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2008-08-12
|
Magnetism Driven by Anion Vacancies in Superconducting $α$--FeSe$_{1-x}$
|
To study the microscopic electronic and magnetic interactions in the
substoichiometric iron chalcogenide FeSe$_{1-x}$ which is observed to
superconduct at x~1/8 up to $T_c$=27 K, we use first principles methods to
study the Se vacancy in this nearly magnetic FeSe system. The vacancy forms a
ferrimagnetic cluster of eight Fe atoms, which for the ordered x=1/8 alloy
leads to half metallic conduction. Similar magnetic clusters are obtained for
FeTe$_{1-x}$ and for BaFe$_2$As$_2$ with an As vacancy, although neither of
these are half metallic. Based on fixed spin density results, we suggest the
low energy excitations in FeSe$_{1-x}$ are antiparamagnon-like with short
correlation length.
|
0808.1733v2
|
2008-08-13
|
Jet modification in 200 AGeV Au-Au collisions
|
The computation of hard processes in hadronic collisions is a major success
of perturbative Quantum Chromodynamics. The environment of heavy-ion collisions
offers the opportunity to embed such hard processes into a soft medium which is
created simultaneously and study the medium-induced modifications. On the level
of single high transverse momentum hadrons, a reduction in yield, the so-called
quenching is observed. However, on the level of jets, the energy-momentum flux
carried by hadrons is conserved, i.e. the effect of the medium is a
redistribution of energy and momentum, and statements about quenching of jets
can only be made for specific cuts used to identify the jet. In this paper, we
present a baseline computation for the expected modification of the
longitudinal momentum distribution in jets by the medium created in 200 AGeV
Au-Au collisions given a realistic set of experimental cuts used to identify
jets in a heavy-iron environment. For this purpose, we use a Monte-Carlo code
developed to simulate in-medium shower evolution following a hard process and a
3-d hydrodynamical evolution to simulate the soft medium.
|
0808.1803v1
|
2008-08-13
|
Ranges of Atmospheric Mass and Composition of Super Earth Exoplanets
|
Terrestrial-like exoplanets may obtain atmospheres from three primary
sources: Capture of nebular gases, degassing during accretion, and degassing
from subsequent tectonic activity. Here we model degassing during accretion to
estimate the range of atmospheric mass and composition on exoplanets ranging
from 1 to 30 Earth masses. We use bulk compositions drawn from primitive and
differentiated meteorite compositions. Degassing alone can create a wide range
of masses of planetary atmospheres, ranging from less than a percent of the
planet's total mass up to ~6 mass% of hydrogen, ~20 mass% of water, and/or ~5
mass% of carbon compounds. Hydrogen-rich atmospheres can be outgassed as a
result of oxidizing metallic iron with water, and excess water and carbon can
produce atmospheres through simple degassing. As a byproduct of our atmospheric
outgassing models we find that modest initial water contents (10 mass% of the
planet and above) create planets with deep surface liquid water oceans soon
after accretion is complete.
|
0808.1909v1
|
2008-08-17
|
Spin Dynamics in Iron-based Layered Superconductor (La_{0.87}Ca_{0.13})FePO Revealed by ^{31}P and ^{139}La NMR Studies
|
We report ^{31}P and ^{139}La NMR studies of (La_{0.87}Ca_{0.13})FePO, which
is a family member of the recently discovered superconductor
LaFeAs(O_{1-x}F_x). In the normal state, Knight shift and $1/T_1T$ show that a
Fermi-liquid state with moderate ferromagnetic fluctuations emerges below 30K.
From 1/T_1T of ^{31}P and ^{139}La, quasi-two dimensional electronic
structure is suggested, in which the FeP layer is more conductive than the LaO
layer. In the superconducting (SC) state, although a clear Meissner signal was
observed, 1/T_1T increases below T_c, in contrast to a decrease of 1/T_1T due
to the opening of a SC gap, suggesting that novel low-energy spin dynamics
develop in the SC state.
|
0808.2293v1
|
2008-08-20
|
Correlation effects of exchange splitting and coexistence of spin-density-wave and superconductivity in single crystalline Sr1-xKxFe2As2
|
The nature of spin-density wave and its relation with superconductivity are
crucial issues in the newly discovered Fe-based high temperature
superconductors. Particularly it is unclear whether the superconducting phase
and spin density wave (SDW) are truly exclusive from each other as suggested by
certain experiments. With angle resolved photoemission spectroscopy, we here
report exchange splittings of the band structures in Sr1-xKxFe2As2
(x=0,0.1,0.2), and the non-rigid-band behaviors of the splitting. Our data on
single crystalline superconducting samples unambiguously prove that SDW and
superconductivity could coexist in iron-pnictides.
|
0808.2738v1
|
2008-08-20
|
Superconducting Properties of Two-Orbital t-t'-J-J' Models
|
Motivated by the recent contradiction of the superconducting pairing symmetry
in the angle-resolved photoemission spectra (ARPES) and the nuclear magnetic
resonance (NMR) data in the FeAs superconductors, we present the theoretical
results on the phase diagram, the temperature dependent Fermi surfaces in
normal state, the ARPES character of quasiparticles and the spin-lattice
relaxation 1/T$_{1}$ of the two-orbital t-t$^{'}$-J-J$^{'}$ models. Our results
show that most of the properties observed in iron-based superconductors could
be comprehensively understood in the present scenario qualitatively, indicating
that the pairing symmetry of the ironpnictides is anisotropic nodeless s-wave,
mainly originating from the band structures and the Fermi surface topology.
|
0808.2763v2
|
2008-08-21
|
High energy spin excitations in BaFe2As2
|
We report neutron scattering measurements of cooperative spin excitations in
antiferromagnetically ordered BaFe2As2, the parent phase of an iron pnictide
superconductor. The data extend up to ~100meV and show that the spin excitation
spectrum is sharp and highly dispersive. By fitting the spectrum to a linear
spin-wave model we estimate the magnon bandwidth to be in the region of 0.17eV.
The large characteristic spin fluctuation energy suggests that magnetism could
play a role in the formation of the superconducting state.
|
0808.2836v2
|
2008-08-26
|
Impurity suppression of the critical temperature in the iron-based superconductors
|
We study the impurity suppression of the critical temperature $T_c$ of the
FeAs superconductors theoretically based on the the $\pm$s-wave pairing state
of a two band model. The effects of non-magnetic and magnetic impurities are
studied with the $\mathcal{T}$-matrix approximation, which can continuously
treat impurity scattering from weak to strong coupling limit. We found that
both magnetic and non-magnetic impurities suppress $T_c$ with a rate that is
practically indistinguishable from the standard d-wave case despite a possibly
large difference of the positive and negative s-wave order parameter (OP)
magnitudes. This is because the density of states enters together with the OP
magnitude for the scattering process.
|
0808.3473v2
|
2008-08-28
|
Orbital Selective Superconductivity in Two-Orbital Asymmetric t-J Models
|
We present the zero-temperature superconducting (SC) ground states of the
two-orbital asymmetric $t-J$ model on a square lattice by means of the
auxiliary-boson approach. Besides the two-gap SC phase, we find an orbital
selective SC (OSSC) phase, which is simultaneously SC in one orbit and normal
in another orbit. The novel OSSC phase is stable only for sufficient asymmetric
degree in orbital space and doping concentration. The pairing symmetry of the
SC phase is s-wave-like in most doping regime, against the d-wave symmetry of
the single-orbital $t-J$ model in a square lattice. The implication of the
present scenario on multi-orbital heavy fermion and iron-based superconductors
is also discussed.
|
0808.3809v1
|
2008-09-02
|
Near-Infrared Photometry of Four Stellar Clusters in the Small Magellanic Cloud
|
We present high-quality J, H and K photometry of four Small Magellanic Cloud
stellar clusters with intermediate ages in the 1-7 Gyr range (namely NGC 339,
361, 416 and 419) . We obtained deep Color-Magnitude Diagrams to study the
evolved sequences and providing a detailed census of the Red Giant Branch
(RGB), Asymptotic Giant Branch (AGB) and Carbon star populations in each
cluster and their contribution to the total cluster light. We find that in the
5-7 Gyr old clusters AGB stars account for ~6 % of the total light in K-band,
Carbon stars are lacking and RGB stars account for ~45 % of the total
bolometric luminosity. These empirical findings are in good agreement with the
theoretical predictions. Finally, we derived photometric metallicities computed
by using the properties of the RGB and finding an iron content of [Fe/H]=-1.18,
-1.08, -0.99 and -0.96 dex for NGC 339, 361, 416 and 419 respectively.
|
0809.0495v1
|
2008-09-10
|
Kramers degeneracy in a magnetic field and Zeeman spin-orbit coupling in antiferromagnets
|
In this article, I analyze the symmetries and degeneracies of electron
eigenstates in a commensurate collinear antiferromagnet. In a magnetic field
transverse to the staggered magnetization, a hidden anti-unitary symmetry
protects double degeneracy of the Bloch eigenstates at a special set of
momenta. In addition to this `Kramers degeneracy' subset, the manifold of
momenta, labeling the doubly degenerate Bloch states in the Brillouin zone, may
also contain an `accidental degeneracy' subset, that is not protected by
symmetry and that may change its shape under perturbation. These degeneracies
give rise to a substantial momentum dependence of the transverse g-factor in
the Zeeman coupling, turning the latter into a spin-orbit interaction.
I discuss a number of materials, where Zeeman spin-orbit coupling is likely
to be present, and outline the simplest properties and experimental
consequences of this interaction, that may be relevant to systems from chromium
to borocarbides, cuprates, hexaborides, iron pnictides, as well as organic and
heavy fermion conductors.
|
0809.1893v3
|
2008-09-13
|
Chemical Abundances from the Continuum
|
The calculation of solar absolute fluxes in the near-UV is revisited,
discussing in some detail recent updates in theoretical calculations of
bound-free opacity from metals. Modest changes in the abundances of elements
such as Mg and the iron-peak elements have a significant impact on the
atmospheric structure, and therefore self-consistent calculations are
necessary. With small adjustments to the solar photospheric composition, we are
able to reproduce fairly well the observed solar fluxes between 200 and 270 nm,
and between 300 and 420 nm, but find too much absorption in the 270-290 nm
window. A comparison between our reference 1D model and a 3D time-dependent
hydrodynamical simulation indicates that the continuum flux is only weakly
sensitive to 3D effects, with corrections reaching <10% in the near-UV, and <2%
in the optical.
|
0809.2364v1
|
2008-09-15
|
Role of covalent Fe-As bonding in the magnetic moment formation and exchange mechanisms in iron-pnictide superconductors
|
The electronic origin of the huge magnetostructural effect in layered Fe-As
compounds is elucidated using LiFeAs as a prototype. The crucial feature of
these materials is the strong covalent bonding between Fe and As, which tends
to suppress the exchange splitting. The bonding-antibonding splitting is very
sensitive to the distance between Fe and As nuclei. We argue that the fragile
interplay between bonding and magnetism is universal for this family of
compounds. The exchange interaction is analyzed in real space, along with its
correlation with covalency and doping. The range of interaction and itinerancy
increase as the Fe-As distance is decreased. Superexchange makes a large
antiferromagnetic contribution to the nearest-neighbor coupling, which develops
large anisotropy when the local moment is not too small. This anisotropy is
very sensitive to doping.
