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2013-11-20 | X-ray time delays in the Narrow Line Seyfert 1 galaxy PG 1244+026 | We analyse the X-ray time-lags in the Narrow Line Seyfert 1 (NLS1) galaxy PG
1244+026 ($M_{\rm BH}\sim 10^7 M_{\rm sun}$, $L/L_{\rm Edd}\sim 1$). The time
delay between the soft (0.3--0.7~keV) and harder (1.2--4.0~keV) variations
shows shows the well established switch from hard lags at low frequencies to
soft lags at high frequencies. The low frequency hard lags are qualitatively
consistent with the propagation of fluctuations model, with some long-timescale
response of the reflection component. The high frequency soft lag appears to
extend over a wide frequency band, that we divide this into two narrow
frequency ranges, and examine the lag as a function of energy for each of
these. At high frequencies the soft excess emission is delayed with respect to
the harder energy bands, without any corresponding strong, hard X-ray
reflection signature. At even higher frequencies a soft lag is seen at the
softest energies, as well as tentative evidence for an iron K$\alpha$
reverberation signal. These results point to the importance of reprocessing as
well as reflection in determining the lags in NLS1s. | 1311.5165v2 |
2013-11-21 | L4Fe2As2Te1-xO4-yFy (L = Pr, Sm, Gd): a layered oxypnictide superconductor with Tc up to 45 K | The synthesis, structural and physical properties of iron lanthanide
oxypnictide superconductors, L4Fe2As2Te1-xO4 (L = Pr, Sm, Gd), with transition
temperature at ~ 25 K are reported. Single crystals have been grown at high
pressure using cubic anvil technique. The crystal structure consists of layers
of L2O2 tetrahedra separated by alternating layers of chains of Te and of
Fe2As2 tetrahedra: -L2O2-Te-L2O2-Fe2As2-L2O2-Te-L2O2- (space group: I4/mmm, a ~
4.0, c ~ 29.6 {\AA}). Substitution of oxygen by fluorine increases the critical
temperature, e.g. in Gd4Fe2As2Te1-xOyF4-y up to 45 K. Magnetic torque
measurements reveal an anisotropy of the penetration depths of ~31. | 1311.5437v2 |
2013-11-22 | Magnetic phase transitions and superconductivity in strained FeTe | The influence of hydrostatic pressure and ab-plane strain on the magnetic
structure of FeTe is investigated from first principles. The results of
calculations reveal a phase transition from antiferromagnetic double-stripe
ordering at ambient pressure to ferromagnetic ordering at 2 GPa, or under
compressive strain reducing the lattice parameter a by about 3%. In turn, a
tensile strain of less than 2% induces the phase transition to
antiferromagnetic single-stripe ordering. It corresponds to the superconducting
FeTe thin films, thereby confirming that the superconducting state is
positively linked to single-stripe antiferromagnetic fluctuations. Both types
of transition indicate that the position of Te atoms in the crystal is crucial
for the magnetic and superconducting properties of iron chalcogenides. | 1311.5732v1 |
2013-11-22 | Coherent emission of $γ$ quanta by synchrotron radiation excited nuclei: geometry of nearly backward scattering | A possibility of further development of Synchrotron M\"{o}ssbauer Source
(SMS) of $^{57}$Fe 14.4 keV radiation is considered. The principles and
detailed description of the SMS device is given in Refs. Phys. Rev. A 84,
053851 (2011) G. V. Smirnov et al, J. Synchrotron Rad., v. 19, 559 (2012) V.
Potapkin et al. The perfect crystal of Iron Borate, FeBO$_{3}$, is the central
element of this device. The coherent nuclear fluorescence of IB crystal
illuminated by synchrotron radiation produces the sharply directed beam of 14.4
keV M\"{o}ssbauer radiation from the crystal set at the pure nuclear Bragg
reflection. Up to now the low angle scattering geometry was used for generation
of the coherent $\gamma $ radiation. The analysis performed in the present
paper shows that the source of about two times larger power can be obtained
when nearly backward scattering geometry is employed. This result can be
efficiently applied in development of high resolution spectroscopy using
synchrotron radiation. | 1311.5771v1 |
2013-11-24 | Temperature and high strain rate dependence of tensile deformation behavior in single crystal iron from dislocation dynamics simulations | We conduct dislocation dynamics (DD) simulations of Fe periodic single
crystals under tensile load at several high strain rates and temperatures. The
simulations are enabled by the recent development of temperature-dependent
dislocation mobility relations obtained from atomistic calculations. The
plastic evolution in the simulations is governed by rapid initial dislocation
multiplication, followed by a saturation of the flow stress when the
subpopulation of slow plastic carriers becomes stabilized by dislocation
annihilation. Above 500 K, edge dislocations coexist with screw dislocations
and contribute proportionaly to the value of the flow stress. The DD
simulations are used to interpret shock-loading experiments in Fe in terms of
the relative importance of different strengthening mechanisms. We find that in
the $10^4$-to-$10^6$ s$^{-1}$ strain rate regime, work hardening explains the
hardening of shock-loaded bulk Fe crystals. | 1311.6173v1 |
2013-11-26 | Field Measurement for Superconducting Magnets of ADS Injector I | The superconducting solenoid magnet prototype for ADS injection-I had been
fabricated in Beijing Qihuan Mechanical and Electric Engineer Company and
tested in Haerbin Institute of Technology (HIT) in Nov, 2012. Batch magnet
production was processed after some major revision from the magnet prototype,
they include: removing off the perm-alloy shield, extending the iron yoke,
using thin superconducting cable, etc. The first one of the batch magnets was
tested in the vertical Dewar in HIT in Sept. 2013. Field measurement was
carried out at the same time by the measurement platform that seated on the top
of the vertical Dewar. This paper will present the field measurement system
design, measurement results and discussion on the residual field from the
persistent current effect. | 1311.6596v1 |
2013-11-28 | From supernovae to neutron stars | The gravitational collapse, bounce, the explosion of an iron core of an 11.2
$M_{\odot}$ star is simulated by two-dimensional neutrino-radiation
hydrodynamic code. The explosion is driven by the neutrino heating aided by
multi-dimensional hydrodynamic effects such as the convection. Following the
explosion phase, we continue the simulation focusing on the thermal evolution
of the protoneutron star up to $\sim$70 s when the crust of the neutron star is
formed using one-dimensional simulation. We find that the crust forms at
high-density region ($\rho\sim10^{14}$ g cm$^{-3}$) and it would proceed from
inside to outside. This is the first self-consistent simulation that
successfully follows from the collapse phase to the protoneutron star cooling
phase based on the multi-dimensional hydrodynamic simulation. | 1311.7249v2 |
2013-12-01 | Chemical Bonding in Aqueous Ferrocyanide: Experimental and Theoretical X-ray Spectroscopic Study | Resonant inelastic X-ray scattering (RIXS) and X-ray absorption (XA)
experiments at the iron L- and nitrogen K-edge are combined with high-level
first principles restricted active space self-consistent field (RASSCF)
calculations for a systematic investigation of the nature of the chemical bond
in potassium ferrocyanide in aqueous solution. The atom- and site-specific RIXS
excitations allow for direct observation of ligand-to-metal (Fe L-edge) and
metal-to-ligand (N K-edge) charge transfer bands and thereby evidence for
strong {\sigma}-donation and {\pi}-back-donation. The effects are identified by
comparing experimental and simulated spectra related to both the unoccupied and
occupied molecular orbitals in solution. | 1312.0236v1 |
2013-12-04 | Ray-tracing in pseudo-complex General Relativity | Motivated by possible observations of the black hole candidate in the center
of our galaxy and the galaxy M87, ray-tracing methods are applied to both
standard General Relativity (GR) and a recently proposed extension, the
pseudo-complex General Relativity (pc-GR). The correction terms due to the
investigated pc-GR model lead to slower orbital motions close to massive
objects. Also the concept of an innermost stable circular orbit (ISCO) is
modified for the pc-GR model, allowing particles to get closer to the central
object for most values of the spin parameter $a$ than in GR. Thus, the
accretion disk, surrounding a massive object, is brighter in pc-GR than in GR.
Iron K$\alpha$ emission line profiles are also calculated as those are good
observables for regions of strong gravity. Differences between the two theories
are pointed out. | 1312.1170v2 |
2013-12-09 | Point contact spectroscopy in Fe-based superconductors: recent advancements and future challenges | Iron-based superconductors (FeSC) present an unprecedented variety of
features both in the superconducting and in the normal state. Different
families differ in the value of the critical temperature, in the shape of the
Fermi surface, in the existence or absence of quasi-nesting conditions, in the
range of doping in which the antiferromagnetic (AFM) and the superconducting
phase coexist and in the structure of the order parameter in the reciprocal
space, and so on. In this paper the most important results of point-contact
spectroscopy (PCS) in Fe-based superconductors are reviewed, and the most
recent advances are described with the aim to discuss the future perspectives
and challenges of this spectroscopic technique in the characterization of the
superconducting properties of these complex compounds. One of the main
challenges, faced so far only by a few researchers in the PCS field, is to
fully explore the phase diagram of these materials, as a function of doping or
pressure, to understand the interplay between superconductivity and magnetism,
the effect of intrinsic or extrinsic inhomogeneities, the role of spin
fluctuations (SFs) in the pairing, the symmetry and the structure of the order
parameter(s). | 1312.2527v1 |
2013-12-10 | On the origin of variable structures in the winds of hot luminous stars | Examination of the temporal variability properties of several strong optical
recombination lines in a large sample of Galactic Wolf-Rayet (WR) stars reveals
possible trends, especially in the more homogeneous WC than the diverse WN
subtypes, of increasing wind variability with cooler subtypes. This could imply
that a serious contender for the driver of the variations is stochastic,
magnetic subsurface convection associated with the 170 kK partial-ionization
zone of iron, which should occupy a deeper and larger zone of greater mass in
cooler WR subtypes. This empirical evidence suggests that the heretofore
proposed ubiquitous driver of wind variability, radiative instabilities, may
not be the only mechanism playing a role in the stochastic multiple
small-scaled structures seen in the winds of hot luminous stars. In addition to
small-scale stochastic behaviour, subsurface convection guided by a global
magnetic field with localized emerging loops may also be at the origin of the
large-scale corotating interaction regions as seen frequently in O stars and
occasionally in the winds of their descendant WR stars. | 1312.2864v1 |
2013-12-10 | Spectrum splitting of bimagnon excitations in a spatially frustrated Heisenberg antiferromagnet revealed by resonant inelastic x-ray scattering | We perform a comprehensive analysis of the bimagnon resonant inelastic x-ray
scattering (RIXS) intensity spectra of the spatially frustrated Jx-Jy-J2
Heisenberg model on a square lattice in both the antiferromagnetic and the
collinear antiferromagnetic phase. We study the model for strong frustration
and significant spatial anisotropy to highlight the key signatures of RIXS
spectrum splitting which may be experimentally discernible. Based on an
interacting spin wave theory study within the ladder approximation
Bethe-Salpeter scheme, we find the appearance of a robust two-peak structure
over a wide range of the transferred momenta in both magnetically ordered
phases. The unfrustrated model has a single-peak structure with a two-peak
splitting originating due to spatial anisotropy and frustrated interactions.
