital degeneracy introduced by the crystal-field splitting also pushes
this materials close to an orbital-selective Mott transition. We predict that
either doping or uniaxial external pressure can drive the material into an
orbital-selective Mott state, where only one or few orbitals are metallized
while the others remain insulating. | 1501.00118v1 |
2015-01-02 | Mott-Kondo Insulator Behavior in the Iron Oxychalcogenides | We perform a combined experimental-theoretical study of the
Fe-oxychalcogenides (FeO$\emph{Ch}$) series
La$_{2}$O$_{2}$Fe$_{2}$O\emph{M}$_{2}$ (\emph{M}=S, Se), which is the latest
among the Fe-based materials with the potential \ to show unconventional
high-T$_{c}$ superconductivity (HTSC). A combination of incoherent Hubbard
features in X-ray absorption (XAS) and resonant inelastic X-ray scattering
(RIXS) spectra, as well as resitivity data, reveal that the parent
FeO$\emph{Ch}$ are correlation-driven insulators. To uncover microscopics
underlying these findings, we perform local density
approximation-plus-dynamical mean field theory (LDA+DMFT) calculations that
unravel a Mott-Kondo insulating state. Based upon good agreement between theory
and a range of data, we propose that FeO$\emph{Ch}$ may constitute a new, ideal
testing ground to explore HTSC arising from a strange metal proximate to a
novel selective-Mott quantum criticality. | 1501.00332v1 |
2015-01-02 | Magnetism in Quasi-One-Dimensional A$_2$Cr$_3$As$_3$ (A=K,Rb) superconductors | We predict that the recently discovered quasi-one dimensional
superconductors, A$_2$Cr$_3$As$_3$(A=K,Rb), possess strong frustrated magnetic
fluctuations and are nearby a novel in-out co-planar magnetic ground state. The
frustrated magnetism is very sensitive to c-axis lattice constant and can thus
be suppressed by increasing pressure. Our results qualitatively explain strong
non-Fermi liquid behaviors observed in the normal state of the superconductors
as the intertwining between the magnetism and superconductivity can create a
large quantum critical region in quasi-one dimensional systems and also suggest
that the materials share similar phase diagrams and superconducting mechanism
with other unconventional superconductors, such as cuprates and iron-based
superconductors. | 1501.00412v2 |
2015-01-03 | Single Phase Synthesis and Room temperature Neutron Diffraction Studies on Multiferroic PbFe0.5Nb0.5O3 | The Lead-iron-niobate, (PbFe0.5Nb0.5O3 or PFN) was synthesized by low
temperature sintering Single Step / Solid State Reaction Method. The 700 C / 2
hrs calcined powder was sintered at 1050 C / 1 hr. The sintered pellets were
characterized through X-Ray Diffraction and Neutron Diffraction at room
temperature. It is found from the XRD pattern that the materials is in single
phase with no traces of pyrochlore phase. It was also confirmed from the
neutron diffraction pattern, the structure of PFN to be monoclinic, belongs to
space group C m. Structural studies has been carried out by refining the
obtained neutron diffraction data by Reitveld refinement method using Full Prof
Program. The neutron diffraction pattern at 290K (near room temperature) was
selected to refine the structure. The Lattice parameters obtained are; a =
5.6709 A, b = 5.6732, c = 4.0136 and {\alpha} = 90, \b{eta} = 89.881, {\gamma}
= 90.000. The P-E measurements showed hysteretic behavior with high remnant
polarization. | 1501.00553v1 |
2015-01-05 | Nematicity and quantum paramagnetism in FeSe | In common with other iron-based high temperature superconductors, FeSe
exhibits a transition to a ``nematic'' phase below 90Kelvin in which the
crystal rotation symmetry is spontaneously broken. However, the absence of
strong low-frequency magnetic fluctuations near or above the transition has
been interpreted as implying the primacy of orbital ordering. In contrast, we
establish that quantum fluctuations of spin-1 local moments with strongly
frustrated exchange interactions can lead to a nematic quantum paramagnetic
phase consistent with the observations in FeSe. We show that this phase is a
fundamental expression of the existence of a Berry's phase associated with the
topological defects of a N\'eel antiferromagnet, in a manner analogous to that
which gives rise to valence bond crystal order for spin 1/2 systems. We present
an exactly solvable model realizing the nematic quantum paramagnetic phase,
discuss its relation with the spin-1 $J_1-J_2$ model, and construct a field
theory of the Landau-forbidden transition between the N\'eel state and this
nematic quantum paramagnet. | 1501.00844v2 |
2015-01-05 | A novel platform for two-dimensional chiral topological superconductivity | We show that the surface of an $s$-wave superconductor decorated with a
two-dimensional lattice of magnetic impurities can exhibit chiral topological
superconductivity. If impurities order ferromagnetically and the
superconducting surface supports a sufficiently strong Rashba-type spin-orbit
coupling, Shiba sub-gap states at impurity locations can hybridize into
Bogoliubov bands with non-vanishing, sometimes large, Chern number $C$. This
