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2010-01-19
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High-Resolution Spectroscopy of Extremely Metal-Poor Stars in the Least Evolved Galaxies: Leo IV
|
We present high-resolution Magellan/MIKE spectroscopy of the brightest star
in the ultra-faint dwarf galaxy Leo IV. We measure an iron abundance of [Fe/H]
= -3.2, adding to the rapidly growing sample of extremely metal-poor stars
being identified in Milky Way satellite galaxies. The star is enhanced in the
alpha elements Mg, Ca, and Ti by ~0.3 dex, very similar to the typical Milky
Way halo abundance pattern. All of the light and iron-peak elements follow the
trends established by extremely metal-poor halo stars, but the neutron-capture
elements Ba and Sr are significantly underabundant. These results are quite
similar to those found for stars in the ultra-faint dwarfs Ursa Major II, Coma
Berenices, Bootes I, and Hercules, suggesting that the chemical evolution of
the lowest luminosity galaxies may be universal. The abundance pattern we
observe is consistent with predictions for nucleosynthesis from a Population
III supernova explosion. The extremely low metallicity of this star also
supports the idea that a significant fraction (>10%) of the stars in the
faintest dwarfs have metallicities below [Fe/H] = -3.0.
|
1001.3137v2
|
2010-02-16
|
Electron-doping evolution of the low-energy spin excitations in the iron arsenide BaFe$_{2-x}$Ni$_{x}$As$_{2}$ superconductors
|
We use elastic and inelastic neutron scattering to systematically investigate
the evolution of the low-energy spin excitations of the iron arsenide
superconductor BaFe2-xNixAs2 as a function of nickel doping x. In the undoped
state, BaFe2As2 exhibits a tetragonal-to-orthorhombic structural phase
transition and simultaneously develops a collinear antiferromagnetic (AF) order
below TN = 143 K. Upon electron-doping of x = 0.075 to induce bulk
superconductivity with Tc = 12.3 K, the AF ordering temperature reduces to TN =
58 K.We show that the appearance of bulk superconductivity in
BaFe1.925Ni0.075As2 coincides with a dispersive neutron spin resonance in the
spin excitation spectra, and a reduction in the static ordered moment. For
optimally doped BaFe1.9Ni0.1As2 (Tc = 20 K) and overdoped BaFe1.85Ni0.15As2 (Tc
= 15 K) superconductors, the static AF long-range order is completely
suppressed and the spin excitation spectra are dominated by a resonance and
spin-gap at lower energies. We determine the electron-doping dependence of the
neutron spin resonance and spin gap energies, and demonstrate that the
three-dimensional nature of the resonance survives into the overdoped regime.
If spin excitations are important for superconductivity, these results would
suggest that the three-dimensional character of the electronic superconducting
gaps are prevalent throughout the phase diagram, and may be critical for
superconductivity in these materials.
|
1002.3133v2
|
2010-04-07
|
Iron oxide doped boron nitride nanotubes: structural and magnetic properties
|
A first-principles formalism is employed to investigate the interaction of
iron oxide (FeO) with a boron nitride (BN) nanotube. The stable structure of
the FeO-nanotube has Fe atoms binding N atoms, with bond length of roughly
$\sim$2.1 \AA, and binding between O and B atoms, with bond length of 1.55 \AA.
In case of small FeO concentrations, the total magnetic moment is
(4$\mu_{Bohr}$) times the number of Fe atoms in the unit cell and it is
energetically favorable to FeO units to aggregate rather than randomly bind to
the tube. As a larger FeO concentration case, we study a BN nanotube fully
covered by a single layer of FeO. We found that such a structure has square FeO
lattice with Fe-O bond length of 2.11 \AA, similar to that of FeO bulk, and
total magnetic moment of 3.94$\mu_{Bohr}$ per Fe atom. Consistently with
experimental results, the FeO covered nanotube is a semi-half-metal which can
become a half-metal if a small change in the Fermi level is induced. Such a
structure may be important in the spintronics context.
|
1004.1151v2
|
2010-04-20
|
Terzan 5: an alternative interpretation for the split horizontal branch
|
We consider the horizontal branch (HB) of the Globular Cluster Terzan 5,
recently shown to be split into two parts, the fainter one (delta M_K ~ 0.3mag)
having a lower metallicity than the more luminous. Both features show that it
contains at least two stellar populations. The separation in magnitude has been
ascribed to an age difference of ~6 Gyr and interpreted as the result of an
atypical evolutionary history for this cluster. We show that the observed HB
morphology is also consistent with a model in which the bright HB is composed
of second generation stars that are metal enriched and with a helium mass
fraction larger (by delta Y ~ 0.07) than that of first generation stars
populating the fainter part of the HB. Terzan 5 would therefore be anomalous,
compared to most "normal" clusters hosting multiple populations, only because
its second generation is strongly contaminated by supernova ejecta; the
previously proposed prolonged period of star formation, however, is not
required. The iron enrichment of the bright HB can be ascribed either to
contamination from Type Ia supernova ejecta of the low-iron, helium rich,
ejecta of the massive asympotic giant branch stars of the cluster, or to its
mixing with gas, accreting on the cluster from the environment, that has been
subject to fast metal enrichment due to its proximity with the galactic bulge.
The model here proposed requires only a small age difference, of ~100Myr.
|
1004.3426v1
|
2010-04-26
|
A strong and broad iron line in the XMM-Newton spectrum of the new X-ray transient and black-hole candidate XTE J1652-453
|
We observed the new X-ray transient and black-hole candidate XTE J1652-453
simultaneously with XMM-Newton and the Rossi X-ray Timing Explorer (RXTE). The
observation was done during the decay of the 2009 outburst, when XTE J1652-453
was in the hard-intermediate state. The spectrum shows a strong and broad iron
emission line with an equivalent width of ~ 450 eV. The profile is consistent
with that of a line being produced by reflection off the accretion disk,
broadened by relativistic effects close to the black hole. The best-fitting
inner radius of the accretion disk is ~ 4 gravitational radii. Assuming that
the accretion disk is truncated at the radius of the innermost stable circular
orbit, the black hole in XTE J1652-453 has a spin parameter of ~ 0.5. The power
spectrum of the RXTE observation has an additional variability component above
50 Hz, which is typical for the hard-intermediate state. No coherent
quasi-periodic oscillations at low frequency are apparent in the power
spectrum, which may imply that we view the system at a rather low inclination
angle.
|
1004.4442v2
|
2010-04-28
|
Are C-rich ultra iron poor stars also He-rich?
|
(abridged) The three most iron poor stars presently known ([Fe/H] equal to
-5.96, -5.4 and -4.75) are carbon-rich, they are called C-Rich Ultra-Metal Poor
Stars (CRUMPS). The origin of their peculiar surface abundances is not
understood. We propose a synthetic view of the different models so far proposed
to explain the peculiar abundances observed at the surface of the CRUMP stars.
We deduce some expected trends based on nucleosynthetic arguments and look for
signatures allowing to discriminate among models. We discuss the conditions for
having CRUMP stars which are He-rich, i.e. with a mass fraction of helium
greater than 0.30 and up to 0.60. We discuss the chemical composition of stars
made of interstellar medium mixed with wind material of very metal poor massive
stars, with wind plus supernova ejecta and with material extracted from the
envelope of early AGB stars. Rotating and non-rotating models are considered.
CRUMP stars formed from wind material of massive stars mixed with small amounts
of pristine interstellar medium are He-rich (helium mass fraction between 0.30
and 0.60), Li-depleted and present low $^{12}$C/$^{13}$C ratios (inferior to 10
in number). Such He-rich stars, if discovered, would confirm that the most
metal poor CRUMPs formed from essentially pure wind/envelope material. They
would provide the most direct way to probe the nucleosynthetic outputs of the
first generations of stars. We show that rotation is a key ingredient to
explain the abundance patterns of CRUMPS stars.
|
1004.5024v1
|
2010-05-08
|
Non-Resonant X-ray Magnetic Scattering on Rare-Earth Iron Borates RFe$_3$(BO$_3$)$_4$
|
Hard x-ray scattering (HXS) experiments with a photon energy of 100keV were
performed as a function of temperature and applied magnetic field on selected
compounds of the RFe$_3$(BO$_3$)$_4$ family. The results show the presence of
several unexpected diffraction features, in particular non-resonant magnetic
reflections in the magnetically ordered phase, and structural reflections that
violate the diffraction conditions for the low temperature phase $P3_121$ of
the rare-earth iron borates. The temperature and field dependence of the
magnetic superlattice reflections corroborate the magnetic structures of the
borate compounds obtained by neutron diffraction. The detailed analysis of the
intensity and scattering cross section of the magnetic reflection reveals
details of the magnetic structure of these materials such as the spin domain
structure of NdFe$_3$(BO$_3$)$_4$ and GdFe$_3$(BO$_3$)$_4$. Furthermore we find
that the correlation length of the magnetic domains is around 100 \AA{} for all
the compounds and that the Fe moments are rotated $53^\circ\pm3^\circ$ off from
the hexagonal basal plane in GdFe$_3$(BO$_3$)$_4$
|
1005.1333v2
|
2010-05-13
|
Vacancy ordering and electronic structure of gamma-Fe2O3 (maghemite): a theoretical investigation
|
The crystal structure of the iron oxide gamma-Fe2O3 is usually reported in
either the cubic system (space group P4332) with partial Fe vacancy disorder or
in the tetragonal system (space group P41212) with full site ordering and
c/a\approx 3. Using a supercell of the cubic structure, we obtain the spectrum
of energies of all the ordered configurations which contribute to the partially
disordered P4332 cubic structure. Our results show that the configuration with
space group P41212 is indeed much more stable than the others, and that this
stability arises from a favourable electrostatic contribution, as this
configuration exhibits the maximum possible homogeneity in the distribution of
iron cations and vacancies. Maghemite is therefore expected to be fully ordered
in equilibrium, and deviations from this behaviour should be associated with
metastable growth, extended anti-site defects and surface effects in the case
of small nanoparticles. The confirmation of the ordered tetragonal structure
allows us to investigate the electronic structure of the material using density
functional theory (DFT) calculations. The inclusion of a Hubbard (DFT+U)
correction allows the calculation of a band gap in good agreement with
experiment. The value of the gap is dependent on the electron spin, which is
the basis for the spin-filtering properties of maghemite.
|
1005.2370v1
|
2010-05-13
|
Competing order and nature of the pairing state in the iron pnictides
|
We show that the competition between magnetism and superconductivity can be
used to determine the pairing state in the iron arsenides. To this end we
demonstrate that the itinerant antiferromagnetic phase (AFM) and the
unconventional $s^{+-}$ sign-changing superconducting state (SC) are near the
borderline of microscopic coexistence and macroscopic phase separation,
explaining the experimentally observed competition of both ordered states. In
contrast, conventional $s^{++}$ pairing is not able to coexist with magnetism.
Expanding the microscopic free energy of the system with competing orders
around the multicritical point, we find that static magnetism plays the role of
an intrinsic interband Josephson coupling, making the phase diagram sensitive
to the symmetry of the Cooper pair wavefunction. We relate this result to the
quasiparticle excitation spectrum and to the emergent SO$(5)$ symmetry of
systems with particle-hole symmetry. Our results rely on the assumption that
the same electrons that form the ordered moment contribute to the
superconducting condensate and that the system is close to particle-hole
symmetry. We also compare the suppression of SC in different regions of the
FeAs phase diagram, showing that while in the underdoped side it is due to the
competition with AFM, in the overdoped side it is related to the disappearance
of pockets from the Fermi surface.
|
1005.2437v3
|
2010-05-18
|
Spectral variability of the peculiar A-type supergiant 3Pup
|
Optical spectra of the peculiar supergiant 3Pup taken in 1997-2008 are used
to analyze the spectral peculiarities and velocity field in its atmosphere. The
profiles of strong FeII lines and of the lines of other iron-group ions have a
specific shape: the wings are raised by emissions, whereas the core is
sharpened by a depression. The latter feature becomes more pronounced with the
increasing line strength, and the increasing wavelength. Line profiles are
variable: the magnitude and sign of the absorption asymmetry, and the
blue-to-red emission intensity ratios vary from one spectrum to another. The
temporal Vr variations are minimal for the forbidden emissions and sharp shell
cores of the absorption features of FeII(42), and other strong lines of
iron-group ions. The average velocity for the above lines can be adopted as the
systemic velocity: Vsys=28.5+/-0.5km/s. The weakest photospheric absorptions
and photospheric MgII, SiII absorptions exhibit well-defined day-to-day
velocity variations of up to 7km/s. Quantitative spectral classification yields
the spectral type of A2.7+/-0.3 Ib. The equivalent widths and profiles of
Hdelta and Hgamma, and the equivalent width of the OI7774A triplet yield an
absolute magnitude estimate of Mv=-5.5+/-0.3mag, implying the heliocentric
distance of 0.7kpc.
|
1005.3098v1
|
2010-05-18
|
A New Limit on the Distances of Nuclei UHECRs Sources
|
Recent evidence from the Pierre Auger Observatory suggests a transition, at 5
EeV-10EeV in the composition of Ultra High Energy Cosmic Rays (UHECRs), from
protons to heavier nuclei such as iron. I consider here the implications of the
heavier composition on the sources of UHECRs. The smaller magnetic rigidity
implies that nuclei UHECRs are: (i) More easily accelerated (ii) Local, as they
can diffuse only a few Mpc from their sources before disintegrating (iii)
Isotropic, because large deflections in the extra galactic and the galactic
magnetic fields erase the directional information. Uncertainties in the
strength and structure of the the extra galactic magnetic field (EGMF) makes it
difficult to estimate the overall effects. However, with typical reasonable
parameters of a few nG and a coherence distance of a Mpc the distance a nuclei
UHECR above the GZK energy traverses before photodisintegrating is only a few
Mpc. In spite of the significantly weaker limits on the luminosity, Cen A is
the only currently active potential source of nuclei UHECRs within this
distance. The large deflections erases the directional anisotropy expected from
a single source. If indeed the composition of above-GZK-UHECRs is iron and if
the EGMF is not too small then Cen A is the dominant source of observed nuclei
UHECRs above the GZK limit.
|
1005.3311v1
|
2010-05-19
|
Thermodynamic Limits on Magnetodynamos in Rocky Exoplanets
|
To ascertain whether magnetic dynamos operate in rocky exoplanets more
massive or hotter than the Earth, we developed a parametric model of a
differentiated rocky planet and its thermal evolution. Our model reproduces the
established properties of Earth's interior and magnetic field at the present
time. When applied to Venus, assuming that planet lacks plate tectonics and has
a dehydrated mantle with an elevated viscosity, the model shows that the dynamo
shuts down or never operated. Our model predicts that at a fixed planet mass,
dynamo history is sensitive to core size, but not to the initial inventory of
long-lived, heat-producing radionuclides. It predicts that rocky planets larger
than 2.5 Earth masses will not develop inner cores because the
temperature-pressure slope of the iron solidus becomes flatter than that of the
core adiabat. Instead, iron "snow" will condense near or at the top of these
cores, and the net transfer of latent heat upwards will suppress convection and
a dynamo. More massive planets can have anemic dynamos due to core cooling, but
only if they have mobile lids (plate tectonics). The lifetime of these dynamos
is shorter with increasing planet mass but longer with higher surface
temperature. Massive Venus-like planets with stagnant lids and more viscous
mantles will lack dynamos altogether. We identify two alternative sources of
magnetic fields on rocky planets: eddy currents induced in the hot or molten
upper layers of planets on very short period orbits, and dynamos in the ionic
conducting layers of "ocean" planets with ~10% mass in an upper mantle of water
(ice).
|
1005.3523v1
|
2010-05-21
|
Observation of Lattice and Andreev Bound States of Vortices in Ba0.6K0.4Fe2As2 Single Crystals with Scanning Tunneling Microscopy/Spectroscopy
|
For a type-II superconductor, when the applied magnetic field is higher than
the lower critical value Hc1, the magnetic flux will penetrate into the
superconductor and form quantized vortices, which usually are arranged in an
Abrikosov lattice. For the newly discovered iron pnictide superconductors,
previous measurements have shown that, in electron-doped BaFe2As2, the vortices
form a highly disordered structure. In addition, the density of states (DOS)
within the vortex cores1 do not exhibit the Andreev bound states in
conventional superconductors. In this Letter, we report the observation of a
triangular vortex lattice and the Andreev bound states in hole-doped BaFe2As2
by using a low temperature scanning tunneling microscope (STM). Detailed study
of the vortex cores reveals that the spectrum of the Andreev bound states
inside the vortex core exhibits a distinct spatial evolution: at the center of
the vortex core, it appears as a single peak at 0.5 mV below the Fermi-energy;
away from the core center, it gradually evolves into two sub-peaks and they
eventually fade out. The drastic differences between the vortex cores of the
electron-doped and hole-doped counterparts are illusive to the pairing
mechanism of the iron pnictide superconductors.
|
1005.4038v1
|
2010-05-25
|
The pairing state in KFe2As2 studied by measurements of the magnetic vortex lattice
|
Understanding the mechanism and symmetry of electron pairing in iron-based
superconductors represents an important challenge in condensed matter physics
[1-3]. The observation of magnetic flux lines - "vortices" - in a
superconductor can contribute to this issue, because the spatial variation of
magnetic field reflects the pairing. Unlike many other iron pnictides, our
KFe2As2 crystals have very weak vortex pinning, allowing
small-angle-neutron-scattering (SANS) observations of the intrinsic vortex
lattice (VL). We observe nearly isotropic hexagonal packing of vortices,
without VL-symmetry transitions up to high fields along the fourfold c-axis of
the crystals, indicating rather small anisotropy of the superconducting
properties around this axis. This rules out gap nodes parallel to the c-axis,
and thus d-wave and also anisotropic s-wave pairing [2, 3]. The strong
temperature-dependence of the intensity down to T<<Tc indicates either widely
different full gaps on different Fermi surface sheets, or nodal lines
perpendicular to the axis.
|
1005.4468v2
|
2010-06-11
|
Scaling of normal-state transport properties of 1111 iron-pnictide superconductors
|
We show that the zero field normal-state resistivity above Tc for various
levels of electron doping-both for underdoped, optimally, and overdoped
LaO1-xFxFeAs, i.e. (x=0.05-0.075 and 0.1-0.2) and underdoped SmO1-xFxFeAs
(x=0.06- 0.1) members of the 1111-iron-pnictide superconductor family-can be
scaled in a broad temperature range from 20 to 300 K onto a single universal
curve. The scaling was performed using the energy scale Delta, the resistivity
rho(Delta) and the residual resistivity rho0 as scaling parameters as well as
applying a recently proposed model-independent scaling method (H. G. Luo, Y. H.