Our predicted two-peak structure from both magnetically ordered regime can be
realized in iron pnictides. | 1312.2876v2 |
2013-12-12 | Impact of electronic correlations on the equation of state and transport in $ε$-Fe | We have obtained the equilibrium volumes, bulk moduli, equations of state of
the ferromagnetic cubic $\alpha$ and paramagnetic hexagonal $\epsilon$ phases
of iron in close agreement with experiment using an ab initio dynamical
mean-field theory approach. The local dynamical correlations are shown to be
crucial for a successful description of the ground-state properties of
paramagnetic $\epsilon$-Fe. Moreover, they enhance the effective mass of the
quasiparticles and reduce their lifetimes across the $\alpha \to \epsilon$
transition leading to a step-wise increase of the resistivity, as observed in
experiment. The calculated magnitude of the jump is significantly
underestimated, which points to non-local correlations. The implications of our
results for the superconductivity and non-Fermi-liquid behavior of
$\epsilon$-Fe are discussed. | 1312.3654v1 |
2013-12-13 | Phosphorus in the Young Supernova Remnant Cassiopeia A | Phosphorus ($^{31}$P), which is essential for life, is thought to be
synthesized in massive stars and dispersed into interstellar space when these
stars explode as supernovae (SNe). Here we report on near-infrared
spectroscopic observations of the young SN remnant Cassiopeia A, which show
that the abundance ratio of phosphorus to the major nucleosynthetic product
iron ($^{56}$Fe) in SN material is up to 100 times the average ratio of the
Milky Way, confirming that phosphorus is produced in SNe. The observed range is
compatible with predictions from SN nucleosynthetic models but not with the
scenario in which the chemical elements in the inner SN layers are completely
mixed by hydrodynamic instabilities during the explosion. | 1312.3807v1 |
2013-12-16 | Optical conductivity of nodal metals | Fermi liquid theory is remarkably successful in describing the transport and
optical properties of metals; at frequencies higher than the scattering rate,
the optical conductivity adopts the well-known power law behavior
$\sigma_1(\omega) \propto \omega^{-2}$. We have observed an unusual non-Fermi
liquid response $\sigma_1(\omega) \propto \omega^{-1\pm 0.2}$ in the ground
states of several cuprate and iron-based materials which undergo electronic or
magnetic phase transitions resulting in dramatically reduced or nodal Fermi
surfaces. The identification of an inverse (or fractional) power-law behavior
in the residual optical conductivity now permits the removal of this
contribution, revealing the direct transitions across the gap and allowing the
nature of the electron-boson coupling to be probed. The non-Fermi liquid
behavior in these systems may be the result of a common Fermi surface topology
of Dirac cone-like features in the electronic dispersion. | 1312.4466v1 |
2013-12-20 | Ultrafast modulation of the chemical potential in BaFe$_2$As$_2$ by coherent phonons | Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is
used to study the electronic structure dynamics in BaFe$_2$As$_2$ around the
high-symmetry points $\Gamma$ and $M$. A global oscillation of the Fermi level
at the frequency of the $A_{1g}$(As) phonon mode is observed. It is argued that
this behavior reflects a modulation of the effective chemical potential in the
photoexcited surface region that arises from the high sensitivity of the band
structure near the Fermi level to the $A_{1g}$ phonon mode combined with a low
electron diffusivity perpendicular to the layers. The results establish a novel
way to tune the electronic properties of iron pnictides: coherent control of
the effective chemical potential. The results further suggest that the
equilibration time for the effective chemical potential needs to be considered
in the ultrafast electronic structure dynamics of materials with weak
interlayer coupling. | 1312.6025v1 |
2013-12-23 | Heat capacity jump at T_c and pressure derivatives of superconducting transition temperature in the Ba(1-x)NaxFe2As2 (0.1 <= x <= 0.9) series | We present the evolution of the initial (up to ~ 10 kbar) hydrostatic,
pressure dependencies of T_c and of the ambient pressure jump in the heat
capacity associated with the superconducting transition as a function of Na -
doping in the Ba(1-x)NaxFe2As2 family of iron-based superconductors. For Na
concentrations 0.15 <= x <= 0.9, the jump in specific heat at T_c, Delta C_p,
follows the Delta C_p ~ T^3 scaling found for most BaFe2As2 - based
superconductors. Pressure dependencies are non-monotonic for x = 0.2 and x =
0.24. For other Na concentrations T_c decreases under pressure in almost linear
fashion. The anomalous behavior of the x = 0.2 and x = 0.24 samples under
pressure are possibly due to the crossing of the phase boundaries of the narrow
antiferromagnetic tetragonal phase, unique for the Ba(1-x)NaxFe2As2 series,
with the application of pressure. | 1312.6705v1 |
2014-01-07 | The polluted atmospheres of cool white dwarfs and the magnetic field connection | We present an analysis of X-Shooter spectra of the polluted hydrogen-rich
white dwarfs (DAZ) NLTT 888 and NLTT 53908. The spectra of NLTT 53908 show
strong, Zeeman-split calcium lines (CaII H&K and Ca I $\lambda 4226$) and the
star appears to be a close relative of the polluted magnetic white dwarf (DAZH)
NLTT 10480, while the spectra of NLTT 888 show narrow lines of calcium and
iron. A comparison of the DAZ NLTT 888 and the DAZH NLTT 53908 with other class
members illustrates the diversity of environment and formation circumstances
surrounding these objects. In particular, we find that the incidence of
magnetism in old, polluted white dwarfs significantly exceeds that found in the
general white dwarf population which suggests an hypothetical link between a
crowded planetary system and magnetic field generation. | 1401.1249v1 |
2014-01-10 | The Iron K Line Profile of IRAS 18325-5926 | IRAS 18325-5926 is an X-ray bright, Compton-thin, type-2 Seyfert galaxy and
it was the first Seyfert 2 in which the presence of a broad Fe K-alpha emission
line was claimed. However, although the structure of the Fe line appears broad,
there is tentative evidence that it may comprise multiple lines. Nevertheless,
previous analyses have only consisted of fitting standalone broad components to
the Fe K band. Here, we have analyzed all available X-ray CCD data from Suzaku,
XMM-Newton and ASCA to fully investigate the nature of the emission complex by
testing broad-band physical models and alternative hypotheses. We find that
both a model consisting of broad, blurred reflection from an ionized accretion
disc and a model consisting of cold, neutral reflection plus narrow emission
lines from highly-ionized photoionized gas (log \xi = 3.5) offer statistically
comparable fits to the data although the true reality of the Fe line cannot
currently be determined with existing data. However, it is hoped that better
quality data and improved photon statistics in the Fe K band will allow a more
robust distinction between models to be made. | 1401.2420v1 |
2014-01-14 | Ipopv2: Photoionization of Ni XIV -- a test case | Several years ago, M. Asplund and coauthors (2004) proposed a revision of the
Solar composition. The use of this new prescription for Solar abundances in
standard stellar models generated a strong disagreement between the predictions
and the observations of Solar observables. Many claimed that the Standard Solar
Model (SSM) was faulty, and more specifically the opacities used in such
models. As a result, activities around the stellar opacities were boosted. New
experiments (J. Bailey at Sandia on Z-Pinch, The OPAC consortium at LULI200) to
measure directly absorbtion coefficients have been realized or are underway.
Several theoretical groups (CEA-OPAS, Los Alamos Nat. Lab., CEA-SCORCG, The
Opacity Project - The Iron Project (IPOPv2)) have started new sets of
calculations using different approaches and codes. While the new results seem
to confirm the good quality of the opacities used in SSM, it remains important
to improve and complement the data currently available. We present recent
results in the case of the photoionization cross sections for Ni XIV (Ni13+ )
from IPOPv2 and possible implications on stellar modelling. | 1401.3122v1 |
2014-01-16 | Terzan 5: a pristine fragment of the Galactic Bulge? | Terzan 5 is a stellar system located in the inner Bulge of the Galaxy and has
been historically catalogued as a globular cluster. However, recent photometric
(Ferraro et al. 2009) and spectroscopic (Origlia et al. 2011; Origlia et al.
2013) investigations have shown that it hosts at least three stellar
populations with different iron abundances (with a total spread of
Delta[Fe/H]>1 dex) thus demonstrating that Terzan 5 is not a genuine globular
cluster. In addition, the striking similarity between the chemical patterns of
this system and those of its surrounding environment, the Galactic Bulge, from
the point of view of both the metallicity distribution and the alpha-element
enrichment, suggests that Terzan 5 could be a pristine fragment of the Bulge
itself. | 1401.3962v1 |
2014-01-20 | Total and Partial Fragmentation Cross-Section of 500 MeV/nucleon Carbon Ions on Different Target Materials | By using an experimental setup based on thin and thick double-sided
microstrip silicon detectors, it has been possible to identify the
fragmentation products due to the interaction of very high energy primary ions
on different targets. Here we report total and partial cross-sections measured
at GSI (Gesellschaft fur Schwerionenforschung), Darmstadt, for 500 MeV/n energy
$^{12}C$ beam incident on water (in flasks), polyethylene, lucite, silicon
carbide, graphite, aluminium, copper, iron, tin, tantalum and lead targets. The
results are compared to the predictions of GEANT4 (v4.9.4) and FLUKA (v11.2)
Monte Carlo simulation programs. | 1401.4897v1 |
2014-01-21 | Tunable VUV laser based spectrometer for Angle Resolved Photoemission Spectroscopy (ARPES) | We have developed an angle-resolved photoemission spectrometer with tunable
VUV laser as a photon source. The photon source is based on the fourth harmonic
generation of a near IR beam from a Ti:sapphire laser pumped by a CW green
laser and tunable between 5.3eV and 7eV. The most important part of the set-up
is a compact, vacuum enclosed fourth harmonic generator based on KBBF crystals,
grown hydrothermally in the US. This source can deliver a photon flux of over
10^14 photons/s. We demonstrate that this energy range is sufficient to measure
the kz dispersion in an iron arsenic high temperature superconductor, which was
previously only possible at synchrotron facilities. | 1401.5404v1 |
2014-01-21 | Dramatic Changes in the Electronic Structure Upon Transition to the Collapsed Tetragonal Phase in CaFe2As2 | We use angle-resolved photoemission spectroscopy (ARPES) and density
functional theory (DFT) calculations to study the electronic structure of
CaFe$_2$As$_2$ in previously unexplored collapsed tetragonal (CT) phase. This
unusual phase of the iron arsenic high temperature superconductors was hard to
measure as it exists only under pressure. By inducing internal strain, via the
post growth, thermal treatment of the single crystals, we were able to
stabilize the CT phase at ambient-pressure. We find significant differences in
the Fermi surface topology and band dispersion data from the more common
orthorhombic-antiferromagnetic or tetragonal-paramagnetic phases, consistent
with electronic structure calculations. The top of the hole bands sinks below
the Fermi level, which destroys the nesting present in parent phases. The
absence of nesting in this phase along with apparent loss of Fe magnetic
moment, are now clearly experimentally correlated with the lack of
superconductivity in this phase. | 1401.5416v1 |
2014-01-22 | The effect of exceptionally high fluorine doping on the anisotropy of single crystalline SmFeAsO1-xFx | We succeeded in growing single crystalline SmFeAsO1-xFx with exceptionally
high fluorine concentration by using a CsCl flux method. Comparing to
conventional flux methods, this method can introduce about double the amount of
fluorine into the oxygen site. The obtained single crystal shows the highest
superconducting transition temperature (Tc = 57.5 K) in single crystalline iron
pnictides. In addition, the residual resistivity ratio (RRR) is almost three
times as large as that of previously reported single crystals. This suggests
that our single crystals are suitable for investigation of the intrinsic
superconducting properties since they have few defects and impurities. Using
both the Werthamer-Helfand-Hohenberg model and the effective mass model, we
demonstrated that a higher fluorine concentration suppresses the anisotropic
superconductivity of SmFeAsO1-xFx. | 1401.5611v2 |
2014-01-28 | Phase Transformation Dynamics in Porous Battery Electrodes | Porous electrodes composed of multiphase active materials are widely used in
Li-ion batteries, but their dynamics are poorly understood. Two-phase models
are largely empirical, and no models exist for three or more phases. Using a
modified porous electrode theory based on non-equilibrium thermodynamics, we
show that experimental phase behavior can be accurately predicted from free
energy models, without artificially placing phase boundaries or fitting the
open circuit voltage. First, we simulate lithium intercalation in porous iron
phosphate, a popular two-phase cathode, and show that the zero-current voltage
gap, sloping voltage plateau and under-estimated exchange currents all result
from size-dependent nucleation and mosaic instability. Next, we simulate porous
graphite, the standard anode with three stable phases, and reproduce
experimentally observed fronts of color-changing phase transformations. These
results provide a framework for physics-based design and control for
electrochemical systems with complex thermodynamics. | 1401.7072v2 |