topological superconductor supports $C$ chiral Majorana edge modes. We
construct phase diagrams for model two-dimensional superconductors, accessing
the dilute and dense magnetic impurity limits analytically and the intermediate
regime numerically. To address potential experimental systems, we identify
stable configurations of ferromagnetic iron atoms on the Pb (111) surface and
conclude that ferromagnetic adatoms on Pb surfaces can provide a versatile
platform for two-dimensional topological superconductivity. | 1501.00999v1 |
2015-01-07 | Pulse radiolysis studies on superoxide reductase from Treponema pallidum | Superoxide reductases (SORs) are small metalloenzymes, which catalyze
reduction of O2*- to H2O2. The reaction of the enzyme from Treponema pallidum
with superoxide was studied by pulse radiolysis methods. The first step is an
extremely fast bi-molecular reaction of the ferrous center with O2, with a rate
constant of 6 x 10 (8) M(-1) s(-1). A first intermediate is formed which is
converted to a second one with a slower rate constant of 4800 s(-1). This
latter value is 10 times higher than the corresponding one previously reported
in the case of SOR from Desulfoarculus baarsii. The reconstituted spectra for
the two intermediates are consistent with formation of transient iron-peroxide
species. | 1501.01443v1 |
2015-01-07 | The flavin reductase ActVB from Streptomyces coelicolor: characterization of the electron transferase activity of the flavoprotein form | The flavin reductase ActVB is involved in the last step of actinorhodin
biosynthesis in Streptomyces coelicolor. Although ActVB can be isolated with
some FMN bound, this form was not involved in the flavin reductase activity. By
studying the ferric reductase activity of ActVB, we show that its FMN-bound
form exhibits a proper enzymatic activity of reduction of iron complexes by
NADH. This shows that ActVB active site exhibits a dual property with regard to
the FMN. It can use it as a substrate that goes in and off the active site or
as a cofactor to provide an electron transferase activity to the polypeptide. | 1501.01465v1 |
2015-01-07 | Identification of iron(III) peroxo species in the active site of the superoxide reductase SOR from Desulfoarculus baarsii | The active site of superoxide reductase SOR consists of an Fe2+ center in an
unusual [His4 Cys1] square-pyramidal geometry. It specifically reduces
superoxide to produce H2O2. Here, we have reacted the SOR from Desulfoarculus
baarsii directly with H2O2. We have found that its active site can transiently
stabilize an Fe3+-peroxo species that we have spectroscopically characterized
by resonance Raman. The mutation of the strictly conserved Glu47 into alanine
results in a stabilization of this Fe3+-peroxo species, when compared to the
wild-type form. These data support the hypothesis that the reaction of SOR
proceeds through such a Fe3+-peroxo intermediate. This also suggests that Glu47
might serve to help H2O2 release during the reaction with superoxide. | 1501.01470v1 |
2015-01-07 | Maximum screening fields of superconducting multilayer structures | It is shown that a multilayer comprised of alternating thin superconducting
and insulating layers on a thick substrate can fully screen the applied
magnetic field exceeding the superheating fields $H_s$ of both the
superconducting layers and the substrate, the maximum Meissner field is
achieved at an optimum multilayer thickness. For instance, a dirty layer of
thickness $\sim 0.1\; \mu$m at the Nb surface could increase $H_s\simeq 240$ mT
of a clean Nb up to $H_s\simeq 290$ mT. Optimized multilayers of Nb$_3$Sn, NbN,
some of the iron pnictides, or alloyed Nb deposited onto the surface of the Nb
resonator cavities could potentially double the rf breakdown field, pushing the
peak accelerating electric fields above 100 MV/m while protecting the cavity
from dendritic thermomagnetic avalanches caused by local penetration of
vortices. | 1501.01512v1 |
2015-01-08 | Sulfurization of Iron in the Dynamic Solar Nebula and Implications for Planetary Compositions | One explanation for the enhanced ratio of volatiles to hydrogen in Jupiter's
atmosphere compared to a a gas of solar composition is that the planet accreted
volatile-bearing clathrates during its formation. Models, however, suggest that
S would be over abundant if clathrates were the primary carrier of Jupiter's
volatiles. This led to the suggestion that S was depleted in the outer nebula
due to the formation troilite (FeS). Here, this depletion is quantitatively
explored by modeling the coupled dynamical and chemical evolution of Fe grains
in the solar nebula. It is found that disks that undergo rapid radial expansion
from an initially compact state may allow sufficient production of FeS and
carry H$_{2}$S-depleted gas outward where ices would form, providing the
conditions needed for S-depleted clathrates to form. However, this expansion
would also carry FeS grains to this region, which could also be incorporated
into planetesimals. Thus for clathrates to be a viable source of volatiles,
models must account for the presence of both H$_{2}$S in FeS in the outer solar
