publicationDate
stringlengths 1
2.79k
| title
stringlengths 1
36.5k
⌀ | abstract
stringlengths 1
37.3k
⌀ | id
stringlengths 9
47
|
|---|---|---|---|
2011-03-30
|
Biaxially textured cobalt-doped BaFe2As2 films with high critical current density over 1 MA/cm2 on MgO-buffered metal-tape flexible substrates
|
High critical current densities (Jc) > 1 MA/cm2 were realized in cobalt-doped
BaFe2As2 (BaFe2As2:Co) films on flexible metal substrates with
biaxially-textured MgO base-layers fabricated by an ion-beam assisted
deposition technique. The BaFe2As2:Co films showed small in-plane crystalline
misorientations (delta fai BaFe2As2:Co) of ~3o regardless of doubly larger
misorientaions of the MgO base-layers (delta fai MgO = 7.3o), and exhibited
high self-field Jc up to 3.5 MA/cm2 at 2 K. These values are comparable to that
on MgO single crystals and the highest Jc among iron pnictide superconducting
tapes and wires ever reported. High in-field Jc suggests the existence of
c-axis correlated vortex pinning centers.
|
1103.5815v2
|
2011-04-04
|
Epitaxial growth of FeSe$_{0.5}$Te$_{0.5}$ thin films on CaF$_2$ substrates with high critical current density
|
In-situ epitaxial growth of FeSe$_{0.5}$Te$_{0.5}$ thin films is demonstrated
on a non-oxide substrate CaF$_2$. Structural analysis reveals that compressive
stress is moderately added to 36-nm thick FeSe$_{0.5}$Te$_{0.5}$, which pushes
up the critical temperature above 15 K, showing higher values than that of bulk
crystals. Critical current density at $T$ = 4.5 K reaches 5.9 x 10$^4$
Acm$^{-2}$ at $\mu_0H$ = 10 T, and 4.2 x 10$^4$ Acm$^{-2}$ at $\mu_0H$ = 14 T.
These results indicate that fluoride substrates have high potential for the
growth of iron-based superconductors in comparison with popular oxide
substrates.
|
1104.0477v2
|
2011-04-04
|
The NLTE Barium Abundance in Dwarf Stars in the Metallicity Range of -1 < [Fe/H] < +0.3
|
We present the results of determination of the barium abundance considering
the non-LTE (NLTE) effects in 172 dwarf stars in the metallicity range of -1<
[Fe/H] <+0.3, assigned to different Galactic substructures by kinematic
criteria. We used a model of the Ba atom with 31 levels of Ba I and 101 levels
of Ba II. The atmosphere models for the investigated stars were computed using
the ATLAS9 code modified by new opacity distribution functions. The NLTE
profiles of the unblended Ba II (4554 A, 5853 A, 6496 A) were computed and then
compared to those observed. The line 6141 A was also used, but with an
allowance for its correlation with the iron line. The average barium abundances
in the thin and thick discs are 0.01 +/- 0.08 and -0.03 +/- 0.07, respectively.
The comparison to the calculations of the Galactic chemical evolution by
Serminato et al. (2009) was conducted. The trend obtained for the Ba abundance
versus [Fe/H] suggests a complex barium production process in the thin and
thick discs.
|
1104.0534v1
|
2011-04-05
|
Incommensurate spin-density wave order in electron-doped BaFe2As2 superconductors
|
Neutron diffraction studies of Ba(Fe[1-x]Co[x])2As2 reveal that commensurate
antiferromagnetic order gives way to incommensurate magnetic order for Co
compositions between 0.056 < x < 0.06. The incommensurability has the form of a
small transverse splitting (0, +-e, 0) from the nominal commensurate
antiferromagnetic propagation vector Q[AFM] = (1, 0, 1) (in orthorhombic
notation) where e = 0.02-0.03 and is composition dependent. The results are
consistent with the formation of a spin-density wave driven by Fermi surface
nesting of electron and hole pockets and confirm the itinerant nature of
magnetism in the iron arsenide superconductors.
|
1104.0717v1
|
2011-04-08
|
Magnetic nanocomposites at microwave frequencies
|
Most conventional magnetic materials used in the electronic devices are
ferrites, which are composed of micrometer-size grains. But ferrites have small
saturation magnetization, therefore the performance at GHz frequencies is
rather poor. That is why functionalized nanocomposites comprising magnetic
nanoparticles (e.g. Fe, Co) with dimensions ranging from a few nm to 100 nm,
and embedded in dielectric matrices (e.g. silicon oxide, aluminium oxide) have
a significant potential for the electronics industry. When the size of the
nanoparticles is smaller than the critical size for multidomain formation,
these nanocomposites can be regarded as an ensemble of particles in
single-domain states and the losses (due for example to eddy currents) are
expected to be relatively small. Here we review the theory of magnetism in such
materials, and we present a novel measurement method used for the
characterization of the electromagnetic properties of composites with
nanomagnetic insertions. We also present a few experimental results obtained on
composites consisting of iron nanoparticles in a dielectric matrix.