|
0809.2586v3
|
2008-09-19
|
An ab-initio calculation of the core-level x-ray photoemission spectra -Fe 3s- and 1s-core levels-
|
We develop a method of the ab-initio calculation for the core-level x-ray
photoemission spectroscopy (XPS). By calculating one-electron states in the
presence of core-hole potential, excited states are constructed by distributing
electrons on these one-electron states. The overlap integrals between the
excited states and the ground state are evaluated by using the wavefunctions
with and without the core-hole potential, and finally the XPS spectra are
obtained. Implementing the procedure, we obtain the spin-resolved 3s-core XPS
spectra in ferromagnetic iron without any adjustable parameters, in good
agreement with the experiment. The spectral shapes are quite different for
different spin channels. We explain the behavior in terms of the difference in
the one-electron states screening the core-hole potential.
|
0809.3296v1
|
2008-09-23
|
Ab initio exchange interactions and magnetic properties of Gd2Fe17 iron sublattice: rhombohedral vs. hexagonal phases
|
In the framework of the LSDA+U method electronic structure and magnetic
properties of the intermetallic compound Gd2Fe17 for both rhombohedral and
hexagonal phases have been calculated. On top of that, ab initio exchange
interaction parameters within the Fe sublattice for all present nearest and
some next nearest Fe ions have been obtained. It was found that for the first
coordination sphere direct exchange interaction is ferromagnetic. For the
second coordination sphere indirect exchange interaction is observed to be
weaker and of antiferromagnetic type. Employing the theoretical values of
exchange parameters Curie temperatures Tc of both hexagonal and rhombohedral
phases of Gd2Fe17 within Weiss mean-field theory were estimated. Obtained
values of Tc and its increase going from the hexagonal to rhombohedral crystal
structure of Gd2Fe17 agree well with experiment. Also for both structures
LSDA+U computed values of total magnetic moment coincide with experimental
ones.
|
0809.3905v2
|
2008-10-01
|
Anomalous behavior of the Debye temperature in Fe-rich Fe-Cr alloys
|
Debye temperature, $\Theta_D$, of Fe-rich Fe$_{100-x}$Cr$_x$ disordered
alloys with $0\le x \le 22.3$ was determined from the temperature dependence of
the central shift of M\"ossbauer spectra recorded in the temperature range of
60 -- 300 K. Its compositional dependence shows a maximum at $x \approx 5$ with
a relative increase of $\sim 30$% compared to a pure iron. The composition at
which the effect occurs correlates well with that at which several other
quantities, e. g. the Curie temperature and the spin-wave stiffness
coefficient, $D_0$, show their maxima, but the enhancement of $\Theta_D$ is
significantly greater and comparable with the enhancement of the hyperfine
field (spin-density of itinerant $s$-like electrons) in the studied system. The
results suggest that the electron-phonon interaction is important in this alloy
system.
|
0810.0123v1
|
2008-10-01
|
Pressure effects on the electron-doped high Tc superconductor BaFe(2-x)Co(x)As(2)
|
Application of pressures or electron-doping through Co substitution into Fe
sites transforms the itinerant antiferromagnet BaFe(2)As(2) into a
superconductor with the Tc exceeding 20K. We carried out systematic transport
measurements of BaFe(2-x)Co(x)As(2) superconductors in pressures up to 2.5GPa,
and elucidate the interplay between the effects of electron-doping and
pressures. For the underdoped sample with nominal composition x = 0.08,
application of pressure strongly suppresses a magnetic instability while
enhancing Tc by nearly a factor of two from 11K to 21K. In contrast, the
optimally doped x=0.20 sample shows very little enhancement of Tc=22K under
applied pressure. Our results strongly suggest that the proximity to a magnetic
instability is the key to the mechanism of superconductivity in iron-pnictides.
|
0810.0287v1
|
2008-10-01
|
NMR measurements of intrinsic spin susceptibility in LaFeAsO(0.9)F(0.1)
|
We will probe the intrinsic behavior of spin susceptibility chi_(spin) in the
LaFeAsO(1-x)F(x) superconductor (x ~ 0.1, Tc ~ 27K) using 19-F and 75-As NMR
techniques. Our new results firmly establish the pseudo-gap behavior with
Delta_(PG)/kB ~ 140K. The estimated magnitude of chi_(spin) at 290K,
~1.8x10^(-4) [emu/mol-Fe], is approximately twice larger than that in high Tc
cuprates. We also show that chi_(spin) levels off below ~50K down to Tc.
|
0810.0305v1
|
2008-10-06
|
Pairing State with a time-reversal symmetry breaking in FeAs based superconductors
|
We investigate the competition between the extended $s_{\pm}$-wave and
$d_{x^2-y^2}$-wave pairing order parameters in the iron-based superconductors.
Because of the frustrating pairing interactions among the electron and the hole
fermi pockets, a time reversal symmetry breaking $s+id$ pairing state could be
favored. We analyze this pairing state within the Ginzburg-Landau theory, and
explore the experimental consequences. In such a state, spatial inhomogeneity
induces supercurrent near a non-magnetic impurity and the corners of a square
sample. The resonance mode between the $s_{\pm}$ and $d_{x^2-y^2}$-wave order
parameters can be detected through the $B_{1g}$-Raman spectroscopy.
|
0810.0887v3
|
2008-10-06
|
Scanning Tunneling Spectroscopy and Vortex Imaging in the Iron-Pnictide Superconductor BaFe$_{1.8}$Co$_{0.2}$As$_2$
|
We present an atomic resolution scanning tunneling spectroscopy study of
superconducting BaFe$_{1.8}$Co$_{0.2}$As$_2$ single crystals in magnetic fields
up to $9 \text{Tesla}$. At zero field, a single gap with coherence peaks at
$\overline{\Delta}=6.25 \text{meV}$ is observed in the density of states. At $9
\text{T}$ and $6 \text{T}$, we image a disordered vortex lattice, consistent
with isotropic, single flux quantum vortices. Vortex locations are uncorrelated
with strong scattering surface impurities, demonstrating bulk pinning. The
vortex-induced sub-gap density of states fits an exponential decay from the
vortex center, from which we extract a coherence length $\xi=27.6\pm 2.9
\text{\AA}$, corresponding to an upper critical field $H_{c2}=43 \text{T}$.
|
0810.1048v2
|
2008-10-08
|
Dielectric anomalies induced by different mechanisms in Pb(Fe1/2Nb1/2)1-xTixO3 single crystals
|
We investigated the dielectric properties of Pb(Fe1/2Nb1/2)1-xTixO3 single
crystals below room temperature. Two dielectric anomalies were detected in
sample A while only one was detected in sample B in the temperature range
90~300 K. A Debye-like relaxation with strong frequency dispersion was detected
in both samples. The pre-edge XAFS suggests that this dielectric anomaly is
induced by the hopping conductivity between Fe2+ and Fe3+. The EXAFS results
give us a clear picture of the local structure of iron ions. The weak frequency
dependent dielectric anomaly only observed in sample A is supposed to be due to
the dipole glass behavior.
|
0810.1346v3
|
2008-10-08
|
Interband superconductivity: contrasts between BCS and Eliashberg theory
|
The newly discovered iron pnictide superconductors apparently present an
unusual case of interband-channel pairing superconductivity. Here we show that,
in the limit where the pairing occurs within the interband channel, several
surprising effects occur quite naturally and generally: different
density-of-states on the two bands lead to several unusual properties,
including a gap ratio which behaves inversely to the ratio of
density-of-states; the weak-coupling limit of the Eliashberg and the BCS
theory, commonly taken as equivalent, in fact predict qualitatively different
dependence of the $\Delta_{1}/\Delta_{2}$ and $\Delta/T_{c}$ ratios on coupling
constants. We show analytically that these effects follow directly from the
interband character of superconductivity. Our results show that in the
interband-only pairing model the maximal gap ratio is $\sqrt{N_{2}/N_{1}}$ as
strong-coupling effects act only to reduce this ratio. This suggests that if
the large experimentally reported gap ratios (up to a factor 2) are correct,
the pairing mechanism must include more intraband interaction than is usually
assumed.
|
0810.1476v1
|
2008-10-09
|
Muon-spin rotation study of the in-plane magnetic penetration depth of FeSe_0.85: evidence for nodeless superconductivity
|
The in-plane magnetic penetration depth \lambda_{ab} of the iron selenide
superconductor with the nominal composition FeSe_0.85 was studied by means of
muon-spin rotation. The measurements of \lambda_{ab}^{-2}(T) are inconsistent
with the presence of nodes in the gap as well as with a simple isotropic s-wave
type of the order parameter, but can be equally well described within a two-gap
(s+s) and an anisotropic s-wave gap picture. This implies that the
superconducting energy gap in FeSe_0.85 contains no nodes.
|
0810.1716v1
|
2008-10-09
|
Kondo decoherence: finding the right spin model for iron impurities in gold and silver
|
We exploit the decoherence of electrons due to magnetic impurities, studied
via weak localization, to resolve a longstanding question concerning the
classic Kondo systems of Fe impurities in the noble metals gold and silver:
which Kondo-type model yields a realistic description of the relevant multiple
bands, spin and orbital degrees of freedom? Previous studies suggest a fully
screened spin $S$ Kondo model, but the value of $S$ remained ambiguous. We
perform density functional theory calculations that suggest $S = 3/2$. We also
compare previous and new measurements of both the resistivity and decoherence
rate in quasi 1-dimensional wires to numerical renormalization group
predictions for $S=1/2,1$ and 3/2, finding excellent agreement for $S=3/2$.
|
0810.1771v2
|
2008-10-10
|
Inverse Raman Scattering in Silicon
|
Stimulated Raman scattering is a well-known nonlinear process that can be
harnessed to produce optical gain in a wide variety of media. This effect has
been used to produce the first silicon-based lasers and high-gain amplifiers.