Su, and T. Xiang, Phys. Rev. B 77, 014529 (2008)). The scaling parameters have
been calculated and the compositional variation of Delta has been determined.
The existence of a universal metallic rho(T) curve is interpreted as an
indication of a single mechanism which dominates the scattering of the charge
carriers in LaO1-xFxFeAs (x= 0.05 to 0.075 and 0.1 to 0.2) and SmO1-xFxFeAs (x
= 0.06 to 0.1) .
|
1006.2350v2
|
2010-07-12
|
Magnetic order in orbital models of the iron pnictides
|
We examine the appearance of the experimentally-observed stripe
spin-density-wave magnetic order in five different orbital models of the iron
pnictide parent compounds. A restricted mean-field ansatz is used to determine
the magnetic phase diagram of each model. Using the random phase approximation,
we then check this phase diagram by evaluating the static spin susceptibility
in the paramagnetic state close to the mean-field phase boundaries. The momenta
for which the susceptibility is peaked indicate in an unbiased way the actual
ordering vector of the nearby mean-field state. The dominant orbitally resolved
contributions to the spin susceptibility are also examined to determine the
origin of the magnetic instability. We find that the observed stripe magnetic
order is possible in four of the models, but it is extremely sensitive to the
degree of the nesting between the electron and hole Fermi pockets. In the more
realistic five-orbital models, this order competes with a strong-coupling
incommensurate state which appears to be controlled by details of the
electronic structure below the Fermi energy. We conclude by discussing the
implications of our work for the origin of the magnetic order in the pnictides.
|
1007.1949v2
|
2010-07-15
|
Spectroscopic survey of M--type asteroids
|
M-type asteroids, as defined in the Tholen taxonomy (Tholen, 1984), are
medium albedo bodies supposed to have a metallic composition and to be the
progenitors both of differentiated iron-nickel meteorites and enstatite
chondrites. We carried out a spectroscopic survey in the visible and near
infrared wavelength range (0.4-2.5 micron) of 30 asteroids chosen from the
population of asteroids initially classified as Tholen M -types, aiming to
investigate their surface composition. The data were obtained during several
observing runs during the years 2004-2007 at the TNG, NTT, and IRTF telescopes.
We computed the spectral slopes in several wavelength ranges for each observed
asteroid, and we searched for diagnostic spectral features. We confirm a large
variety of spectral behaviors for these objects as their spectra are extended
into the near-infrared, including the identification of weak absorption bands,
mainly of the 0.9 micron band tentatively attributed to orthopyroxene, and of
the 0.43 micron band that may be associated to chlorites and Mg-rich
serpentines or pyroxene minerals such us pigeonite or augite. A comparison with
previously published data indicates that the surfaces of several asteroids
belonging to the M-class may vary significantly. We attempt to constrain the
asteroid surface compositions of our sample by looking for meteorite spectral
analogues in the RELAB database and by modelling with geographical mixtures of
selected meteorites/minerals. We confirm that iron meteorites, pallasites, and
enstatite chondrites are the best matches to most objects in our sample, as
suggested for M-type asteroids. The presence of subtle absorption features on
several asteroids confirms that not all objects defined by the Tholen M-class
have a pure metallic composition.
|
1007.2582v1
|
2010-07-16
|
High-Resolution X-ray Spectroscopy of the Interstellar Medium
|
The interstellar medium (ISM) has a multiphase structure characterized by
gas, dust and molecules. The gas can be found in different charge states:
neutral, low-ionized (warm) and high-ionized (hot). It is possible to probe the
multiphase ISM through the observation of its absorption lines and edges in the
X-ray spectra of background sources. We present a high-quality RGS spectrum of
the low-mass X-ray binary GS 1826-238 with an unprecedent detailed treatment of
the absorption features due to the dust and both the neutral and ionized gas of
the ISM. We constrain the column density ratios within the different phases of
the ISM and measure the abundances of elements such as O, Ne, Fe and Mg. We
found significant deviations from the proto-Solar abundances: oxygen is
over-abundant by a factor 1.23 +/- 0.05, neon 1.75 +/- 0.11, iron 1.37 +/- 0.17
and magnesium 2.45 +/- 0.35. The abundances are consistent with the measured
metallicity gradient in our Galaxy: the ISM appears to be metal-rich in the
inner regions. The spectrum also shows the presence of warm/hot ionized gas.
The gas column has a total ionization degree less than 10%. We also show that
dust plays an important role as expected from the position of GS 1826-238: most
iron appears to be bound in dust grains, while 10-40% of oxygen consists of a
mixture of dust and molecules.
|
1007.2796v1
|
2010-07-16
|
Crystal Symmetry and Magnetic Order in Iron Pnictides: a Tight Binding Wannier Function Analysis
|
To perform a local orbital analysis of electronic and magnetic interactions,
we construct the Wannier functions (WFs) of the Fe $3d$ orbitals in the parent
compound of the recently discovered iron pnictide superconductors, LaFeAsO, and
a comparison material LaFePO. Comparing the WFs for the stripe
antiferromagnetic order with those for no magnetic order, the difference is a
significant spreading (``{\it de}localization'') of specifically the $d_{xy}$
and $d_{xz}$ (but not $d_{yz}$) WFs, where parallel Fe spins lie along the $x$
direction. The WF basis gives a tight-binding representation of the first
principles, density functional based Fe-derived bands. Comparing hopping
parameters, it is found that changes due to stripe antiferromagnetism, even if
it is weak, enables more isotropic hopping involving spin-majority electrons in
the Fe $3d_{xz}$ (but not the $3d_{yz}$) orbital. This change,
counterintuitively, actually reinforces electronic anisotropy. Further insight
is gained by comparing the WFs of LaFeAsO and LaFePO, identifying how the
difference in WFs is related to the difference in hopping integrals and showing
how the pnictide atom is influential in forming the stripe antiferromagnetism.
Kinetic energy considerations suggest that orbital fluctuation, in addition to
spin fluctuation, may contribute to the decrease in observed ordered moment
compared to the calculated values.
|
1007.2878v1
|
2010-07-21
|
Pseudogap of Superconducting correlation origin in iron-pnictide superconductor Ba0.6+δK0.4-δFe2As2
|
Pseudogap (PG), a phenomenon of opening of gap like features above
superconducting (SC) transition temperature (Tc), has been universally observed
in the high-Tc cuprates (HTC) (1,2), some heavy fermion superconductors (3),
and iron pnictides (4-7). Here, we report the observation of PG behavior in
optical conductivity in an underdoped Ba0.6+{\delta}K0.4-{\delta}Fe2As2
(Tconset =36 K) single crystal far above Tc, up to 100 K (~ 3Tc). Unique scale
separation in Ba0.6+{\delta}K0.4-{\delta}Fe2As2 compound - magnetic and SC
correlation energy scales being widely separated - enabled us to establish that
the PG structures observed in the range of 50 cm-1 - 150 cm-1 in optical
conductivity is caused by the SC correlation and the magnetic correlation
origin is ruled out. Theoretical calculations, based on the preformed Cooper
pair model (8), provided an excellent description to the temperature evolution
of the optical conductivity data from below to above Tc.
|
1007.3617v1
|
2010-07-29
|
Dissimilarities between the electronic structure of chemically doped and chemically pressurized iron pnictides from an angle-resolved photoemission spectroscopy study
|
We have studied the electronic structure of EuFe2As2-xPx using high
resolution angle-resolved photoemission spectroscopy. Upon substituting As with
the isovalent P, which leads to a chemical pressure and to superconductivity,
we observe a non-rigid-band like change of the electronic structure along the
center of the Brillouin zone (BZ): an orbital and kz dependent increase or
decrease in the size of the hole pockets near the Gamma - Z line. On the other
hand, the diameter of the Fermi surface cylinders at the BZ corner forming
electron pockets, hardly changes. This is in stark contrast to p and n-type
doped iron pnictides where, on the basis of ARPES experiments, a more
rigid-band like behavior has been proposed. These findings indicate that there
are different ways in which the nesting conditions can be reduced causing the
destabilization of the antiferromagnetic order and the appearance of the
superconducting dome.
|
1007.5205v1
|
2010-07-29
|
Spectral analysis for the iron-based superconductors: Anisotropic spin fluctuations and fully gapped s^{\pm}-wave superconductivity
|
Spin fluctuations are considered to be one of the candidates that drive a
sign-reversed s^{\pm} superconducting state in the iron pnictides. In the
magnetic scenario, whether the spin fluctuation spectrum exhibits certain
unique fine structures is an interesting aspect for theoretical study in order
to understand experimental observations. We investigate the detailed momentum
dependence of the short-range spin fluctuations using a 2-orbital model in the
self-consistent fluctuation exchange approximation and find that a common
feature of those fluctuations that are capable of inducing a fully gapped
s^{\pm} state is the momentum anisotropy with lengthened span along the
direction transverse to the antiferromagnetic momentum transfer. Performing a
qualitative analysis based on the orbital character and the deviation from
perfect nesting of the electronic structure for the 2-orbital and a more
complete 5-orbital model, we gain the insight that this type of anisotropic
spin fluctuations favor superconductivity due to their enhancement of
intra-orbital, but inter-band, pair scattering processes. The momentum
anisotropy leads to elliptically shaped magnetic responses which have been
observed in inelastic neutron scattering measurements. Meanwhile, our detailed
study on the magnetic and the electronic spectrum shows that the dispersion of
the magnetic resonance mode in the nearly isotropic s^{\pm} superconducting
state exhibits anisotropic propagating behavior in an upward pattern and the
coupling of the resonance mode to fermions leads to a dip feature in the
spectral function.
|
1007.5321v3
|
2010-09-22
|
3D MHD simulations of subsurface convection in OB stars
|
During their main sequence evolution, massive stars can develop convective
regions very close to their surface. These regions are caused by an opacity
peak associated with iron ionization. Cantiello et al. (2009) found a possible
connection between the presence of sub-photospheric convective motions and
small scale stochastic velocities in the photosphere of early-type stars. This
supports a physical mechanism where microturbulence is caused by waves that are
triggered by subsurface convection zones. They further suggest that clumping in
the inner parts of the winds of OB stars could be related to subsurface
convection, and that the convective layers may also be responsible for
stochastic excitation of non-radial pulsations. Furthermore, magnetic fields
produced in the iron convection zone could appear at the surface of such
massive stars. Therefore subsurface convection could be responsible for the
occurrence of observable phenomena such as line profile variability and
discrete absorption components. These phenomena have been observed for decades,
but still evade a clear theoretical explanation. Here we present preliminary
results from 3D MHD simulations of such subsurface convection.
|
1009.4462v1
|
2010-10-05
|
Renormalization group flow, competing phases, and the structure of superconducting gap in multi-band models of Iron based superconductors
|
We perform an analytical renormalization group (RG) study to address the role
of Coulomb repulsion, the competition between extended s-wave superconducting
order (s+-) and the spin-density wave (SDW) order and the angular dependence of
the superconducting gap in multi-pocket models of Iron based superconductors.
Previous analytic RG studies considered a toy model of one hole and one
electron pocket. We consider more realistic models of two electron pockets and
either two or three hole pockets, and also incorporate the angular dependence
of interaction. We neglect for simplicity one of the two hole pockets centered
at k=0, which is less nested with electron pockets, i.e., consider 3-pocket and
4-pocket models. In a toy 2-pocket model, SDW order always wins over s+- order
at perfect nesting, and s+- order only appears at a finite doping, and only if
RG flow extends long enough to overcome intra-pocket Coulomb repulsion. For
multi-pocket models, we find two new effects. First, there always exists an
attractive component of the interaction in s+- channel, such that the system
necessary becomes a superconductor once it overcomes the competition from the
SDW state. Second, in 3-pocket case (but not in 4-pocket case), there are
situations when s+- order wins over SDW order even for perfect nesting,
suggesting that SDW order is not a necessary pre-condition for the s+- order.
Our results are in good agreement with recent numerical functional RG studies
by Thomale et al. [arXiv:1002.3599]
|
1010.0984v3
|
2010-10-08
|
Parity-odd multipoles, magnetic charges and chirality in haematite (alfa-Fe2O3)
|
Collinear and canted magnetic motifs in haematite were investigated by
Kokubun et al. (2008) using x-ray Bragg diffraction magnified at the iron
K-edge, and analyses of observations led to various potentially interesting
conclusions. We demonstrate that the reported analyses for both non-resonant
and resonant magnetic diffraction at low energies near the absorption K-edge
are not appropriate. In its place, we apply a radically different formulation,
thoroughly tried and tested, that incorporates all magnetic contributions to
resonant x-ray diffraction allowed by the established chemical and magnetic
structures. Essential to a correct formulation of diffraction by a magnetic
crystal with resonant ions at sites that are not centres of inversion symmetry
are parity-odd atomic multipoles, time-even (polar) and time-odd
(magneto-electric), that arise from enhancement by the electric-dipole (E1) -
electric-quadrupole (E2) event. Analyses of azimuthal-angle scans on two
space-group forbidden reflections, hexagonal (0, 0, 3)h and (0, 0, 9)h,
collected by Kokubun et al. above and below the Morin temperature (TM = 250K),
allow us to obtain good estimates of contributing polar and magneto-electric
multipoles, including the iron anapole. We show, beyond reasonable doubt, that
available data are inconsistent with parity-even events only (E1-E1 and E2-
E2). For future experiments, we show that chiral states of haematite couple to
circular polarization and differentiate E1-E2 and E2-E2 events, while the
collinear motif supports magnetic charges.
|
1010.1686v3
|
2010-11-02
|
Electron transport and anisotropy of the upper critical magnetic field in a Ba0.68K0.32Fe2As2 single crystals
|
Early work on the iron-arsenide compounds supported the view, that a reduced
dimensionality might be a necessary prerequisite for high-Tc superconductivity.