|
1104.1535v1
|
2011-04-11
|
Collinear to Anti-collinear Quantum Phase Transition by Vacancies
|
We study static vacancies in the collinear magnetic phase of a frustrated
Heisenberg $J_1$-$J_2$ model. It is found that vacancies can rapidly suppress
the collinear antiferromagnetic state (CAFM)and generate a new magnetic phase,
an anti-collinear magnetic phase (A-CAFM), due to magnetic frustration. We
investigate the quantum phase transition between these two states by studying a
variety of vacancy superlattices. We argue that the anti-collinear magnetic
phase can exist in iron-based superconductors in the absence of any preceding
structural transitions and an observation of this novel phase will
unambiguously resolve the relation between the magnetic and structural
transitions in these materials.
|
1104.1848v2
|
2011-04-12
|
Reply to "Comment on 'Isotope effect in multi-band and multi-channel attractive systems and inverse isotope effect in iron-based superconductors'"
|
The Comment insists on the following: in our model it is assumed that the
effective interactions have specific energy ranges within the single band with
a cutoff at \omega_1 for the phononic part and a range from \omega_1 to
\omega_2 in the AF channel. Our reply is that we assume that V_i(k,k')\neq 0 if
|\xi_k|<\omega_i and |\xi_{k'}|<\omega_i, and otherwise V_i(k,k')= 0 (i=1,2),
as stated in our paper. This is the model of BCS type with two attractive
interactions, and this assumption is the characteristic of the BCS
approximation. The claim "the integration limits have been modified such that
the AF channel mediated pairing sets in where the ph-channel pairing terminates
and is limited at an energy given by \omega_j=\omega_{AF}" in the Comment is
wrong. We describe the model and the method to solve the gap equation in more
detail.
|
1104.2146v1
|
2011-03-26
|
Parametrization for Cherenkov light lateral distribution function in Extensive Air Showers
|
The simulation of the Cherenkov light lateral distribution function (LDF) in
Extensive Air Showers (EAS) was perfomed with the CORSIKA code in the energy
range 10^13-10^16 eV for configuration of the Tunka-25 EAS array. On the basis
of this simulation we obtained sets of approximating functions for primary
protons, iron nuclei and gamma-quanta for zenith angles Theta<=20. The
comparison of the calculated Cherenkov light LDF with that measured with the
Tunka-25 array has shown an opportunity of primary particle identification and
definition of its energy around the "knee".
|
1104.2510v1
|
2011-04-15
|
The effect of hydrogen on the magnetic properties of FeV superlattice
|
The electronic and magnetic structures of a hydrogenated and hydrogen free
superlattice of 3 iron monolayers and 9 vanadium monolayers are studied using
the first principle full-potential augmented-plane-wave method as implemented
in WIEN2k package. The volume, the total energy and the magnetic moments of the
system are studied versus the hydrogen positions at the octahedral sites within
the superlattice and also versus the filling of the vanadium octahedral
location by hydrogen atoms. It is found that the hydrogen locations at the
interior of vanadium layer are energetically more favourable. The local Fe
magnetic moment and the average magnetic moment per supercell are found to
increase as the H position moves towards the Fe-V interface. On the other hand,
the average magnetic moment per supercell is found to initially decrease up to
filling by 3 H atoms and then increases afterwards. To our knowledge, this is
the first reporting on the increase in the computed magnetic moment with
hydrogenation. These trends of magnetic moments are attributed to the volume
changes resulting from hydrogenation and not to electronic hydrogen-metal
interaction.
|
1104.2965v1
|
2011-04-18
|
A new composition-sensitive parameter for Ultra-High Energy Cosmic Rays
|
A new family of parameters intended for composition studies in cosmic ray
surface array detectors is proposed. The application of this technique to
different array layout designs has been analyzed. The parameters make exclusive
use of surface data combining the information from the total signal at each
triggered detector and the array geometry. They are sensitive to the combined
effects of the different muon and electromagnetic components on the lateral
distribution function of proton and iron initiated showers at any given primary
energy. Analytical and numerical studies have been performed in order to assess
the reliability, stability and optimization of these parameters. Experimental
uncertainties, the underestimation of the muon component in the shower
simulation codes, intrinsic fluctuations and reconstruction errors are
considered and discussed in a quantitative way. The potential discrimination
power of these parameters, under realistic experimental conditions, is compared
on a simplified, albeit quantitative way, with that expected from other surface
and fluorescence estimators.