nebula. | 1501.01878v1 |
2015-01-09 | Study of the chemical composition of high energy cosmic rays using the muon LDF of EAS between $10^{17.25}$ eV and $10^{17.75}$ eV | We explore the feasibility of estimating primary cosmic ray composition at
high energies from the study of two parameters of Extensive Air Showers (EAS)
at ground and underground level with Monte Carlo simulations using the new EPOS
and QGSJETII hadronic models tuned with LHC data. Namely, the slope and density
at a given distance of the muon lateral distribution function are analysed in
this work. The power to discriminate primary masses is quantified in terms of
merit factor for each parameter. The analysis considers three different primary
particles (proton, iron and gamma), four different zenith angles (0$^{\circ}$,
15$^{\circ}$, 30$^{\circ}$ and 45$^{\circ}$) and primary energies of
$10^{17.25}$ eV, $10^{17.50}$ eV and $10^{17.75}$ eV. | 1501.02217v1 |
2015-01-11 | Hidden Order as a Source of Interface Superconductivity | Interfacial superconductivity is observed in a variety of heterostructures
composed of different materials including superconducting and
nonsuperconducting (at appropriate doping and temperatures) cuprates and
iron-based pnictides. The origin of this superconductivity remains in many
cases unclear. Here, we propose a general mechanism of interfacial
superconductivity for systems with competing order parameters. We assume that
parameters characterizing the material allow formation of another order like
charge- or spin-density wave competing and prevailing superconductivity in the
bulk (hidden superconductivity). Diffusive electron scattering on the interface
results in a suppression of this order and releasing the superconductivity. Our
theory is based on the use of Ginzburg--Landau equations applicable to a broad
class of systems. We demonstrate that the local superconductivity appears in
the vicinity of the interface and the spatial dependence of the superconducting
order parameter~$\Delta(x)$ is described by the Gross--Pitaevskii equation.
Solving this equation we obtain quantized values of temperature and doping
levels at which~$\Delta(x)$ appears. Remarkably, the local superconductivity
shows up even in the case when the rival order is only slightly suppressed and
may arise also on the surface of the sample (surface superconductivity). | 1501.02505v2 |
2015-01-16 | Slow magnetic fluctuations and superconductivity in fluorine-doped NdFeAsO | Among the widely studied superconducting iron-pnictide compounds belonging to
the Ln1111 family (with Ln a lanthanide), a systematic investigation of the
crossover region between the superconducting and the antiferromagnetic phase
for the Ln = Nd case has been missing. We fill this gap by focusing on the
intermediate doping regime of NdFeAsO(1-x)F(x) by means of dc-magnetometry and
muon-spin spectroscopy measurements. The long-range order we detect at low
fluorine doping is replaced by short-range magnetic interactions at x = 0.08,
where also superconductivity appears. In this case, longitudinal-field
muon-spin spectroscopy experiments show clear evidence of slow magnetic
fluctuations that disappear at low temperatures. This fluctuating component is
ascribed to the glassy-like character of the magnetically ordered phase of
NdFeAsO at intermediate fluorine doping. | 1501.04005v1 |
2015-01-19 | Raman scattering in superconducting NdO1-xFxBiS2 crystals | The recently discovered layered BiS2-based superconductors have attracted a
great deal of interest due to their structural similarity to cuprate and
iron-pnictide superconductors. We have performed Raman scattering measurements
on two superconducting crystals NdO0.5F0.5BiS2 (Tc = 4.5 K) and NdO0.7F0.3BiS2
(Tc = 4.8 K). The observed Raman phonon modes are assigned with the aid of
first-principles calculations. The asymmetrical phonon mode around 118 cm-1
reveals a small electron-phonon (e-ph) coupling constant 0.16, which is
insufficient to generate superconductivity at ~ 4.5 K. In the Raman spectra
there exists a clear temperature-dependent hump around 100 cm-1, which can be
well understood in term of inter-band vertical transitions around Fermi
surface. The transitions get boosted when the particular rectangular-like Fermi
surface meets band splitting caused by spin-orbit coupling. It enables a unique
and quantitative insight into the band splitting. | 1501.04453v2 |
2015-01-20 | Shape optimization of an electric motor subject to nonlinear magnetostatics | The goal of this paper is to improve the performance of an electric motor by
modifying the geometry of a specific part of the iron core of its rotor. To be
more precise, the objective is to smooth the rotation pattern of the rotor. A
shape optimization problem is formulated by introducing a tracking-type cost
functional to match a desired rotation pattern. The magnetic field generated by
permanent magnets is modeled by a nonlinear partial differential equation of
magnetostatics. The shape sensitivity analysis is rigorously performed for the
nonlinear problem by means of a new shape-Lagrangian formulation adapted to