2014-01-29 | The poisoning effect of Mn in LaFe(1-x)Mn(x)AsO(0.89)F(0.11): unveiling a quantum critical point in the phase diagram of iron-based superconductors | A superconducting-to-magnetic transition is reported for
LaFe$_{1-x}$Mn$_x$AsO$_{0.89}$F$_{0.11}$ where a per thousand amount of Mn
impurities is dispersed. By employing local spectroscopic techniques like muon
spin rotation (muSR) and nuclear quadrupole resonance (NQR) on compounds with
Mn contents ranging from x=0.025% to x=0.75%, we find that the electronic
properties are extremely sensitive to the Mn impurities. In fact, a small
amount of Mn as low as 0.2% suppresses superconductivity completely. Static
magnetism, involving the FeAs planes, is observed to arise for x > 0.1% and
becomes further enhanced upon increasing Mn substitution. Also a progressive
increase of low energy spin fluctuations, leading to an enhancement of the NQR
spin-lattice relaxation rate 1/T1, is observed upon Mn substitution. The
analysis of 1/T1 for the sample closest to the the crossover between
superconductivity and magnetism (x = 0.2%) points towards the presence of an
antiferromagnetic quantum critical point around that doping level. | 1401.7576v1 |
2014-01-30 | Electronic structure of BaNi$_2$P$_2$ observed by angle-resolved photoemission spectroscopy | We have performed an angle-resolved photoemission spectroscopy (ARPES) study
of BaNi$_2$P$_2$ which shows a superconducting transition at $T_c$ $\sim$ 2.5
K. We observed hole and electron Fermi surfaces (FSs) around the Brillouin zone
center and corner, respectively, and the shapes of the hole FSs dramatically
changed with photon energy, indicating strong three-dimensionality. The
observed FSs are consistent with band-structure calculation and de Haas-van
Alphen measurements. The mass enhancement factors estimated in the normal state
were $m^*$/$m_b$ $\leq$ 2, indicating weak electron correlation compared to
typical iron-pnictide superconductors. An electron-like Fermi surface around
the Z point was observed in contrast with BaNi$_2$As$_2$ and may be related to
the higher $T_c$ of BaNi$_2$P$_2$. | 1401.7897v2 |
2014-01-31 | A global cloud map of the nearest known brown dwarf | Brown dwarfs -- substellar bodies more massive than planets but not massive
enough to initiate the sustained hydrogen fusion that powers self-luminous
stars -- are born hot and slowly cool as they age. As they cool below about
2,300 K, liquid or crystalline particles composed of calcium aluminates,
silicates and iron condense into atmospheric 'dust', which disappears at still
cooler temperatures (around 1,300 K). Models to explain this dust dispersal
include both an abrupt sinking of the entire cloud deck into the deep, unob-
servable atmosphere and breakup of the cloud into scattered patches (as seen on
Jupiter and Saturn). Thus far, observations of brown dwarfs have been limited
to globally integrated measurements, which can reveal surface inhomogeneities
but cannot unambiguously resolve surface features. Here we report a
two-dimensional map of a brown dwarf's surface that allows identification of
large-scale bright and dark features, indicative of patchy clouds. | 1401.8145v1 |
2014-02-01 | Phase stability and Nucleation effect upon Iron addition to SmCo5 bulk magnets | Change in structure and magnetic properties of SmCo5-xFex permanent magnets
for different values of x ranging from 0 to 2 have been studied. Structural
investigation from X-ray diffraction (XRD) patterns confirms the hexagonal
CaCu5-type structure of the SmCo5-xFex ribbons for 0<x<2. The decrease in
angular position of the diffraction peaks points to the lattice expansion due
to the substitution of Co atoms by larger Fe atoms. Mixture of phases occurs
for x>2 and has been confirmed by both XRD studies and magnetic measurements.
Nucleation effect induced by the additive Fe enhances the coercivity (Hc)up to
27 kOe which is much larger than 4.5 kOe obtained for pure SmCo5. | 1402.0138v1 |
2014-02-03 | Hemispherical Parker waves driven by thermal shear in planetary dynamos | Planetary and stellar magnetic fields are thought to be sustained by helical
motions ($\alpha$-effect) and, if present, differential rotation
($\Omega$-effect). In the Sun, the strong differential rotation in the
tachocline is responsible for an efficient $\Omega$-effect creating a strong
axisymmetric azimuthal magnetic field. This is a prerequisite for Parker dynamo
waves that may be responsible for the solar cycle. In the liquid iron cores of
terrestrial planets, the Coriolis force organizes convection into columns with
a strong helical flow component. These likely dominate magnetic field
generation while the $\Omega$-effect is of secondary importance. Here we use
numerical simulations to show that the planetary dynamo scenario may change
when the heat flux through the outer boundary is higher in one hemisphere than
in the other. A hemispherical dynamo is promoted that is dominated by fierce
thermal wind responsible for a strong $\Omega$-effect. As a consequence Parker
dynamo waves are excited equivalent to those predicted for the Sun. They obey
the same dispersion relation and propagation characteristics. We suggest that
Parker waves may therefore also play a role in planetary dynamos for all
scenarios where zonal flows become an important part of convective motions. | 1402.0343v1 |
2014-02-06 | Temperature-induced Change in the Fermi Surface Topology in the Spin Density Wave Phase of Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ | We report electronic Raman scattering measurements of
Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals in their magnetic - Spin Density
Wave (SDW) phase. The spectra display multiple, polarization-resolved SDW gaps
as expected in a band-folding itinerant picture for a multiband system. The
temperature dependence of the SDW gaps reveals an unusual evolution of the
reconstructed electronic structure with at least one gap being activated only
well below the magnetic SDW transition $T_N$. A comparison with temperature
dependent Hall measurements allows us to assign this activated behavior to a
change in the Fermi surface topology deep in the SDW phase, which we attribute
to the disappearance of a hole-like Fermi pocket. Our results highlight the
strong sensitivity of the low energy electronic structure to temperature in
iron-arsenide superconductors. | 1402.1422v2 |
2014-02-07 | Signatures of superconducting and pseudogap phases in ultrafast transient reflectivity of Ca(Fe0.927Co0.073)2As2 | We present femtosecond time-resolved pump-probe spectroscopic studies of a
pseudogap (PG) along with the superconducting (SC) gap in an overdoped iron
pnictide Ca(Fe0.927Co0.073)2As2. It is seen that the temperature evolution of
the photoexcited quasiparticle (QP) relaxation dynamics, coherently excited
A1g-symmetric optical phonon and two acoustic phonon dynamics behave
anomalously in the vicinity of the superconducting transition temperature Tc. A
continuous change in the sign of the experimentally measured transient
differential reflectivity Delta_R/R signal at the zero time-delay between the
pump and probe pulses at a temperature of ~200 K is inferred as an evidence of
the emergence of the PG phase around that temperature. This behavior is
independent of the pump photon energy and occurs for crystals without the spin
density wave phase transition. | 1402.1627v1 |
2014-02-07 | Suzaku Discovery of Fe K-Shell Line from the O-Rich SNR G292.0+1.8 | We report the Suzaku/XIS results of the Galactic oxygen-rich supernova
remnant (SNR), G292.0+1.8, a remnant of a core-collapse supernova. The X-ray
spectrum of G292.0+1.8 consists of two type plasmas, one is in collisional
ionization equilibrium (CIE) and the other is in non-equilibrium ionization
(NEI). The CIE plasma has nearly solar abundances, and hence would be
originated from the circumstellar and interstellar mediums. The NEI plasma has
super-solar abundances, and the abundance pattern indicates that the plasma
originates from the supernova ejecta with a main sequence of 30-35 Msolar. Iron
K-shell line at energy of 6.6 keV is detected for the first time in the NEI
plasma. | 1402.1633v1 |
2014-02-12 | Electron Impact Ionization of Stored Highly Charged Ions | Accurate cross section data for electron impact ionization (EII) are needed
in order to interpret the spectra of collisionally ionized plasmas both in
astrophysics and in the laboratory. Models and spectroscopic diagnostics of
such plasmas rely on accurate ionization balance calculations, which depend, in
turn, on the underlying rates for EII and electron-ion recombination. EII
measurements have been carried out using the TSR storage ring located at the
Max-Planck-Institut fuer Kernphysik in Heidelberg, Germany. Storage ring
measurements are largely free of metastable contamination, resulting in
unambiguous EII data, unlike what is encountered with other experimental
geometries. As it is impractical to perform experiments for every ion, theory
must provide the bulk of the necessary EII data. In order to guide theory, TSR
experiments have focused on providing at least one measurement for every
isoelectronic sequence. EII data have been measured for ions from 13
isoelectronic sequences: Li-like silicon and chlorine, Be-like sulfur, B-like
magnesium, and F-like through K-like iron. These experimental results provide
an important benchmark for EII theory. | 1402.2964v1 |
2014-02-21 | Enhancement of Superconducting Transition Temperature Due to Antiferromagnetic Spin Fluctuations in Iron-pnictides LaFe(As_{1-x}P_x)(O_{1-y}F_y) : 31P-NMR Studies | Systematic P-NMR studies on LaFe(As_{1-x}P_x)(O_{1-y}F_y) with y=0.05 and 0.1
have revealed that the antiferromagnetic spin fluctuations (AFMSFs) at low
energies are markedly enhanced around x=0.6 and 0.4, respectively, and as a
result, Tc exhibits respective peaks at 24 K and 27 K against the
P-substitution for As. This result demonstrates that the AFMSFs are responsible
for the increase in Tc for LaFe(As_{1-x}P_x)(O_{1-y}F_y) as a primary mediator
of the Cooper pairing. From a systematic comparison of AFMSFs with a series of
(La_{1-z}Y_z)FeAsO_{\delta} compounds in which Tc reaches 50 K for z=0.95, we
remark that a moderate development of AFMSFs causes the Tc to increase up to 50
K under the condition that the local lattice parameters of FeAs tetrahedron
approaches those of the regular tetrahedron. We propose that the T_c of
Fe-pnictides exceeding 50 K is maximized under an intimate collaboration of the
AFMSFs and other factors originating from the optimization of the local
structure. | 1402.5220v1 |
2014-03-02 | Transport in multiband systems with hot spots on the Fermi surface: Forward-scattering corrections | Multiband models with hot spots are of current interest partly because of
their relevance for the iron-based superconductors. In these materials, the
momentum-dependent scattering off spin fluctuations and the ellipticity of the
electron Fermi pockets are responsible for anisotropy of the lifetimes of
excitations around the Fermi surface. The deep minima of the lifetimes---the
so-called hot spots---have been assumed to contribute little to the transport
as is indeed predicted by a simple relaxation-time approach. Calculating
forward-scattering corrections to this approximation, we find that the
effective transport times are much more isotropic than the lifetimes and that,
therefore, the hot spots contribute to the transport even in the case of strong
spin-fluctuation scattering. We discuss this effect on the basis of an
analytical solution of the Boltzmann equation and calculate numerically the
temperature and doping dependence of the resistivity and the Hall, Seebeck, and
Nernst coefficients. | 1403.0144v2 |
2014-03-02 | Heat transport in RbFe2As2 single crystal: evidence for nodal superconducting gap | The in-plane thermal conductivity of iron-based superconductor RbFe$_2$As$_2$
single crystal ($T_c \approx$ 2.1 K) was measured down to 100 mK. In zero
field, the observation of a significant residual linear term $\kappa_0/T$ =
0.65 mW K$^{-2}$ cm$^{-1}$ provides clear evidence for nodal superdonducting
gap. The field dependence of $\kappa_0/T$ is similar to that of its sister
compound CsFe$_2$As$_2$ with comparable residual resistivity $\rho_0$, and lies
between the dirty and clean KFe$_2$As$_2$. These results suggest that the
(K,Rb,Cs)Fe$_2$As$_2$ serial superconductors have a common nodal gap structure. | 1403.0191v3 |
2014-03-02 | Nodeless superconducting gaps in Ca$_{10}$(Pt$_{4-δ}$As$_8$)((Fe$_{1-x}$Pt$_{x}$)$_2$As$_2$)$_5$ probed by quasiparticle heat transport | The in-plane thermal conductivity of iron-based superconductor
Ca$_{10}$(Pt$_{4-\delta}$As$_8$)((Fe$_{1-x}$Pt$_{x}$)$_2$As$_2$)$_5$ single
crystal (``10-4-8", $T_c$ = 22 K) was measured down to 80 mK. In zero field,
the residual linear term $\kappa_0/T$ is negligible, suggesting nodeless
superconducting gaps in this multiband compound. In magnetic fields,
$\kappa_0/T$ increases rapidly, which mimics those of multiband superconductor
NbSe$_2$ and LuNi$_2$B$_2$C with highly anisotropic gap. Such a field
dependence of $\kappa_0/T$ is an evidence for multiple superconducting gaps
with quite different magnitudes or highly anisotropic gap. Comparing with the
London penetration depth results of
Ca$_{10}$(Pt$_3$As$_8$)((Fe$_{1-x}$Pt$_{x}$)$_2$As$_2$)$_5$ (``10-3-8")
compound, the 10-4-8 and 10-3-8 compounds may have similar superconducting gap
structure. | 1403.0194v1 |
2014-03-04 | Tunnel diode oscillator measurements of the upper critical magnetic field of FeTe0:5Se0:5 | Temperature dependence of the upper critical magnetic field (Hc2) of single
crystalline FeTe0.5Se0.5 (Tc = 14.5 K) have been determined by tunnel diode
oscillator-based measurements in magnetic fields of up to 55 T and temperatures
down to 1.6 K. The Werthamer-Helfand-Hohenberg model accounts for the data for
magnetic field applied both parallel (H || ab) and perpendicular (H || c) to
the iron conducting plane, in line with a single band superconductivity.