Later, however, it was found that the zero-temperature upper critical magnetic
field, Hc2(0), for the 122 iron pnictides is in fact rather isotropic. Here, we
report measurements of the temperature dependence of the electrical
resistivity, \Gamma(T), in Ba0.5K0.5Fe2As2 and Ba0.68K0.32Fe2As2 single
crystals in zero magnetic field and for Ba0.68K0.32Fe2As2 as well in static and
pulsed magnetic fields up to 60 T. We find that the resistivity of both
compounds in zero field is well described by an exponential term due to
inter-sheet umklapp electron-phonon scattering between light electrons around
the M point to heavy hole sheets at the \Gamma point in reciprocal space. From
our data, we construct an H-T phase diagram for the inter-plane (H || c) and
in-plane (H || ab) directions for Ba0.68K0.32Fe2As2. Contrary to published data
for underdoped 122 FeAs compounds, we find that Hc2(T) is in fact anisotropic
in optimally doped samples down to low temperatures. The anisotropy parameter,
{\gamma} = Habc2/Hcc2, is about 2.2 at Tc. For both field orientations we find
a concave curvature of the Hc2 lines with decreasing anisotropy and saturation
towards lower temperature. Taking into account Pauli spin paramagnetism we
perfectly can describe Hc2(T) and its anisotropy.
|
1011.0698v1
|
2011-01-18
|
Microturbulent velocity from stellar spectra: a comparison between different approaches (Research Note)
|
Context --- The classical method to infer microturbulent velocity in stellar
spectra requires that the abundances of the iron lines are not correlated with
the observed equivalent widths. An alternative method, requiring the use of the
expected line strength, is often used to by-pass the risk of spurious slopes
due to the correlation between the errors in abundance and equivalent width.
Aims --- To compare the two methods and identify pros and cons and
applicability to the typical practical cases. Methods --- I performed a test
with a grid of synthetic spectra, including instrumental broadening and
Poissonian noise. For all these spectra, microturbulent velocity has been
derived by using the two approaches and compared with the original value with
which the synthetic spectra have been generated. Results --- The two methods
provide similar results for spectra with SNR$ > 70, while for lower SNR both
approaches underestimate the true microturbulent velocity, depending of the SNR
and the possible selection of the lines based on the equivalent width errors.
Basically, the values inferred by using the observed equivalent widths better
agree with those of the synthetic spectra. In fact, the method based on the
expected line strength is not totally free from a bias that can heavily affect
the determination of microturbulent velocity. Finally, I recommend to use the
classical approach (based on the observed equivalent widths) to infer this
parameter. In cases of full spectroscopical determination of all the
atmospherical parameters, the difference between the two approaches is reduced,
leading to an absolute difference in the derived iron abundances of less than
0.1 dex.
|
1101.3426v1
|
2011-01-24
|
Distinct Fermi Surface Topology and Nodeless Superconducting Gap in (Tl0.58Rb0.42)Fe1.72Se2 Superconductor
|
High resolution angle-resolved photoemission measurements have been carried
out to study the electronic structure and superconducting gap of the
(Tl$_{0.58}$Rb$_{0.42}$)Fe$_{1.72}$Se$_2$ superconductor with a T$_c$=32 K. The
Fermi surface topology consists of two electron-like Fermi surface sheets
around $\Gamma$ point which is distinct from that in all other iron-based
compounds reported so far. The Fermi surface around the M point shows a nearly
isotropic superconducting gap of $\sim$12 meV. The large Fermi surface near the
$\Gamma$ point also shows a nearly isotropic superconducting gap of $\sim$15
meV while no superconducting gap opening is clearly observed for the inner tiny
Fermi surface. Our observed new Fermi surface topology and its associated
superconducting gap will provide key insights and constraints in understanding
superconductivity mechanism in the iron-based superconductors.
|
1101.4556v1
|
2011-02-10
|
An effective quantum parameter for strongly correlated metallic ferromagnets
|
The correlated motion of electrons in multi-orbital metallic ferromagnets is
investigated in terms of a realistic Hubbard model with {\cal N}-fold orbital
degeneracy and arbitrary intra- and inter-orbital Coulomb interactions U and J
using a Goldstone-mode-preserving non-perturbative scheme. An effective quantum
parameter '\hbar'=\frac{U^2+({\cal N}-1)J^2}{(U+({\cal N}-1)J)^2} is obtained
which determines, in analogy with 1/S for quantum spin systems and 1/N for the
N-orbital Hubbard model, the strength of correlation-induced quantum
corrections to magnetic excitations. The rapid suppression of this quantum
parameter with Hund's coupling J, especially for large {\cal N}, provides
fundamental insight into the phenomenon of strong stabilization of metallic
ferromagnetism by orbital degeneracy and Hund's coupling. This approach is
illustrated for the case of ferromagnetic iron and the half metallic Heusler
alloy Co_2 Mn Si. For realistic values for iron, the calculated spin stiffness
and Curie temperature values obtained are in quantitative agreement with
measurements. Significantly, the contribution of long wavelength modes is shown
to yield a nearly ~25% reduction in the calculated Curie temperature. Finally,
an outline is presented for extending the approach to generic multi-band
metallic ferromagnets including realistic band-structure features of
non-degenerate orbitals and inter-orbital hopping as obtained from LDA
calculations.
|
1102.2115v1
|
2011-03-05
|
Physics and measurements of magnetic materials
|
Magnetic materials, both hard and soft, are used extensively in several
components of particle accelerators. Magnetically soft iron-nickel alloys are
used as shields for the vacuum chambers of accelerator injection and extraction
septa; Fe-based material is widely employed for cores of accelerator and
experiment magnets; soft spinel ferrites are used in collimators to damp
trapped modes; innovative materials such as amorphous or nanocrystalline core
materials are envisaged in transformers for high-frequency polyphase resonant
convertors for application to the International Linear Collider (ILC). In the
field of fusion, for induction cores of the linac of heavy-ion inertial fusion
energy accelerators, based on induction accelerators requiring some 107 kg of
magnetic materials, nanocrystalline materials would show the best performance
in terms of core losses for magnetization rates as high as 105 T/s to 107 T/s.
After a review of the magnetic properties of materials and the different types
of magnetic behaviour, this paper deals with metallurgical aspects of
magnetism. The influence of the metallurgy and metalworking processes of
materials on their microstructure and magnetic properties is studied for
different categories of soft magnetic materials relevant for accelerator
technology. Their metallurgy is extensively treated. Innovative materials such
as iron powder core materials, amorphous and nanocrystalline materials are also
studied. A section considers the measurement, both destructive and
non-destructive, of magnetic properties. Finally, a section discusses magnetic
lag effects.
|
1103.1069v1
|
2011-03-22
|
Dipolar Interactions between Iron-Oxide Nanoparticles in Frozen Ferrofluids and Ferronematics
|
We present a detailed study of the magnetic behavior of iron-oxide
(gamma-Fe2O3 and Fe3O4) nanoparticles constituents of ferrofluids (FF's) with
average particle sizes <d> = 2.5 and 10 nm. The particles were dispersed in the
frozen liquid carrier (pure FF) and in a frozen lyotropic liquid crystalline
matrix in the nematic phase or ferronematic (FN) (ferrolyomesophase). Both FF
and FN phases displayed superparamagnetic (SPM) behaviour at room temperature,
with blocking temperatures T_B ~ 10 and 100 K for <d> = 2.5 and 10 nm,
respectively. Dynamic ac susceptibility measurements showed a thermally
activated N\'eel-Brown dependence of the blocking temperature with applied
frequency. Our results show that dipolar interactions are small, but
non-negligible, as compared to the single-particle energy barriers from
magnetic anisotropy. From the fit of ac susceptibility we calculated the
effective magnetic anisotropy constant K_{eff} for 2.5 nm maghemite particles.
Although interparticle interactions present in highly diluted samples do not
appreciably modify the dynamic magnetic behavior of isolated particles, the
calculated magnetic anisotropy were abut one order of magnitude larger that the
bulk materials, suggesting the existence of large surface anisotropy. Using the
thermally activated model to fit the dynamic data yielded effective energy
barriers Ea = 3.5x10^{-21} J. From these data, we obtained K_{eff} = 422 kJ/m^3
for the single-particle effective magnetic anisotropy.
|
1103.4403v1
|
2011-04-04
|
X-ray Spectroscopy of MXB 1728-34 with XMM-Newton
|
We have analysed an XMM-Newton observation of the low mass X-ray binary and
atoll source MXB 1728-34. The source was in a low luminosity state during the
XMM-Newton observation, corresponding to a bolometric X-ray luminosity of
5*10E36 d^2 erg/s, where d is the distance in units of 5.1 kpc. The 1-11 keV
X-ray spectrum of the source, obtained combining data from all the five
instruments on-board XMM-Newton, is well fitted by a Comptonized continuum.
Evident residuals are present at 6-7 keV which are ascribed to the presence of
a broad iron emission line. This feature can be equally well fitted by a
relativistically smeared line or by a self-consistent, relativistically
smeared, reflection model. Under the hypothesis that the iron line is produced
by reflection from the inner accretion disk, we can infer important information
on the physical parameters of the system, such as the inner disk radius, Rin =
25-100 km, and the inclination of the system, 44{\deg} < i < 60{\deg}.
|
1104.0566v1
|
2011-04-22
|
Non-LTE line formation for Fe I and Fe II in the atmospheres of A-F type stars
|
Non-local thermodynamical equilibrium (non-LTE) line formation for neutral
and singly-ionized iron is considered through a range of spectral types when
the effective temperature varies from 6500 K up to 8500 K, the gravity from log
g = 4 down to log g = 3, and the metallicity is solar one. The non-LTE
calculations were performed with a comprehensive model atom for iron which was
treated in our earlier paper (Mashonkina et al. 2011, A&A, 528, A87). The
departures from LTE lead to systematically depleted total absorption in the Fe
I lines and positive abundance corrections, in qualitative agreement with the
Rentzsch-Holm (1996, A&A, 312, 966) study. However, our predicted magnitude of
the non-LTE effects is significantly smaller compared to the previous results
due to the use of rather complete model atom of Fe I. The non-LTE abundance
corrections do not exceed 0.1 dex for the dwarf models and 0.20 dex for the
giant ones. Non-LTE leads to strengthening the Fe II lines, however, the effect
is small, such that the abundance correction is at the level of -0.01 to -0.03
dex over the whole range of stellar parameters being considered. No firm
conclusion can be drawn with respect to whether or not the Fe I/Fe II
ionization equilibrium is fulfilled in the atmosphere of the Sun and Procyon
due to uncertainty in available gf-values for visible lines of Fe I and Fe II.
|
1104.4403v1
|
2011-05-11
|
Doping-induced vertical line nodes in the superconducting gap of the iron arsenide K-Ba122 from directional thermal conductivity
|
The thermal conductivity k of the iron-arsenide superconductor K-Ba122 was
measured down to 50 mK in a magnetic field up to 15 T, for a heat current
parallel and perpendicular to the tetragonal c axis. In the range from optimal
doping (x ~ 0.4) down to x = 0.16, there is no residual linear term in k(T) at
T = 0, showing that there are no nodes in the superconducting gap anywhere on
the Fermi surface. Upon crossing below x = 0.16, a large residual linear term
suddenly appears, signaling the onset of nodes in the superconducting gap, most
likely vertical line nodes running along the c axis. We discuss two scenarios:
1) accidental nodes in an s-wave gap, resulting from a strong modulation of the
gap around the Fermi surface, in which minima deepen rapidly with underdoping;
2) a phase transition from a nodeless s-wave state to a d-wave state, in which
nodes are imposed by symmetry.
|
1105.2232v2
|
2011-05-21
|
Specific Heat Discontinuity, deltaC, at Tc in BaFe2(As0.7P0.3)2 - Consistent with Unconventional Superconductivity
|
We report the specific heat discontinuity, deltaC/Tc, at Tc = 28.2 K of a
collage of single crystals of BaFe2(As0.7P0.3)2 and compare the measured value
of 38.5 mJ/molK**2 with other iron pnictide and iron chalcogenide (FePn/Ch)
superconductors. This value agrees well with the trend established by Bud'ko,
Ni and Canfield who found that deltaC/Tc ~ a*Tc**2 for 14 examples of doped
Ba1-xKxFe2As2 and BaFe2-xTMxAs2, where the transition metal TM=Co and Ni. We
extend their analysis to include all the FePn/Ch superconductors for which
deltaC/Tc is currently known and find deltaC/Tc ~ a*Tc**1.9 and a=0.083
mJ/molK**4. A comparison with the elemental superconductors with Tc>1 K and
with A-15 superconductors shows that, contrary to the FePn/Ch superconductors,
electron-phonon-coupled conventional superconductors exhibit a significantly
different dependence of deltaC on Tc, namely deltaC/Tc ~ Tc**0.9. However
deltaC/gamma*Tc appears to be comparable in all three classes (FePn/Ch,
elemental and A-15) of superconductors with, e. g., deltaC/gamma*Tc=2.4 for
BaFe2(As0.7P0.3)2. A discussion of the possible implications of these
phenomenological comparisons for the unconventional superconductivity believed
to exist in the FePn/Ch is given.
|
1105.4289v1
|
2011-06-03
|
Iron-based n-type electron-induced ferromagnetic semiconductor
|
Carrier-induced ferromagnetic semiconductors (FMSs) have been intensively
studied for decades as they have novel functionalities that cannot be achieved
with conventional metallic materials. These include the ability to control
magnetism by electrical gating or light irradiation, while fully inheriting the
advantages of semiconductor materials such as band engineering. Prototype FMSs
such as (In,Mn)As or (Ga,Mn)As, however, are always p-type, making it difficult
to be used in real spin devices. This is because manganese (Mn) atoms in those
materials work as local magnetic moments and acceptors that provide holes for
carrier-mediated ferromagnetism. Here we show that by introducing iron (Fe)
into InAs, it is possible to fabricate a new FMS with the ability to control
ferromagnetism by both Fe and independent carrier doping. Despite the general
belief that the tetrahedral Fe-As bonding is antiferromagnetic, we demonstrate
that (In,Fe)As doped with electrons behaves as an n-type electron-induced FMS,
a missing piece of semiconductor spintronics for decades. This achievement
opens the way to realise novel spin-devices such as spin light-emitting diodes
or spin field-effect transistors, as well as helps understand the mechanism of
carrier-mediated ferromagnetism in FMSs.
|
1106.0561v3
|
2011-06-06
|
Correlation of conductivity and angle integrated valence band photoemission characteristics in single crystal iron perovskites for 300 K < T < 800 K: Comparison of surface and bulk sensitive methods
|
A single crystal monolith of La0.9Sr0.1FeO3 and thin pulsed laser deposited
film of La0.8Sr0.2Fe0.8Ni0.2O3 were subject to angle integrated valence band
photoemission spectroscopy in ultra high vacuum and conductivity experiments in
ambient air at temperatures from 300 K to 800 K. Except for several sputtering
and annealing cycles, the specimen were not prepared in-situ.. Peculiar changes
in the temperature dependent, bulk representative conductivity profile as a
result of reversible phase transitions, and irreversible chemical changes are
semi-quantitatively reflected by the intensity variation in the more surface
representative valence band spectra near the Fermi energy. X-ray photoelectron
diffraction images reflect the symmetry as expected from bulk iron perovskites.
The correlation of spectral details in the valence band photoemission spectra
(VB PES) and details of the conductivity during temperature variation suggest
that valuable information on electronic structure and transport properties of
complex materials may be obtained without in-situ preparation.
|
1106.1034v1
|
2011-06-15
|
No X-Ray Excess from the HESS J1741-302 Region except a New Intermediate Polar Candidate
|
With the Suzaku satellite, we observed an unidentified TeV gamma-ray source
HESS J1741$-$302 and its surroundings. No diffuse or point-like X-ray sources
are detected from the bright southern emission peak of HESS J1741$-$302. From
its neighborhood, we found a new intermediate polar candidate at the position
of $(\alpha, \delta)_{\rm J2000.0} = (\timeform{17h40m35.6s},
\timeform{-30D14m16s})$, which is designated as Suzaku J174035.6$-$301416. The
spectrum of Suzaku J174035.6$-$301416 exhibits emission lines at the energy of
6.4, 6.7 and 7.0 keV, which can be assigned as the K$\alpha$ lines from
neutral, He-like and H-like iron, respectively. A coherent pulsation is found
at a period of 432.1 $\pm$ 0.1 s. The pulse profile is quasi-sinusoidal in the
hard X-ray band (4$-$8 keV), but is more complicated in the soft X-ray band
(1$-$3 keV). The moderate period of pulsation, the energy flux, and the
presence of the iron K$\alpha$ lines indicate that Suzaku J174035.6$-$301416 is
likely an intermediate polar, a subclass of magnetized white dwarf binaries
(cataclysmic variables).