|
1104.3399v1
|
2011-04-22
|
Modelling the Broad-band Spectra of MCG-6-30-15 with a Relativistic Reflection Model
|
The spectrum of the bright Seyfert I galaxy MCG-6-30-15 shows a broad Fe
K-alpha emission line, generally interpreted to originate very close to the
central black hole which must then have a high spin. The observed X-ray
variability is driven by a powerlaw component, with little variability found in
the iron line energy band. The disconnection of continuum and line variability
may be a consequence of strong gravitational light bending. The X-ray spectrum
of MCG-6-30-15 is however complex, strongly modified at soft X-ray energies by
warm absorbers, which some workers have extended to build an
absorption-dominated model of the whole source behaviour. The absorption model
interprets the whole X-ray spectrum without a relativistic reflection component
and attributes most variability to the warm absorbers. We re-examine the
XMM-Newton, Chandra and Suzaku data taken in 2001, 2004 and 2006, respectively,
and construct a model consisting of several warm absorbers confirmed by
spectral analysis, together with a relativistically blurred reflection
component that explains both the soft and hard excesses as well. The model
works well on data from all epochs, demonstrating that the reflection model
provides a consistent interpretation of the broadband spectrum of MCG-6-30-15.
|
1104.4483v1
|
2011-04-27
|
Study of the X-ray/gamma source AX J1910.7+0917 and three newly discovered INTEGRAL sources
|
AX J1910.7+0917 is a still unidentified source discovered with ASCA and
observed more recently with IBIS/ISGRI, mainly noticeable for its rather hard
spectrum. We analyzed all the public available data on this source, and we took
advantage of the recent improvements performed in the INTEGRAL data analysis
software to fully exploit the IBIS/ISGRI data. In the data collected from
INTEGRAL, XMM-Newton, Chandra and ASCA the source is clearly variable. The
spectrum can be modelled as an absorbed powerlaw (NH~6x10^22 cm^-2, Gamma=1.5)
with an iron line at 6.4 keV. The present data still do not allow for a unique
classification of the source. In the IBIS/ISGRI field of view around AX
J1910.7+0917, we discovered three new sources: IGR J19173+0747, IGR J19294+1327
and IGR J19149+1036, where the latter is positionally coincident with the
Einstein source 2E 1912.5+1031. For the first two sources we report results
obtained from follow-up observations carried out with Swift/XRT.
|
1104.5102v1
|
2011-04-28
|
Electronic inhomogeneity in a Kondo lattice
|
Inhomogeneous electronic states resulting from entangled spin, charge, and
lattice degrees of freedom are hallmarks of strongly correlated electron
materials; such behavior has been observed in many classes of d-electron
materials, including the high-Tc copper-oxide superconductors, manganites, and
most recently the iron-pnictide superconductors. The complexity generated by
competing phases in these materials constitutes a considerable theoretical
challenge-one that still defies a complete description. Here, we report a new
manifestation of electronic inhomogeneity in a strongly correlated f-electron
system, using CeCoIn5 as an example. A thermodynamic analysis of its
superconductivity, combined with nuclear quadrupole resonance measurements,
shows that nonmagnetic impurities (Y, La, Yb, Th, Hg and Sn) locally suppress
unconventional superconductivity, generating an inhomogeneous electronic "Swiss
cheese" due to disrupted periodicity of the Kondo lattice. Our analysis may be
generalized to include related systems, suggesting that electronic
inhomogeneity should be considered broadly in Kondo lattice materials.
|
1104.5457v1
|
2011-04-29
|
Evolution of correlation strength in KxFe(2-y)Se2 superconductor doped with S
|
We report the evolution of thermal transport properties of iron-based
superconductor K$_x$Fe$_{2-y}$Se$_2$ with sulfur substitution at Se sites.
Sulfur doping suppresses the superconducting $T_c$ as well as the Seebeck
coefficient. The Seebeck coefficient of all crystals in the low temperature
range can be described very well by diffusive thermoelectric response model.
The zero-temperature extrapolated value of Seebeck coefficient divided by
temperature $S/T$ gradually decreases from $-0.48 \mu V/K^2$ to a very small
value $\sim$ 0.03 $\mu$V/K$^2$ where $T_c$ is completely suppressed. The normal
state electron Sommerfeld term ($\gamma_n$) of specific heat also decreases
with the increase of sulfur content. The dcrease of $S/T$ and $\gamma_n$
reflects a suppression of the density of states at the Fermi energy, or a
change in the Fermi surface that would induce the suppression of correlation
strength.
|
1104.5711v1
|
2011-04-30
|
Magnetic hydrogels derived from polysaccharides with improved specific power absorption: potential devices for remotely triggered drug delivery
|
We report on novel ferrogels derived from polysaccharides (sodium alginate
and chitosan) with embedded iron oxide nanoparticles synthesized in situ and
their combination with thermally responsive poly (N-isopropylacrylamide) for
externally-driven drug release using AC magnetic fields. Samples were
characterized by Raman spectroscopy, transmission electron microscopy (TEM) and
magnetic measurements. The obtained nanoparticles were found to be of ca. 10 nm
average size, showing magnetic properties very close to those of the bulk
material. The thermal response was measured by power absorption experiments,
finding specific power absorption (SPA) values between 100-300 W/g, which was
enough for attaining the lower critical solution temperature (LCST) of the
polymeric matrix within few minutes. This fast response makes these materials
good candidates for externally controlled drug release.