nonlinear problems. | 1501.04752v1 |
2015-01-25 | Sensorless Battery Internal Temperature Estimation using a Kalman Filter with Impedance Measurement | This study presents a method of estimating battery cell core and surface
temperature using a thermal model coupled with electrical impedance
measurement, rather than using direct surface temperature measurements. This is
advantageous over previous methods of estimating temperature from impedance,
which only estimate the average internal temperature. The performance of the
method is demonstrated experimentally on a 2.3 Ah lithium-ion iron phosphate
cell fitted with surface and core thermocouples for validation. An extended
Kalman filter, consisting of a reduced order thermal model coupled with
current, voltage and impedance measurements, is shown to accurately predict
core and surface temperatures for a current excitation profile based on a
vehicle drive cycle. A dual extended Kalman filter (DEKF) based on the same
thermal model and impedance measurement input is capable of estimating the
convection coefficient at the cell surface when the latter is unknown. The
performance of the DEKF using impedance as the measurement input is comparable
to an equivalent dual Kalman filter using a conventional surface temperature
sensor as measurement input. | 1501.06160v1 |
2015-01-26 | Measurement of the chemical composition of the ultra-high-energy cosmic rays with the Pierre Auger Observatory | The Pierre Auger Observatory infers the chemical composition of
ultra-high-energy cosmic rays through two independent detection techniques. The
Fluorescence Detector (FD) measures the longitudinal profile of high energy air
showers and can determine the depth of the shower maximum $X_{max}$, which is
sensitive to the chemical composition of the primary cosmic rays. Additionally,
measurements by the Surface Detector (SD) provide independent experimental
observables based on the muonic shower component to analyze the chemical
composition. We present the results for the $X_{max}$ distributions and the
mass composition results measured by the FD and the SD for the energies $E \geq
10^{18}$\,eV. The data will be compared with the expectations for proton and
iron primaries according to different hadronic interaction models. | 1501.06325v1 |
2015-01-28 | The Bubble-like Interior of the Core-Collapse Supernova Remnant Cassiopeia A | The death of massive stars is believed to involve aspheric explosions
initiated by the collapse of an iron core. The specifics of how these
catastrophic explosions proceed remain uncertain due, in part, to limited
observational constraints on various processes that can introduce asymmetries
deep inside the star. Here we present near-infrared observations of the young
Milky Way supernova remnant Cassiopeia A, descendant of a type IIb
core-collapse explosion, and a three-dimensional map of its interior, unshocked
ejecta. The remnant's interior has a bubble-like morphology that smoothly
connects to and helps explain the multi-ringed structures seen in the remnant's
bright reverse shocked main shell of expanding debris. This internal structure
may have originated from turbulent mixing processes that encouraged the
development of outwardly expanding plumes of radioactive 56Ni-rich ejecta. If
this is true, substantial amounts of its decay product, 56Fe, may still reside
in these interior cavities. | 1501.07283v1 |
2015-02-02 | Cold condensation of dust in the ISM | The condensation of complex silicates with pyroxene and olivine composition
at conditions prevailing in molecular clouds has been experimentally studied.
For this purpose, molecular species comprising refractory elements were forced
to accrete on cold substrates representing the cold surfaces of surviving dust
grains in the interstellar medium. The efficient formation of amorphous and
homogeneous magnesium iron silicates at temperatures of about 12 K has been
monitored by IR spectroscopy. The gaseous precursors of such condensation
processes in the interstellar medium are formed by erosion of dust grains in
supernova shock waves. In the laboratory, we have evaporated glassy silicate
dust analogs and embedded the released species in neon ice matrices that have
been studied spectroscopically to identify the molecular precursors of the
condensing solid silicates. A sound coincidence between the 10 micron band of
the interstellar silicates and the 10 micron band of the low-temperature
siliceous condensates can be noted. | 1502.00388v1 |
2015-02-04 | Is inner core seismic anisotropy a marker of plastic flow of cubic iron? | This paper investigates whether observations of seismic anisotropy are
compatible with a cubic structure of the inner core Fe alloy. We assume that
anisotropy is the result of plastic deformation within a large scale flow
induced by preferred growth at the inner core equator. Based on elastic moduli
from the literature, bcc- or fcc-Fe produce seismic anisotropy well below
seismic observations ($\textless{}0.4\%$). A Monte-Carlo approach allows us to
generalize this result to any form of elastic anisotropy in a cubic system.