Based on these discoveries, we give some implications on the origin of GCDX
and brief comments on HESS J1741$-$302 and PSR B1737$-$30.
|
1106.2861v1
|
2011-06-15
|
Electronic identification of the actual parental phase of KxFe2-ySe2 superconductor and its intrinsic mesoscopic phase separation
|
While the parent compounds of the cuprate high temperature superconductors
(high-Tc's) are Mott insulators, the iron-pnictide high-Tc's are in the
vicinity of a metallic spin density wave (SDW) state, which highlights the
difference between these two families. However, insulating parent compounds
were identified for the newly discovered KxFe2-ySe2. This raises an intriguing
question as to whether the iron-based high-Tc's could be viewed as doped Mott
insulators like the cuprates. Here we report angle-resolved photoemission
spectroscopy (ARPES) evidence of two insulating and one semiconducting phases
of KxFe2-ySe2, and the mesoscopic phase separation between the
superconducting/semiconducting phase and the insulating phases. The insulating
phases are characterized by the depletion of electronic states over a 0.5 eV
window below the chemical potential, giving a compelling evidence for the
presence of Mott-like physics. The charging effects and the absence of band
folding in the superconducting/semiconducting phase further prove that the
static magnetic and vacancy orders are not related to the superconductivity.
Instead, the electronic structure of the superconducting phase is much closer
to the semiconducting phase, indicating the superconductivity is likely
developed by doping the semiconducting phase rather than the insulating phases.
|
1106.3026v1
|
2011-06-19
|
Iron Based Superconductors: Pnictides versus Chalcogenides
|
We present a brief review of the present day situation with studies of
high-temperature superconductivity in iron pnictides and chalcogenides. Recent
discovery of superconductivity with T_c > 30 K in A_xFe_{2-x/2}Se_2
(A=K,Cs,Tl,...) represents the major new step in the development of new
concepts in the physics of Fe - based high-temperature superconductors. We
compare LDA and ARPES data on the band structure and Fermi surfaces of novel
superconductors and those of the previously studied FeAs superconductors,
especially isostructural 122 - superconductors like BaFe_2As_2. It appears that
electronic structure of new superconductors is rather different from that of
FeAs 122 - systems. In particular, no nesting properties of electron and hole -
like Fermi surfaces is observed, casting doubts on most popular theoretical
schemes of Cooper pairing for these systems. The discovery of Fe vacancies
ordering and antiferromagnetic (AFM) ordering at pretty high temperatures (T_N>
500 K), much exceeding superconducting T_c makes these systems unique
antiferromagnetic superconductors with highest T_N observed up to now. We
discuss the role of both vacancies and AFM ordering in transformations of band
structure and Fermi surfaces, as well as their importance for
superconductivity. In particular, we show that system remains metallic with
unfolded Fermi surfaces quite similar to that in paramagnetic state.
Superconducting transition temperature T_c of new superconductors is discussed
within the general picture of superconductivity in multiple band systems. It is
demonstrated that both in FeAs - superconductors and in new FeSe - systems the
value of T_c correlates with the value of the total density of states (DOS) at
the Fermi level.
|
1106.3707v1
|
2011-06-24
|
Unprecedented anisotropic metallic state in BaFe2As2 revealed by optical spectroscopy
|
An ordered phase showing remarkable electronic anisotropy in proximity to the
superconducting phase is now a hot issue in the field of
high-transition-temperature superconductivity. As in the case of copper oxides,
superconductivity in iron arsenides competes or coexists with such an ordered
phase. Undoped and underdoped iron arsenides have a magnetostructural ordered
phase exhibiting stripe-like antiferromagnetic spin order accompanied by an
orthorhombic lattice distortion; both the spin order and lattice distortion
break the tetragonal symmetry of crystals of these compounds. In this ordered
state, anisotropy of in-plane electrical resistivity is anomalous and difficult
to attribute simply to the spin order and/or the lattice distortion. Here, we
present the anisotropic optical spectra measured on detwinned BaFe2As2 crystals
with light polarization parallel to the Fe planes. Pronounced anisotropy is
observed in the spectra, persisting up to an unexpectedly high photon energy of
about 2 eV. Such anisotropy arises from an anisotropic energy gap opening below
and slightly above the onset of the order. Detailed analysis of the optical
spectra reveals an unprecedented electronic state in the ordered phase.
|
1106.4967v1
|
2011-06-27
|
Silicon and Oxygen Abundances in Planet-Host Stars
|
The positive correlation between planet detection rate and host star iron
abundance lends strong support to the core accretion theory of planet
formation. However, iron is not the most significant mass contributor to the
cores of giant planets. Since giant planet cores are thought to grow from
silicate grains with icy mantles, the likelihood of gas giant formation should
depend heavily on the oxygen and silicon abundance of the planet formation
environment. Here we compare the silicon and oxygen abundances of a set of 76
planet hosts and a control sample of 80 metal-rich stars without any known
giant planets. Our new, independent analysis was conducted using high
resolution, high signal-to-noise data obtained at McDonald Observatory. Because
we do not wish to simply reproduce the known planet-metallicity correlation, we
have devised a statistical method for matching the underlying [Fe/H]
distributions of our two sets of stars. We find a 99% probability that planet
detection rate depends on the silicon abundance of the host star, over and
above the observed planet-metallicity correlation. We do not detect any such
correlation for oxygen. Our results would thus seem to suggest that grain
nucleation, rather than subsequent icy mantle growth, is the important limiting
factor in forming giant planets via core accretion. Based on our results and
interpretation, we predict that planet detection should correlate with host
star abundance for refractory elements responsible for grain nucleation and
that no such trends should exist for the most abundant volatile elements
responsible for icy mantle growth.
|
1106.5509v1
|
2011-06-28
|
Pressure-induced isostructural phase transition and correlation of FeAs coordination with the superconducting properties of 111-type Na1-xFeAs
|
The effect of pressure on the crystalline structure and superconducting
transition temperature (Tc) of the 111-type Na1-xFeAs system using in situ high
pressure synchrotron x-ray powder diffraction and diamond anvil cell techniques
is studied. A pressure-induced tetragonal to tetragonal isostructural phase
transition was found. The systematic evolution of the FeAs4 tetrahedron as a
function of pressure based on Rietveld refinements on the powder x-ray
diffraction patterns was obtained. The non-monotonic Tc(P) behavior of
Na1-xFeAs is found to correlate with the anomalies of the distance between the
anion (As) and the iron layer as well as the bond angle between As-Fe-As for
the two tetragonal phases. This behavior provides the key structural
information in understanding the origin of the pressure dependence of Tc for
111-type iron pnictide superconductors. A pressure-induced structural phase
transition is also observed at 20 GPa.
|
1106.5640v1
|
2011-06-30
|
Suzaku Observation of the Intermediate Polar V1223 Sagittarii
|
We report on the Suzaku observation of the intermediate polar V1223
Sagittarii. Using a multi-temperature plasma emission model with its reflection
from a cold matter, we obtained the shock temperature to be 37.9^{+5.1}_{-4.6}
keV. This constrains the mass and the radius of the white dwarf (WD) in the
ranges 0.82^{+0.05}_{-0.06} solar masses and (6.9+/-0.4)x10^8 cm, respectively,
with the aid of a WD mass-radius relation. The solid angle of the reflector
viewed from the post-shock plasma was measured to be Omega/2pi = 0.91+/-0.26. A
fluorescent iron Kalpha emission line is detected, whose central energy is
discovered to be modulated with the WD rotation for the first time in
magnetic-CVs. Detailed spectral analysis indicates that the line comprises of a
stable 6.4 keV component and a red-shifted component, the latter of which
appears only around the rotational intensity-minimum phase. The equivalent
width (EW) of the former stable component ~80 eV together with the measured
Omega indicates the major reflector is the WD surface, and the shock height is
not more than 7% of the WD radius. Comparing this limitation to the height
predicted by the Aizu model (1973), we estimated the fractional area onto which
the accretion occurs to be < 7x10^{-3}$ of the WD radius, which is the most
severe constraint in non-eclipsing IPs. The red-shifted iron line component, on
the other hand, can be interpreted as emanating from the pre-shock accretion
flow via fluorescence. Its EW (28^{+44}_{-13} eV) and the central energy
(6.30_{-0.05}^{+0.07} keV) at the intensity-minimum phase are consistent with
this interpretation.
|
1106.6111v1
|
2011-08-09
|
Magnetic spots on hot massive stars
|
Hot luminous stars show a variety of phenomena in their photospheres and
winds which still lack clear physical explanation. Among these phenomena are
photospheric turbulence, line profile variability (LPV), non-thermal emission,
non-radial pulsations, discrete absorption components (DACs) and wind clumping.
Cantiello et al. (2009) argued that a convection zone close to the stellar
surface could be responsible for some of these phenomena. This convective zone
is caused by a peak in the opacity associated with iron-group elements and is
referred to as the "iron convection zone" (FeCZ). Assuming dynamo action
producing magnetic fields at equipartition in the FeCZ, we investigate the
occurrence of subsurface magnetism in OB stars. Then we study the surface
emergence of these magnetic fields and discuss possible observational
signatures of magnetic spots. Simple estimates are made using the subsurface
properties of massive stars, as calculated in 1D stellar evolution models. We
find that magnetic fields of sufficient amplitude to affect the wind could
emerge at the surface via magnetic buoyancy. While at this stage it is
difficult to predict the geometry of these features, we show that magnetic
spots of size comparable to the local pressure scale height can manifest
themselves as hot, bright spots. Localized magnetic fields could be widespread
in those early type stars that have subsurface convection. This type of surface
magnetism could be responsible for photometric variability and play a role in
X-ray emission and wind clumping.
|
1108.2030v1
|
2011-08-23
|
Fine-Grid Calculations for Stellar Electron and Positron Capture Rates on Fe-Isotopes
|
The acquisition of precise and reliable nuclear data is a prerequisite to
success for stellar evolution and nucleosynthesis studies. Core-collapse
simulators find it challenging to generate an explosion from the collapse of
the core of massive stars. It is believed that a better understanding of the
microphysics of core-collapse can lead to successful results. The weak
interaction processes are able to trigger the collapse and control the
lepton-to-baryon ratio ($Y_{e}$) of the core material. It is suggested that the
temporal variation of $Y_{e}$ within the core of a massive star has a pivotal
role to play in the stellar evolution and a fine-tuning of this parameter at
various stages of presupernova evolution is the key to generate an explosion.
During the presupernova evolution of massive stars, isotopes of iron, mainly
$^{54,55,56}$Fe, are considered to be key players in controlling $Y_{e}$ ratio
via electron capture on these nuclide. Recently an improved microscopic
calculation of weak interaction mediated rates for iron isotopes was introduced
using the proton-neutron quasiparticle random phase approximation (pn-QRPA)
theory. The pn-QRPA theory allows a microscopic \textit{state-by-state}
calculation of stellar capture rates which greatly increases the reliability of
calculated rates. The results were suggestive of some fine-tuning of the
$Y_{e}$ ratio during various phases of stellar evolution. Here we present for
the first time the fine-grid calculation of the electron and positron capture
rates on $^{54,55,56}$Fe. Core-collapse simulators may find this calculation
suitable for interpolation purposes and for necessary incorporation in the
stellar evolution codes.
|
1108.4569v1
|
2011-10-09
|
Preemptive nematic order, pseudogap, and orbital order in the iron pnictides
|
Starting from a microscopic itinerant model, we derive and analyze the
effective low-energy model for collective magnetic excitations in the iron
pnictides. We show that the stripe magnetic order is generally preempted by an
Ising-nematic order which breaks $C_{4}$ lattice symmetry but preserves O(3)
spin-rotational symmetry. This leads to a rich phase diagram as function of
doping, pressure, and elastic moduli, displaying split magnetic and nematic
tri-critical points. The nematic transition may instantly bring the system to
the verge of a magnetic transition, or it may occur first, being followed by a
magnetic transition at a lower temperature. In the latter case, the preemptive
nematic transition is accompanied by either a jump or a rapid increase of the
magnetic correlation length, triggering a pseudogap behavior associated with
magnetic precursors. Furthermore, due to the distinct orbital character of each
Fermi pocket, the nematic transition also induces orbital order. We compare our
results to various experiments, showing that they correctly address the changes
in the character of the magneto-structural transition across the phase diagrams
of different compounds, as well as the relationship between the orthorhombic
and magnetic order parameters.
|
1110.1893v2
|
2011-11-08
|
Two Populations of X-ray Pulsars Produced by Two Types of Supernovae
|
Two types of supernova are thought to produce the overwhelming majority of
neutron stars in the Universe. The first type, iron-core collapse supernovae,
occurs when a high-mass star develops a degenerate iron core that exceeds the
Chandrasekhar limit. The second type, electron-capture supernovae, is
associated with the collapse of a lower-mass oxygen-neon-magnesium core as it
loses pressure support owing to the sudden capture of electrons by neon and/or
magnesium nuclei. It has hitherto been impossible to identify the two distinct
families of neutron stars produced in these formation channels. Here we report
that a large, well-known class of neutron-star-hosting X-ray pulsars is
actually composed of two distinct sub-populations with different characteristic
spin periods, orbital periods and orbital eccentricities. This class, the
Be/X-ray binaries, contains neutron stars that accrete material from a more
massive companion star. The two sub-populations are most probably associated
with the two distinct types of neutron-star-forming supernovae, with
electron-capture supernovae preferentially producing system with short spin
period, short orbital periods and low eccentricity. Intriguingly, the split
between the two sub-populations is clearest in the distribution of the
logarithm of spin period, a result that had not been predicted and which still
remains to be explained.
|
1111.2051v1
|
2011-11-22
|
Microstructural magnetic phases in superconducting FeTe0.65Se0.35
|
In this paper, we address a number of outstanding issues concerning the
nature and the role of magnetic inhomogenities in the iron chalcogenide system
FeTe1-xSex and their correlation with superconductivity in this system. We
report morphology of superconducting single crystals of FeTe0.65Se0.35 studied
with transmission electron microscopy, high angle annular dark field scanning
transmission electron microscopy and their magnetic and superconducting
properties characterized with magnetization, specific heat and magnetic
resonance spectroscopy. Our data demonstrate a presence of nanometre scale
hexagonal regions coexisting with tetragonal host lattice, a chemical disorder
demonstrating non homogeneous distribution of host atoms in the crystal
lattice, as well as hundreds-of-nanometres-long iron-deficient bands. From
magnetic data and ferromagnetic resonance temperature dependence, we attribute
magnetic phases in Fe-Te-Se to Fe3O4 inclusions and to hexagonal symmetry
nanometre scale regions with structure of Fe7Se8 type. Our results suggest that
nonhomogeneous distribution of host atoms might be an intrinsic feature of
superconducting Fe-Te-Se chalcogenides and we find a surprising correlation
indicating that faster grown crystal of inferior crystallographic properties is
a better superconductor.
|
1111.5176v2
|
2011-11-27
|
Evolution of structural properties of iron oxide nano particles during temperature treatment from 250°C - 900°C: X-ray diffraction and Fe K-shell pre-edge X-ray absorption study
|
Iron oxide nano particles with nominal Fe2O3 stoichiometry were synthesized
by a wet, soft chemical method with the heat treatment temperatures from
250{\deg}C to 900{\deg}C in air. The variation in the structural properties of
the nano particles with the heat treatment temperature was studied by X-ray
diffraction and Fe K shell X-ray absorption study. X-ray diffractograms show
that at lower annealing temperatures nano particle comprises both maghemite and
hematite phases. With increasing temperature, the remainder of the maghemite
phase transformed completely to hematite. Profile analysis of the leading Bragg
reflections reveals that the average crystallite size increases from 50 nm to
150 nm with increasing temperature. The mean primary particle size decreased
from 105 nm to 90 nm with increasing heat treatment temperature. The X-ray
diffraction results are paralleled by systematic changes in the pre-edge
structure of the Fe K-edge X-ray absorption spectra, in particular by a gradual
decrease of the t2g/eg peak height ratio of the two leading pre-edge
resonances, confirming oxidation of the Fe from Fe2+ towards Fe3+. Transmission
electron microscopy (TEM) on the samples treated at temperatures as high as
900{\deg}C showed particles with prismatic morphology along with the formation
of stacking fault like defects. High resolution TEM with selected area electron
diffraction (SAED) of samples heat treated above 350{\deg}C showed that the
nano particles have well developed lattice fringes corresponding to that of
(110) plane of hematite.
|
1111.6204v1
|
2011-11-29
|
A J-band detection of the donor star in the dwarf nova OY Carinae, and an optical detection of its `iron curtain'
|
Purely photometric models can be used to determine the binary parameters of
eclipsing cataclysmic variables with a high degree of precision. However, the
photometric method relies on a number of assumptions, and to date there have
been very few independent checks of this method in the literature. We present
time-resolved spectroscopy of the P=90.9 min eclipsing cataclysmic variable OY
Carinae obtained with X-shooter on the VLT, in which we detect the donor star
from K I lines in the J-band. We measure the radial velocity amplitude of the
donor star K2 = 470.0 +/- 2.7 km/s, consistent with predictions based upon the
photometric method (470 +/- 7 km/s). Additionally, the spectra obtained in the
UVB arm of X-shooter show a series of Fe I and Fe II lines with a phase and
velocity consistent with an origin in the accretion disc. This is the first
unambiguous detection at optical wavelengths of the `iron curtain' of disc
material which has been previously reported to veil the white dwarf in this
system. The velocities of these lines do not track the white dwarf, reflecting
a distortion of the outer disc that we see also in Doppler images. This is
evidence for considerable radial motion in the outer disk, at up to 90 km/s
towards and away from the white dwarf.
|
1111.6775v1
|
2011-12-08
|
Influence of high permeability disks in an axisymmetric model of the Cadarache dynamo experiment
|
Numerical simulations of the kinematic induction equation are performed on a
model configuration of the Cadarache von-K\'arm\'an-Sodium dynamo experiment.