Interestingly, the Raman effect can also produce intensity-dependent nonlinear
loss through a corollary process known as inverse Raman scattering (IRS). Here,
we demonstrate IRS in silicon--a process that is substantially modified by the
presence of optically-generated free carriers--achieving attenuation levels >15
dB with a pump intensity of 4 GW/cm^2. Ironically, we find that free-carrier
absorption, the detrimental effect that suppresses other nonlinear effects in
silicon, actually facilitates IRS by delaying the onset of contamination from
coherent anti-Stokes Raman scattering. The carriers allow significant IRS
attenuation over a wide intensity range. Silicon-based IRS could be used to
produce chip-scale wavelength-division multiplexers, optical signal inverters,
and fast optical switches.
|
0810.1939v1
|
2008-10-14
|
SrFeAsF as a parent compound for iron pnictide superconductors
|
We have successfully synthesized the fluo-arsenide SrFeAsF, a new parent
phase with the ZrCuAsSi structure. The temperature dependence of resistivity
and dc magnetization both reveal an anomaly at about T_{an} = 173 K, which may
correspond to the structural and/or Spin-Density-Wave (SDW) transition. Strong
Hall effect and magnetoresistance were observed below T_{an}. Interestingly,
the Hall coefficient R_H is positive below T_{an}, which is opposite to the
cases in the two parent phases of FeAs-based systems known so far, i.e.,
LnFeAsO (Ln = rare earth elements) and (Ba, Sr)Fe_2As_2 where the Hall
coefficient R_H is negative. This strongly suggests that the gapping to the
Fermi surfaces induced by the SDW order is more complex than we believed before
that it removes the density of states on some Fermi pockets and leaves one of
the electron pockets less-gapped or un-gapped. Our data clearly show that it is
possible for the parent phase to have electron-like or hole-like charge
carriers.
|
0810.2475v2
|
2008-10-15
|
On the origin of microturbulence in hot stars
|
We present results from the first extensive study of convection zones in the
envelopes of hot massive stars, which are caused by opacity peaks associated
with iron and helium ionization. These convective regions can be located very
close to the stellar surface. Recent observations of microturbulence in massive
stars from the VLT-Flames survey are in good agreement with our predictions
concerning the occurrence and the strength of sub-surface convection in hot
stars. We argue further that convection close to the surface may trigger
clumping at the base of the stellar wind of massive stars.
|
0810.2546v1
|
2008-10-17
|
Band Structure and Fermi Surface of an Extremely Overdoped Iron-Based Superconductor KFe2As2
|
We have performed high-resolution angle-resolved photoemission spectroscopy
on heavily overdoped KFe_2As_2 (transition temperature (Tc = 3 K). We observed
several renormalized bands near the Fermi level with a renormalization factor
of 2-4. While the Fermi surface (FS) around the Brillouin-zone center is
qualitatively similar to that of optimally-doped Ba_{1-x}K_xFe_2As_2 (x = 0.4;
Tc = 37 K), the FS topology around the zone corner (M point) is markedly
different: the two electron FS pockets are completely absent due to excess of
hole doping. This result indicates that the electronic states around the M
point play an important role in the high-Tc superconductivity of
Ba$_{1-x}$K$_x$Fe$_2$As$_2$ and suggests that the interband scattering via the
antiferromagnetic wave vector essentially controls the Tc value in the
overdoped region.
|
0810.3047v1
|
2008-10-19
|
Cosmic Ray Results from the IceTop Air Shower Array
|
We report on the first results obtained with the IceTop air shower array on
the cosmic ray energy spectrum and mass composition in the range of 1 PeV to 80
PeV. IceTop is the surface detector of the IceCube neutrino telescope currently
under construction at the South Pole. A high sensitivity to the primary mass
composition was observed by reconstructing showers at different zenith angles.
Assuming only protons or iron nuclei as primary particles yields significantly
different energy spectra for different zenith angle ranges, while only models
with mixed composition, like the poly-gonato model, lead to the expected
isotropic flux. The prospects of composition measurements with different,
alternative methods using the full IceCube detector are also discussed.
|
0810.3409v1
|
2008-10-20
|
Systematic ^{75}As NMR study of the dependence of low-lying excitations on F doping in the iron oxypnictide LaFeAs(O_{1-x}F_{x})
|
We report systematic $^{75}$As NMR studies on LaFeAs(O$_{1-x}$F$_{x}$) ($0\le
x\le0.14$). At $x=0.04$ near the phase boundary, from resistivity, spin-lattice
relaxation rate $1/T_1$, and NMR spectrum measurements, we found weak magnetic
order at $T_N\simeq 30$ K. Antiferromagnetic (AFM) fluctuations proved through
$1/T_1$ are suppressed significantly with F-doping, and pseudogap behavior
without pronounced AFM fluctuations is observed at $x=0.11$ where $T_c$ is
maximum. This significant suppression of $1/T_1T$ upon F-doping while $T_c$
remains nearly unchanged suggests that low-energy AFM fluctuations probed with
$^{75}$As NMR do not play a crucial role in the superconductivity.
|
0810.3569v2
|
2008-10-23
|
The Age and Metallicity of the Bootes I System
|
We present Washington photometry of a field central to the Bootes I dwarf
spheroidal galaxy, which was discovered as a stellar overdensity in the Sloan
Digital Sky Survey (DR5). We show that the Washington filters are much more
effective than the Sloan filters in separating the metal-poor turn-off stars in
the dwarf galaxy from the foreground stars. We detect 165 objects in the field,
and statistically determine that just over 40% of the objects are non-members.
Our statistical analysis mostly agrees with radial velocity measurements of the
brighter stars. We find that that there is a distinct main-sequence turn-off
and subgiant branch, where there is some evidence of a spread in chemical
abundance. Any evidence of an age spread is limited to a few billion years. The
brightest 7 Bootes I members give a (photometric-color derived) weighted mean
iron-abundance of [Fe/H]=-2.1+/-0.4, and the best-fit isochrone is the 14.1
Gyr, Z=0.0002 model, with DM=19.11 and E(B-V)=0.02.
|
0810.4353v1
|
2008-10-25
|
Detection of Hard X-Rays from the Compton-Thick Seyfert 2 Galaxy NGC 2273 with Suzaku
|
We have obtained a broad-band spectrum of the Compton-thick Seyfert 2 galaxy
NGC 2273 with Suzaku. The spectrum reveals the first detection of hard X-rays
above ~10 keV from NGC 2273. The broad-band spectrum is well represented by a
three-component model, accompanied by both a strong iron Ka line with an
equivalent width of ~ 1.8 keV and several weak lines. The three-component model
consists of a soft component, a reflection component from cold matter, and an
absorbed power-law component. The soft component can be represented by thin
thermal emission with kT~0.56 keV or by a scattered component with a scattering
fraction of 0.4%. Fixing the photon indices of the power law and reflection
components at 1.9, we found that the power law component, heavily absorbed by
gas with a column density of ~1.5x10^24 cm^-2, has an intrinsic 2-10 keV
luminosity of ~1.7x10^42 erg/s. We also apply a reflection model based on a
Monte Carlo simulation, assuming a simple torus geometry. We found that the
model fits the broad band spectrum well, and we place some tentative
constraints on the geometry of the putative torus in NGC2273.
|
0810.4570v1
|
2008-10-26
|
Development of Glass Resistive Plate Chambers for INO
|
The India-based Neutrino Observatory (INO) collaboration is planning to build
a massive 50kton magnetised Iron Calorimeter (ICAL) detector, to study
atmospheric neutrinos and to make precision measurements of the parameters
related to neutrino oscillations. Glass Resistive Plate Chambers (RPCs) of
about 2m X 2m in size are going to be used as active elements for the ICAL
detector. We have fabricated a large number of glass RPC prototypes of 1m X 1m
in size and have studied their performance and long term stability. In the
process, we have developed and produced a number of materials and components
required for fabrication of RPCs. We have also designed and optimised a number
of fabrication and quality control procedures for assembling the gas gaps. In
this paper we will review our activities towards development of glass RPCs for
the INO ICAL detector and will present results of the characterisation studies
of the RPCs.
|
0810.4693v1
|
2008-10-27
|
Anisotropic itinerant magnetism and spin fluctuations in BaFe2As2: A neutron scattering study
|
Neutron scattering measurements were performed to investigate magnetic
excitations in a single-crystal sample of the ternary iron arsenide BaFe2As2, a
parent compound of a recently discovered family of Fe-based superconductors. In
the ordered state, we observe low energy spin-wave excitations with a gap
energy of 9.8(4) meV. The in-plane spin-wave velocity v_ab and out-of-plane
spin-wave velocity v_c measured at 12 meV are 280(150) and 57(7) meV A,
respectively. At high energy, we observe anisotropic scattering centered at the
antiferromagnetic wave vectors. This scattering indicates two-dimensional spin
dynamics, which possibly exist inside the Stoner continuum. At T_N=136(1) K,
the gap closes, and quasi-elastic scattering is observed above T_N, indicative
of short-range spin fluctuations. In the paramagnetic state, the scattering
intensity along the L direction becomes "rodlike," characteristic of
uncorrelated out-of-plane spins, attesting to the two-dimensionality of the
system.
|
0810.4790v2
|
2008-10-31
|
The minimum stellar metallicity observable in the Galaxy
|
The first stars fundamentally transformed the early Universe through their
production of energetic radiation and the first heavy chemical elements. The
impact on cosmic evolution sensitively depends on their initial mass function
(IMF), which can be empirically constrained through detailed studies of
ancient, metal-poor halo stars in our Galaxy. We compare the lowest magnesium
and iron abundances measured in Galactic halo stars with theoretical
predictions for the minimum stellar enrichment provided by Population III stars
under the assumption of a top-heavy IMF. To demonstrate that abundances
measured in metal-poor stars reflect the chemical conditions at their
formation, and that they can thus be used to derive constraints on the
primordial IMF, we carry out a detailed kinematic analysis of a large sample of
metal-poor stars drawn from the SDSS survey. We assess whether interstellar
accretion has altered their surface abundances. We find that accretion is
generally negligible, even at the extremely low levels where the primordial IMF
can be tested. We conclude that the majority of the first stars were very
massive, but had likely masses below ~140 M.
|
0811.0020v1
|
2008-10-31
|
Anisotropy, Itineracy, and Magnetic Frustration in High-Tc Iron Pnictides
|
Using first-principle density functional theory calculations combined with
insight from a tight-binding representation, dynamical mean field theory, and
linear response theory, we have extensively investigated the electronic
structures and magnetic interactions of nine ferropnictides representing three
different structural classes. The calculated magnetic interactions are found to
be short-range, and the nearest ($J_{1a}$) and next-nearest ($J_{2}$) exchange
constants follow the universal trend of J_{1a}/2J_{2}\sim 1, despite their
itinerant origin and extreme sensitivity to the z-position of As. These results
bear on the discussion of itineracy versus magnetic frustration as the key
factor in stabilizing the superconducting ground state. The calculated spin
wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast
to cuprates.
|
0811.0034v3
|
2008-11-05
|
Superconductivity at 56 K in Samarium-doped SrFeAsF
|
We synthesized the samples Sr$_{1-x}$Sm$_x$FFeAs with ZrCuSiAs-type
structure. These samples were characterized by resistivity and susceptibility.
It is found that substitution of rare earth metal for alkaline earth metal in
this system suppresses the anomaly in resistivity and induces
superconductivity. Superconductivity at 56 K in nominal composition
Sr$_{0.5}$Sm$_{0.5}$FFeAs is realized, indicating that the superconducting
transition temperatures in the iron arsenide fluorides can reach as high as
that in oxypnictides with the same structure.
|
0811.0761v3
|
2008-11-06
|
Resolution-dependent mechanisms for bimodal switching-time distributions in simulated Fe nanopillars
|
We study the magnetization-switching statistics following reversal of the
applied field for three separate computational models representing the same
physical system, an iron nanopillar. The primary difference between the models
is the resolution of the computational lattice and, consequently, the intrinsic
parameters that must be rescaled to retain similarity to the physical system.
Considering the first-passage time to zero for the magnetization component in
the longitudinal (easy-axis) direction, we look for applied fields that result
in bimodal distributions of this time for each system and compare the results
to the experimental system. We observe that the relevant fluctuations leading
to bimodal distributions are different for each lattice resolution and result
in magnetization-switching behavior that is unique to each computational model.