Whereas Pauli pair breaking is negligible for H || c, Pauli contribution is
evidenced for H || ab with Maki parameter alpha= 1.4, corresponding to Pauli
field HP = 79 T. As a result, the Hc2 anisotropy (= Habc2 /Hcc2) which is
already rather small at Tc (gamma = 1.6) further decreases as the temperature
decreases and becomes smaller than 1 at liquid helium temperatures. | 1403.0844v1 |
2014-03-05 | Design and demonstration of a quasi-monoenergetic neutron source | The design of a neutron source capable of producing 24 and 70 keV neutron
beams with narrow energy spread is presented. The source exploits
near-threshold kinematics of the $^{7}$Li(p,n)$^{7}$Be reaction while taking
advantage of the interference `notches' found in the scattering cross-sections
of iron. The design was implemented and characterized at the Center for
Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory.
Alternative filters such as vanadium and manganese are also explored and the
possibility of studying the response of different materials to low-energy
nuclear recoils using the resultant neutron beams is discussed. | 1403.1285v2 |
2014-03-08 | High critical-current density with less anisotropy in BaFe2(As,P)2 epitaxial thin films: Effect of intentionally grown c-axis vortex-pinning centers | We report herein a high and isotropic critical-current density Jc for
BaFe2(As,P)2 epitaxial films. The isotropy of Jc with respect to the
magnetic-field direction was improved significantly by decreasing the film
growth rate to 2.2 {\AA}/s. The low growth rate served to preferentially align
dislocations along the c-axis, which work well as c-axis vortex-pinning
centers. Because of the intentional introduction of effective pinning, the
absolute Jc at 9 T was larger than that obtained for other iron-based
superconductors and conventional alloy superconducting wires. | 1403.1947v2 |
2014-03-08 | Challenges in nucleosynthesis of trans-iron elements | Nucleosynthesis beyond Fe poses additional challenges not encountered when
studying astrophysical processes involving light nuclei. Astrophysical sites
and conditions are not well known for some of the processes involved. On the
nuclear physics side, different approaches are required, both in theory and
experiment. The main differences and most important considerations are
presented for a selection of nucleosynthesis processes and reactions,
specifically the $s$-, $r$-, $\gamma$-, and $\nu p$-processes. Among the
discussed issues are uncertainties in sites and production conditions, the
difference between laboratory and stellar rates, reaction mechanisms, important
transitions, thermal population of excited states, and uncertainty estimates
for stellar rates. The utility and limitations of indirect experimental
approaches are also addressed. The presentation should not be viewed as
confining the discussed problems to the specific processes. The intention is to
generally introduce the concepts and possible pitfalls along with some
examples. Similar problems may apply to further astrophysical processes
involving nuclei from the Fe region upward and/or at high plasma temperatures.
The framework and strategies presented here are intended to aid the conception
of future experimental and theoretical approaches. | 1403.2015v1 |
2014-03-11 | Chemical abundance analysis of symbiotic giants. I. RW Hya and SY Mus | The study of symbiotic systems is of considerable importance in our
understanding of binary system stellar evolution in systems where mass loss or
transfer takes place. Elemental abundances are of special significance since
they can be used to track mass exchange. However, there are few symbiotic
giants for which the abundances are fairly well determined. Here we present for
the first time a detailed analysis of the chemical composition for the giants
in the RW Hya and SY Mus systems. The analysis is based on high resolution (R
50000), high S/N, near-IR spectra. Spectrum synthesis employing standard LTE
analysis and atmosphere models was used to obtain photospheric abundances of
CNO and elements around the iron peak (Sc, Ti, Fe, and Ni). Our analysis
reveals a significantly sub-solar metallicity, [Fe/H]-0.75}, for the RW Hya
giant confirming its membership in the Galactic halo population and a
near-solar metallicity for the SY Mus giant. The very low 12C/13C isotopic
ratios, 6-10, derived for both objects indicate that the giants have
experienced the first dredge-up | 1403.2659v1 |
2014-03-22 | Neutron Reactions in Astrophysics | The quest for the origin of matter in the Universe had been the subject of
philosophical and theological debates over the history of mankind, but
quantitative answers could be found only by the scientific achievements of the
last century. A first important step on this way was the development of
spectral analysis by Kirchhoff and Bunsen in the middle of the 19$^{\rm th}$
century, which provided first insight in the chemical composition of the sun
and the stars. The energy source of the stars and the related processes of
nucleosynthesis, however, could be revealed only with the discoveries of
nuclear physics. A final breakthrough came eventually with the compilation of
elemental and isotopic abundances in the solar system, which are reflecting the
various nucleosynthetic processes in detail.
This review is focusing on the mass region above iron, where the formation of
the elements is dominated by neutron capture, mainly in the slow ($s$) and
rapid ($r$) processes. Following a brief historic account and a sketch of the
relevant astrophysical models, emphasis is put on the nuclear physics input,
where status and perspectives of experimental approaches are presented in some
detail, complemented by the indispensable role of theory. | 1403.5670v1 |
2014-03-24 | Dust formation in macronovae | We examine dust formation in macronovae (as known as kilonovae), which are
the bright ejecta of neutron star binary mergers and one of the leading sites
of r-process nucleosynthesis. We find that dust grains of r-process elements
are difficult to form because of the low number density of the r-process atoms,
while carbon or elements lighter than irons can condense into dust if they are
abundant, in light of the first macronova candidate associated with GRB
130603B. Dust grains absorb emission from ejecta with opacity even greater than
that of the r-process elements, and re-emit photons at infrared wavelengths.
Such dust emission can potentially account for the macronova without r-process
nucleosynthesis as an alternative model. This dust scenario predicts a more
featureless spectrum than the r-process model and day-scale
optical-to-ultraviolet emission. | 1403.5872v2 |
2014-03-26 | Photoconductivity and photo-detection response of multiferroic bismuth iron oxide | We report visible light detection with in-plane BiFeO3 (BFO) thin films grown
on pre-patterned inter-digital electrodes. In-plane configured BFO film
displayed photocurrents with a 40:1 photo-to-dark-current ratio and improved
photo-sensing ability for >15000 s (4 hrs) under small bias voltage (42V).
Nearly sixty percent of the photo-induced charge carriers decay in 1.0 s and
follow a double-exponential decay model. At 373 K the effect of light does not
significantly increase the dark current, probably due to reduced mobility.
Sub-bandgap weak monochromatic light (1 mw/cm2) shows one fold increase in
photo-charge carriers. | 1403.6748v1 |
2014-03-26 | Simulating a two-dimensional frustrated spin system with fermionic resonating-valence-bond states | The frustrated Heisenberg $J_{1}-J_{2}$ model on a square lattice is
numerically investigated by variational Monte Carlo simulations. We propose a
antiferromagnetic fermion resonating-valence-bond (AF-fRVB) state that has
ability to examine the entire phase diagram in the $J_{1}-J_{2}$ model. Two
phase transition points, the second order around $J_{2}/J_{1}=0.45$ and the
first order around $J_{2}/J_{1}=0.6$, can be extracted more clearly than the
conventional bosonic RVB state. At the maximally frustrated point
($J_{2}/J_{1}=0.5$), the AF-fRVB state shows the variational ground-state
energy in the thermodynamic limit very close to the one estimated by the
projected entangled pair state at the largest bond dimension available. On the
other hand, in the frustrated regime $0.4\lesssim J_{2}/J_{1}\leq0.5$, AF-fRVB
states with $s_{+-}$ (using the terminology in the field of iron-based
superconductors) and $d_{xy}$ pairing symmetries are degenerate in the
thermodynamic limit, implying the existence of gapless Dirac excitations in the
spinon spectrum. | 1403.6879v1 |
2014-03-30 | Scattering of electron vortex beams on a magnetic crystal: towards atomic resolution magnetic measurements | Use of electron vortex beams (EVB), that is convergent electron beams
carrying an orbital angular momentum (OAM), is a novel development in the field
of transmission electron microscopy. They should allow measurement of
element-specific magnetic properties of thin crystals using electron magnetic
circular dichroism (EMCD)---a phenomenon similar to the x-ray magnetic circular
dichroism. Recently it has been shown computationally that EVBs can detect
magnetic signal in a scanning mode only at atomic resolution. In this follow-up
work we explore in detail the elastic and inelastic scattering properties of
EVBs on crystals, as a function of beam diameter, initial OAM, acceleration
voltage and beam displacement from an atomic column. We suggest that for a 10
nm layer of bcc iron oriented along (001) zone axis an optimal configuration
for a detection of EMCD is an EVB with OAM of $1\hbar$ and a diameter of 1.6
\AA, acceleration voltage 200 keV and an annular detector with inner and outer
diameters of $G$ and $5G$, respectively, where $\mathbf{G}=(100)$. | 1403.7730v1 |
2014-03-31 | Strain effects on electronic structure and superconductivity in the iron telluride | The influence of tensile strain in the ab-plane on crystal and electronic
structure of FeTe has been studied ab initio. In superconducting FeSe the Fermi
surface nesting with a vector q~(0.5,0.5)(2\pi /a) is believed to be crucial
for rising superconductivity mediated by spin-fluctuations. The results
presented here indicate that tensile-strained FeTe also exhibits such
conditions. Furthermore, the Fermi surface changes, related to the increase of
the lattice parameter a of this telluride, are opposite to analogous effects
reported for FeSe. Since a recently reported transition from the double-stripe
to the single-stripe magnetic order in FeTe under tensile strain in the
ab-plane is associated with an occurence of superconductivity in corresponding
thin films, these findings allow for drawing a consistent picture of
superconductivity in FeSe(1-x)Te(x) systems, in general. | 1403.8005v2 |
2014-04-02 | Spurious peaks arising from multiple scattering events involving cryostat walls in inelastic neutron scattering | Well defined peaks with energies of about 18 meV have been observed in a
variety of inelastic neutron scattering experiments on single crystals as well
as on powders of iron-based superconductors and their parent compounds using
either the triple-axis or the time-of-flight technique. They can easily be
mistaken for signatures of real excitations. We have found that they are due to
multiple scattering events involving primarily walls of the sample environment.