The effect of a localized axisymmetric distribution of relative permeability
{\mu} that represents soft iron material within the conducting fluid flow is
investigated. The critical magnetic Reynolds number Rm^c for dynamo action of
the first non-axisymmetric mode roughly scales like
Rm^c({\mu})-Rm^c({\mu}->infinity) ~ {\mu}^(-1/2) i.e. the threshold decreases
as {\mu} increases. This scaling law suggests a skin effect mechanism in the
soft iron disks. More important with regard to the Cadarache dynamo experiment,
we observe a purely toroidal axisymmetric mode localized in the high
permeability disks which becomes dominant for large {\mu}. In this limit, the
toroidal mode is close to the onset of dynamo action with a (negative)
growth-rate that is rather independent of the magnetic Reynolds number. We
qualitatively explain this effect by paramagnetic pumping at the fluid/disk
interface and propose a simplified model that quantitatively reproduces
numerical results. The crucial role of the high permeability disks for the mode
selection in the Cadarache dynamo experiment cannot be inferred from
computations using idealized pseudo-vacuum boundary conditions (H x n = 0).
|
1112.1780v2
|
2011-12-20
|
Spin Dynamics of a J1-J2-K Model for the Paramagnetic Phase of Iron Pnictides
|
We study the finite-temperature spin dynamics of the paramagnetic phase of
iron pnictides within an antiferromagnetic J_1-J_2 Heisenberg model on a square
lattice with a biquadratic coupling $-K (S_i \cdot S_j)^2$ between the
nearest-neighbor spins. Our focus is on the paramagnetic phase in the parameter
regime of this J_1-J_2-K model where the ground state is a (\pi,0) collinear
antiferromagnet. We treat the biquadratic interaction via a
Hubbard-Stratonovich decomposition, and study the resulting effective
quadratic-coupling model using both modified spin wave and Schwinger boson
mean-field theories; the results for the spin dynamics derived from the two
methods are very similar. We show that the spectral weight of dynamical
structure factor S(q,\omega) is peaked at ellipses in the momentum space at low
excitation energies. With increasing energy, the elliptic features expand
towards the zone boundary, and gradually split into two parts, forming a
pattern around (\pi,\pi). Finally, the spectral weight is anisotropic, being
larger along the major axis of the ellipse than along its minor axis. These
characteristics of the dynamical structure factor are consistent with the
recent measurements of the inelastic neutron scattering spectra on BaFe_2As_2
and SrFe_2As_2.
|
1112.4785v3
|
2012-01-16
|
Universal heat conduction in the iron-arsenide superconductor KFe2As2 : Evidence of a d-wave state
|
The thermal conductivity of the iron-arsenide superconductor KFe2As2 was
measured down to 50 mK for a heat current parallel and perpendicular to the
tetragonal c-axis. A residual linear term (RLT) at T=0 is observed for both
current directions, confirming the presence of nodes in the superconducting
gap. Our value of the RLT in the plane is equal to that reported by Dong et al.
[Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity
was ten times larger. This independence of the RLT on impurity scattering is
the signature of universal heat transport, a property of superconducting states
with symmetry-imposed line nodes. This argues against an s-wave state with
accidental nodes. It favors instead a d-wave state, an assignment consistent
with five additional properties: the magnitude of the critical scattering rate
for suppressing Tc to zero; the magnitude of the RLT, and its dependence on
current direction and on magnetic field; the temperature dependence of the
thermal conductivity.
|
1201.3376v3
|
2012-01-26
|
Very fast photometric and X-ray observations of the intermediate polar V2069 Cygni (RX J2123.7+4217)
|
We present fast timing photometric observations of the intermediate polar
V2069 Cygni (RX J2123.7+4217) using the Optical Timing Analyzer (OPTIMA) at the
1.3 m telescope of Skinakas Observatory. The optical (450-950 nm) light curve
of V2069 Cygni was measured with sub-second resolution for the first time
during July 2009 and revealed a double-peaked pulsation with a period of 743.38
+0.25. A similar double-peaked modulation was found in the simultaneous Swift
satellite observations. We suggest that this period represents the spin of the
white dwarf accretor. Moreover, we present the results from a detailed analysis
of the XMM-Newton observation that also shows a double-peaked modulation,
however shifted in phase, with 742.35 +0.23 s period. The X-ray spectra
obtained from the XMM-Newton EPIC (European Photon Imaging Camera) instruments
were modelled by a plasma emission and a soft black body component with a
partial covering photo-electric absorption model with covering fraction of
0.65. An additional Gaussian emission line at 6.385 keV with an equivalent
width of 243 eV is required to account for fluorescent emission from neutral
iron. The iron fluorescence (~6.4 keV) and FeXXVI lines (~6.95 keV) are clearly
resolved in the EPIC spectra. In the Porb-Pspin diagram of IPs, V2069 Cyg shows
a low spin to orbit ratio of ~0.0276 in comparison with ~0.1 for other
intermediate polars.
|
1201.5629v1
|
2012-02-14
|
Nernst effect of iron pnictide and cuprate superconductors: signatures of spin density wave and stripe order
|
The Nernst effect has recently proven a sensitive probe for detecting unusual
normal state properties of unconventional superconductors. In particular, it
may sensitively detect Fermi surface reconstructions which are connected to a
charge or spin density wave (SDW) ordered state, and even fluctuating forms of
such a state. Here we summarize recent results for the Nernst effect of the
iron pnictide superconductor $\rm LaO_{1-x}F_xFeAs$, whose ground state evolves
upon doping from an itinerant SDW to a superconducting state, and the cuprate
superconductor $\rm La_{1.8-x}Eu_{0.2}Sr_xCuO_4$ which exhibits static stripe
order as a ground state competing with the superconductivity. In $\rm
LaO_{1-x}F_xFeAs$, the SDW order leads to a huge Nernst response, which allows
to detect even fluctuating SDW precursors at superconducting doping levels
where long range SDW order is suppressed. This is in contrast to the impact of
stripe order on the normal state Nernst effect in $\rm
La_{1.8-x}Eu_{0.2}Sr_xCuO_4$. Here, though signatures of the stripe order are
detectable in the temperature dependence of the Nernst coefficient, its overall
temperature dependence is very similar to that of $\rm La_{2-x}Sr_xCuO_4$,
where stripe order is absent. The anomalies which are induced by the stripe
order are very subtle and the enhancement of the Nernst response due to static
stripe order in $\rm La_{1.8-x}Eu_{0.2}Sr_xCuO_4$ as compared to that of the
pseudogap phase in $\rm La_{2-x}Sr_xCuO_4$, if any, is very small.
|
1202.2959v1
|
2012-04-04
|
A New Disintegrative Capture Theory for the Origin of the Moon
|
The object that resulted in the creation of the Moon started in the same
orbital path as Earth around the Sun, but at Earth's L4. This proto-Moon (PM)
was 4 times less massive than the usual Giant Impact (GI) object "Theia" and
was captured into Earth orbit. It had a 32% Iron-Nickel-Sulfur core supporting
a dynamo, which explains magnetized lunar rocks. Following capture, it was torn
apart by tidal forces and its core of iron plastered itself, with some of its
rock mantle, on the surface of Earth at a very flat angle (producing the "Late
Veneer"). After tidal stripping, the remaining PM rock was driven away from
Earth to about 3.8 times Earth's radius and formed into what is now the Moon.
The GI theory has several troubles: The violent collision melts the entire
Earth, contrary to geological evidence. The Moon itself also has to condense
out of the vapor cloud generated in the collision, but there is evidence that
the Moon was not condensed out of vapor. In the new theory, the Moon as we know
it may be only 3.8 - 3.9 billion years old, not 4.56 as usually assumed. That
is the age of the PM. The minerals in the Moon would be about as old as the
Earth, but would have been re-arranged in the capture and temporary
disintegration process. If the Moon is as young as suggested, its origin would
coincide with the beginning of life on Earth, which is unexplained in the GI
theory. The manuscript asks, "Was the Moon Turned Inside-Out" and the answer is
"Essentially, Yes."
|
1204.0980v2
|
2012-05-02
|
Negative thermal expansion and antiferromagnetism in the actinide oxypnictide NpFeAsO
|
A neptunium analogue of the LaFeAsO tetragonal layered compound has been
synthesized and characterized by a variety of experimental techniques. The
occurrence of long-range magnetic order below a critical temperature T_N = 57 K
is suggested by anomalies in the temperature-dependent magnetic susceptibility,
electrical resistivity, Hall coefficient, and specific heat curves. Below T_N,
powder neutron diffraction measurements reveal an antiferromagnetic structure
of the Np sublattice, with an ordered magnetic moment of 1.70(0.07) \mu_B
aligned along the crystallographic c-axis. No magnetic order has been observed
on the Fe sublattice, setting an upper limit of about 0.3 \mu_B for the ordered
magnetic moment on the iron. High resolution x-ray powder diffraction
measurements exclude the occurrence of lattice transformations down to 5 K, in
sharp contrast to the observation of a tetragonal-to-orthorhombic distortion in
the rare-earth analogues, which has been associated with the stabilization of a
spin density wave on the iron sublattice. Instead, a significant expansion of
the NpFeAsO lattice parameters is observed with decreasing temperature below
T_N, corresponding to a relative volume change of about 0.2% and to an invar
behavior between 5 and 20 K. First-principle electronic structure calculations
based on the local-spin density plus Coulomb interaction and the local density
plus Hubbard-I approximations provide results in good agreement with the
experimental findings.
|
1205.0438v1
|
2012-05-14
|
The Central Engine Structure of 3C120: Evidence for a Retrograde Black Hole or a Refilling Accretion Disk
|
The broad-line radio galaxy 3C120 is a powerful source of both X-ray and
radio emission including superluminal jet outflows. We report on our reanalysis
of 160 ks of Suzaku data taken in 2006, previously examined by Kataoka et al.
(2007). Spectral fits to the XIS and HXD/PIN data over a range of 0.7-45 keV
reveal a well-defined iron K line complex with a narrow Ka core and
relativistically broadened features consistent with emission from the inner
regions of the accretion disk. Furthermore, the inner region of the disk
appears to be truncated with an inner radius of r_in = 11.7^{+3.5}_{-5.2} r_g.
If we assume that fluorescent iron line features terminate at the inner-most
stable circular orbit (ISCO), we measure a black hole spin of a < -0.1 at a 90%
confidence level. A rapidly spinning prograde black hole (a > 0.8) can be ruled
out at the 99% confidence level. Alternatively, the disk may be truncated well
outside of the ISCO of a rapid prograde hole. The most compelling scenario is
the possibility that the inner regions of the disk were destroyed/ejected by
catastrophic instabilities just prior to the time these observations were made.
|
1205.3175v1
|
2012-05-21
|
Impact of the 2 Fe unit cell on the electronic structure measured by ARPES in iron pnictides
|
In all iron pnictides, the positions of the ligand alternatively above and
below the Fe plane create 2 inequivalent Fe sites. This results in 10 Fe 3d
bands in the electronic structure. However, they do not all have the same
status for an ARPES experiment. There are interference effects between the 2 Fe
that modulate strongly the intensity of the bands and that can even switch
their parity. We give a simple description of these effects, notably showing
that ARPES polarization selection rules in these systems cannot be applied by
reference to a single Fe ion. We show that ARPES data for the electron pockets
in Ba(Fe0.92Co0.08)2As2 are in excellent agreement with this model. We observe
both the total suppression of some bands and the parity switching of some other
bands. Once these effects are properly taken into account, the structure of the
electron pockets, as measured by ARPES, becomes very clear and simple. By
combining ARPES measurements in different experimental configurations, we
clearly isolate each band forming one of the electron pockets. We identify a
deep electron band along one ellipse axis with the dxy orbital and a shallow
electron band along the perpendicular axis with the dxz/dyz orbitals, in good
agreement with band structure calculations. We show that the electron pockets
are warped as a function of kz as expected theoretically, but that they are
much smaller than predicted by the calculation.
|
1205.4513v1
|
2012-05-27
|
Neutron-Diffraction Measurements of an Antiferromagnetic Semiconducting Phase in the Vicinity of the High-Temperature Superconducting State of K$_x$Fe$_{2-y}$Se$_2$
|
The recently discovered K-Fe-Se high temperature superconductor has caused
heated debate regarding the nature of its parent compound. Transport,
angle-resolved photoemission spectroscopy, and STM measurements have suggested
that its parent compound could be insulating, semiconducting or even metallic
[M. H. Fang, H.-D. Wang, C.-H. Dong, Z.-J. Li, C.-M. Feng, J. Chen, and H. Q.
Yuan, Europhys. Lett. 94, 27009 (2011); F. Chen et al. Phys. Rev. X 1, 021020
(2011); and W. Li et al.,Phys. Rev. Lett. 109, 057003 (2012)]. Because the
magnetic ground states associated with these different phases have not yet been
identified and the relationship between magnetism and superconductivity is not
fully understood, the real parent compound of this system remains elusive.
Here, we report neutron-diffraction experiments that reveal a semiconducting
antiferromagnetic (AFM) phase with rhombus iron vacancy order. The magnetic
order of the semiconducting phase is the same as the stripe AFM order of the
iron pnictide parent compounds. Moreover, while the root5*root5 block AFM phase
coexists with superconductivity, the stripe AFM order is suppressed by it. This
leads us to conjecture that the new semiconducting magnetic ordered phase is
the true parent phase of this superconductor.
|
1205.5992v2
|
2012-05-30
|
The silicate model and carbon rich model of CoRoT-7b, Kepler-9d and Kepler-10b
|
Possible bulk compositions of the super-Earth exoplanets, CoRoT-7b,
Kepler-9d, and Kepler-10b are investigated by applying a commonly used silicate
and a non-standard carbon model. Their internal structures are deduced using
the suitable equation of state of the materials. The degeneracy problems of
their compositions can be partly overcome, based on the fact that all three
planets are extremely close to their host stars. By analyzing the numerical
results, we conclude: 1) The iron core of CoRoT-7b is not more than 27% of its
total mass within 1 $\sigma$ mass-radius error bars, so an Earth-like
composition is less likely, but its carbon rich model can be compatible with an
Earth-like core/mantle mass fraction; 2) Kepler-10b is more likely with a
Mercury-like composition, its old age implies that its high iron content may be
a result of strong solar wind or giant impact; 3) the transiting-only
super-Earth Kepler-9d is also discussed. Combining its possible composition
with the formation theory, we can place some constraints on its mass and bulk
composition.
|
1205.6554v2
|
2012-06-18
|
Impact of Dynamic Orbital Correlations on Magnetic Excitations in the Normal State of Iron-Based Superconductors
|
We show here that orbital degrees of freedom produce a distinct signature in
the magnetic excitation spectrum of iron-based superconductors above the
magnetic ordering temperature. Because $d_{xz}$ and $d_{yz}$ orbitals are
strongly connected with the Fermi surface topology, the nature of magnetic
excitations can be modified significantly due to the presence of either static
or fluctuating orbital correlations. Within a five-orbital itinerant model, we
show that static orbital order generally leads to an enhancement of
commensurate magnetic excitations even when the original Fermi surface lacks
nesting at $(\pi,0)$ or $(0,\pi)$. When long-range orbital order is absent,
Gaussian fluctuations beyond the standard random-phase approximation (RPA)
capture the effects of fluctuating orbital correlations on the magnetic
excitations. We find that commensurate magnetic excitations can also be
enhanced if the orbital correlations are strong. Our results offer a natural
explanation for the incommensurate-to-commensurate transformation observed in a
recent neutron scattering measurement (Z. Xu, et. al., arXiv:1201.4404), and we
propose that this unusual transformation is an important signature to
distinguish orbital from spin physics in the normal state of the pnictides.