|
1105.0075v1
|
2011-05-11
|
Systematics of the temperature-dependent inter-plane resistivity in Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ with T= Rh, Ni, and Pd
|
Temperature-dependent inter-plane resistivity, $\rho_c(T)$, was measured
systematically as a function of transition metal substitution in the
iron-arsenide superconductors Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$, $T$= Ni, Pd, Rh.
The data are compared with the behavior found in
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, revealing resistive signatures of pseudogap. In
all compounds we find resistivity crossover at a characteristic pseudogap
temperature $T^*$ from non-metallic to metallic temperature dependence on
cooling. Suppression of $T^*$ proceeds very similar in cases of Ni and Pd
doping and much faster than in similar cases of Co and Rh doping. In cases of
Co and Rh doping an additional minimum in the temperature-dependent $\rho_c$
emerges for high dopings, when superconductivity is completely suppressed.
These features are consistent with the existence of a charge gap covering part
of the Fermi surface. The part of the Fermi surface affected by this gap is
notably larger for Ni and Pd doped compositions than in Co and Rh doped
compounds.
|
1105.2277v1
|
2011-05-13
|
Scaling of nascent nodes in extended s-wave superconductors
|
We analyze the low-energy properties of superconductors near the onset of
accidental nodes, i.e. zeroes of the gap function not enforced by symmetry. The
existence of such nodes has been motivated by recent experiments suggesting a
transition between nodeless and nodal superconductivity in iron-based
compounds. We find that the low-temperature behavior of the penetration depth,
the specific heat, and the NMR spin-lattice relaxation rate are determined by
the scaling properties of a quantum critical point associated with the nascent
nodes. Although the power-law exponents are insensitive to weak short-range
electronic interactions, they can be significantly altered by the curvature of
the Fermi surface or by the three-dimensional character of the gap.
Consequently, the behavior of macroscopic quantities near the onset of nodes
can be used as a criterion to determine the nodal structure of the gap
function.
|
1105.2785v2
|
2011-05-17
|
Superconductivity and ferromagnetism in EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$
|
Superconductivity and ferromagnetism are two antagonistic cooperative
phenomena, which makes it difficult for them to coexist. Here we demonstrate
experimentally that they do coexist in EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ with
$0.2\leq x\leq0.4$, in which superconductivity is associated with Fe-3$d$
electrons and ferromagnetism comes from the long-range ordering of Eu-4$f$
moments via Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. The coexistence
is featured by large saturated ferromagnetic moments, high and comparable
superconducting and magnetic transition temperatures, and broad coexistence
ranges in temperature and field. We ascribe this unusual phenomenon to the
robustness of superconductivity as well as the multi-orbital characters of iron
pnictides.
|
1105.3255v1
|
2011-05-17
|
Effective Ex-Situ Fabrication of F-Doped SmFeAsO Wire for High Transport Critical Current Density
|
We demonstrate the fabrication of superconducting SmFeAsO1-xFx (Sm-1111)
wires by using the ex-situ powder-in-tube technique. Sm-1111 powder and a
binder composed of SmF3, samarium arsenide, and iron arsenide were used to
synthesize the superconducting core. Although the F content of Sm-1111 is
reduced in the process of ex-situ fabrication, the binder compensates by
sufficiently supplementing the F content, thereby preventing a decrease in the
superconducting transition temperature and a shrinkage of the superconducting
volume fraction. Thus, in the superconducting Sm-1111 wire with the binder, the
transport critical current density reaches the highest value of ~4 kA/cm2 at
4.2 K.
|
1105.3342v2
|
2011-05-19
|
High energy pseudogap and its evolution with doping in Fe-based superconductors as revealed by optical spectroscopy
|
We report optical spectroscopic measurements on electron- and hole-doped
BaFe2As2. We show that the compounds in the normal state are not simple metals.