Within our model, inner core global anisotropy is not compatible with a cubic
structure of Fe alloy. Hence, if the inner core material is indeed cubic, large
scale coherent anisotropic structures, incompatible with plastic deformation
induced by large scale flow, must be present. | 1502.01270v1 |
2015-02-05 | Code intercomparison and benchmark for muon fluence and absorbed dose induced by an 18-GeV electron beam after massive iron shielding | In 1974, Nelson, Kase, and Svenson published an experimental investigation on
muon shielding using the SLAC high energy LINAC. They measured muon fluence and
absorbed dose induced by a 18 GeV electron beam hitting a copper/water beam
dump and attenuated in a thick steel shielding. In their paper, they compared
the results with the theoretical mode ls available at the time. In order to
compare their experimental results with present model calculations, we use the
modern transport Monte Carlo codes MARS15, FLUKA2011 and GEANT4 to model the
experimental setup and run simulations. The results will then be compared
between the codes, and with the SLAC data. | 1502.01681v1 |
2015-02-18 | UV spectra of iron-doped carbon clusters FeC_n n = 3-6 | Electronic transitions of jet-cooled FeC$_n$ clusters ($n = 3 - 6$) were
measured between 230 and 300 nm by a mass-resolved 1+1 resonant two-photon
ionization technique. Rotational profiles were simulated based on previous
calculations of ground state geometries and compared to experimental
observations. Reasonable agreement is found for the planar fan-like structure
of FeC$_3$. The FeC$_4$ data indicate a shorter distance between the Fe atom
and the bent C$_4$ unit of the fan. The transitions are suggested to be
$^{3}$A$_{2} \leftarrow ^{3}$B$_{1}$ for FeC$_3$ and $^{5}$A$_{1} \leftarrow
^{5}$A$_{1}$ for FeC$_4$. In contrast to the predicted C$_{\infty \text{v}}$
geometry, non-linear FeC$_5$ is apparently observed. Line width broadening
prevents analysis of the FeC$_6$ spectrum. | 1502.05310v1 |
2015-02-19 | Low 60Fe abundance in Semarkona and Sahara 99555 | Iron-60 (t1/2=2.62 Myr) is a short-lived nuclide that can help constrain the
astrophysical context of solar system formation and date early solar system
events. A high abundance of 60Fe (60Fe/56Fe= 4x10-7) was reported by in situ
techniques in some chondrules from the LL3.00 Semarkona meteorite, which was
taken as evidence that a supernova exploded in the vicinity of the birthplace
of the Sun. However, our previous MC-ICPMS measurements of a wide range of
meteoritic materials, including chondrules, showed that 60Fe was present in the
early solar system at a much lower level (60Fe/56Fe=10-8). The reason for the
discrepancy is unknown but only two Semarkona chondrules were measured by
MC-ICPMS and these had Fe/Ni ratios below ~2x chondritic. Here, we show that
the initial 60Fe/56Fe ratio in Semarkona chondrules with Fe/Ni ratios up to
~24x chondritic is 5.4x10-9. We also establish the initial 60Fe/56Fe ratio at
the time of crystallization of the Sahara 99555 angrite, a chronological
anchor, to be 1.97x10-9. These results demonstrate that the initial abundance
of 60Fe at solar system birth was low, corresponding to an initial 60Fe/56Fe
ratio of 1.01x10-8. | 1502.05611v1 |
2015-02-20 | Magnetic and Dielectric Properties of Multiferroic BiFeO3 Nanoparticles Synthesized by a Novel Citrate Combustion Method | Single phase BiFeO3 nanoparticles have been successfully synthesized for the
first time by a novel citrate combustion method without using any solvent. Well
mixed metal nitrates along with citric acid which is used as fuel combust to
give BiFeO3 nanoparticles after annealing. These particles are single phase in
nature and crystallize in the rhombohedral distorted perovskite structure
(space group-R3c) which has been confirmed by the Rietveld refinement of the
room temperature powder x-ray diffraction data. Nearly spherical particles of
average particle size 47 nm have been seen from transmission electron
micrograph. Room temperature magnetic hysteresis measurement shows weak
ferromagnetism though the magnetization does not saturate upto 1.75 T applied
field. The coercive field value is calculated to be 180 Oe which is 3 times
higher than that prepared by solvent free combustion method using Glycine. 57Fe
M\"ossbauer spectrum can be fitted with a sextet corresponding to single
magnetic state of hyperfine field about 49.5 T corresponding to Fe3+ state of
the iron atom. The dielectric relaxation and ac conductivity as a function of
frequency have been discussed. High dielectric permittivity has not been found
in these nanoparticles like other reported BiFeO3 ceramics. | 1502.05797v1 |
2015-02-21 | Spin liquid polymorphism in a correlated electron system on the threshold of superconductivity | We report neutron scattering measurements, which reveal spin-liquid
polymorphism in a '11' iron chalcogenide superconductor, a poorly-metallic
magnetic FeTe tuned towards superconductivity by substitution of a small amount
of Tellurium with iso-electronic Sulphur. We observe liquid-like magnetic
dynamics, which is described by a competition of two phases with different
local structure, whose relative abundance depends on temperature. One is the
ferromagnetic (FM) plaquette phase observed in the non-superconducting FeTe,
which preserves the C$_4$ symmetry of the underlying square lattice and is
favored at high temperatures. The other is the antiferromagnetic plaquette
phase with broken C$_4$ symmetry, which emerges with doping and is predominant
at low temperatures. These findings suggest a first-order liquid-liquid phase
transition in the electronic spin system of FeTe$_{1-x}$(S,Se)$_x$. We thus
discover remarkable new physics of competing spin liquid polymorphs in a
correlated electron system approaching superconductivity. Our results
facilitate an understanding of large swaths of recent experimental data in
unconventional superconductors. | 1502.06051v1 |
2015-02-25 | HD188112: Supernova Ia progenitor? | HD188112 is an extremely low mass white dwarf in a close binary system.