Correct model resolution is thus essential for obtaining reliable numerical
results for the system dynamics.
|
0811.1028v1
|
2008-11-07
|
Substitution Effects on FeSe Superconductor
|
We have investigated the effect of atomic substitutions in the FeSe system,
which exhibits the simplest crystal structure among the iron-based
superconductors. An enhancement of the superconducting transition temperature
Tc was observed with the substitution of S or Te for Se; the Tc increased with
S substitution by up to 20 %, and also increased with Te substitution up to 75
%. In contrast, Co or Ni substitutions for the Fe site significantly suppressed
superconductivity. In this work we present a detailed description of the
substitution technique employed to determine Tc in the FeSe system.
|
0811.1123v3
|
2008-11-10
|
Strong short-range magnetic order in a frustrated FCC lattice and its possible role in the iron structural transformation
|
We investigate magnetic properties of a frustrated Heisenberg antiferromagnet
with a face-centered cubic (FCC) lattice and exchange interactions between the
nearest- and next-nearest neighbours, J1 and J2. In a collinear phase with the
wave vector Q = (pi,pi,pi) the equations of the self-consistent spin-wave
theory for the sublattice magnetization and the average short range order
parameter are obtained and numerically solved. The dependence of the Neel
temperature T_N on the ratio J2/J1 is obtained. It is shown, that at strong
enough frustration there is a wide temperature region above T_N with strong
short range magnetic order. Application of this result to description of
structural phase transition between alpha and gamma-phase of Fe is considered.
|
0811.1472v1
|
2008-11-10
|
The XMM-Newton long look of NGC 1365: uncovering of the obscured X-ray source
|
We present an analysis of the extreme obscuration variability observed during
an XMM-Newton 5-days continuous monitoring of the AGN in NGC 1365. The source
was in a reflection-dominated state in the first ~1.5 days, then a strong
increase of the 7-10 keV emission was observed in ~10 hours, followed by a
symmetric decrease. The spectral analysis of the different states clearly shows
that this variation is due to an uncovering of the X-ray source. From this
observation we estimate a size of the X-ray source D_S<10^13 cm, a distance of
the obscuring clouds R~10^16 cm and a density n~10^11 cm^{-3}. These values
suggest that the X-ray absorption/reflection originate from the broad line
region clouds. This is also supported by the resolved width of the iron narrow
Kalpha emission line, consistent with the width of the broad Hbeta line.
|
0811.1594v2
|
2008-11-12
|
CERN-INO magical Beta-beam experiment: A high precision probe for neutrino parameters
|
This talk is an attempt to underscore in detail the physics reach of an
experimental set-up where neutrinos produced in a beta-beam facility at CERN
would be observed in the proposed large magnetized iron calorimeter detector
(ICAL) at the India-based Neutrino Observatory (INO). The "magical" CERN-INO
beta-beam set-up offers an excellent avenue to use the "Golden" channel ($\nu_e
\to \nu_{\mu}$) oscillation probability for a simultaneous determination of the
neutrino mass ordering and $\theta_{13}$ avoiding the impact of the CP phase
$\delta_{CP}$ on these measurements. With Lorentz boost $\gamma=650$ and
irrespective of the true value of $\delta_{CP}$, the neutrino mass hierarchy
could be determined at $3\sigma$ C.L. if $\sin^22\theta_{13}{\rm {(true)}} >
5.6 \times 10^{-4}$ and we can expect an unambiguous signal for $\theta_{13}$
at $3\sigma$ C.L. if $\sin^22\theta_{13}{\rm {(true)}} > 5.1 \times 10^{-4}$
independent of the true neutrino mass hierarchy.
|
0811.1822v1
|
2008-11-12
|
X-Ray Reflection Nebulae with Large Equivalent Widths of Neutral Iron Ka Line in the Sgr C Region
|
This paper reports on the first results of the Suzaku observation in the Sgr
C region. We detected four diffuse clumps with strong line emission at 6.4keV,
Ka from neutral or low-ionized Fe. One of them, M359.38-0.00, is newly
discovered with Suzaku. The X-ray spectra of the two bright clumps,
M359.43-0.07 and M359.47-0.15, after subtracting the Galactic center diffuse
X-ray emission (GCDX), exhibit strong Ka line from FeI with large equivalent
widths (EWs) of 2.0-2.2keV and clear Kb of FeI. The GCDX in the Sgr C region is
composed of the 6.4keV- and 6.7keV-associated components. These are
phenomenologically decomposed by taking relations between EWs of the 6.4keV and
6.7keV lines. Then the former EWs against the associated continuum in the
bright clump regions are estimated to be 2.4(+2.3_-0.7)keV. Since the two
different approaches give similar large EWs of 2keV, we strongly suggest that
the 6.4keV clumps in the Sgr C region are due to X-ray reflection/fluorescence
(the X-ray reflection nebulae).
|
0811.1830v1
|
2008-11-12
|
Bond analysis of cobalt and iron based skutterudites: elongated lanthanum bonds in LaFe4P12
|
Motivated by the possibility of further improving the thermoelectric
properties of skutterudites we investigate charge transfer and bonding in this
class of materials using density functional calculations. Results for the CoP3,
CoSb3, LaFe4P12 and the hypothetical FeP3 compounds are presented using the
procrystal as the non-binding reference. Spherical integration and Bader
analysis are performed to illustrate charge transfer differences between these
compounds. The results are in good qualitative agreement with simple
electronegativity considerations. The calculations confirm that the transition
metal-pnictogen and the pnictogen-pnictogen bonds are covalent, while the
filler atom-pnictogen bond is of a more polar and complex nature. The success
of the "rattling" cage as phonon inhibitor is explained by a unique
semi-correlated bonding scheme between lanthanum and phosphorus. Elongated
bonds along the crystal axes surrounds the lanthanum ion and generate a
dodecahedra grid. Vibrations along the crystal axes are then closely connected
to and scatter from the phosphorus rings. In the other directions, a more
uncorrelated vibration is possible. This duality widens the phonon dampening
possibilities.
|
0811.1889v1
|
2008-11-14
|
Metallicities for 13 nearby open clusters from high-resolution spectroscopy of dwarf and giant stars. Stellar metallicity, stellar mass, and giant planets
|
We present a study of accurate stellar parameters and iron abundances for 39
giants and 16 dwarfs in the 13 open clusters IC2714, IC4651, IC4756, NGC2360,
NGC2423, NGC2447 (M93), NGC2539, NGC2682 (M67), NGC3114, NGC3680, NGC4349,
NGC5822, NGC6633. The analysis was done using a set of high-resolution and
high-S/N spectra obtained with the UVES spectrograph (VLT). These clusters are
currently being searched for planets using precise radial velocities. For all
the clusters, the derived average metallicities are close to solar.
Interestingly, the values derived seem to depend on the line-list used. This
dependence and its implications for the study of chemical abundances in giants
stars are discussed. We show that a careful choice of the lines may be crucial
for the derivation of metallicities for giant stars on the same metallicity
scale as those derived for dwarfs. Finally, we discuss the implications of the
derived abundances for the metallicity- and mass-giant planet correlation. We
conclude that a good knowledge of the two parameters is necessary to correctly
disentangle their influence on the formation of giant planets.
|
0811.2392v1
|
2008-11-16
|
Solar neutrinos and the solar composition problem
|
Standard solar models (SSM) are facing nowadays a new puzzle: the solar
composition problem. New determinations of solar metal abundances lead SSM
calculations to conflict with helioseismological measurements, showing
discrepancies that extend from the convection zone to the solar core and can
not be easily assigned to deficiencies in the modelling of the solar convection
zone. We present updated solar neutrino fluxes and uncertainties for two SSM
with high (old) and low (new) solar metallicity determinations. The
uncertainties in iron and carbon abundances are the largest contribution to the
uncertainties of the solar neutrino fluxes. The uncertainty on the ^14N+p ->
^15O+g rate is the largest of the non-composition uncertainties to the CNO
neutrino fluxes. We propose an independent method to help identify which SSM is
the correct one. Present neutrino data can not distinguish the solar neutrino
predictions of both models but ongoing measurements can help to solve the
puzzle.
|
0811.2424v1
|
2008-11-16
|
The Absence of Superconductivity in Single Phase CaFe2As2 under Hydrostatic Pressure
|
Recent high-pressure studies found that superconductivity can be achieved
under very low pressure in the parent iron arsenide compound CaFe2As2, although
details of the sharpness and temperature of transitions vary between liquid
medium and gas medium measurements. To better understand this issue, we
performed high-pressure susceptibility and transport studies on CaFe2As2, using
helium as the pressure medium. The signatures of the transitions to the
low-temperature orthorhombic and collapsed tetragonal phase remained
exceptionally sharp and no signature of bulk superconductivity was found under
our hydrostatic conditions. Our results suggest that phase separation and
superconductivity in CaFe2As2 are induced by non-hydrostatic conditions
associated with the frozen liquid media.
|
0811.2554v2
|
2008-11-19
|
The location and kinematics of the coronal-line emitting regions in AGN
|
We use the photoionisation code Cloudy to determine both the location and the
kinematics of the optical forbidden, high ionisation line (hereafter, FHIL)
emitting gas in the narrow line Seyfert 1 galaxy Ark 564. The results of our
models are compared with the observed properties of these emission lines to
produce a physical model that is used to explain both the kinematics and the
source of this gas. The main features of this model are that the FHIL emitting
gas is launched from the putative dusty torus and is quickly accelerated to its
terminal velocity of a few hundred km/s. Iron-carrying grains are destroyed
during this initial acceleration. This velocity is maintained by a balance
between radiative forces and gravity in this super-Eddington source. Eventually
the outflow is slowed at large radii by the gravitational forces of and
interactions with the host galaxy. In this model, FHIL emission traces the
transition between the AGN and bulge zones of influence.
|
0811.3204v1
|
2008-11-20
|
A critical re-examination of resonant soft x-ray Bragg forbidden reflections in magnetite
|
Magnetite, Fe$_3$O$_4$, displays a highly complex low temperature crystal
structure that may be charge and orbitally ordered. Many of the recent
experimental claims of such ordering rely on resonant soft x-ray diffraction at
the oxygen K and iron L edges. We have re-examined this system and undertaken
soft x-ray diffraction experiments on a high-quality single crystal. Contrary
to previous claims in the literature, we show that the intensity observed at
the Bragg forbidden (001/2)$_c$ reflection can be explained purely in terms of
the low-temperature structural displacements around the resonant atoms. This
does not necessarily mean that magnetite is not charge or orbitally ordered,
but rather that the present sensitivity of resonant soft x-ray experiments does
not allow conclusive demonstration of such ordering.
|
0811.3350v2
|
2008-11-22
|
75As-NMR studies on La FeAsO1-xFx (x=0.14) under a pressure of 3GPa
|
75As-nuclear magnetic resonance (NMR) on an iron-based superconductor
LaFeAsO1-xFx (x=0.14) was performed under a pressure of 3GPa. Enhancement of
superconducting transition temperature Tc was confirmed from the relaxation
rate 1/T1; Tc goes up to 40K by applying pressure up to 3GPa. 1/T1T, which is
temperature independent just above Tc and gives a measure of the density of
states (DOS) at the Fermi energy, enhances by applying pressure. These facts
suggest that the increase of the DOS leads to the enhancement of Tc. On the
other hand, anomalous behavior of 1/T1T observed at high temperatures is
suppressed by applying pressure.