Hence, they are particularly troublesome in experiments using very small
samples as have been used with recently developed high intensity neutron
spectrometers. It will be discussed what needs to be done to reduce the
unwanted scattering to a minimum. | 1404.0522v2 |
2014-04-03 | Charge Transfer Kinetics at the Solid-Solid Interface in Porous Electrodes | Interfacial charge transfer is widely assumed to obey Butler-Volmer kinetics.
For certain liquid-solid interfaces, Marcus-Hush-Chidsey theory is more
accurate and predictive, but it has not been applied to porous electrodes. Here
we report a simple method to extract the charge transfer rates in carbon-coated
LiFePO4 porous electrodes from chronoamperometry experiments, obtaining curved
Tafel plots that contradict the Butler-Volmer equation but fit the
Marcus-Hush-Chidsey prediction over a range of temperatures. The fitted
reorganization energy matches the Born solvation energy for electron transfer
from carbon to the iron redox site. The kinetics are thus limited by electron
transfer at the solid-solid (carbon-LixFePO4) interface, rather than by ion
transfer at the liquid-solid interface, as previously assumed. The proposed
experimental method generalizes Chidsey's method for phase-transforming
particles and porous electrodes, and the results show the need to incorporate
Marcus kinetics in modeling batteries and other electrochemical systems. | 1404.0859v1 |
2014-04-03 | Fermi-Surface Reconstruction and Complex Phase Equilibria in CaFe$_{2}$As$_{2}$ | Fermi-surface topology governs the relationship between magnetism and
superconductivity in iron-based materials. Using low-temperature transport,
angle-resolved photoemission, and x-ray diffraction we show unambiguous
evidence of large Fermi surface reconstruction in CaFe$_{2}$As$_{2}$ at
magnetic spin-density-wave and nonmagnetic collapsed-tetragonal ($cT$)
transitions. For the $cT$ transition, the change in the Fermi surface topology
has a different character with no contribution from the hole part of the Fermi
surface. In addition, the results suggest that the pressure effect in
CaFe$_{2}$As$_{2}$ is mainly leading to a rigid-band-like change of the valence
electronic structure. We discuss these results and their implications for
magnetism and superconductivity in this material. | 1404.1095v1 |
2014-04-07 | Fingerprints of Galactic Loop I on the Cosmic Microwave Background | We investigate possible imprints of galactic foreground structures such as
the "radio loops" in the derived maps of the cosmic microwave background.
Surprisingly there is evidence for these not only at radio frequencies through
their synchrotron radiation, but also at microwave frequencies where emission
by dust dominates. This suggests the mechanism is magnetic dipole radiation
from dust grains enriched by metallic iron or ferrimagnetic materials. This new
foreground we have identified is present at high galactic latitudes, and
potentially dominates over the expected $B$-mode polarization signal due to
primordial gravitational waves from inflation. | 1404.1899v3 |
2014-04-09 | Zinc abundances of planetary nebulae | Zinc is a useful surrogate element for measuring Fe/H as, unlike iron, it is
not depleted in the gas phase media. Zn/H and O/Zn ratios have been derived
using the [Zn IV] emission line at 3.625um for a sample of nine Galactic
planetary nebulae, seven of which are based upon new observations using the
VLT. Based on photoionization models, O/O++ is the most reliable ionisation
correction factor for zinc that can readily be determined from optical emission
lines, with an estimated accuracy of 10% or better for all targets in our
sample. The majority of the sample is found to be sub-solar in [Zn/H]. [O/Zn]
in half of the sample is found to be consistent with Solar within
uncertainties, whereas the remaining half are enhanced in [O/Zn]. [Zn/H] and
[O/Zn] as functions of Galactocentric distance have been investigated and there
is little evidence to support a trend in either case. | 1404.2524v1 |
2014-04-10 | Synthesis and physical properties of Ca1-xRExFeAs2 with RE = La ~ Gd | Synthesis of a series of layered iron arsenides Ca1-xRExFeAs2 (112) was
attempted by heating at 1000 C under a high-pressure of 2 GPa. The 112 phase
successfully forms with RE = La, Ce, Nd, Sm, Eu and Gd, while Tb, Dy and Ho
substituted and RE free samples does not contain the 112 phase. The Ce, Nd, Sm,
Eu and Gd doped Ca1-xRExFeAs2 are new compounds. All of them exhibit
superconducting transition except for the Ce doped sample. The behaviour of the
critical temperature, with the RE ionic radii have been investigated. | 1404.2738v1 |
2014-04-24 | Superconductivity driven by the screening of long-distance Coulomb interaction | The pair-fluctuation contribution reduces the electrostatic screening length
in superconductivity as compared to the normal state. When a conductor
possesses a static background charge distribution, superconductivity arises
even in the absence of an explicit pairing interaction, such that the Coulomb
repulsion is reduced and the total energy is lowered. We demonstrate that the
superconducting gap increases with increased background charge at first, after
which the mixing of the Higgs and plasma modes suppresses superconductivity in
the pseudogap phase. This indicates that the mechanism may be relevant to the
cuprates and iron pnictides. When the background charge is identified with the
incoherent component of optical conductivity in the cuprates, our results
reproduce the shape, size and position of the superconducting dome with zero
free parameters. A superconducting critical temperature of about 1000 K is
possible in ion-doped conductors. | 1404.6292v1 |
2014-04-25 | An all-electrical torque differential magnetometer operating under ambient conditions | An all-electrical torque differential magnetometry (also known as cantilever
magnetometry) setup employing piezoelectric quartz tuning forks is
demonstrated. The magnetometer can be operated under ambient conditions as well
as low temperatures and pressures. It extends the allowed specimen mass range
up to several 10 $\mu$g without any significant reduction in the sensitivity.
Operation under ambient conditions and a simple all-electrical design of the
magnetometer should allow for an easy integration with other experimental
setups. The uniaxial magnetic anisotropy of a 25 $\mu$m diameter iron wire,
measured under ambient conditions with a high signal to noise ratio, was found
to be in good agreement with its literature value. Further applications of the
technique are discussed. | 1404.6341v2 |
2014-05-02 | Spectral line broadening in magnetized black holes | We consider weakly magnetized non-rotating black holes. In the presence of a
regular magnetic field the motion of charged particles in the vicinity of a
black hole is modified. As a result, the position of the innermost stable
circular orbit (ISCO) becomes closer to the horizon. When the Lorentz force is
repulsive (directed from the black hole) the ISCO radius can reach the
gravitational radius. In the process of accretion charged particles (ions) of
the accreting matter can be accumulated near their ISCO, while neutral
particles fall down to the black hole after they reach $6M$ radius. The sharp
spectral line Fe K$\alpha$, emitted by iron ions at such orbits, is broadened
when the emission is registered by a distant observer. In this paper we study
this broadening effect and discuss how one can extract information concerning
the strength of the magnetic field from the observed spectrum. | 1405.0510v3 |
2014-05-06 | Evolution from incoherent to coherent electronic states and its implications to superconductivity in FeTe1-xSex | We have performed systematic angle-resolved photoemission spectroscopy
(ARPES) of iron-chalcogenide superconductor FeTe1-xSex to elucidate the
electronic states relevant to the superconductivity. While the Fermi-surface
shape is nearly independent of x, we found that the ARPES spectral line shape
shows prominent x dependence. A broad ARPES spectrum characterized by a small
quasiparticle weight at x = 0, indicative of incoherent electronic states,
becomes progressively sharper with increasing x, and a well-defined
quasiparticle peak appears around x = 0.45 where bulk superconductivity is
realized. The present result suggests the evolution from incoherent to coherent
electronic states and its close relationship to the emergence of
superconductivity. | 1405.1172v1 |
2014-05-05 | Mathematical Modeling on Open Limestone Channel | Acid mine drainage (AMD) is the outflow of acidic water from metal mines or
coal mines. When exposed to air and water, metal sulfides from the deposits of
the mines are oxidized and produce acid, metal ions and sulfate, which lower
the pH value of the water. An open limestone channel (OLC) is a passive and low
cost way to neutralize AMD. The dissolution of calcium into the water increases
the pH value of the solution. A differential equation model is numerically
solved to predict the variation of concentration of each species in the OLC
solution. The diffusion of Calcium due to iron precipitates is modeled by a
linear equation. The results give the variation of pH value and the
concentration of Calcium. | 1405.1338v1 |
2014-05-08 | Effect of nominal substitution of transition metals for excess Fe in Fe_{1+x}Se superconductor | Taking cue from the increase in the superconducting transition temperature
(Tc) of Fe1+xSe via nominal (2 wt%) substitution of Cr instead of excess Fe, we
have now extended our study with nominal substitution (<=5 wt%) with other
transition metals (Ni, Co, Fe, Mn, Cr, V and Ti) in place of excess iron. The
Tc is found to increase (maximum ~11 K) or get suppressed depending on the
substituted transition metal. Our studies indicate that the superconducting
transition temperature depends on various parameters like the ionic size of the
transition metal, its magnetic moment as well as the amount of hexagonal phase
present as impurity. | 1405.2050v3 |
2014-05-13 | Magnetism and superconductivity in Sr$_2$VFeAsO$_3$ revealed by $^{75}$As- and $^{51}$V-NMR under elevated pressures | We report $^{75}$As- and $^{51}$V-nuclear magnetic resonance (NMR)
measurements on the iron-based superconductor Sr$_2$VFeAsO$_3$ with alternating
stacks structure. We find that the $^{75}$As nuclear spin-spin relaxation rate
($1/T_2$) shows a pronounced peak at $T_N$ = 165 K, below which the resonance
peak shifts to a higher frequency due to the onset of an internal magnetic
field. The $^{51}$V spectrum does not shift, but is broadened below $T_N$. We
conclude that the Fe electrons oder antiferromagnetically below $T_N$ with a
magnetic moment $m_{Fe}$ $\sim$ 0.4 $\mu_B$. Application of external pressure
up to 2.4 GPa reduces $T_N$ in a rate of $-$40 K/GPa, and enhances the
superconducting transition temperature $T_c$ in a rate of 2 K/GPa. The
pressure-temperature phase diagram for Sr$_2$VFeAsO$_3$ shows that
superconductivity coexists with antiferromagnetism over a wide pressure range
with an unprecedented high $T_c$ up to 36.5 K. | 1405.3048v1 |
2014-05-15 | Nuclear Target Cross Section Ratios at MINERvA | Measurements of $\nu_{\mu}$ inclusive charged-current cross section ratios on
carbon, iron, and lead relative to scintillator are presented. Data for the
analysis were collected by the fine-grained MINERvA detector in the NuMI
beamline at Fermilab. This is the first direct measurement of nuclear
dependence in neutrino scattering. The ratios show a depletion at low Bjorken
$x$ and enhancement at large $x$, both of which increase with the nucleon
number of the target. The data exhibit trends not found in GENIE, a standard
neutrino-nucleus event generator, or alternative models of nuclear modification
to inelastic structure functions. | 1405.3960v1 |
2014-05-19 | Neutron Scattering Measurements of Spatially Anisotropic Magnetic Exchange Interactions in Semiconducting K0.85Fe1.54Se2 (TN=280 K) | We use neutron scattering to study the spin excitations associated with the
stripe antiferromagnetic (AFM) order in semiconducting
K$_{0.85}$Fe$_{1.54}$Se$_2$ ($T_N$=$280$ K). We show that the spin wave spectra
can be accurately described by an effective Heisenberg Hamiltonian with highly
anisotropic in-plane couplings at $T$= $5$ K. At high temperature ($T$= $300$
K) above $T_N$, short range magnetic correlation with anisotropic correlation
lengths are observed. Our results suggest that, despite the dramatic difference
in the Fermi surface topology, the in-plane anisotropic magnetic couplings are
a fundamental property of the iron based compounds; this implies that their
antiferromagnetism may originate from local strong correlation effects rather
than weak coupling Fermi surface nesting. | 1405.4737v1 |
2014-05-19 | Multiferroic Iron Oxide Thin Films at Room-Temperature | In spite of being highly relevant for the development of a new generation of