The optical conductivity spectra contain, in addition to the free carrier
response at low frequency, a temperature-dependent gap-like suppression at
rather high energy scale near 0.6 eV. This suppression evolves with the
As-Fe-As bond angle induced by electron- or hole-doping. Furthermore, the
feature becomes much weaker in the Fe-chalcogenide compounds. We elaborate that
the feature is caused by the strong Hund's rule coupling effect between the
itinerant electrons and localized electron moment arising from the multiple Fe
3d orbitals. Our experiments demonstrate the coexistence of itinerant and
localized electrons in iron-based compounds, which would then lead to a more
comprehensive picture about the metallic magnetism in the materials.
|
1105.3939v1
|
2011-05-20
|
Neutrino Probes of the Nature of Light Dark Matter
|
Dark matter particles gravitationally trapped inside the Sun may annihilate
into Standard Model particles, producing a flux of neutrinos. The prospects of
detecting these neutrinos in future multi-\kton{} neutrino detectors designed
for other physics searches are explored here. We study the capabilities of a
34/100 \kton{} liquid argon detector and a 100 \kton{} magnetized iron
calorimeter detector. These detectors are expected to determine the energy and
the direction of the incoming neutrino with unprecedented precision allowing
for tests of the dark matter nature at very low dark matter masses, in the
range of 5-50 GeV. By suppressing the atmospheric background with angular cuts,
these techniques would be sensitive to dark matter - nucleon spin dependent
cross sections at the fb level, reaching down to a few ab for the most
favorable annihilation channels and detector technology.
|
1105.4077v2
|
2011-05-26
|
Phosphor induced significant hole-doping in ferropnictide superconductor BaFe2(As(1-x)Px)2
|
The superconductivity in high temperature superconductors ordinarily arises
when doped with hetero-valent ions that introduce charge carriers. However, in
ferropnictides, "iso-valent" doping, which is generally believed not to
introduce charge carriers, can induce superconductivity as well. Moreover,
unlike other ferropnictides, the superconducting gap in BaFe2(As1-xPx)2 has
been found to contain nodal lines. The exact nature of the "iso-valent" doping
and nodal gap here are key open issues in building a comprehensive picture of
the iron-based high temperature superconductors. With angle-resolved
photoemission spectroscopy (ARPES), we found that the phosphor substitution in
BaFe2(As1-xPx)2 induces sizable amount of holes into the hole Fermi surfaces,
while the dxy-originated band is relatively intact. This overturns the previous
common belief of "iso-valent" doping, explains why the phase diagram of
BaFe2(As1-xPx)2 is similar to those of the holedoped compounds, and rules out
theories that explain the nodal gap based on vanishing dxy hole pocket.
|
1105.5242v1
|
2011-05-27
|
Orbitally resolved lifetimes in Ba(Fe0.92Co0.08)2As2 measured by ARPES
|
Despite many ARPES investigations of iron pnictides, the structure of the
electron pockets is still poorly understood. By combining ARPES measurements in
different experimental configurations, we clearly resolve their elliptic shape.
Comparison with band calculation identify a deep electron band with the dxy
orbital and a shallow electron band along the perpendicular ellipse axis with
the dxz/dyz orbitals. We find that, for both electron and hole bands, the
lifetimes associated with dxy are longer than for dxz/dyz. This suggests that
the two types of orbitals play different roles in the electronic properties and
that their relative weight is a key parameter to determine the ground state.
|
1105.5604v2
|
2011-05-31
|
Micromagnetic study of flux-closure states in Fe dots using quantitative Lorentz Microscopy
|
A micromagnetic study of epitaxial micron-sized iron dots is reported through
the analysis of Fresnel contrast in Lorentz Microscopy. Their use is reviewed
and developed through analysis of various magnetic structures in such dots.
Simple Landau configuration is used to investigate various aspects of
asymmetric Bloch domain walls. The experimental width of such a complex wall is
first derived and its value is discussed with the help of micromagnetic
simulations. Combination of these two approaches enables us to define what is
really extracted when estimating asymmetric wall width in Lorentz Microscopy.
Moreover, quantitative data on the magnetization inside the dot is retrieved
using phase retrieval as well as new informations on the degrees of freedom of
such walls. Finally, it is shown how the existence and the propagation of a
surface vortex can be characterized and monitored. This demonstrates the
ability to reach a magnetic sensitivity a priori hidden in Fresnel contrast,
based on an original image treatment and backed-up by the evaluation of
contrasts obtained from micromagnetic simulations.
|
1105.6295v2
|
2011-06-05
|
Novel Insulating Magnetism in Vacancy-Ordered K2Fe4Se5
|
We unveil the novel physical origin of the insulating block checkerboard
antiferromagnetism in vacancy-ordered K2Fe4Se5. Our first-principles electronic
structure analysis reveals its incompatibility with a simple Fermi-surface
nesting or Mott insulator scenario, and suggests the picture of coexisting
itinerant and localized electronic states. Consistently, we demonstrate that it
can be unified with the metallic collinear or bicollinear antiferromagnetism of
the vacancy-free parent compounds LaOFeAs, BaFe2As2, or FeTe in the
spin-fermion model. These results indicate that the blocking effects of Hund's
rule coupling and the resulting electron correlation are crucial to the
electronic and magnetic structures of iron-based superconductors.