|
0811.3672v3
|
2008-11-26
|
Ironing out primordial temperature fluctuations with polarisation: optimal detection of cosmic structure imprints
|
Secondary anisotropies of the cosmic microwave background (CMB) can be
detected by using the cross-correlation between the large-scale structure (LSS)
and the CMB temperature fluctuations. In such studies, chance correlations of
primordial CMB fluctuations with the LSS are the main source of uncertainty. We
present a method for reducing this noise by exploiting information contained in
the polarisation of CMB photons. The method is described in general terms and
then applied to our recently proposed optimal method for measuring the
integrated Sachs-Wolfe (ISW) effect. We obtain an expected signal-to-noise
ratio of up to 8.5. This corresponds to an enhancement of the signal-to-noise
by 23 per cent as compared to the standard method for ISW detection, and by 16
per cent w.r.t. our recently proposed method, both for the best-case scenario
of having perfect (noiseless) CMB and LSS data.
|
0811.4433v2
|
2008-11-27
|
A unified theory for the cuprates, iron-based and similar superconducting systems: non-Fermi-liquid to Fermi-liquid crossover, low-energy and waterfall anomalies
|
A unified theory is outlined for the cuprates, Fe-based, and related
superconductors. Their low-energy excitations are approached in terms of
auxiliary particles representing combinations of atomic-like electron
configurations, and the introduction of a Lagrange Bose field enables their
treatment as bosons or fermions. This theory correctly describes the observed
phase diagram of the cuprates, including the non-Fermi-liquid to FL crossover
in the normal state, the existence of Fermi arcs below T^* and of "marginal-FL"
behavior above it. The anomalous behavior of numerous physical quantities is
accounted for, including kink- and waterfall-like spectral features, the drop
in the scattering rates below T^* and more radically below T_c, and an
effective increase in the density of carriers with T and \omega, reflected in
transport, optical and other properties. Also is explained the correspondence
between T_c, the resonance-mode energy, and the increase in the gap below T_c.
|
0811.4561v1
|
2008-11-27
|
Two-dimensional magnetism in the pnictide superconductor parent material SrFeAsF probed by muon-spin relaxation
|
We report muon-spin relaxation measurements on SrFeAsF, which is the parent
compound of a newly discovered iron-arsenic-fluoride based series of
superconducting materials. We find that this material has very similar magnetic
properties to LaFeAsO, such as separated magnetic and structural transitions
(TN = 120 K, Ts = 175 K), contrasting with SrFe2As2 where they are coincident.
The muon oscillation frequencies fall away very sharply at TN, which suggests
that the magnetic exchange between the layers is weaker than in comparable
oxypnictide compounds. This is consistent with our specific heat measurements,
which find that the entropy change S = 0.05 J/mol/K largely occurs at the
structural transition and there is no anomaly at TN.
|
0811.4598v1
|
2008-11-30
|
Andreev Bound states as a phase sensitive probe of the pairing symmetry of the iron pnictide superconductors
|
A leading contender for the pairing symmetry in the Fe-pnictide high
temperature superconductors is extended s-wave $s_\pm$, a nodeless state in
which the pairing changes sign between Fermi surfaces. Verifying such a pairing
symmetry requires a special phase sensitive probe that is also momentum
selective. We show that the sign structure of $s_\pm$ pairing leads to surface
Andreev bound states at the sample edge. In the clean limit they only occur
when the edge is along the nearest neighbor Fe-Fe bond, but not for a diagonal
edge or a surface orthogonal to the c-axis. In contrast to d-wave Andreev bound
states, they are not at zero energy and, in general, do not produce a zero bias
tunneling peak. Consequences for tunneling measurements are derived, within a
simplified two band model and also for a more realistic five band model.
|
0812.0015v1
|
2008-11-28
|
Small RNAs Establish Delays and Temporal Thresholds in Gene Expression
|
Non-coding RNAs are crucial regulators of gene expression in prokaryotes and
eukaryotes, but it remains poorly understood how they affect the dynamics of
transcriptional networks. We analyzed the temporal characteristics of the
cyanobacterial iron stress response by mathematical modeling and quantitative
experimental analyses, and focused on the role of a recently discovered small
non-coding RNA, IsrR. We found that IsrR is responsible for a pronounced delay
in the accumulation of isiA mRNA encoding the late-phase stress protein, IsiA,
and that it ensures a rapid decline in isiA levels once external stress
triggers are removed. These kinetic properties allow the system to selectively
respond to sustained (as opposed to transient) stimuli, and thus establish a
temporal threshold, which prevents energetically costly IsiA accumulation under
short-term stress conditions. Biological information is frequently encoded in
the quantitative aspects of intracellular signals (e.g., amplitude and
duration). Our simulations reveal that competitive inhibition and regulated
degradation allow intracellular regulatory networks to efficiently discriminate
between transient and sustained inputs.
|
0812.0025v1
|
2008-12-01
|
Near-degeneracy of several pairing channels in multiorbital models for the Fe-pnictides
|
Weak-coupling approaches to the pairing problem in the iron pnictide
superconductors have predicted a wide variety of superconducting ground states.
We argue here that this is due both to the inadequacy of certain approximations
to the effective low-energy band structure, and to the natural near-degeneracy
of different pairing channels in superconductors with many distinct Fermi
surface sheets. In particular, we review attempts to construct two-orbital
effective band models, the argument for their fundamental inconsistency with
the symmetry of these materials, and the comparison of the dynamical
susceptibilities in two- and five-orbital models. We then present results for
the magnetic properties, pairing interactions, and pairing instabilities within
a five-orbital Random Phase Approximation model. We discuss the robustness of
these results for different dopings, interaction strengths, and variations in
band structure. Within the parameter space explored, an anisotropic,
sign-changing s-wave state and a d_x2-y2 state are nearly degenerate, due to
the near nesting of Fermi surface sheets.
|
0812.0343v2
|
2008-12-04
|
Medium Effects in rho-Meson Photoproduction
|
We compute dilepton invariant mass spectra from the decays of rho mesons
produced by photon reactions off nuclei. Our calculations employ a realistic
model for the rho photoproduction amplitude on the nucleon which provides fair
agreement with measured cross sections. Medium effects are implemented via an
earlier constructed rho propagator based on hadronic many-body theory. At
incoming photon energies of 1.5 -3 GeV as used by the CLAS experiment at JLAB,
the average density probed for iron targets is estimated at about half
saturation density. At the pertinent rho-meson 3-momenta the predicted medium
effects on the rho propagator are rather moderate. The resulting dilepton
spectra approximately agree with recent CLAS data.
|
0812.0987v3
|
2008-12-09
|
Insular superconductivity in Co-doped iron pnictide CaFe$_{1-x}$Co$_x$AsF
|
The presence of macroscopic phase separation between the superconducting and
magnetic phases in \cfcaf is demonstrated by muon spin rotation (muSR)
measurements conducted across their phase boundaries (x=0.05-0.15). The
magnetic phase tends to retain the high transition temperature (T_m > T_c),
while Co-doping induces strong randomness. The volumetric fraction of
superconducting phase is nearly proportional to the Co content $x$ with
constant superfluid density. These observations suggest the formation of
superconducting "islands" (or domains) associated with Co ions in the
Fe$_2$As$_2$ layers, indicating a very short coherence length.
|
0812.1670v3
|
2008-12-10
|
Disk reflection signatures in the spectrum of the bright Z-source GX 340+0
|
We present the preliminary results of a 50 ks long XMM-Newton observation of
the bright Z-source GX 340+0. In this Letter we focus on the study of a broad
asymmetric emission line in the Fe K alpha energy band, whose shape is clearly
resolved and compatible with a relativistically smeared profile arising from
reflection on a hot accretion disk extending close to the central accreting
neutron star. By combining temporal and spectral analysis, we are able to
follow the evolution of the source along its Horizontal Branch. However,
despite a significant change in the continuum emission and luminosity, the line
profile does not show any strong correlated variation. This broad line is
produced by recombination of highly ionized iron (Fe XXV) at an inferred inner
radius close to 13 gravitational radii while the fit requires a high value for
the outer disk radius. The inclination of the source is extremely well
constrained at 35 deg, while the emissivity index is -2.50.
|
0812.1974v1
|
2008-12-11
|
Full Gap Superconductivity in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Probed by Muon Spin Rotation
|
Superfluid density ($n_s$) in the mixed state of an iron pnictide
superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ is determined by muon spin
rotation for a sample with optimal doping ($x=0.4$). The temperature dependence
of $n_s$ is perfectly reproduced by the conventional BCS model for s-wave
paring, where the order parameter can be either a single-gap with
$\Delta=8.35(6)$ meV [$2\Delta/k_BT_c=5.09(4)$], or double-gap structure with
$\Delta_1=12$ meV (fixed) [$2\Delta_1/k_BT_c=7.3$] and $\Delta_2=6.8(3)$ meV
[$2\Delta_2/k_BT_c=4.1(2)$]. The latter is consistent with the recent result of
angle-resolved photo-emssion spectroscopy. The large gap parameters
($2\Delta/k_BT_c$) indicate extremely strong coupling of carriers to bosons
that mediate the Cooper pairing.
|
0812.2069v2
|
2008-12-11
|
Impurity-induced in-gap state and Tc in sign-reversing s-wave superconductors: analysis of iron oxypnictide superconductors
|
The sign-reversing fully gapped superconducting state, which is expected to
be realized in oxypnictide superconductors, can be prominently affected by
nonmagnetic impurities due to the interband scattering of Cooper pairs. We
study this problem based on the isotropic two-band BCS model: In oxypnictide
superconductors, the interband impurity scattering $I'$ is not equal to the
intraband one $I$. In the Born scattering regime, the reduction in Tc is
sizable and the impurity-induced density of states (DOS) is prominent if $I\sim
I'$, due to the interband scattering. Although impurity-induced DOS can yield a
power-law temperature dependence in $1/T_1$, a sizable suppression in Tc is
inevitably accompanied. In the unitary scattering regime, in contrast, impurity
effect is very small for both Tc and DOS except at $I=I'$. By comparing theory
and experiments, we expect that the degree of anisotropy in the $s_\pm$-wave
gap function strongly depends on compounds.
|
0812.2100v2
|
2008-12-11
|
Quantum and classical structures in nondeterminstic computation
|
In categorical quantum mechanics, classical structures characterize the
classical interfaces of quantum resources on one hand, while on the other hand
giving rise to some quantum phenomena. In the standard Hilbert space model of
quantum theories, classical structures over a space correspond to its
orthonormal bases. In the present paper, we show that classical structures in
the category of relations correspond to biproducts of abelian groups. Although
relations are, of course, not an interesting model of quantum computation, this
result has some interesting computational interpretations. If relations are
viewed as denotations of nondeterministic programs, it uncovers a wide variety
of non-standard quantum structures in this familiar area of classical
computation. Ironically, it also opens up a version of what in philosophy of
quantum mechanics would be called an ontic-epistemic gap, as it provides no
direct interface to these nonstandard quantum structures.