information storage devices, not many single-phase materials displaying
magnetic and ferroelectric orders above room temperature are known. Moreover,
these uncommon materials typically display insignificant values of the remanent
moment in one of the ferroic orders or are complex multicomponent oxides which
will be very challenging to integrate in devices. Here we report on the
strategy to stabilize the metastable epsilon-Fe2O3 in thin film form, and we
show that besides its already known ferrimagnetic nature, the films are also
ferroelectric at 300 K with a remanent polarization of 1 microC/cm2. The film
polarization shows long retention times and can be switched under small applied
voltages. These characteristics make of epsilon-Fe2O3 the first single-ion
transition-metal oxide which is ferro(ferri)magnetic and ferroelectric at room
temperature. The simple composition of this new multiferroic oxide and the
discovery of a robust path for its thin film growth may boost the exploitation
of epsilon-Fe2O3 in novel devices. | 1405.4909v1 |
2014-05-22 | Layered water Cherenkov detector for the study of ultra high energy cosmic rays | We present a new design for the water Cherenkov detectors that are in use in
various cosmic ray observatories. This novel design can provide a significant
improvement in the independent measurement of the muonic and electromagnetic
component of extensive air showers. From such multi-component data an event by
event classification of the primary cosmic ray mass becomes possible. According
to popular hadronic interaction models, such as EPOS-LHC or QGSJetII-04, the
discriminating power between iron and hydrogen primaries reaches Fisher values
of $\sim$ 2 or above for energies in excess of $10^{19}$ eV with a detector
array layout similar to that of the Pierre Auger Observatory. | 1405.5699v1 |
2014-05-25 | Theory of Point Contact Spectroscopy in Correlated Materials | We develop a microscopic theory for the point-contact conductance between a
metalic electrode and a strongly correlated material using the non-equilibrium
Schwinger-Kadanoff-Baym-Keldysh formalism. We explicitly show that in the
classical limit, contact size shorter than the scattering length of the system,
the microscopic model can be reduced to an effective model with transfer matrix
elements that conserve in-plane momentum. We find that the conductance $dI/dV$
is proportional to the {\it effective density of states}, that is, the
integrated single-particle spectral function $A(\omega=eV)$ over the whole
Brillouin zone. From this conclusion, we are able to establish the conditions
under which a non-Fermi liquid metal exhibits a zero-bias peak in the
conductance. This finding is discussed in the context of recent point-contact
spectroscopy on the iron pnictides and chalcogenides which has exhibited a
zero-bias conductance peak. | 1405.6357v2 |
2014-05-25 | CaFeAs$_2$: a Staggered Intercalation of Quantum Spin Hall and High Temperature Superconductivity | We predict that CaFeAs$_2$, a newly discovered iron-based high temperature
(T$_c$) superconductor, is a staggered intercalation compound that integrates
topological quantum spin hall (QSH) and superconductivity (SC). CaFeAs$_2$ has
a structure with staggered CaAs and FeAs layers. While the FeAs layers are
known to be responsible for high T$_c$ superconductivity, we show that with
spin orbital coupling each CaAs layer is a $Z_{2}$ topologically nontrivial
two-dimensional QSH insulator and the bulk is a 3-dimensional weak topological
insulator. In the superconducting state, the edge states in the CaAs layer are
natural 1D topological superconductors. The staggered intercalation of QSH and
SC provides us an unique opportunity to realize and explore novel physics, such
as Majorana modes and Majorana Fermions chains. | 1405.6401v2 |
2014-05-28 | Contactless measurement of nonlinear conductivity in the radio-frequency range | We have developed a system for contactless measurement of nonlinear
conductivity in the radio-frequency band, and over a wide temperature range. A
non-resonant circuit is used to electrically excite the sample, and the induced
signal is detected by a resonant circuit whose natural frequency matches higher
harmonics of the excitation. A simple modification of the probe allows
non-resonant detection suitable for stronger signals. Two measurement
procedures are proposed that allow significant excitation power variation, up
to 150 W. The apparatus has been validated trough the measurement of the
nonlinear response at the superconducting transition of a high-Tc
superconductor, and the nematic transition of an iron pnictide. | 1405.7282v1 |
2014-05-31 | Point contact spectroscopy in the superconducting and normal state of $\mathrm{NaFe_{1-\textit{x}}Co_\textit{x}As}$ | We use point contact spectroscopy to probe the superconducting and normal
state properties of the iron-based superconductor
$\rm{NaFe_{1-\textit{x}}Co_{\textit{x}}As}$ with $\rm{\textit{x} = 0, 0.02,
0.06}$. Andreev spectra corresponding to multiple superconducting gaps are
detected in the superconducting phase. For $\rm{\textit{x} = 0.02}$, a broad
conductance enhancement around zero bias voltage is detected in both the normal
and the superconducting phase. Such a feature is not present in the
$\rm{\textit{x} = 0.06}$ samples. We suspect that this enhancement is caused by
orbital fluctuations, as previously detected in underdoped
$\rm{Ba(Fe_{1-\textit{x}}Co_\textit{x})_2As_2}$ (Phys. Rev. B 85, 214515
(2012)). Occasionally, the superconducting phase shows a distinct asymmetric
conductance feature instead of Andreev reflection. We discuss the possible
origins of this feature. NaFeAs (the parent compound) grown by two different
techniques is probed. Melt-grown NaFeAs shows a normal state conductance
enhancement. On the other hand, at low temperatures, flux-grown NaFeAs shows a
sharp dip in the conductance at zero bias voltage. The compounds are very
reactive in air and the different spectra are likely a reflection of their
different oxidation and purity levels. | 1406.0038v1 |
2014-06-01 | Hydrogen-Activation Mechanism of [Fe] Hydrogenase Revealed by Multi-Scale Modeling | When investigating the mode of hydrogen activation by [Fe] hydrogenases, not
only the chemical reactivity at the active site is of importance but also the
large-scale conformational change between the so-called open and closed
conformations, which leads to a special spatial arrangement of substrate and
iron cofactor. To study H2 activation, a complete model of the solvated and
cofactor-bound enzyme in complex with the substrate methenyl-H4MPT+ was
constructed. Both the closed and open conformations were simulated with
classical molecular dynamics on the 100 ns time scale.
Quantum-mechanics/molecular-mechanics calculations on snapshots then revealed
the features of the active site that enable the facile H2 cleavage. The
hydroxyl group of the pyridinol ligand can easily be deprotonated. With the
deprotonated hydroxyl group and the structural arrangement in the closed
conformation, H2 coordinated to the Fe center is subject to an ionic and
orbital push-pull effect and can be rapidly cleaved with a concerted hydride
transfer to methenyl-H4MPT+. An intermediary hydride species is not formed. | 1406.0166v1 |
2014-06-04 | Observation of strong electron pairing on bands without Fermi surfaces in LiFe1-xCoxAs | In conventional BCS superconductors, the quantum condensation of
superconducting electron pairs is understood as a Fermi surface (FS)
instability, in which the low-energy electrons are paired by attractive
interactions. Whether this explanation is still valid in high-Tc
superconductors such as cuprates and iron-based superconductors remains an open
question. In particular, a fundamentally different picture of the electron
pairs, which are believed to be formed locally by repulsive interactions, may
prevail. Here we report a high-resolution angle-resolved photoemission
spectroscopy study on LiFe1-xCoxAs. We reveal a large and robust
superconducting (SC) gap on a band sinking below the Fermi energy upon Co
substitution. The observed FS-free SC order is also the largest over the
momentum space, which rules out a proximity effect origin and indicates that
the SC order parameter is not tied to the FS as a result of a FS instability. | 1406.0983v2 |
2014-06-04 | Anomalous Nernst and Hall effects in magnetized platinum and palladium | We study the anomalous Nernst effect (ANE) and anomalous Hall effect (AHE) in
proximity-induced ferromagnetic palladium and platinum which is widely used in
spintronics, within the Berry phase formalism based on the relativistic band
structure calculations. We find that both the anomalous Hall ($\sigma_{xy}^A$)
and Nernst ($\alpha_{xy}^A$) conductivities can be related to the spin Hall
conductivity ($\sigma_{xy}^S$) and band exchange-splitting ($\Delta_{ex}$) by
relations $\sigma_{xy}^A =\Delta_{ex}\frac{e}{\hbar}\sigma_{xy}^S(E_F)'$ and
$\alpha_{xy}^A =
-\frac{\pi^2}{3}\frac{k_B^2T\Delta_{ex}}{\hbar}\sigma_{xy}^s(\mu)"$,
respectively. In particular, these relations would predict that the
$\sigma_{xy}^A$ in the magnetized Pt (Pd) would be positive (negative) since
the $\sigma_{xy}^S(E_F)'$ is positive (negative). Furthermore, both
$\sigma_{xy}^A$ and $\alpha_{xy}^A$ are approximately proportional to the
induced spin magnetic moment ($m_s$) because the $\Delta_{ex}$ is a linear
function of $m_s$. Using the reported $m_s$ in the magnetized Pt and Pd, we
predict that the intrinsic anomalous Nernst conductivity (ANC) in the magnetic
platinum and palladium would be gigantic, being up to ten times larger than,
e.g., iron, while the intrinsic anomalous Hall conductivity (AHC) would also be
significant. | 1406.1029v1 |
2014-06-09 | Magnetic and orbital instabilities in a lattice of SU(4) organometallic Kondo complexes | Motivated by experiments of scanning tunneling spectroscopy (STS) on
self-assembled networks of iron(II)-phtalocyanine (FePc) molecules deposited on
a clean Au(111) surface [FePc/Au(111)] and its explanation in terms of the
extension of the impurity SU(4) Anderson model to the lattice in the Kondo
regime, we study the competition between the Kondo effect and the
magneto-orbital interactions occurring in FePc/Au(111). We explore the quantum
phases and critical points of the model using a large-$N$ slave-boson method in
the mean-field approximation. The SU(4) symmetry in the impurity appears as a
combination of the usual spin and an orbital pseudospin arising from the
degenerate $3d_{xz}$ and $3d_{yz}$ orbitals in the Fe atom. In the case of the
lattice, our results show that the additional orbital degrees of freedom
crucially modify the low-temperature phase diagram, and induce new types of
orbital interactions among the Fe atoms, which can potentially stabilize exotic
quantum phases with magnetic and orbital order. The dominant instability
corresponds to spin ferromagnetic and orbital antiferromagnetic order. | 1406.2347v1 |
2014-06-11 | ARPES experiment in fermiology of quasi-2D metals (Review Article) | Angle resolved photoemission spectroscopy (ARPES) enables direct observation
of the Fermi surface and underlying electronic structure of crystals---the
basic concepts to describe all the electronic properties of solids and to
understand the key electronic interactions involved. The method is the most
effective to study quasi-2D metals, to which the subjects of almost all hot
problems in modern condensed matter physics have happened to belong. This has
forced incredibly the development of the ARPES method which we face now. The
aim of this paper is to introduce to the reader the state-of-the-art ARPES,
reviewing the results of its application to such topical problems as high
temperature superconductivity in cuprates and iron based superconductors, and
electronic ordering in the transition metal dichalcogenides and manganites. | 1406.2948v1 |
2014-06-14 | Critical Behavior of the SDW Transition in Underdoped Ba(Fe1-xCox)2As2 (x <=0.05): 75As NMR Investigation | We investigate the nature of the SDW (Spin Density Wave) transition in the
underdoped regime of an iron-based high Tc superconductor Ba(Fe1-xCox)2As2 by
75As NMR, with primary focus on a composition with x = 0.02 (T_SDW = 99 K).We
demonstrate that critical slowing down toward the three dimensional SDW
transition sets in at the tetragonal to orthorhombic structural phase
transition, Ts = 105 K, suggesting strong interplay between structural
distortion and spin correlations. In the critical regime between Ts and T_SDW,
the dynamical structure factor of electron spins S(q,Wn) measured with the
longitudinal NMR relaxation rate 1/T1 exhibits a divergent behavior obeying a
power law, 1/T1~S(q, Wn)~(T/T_SDW-1)^a with the critical exponent a ~ 0.33. | 1406.3734v1 |
2014-06-16 | Search for Magnetic Monopole using ICAL at INO | Sub-relativistic magnetic monopoles are predicted from the GUT era by theory.