According to a previous study, the mass of HD188112 is $\sim$0.24 Msun and a
lower limit of 0.73 Msun could be put for the mass of its unseen companion, a
compact degenarate object. We used HST STIS spectra to measure the rotational
broadening of UV metallic lines in HD188112, in order to put tighter
constraints on the mass of its companion. By assuming that the system in is
synchronous rotation, we derive a companion mass between 1.05 and 1.25 Msun. We
also measure abundances for magnesium, silicon, and iron, respectively log
$N$(X)/$N$(H) = $-$6.40, $-$7.25, and $-$5.81. The radial velocities measured
from the UV spectra are found to be in very good agreement with the prediction
based on the orbital parameters derived in the previous study made a decade
ago. | 1502.07096v1 |
2015-02-25 | Electron capture cross sections for stellar nucleosynthesis | In the first stage of this work, we perform detailed calculations for the
cross sections of the electron capture on nuclei under laboratory conditions.
Towards this aim we exploit the advantages of a refined version of the
proton-neutron quasi-particle random-phase approximation (pn-QRPA) and carry
out state-by-state evaluations of the rates of exclusive processes that lead to
any of the accessible transitions within the chosen model space. In the second
stage of our present study, we translate the above mentioned $e^-$-capture
cross sections to the stellar environment ones by inserting the temperature
dependence through a Maxwell-Boltzmann distribution describing the stellar
electron gas. As a concrete nuclear target we use the $^{66}Zn$ isotope, which
belongs to the iron group nuclei and plays prominent role in stellar
nucleosynthesis at core collapse supernovae environment. | 1502.07225v1 |
2015-02-27 | Doping evolution of antiferromagnetism and transport properties in the non-superconducting BaFe2-2xNixCrxAs2 | We report elastic neutron scattering and transport measurements on the Ni and
Cr equivalently doped iron pnictide BaFe$_{2-2x}$Ni$_{x}$Cr$_{x}$As$_{2}$.
Compared with the electron-doped BaFe$_{2-x}$Ni$_{x}$As$_{2}$, the long-range
antiferromagnetic (AF) order in BaFe$_{2-2x}$Ni$_{x}$Cr$_{x}$As$_{2}$ is
gradually suppressed with vanishing ordered moment and N\'{e}el temperature
near $x= 0.20$ without the appearance of superconductivity. A detailed analysis
on the transport properties of BaFe$_{2-x}$Ni$_{x}$As and
BaFe$_{2-2x}$Ni$_{x}$Cr$_{x}$As$_{2}$ suggests that the non-Fermi-liquid
behavior associated with the linear resistivity as a function of temperature
may not correspond to the disappearance of the static AF order. From the
temperature dependence of the resistivity in overdoped compounds without static
AF order, we find that the transport properties are actually affected by Cr
impurity scattering, which may induce a metal-to-insulator crossover in highly
doped BaFe$_{1.7-y}$Ni$_{0.3}$Cr$_{y}$As$_{2}$. | 1502.07950v1 |
2015-02-27 | Is HE 0436-4717 Anemic? A deep look at a bare Seyfert 1 galaxy | A multi-epoch, multi-instrument analysis of the Seyfert 1 galaxy HE 0436-4717
is conducted using optical to X-ray data from XMM-Newton and Swift (including
the BAT). Fitting of the UV-to-X-ray spectral energy distribution shows little
evidence of extinction and the X-ray spectral analysis does not confirm
previous reports of deep absorption edges from OVIII. HE 0436-4717 is a "bare"
Seyfert with negligible line-of-sight absorption making it ideal to study the
central X-ray emitting region. Three scenarios were considered to describe the
X-ray data: partial covering absorption, blurred reflection, and soft
Comptonization. All three interpretations describe the 0.5-10.0 keV spectra
well. Extrapolating the models to 100 keV results in poorer fits for the the
partial covering model. When also considering the rapid variability during one
of the XMM-Newton observations, the blurred reflection model appears to
describe all the observations in the most self-consistent manner. If adopted,
the blurred reflection model requires a very low iron abundance in HE
0436-4717. We consider the possibilities that this is an artifact of the
fitting process, but it appears possible that it is intrinsic to the object. | 1502.07958v1 |
2015-03-03 | Magnetic Fields of Uranus and Neptune: Metallic Fluid Hydrogen | The magnetic fields of the Ice Giant Planets Uranus and Neptune (U/N) are
unique in the solar system. Based on a substantial database measured on Earth
for representative planetary fluids at representative dynamic pressures up to
200 GPa (2 Mbar) and a few 1000 K, the complex magnetic fields of U/N are (i)
probably made primarily by degenerate metallic fluid H (MFH) at or near the
crossover from the H-He envelopes to Ice cores at ~100 GPa (Mbar) pressures and
normalized radii of ~90% of the radii of U/N; (ii) because those magnetic
fields are made relatively close to the surfaces of U/N, non-dipolar fields can