|
1106.0881v2
|
2011-06-06
|
Correlation of high temperature X-ray photoemission spectral features and conductivity of epitaxially strained (La0.8Sr0.2)0.95Ni0.2Fe0.8O3/SrTiO3(110)
|
Reversible and irreversible discontinuities at around 573 K and 823 K in the
electric conductivity of a strained 175 nm thin film of
(La0.8Sr0.2)0.95Ni0.2Fe0.8O3-{\delta} grown by pulsed laser deposition on
SrTiO3 (110) are reflected by valence band changes as monitored in
photoemission and oxygen K-edge x-ray absorption spectra. The irreversible jump
at 823 K is attributed to depletion of doped electron holes and reduction of
Fe4+ to Fe3+, as evidenced by oxygen and iron core level soft x-ray
spectroscopy, and possibly of a chemical origin, whereas the reversible jump at
573 K possibly originates from structural changes.
|
1106.1024v1
|
2011-06-06
|
Fe-resonant valence band photoemission and oxygen NEXAFS study on La1-xSrxFe0.75Ni0.25O3-δ
|
Iron resonant valance band photoemission spectra of Sr substituted
LaFe0.75Ni0.25 O3-{\delta} have been recorded across the Fe 2p - 3d absorption
threshold to obtain Fe specific spectral information on the 3d projected
partial density of states. Comparison with La1-xSrxFeO3 resonant VB PES
literature data suggests that substitution of Fe by Ni forms electron holes
which are mainly O 2p character. Substitution of La by Sr increases the hole
concentration to an extent that the eg structure vanishes. The variation of the
eg and t2g structures is paralleled by the changes in the electrical
conductivity.
|
1106.1086v1
|
2011-06-07
|
Analysis of interdiffusion between SmFeAsO0.92F0.08 and metals for ex situ fabrication of superconducting wire
|
We demonstrate the fabrication of superconducting SmFeAsO1-xFx (Sm-1111)
wires by using the ex-situ powder-in-tube technique. Sm-1111 powder and a
binder composed of SmF3, samarium arsenide, and iron arsenide were used to
synthesize the superconducting core. Although the F content of Sm-1111 is
reduced in the process of ex-situ fabrication, the binder compensates by
sufficiently supplementing the F content, thereby preventing a decrease in the
superconducting transition temperature and a shrinking of the superconducting
volume fraction. Thus, in the superconducting Sm-1111 wire with the binder, the
transport critical current density reaches the highest value of ~4000 A/cm2 at
4.2 K.
|
1106.1328v2
|
2011-06-11
|
Fish-tail and vortex dynamics in Ni doped iron-pnictide BaFe{1.82}Ni_{0.18}As_2
|
We study the vortex-dynamics of a BaFe{1.82}Ni_{0.18}As_2 crystal with Tc = 8
K, by measuring flux-creep over the second magnetisation (or fish-tail) peak
for both H//c-axis and H//ab planes. Magnetic relaxation data show an
anomalously long initial stage of relaxation, lasting approximately10 minutes
for H//c-axis and 2-3 min for H//ab, resembling a transient effect with a lower
relaxation rate, which is followed by the usual log(time) relaxation.
Interestingly, study of the relaxation rate R vs H for both stages of
relaxation and for both field directions, are featureless over the field range
associated with the fish-tail. The same trend was confirmed by plotting R vs T
obtained from flux-creep data measured as a function of temperature for a fixed
field (H//$c-axis). A plot of the activation energy U(M,T) calculated from the
time relaxation of the magnetisation at a fixed field also shows a smooth
behavior further supporting the view that the fish-tail peak is not associated
with a crossover in vortex pinning regime within the collective pinning
scenario.
|
1106.2248v2
|
2011-06-17
|
A Three-Dimensional Tight-Binding Model and Magnetic Instability of KFe2e2
|
For a newly discovered iron-based high T_c superconducting parent material
KFe2Se2, we present an effective three-dimensional five-orbital tight-binding
model by fitting the band structures. The three t2g-symmetry orbitals of the
five Fe 3d orbitals mainly contribute to the electron-like Fermi surface, in
agreement with recent angle-resolved photoemission spectroscopy experiments. To
understand the groundstate magnetic structure, the two- and three-dimensional
dynamical spin susceptibilities within the random phase approximation are
investigated. It obviously shows a sharp peak at wave vector $\mathbf{Q}$
$\thicksim$ ($\pi$, $\pi$), indicating the magnetic instability of {\it
N$\acute{e}$el}-type antiferromagnetic rather than ($\pi$/2, $\pi$/2)-type
antiferromagnetic ordering. While along $\emph{c}$ axis, it exhibits a
ferromagnetic coupling between the nearest neighboring FeSe layers. The
difference between the present results and the experimental observation in
KxFe2-ySe2 is attributed to the presence of Fe vacancy in the latter.
|
1106.3485v1
|
2011-06-21
|
Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-based New Superconductor (Ca_4Al_2O_{6-y})(Fe_2As_2):75As-NQR Study
|
We report 75As-nuclear quadrupole resonance (NQR) studies on
(Ca_4Al_2O_{6-y})(Fe_2As_2) with Tc=27K, which unravel unique normal-state
properties and point to unconventional nodeless superconductivity (SC).