|
0812.2266v3
|
2008-12-15
|
Microscopic origin of pressure-induced phase transitions in iron-pnictide $AFe_2As_2$ superconductors: an {ab initio} molecular-dynamics study
|
Using {\it ab initio} molecular dynamics we investigate the electronic and
lattice structure of $A$Fe$_2$As$_2$ ($A$=Ca, Sr, Ba) under pressure. We find
that the structural phase transition (orthorhombic to tetragonal symmetry) is
always accompanied by a magnetic phase transition in all the compounds, while
the nature of the transitions is different for the three systems. Our
calculations explain the origin of the existence of a collapsed tetragonal
phase in CaFe$_2$As$_2$ and its absence in BaFe$_2$As$_2$. We argue that
changes of the Fermi surface nesting features dominate the phase transition
under pressure rather than spin frustration or a Kondo scenario. The
consequences for superconductivity are discussed.
|
0812.2920v3
|
2008-12-16
|
Anomalous Hall effect in granular ferromagnetic metals and effects of weak localization
|
We theoretically investigate the anomalous Hall effect in a system of
dense-packed ferromagnetic grains in the metallic regime. Using the formalism
recently developed for the conventional Hall effect in granular metals, we
calculate the residual anomalous Hall conductivity $\sigma_{xy}$ and
resistivity $\rho_{xy}$ and weak localization corrections to them for both
skew-scattering and side-jump mechanisms. We find that, unlike for
homogeneously disordered metals, the scaling relation between $\rho_{xy}$ and
the longitudinal resistivity $\rho_{xx}$ does not hold. The weak localization
corrections, however, are found to be in agreement with those for homogeneous
metals. We discuss recent experimental data on the anomalous Hall effect in
polycrystalline iron films in view of the obtained results.
|
0812.3085v2
|
2008-12-18
|
Observation of the Josephson effect in Pb/(Ba,K)Fe2As2 single crystal junctions
|
We have fabricated c-axis Josephson junctions on single crystals of
(Ba,K)Fe2As2 by using Pb as the counter electrode in two geometries, planar and
point contact. Junctions in both geometries show resistively shunted junction
I-V curves below the Tc of the counter electrode. Microwave induced steps were
observed in the I-V curves, and the critical currents are suppressed with an
in-plane magnetic field in a manner consistent with the small junction limit.
ICRN products of up to 0.3 mV have been observed in these junctions at 4.2 K.
The observation of Josephson coupling along the c-axis between (Ba,K)Fe2As2 and
a conventional superconductor suggests the existence of a s-wave
superconducting order parameter in this class of iron pnictide superconductors.
|
0812.3605v2
|
2008-12-21
|
Suppression of antiferromagnetic spin fluctuations in the collapsed tetragonal phase of CaFe2As2
|
Inelastic neutron scattering measurements of CaFe2As2 under applied
hydrostatic pressure show that the antiferromagnetic spin fluctuations observed
in the ambient pressure, paramagnetic, tetragonal (T) phase are strongly
suppressed, if not absent, in the collapsed tetragonal (cT) phase. These
results are consistent with a quenched Fe moment in the cT phase and the strong
decrease in resistivity observed upon crossing the boundary from the T to cT
phase. The suppression or absence of static antiferromagnetic order and dynamic
spin fluctuations in the non-superconducting cT phase supports the notion of a
coupling between spin fluctuations and superconductivity in the iron arsenides.
|
0812.4056v1
|
2008-12-22
|
Electronic structure of heavily electron-doped BaFe$_{1.7}$Co$_{0.3}$As$_2$ studied by angle-resolved photoemission
|
We have performed high-resolution angle-resolved photoemission spectroscopy
on heavily electron-doped non-superconducting (SC)
BaFe$_{1.7}$Co$_{0.3}$As$_2$. We find that the two hole Fermi surface pockets
at the zone center observed in the hole-doped superconducting
Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ are absent or very small in this compound,
while the two electron pockets at the M point significantly expand due to
electron doping by the Co substitution. Comparison of the Fermi surface between
non-SC and SC samples indicates that the coexistence of hole and electron
pockets connected via the antiferromagnetic wave vector is essential in
realizing the mechanism of superconductivity in the iron-based superconductors.
|
0812.4111v3
|
2008-12-29
|
Superconducting properties of the oxygen-vacant iron oxyarsenide TbFeAsO1-x from underdoped to overdoped compositions
|
A wide-range doping was achieved by a high-pressure method for TbFeAsO1-x
from "under doped" to "over doped" superconducting compositions throughout the
optimized superconductivity (Tc of 44 K). Tc vs. lattice constant shows a
dome-shaped feature, while Tc vs. the lattice constant likely follows a unique
empirical curve over the doping range. The relatively large amount of oxygen
vacancies up to 0.3 per the formula unit was introduced possibly because of the
smaller replacement Tb than the other Ln (rare-earth element) in the LnFeAsO1-x
system.
|
0812.4907v2
|
2009-01-01
|
75As NMR Study of Hole-Doped Superconductor Ba1-xKxFe2As2 (Tc = 38K)
|
We report the 75As nuclear magnetic resonance (NMR) measurement of the
hole-doped superconductor Ba1-xKxFe2As2 with different lattice parameters and
different superconducting volume fractions (Tc = 38K). 75As-NMR spectra
revealed that the magnetically ordered and superconducting phases are
microscopically separated. The spin-lattice relaxation rate 1/T1 in the normal
state reflects the existence of a large two-dimensional antiferromagnetic spin
fluctuation. The 1/T1 in the superconducting state down to the lowest
measurement temperature T varies close to T^3. In addition, it exhibits no
coherence peak just below Tc. This shows a T dependence similar to those of
other iron pnictides.
|
0901.0177v2
|
2009-01-02
|
Electronic structure and Magnetism in BaMn$_2$As$_2$ and BaMn$_2$Sb$_2$
|
We study the properties of ThCr$_2$Si$_2$ structure BaMn$_2$As$_2$ and
BaMn$_2$Sb$_2$ using density functional calculations of the electronic and
magnetic as well experimental measurements on single crystal samples of
BaMn$_2$As$_2$. These materials are local moment magnets with moderate band gap
antiferromagnetic semiconducting ground states. The electronic structures show
substantial Mn - pnictogen hybridization, which stabilizes an intermediate spin
configuration for the nominally $d^5$ Mn. The results are discussed in the
context of possible thermoelectric applications and the relationship with the
corresponding iron / cobalt / nickel compounds Ba(Fe,Co,Ni)$_2$As$_2$.
|
0901.0272v2
|
2009-01-09
|
Fermi surface topology and low-lying quasiparticle structure of magnetically ordered Fe1+xTe
|
We report the first photoemission study of Fe1+xTe - the host compound of the
newly discovered iron-chalcogenide superconductors. Our results reveal a pair
of nearly electron- hole compensated Fermi pockets, strong Fermi velocity
renormalization and an absence of a spin-density-wave gap. A shadow hole pocket
is observed at the "X"-point of the Brillouin zone which is consistent with a
long-range ordered magneto-structural groundstate. No signature of Fermi
surface nesting instability associated with Q= pi(1/2, 1/2) is observed. Our
results collectively reveal that the Fe1+xTe series is dramatically different
from the undoped phases of the high Tc pnictides and likely harbor unusual
mechanism for superconductivity and quantum magnetic order.
|
0901.1299v1
|
2009-01-11
|
Theory of tunnelling into a multi-band superconductor: decoherence and interference
|
By an exact formulation of tunnelling into a multi-band superconductor in
terms of Green's functions, we demonstrate that the multi-band feature of the
iron-based superconductors can lead to novel interference between Andreev
reflections and decoherence effect of quasi-particles to Andreev reflections in
a tunnelling junction. These effects depend on the relative sign of the gaps
for s-wave pairing, and the sign of the Fermi velocities also matters for
tunnelling along the nodal direction of d-wave gaps. Experimental probe of such
effects could determine both the pairing symmetry and the relative sign of the
gaps on different bands.
|
0901.1419v2
|
2009-01-14
|
Plasma focus based repetitive source of fusion neutrons and hard x-rays
|
A plasma focus device capable of operating at 0.2 pulses per second during
several minutes is used as a source of hard x-rays and fast neutrons. An
experimental demonstration of the use of the neutrons emissions for radiation
probing of hydrogenated substances is presented, showing a particular
application in detecting water concentrations differences in the proximity of
the device by elastic scattering. Moreover, the device produces ultrashort hard
x-rays pulses useful for introspective images of small objects, static or in
fast motion, suitable for the identification of internal submillimetric
defects. Clear images of metallic objects shielded by several millimeters iron
walls are shown.
|
0901.2007v1
|
2009-01-16
|
Star Formation in HII Galaxies. Properties of the ionized gas
|
We propose a methodology to perform a self-consistent analysis of the
physical properties of the emitting gas of HII galaxies adequate to the data
that can be obtained with the XXI century technology. This methodology requires
the production and calibration of empirical relations between the different
line temperatures that should superseed currently used ones based on very
simple, and poorly tested, photo-ionization model sequences. Then, these
observations are analysed applying a methodology designed to obtain accurate
elemental abundances of oxygen, sulphur, nitrogen, neon, argon and iron in the
ionsied gas. Four electron temperatures and one electron density are derived
from the observed forbidden line ratios using the five-level atom
approximation. For our best objects errors of 1% in T([OIII]), 3% in T([OII])
and 5% in T([SIII]) are achieved with a resulting accuracy between 5 and 9% in
total oxygen abundances, O/H. These accuracies are expected to improve as
better calibrations based on more precise measurements, both on electron
temperatures and densities, are produced.
|
0901.2465v1
|
2009-01-19
|
NMR Studies on the Superconducting Symmetry of Iron Pnictide Systems
|
NMR longitudinal relaxation rates 1/T1 and Knight shifts K have been measured
for superconducting samples of LaFe1-yCoyAsO1-xFx with y=0.0 and 0.0075 and for
a nonsuperconducting metallic sample with y=0.1, where the x values are always
fixed at 0.11. The temperature (T) dependence the relaxation rates 1/T1 of the
superconducting samples has been found to be markedly different from the
behavior 1/T1 T^(2.5-3.0) reported by many groups in the entire T range
measured (from the temperature immediately below the superconducting transition
temperature Tc down to (0.1-0.2)Tc). The nonexistence of the coherence peak has
also been found. Based on the results of the measurements and other kinds of
existing data, arguments are presented on the superconducting symmetry of the
Fe pnictide systems, where the several points which cannot be easily understood
by existing theories, are pointed out. Results of the measurements on the
nonsuperconducting metallic samples are also presented.
|
0901.2830v1
|
2009-01-20
|
Broad relativistic iron emission line observed in SAX J1808.4-3658
|
During the September-October 2008 outburst of the accreting millisecond
pulsar SAX J1808.4-3658, the source was observed by both Suzaku and XMM-Newton
approximately 1 day apart. Spectral analysis reveals a broad relativistic Fe
K-alpha emission line which is present in both data-sets, as has recently been
reported for other neutron star low-mass X-ray binaries. The properties of the
Fe K line observed during each observation are very similar. From modeling the
Fe line, we determine the inner accretion disk radius to be 13.2 +/- 2.5
GM/c^2. The inner disk radius measured from the Fe K line suggests that the
accretion disk is not very receded in the island state. If the inner disk (as
measured by the Fe line) is truncated at the magnetospheric radius this implies
a magnetic field strength of ~3E8 G at the magnetic poles, consistent with
other independent estimates.
|
0901.3142v2
|
2009-01-21
|
Superconductivity up to 30 K in the vicinity of quantum critical point in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$
|
We report bulk superconductivity induced by an isovalent doping of phosphorus
in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. The P-for-As substitution results in
shrinkage of lattice, especially for the FeAs block layers. The resistivity
anomaly associated with the spin-density-wave (SDW) transition in the undoped
compound is gradually suppressed by the P doping. Superconductivity with the
maximum $T_c$ of 30 K emerges at $x$=0.32, coinciding with a magnetic quantum
critical point (QCP) which is evidenced by the disappearance of SDW order and
the linear temperature-dependent resistivity in the normal state. The $T_c$
values were found to decrease with further P doping, and no superconductivity
was observed down to 2 K for $x\geq$ 0.77. The appearance of superconductivity
in the vicinity of QCP hints to the superconductivity mechanism in iron-based
arsenides.
|
0901.3227v2
|
2009-01-26
|
RXTE confirmation of the intermediate polar status of IGR J15094-6649
|
Aims. To establish the X-ray properties of the intermediate polar candidate
IGR J15094-6649 and therefore confirm its inclusion into the class.