Implications for the magnetic and superconducting states are discussed.
|
1206.4095v2
|
2012-06-29
|
Laser spectroscopy of finite size and covering effects in magnetite nanoparticles
|
The experiments on the impact of the size of magnetite clusters on various
magnetic properties (magnetic moment, Curie temperature, blocking temperature
etc.) have been carried out. The methods of magnetic separation, centrifuging
of water suspensions of biocompatible iron oxide nanoparticles (NPs) allow
producing fractions with diameter of nanoparticles in the range of 4{\div}22
nm. The size of NPs are controlled by the methods of dynamic light scattering
(DLS), transmission electron microscopy (TEM) and atomic force microscopy
(AFM). For the first time the DLS method is applied in real time to control the
size during the process of the separation of the NPs in aqueous suspensions.
The changes of the size of NPs cause a shift in the Curie temperature and in
the changes in the specific magnetic properties of the iron NPs. The
experimental data is interpreted on the basis of Monte Carlo simulations for
the classical Heisenberg model with different bulk and surface magnetic
moments. It is demonstrated experimentally and by theoretical modeling that
magnetic properties of magnetite NPs are determined not only by their sizes,
but also by the their surface spin states, while both growing and falling
dependences of the magnetic moment (per Fe3O4 formula unit) being possible,
depending on the number of magnetic atoms in the nanoparticle. Both NPs clean
and covered with a bioresorbable layer clusters have been investigated.
|
1206.6985v5
|
2012-07-03
|
Two-dome structure in electron-doped iron arsenide superconductors
|
Iron arsenide superconductors based on the material LaFeAsO1-xFx are
characterized by a two-dimensional Fermi surface (FS) consisting of hole and
electron pockets yielding structural and antiferromagnetic transitions at x =
0. Electron doping by substituting O2- with F- suppresses these transitions and
gives rise to superconductivity with a maximum Tc = 26 K at x = 0.1. However,
the over-doped region cannot be accessed due to the poor solubility of F- above
x = 0.2. Here we overcome this problem by doping LaFeAsO with hydrogen. We
report the phase diagram of LaFeAsO1-xHx (x < 0.53) and, in addition to the
conventional superconducting dome seen in LaFeAsO1-xFx, we find a second dome
in the range 0.21 < x < 0.53, with a maximum Tc of 36 K at x = 0.3. Density
functional theory calculations reveal that the three Fe 3d bands (xy, yz, zx)
become degenerate at x = 0.36, whereas the FS nesting is weakened monotonically
with x. These results imply that the band degeneracy has an important role to
induce high Tc.
|
1207.0583v2
|
2012-07-13
|
The nature of the multi-wavelength emission of 3C 111
|
We attempt to determine the nature of the high energy emission of the radio
galaxy 3C 111, by distinguishing between the effects of the thermal and
non-thermal processes. We study the X-ray spectrum of 3C 111 between 0.4 keV
and 200 keV, and its spectral energy distribution, using data from the Suzaku
satellite combined with INTEGRAL, Swift/BAT data, and Fermi/LAT data. We then
model the overall spectral energy distribution including radio and infrared
data. The combined Suzaku, Swift and INTEGRAL data are represented by an
absorbed exponentially cut-off power-law with reflection from neutral material
with a photon index Gamma = 1.68+-0.03, a high-energy cut-off Ecut = 227+143-67
keV, a reflection component with R = 0.7+-0.3 and a Gaussian component to
account for the iron emission-line at 6.4 keV with an equivalent width of EW =
85+-11 eV. The X-ray spectrum appears dominated by thermal, Seyfert-like
processes, but there are also indications of non-thermal processes. The radio
to gamma-ray spectral energy distribution can be fit with a single-zone
synchrotron-self Compton model, with no need for an additional thermal
component. We suggest a hybrid scenario to explain the broad-band emission,
including a thermal component (iron line, reflection) that dominates in the
X-ray regime and a non-thermal one to explain the spectral energy distribution.
|
1207.3308v2
|
2012-07-16
|
Searching for Massive Outflows in Holmberg IX X-1 and NGC 1313 X-1: The Iron K Band
|
We have analysed all the good quality XMM-Newton data publicly available for
the bright ULXs Holmberg IX X-1 and NGC 1313 X-1, with the aim of searching for
discrete emission or absorption features in the Fe K band that could provide
observational evidence for the massive outflows predicted if these sources are
accreting at substantially super-Eddington rates. We do not find statistically
compelling evidence for any atomic lines, and the limits that are obtained have
interesting consequences. Any features in the immediate Fe K energy band (6-7
keV) must have equivalent widths weaker than ~30 eV for Holmberg IX X-1, and
weaker than ~50 eV for NGC 1313 X-1 (at 99 per cent confidence). In comparison
to the sub-Eddington outflows observed in GRS 1915+105, which imprint iron
absorption features with equivalent widths of ~30 eV, the limits obtained here
appear quite stringent, particularly when Holmberg IX X-1 and NGC 1313 X-1 must
be expelling at least 5-10 times as much material if they host black holes of
similar masses. The difficulty in reconciling these observational limits with
the presence of strong line-of-sight outflows suggests that either these
sources are not launching such outflows, or that they must be directed away
from our viewing angle.
|
1207.3839v1
|
2012-08-01
|
Coronae above accretion disks around black holes: The effect of Compton cooling
|
The geometry of the accretion flow around stellar mass and supermassive black
holes depends on the accretion rate. Broad iron emission lines originating from
the irradiation of cool matter can indicate that there is an inner disk below a
hot coronal flow.These emission lines have been detected in X-ray binaries.
Observations with the Chandra X-ray Observatory, XMM Newton and Suzaku have
confirmed the presence of these emission lines also in a large fraction of
Seyfert-1 active galactic nuclei (AGN). We investigate the accretion flow
geometry for which broad iron emission lines can arise in hard and soft
spectral state. We study an ADAF-type coronal flow, where the ions are
viscously heated and electrons receive their heat only by collisions from the
ions and are Compton cooled by photons from an underlying cool disk. For a
strong mass flow in the disk and the resulting strong Compton cooling only a
very weak coronal flow is possible. This limitation allows the formation of
ADAF-type coronae above weak inner disks in the hard state, but almost rules
them out in the soft state. The observed hard X-ray luminosity in the soft
state, of up to 10% or more of the total flux, indicates that there is a
heating process that directly accelerates the electrons. This might point to
the action of magnetic flares of disk magnetic fields reaching into the corona.
Such flares have also been proposed by observations of the spectra of X-ray
black hole binaries without a thermal cut-off around 200 keV.
|
1208.0265v1
|
2012-08-18
|
Gap-dependent quasiparticle dynamics and coherent acoustic phonons in parent iron pnictide CaFe2As2 across the spin density wave phase transition
|
We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons
in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density
wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165
K using femtosecond time-resolved pump-probe spectroscopy. The contributions in
transient differential reflectivity arising from exponentially decaying QP
relaxation and oscillatory coherent acoustic phonon mode show large variations
in the vicinity of T_SDW. From the temperature-dependence of the QP
recombination dynamics in the SDW phase, we evaluate a BCS-like temperature
dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW,
whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has
been estimated from the linear temperature-dependence of the QP relaxation
time. The long-wavelength coherent acoustic phonons with typical time-period of
~100 ps have been analyzed in the light of propagating strain pulse model
providing important results for the optical constants, sounds velocity and the
elastic modulus of the crystal in the whole temperature range of 3 K to 300 K.
|
1208.3742v2
|
2012-08-29
|
Interpreting the near-infrared spectra of the 'golden standard' Type Ia supernova 2005cf
|
We present nine near-infrared (NIR) spectra of supernova (SN) 2005cf at
epochs from -10 d to +42 d with respect to B-band maximum, complementing the
existing excellent data sets available for this prototypical Type Ia SN at
other wavelengths. The spectra show a time evolution and spectral features
characteristic of normal Type Ia SNe, as illustrated by a comparison with SNe
1999ee, 2002bo and 2003du. The broad-band spectral energy distribution (SED) of
SN 2005cf is studied in combined ultraviolet (UV), optical and NIR spectra at
five epochs between ~ 8 d before and ~ 10 d after maximum light. We also
present synthetic spectra of the hydrodynamic explosion model W7, which
reproduce the key properties of SN 2005cf not only at UV-optical as previously
reported, but also at NIR wavelengths. From the radiative-transfer calculations
we infer that fluorescence is the driving mechanism that shapes the SED of SNe
Ia. In particular, the NIR part of the spectrum is almost devoid of absorption
features, and instead dominated by fluorescent emission of both iron-group
material and intermediate-mass elements at pre-maximum epochs, and pure
iron-group material after maximum light. A single P-Cygni feature of Mg II at
early epochs and a series of relatively unblended Co II lines at late phases
allow us to constrain the regions of the ejecta in which the respective
elements are abundant.
|
1208.5949v1
|
2012-09-03
|
The Black Hole Spin and Soft X-ray Excess of the Luminous Seyfert Galaxy Fairall 9
|
We present an analysis of all XMM-Newton and Suzaku X-ray spectra of the
nearby luminous Seyfert galaxy Fairall 9. Confirming previous analyses, we find
robust evidence for a broad iron line associated with X-ray reflection from the
innermost accretion disk. By fitting a spectral model that includes a
relativistically ionized reflection component, we examine the constraints on
the inclination of the inner accretion disk and the black hole spin, and the
complications introduced by the presence of a photoionized emission line
system. Employing multi-epoch fitting, we attempt to obtain robust and
concordant measures of the accretion disk parameters. We also clearly see a
soft X-ray excess in Fairall 9. During certain epochs, the soft excess can be
described with same disk reflection component that produces the iron line.
However, there are epochs where an additional soft component is required. This
can be attributed to either an additional highly-ionized, strongly blurred disk
reflection component, or a new X-ray continuum component.
|
1209.0468v1
|
2012-09-04
|
The enrichment history of the intracluster medium: a Bayesian approach
|
This work measures the evolution of the iron content in galaxy clusters by a
rigorous analysis of the data of 130 clusters at 0.1<z<1.3. This task is made
difficult by a) the low signal-to-noise ratio of abundance measurements and the
upper limits, b) possible selection effects, c) boundaries in the parameter
space, d) non-Gaussian errors, e) the intrinsic variety of the objects studied,
and f) abundance systematics. We introduce a Bayesian model to address all
these issues at the same time, thus allowing cross-talk (covariance). On
simulated data, the Bayesian fit recovers the input enrichment history, unlike
in standard analysis. After accounting for a possible dependence on X-ray
temperature, for metal abundance systematics, and for the intrinsic variety of
studied objects, we found that the present-day metal content is not reached
either at high or at low redshifts, but gradually over time: iron abundance
increases by a factor 1.5 in the 7 Gyr sampled by the data. Therefore, feedback
in metal abundance does not end at high redshift. Evolution is established with
a moderate amount of evidence, 19 to 1 odds against faster or slower metal
enrichment histories. We quantify, for the first time, the intrinsic spread in
metal abundance, 18+/-3 %, after correcting for the effect of evolution, X-ray
temperature, and metal abundance systematics. Finally, we also present an
analytic approximation of the X-ray temperature and metal abundance likelihood
functions, which are useful for other regression fitting involving these
parameters. The data for the 130 clusters and code used for the stochastic
computation are provided with the paper.
|
1209.0565v1
|
2012-10-04
|
Manipulation and detection of spin state of Iron-Porphyrin by dedicated chemisorption on magnetic substrates
|
One of the key factors behind the rapid evolution of molecular spintronics is
the efficient realization of spin manipulation of organic molecules with a
magnetic center. The spin state of such molecules may depend crucially on the
interaction with the substrate on which they are adsorbed. In this letter, we
demonstrate, using ab initio density functional calculations, that the
stabilization of a high spin state of an iron porphyrin (FeP) molecule can be
achieved via a dedicated chemisorption on magnetic substrates of different
species and orientations. It is shown that the strong covalent interaction with
the substrate increases Fe-N bond lengths in FeP and hence a switching to a
high spin state (S=2) from a low spin state (S=1) is achieved. A ferromagnetic
exchange interaction is established through a direct exchange between Fe and
substrate magnetic atoms as well as through an indirect exchange via the N
atoms in FeP. The mechanism of exchange interaction is further analyzed by
considering structural models constructed from ab initio calculations. Finally,
we illustrate the possibility of detecting a change in the molecular spin state
by x-ray magnetic circular dichroism, Raman spectroscopy and spin-polarized
scanning tunneling microscopy.
|
1210.1376v1
|
2012-12-07
|
Poly(acrylic acid)-coated iron oxide nanoparticles : quantitative evaluation of the coating properties and applications for the removal of a pollutant dye
|
In this work, 6 to 12 nm iron oxide nanoparticles were synthesized and coated
with poly(acrylic acid) chains of molecular weight 2100 g/mol. Based on a
quantitative evaluation of the dispersions, the bare and coated particles were
thoroughly characterized. The number densities of polymers adsorbed at the
particle surface and of available chargeable groups were found to be 1.9 +/-
0.3 nm-2 and 26 +/- 4 nm-2, respectively. Occurring via a multi-site binding
mechanism, the electrostatic coupling leads to a solid and resilient anchoring
of the chains. To assess the efficacy of the particles for pollutant
remediation, the adsorption isotherm of methylene blue molecules, a model of
pollutant, was determined. The excellent agreement between the predicted and
measured amounts of adsorbed dyes suggests that most carboxylates participate
to the complexation and adsorption mechanisms. An adsorption of 830 mg/g was
obtained. This quantity compares well with the highest values available for
this dye.
|
1212.1580v1
|
2012-12-07
|
Magneto-optic Faraday effect in maghemite nanoparticles/silica matrix nanocomposites prepared by the Sol-Gel method
|
Bulk monolithic samples of {\gamma}-Fe2O3/SiO2 composites with different iron
oxide/silica ratios have been prepared by the sol-gel technique. Iron oxide
nanoparticles are obtained in-situ during heat treatment of samples and silica
matrix consolidation. Preparation method was previously optimized to minimize
the percentage of antiferromagnetic {\alpha}-Fe2O3 and parallelepipeds of
roughly 2x5x12 mm3, with good mechanical stability, are obtained. RT
magnetization curves show a non-hysteretic behavior. Thus, magnetization
measurements have been well fitted to an expression that combines the Langevin
equation with an additional linear term, indicating that some of the
nanoparticles are still superparamagnetic as confirmed by X-ray diffraction and
electron microscopy measurements. Zero field cooled /field cooled experiments
show curves with slightly different shapes, depending on the size and shape
distribution of nanoparticles for a given composition. Magneto-optical Faraday
effect measurements show that the Faraday rotation is proportional to
magnetization of the samples, as expected. As a demonstration of their sensing
possibilities, the relative intensity of polarized light, measured at 5{\deg}
from the extinction angle, was plotted versus applied magnetic field.
|
1212.1690v1
|
2012-12-13
|
Dopant Clustering, Electronic Inhomogeneity, and Vortex Pinning in Iron-Based Superconductors
|
We use scanning tunneling microscopy to map the surface structure, nanoscale
electronic inhomogeneity, and vitreous vortex phase in the hole-doped
superconductor Sr$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ with $T_c$=32 K. We find the
low-$T$ cleaved surface is dominated by a half-Sr/K termination with $1\times
2$ ordering and ubiquitous superconducting gap, while patches of gapless,
unreconstructed As termination appear rarely. The superconducting gap varies by
$\sigma/\bar{\Delta}$=16% on a $\sim$3 nm length scale, with average
$2\bar{\Delta}/k_B T_c=3.6$ in the weak coupling limit. The vortex core size
provides a measure of the superconducting coherence length $\xi$=2.3 nm. We
quantify the vortex lattice correlation length at 9 T in comparison to several
iron-based superconductors. The comparison leads us to suggest the importance
of dopant size mismatch as a cause of dopant clustering, electronic
inhomogeneity, and strong vortex pinning.
|
1212.3240v3
|
2013-01-07
|
Persistent high-energy spin excitations in iron pnictide superconductors
|
Motivated by the premise that superconductivity in iron-based superconductors
is unconventional and mediated by spin fluctuations, an intense research effort
has been focused on characterizing the spin excitation spectrum in the
magnetically ordered parent phases of the Fe-pnictides2,3 and - chalcogenides4.