be expected; (iii) the Ice cores are most probably a heterogeneous fluid
mixture of H, N, O, C, Fe/Ni and silicate-oxides and their mutual reaction
products at high pressures and temperatures,as discussed elsewhere. Ironically,
there is probably little nebular Ice in the Ice Giant Planets. | 1503.01042v2 |
2015-03-06 | NMR evidence for inhomogeneous glassy behavior driven by nematic fluctuations in iron arsenide superconductors | We present $^{75}$As nuclear magnetic resonance spin-lattice and spin-spin
relaxation rate data in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and
Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$ as a function of temperature, doping and
magnetic field. The relaxation curves exhibit a broad distribution of
relaxation rates, consistent with inhomogeneous glassy behavior up to 100 K.
The doping and temperature response of the width of the dynamical heterogeneity
is similar to that of the nematic susceptibility measured by elastoresistance
measurements. We argue that quenched random fields which couple to the nematic
order give rise to a nematic glass that is reflected in the spin dynamics. | 1503.01844v2 |
2015-03-09 | Buffer Aided Relaying Improves Both Throughput and End-to-End Delay | Buffer aided relaying has recently attracted a lot of attention due to the
improvement in the system throughput. However, a side effect usually deemed is
that buffering at relay nodes results in the increase in packet delays. In this
paper, we study the effect of buffering relays on the end-to-end delay of
users' data, from the time they arrive at source until delivery to the
destination. We use simple discussions to provide an insight on the overall
waiting time of the packets in the system. By studying the Bernoulli
distributed channel conditions, and using intuitive generalizations, we
conclude that the use of buffers at relays improves not only throughput, but
ironically the end-to-end delay as well. Computer simulations in the settings
of practical systems confirm the above results. | 1503.02348v1 |
2015-03-09 | Itinerancy enhanced quantum fluctuation of magnetic moments in iron-based superconductors | We investigate the influence of itinerant carriers on dynamics and
fluctuation of local moments in Fe-based superconductors, via linear spin-wave
analysis of a spin-fermion model containing both itinerant and local degrees of
freedom. Surprisingly against the common lore, instead of enhancing the
($\pi$,0) order, itinerant carriers with well nested Fermi surfaces is found to
induce significant amount of \textit{spatial} and temporal quantum fluctuation
that leads to the observed small ordered moment. Interestingly, the underlying
mechanism is shown to be intra-pocket nesting-associated long-range coupling,
rather than the previously believed ferromagnetic double-exchange effect. This
challenges the validity of ferromagnetically compensated first-neighbor
coupling reported from short-range fitting to the experimental dispersion,
which turns out to result instead from the ferro-orbital order that is also
found instrumental in stabilizing the magnetic order. | 1503.02653v1 |
2015-03-12 | Anomalous phonon redshift in K-doped BaFe2As2 iron pnictides | The effect of K, Co and P dopings on the lattice dynamics in the
BaFe$_2$As$_2$ system is studied by infrared spectroscopy. We focus on the
phonon at $\sim$ 253 cm$^{-1}$, the highest energy in-plane infrared-active
Fe-As mode in BaFe$_2$As$_2$. Our studies show that the Co and P dopings lead
to a blue shift of this phonon in frequency, which can be simply interpreted by
the change of lattice parameters induced by doping. In sharp contrast, an
unusual red shift of the same mode was observed in the K-doped compound, at
odds with the above explanation. This anomalous behavior in K-doped
BaFe$_2$As$_2$ is more likely associated with the coupling between lattice
vibrations and other channels, such as charge or spin. This coupling scenario
is also supported by the asymmetric line shape and intensity growth of the
phonon in the K-doped compound. | 1503.03838v1 |
2015-03-13 | MMonCa: An Object Kinetic Monte Carlo simulator for damage irradiation evolution and defect diffusion | In this work we introduce the Object Kinetic Monte Carlo (OKMC) simulator
MMonCa and simulate the defect evolution in three different materials. We start
by explaining the theory of OKMC and showing some details of how such theory is
implemented by creating generic structures and algorithms in the objects that
we want to simulate. Then we successfully reproduce simulated results for
defect evolution in iron, silicon and tungsten using our simulator and compare
with available experimental data and similar simulations. The comparisons
validate MMonCa showing that it is powerful and flexible enough to be
customized and used to study the damage evolution of defects in a wide range of
solid materials. | 1503.03987v1 |