Methods. 42 856 s of X-ray data from RXTE was analysed. Frequency analysis
was used to constrain temporal variations and spectral analysis used to
characterise the emission and absorption properties.
Results. A spin period of 809.7+-0.6 s is present, revealed as a complex
pulse profile whose modulation depth decreases with increasing X-ray energy.
The spectrum is well fitted by either a 19+-4 keV Bremsstrahlung or
Gamma=1.8+-0.1 power law, with an iron emission line feature and significant
absorption in each case.
Conclusions. IGR J15094-6649 is confirmed to be an intermediate polar.
|
0901.3983v1
|
2009-01-28
|
Chemical evolution of the galactic bulge: single and double infall models
|
Recent work has produced a wealth of data concerning the chemical evolution
of the galactic bulge, both for stars and nebulae. Present theoretical models
generally adopt a limited range of such constraints, frequently using a single
chemical element (usually iron), which is not enough to describe it
unambiguously. In this work, we take into account constraints involving as many
chemical elements as possible, basically obtained from bulge nebulae and stars.
Our main goal is to show that different scenarios can describe, at least
partially, the abundance distribution and several distance-independent
correlationss for these objects. Three classes of models were developed. The
first is a one-zone, single-infall model, the second is a one-zone,
double-infall model and the third is a multizone, double infall model. We show
that a one-zone model with a single infall episode is able to reproduce some of
the observational data, but the best results are achieved using a multizone,
double infall model.
|
0901.4382v1
|
2009-02-06
|
Superconductivity, magnetism and crystal chemistry of Ba1-xKxFe2As2
|
BaFe2As2 is the parent compound of the '122' iron arsenide superconductors
and crystallizes with the tetragonal ThCr2Si2 type structure, space group
I4/mmm. A spin density wave transition at 140 K is accompanied by a symmetry
reduction to space group Fmmm and simultaneously by antiferromagnetic ordering.
Hole-doping induces superconductivity in Ba1-xKxFe2As2 with a maximum Tc of 38
K at x = 0.4. The upper critical fields approach 75 T with rather small
anisotropy of Hc2. At low potassium concentrations (x <= 0.2),
superconductivity apparently co-exists with the orthorhombic distorted and
magnetically ordered phase. At doping levels x >= 0.3, the structural
distortion and antiferromagnetic ordering is completely suppressed and the Tc
is maximized. No magnetically ordered domains could be detected in optimally
doped Ba1-xKxFe2As2 (x >= 0.3) by 57Fe-Moessbauer spectroscopy in contrast muSR
results obtained with single crystals. The magnetic hyperfine interactions
investigated by 57Fe Moessbauer spectroscopy are discussed and compared to the
ZrCuSiAs-type materials.
|
0902.1085v1
|
2009-02-07
|
Superconducting fluctuations in the reversible magnetization of the iron-pnictide $Ba_{1-x}K_xFe_2As_2$
|
We report on isofield magnetization curves obtained as a function of
temperature in two single crystals of $Ba_{1-x}K_xFe_2As_2$ with
superconducting transition temperature $T_c$=28K and 32.7 K. Results obtained
for fields above 20 kOe show a well defined rounding effect on the reversible
region extending 1-3 K above $T_c(H)$ masking the transition. This rounding
appears to be due to three-dimensional critical fluctuations, as the higher
field curves obey a well know scaling law for this type of critical
fluctuations. We also analysed the asymptotic behavior of $\sqrt M$vs.T curves
in the reversible region which probes the shape of the gap near $T_c(H)$.
Results of the analysis suggests that phase fluctuations are important in
$Ba_{1-x}K_xFe_2As_2$ which is consistent with nodes in the gap.
|
0902.1252v2
|
2009-02-10
|
Enhancement of electronic anomalies in iron-substituted La_2-x_Sr_x_Cu_1-y_Fe_y_O_4_ around x=0.22
|
We have measured the temperature dependences of Rho and Chi for
Fe-substituted La_2-x_Sr_x_Cu_1-y_Fe_y_O_4_ in the overdoped regime, in order
to investigate Fe-substitution effects on electronic properties around x=0.22.
From the Rho measurements, it has been found around x=0.22 that the values of
Rho are large at room temperature and that Rho exhibits a pronounced upturn at
low temperatures. Moreover, from the Rho and Chi measurements, it has been
found that T_c_ is anomalously depressed around x=0.22. These results indicate
that the electronic anomalies around x=0.22 are enhanced by Fe substitution,
which might be related to the development of stripe correlations by Fe
substitution.
|
0902.1618v1
|
2009-02-10
|
The Role of Dust Clouds in the Atmospheres of Brown Dwarfs
|
The new spectroscopic classes, L and T, are defined by the role of dust
clouds in their atmospheres, the former by their presence and the latter by
their removal and near absence. Moreover, the M to L and L to T transitions are
intimately tied to the condensation and character of silicate and iron grains,
and the associated clouds play pivotal roles in the colors and spectra of such
brown dwarfs. Spanning the effective temperature range from $\sim$2200 K to
$\sim$600 K, these objects are being found in abundance and are a new arena in
which condensation chemistry and the optical properties of grains is assuming
astronomical importance. In this short paper, I summarize the role played by
such refractories in determining the properties of these "stars" and the
complexities of their theoretical treatment.
|
0902.1777v1
|
2009-02-13
|
Growth and Superconductivity of FeSex Crystals
|
Iron selenide (FeSex) crystals with lateral dimensions up to millimeters were
grown via a vapor self-transport method. The crystals consist of the dominant
alpha - phase with trace amounts of beta- phase as identified by powder x-ray
diffraction. With four-probe resistance measurements we obtained a
zero-resistance critical temperature of 7.5 K and a superconducting onset
transition temperature of up to 11.8 K in zero magnetic field as well as an
anisotropy of 1.5 +- 0.1 for the critical field. Magnetization measurements on
individual crystals reveal the co-existence of superconductivity and
ferromagnetism.
|
0902.2240v1
|
2009-02-14
|
ARPES studies of the electronic structure of LaOFe(P,As)
|
We report a comparison study of LaOFeP and LaOFeAs, two parent compounds of
recently discovered iron-pnictide superconductors, using angle-resolved
photoemission spectroscopy. Both systems exhibit some common features that are
very different from well-studied cuprates. In addition, important differences
have also been observed between these two ferrooxypnictides. For LaOFeP,
quantitative agreement can be found between our photoemission data and the LDA
band structure calculations, suggesting that a weak coupling approach based on
an itinerant ground state may be more appropriate for understanding this new
superconducting compound. In contrast, the agreement between LDA calculations
and experiments in LaOFeAs is relatively poor, as highlighted by the unexpected
Fermi surface topology around (pi,pi). Further investigations are required for
a comprehensive understanding of the electronic structure of LaOFeAs and
related compounds.
|
0902.2503v1
|
2009-02-18
|
Ultra High Energy Cosmic Ray, Neutrino, and Photon Propagation and the Multi-Messenger Approach
|
The propagation of UHECR nuclei for A=1 (protons) to A=56 (iron) from
cosmological sources through extragalactic space is discussed in the first
lecture. This is followed in the second and third lectures by a consideration
of the generation and propagation of secondary particles produced via the UHECR
loss interactions. In the second lecture we focus on the generation of the
diffuse cosmogenic UHE-neutrino flux. In the third lecture we investigate the
arriving flux of UHE-photon flux at Earth. In the final lecture the results of
the previous lectures are put together in order to provide new insights into
UHECR sources. The first of these providing a means with which to investigate
the local population of UHECR sources through the measurement of the UHECR
spectrum and their photon fraction at Earth. The second of these providing
contraints on the UHECR source radiation fields through the possible
observation at Earth of UHECR nuclei.
|
0902.3012v1
|
2009-02-18
|
Doppler-Broadened Iron X-ray Lines from Tycho's Supernova Remnant
|
We use \suzaku observations to measure the spatial variation of the Fe
K$\alpha$ line with radius in the \tycho supernova remnant. The Fe line widths
show a significant decrease from a FWHM value of 210 eV at the center to 130 eV
at the rim. Over the same radial range the line center energy remains nearly
constant. These observations are consistent with a scenario in which the shell
of Fe-emitting ejecta in \tycho is expanding at speeds of 2800--3350 km
s$^{-1}$. The minimum line width we measure is still a factor of two larger
than expected from a single component plasma emission model. If thermal Doppler
broadening is the dominant additional source of broadening, we infer an ion
temperature of $(1--3) \times 10^{10}$ K.
|
0902.3049v1
|
2009-02-23
|
Why Does Undoped FeSe Become A High Tc Superconductor Under Pressure?
|
Unlike the parent phases of the iron-arsenide high Tc superconductors,
undoped FeSe is not magnetically ordered and exhibits superconductivity with
Tc~9K. Equally surprising is the fact that applied pressure dramatically
enhances the modest Tc to ~37K. We investigate the electronic properties of
FeSe using 77Se NMR to search for the key to the superconducting mechanism. We
demonstrate that the electronic properties of FeSe are very similar to those of
electron-doped FeAs superconductors, and that antiferromagnetic spin
fluctuations are strongly enhanced near Tc. Furthermore, applied pressure
enhances spin fluctuations. Our findings suggest a link between spin
fluctuations and the superconducting mechanism in FeSe.
|
0902.3832v2
|
2009-02-23
|
Superconductivity at 22.3 K in SrFe2-xIrxAs2
|
By substituting the Fe with the 5d-transition metal Ir in SrFe2As2, we have
successfully synthesized the superconductor SrFe2-xIrxAs2 with Tc = 22.3 K at x
= 0.5. X-ray diffraction indicates that the material has formed the
ThCr2Si2-type structure with a space group I4/mmm. The temperature dependence
of resistivity and dc magnetization both reveal sharp superconducting
transitions at around 22 K. An estimate on the diamagnetization signal reveals
a high Meissner shielding volume. Interestingly, the normal state resistivity
exhibits a roughly linear behavior up to 300 K. The superconducting transitions
at different magnetic fields were also measured yielding a slope of -dHc2/dT =
3.8 T/K near Tc. Using the Werthamer-Helfand-Hohenberg (WHH) formula, the upper
critical field at zero K is found to be about 58 T. Counting the possible
number of electrons doped into the system in SrFe2-xIrxAs2, we argue that the
superconductivity in the Ir-doped system is different from the Co-doped case,
which should add more ingredients to the underlying physics of the iron
pnictide superconductors.
|
0902.3957v2
|
2009-02-24
|
Thermal instability in X-ray photoionized media in Active Galactic Nuclei: II. Role of the thermal conduction in warm absorber
|
A photoionized gas under constant pressure can display a thermal instability,
with three or more solutions for possible thermal equilibrium. A unique
solution of the structure of the irradiated medium is obtained only if electron
conduction is considered. The subject of our study is to estimate how the
effect of thermal conduction affects the structure and transmitted spectrum of
the warm absorber computed by solving radiative transfer with the code TITAN.