To date there have been no confirmed observations of such exotic particles. The
Iron CALorimeter (ICAL) at India-based Neutrino Observatory (INO) aims to
measure the neutrino oscillation parameters precisely. As it is a tracking
detector there is also the possibility of detecting magnetic monopoles in the
sub-relativistic region. Using ICAL the magnetic monopole event is
characterised by the large time intervals of upto 30 microsec between the
signals in successive layers of the active detectors. The aim of this study is
to identify the sensitivity of ICAL for a particle carrying magnetic charge in
the mass range from 10^{5} to 10^{17} GeV with beta ranging from 10^{-5} to 9 x
10^{-1} for ICAL at INO. A similar study has also been carried out for the ICAL
prototype which will be placed overground. Due to the rock cover of
approximately 1.3 km, ICAL at INO will not be able to place bounds on the flux
of the lower mass magnetic monopoles. This mass region is however addressed by
the prototype ICAL. | 1406.3938v1 |
2014-06-16 | An air-cooled Litz wire coil for measuring the high frequency hysteresis loops of magnetic samples : a useful setup for magnetic hyperthermia applications | A low-cost and simple setup for measuring the high-frequency hysteresis loops
of magnetic samples is described. An AMF in the range 6-100 kHz with amplitude
up to 80 mT is produced by a Litz wire coil. The latter is air-cooled using a
forced-air approach so no water flow is required to run the setup.
High-frequency hysteresis loops are measured using a system of pick-up coils
and numerical integration of signals. Reproducible measurements are obtained in
the frequency range of 6-56 kHz. Measurement examples on ferrite cylinders and
on iron oxide nanoparticle ferrofluids are shown. Comparison with other
measurement methods of the hysteresis loop area (complex susceptibility,
quasi-static hysteresis loops and calorific measurements) is provided and shows
the coherency of the results obtained with this setup. This setup is well
adapted to the magnetic characterization of colloidal solutions of MNPs for
magnetic hyperthermia applications. | 1406.4013v1 |
2014-06-16 | Infrared dichroism of gold nanorods controlled using a magnetically addressable mesophase | Gold nanorods have unique optical properties, which make them promising
candidates for building nano-structured materials using a "bottom-up" strategy.
We formulate stable bulk materials with anisotropic optical properties by
inserting gold and iron oxide nanorods within a lamellar mesophase.
Quantitative measurements of the order parameter by modelling the absorbance
spectra show that the medium is macroscopically aligned in a direction defined
by an external magnetic field. Under field, the system exhibits significant
absorption dichroism in the infrared range, at the position of the longitudinal
plasmon peak of the gold nanorods (about 1200 nm), indicating strong
confinement of these particles within the water layers of the lamellar phase.
This approach can yield soft and addressable optical elements. | 1406.4014v1 |
2014-06-21 | The Deaths of Very Massive Stars | The theory underlying the evolution and death of stars heavier than 10 Msun
on the main sequence is reviewed with an emphasis upon stars much heavier than
30 Msun. These are stars that, in the absence of substantial mass loss, are
expected to either produce black holes when they die, or, for helium cores
heavier than about 35 Msun, encounter the pair instability. A wide variety of
outcomes is possible depending upon the initial composition of the star, its
rotation rate, and the physics used to model its evolution. These heavier stars
can produce some of the brightest supernovae in the universe, but also some of
the faintest. They can make gamma-ray bursts or collapse without a whimper.
Their nucleosynthesis can range from just CNO to a broad range of elements up
to the iron group. Though rare nowadays, they probably played a
disproportionate role in shaping the evolution of the universe following the
formation of its first stars. | 1406.5657v1 |
2014-06-22 | Variation of the X-ray non-thermal emission in the Arches cloud | The origin of the iron fluorescent line at 6.4 keV from an extended region
surrounding the Arches cluster is debated and the non-variability of this
emission up to 2009 has favored the low-energy cosmic-ray origin over a
possible irradiation by hard X-rays. By probing the variability of the Arches
cloud non-thermal emission in the most recent years, including a deep
observation in 2012, we intend to discriminate between the two competing
scenarios. We perform a spectral fit of XMM-Newton observations collected from
2000 to 2013 in order to build the Arches cloud lightcurve corresponding to
both the neutral Fe Kalpha line and the X-ray continuum emissions. We reveal a
30% flux drop in 2012, detected with more than 4 sigma significance for both
components. This implies that a large fraction of the studied non-thermal
emission is due to the reflection of an X-ray transient source. | 1406.5727v1 |
2014-06-22 | Putnam looks at quantum mechanics (again and again) | Hilary Putnam (1965, 2005) has argued that from a realist perspective,
quantum mechanics stands in need of an interpretation. Ironically, this
hypothesis may appear vulnerable against arguments drawing on Putnam's own
work. Nancy Cartwright (2005) has urged that his 1962 essay on the meaning of
theoretical terms suggests that quantum mechanics needs no interpretation and
thus stands in tension with his claim of three years later. She furthermore
contends that this conflict should be resolved in favour of the earlier work,
as quantum mechanics, like all successful theories, does not need an
interpretation. The first part of this essay deflates both of these objections.
The second part addresses and evaluates Putnam's own assessments of the main
interpretative options available in 1965 and 2005. Although we may disagree on
some aspects, his pessimistic conclusion will come out largely unscathed, and,
in fact, enhanced. I will close by briefly stating the historical relevance of
this work. | 1406.5737v1 |
2014-06-23 | Phonon Anomalies, Orbital-Ordering and Electronic Raman Scattering in iron-pnictide Ca(Fe0.97Co0.03)2As2: Temperature-dependent Raman Study | We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a
wide spectral range of 120-5200 cm-1 from 5K to 300K, covering the tetragonal
to orthorhombic structural transition as well as magnetic transition at Tsm ~
160K. The mode frequencies of two first-order Raman modes B1g and Eg, both
involving displacement of Fe atoms, show sharp increase below Tsm.
Concomitantly, the linewidths of all the first-order Raman modes show anomalous
broadening below Tsm, attributed to strong spin-phonon coupling. The high
frequency modes observed between 400-1200 cm-1 are attributed to the electronic
Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The
splitting between xz and yz d-orbital levels is shown to be ~ 25 meV which
increases as temperature decreases below Tsm. A broad Raman band observed at ~
3200 cm-1 is assigned to two-magnon excitation of the itinerant Fe 3d
antiferromagnet. | 1406.5844v1 |
2014-06-30 | Interplay of magnetism and superconductivity in EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ single crystals probed by muon spin rotation and ${}^{57}$Fe Mössbauer spectroscopy | We present our results of a local probe study on
EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ single crystals with $x$=0.13, 0.19 and
0.28 by means of muon spin rotation and ${}^{57}$Fe M\"ossbauer spectroscopy.
We focus our discussion on the sample with $x$=0.19 viz. at the optimal
substitution level, where bulk superconductivity ($T_{\text{SC}}=28$ K) sets in
above static europium order ($T^{\text{Eu}}=20$K) but well below the onset of
the iron antiferromagnetic (AFM) transition ($\sim$100 K). We find enhanced
spin dynamics in the Fe sublattice closely above $T_{\text{SC}}$ and propose
that these are related to enhanced Eu fluctuations due to the evident coupling
of both sublattices observed in our experiments. | 1406.7715v1 |
2014-06-30 | Theoretical Analysis of Bayesian Optimisation with Unknown Gaussian Process Hyper-Parameters | Bayesian optimisation has gained great popularity as a tool for optimising
the parameters of machine learning algorithms and models. Somewhat ironically,
setting up the hyper-parameters of Bayesian optimisation methods is notoriously
hard. While reasonable practical solutions have been advanced, they can often
fail to find the best optima. Surprisingly, there is little theoretical
analysis of this crucial problem in the literature. To address this, we derive
a cumulative regret bound for Bayesian optimisation with Gaussian processes and
unknown kernel hyper-parameters in the stochastic setting. The bound, which
applies to the expected improvement acquisition function and sub-Gaussian
observation noise, provides us with guidelines on how to design hyper-parameter
estimation methods. A simple simulation demonstrates the importance of
following these guidelines. | 1406.7758v1 |
2014-07-01 | Emergent defect states as a source of resistivity anisotropy in the nematic phase of iron pnictides | We consider the role of potential scatterers in the nematic phase of Fe-based
superconductors above the transition temperature to the (pi,0) magnetic state
but below the orthorhombic structural transition. The anisotropic spin
fluctuations in this region can be frozen by disorder, to create elongated
magnetic droplets whose anisotropy grows as the magnetic transition is
approached. Such states act as strong anisotropic defect potentials which
scatter with much higher probability perpendicular to their length than
parallel, although the actual crystal symmetry breaking is tiny. We calculate
the scattering potentials, relaxation rates, and conductivity in this region,
and show that such emergent defect states are essential for the transport
anisotropy observed in experiments. | 1407.0117v1 |
2014-07-01 | Anomalous ideal tensile strength of ferromagnetic Fe and Fe-rich alloys | Within the same failure mode, iron has the lowest ideal tensile strength
among the transition metals crystallizing in the body-centered cubic structure.