2015-03-13 | Microscopic Parameters from High-Resolution Specific Heat Measurements on Overdoped BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ Single Crystals | We investigate the electronic specific heat of overdoped
BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ single crystals in the superconducting
state using high-resolution nanocalorimetry. From the measurements, we extract
the doping dependence of the condensation energy, superconducting gap $\Delta$,
and related microscopic parameters. We find that the anomalous scaling of the
specific heat jump $\Delta C \propto T_{\mathrm{c}}^3$, found in many
iron-based superconductors, in this system originates from a
$T_\mathrm{c}$-dependent ratio $\Delta/k_\mathrm{B}T_\mathrm{c}$ in combination
with a doping-dependent density of states $N(\varepsilon_\mathrm{F})$. A clear
enhancement is seen in the effective mass $m^{*}$ as the composition approaches
the value that has been associated with a quantum critical point at optimum
doping. However, a simultaneous increase in the superconducting carrier
concentration $n_\mathrm{s}$ maintains the superfluid density, yielding an
apparent penetration depth $\lambda$ that decreases with increasing
$T_\mathrm{c}$ without sharp divergence at the quantum critical point. Uemura
scaling indicates that $T_\mathrm{c}$ is governed by the Fermi temperature
$T_\mathrm{F}$ for this multi-band system. | 1503.04088v1 |
2015-03-18 | Super-Hydrophobic Multi-Walled Carbon Nanotube Coatings for Stainless Steel | We have taken advantage of the native surface roughness and the iron content
of AISI 316 stainless steel to direct grow multi-walled carbon nanotube (MWCNT)
random networks by chemical vapor deposition (CVD) at low-temperature ($<
1000^{\circ}$C), without the addition of any external catalysts or
time-consuming pre-treatments. In this way, super-hydrophobic MWCNT films on
stainless steel sheets were obtained, exhibiting high contact angle values
($154^{\circ}$) and high adhesion force (high contact angle hysteresis).
Furthermore, the investigation of MWCNT films at scanning electron microscopy
(SEM) reveals a two-fold hierarchical morphology of the MWCNT random networks
made of hydrophilic carbonaceous nanostructures on the tip of hydrophobic
MWCNTs. Owing to the Salvinia effect, the hydrophobic and hydrophilic composite
surface of the MWCNT films supplies a stationary super-hydrophobic coating for
conductive stainless steel. This biomimetical inspired surface not only may
prevent corrosion and fouling but also could provide low-friction and
drag-reduction. | 1503.05482v1 |
2015-03-20 | Collapse of the magnetic moment under pressure of AFe$_2$ (A = Y, Zr, Lu and Hf) in the cubic Laves phase | The electronic structures of four Laves phase iron compounds (e.g. YFe$_2$,
ZrFe$_2$, LuFe$_2$ and HfFe$_2$) have been calculated by the state-of-the-art
full potential electronic structure code. The magnetic moments collapse under
hydrostatic pressure. This feature is found to be universal in these materials.
Its electronic origin is provided by the sharp peaks in the density of states
near the Fermi level. It is shown that a first order quantum phase transition
can be expected under pressure in Y(Zr, or Lu)Fe$_2$, while a second order one
in HfFe$_2$. The bonding characteristics are discussed to elucidate the
equilibrium lattice constant variation. The large spontaneous volume
magnetostriction gives one of the most important character of these compounds.
Invar anomalies in these compounds can be partly explained by the current work
when the fast continuous magnetic moment decrease at the decrease of the
lattice constant was properly considered. This work may remind the
experimentalists of these "old" compounds and exploration of the quantum
properties under high pressures are greatly encouraged. | 1503.06017v1 |
2015-03-25 | Fractional Flux Plateau in Magnetization Curve of Multicomponent Superconductor Loop | Time-reversal symmetry (TRS) may be broken in superconductors with three or
more condensates interacting repulsively, yielding two degenerate states
specified by chirality of gap functions. We consider a loop of such
superconductor with two halves occupied by the two states with opposite
chiralities. Fractional flux plateaus are found in magnetization curve
associated with free-energy minima, where the two domain walls between the two
halves accommodate different inter-component phase kinks leading to finite
winding numbers in a part of the whole condensates around the loop. Fractional
flux plateaus form pairs with their heights related to the flux quantum {\Phi}0
= hc/2e. This phenomenon is a clear evidence of time-reversal symmetry broken
(TRSB) superconductivity, which in a general point of view provides a novel
chance to explore relative phase difference, phase kink and soliton in
ubiquitous multi-component superconductivity such as that in iron pnicitides. | 1503.07267v3 |