We developed a new computational mode for the code TITAN to obtain several
solutions for a given external conditions and we test a posteriori which
solution is the closest one to the required integral condition based on
conduction. We demonstrate that the automatic mode of the code TITAN provides
the solution to the radiative transfer which is generally consistent with the
estimated exact solution within a few per cent accuracy, with larger errors for
some line intensities (up to 20 per cent) for iron lines at intermediate
ionization state.
|
0902.4083v1
|
2009-02-24
|
Frontiers of the physics of dense plasmas and planetary interiors: experiments, theory, applications
|
Recent developments of dynamic x-ray characterization experiments of dense
matter are reviewed, with particular emphasis on conditions relevant to
interiors of terrestrial and gas giant planets. These studies include
characterization of compressed states of matter in light elements by x-ray
scattering and imaging of shocked iron by radiography. Several applications of
this work are examined. These include the structure of massive "Super Earth"
terrestrial planets around other stars, the 40 known extrasolar gas giants with
measured masses and radii, and Jupiter itself, which serves as the benchmark
for giant planets.
|
0902.4115v1
|
2009-02-24
|
Heating and cooling of magnetars with accreted envelopes
|
We study the thermal structure and evolution of magnetars as cooling neutron
stars with a phenomenological heat source in an internal layer. We focus on the
effect of magnetized (B > 10^{14} G) non-accreted and accreted outermost
envelopes composed of different elements, from iron to hydrogen or helium. We
discuss a combined effect of thermal conduction and neutrino emission in the
outer neutron star crust and calculate the cooling of magnetars with a dipole
magnetic field for various locations of the heat layer, heat rates and magnetic
field strengths. Combined effects of strong magnetic fields and light-element
composition simplify the interpretation of magnetars in our model: these
effects allow one to interpret observations assuming less extreme (therefore,
more realistic) heating. Massive magnetars, with fast neutrino cooling in their
cores, can have higher thermal surface luminosity.
|
0902.4213v1
|
2009-02-26
|
Tricritical Points and Liquid-Solid Critical Lines
|
Tricritical points separate continuous and discontinuous symmetry breaking
transitions. They occur in a variety of physical systems and their mathematical
models. A tricritical point is used to determine a liquid-solid phase
transition line in the pressure-temperature plane [Aitta, J. Stat. Mech.,
2006]. Excellent experimental agreement has been obtained for iron, the
material having the most high pressure data. This allows extrapolation to much
higher pressures and temperatures than available experimentally. One can
predict the temperature at the liquid-solid boundary in the core of the Earth
where the pressure is 329 GPa. Light matter, present as impurities in the core
fluid, is found to generate about a 600 K reduction of this temperature.
|
0902.4578v1
|
2009-02-27
|
Looking outside the Galaxy: the discovery of chemical anomalies in 3 old Large Magellanic Cloud clusters
|
By using the multifiber spectrograph FLAMES mounted at the ESO-VLT, we have
obtained high-resolution spectra for 18 giant stars, belonging to 3 old
globular clusters of the Large Magellanic Cloud (namely NGC 1786, 2210 and
2257). While stars in each cluster showed quite homogeneous iron content,
within a few cents of dex (the mean values being Fe/H]= -1.75+-0.01 dex,
-1.65+-0.02 dex and -1.95+-0.02 dex for NGC 1786, 2210 and 2257, respectively),
we have detected significant inhomogeneities for the [Na/Fe], [Al/Fe], [O/Fe]
and [Mg/Fe] abundance ratios, with evidence of [O/Fe] vs [Na/Fe] and [Mg/Fe] vs
[Al/Fe] anticorrelations. The trends detected nicely agree with those observed
in Galactic Globular Clusters, suggesting that such abundance anomalies are
ubiquitous features of old stellar systems and they do not depend on the parent
galaxy environment. In NGC 1786 we also detected two extreme O-poor, Na-rich
stars. This is the first time that a firm signature of extreme chemical
abundance anomalies has been found in an extragalactic stellar cluster.
|
0902.4778v1
|
2009-02-27
|
The superconductor KxSr(1-x)Fe2As2: Normal state and superconducting properties
|
The normal state and superconducting properties are investigated in the phase
diagram of K_xSr_{1-x}Fe_2As_2 for 0<x<1. The ground state upper critical
field, H_{c2}(0), is extrapolated from magnetic field dependent resistivity
measurements. H_{c2}(0) scales with the critical temperature, T_c, of the
superconducting transition. In the normal state the Seebeck coefficient is
shown to experience a dramatic change near a critical substitution of x=0.3.
This is associated with the formation of a spin density wave state above the
superconducting transition temperature. The results provide strong evidence for
the reconstruction of the Fermi surface with the onset of magnetic order.
|
0903.0013v1
|
2009-03-09
|
Cohesive and magnetic properties of grain boundaries in bcc Fe with Cr additions
|
Structural, cohesive, and magnetic properties of two symmetric $\Sigma3(111)$
and $\Sigma5(210)$ tilt grain boundaries (GBs) in pure bcc Fe and in dilute
FeCr alloys are studied from first principles. Different concentration and
position of Cr solute atoms are considered. We found that Cr atoms placed in
the GB interstice enhance the cohesion by 0.5-1.2 J/m$^2$. Substitutional Cr in
the layers adjacent to the boundary shows anisotropic effect on the GB
cohesion: it is neutral when placed in the (111) oriented Fe grains, and
enhances cohesion (by 0.5 J/m$^2$) when substituted in the boundary layer of
the (210) grains. The strengthening effect of the Cr solute is dominated by the
chemical component of the adhesive binding energy. Our calculations show that
unlike the free iron surfaces, Cr impurities segregate to the boundaries of the
Fe grains. The magnetic moments on GB atoms are substantially changed and their
variation correlates with the corresponding relaxation pattern of the GB
planes. The moments on Cr additions are 2-4 times enhanced in comparison with
that in a Cr crystal and are antiparallel to the moments on the Fe atoms.
|
0903.1618v2
|
2009-03-12
|
Pressure evolution of low-temperature crystal structure and bonding of 37 K $T_c$ FeSe superconductor
|
FeSe with the PbO structure is a key member of the family of new high-$T_c$
iron pnictide and chalcogenide superconductors, as while it possesses the basic
layered structural motif of edge-sharing distorted FeSe$_4$ tetrahedra, it
lacks interleaved ion spacers or charge-reservoir layers. We find that
application of hydrostatic pressure first rapidly increases $T_c$ which attains
a broad maximum of 37 K at $\sim$7 GPa (this is one of the highest $T_c$ ever
reported for a binary solid) before decreasing to 6 K upon further compression
to $\sim$14 GPa. Complementary synchrotron X-ray diffraction at 16 K was used
to measure the low-temperature isothermal compressibility of $\alpha$-FeSe,
revealing an extremely soft solid with a bulk modulus, $K_0$ = 30.7(1.1) GPa
and strong bonding anisotropy between inter- and intra-layer directions that
transforms to the more densely packed $\beta$-polymorph above $\sim$9 GPa. The
non-monotonic $T_c$($P$) behavior of FeSe coincides with drastic anomalies in
the pressure evolution of the interlayer spacing, pointing to the key role of
this structural feature in modulating the electronic properties.
|
0903.2204v1
|
2009-03-12
|
Universal relation between magnetic resonance and superconducting gap in unconventional superconductors
|
Unconventional superconductors such as the high-transition temperature
cuprates, heavy-fermion systems and iron arsenide-based compounds exhibit
antiferromagnetic fluctuations that are dominated by a resonance, a collective
spin-one excitation mode in the superconducting state. Here we demonstrate the
existence of a universal linear relation, $Er \propto 2\Delta$, between the
magnetic resonance energy (Er) and the superconducting pairing gap ($\Delta$),
spanning two orders of magnitude in energy. This relation is valid for
materials that range from being close to the Mott-insulating limit to being on
the border of itinerant magnetism. Since the common excitonic picture of the
resonance has not led to such universality, our observation suggests a much
deeper connection between antiferromagnetic fluctuations and unconventional
superconductivity.
|
0903.2291v1
|
2009-03-13
|
The nature of the magnetic and structural phase transitions in BaFe$_{2}$As$_{2}$
|
We present the results of an investigation of both the magnetic and
structural phase transitions in a high quality single crystalline sample of the
undoped, iron pnictide compound BaFe$_2$As$_2$. Both phase transitions are
characterized via neutron diffraction measurements which reveal simultaneous,
continuous magnetic and structural orderings with no evidence of hysteresis,
consistent with a single second order phase transition. The onset of long-range
antiferromagnetic order can be described by a simple power law dependence
$\phi(T)^2\propto(1-\frac{T}{T_N})^{2\beta}$ with $\beta=0.103\pm0.018$; a
value near the $\beta=0.125$ expected for a two-dimensional Ising system.
Biquadratic coupling between the structural and magnetic order parameters is
also inferred along with evidence of three-dimensional critical scattering in
this system.
|
0903.2430v2
|
2009-03-23
|
On Estimating the Flux of the Brightest Cosmic Ray Source above 57x10^18 eV
|
The sources of ultra-high energy cosmic rays are not yet known. However, the
discovery of anisotropic cosmic rays above 57x10^18 eV by the Pierre Auger
Observatory suggests that a direct source detection may soon be possible. The
near-future prospects for such a measurement are heavily dependent on the flux
of the brightest source. In this work, we show that the flux of the brightest
source above 57x10^18 eV is expected to comprise 10% or more of the total flux
if two general conditions are true. The conditions are: 1.) the source objects
are associated with galaxies other than the Milky Way and its closest
neighbors, and 2.) the cosmic ray particles are protons or heavy nuclei such as
iron and the Greisen-Zatsepin-Kuz'min effect is occurring. The Pierre Auger
Observatory collects approximately 23 events above 57x10^18 eV per year.
Therefore, it is plausible that, over the course of several years, tens of
cosmic rays from a single source will be detected.
|
0903.3981v1
|
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