Measurement of nuclear-spin-relaxation rate 1/T_1 has revealed a significant
development of two dimensional (2D) antiferromagnetic (AFM) spin fluctuations
down to Tc, in association with the fact that FeAs layers with the smallest
As-Fe-As bond angle are well separated by thick perovskite-type blocking layer.
Below Tc, the temperature dependence of 1/T_1 without any trace of the
coherence peak is well accounted for by an s(+-)-wave multiple gaps model. From
the fact that Tc=27K in this compound is comparable to Tc=28K in the
optimally-doped LaFeAsO_{1-y} in which AFM spin fluctuations are not dominant,
we remark that AFM spin fluctuations are not a unique factor for enhancing Tc
among existing Fe-based superconductors, but a condition for optimizing SC
should be addressed from the lattice structure point of view.
|
1106.4105v1
|
2011-06-22
|
Evidence for conventional superconductivity in SrPd2Ge2
|
Electronic structure of SrPd2Ge2 single crystals is studied by angle-resolved
photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS) and
band-structure calculations within the local-density approximation (LDA). The
STS measurements show single s-wave superconducting energy gap \Delta(0) = 0.5
meV. Photon-energy dependence of the observed Fermi surface reveals a strongly
three-dimensional character of the corresponding electronic bands. By comparing
the experimentally measured and calculated Fermi velocities a renormalization
factor of 0.95 is obtained, which is much smaller than typical values reported
in Fe-based superconductors. We ascribe such an unusually low band
renormalization to the different orbital character of the conduction electrons
and using ARPES and STS data argue that SrPd2Ge2 is likely to be a conventional
superconductor, which makes it clearly distinct from isostructural iron
pnictide superconductors of the "122" family.
|
1106.4585v1
|
2011-06-26
|
Anisotropic two-orbital Hubbard model: single-site versus cluster dynamical mean-field theory
|
The anisotropic two-orbital Hubbard model with different bandwidths and
degrees of frustration in each orbital is investigated in the framework of both
single-site dynamical mean-field theory (DMFT) as well as its cluster extension
(DCA) for clusters up to four sites combined with a continuous-time quantum
Monte Carlo algorithm. This model shows a rich phase diagram which includes the
appearance of orbital selective phase transitions, non-Fermi liquid behavior as
well as antiferromagnetic metallic states. We discuss the advantages and
drawbacks of employing the single-site DMFT with respect to DCA and the
consequences for the physical picture obtained out of these calculations.
Finally, we argue that such a minimal model may be of relevance to understand
the nature of the antiferromagnetic metallic state in the iron-pnictide
superconductors as well as the origin of the small staggered magnetization
observed in these systems.
|
1106.5255v1
|
2011-06-27
|
Comment on "Infrared signature of the superconducting gap symmetry in iron-arsenide superconductors", Y. M. Dai et al. arXiv:1106.4430
|
Y. M. Dai et al. in their recent work arXiv:1106.4430 presented a
reflectivity study of the in-plance optical conductivity of a
$\textrm{Ba}_{0.6}\textrm{K}_{0.4}\textrm{Fe}_2\textrm{As}_2$ (BKFA)
superconductor with $T_{\textrm{c}}\approx39K$. The single crystals used in
this study are of high quality and the measurements are rather accurate. The
authors analyzed the optical conductivity of BKFA in the framework of the
Mattis-Bardeen theory and the BCS theory of superconductivity. This analysis,
however, generates a series of severe contradictions with a large body of
experimental data obtained with multiple probes as well as misuses both the
Mattis-Bardeen and the BCS theories of superconductivity beyond their
approximation regions.
|
1106.5343v1
|
2011-06-27
|
Nodal superconductivity in Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ induced by isovalent Ru substitution
|
We present the ultra-low-temperature heat transport study of an iron-based
superconductor Ba(Fe$_{0.64}$Ru$_{0.36}$)$_2$As$_2$ ($T_c$ = 20.2 K), in which
the superconductivity is induced by isovalent Ru substitution. In zero field we
find a large residual linear term $\kappa_0/T$, more than 40% of the
normal-state value. At low field, the $\kappa_0/T$ shows an $H^{1/2}$
dependence. These provide strong evidences for nodes in the superconducting gap
of Ba(Fe$_{0.64}$Ru$_{0.36}$)$_2$As$_2$, which mimics that in another
isovalently substituted superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Our
results show that the isovalent Ru substitution can also induce nodal
superconductivity in BaFe$_2$As$_2$, as P does, and they may have the same
origin. We further compare them with other two nodal superconductors LaFePO and
LiFeP.