For these undoped materials it is well-established that the spin excitation
spectrum consists of a sharp, highly dispersive magnon spanning an energy range
of up to 200 meV (ref. 3). The fate of these high-energy magnetic modes upon
sizable doping is hitherto unresolved. Using resonant inelastic x-ray
scattering we show that optimally doped superconducting Ba0.6K0.4Fe2As2 retains
well defined, dispersive high-energy modes of magnetic origin. These paramagnon
modes are softer than, though as intense as, the magnon of undoped
antiferromagnetic BaFe2As2. The persistence of spin excitations well into the
superconducting phase suggests that, if spin fluctuations are responsible for
superconducting pairing, they originate from a distinctly correlated
spin-state. This connects Fe-pnictide superconductors to the high-Tc cuprates,
for which in spite of fundamental differences in the electronic structure,
similar paramagnon modes are present5.
|
1301.1289v1
|
2013-02-21
|
The Mid-Infrared and Optical Decay of SN 2011fe
|
We measure the decay rate of the mid-IR luminosity from type Ia supernova
2011fe between six months and one year after explosion using Spitzer/IRAC
observations. The fading in the 3.6 micron channel is 1.48+/-0.02 mag/100d,
which is similar to that seen in blue optical bands. The supernova brightness
fades at 0.78+/-0.02 mag/100d in the 4.5 micron channel which is close to that
observed in the near-IR. We argue that the difference is a result of doubly
ionized iron-peak elements dominating the bluer IRAC band while singly ionized
species are controlling the longer wavelength channel. To test this, we use
Large Binocular Telescope spectra taken during the same phases to show that
doubly ionized emission lines do fade more slowly than their singly ionized
cousins. We also find that [Co III] emission fades at more than twice the
radioactive decay rate due to the combination of decreasing excitation in the
nebula, recombination and cobalt decaying to iron. The nebular emission
velocities of [Fe III] and [Co III] lines show a smaller blue-shift than
emission from singly ionized atoms. The Si II velocity gradient near maximum
light combined with our nebular velocity measurements suggest SN 2011fe was a
typical member of the `low velocity gradient' class of type Ia. Analyzing IRAC
photometry from other supernovae we find that mid-IR color of type Ia events is
correlated with the early light curve width and can be used as an indicator of
the radioactive nickel yield.
|
1302.5421v1
|
2013-02-24
|
The X-ray Spectrum Of The Black Hole Candidate Swift J1753.5-0127
|
We present a spectral analysis of the black hole candidate and X-ray
transient source Swift J1753.5 0127 making use of simultaneous observations of
XMM-Newton and Rossi X-ray Timing Explorer (RXTE) in 2006, when the source was
in outburst. The aim of this paper is to test whether a thermal component due
to the accretion disc is present in the X-ray spectrum. We fit the data with a
range of spectral models, and we find that for all of these models the fits to
the X-ray energy spectra significantly require the addition of the disc
black-body component. We also find a broad iron emission line at around 6.5
keV, most likely due to iron in the accretion disc. Our results confirm the
existence of a cool inner disc extending near or close to the innermost
circular orbit (ISCO).We further discovered broad emission lines of NVII and
OVIII at ~ 0.52 keV and 0.65 keV, respectively in the RGS spectrum of Swift
J1753.5-0127.
|
1302.5882v2
|
2013-03-04
|
Soft lags in neutron star kHz Quasi Periodic Oscillations: evidence for reverberation?
|
High frequency soft reverberation lags have now been detected from stellar
mass and super massive black holes. Their interpretation involves reflection of
a hard source of photons onto an accretion disk, producing a delayed reflected
emission, with a time lag consistent with the light travel time between the
irradiating source and the disk. Independently of the location of the clock,
the kHz Quasi-Periodic Oscillation (QPO) emission is thought to arise from the
neutron star boundary layer. Here, we search for the signature of reverberation
of the kHz QPO emission, by measuring the soft lags and the lag energy spectrum
of the lower kHz QPOs from 4U1608-52. Soft lags, ranging from ~15 to ~40
microseconds, between the 3-8 keV and 8-30 keV modulated emissions are detected
between 565 and 890 Hz. The soft lags are not constant with frequency and show
a smooth decrease between 680 Hz and 890 Hz. The broad band X-ray spectrum is
modeled as the sum of a disk and a thermal comptonized component, plus a broad
iron line, expected from reflection. The spectral parameters follow a smooth
relationship with the QPO frequency, in particular the fitted inner disk radius
decreases steadily with frequency. Both the bump around the iron line in the
lag energy spectrum, and the consistency between the lag changes and the
inferred changes of the inner disk radius, from either spectral fitting or the
QPO frequency, suggest that the soft lags may indeed involve reverberation of
the hard pulsating QPO source on the disk.
|
1303.0750v2
|
2013-03-07
|
Abundances of neutron-capture elements in stars of the galactic disk substructures
|
The aim of this work is to present and discuss the observations of the iron
peak (Fe, Ni) and neutron-capture element (Y, Zr, Ba, La, Ce, Nd, Sm, and Eu)
abundances for 276 FGK dwarfs, located in the galactic disk with metallicity -1
< [Fe/H] < +0.3. Atmospheric parameters and chemical composition of the studied
stars were determined from an high resolution, high signal-to-noise echelle
spectra obtained with the echelle spectrograph ELODIE at the Observatoire de
Haute-Provence (France). Effective temperatures were estimated by the line
depth ratio method and from the H_{\alpha} line-wing fitting. Surface gravities
(log g) were determined by parallaxes and the ionization balance of iron.
Abundance determinations were carried out using the LTE approach, taking the
hyperfine structure for Eu into account, and the abundance of Ba was computed
under the NLTE approximation. We are able to assign most of the stars in our
sample to the substructures of the Galaxy thick disk, thin disk, or Hercules
stream according to their kinematics. The classification of 27 stars is
uncertain. For most of the stars in the sample, the abundances of
neutron-capture elements have not been measured earlier. For all of them, we
provide the chemical composition and discuss the contribution from different
nucleosynthesis processes.
|
1303.1730v1
|
2013-03-28
|
New Diluted Ferromagnetic Semiconductor isostructural to 122 type iron pnictide superconductor with TC up to 180 K
|
Diluted magnetic semiconductors (DMS) have received much attention due to its
potential applications to spintronics devices. A prototypical system (Ga,Mn)As
has been widely studied since 1990s. The simultaneous spin and charge doping
via hetero-valence (Ga3+,Mn2+) substitution, however, resulted in severely
limited solubility without availability of bulk specimens. Previously we
synthesized a new diluted ferromagnetic semiconductor of bulk Li(Zn,Mn)As with
Tc up to 50K, where isovalent (Zn,Mn) spin doping was separated from charge
control via Li concentrations. Here we report the synthesis of a new diluted
ferromagnetic semiconductor (Ba1-xKx)(Zn1-yMny)2As2, isostructural to iron 122
system, where holes are doped via (Ba2+, K1+), while spins via (Zn2+,Mn2+)
substitutions. Bulk samples with x=0.1-0.3 and y=0.05-0.15 exhibit
ferromagnetic order with TC up to 180K, comparable to that of record high Tc
for Ga(MnAs), significantly enhanced than Li(Zn,Mn)As. Moreover the
(Ba,K)(Zn,Mn)2As2 shares the same 122 crystal structure with semiconducting
BaZn2As2, antiferromagnetic BaMn2As2, and superconducting (Ba,K)Fe2As2, which
makes them promising to the development of multilayer functional devices.
|
1303.7157v1
|
2013-03-29
|
Seeing the orbital ordering in Iron-based superconductors with magnetic anisotropy
|
The orbital fluctuation of the conduction electrons in the Iron-based
superconductors is found to contribute significantly to the magnetic response
of the system. With the use of a realistic five-band model and group
theoretical analysis, we have determined the orbital magnetic susceptibility in
such a multi-orbital system. At $n=6.1$, the in-plane orbital magnetic
susceptibility is predicted to be about 10$\mu_{\mathrm{B}}^{2}/\mathrm{eV}$,
which is more than 2/3 of the observed total susceptibility around 200 K in 122
systems(of about 14$\mu_{\mathrm{B}}^{2}/\mathrm{eV}$ or
$4.5\times10^{-4}\mathrm{erg}/\mathrm{G}^{2}\mathrm{mol}_{\mathrm{AS}}$\cite{Klingeler}).
We find the in-plane orbital magnetic response is sensitive to the breaking of
the tetragonal symmetry in the orbital space. In particular, when the observed
band splitting(between the $3d_{xz}$ and the $3d_{yz}$-dominated band) is used
to estimate the strength of the symmetry breaking perturbation\cite{Shen}, a
4.5% modulation in the in-plane orbital magnetic susceptibility can be
produced, making the latter a useful probe of the orbital ordering in such a
multi-orbital system. As a by product, the theory also explains the large
anisotropy between the in-plane and the out-of-plane magnetic response observed
universally in susceptibility and NMR measurements.
|
1303.7302v2
|
2013-04-01
|
Carbon and Oxygen Abundances in the Hot Jupiter Exoplanet Host Star XO-2N and its Binary Companion
|
With the aim of connecting the compositions of stars and planets, we present
the abundances of carbon and oxygen, as well as iron and nickel, for the
transiting exoplanet host star XO-2N and its wide-separation binary companion
XO-2S. Stellar parameters are derived from high-resolution,
high-signal-to-noise spectra, and the two stars are found to be similar in
their Teff, log g, iron ([Fe/H]), nickel ([Ni/H]) abundances. Their carbon
([C/H]) and oxygen ([O/H]) abundances also overlap within errors, although
XO-2N may be slightly more C-rich and O-rich than XO-2S. The C/O ratios of both
stars (~0.60+/-0.20) may also be somewhat larger than solar (C/O~0.50). The
XO-2 system has a transiting hot Jupiter orbiting one binary component but not
the other, allowing us to probe the potential effects planet formation might
have on the host star composition. Additionally, with multiple observations of
its atmosphere the transiting exoplanet XO-2b lends itself to compositional
analysis, which can be compared to the natal chemical environment established
by our binary star elemental abundances. This work sets the stage for
determining how similar/different exoplanet and host star compositions are, and
the implications for planet formation, by discussing the C/O ratio measurements
in the unique environment of a visual binary system with one star hosting a
transiting hot Jupiter.
|
1304.0395v1
|
2013-04-03
|
Molybdenum, Ruthenium, and the Heavy r-process Elements in Moderately Metal-Poor Main-Sequence Turnoff Stars
|
The ratios of elemental abundances observed in metal-poor stars of the
Galactic halo provide a unique present-day record of the nucleosynthesis
products of its earliest stars. While the heaviest elements were synthesized by
the r- and s-processes, dominant production mechanisms of light trans-ironic
elements were obscure until recently. This work investigates further our 2011
conclusion that the low-entropy regime of a high-entropy wind (HEW) produced
molybdenum and ruthenium in two moderately metal-poor turnoff stars that showed
extreme overabundances of those elements with respect to iron. Only a few, rare
nucleosynthesis events may have been involved.
Here we determine abundances for Mo, Ru, and other trans-Fe elements for 28
similar stars by matching spectral calculations to well-exposed near-UV Keck
HIRES spectra obtained for beryllium abundances. In each of the 26 turnoff
stars with Mo or Ru line detections and no evidence for s-process production
(therefore old), we find Mo and Ru to be three to six times overabundant. In
contrast, the maximum overabundance is reduced to factors of three and two for
the neighboring elements zirconium and palladium. Since the overproduction
peaks sharply at Mo and Ru, a low-entropy HEW is confirmed as its origin.
The overabundance level of the heavy r-process elements varies significantly,
from none to a factor of four, but is uncorrelated with Mo and Ru
overabundances. Despite their moderate metallicity, stars in this group trace
the products of different nucleosynthetic events: possibly very few events,
possibly events whose output depended on environment, metallicity, or time.
|
1304.1050v1
|
2013-04-04
|
Learning from Nature to Improve the Heat Generation of Iron-Oxide Nanoparticles for Magnetic Hyperthermia Applications
|
The performance of magnetic nanoparticles is intimately entwined with their
structure, mean size and magnetic anisotropy. Besides, ensembles offer a unique
way of engineering the magnetic response by modifying the strength of the
dipolar interactions between particles. Here we report on an experimental and
theoretical analysis of magnetic hyperthermia, a rapidly developing technique
in medical research and oncology. Experimentally, we demonstrate that
single-domain cubic iron oxide particles resembling bacterial magnetosomes have
superior magnetic heating efficiency compared to spherical particles of similar
sizes. Monte Carlo simulations at the atomic level corroborate the larger
anisotropy of the cubic particles in comparison with the spherical ones, thus
evidencing the beneficial role of surface anisotropy in the improved heating
power. Moreover we establish a quantitative link between the particle
assembling, the interactions and the heating properties. This knowledge opens
new perspectives for improved hyperthermia, an alternative to conventional
cancer therapies.
|
1304.1298v1
|
2013-04-18
|
Hadron energy response of the Iron Calorimeter detector at the India-based Neutrino Observatory
|
The results of a Monte Carlo simulation study of the hadron energy response
for the magnetized Iron CALorimeter detector, ICAL, proposed to be located at
the India-based Neutrino Observatory (INO) is presented. Using a GEANT4
modeling of the detector ICAL, interactions of atmospheric neutrinos with
target nuclei are simulated. The detector response to hadrons propagating
through it is investigated using the hadron hit multiplicity in the active
detector elements. The detector response to charged pions of fixed energy is
studied first, followed by the average response to the hadrons produced in
atmospheric neutrino interactions using events simulated with the NUANCE event
generator. The shape of the hit distribution is observed to fit the Vavilov
distribution, which reduces to a Gaussian at high energies. In terms of the
parameters of this distribution, we present the hadron energy resolution as a
function of hadron energy, and the calibration of hadron energy as a function
of the hit multiplicity. The energy resolution for hadrons is found to be in
the range 85% (for 1GeV) -- 36% (for 15 GeV).
|
1304.5115v2
|
2013-04-19
|
Understanding the re-entrant superconducting phase diagram of an iron-pnictide Ca$_4$Al$_2$O$_6$Fe$_2$(As$_{1-x}$P$_x$)$_2$
|
Recently, a very rich phase diagram has been obtained for an iron-based
superconductor Ca4Al2O6Fe2(As1-xPx)2. It has been revealed that nodeless (x=0)
and nodal (x = 1) superconductivity are separated by an antiferromagnetic
phase. Here we study the origin of this peculiar phase diagram using a five
orbital model constructed from first principles band calculation, and applying
the fluctuation exchange approximation assuming spin fluctuation mediated
pairing. Based on the calculation results, we propose a scenario where the
frustration in momentum space degrades superconductivity in the intermediate x
regime, while antiferromangetism takes place due to a very good nesting. In
order to see whether the present theoretical scenario is consistent with the
actual nature of the competition between superconductivity and
antiferromagnetism, we also perform hydrostatic pressure experiment for
Ca4Al2O6Fe2(As1-xPx)2. In the intermediate x regime where antiferromagnetism
occurs at ambient pressure, applying hydrostatic pressure smears out the
antiferromagnetic transition, but superconductivity does not take place. This
supports our scenario that superconductivity is suppressed by the momentum
space frustration in the intermediate x regime, apart from the presence of the
antiferromangnetism.
|
1304.5360v1
|
2013-04-24
|
Deriving precise parameters for cool solar-type stars. Optimizing the iron line list
|
Temperature, surface gravity, and metallicitity are basic stellar atmospheric
parameters necessary to characterize a star. We aim to improve the description
of the spectroscopic temperatures especially for the cooler stars where the
differences with the Infrared Flux Method are higher, as presented in previous
work. Our spectroscopic analysis is based on the iron excitation and ionization
balance, assuming Kurucz model atmospheres in LTE. The abundance analysis is
determined using the code MOOG. We optimize the line list using a cool star
with high resolution and high signal-to-noise spectrum, as a reference in order
to check for weak, isolated lines. We test the quality of the new line list by
re-deriving stellar parameters for 451 stars with high resolution and
signal-to-noise HARPS spectra, that were analyzed in a previous work with a
larger line list. The comparison in temperatures between this work and the
latest IRFM shows that the differences for the cooler stars are significantly
smaller and more homogeneously distributed than in previous studies for stars
with temperatures below 5000 K. We use the new line list to re-derive
parameters for some of the cooler stars that host planets. Finally, we present
the impact of the new temperatures on the [Cr I/Cr II] and [Ti I/Ti II]
abundance ratios that previously showed systematic trends with temperature.
|
1304.6639v1
|
2013-04-28
|
Point Contact Spectroscopy of Fe Pnictides & Chalcogenides In The Normal State
|
We review the current status of point contact spectroscopy on the iron based
superconductors, focusing on their normal state. Point contact spectroscopy is
generally used to study superconductors via Andreev reflection, but in recent
years it has also proved to be a useful bulk probe of strongly correlated
electron systems. Point contact spectroscopy picks up a conductance enhancement
in the normal state, above the structural phase transition, of certain iron
based compounds. These include Co doped $\rm{BaFe_2As_2}$, $\rm{SrFe_2As_2}$,
$\rm{Fe_{1+y}Te}$ and F doped $\rm{SmFeAsO}$ and $\rm{LaFeAsO}$. Two materials
which do not show this conductance enhancement are $\rm{CaFe_2As_2}$ and K
doped $\rm{BaFe_2As_2}$. This conductance enhancement is thought to be tied to
orbital fluctuations. Orbital fluctuations in the normal state of these
compounds increases the single particle density of states at the Fermi level,
indicating that PCS is sensitive to this excess density of states. The
enhancement is only observed at those temperatures and dopings where an
in-plane resisitve anisotropy in the detwinned compounds is known to occur.