Here, we demonstrate that this anomalously low strength of Fe originates partly
from magnetism and is reflected in unexpected alloying effects in dilute
Fe(\emph{M}) (\emph{M} = Al, V, Cr, Mn, Co, Ni) binaries. We employ the
structural energy difference and the magnetic pressure to disentangle the
magnetic effect on the ideal tensile strength from the chemical effect. We find
that the investigated solutes strongly alter the magnetic response of the Fe
host from the weak towards a stronger ferromagnetic behavior, which is
explained based on single-particle band energies. | 1407.0289v1 |
2014-07-03 | The electronic structure of Co-substituted $\textrm{Fe}\textrm{Se}$ superconductor probed by soft X-ray spectroscopy and density functional theory | We study the crystalline and electronic properties of the
$\textrm{Fe}_{1-x}\textrm{Co}_x\textrm{Se}$ system ($x=0$, 0.25, 0.5, 0.75, and
1.0) using X-ray diffraction, X-ray spectroscopy and density functional theory.
We show that the introduction of Co $3d$ states in FeSe relaxes the bond
strengths and induces a structural transition from tetragonal to hexagonal
whose crossover takes place at $x\approx0.38$. This structural transition in
turn modifies the magnetic order which can be related to the spin state. Using
resonant inelastic X-ray spectroscopy we estimate the spin state of the system;
FeSe is found to be in a high spin state (S=2), but Fe is reduced to a low spin
state upon Co substitution of $x \le 0.25$, well below the structural
transition. Finally, we show evidence that FeSe is a moderately correlated
system but the introduction of Co into the host lattice weakens the correlation
strength for $x\ge0.25$. These novel findings are important to unravel the
mechanisms responsible for the superconducting state in iron-chalcogenide
superconductors. | 1407.0746v1 |
2014-07-03 | A Tale of Three Galaxies: Anomalous Dust Properties in IRASF10398+1455, IRASF21013-0739 and SDSSJ0808+3948 | On a galactic scale the 9.7um silicate emission is usually only seen in type
1 active galactic nuclei (AGNs). They usually also display a flat emission
continuum at ~5--8um and the absence of polycyclic aromatic hydrocarbon (PAH)
emission bands. In contrast, starburst galaxies, luminous infrared (IR)
galaxies (LIRGs), and ultraluminous IR galaxies (ULIRGs) exhibit a red 5--8um
emission continuum, strong 10um and 18um silicate absorption features, and
strong PAH emission bands. Here we report the detection of anomalous dust
properties by Spitzer/Infrared Spectrograph in three galaxies (IRASF10398+1455,
IRASF21013-0739 and SDSSJ0808+3948) which are characterized by the simultaneous
detection of a red 5--8um emission continuum, the 9.7 and 18um silicate
emission features as well as strong PAH emission bands. These apparently
contradictory dust IR emission properties are discussed in terms of iron-poor
silicate composition, carbon dust deficit, small grain size and low dust
temperature in the young AGN phase of these three galaxies. | 1407.0914v3 |
2014-07-03 | Superconductivity by rare earth doping in the 1038-type compounds (Ca$_{1-x}$RE$_x$)$_{10}$(FeAs)$_{10}$(Pt$_3$As$_8$) with RE = Y, La-Nd, Sm-Lu | We report superconductivity in polycrystalline samples of the 1038-type
compounds (Ca$_{1-x}$RE$_x$)$_{10}$(FeAs)$_{10}$(Pt$_3$As$_8$) up to T$_c$ = 35
K with RE = Y, La-Nd, Sm, Gd-Lu. The critical temperatures are independent of
the trivalent rare earth element used, yielding an universal T$_c$($x$) phase
diagram for electron doping in all these systems. The absence of
superconductivity in Eu$^{2+}$ doped samples, as well as the close resemblance
of (Ca$_{1-x}$RE$_x$)$_{10}$(FeAs)$_{10}$(Pt$_3$As$_8$) to the 1048 compound
substantiate that the electron doping scenario in the RE-1038 and 1048 phases
is completely analogous to other iron-based superconductors with simpler
crystal structures. | 1407.1007v1 |
2014-07-05 | Self-organized electronic superlattices in layered materials | We show that in layered systems with electronic phase separation tendency,
the long-range Coulomb interaction can drive the spontaneous formation of
unidirectional superlattices of electronic charge in a completely homogeneous
crystalline background. In this self-organized electronic heterostructure, the
ratio among the number of crystalline planes in the minority and majority
electronic phases corresponds to Farey fractions with the superlattice period
controlled by the background charge density and the frustrating Coulomb
interaction strength. The phase diagram displays Arnold tongues obeying a
modified Farey tree hierarchy and a devil's staircase typical of systems with
frustration among different scales. We further discuss the competition of these
electronic superlattices, recently observed in iron-based superconductors and
mixed valence compounds, with in-plane electronically modulated phases. | 1407.1378v1 |
2014-07-10 | NMR investigation of spin correlations in BaCo2As2 | We use NMR techniques to investigate the magnetic properties of BaCo2As2
single crystals, the non-superconducting end member of the Co-substituted
iron-pnictide high Tc superconductor Ba(Fe1-xCox)2As2 with x = 1. We present
75As NMR evidence for enhancement of low frequency spin fluctuations below ~
100 K. This enhancement is accompanied by that of static uniform spin
susceptibility at the wave vector q = 0, suggesting that the primary channel of
the spin correlations is ferromagnetic rather than antiferromagnetic.
Comparison between the NMR Knight shift 75K and bulk susceptibility chi(bulk)
data uncovers the presence of two separate components of spin susceptibility
with distinct temperature dependences, presumably because multiple electronic
bands crossing the Fermi energy play different roles in the electronic
properties of BaCo2As2. | 1407.2750v1 |
2014-07-13 | Accurate bare susceptibilities from full-potential $\textit{ab initio}$ calculations | Electronic susceptibilities are a very popular tool to study electronic and
magnetic properties of materials, both in experiment and theory. Unfortunately,
the numerical evaluation of even the bare susceptibility, which depends on the
computation of matrix elements and sums over energy bands, is very
work-intensive and therefore various approximations have been introduced to
speed up the calculations. We present a reliable and efficient implementation
to compute static as well as dynamic bare susceptibilities based on
full-potential density functional theory (DFT) calculations. Based on the exact
results we will assess the accuracy of replacing the matrix elements with a
constant and the impact of truncating the sum over the energy bands. Results
will be given for representative and topical materials, such as Cr, a classical
transition metal, as well as for FeSe and LaFeAsO, examples of iron-based
superconductors. | 1407.3444v2 |
2014-07-14 | Observation of pseudogap-like feature above Tc in LiFeAs and (Ba0.6K0.4)Fe2As2 by ultrafast optical measurement | We utilize ultrafast optical measurement to study the quasiparticle
relaxation in stoichiometric LiFeAs and nearly optimally doped (BaK)Fe2As2
crystals. According to our temperature-dependent studies of LiFeAs, we have
observed pseudogap-like feature at onset temperature of ~ 55 K, which is above
Tc = 15 K. In addition, the onset temperature of pseudogap ~90K was also
observed in Ba0.6K0.4Fe2As2 (Tc = 36 K). Our findings seem implying that the
pseudogap feature, which is due to antiferromagnetic fluctuations, is universal
for the largely studied 11, 111, 122, and 1111 iron-based superconductors. | 1407.3539v1 |
2014-07-16 | The Effect of Doppler Broadening on the $6.3 \ PeV$ $W^-$ Resonance in $\barν_e e^-$ Collisions | We calculate the Doppler broadening of the $W^-$ resonance produced in
$\bar{\nu}_e e^-$ collisions of cosmic anti-neutrinos with $E_{\nu}\approx 6.3
\ PeV$ with electrons in atoms up to Iron. Revisiting this issue is prompted by
recent observations of PeV neutrinos by Ice-Cube. Despite its poor energy
resolution, the $20\%$ Doppler broadening of the resonance due to electronic
motions can produce observable effects via non-linear neutrino absorption near
the resonance. The attendant suppression of the peak cross section allows
$\bar{\nu}_e$ to travel correspondingly longer distances. While this effect is
unlikely to be directly detected in the near future, it may facilitate
terrestrial tomography at depths of $\sim 10 \ km$, complementing deeper
explorations using the more frequent nuclear interactions at lower energies. | 1407.4415v1 |
2014-07-17 | Spatial ordering of nano-dislocation loops in ion-irradiated materials | Defect microstructures formed in ion-irradiated metals, for example iron or
tungsten, often exhibit patterns of spatially ordered nano-scale dislocation
loops. We show that such ordered dislocation loop structures may form
spontaneously as a result of Brownian motion of loops, biased by the
angular-dependent elastic interaction between the loops. Patterns of spatially
ordered loops form once the local density of loops produced by ion irradiation
exceeds a critical threshold value. | 1407.4683v1 |
2014-07-18 | Estimating R-Process Yields from Abundances of the Metal-Poor Stars | The chemical abundances of metal-poor stars provide important clues to
explore stellar formation history and set significant constraints on models of
the r-process. In this work, we find that the abundance patterns of the light
and iron group elements of the main r-process stars are very close to those of
the weak r-process stars. Based on a detailed abundance comparison, we find
that the weak r-process occurs in supernovae with a progenitor mass range of
$\sim11-26M_{\odot}$. Using the SN yields given by Heger & Woosley and the
abundances of the weak r-process stars, the weak r-process yields are derived.
The SNe with a progenitor mass range of $15M_{\odot}<M<26M_{\odot}$ are the
main sites of the weak r-process and their contributions are larger than 80%.
Using the abundance ratios of the weak r-process and the main r-process in the
solar system, the average yields of the main r-process are estimated. The
observed correlations of the [neutron-capture/Eu] versus [Eu/Fe] can be
explained by mixing of the two r-process abundances in various fractions. | 1407.4878v1 |
2014-07-22 | Orbital Selective Mott Transition Induced by Orbitals with Distinct Noninteracting Densities of States | By applying dynamical mean-field theory in combination with exact
diagonalization at zero temperature to a half-filled Hubbard model with two
orbitals having distinct noninteracting densities of states, we show that an
orbital selective Mott transition (OSMT) will take place even without crystal
field splitting, differences in bandwidth and orbital degeneracy. We find that
formation of local spin triplet states followed by a two-stage breakdown of the
Kondo effect, rather than decoupling of charge degrees of freedom among
different orbitals, is the underlying physics for the OSMT. The relevance of
our findings to Ca$_{2-x}$Sr$_x$RuO$_4$ and the iron-based superconductors is
discussed, and a decent candidate to detect such an origin for the OSMT is
proposed. | 1407.5795v1 |
2014-07-31 | Improving the Scalability of DPWS-Based Networked Infrastructures | The Devices Profile for Web Services (DPWS) specification enables seamless
discovery, configuration, and interoperability of networked devices in various
settings, ranging from home automation and multimedia to manufacturing
equipment and data centers. Unfortunately, the sheer simplicity of event
notification mechanisms that makes it fit for resource-constrained devices,
makes it hard to scale to large infrastructures with more stringent
dependability requirements, ironically, where self-configuration would be most
useful. In this report, we address this challenge with a proposal to integrate
gossip-based dissemination in DPWS, thus maintaining compatibility with
original assumptions of the specification, and avoiding a centralized
configuration server or custom black-box middleware components. In detail, we
show how our approach provides an evolutionary and non-intrusive solution to
the scalability limitations of DPWS and experimentally evaluate it with an
implementation based on the the Web Services for Devices (WS4D) Java Multi
Edition DPWS Stack (JMEDS). | 1407.8546v1 |