Thus point contact spectroscopy provides strong indications of electronic
nematicity in such materials. We also present diagnostics on how to judge if a
junction is impacted by joule heating or not. We conclude with the outstanding
challenges in the field and the new experiments that need to be carried out.
|
1304.7426v1
|
2013-05-08
|
A fully quantum mechanical calculation of the diffusivity of hydrogen in iron using the tight binding approximation and path integral theory
|
We present calculations of free energy barriers and diffusivities as
functions of temperature for the diffusion of hydrogen in bcc-Fe. This is a
fully quantum mechanical approach since the total energy landscape is computed
using a new self consistent, transferable tight binding model for interstitial
impurities in magnetic iron. Also the hydrogen nucleus is treated quantum
mechanically and we compare here two approaches in the literature both based in
the Feynman path integral formulation of statistical mechanics. We find that
the quantum transition state theory which admits greater freedom for the proton
to explore phase space gives result in better agreement with experiment than
the alternative which is based on fixed centroid calculations of the free
energy barrier. We also find results in better agreement compared to recent
centroid molecular dynamics (CMD) calculations of the diffusivity which
employed a classical interatomic potential rather than our quantum mechanical
tight binding theory. In particular we find first that quantum effects persist
to higher temperatures than previously thought, and conversely that the low
temperature diffusivity is smaller than predicted in CMD calculations and
larger than predicted by classical transition state theory. This will have
impact on future modeling and simulation of hydrogen trapping and diffusion.
|
1305.1772v1
|
2013-05-10
|
Carbon Deflagration in Type Ia Supernova: I. Centrally Ignited Models
|
A leading model for Type Ia supernovae (SNe Ia) begins with a white dwarf
near the Chandrasekhar mass that ignites a degenerate thermonuclear runaway
close to its center and explodes. In a series of papers, we shall explore the
consequences of ignition at several locations within such dwarfs. Here we
assume central ignition, which has been explored before, however, the problem
is worth revisiting, if only to validate those previous studies and to further
elucidate the relevant physics for future work. A perturbed sphere of hot iron
ash with a radius of ~100 km is initialized at the middle of the star. The
subsequent explosion is followed in several simulations using a thickened flame
model in which the flame speed is either fixed --- within the range expected
from turbulent combustion --- or based on the local turbulent intensity. Global
results, including the explosion energy and bulk nucleosynthesis (e.g. 56Ni of
0.48--0.56 $\Msun$) turn out to be insensitive to this speed. In all completed
runs, the energy released by the nuclear burning is adequate to unbind the
star, but not enough to give the energy and brightness of typical SNe Ia. As
found previously, the chemical stratification observed in typical events is not
reproduced. These models produce a large amount of unburned carbon and oxygen
in central low velocity regions, which is inconsistent with spectroscopic
observations, and the intermediate mass elements and iron group elements are
strongly mixed during the explosion.
|
1305.2433v1
|
2013-05-16
|
Effect of doping on the magnetostructural ordered phase of iron arsenides: A comparative study of the resistivity anisotropy in the doped BaFe$_2$As$_2$ with doping into three different sites
|
In order to unravel a role of doping in the iron-based superconductors, we
investigated the in-plane resistivity for BaFe$_2$As$_2$ doped at either of the
three different lattice sites, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$,
BaFe$_2$(As$_{1-x}$P$_x$)$_2$, and Ba$_{1-x}$K$_x$Fe$_2$As$_2$, focusing on the
doping effect in the low-temperature antiferromagnetic/orthorhombic (AFO)
phase. A major role of doping in the high-temperature paramagnetic/tetragonal
(PT) phase is known to change the Fermi surface by supplying charge carriers or
by exerting chemical pressure. In the AFO phase, we found a clear correlation
between the magnitude of residual resistivity and resistivity anisotropy. This
indicates that the resistivity anisotropy originates from the anisotropic
impurity scattering from dopant atoms. The magnitude of residual resistivity is
also found to be a parameter controlling the suppression rate of AFO ordering
temperature $T_s$. Therefore, the dominant role of doping in the AFO phase is
to introduce disorder to the system, distinct from that in the PT phase.
|
1305.3744v1
|
2013-05-31
|
Supernova Ejecta in the Youngest Galactic Supernova Remnant G1.9+0.3
|
G1.9+0.3 is the youngest known Galactic supernova remnant (SNR), with an
estimated supernova (SN) explosion date of about 1900, and most likely located
near the Galactic Center. Only the outermost ejecta layers with free-expansion
velocities larger than about 18,000 km/s have been shocked so far in this
dynamically young, likely Type Ia SNR. A long (980 ks) Chandra observation in
2011 allowed spatially-resolved spectroscopy of heavy-element ejecta. We
denoised Chandra data with the spatio-spectral method of Krishnamurthy et al.,
and used a wavelet-based technique to spatially localize thermal emission
produced by intermediate-mass elements (IMEs: Si and S) and iron. The spatial
distribution of both IMEs and Fe is extremely asymmetric, with the strongest
ejecta emission in the northern rim. Fe Kalpha emission is particularly
prominent there, and fits with thermal models indicate strongly oversolar Fe
abundances. In a localized, outlying region in the northern rim, IMEs are less
abundant than Fe, indicating that undiluted Fe-group elements (including 56Ni)
with velocities larger than 18,000 km/s were ejected by this SN. But in the
inner west rim, we find Si- and S-rich ejecta without any traces of Fe, so
high-velocity products of O-burning were also ejected. G1.9+0.3 appears similar
to energetic Type Ia SNe such as SN 2010jn where iron-group elements at such
high free-expansion velocities have been recently detected. The pronounced
asymmetry in the ejecta distribution and abundance inhomogeneities are best
explained by a strongly asymmetric SN explosion, similar to those produced in
some recent 3D delayed-detonation Type Ia models.
|
1305.7399v1
|
2013-05-31
|
Space Velocities of Southern Globular Clusters VII. NGC 6397, NGC 6626 (M 28) and NGC 6656 (M 22)
|
We have measured the absolute proper motions of globular clusters NGC 6397,
NGC 6626 (M 22) and NGC 6656 (M 28) as part of our ongoing Southern
Proper-Motion Program. The reference system is the ICRS via Hipparcos stars for
these three low galactic latitude clusters. Formal errors range between ~0.3
and 0.7 mas/yr. Notable is the result for NGC 6397 which differs by 2.5 mas/yr
from two HST determinations, while agreeing with previous ground-based ones. We
determine orbits for all three clusters in an axisymmetric and barred model of
the Galaxy and discuss these in the context of globular-cluster formation. M 22
is a well-known cluster with an iron abundance spread; such clusters are now
believed to have formed in massive parent systems that can retain ejecta of
core-collapsed SNe. We find that the five currently-accepted globular clusters
with iron/calcium abundance spread show orbits unrelated to each other, thus
suggesting at least five independent, massive progenitors that have contributed
to the build-up of the Milky-Way halo.
|
1305.7431v1
|
2013-06-26
|
Are there missing bond paths in Trimethylenemethane-Iron-tri-carbonyl [(CO)3Fe-(C4H6)] complex?
|
In a recent paper [J. Organomet. Chem. (2013) doi: 10.1016 /j.jorganchem.
2013.03.047] analyzing the bonding mode of Trimethylenemethane (TMM) with some
metal carbonyls, Mousavi and Frenking have declared the absence of bond paths,
the so-called missed bond paths, between metal atoms and terminal carbon atoms
in several complexes. In this communication, it is first demonstrated that the
presupposition of the equivalence of a bond path and a chemical bond within the
context of the quantum theory of atoms in molecules (QTAIM) is superficial and
basically flawed which is not only against with the recent strict declaration
on the contrary [R.F.W. Bader, J. Phys. Chem. A 113 (2009) 10396], but also in
odd with the foundations of the QTAIM. Then, it is demonstrated that the
so-called missed bond paths indeed appear in molecular graphs of some
non-equilibrium geometries that are energetically quite accessible at room
temperatures. To emphasize on the importance of this observation the term
passionate neighbors is coined referring to the atomic basins that do not share
an inter-atomic surface at the equilibrium geometry but are neighbors, share an
inter-atomic surface, at non-equilibrium geometries accessible by nuclear
vibrations. Using the delocalization index as well as other evidence from
previous literature it is demonstrated that the QTAIM analysis is indeed
compatible with the presence of chemical bonds between iron metal and terminal
carbons in (CO)3Fe-TMM complex. Finally, some general discussions are done to
unravel the delicate relationship between the QTAIM proposed concepts, e.g.
bond paths and molecular graphs, and some orthodox concepts of chemistry, e.g.
chemical bonds and chemical structures, emphasizing that there is no one-to-one
relationship between these two categorizes.
|
1306.6350v2
|
2013-07-01
|
SWEET-Cat: A catalogue of parameters for Stars With ExoplanETs I. New atmospheric parameters and masses for 48 stars with planets
|
Due to the importance that the star-planet relation has to our understanding
of the planet formation process, the precise determination of stellar
parameters for the ever increasing number of discovered extra-solar planets is
of great relevance. Furthermore, precise stellar parameters are needed to fully
characterize the planet properties. It is thus important to continue the
efforts to determine, in the most uniform way possible, the parameters for
stars with planets as new discoveries are announced. In this paper we present
new precise atmospheric parameters for a sample of 48 stars with planets. We
then take the opportunity to present a new catalogue of stellar parameters for
FGK and M stars with planets detected by radial velocity, transit, and
astrometry programs. Stellar atmospheric parameters and masses for the 48 stars
were derived assuming LTE and using high resolution and high signal-to-noise
spectra. The methodology used is based on the measurement of equivalent widths
for a list of iron lines and making use of iron ionization and excitation
equilibrium principles. For the catalog, and whenever possible, we used
parameters derived in previous works published by our team, using well defined
methodologies for the derivation of stellar atmospheric parameters. This set of
parameters amounts to over 65% of all planet host stars known, including more
than 90% of all stars with planets discovered through radial velocity surveys.
For the remaining targets, stellar parameters were collected from the
literature.
|
1307.0354v1
|
2013-07-17
|
The role of the Gamma-Eddington ratio relation on the X-ray Baldwin effect in Active Galactic Nuclei
|
The X-ray Baldwin effect is the inverse correlation between the equivalent
width (EW) of the narrow component of the iron Kalpha line and the X-ray
luminosity of active galactic nuclei (AGN). A similar trend has also been
observed between Fe Kalpha EW and the Eddington ratio (lambda_Edd). Using
Chandra/HEG results of Shu et al. (2010) and bolometric corrections we study
the relation between EW and the lambda_Edd, and find that log EW =
(-0.13+/-0.03)log(lambda_Edd) + 1.47. We explore the role of the known positive
correlation between the photon index of the primary X-ray continuum (Gamma) and
lambda_Edd on the X-ray Baldwin effect. We simulate the iron Kalpha line
emitted by populations of unabsorbed AGN considering 3 different geometries of
the reflecting material: toroidal, spherical-toroidal and slab. We find that
the Gamma-lambda_Edd correlation cannot account for the whole X-ray Baldwin
effect, unless a strong dependence of Gamma on lambda_Edd, such as the one
recently found by Risaliti et al. (2009) and Jin et al. (2012), is assumed. No
clear correlation is found between EW and Gamma. We conclude that a good
understanding of the slope of the Gamma-lambda_Edd relation is critical to
assess whether the trend plays a leading or rather a marginal role in the X-ray
Baldwin effect.
|
1307.4507v2
|
2013-07-23
|
Variable X-ray reflection from 1H~0419--577
|
We present detailed broadband X-ray spectral variability of a Seyfert 1
galaxy 1H~0419--577 based on an archival \suzaku{} observation in July 2007, a
new \suzaku{} observation performed in January 2010 and the two latest \xmm{}
observations from May 2010. All the observations show soft X-ray excess
emission below $2\kev$ and both \suzaku{} observations show a hard X-ray excess
emission above $10\kev$ when compared to a power-law. We have tested three
physical models -- a complex partial covering absorption model, a blurred
reflection model and an intrinsic disk Comptonization model. Among these three
models, the blurred reflection model provided statistically the best-fit to all
the four observations. Irrespective of the models used, the soft X-ray excess
emission requires contribution from a thermal component similar to that
expected from an accretion disk. The partial covering absorption model results
in a nonphysical high temperature ($kT_{in} \sim 100\ev$) for an accretion disk
and is also statistically the worst fit among the three models. 1H~0419--577
showed remarkable X-ray spectral variability. The soft X-ray excess and the
power-law both became weaker in January 2010 as well as in May 2010. A
moderately broad iron line, detected in July 2007, is absent in the January
2010 observation. Correlated variability of the soft X-ray excess and the iron
$K\alpha$ line strongly suggest reflection origin for both the components.
However, such spectral variability cannot be explained by the light bending
model alone and requires changes in the accretion disk/corona geometry possibly
arising from changes in the accretion rate.
|
1307.6223v1
|
2013-08-15
|
Structural and optical investigations of the iron-chalcogenide superconductor Fe$_{1.03}$Se$_{0.5}$Te$_{0.5}$ under high pressure
|
Iron-chalcogenide superconductor Fe$_{1.03}$Se$_{0.5}$Te$_{0.5}$ has been
investigated under high pressure using synchrotron based x-ray diffraction and
mid-infrared reflectance measurements at room temperature. Pressure dependence
of the superconducting transition temperature (T$_c$) of the same sample has
been determined by temperature-dependent resistance measurements up to 10 GPa.
Although the high pressure orthorhombic phase ($\textit{Pbnm}$) starts emerging
at 4 GPa, structural transition becomes clearly observable above 10 GPa. A
strong correlation is observed between the Fe(Se,Te)$_{4}$ tetrahedral
deformation in the tetragonal phase ($\textit{P4/nmm}$) and the sharp rise of
T$_c$ up to $\sim$4 GPa, above which T$_c$ is found to be almost pressure
independent at least up to 10 GPa. A subtle structural modification of the
tetragonal phase is noticed above 10 GPa, suggesting a structural transition
with possible Fe$^{2+}$ spin-state transition. The evolution with pressure of
the optical conductivity shows that the Drude term increases systematically
with pressure up to 5.4 GPa, indicating the evolution of the tetragonal phase
towards a conventional metallic state. At further higher pressures the Drude
term reduces drastically implying a poor metallic character of the high
pressure orthorhombic phase. Our results suggest that occurrence of large onset
T$_c$ above 4 GPa is due to a systematic increase of high pressure orthorhombic
phase fraction.
|
1308.3367v1
|
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