|
1106.5417v2
|
2011-06-29
|
Suzaku Discovery of a Hard Component Varying Independently of the Power-Law Emission in MCG-6-30-15
|
Focusing on hard X-ray variability, we reanalyzed Suzaku data of Type I
Seyfert galaxy MCG-6-30-15 obtained in 2006. Intensity-sorted spectroscopy and
a principal component analysis consistently revealed a very hard component that
varies independently of the dominant power-law emission. Although the exact
nature of this hard component is not yet identified, it can be modeled as a
power-law with a photon index ~2 affected by a partial covering absorption, or
as a thermal Comptonization emission with a relatively large optical depth.
When this component is included in the fitting model, the time-averaged 2.5-55
keV spectrum of MCG-6-30-15 can be reproduced successfully by invoking a mildly
broadened iron line with its emission region located at > 8 times the
gravitational radii from the central black hole, and a moderate reflection with
a covering fraction of ~3.4. This result implies that the solution of a highly
spinning black hole in MCG-6-30-15, obtained by Miniutti et al. (2007, PASJ,
59, S315) using the same Suzaku data, is a model dependent result.
|
1106.5872v1
|
2011-07-02
|
Nanoscale phase separation in the iron chalcogenide superconductor K0.8Fe1.6Se2 as seen via scanning nanofocused x-ray diffraction
|
Advanced synchrotron radiation focusing down to a size of 300 nm has been
used to visualize nanoscale phase separation in the K0.8Fe1.6Se2
superconducting system using scanning nanofocus single-crystal X-ray
diffraction. The results show an intrinsic phase separation in K0.8Fe1.6Se2
single crystals at T< 520 K, revealing coexistence of i) a magnetic phase
characterized by an expanded lattice with superstructures due to Fe vacancy
ordering and ii) a non-magnetic phase with an in-plane compressed lattice. The
spatial distribution of the two phases at 300 K shows a frustrated or arrested
nature of the phase separation. The space-resolved imaging of the phase
separation permitted us to provide a direct evidence of nanophase domains
smaller than 300 nm and different micrometer-sized regions with percolating
magnetic or nonmagnetic domains forming a multiscale complex network of the two
phases.
|
1107.0412v2
|
2011-07-03
|
Linear decrease of critical temperature with increasing Zn substitution in the iron-based superconductor BaFe1.89-2xZn2xCo0.11As2
|
The nonmagnetic impurity effect is studied on the Fe-based BaFe1.89Co0.11As2
superconductor (Tc = 25 K) with Zn substitution for Fe up to 8 at. %, which is
achieved by means of high-pressure and high-temperature heating. Tc decreases
almost linearly with increasing the Zn content and disappears at ~8 atomic %,
being different in the shared phenomenology of the early Zn doping studies,
where Tc decreases little. The Tc decreasing rate, however, remains much lower
(3.63 K/%) than what is expected for the s(+-)-wave model, implying the model
is unlikely. Another symmetry model such as the non-sign reversal s-wave model
may better account for the result.
|
1107.0423v1
|
2011-07-04
|
London Penetration Depth in Iron - based Superconductors
|
Measurements of London penetration depth are a sensitive tool to study
multi-band superconductivity and it has provided several important insights to
the behavior of Fe-based superconductors. We first briefly review the
"experimentalist - friendly" self-consistent Eilenberger model that relates the
measurable superfluid density and structure of superconducting gaps. Then we
focus on the \BaFe -derived materials, for which the results are consistent
with 1) two distinct superconducting gaps; 2) development of strong in-plane
gap anisotropy with the departure from the optimal doping; 3) appearance of gap
nodes along the $c-$direction in a highly overdoped regime; 4) significant
pair-breaking, presumably due to charge doping; 5) fully gapped (exponential)
intrinsic behavior at the optimal doping if scattering is removed (probed in
the "self-doped" stoichiometric LiFeAs); 6) competition between magnetically
ordered state and superconductivity, which do coexist in the underdoped
compounds. Overall, it appears that while there are common trends in the
behavior of Fe-based superconductors, the gap structure is non-universal and is
sensitive to the doping level. It is plausible that the rich variety of
possible gap structures within the general $s_{\pm}$ framework is responsible
for the observed behavior.
|
1107.0675v1
|
2011-07-04
|
Enhancement of the Superconducting Transition Temperature under Pressure in Rare-Earth Doped Ca$_{1-x}$La$_x$Fe$_2$As$_2$ (x=0.27)
|
We report measurements of the pressure dependence of the superconducting
transition temperature T_c in single crystal samples of the rare-earth doped
superconductor Ca$_{0.73}$La$_{0.27}$Fe$_2$As$_2$. We track T_c with two
techniques, via in-plane resistivity measurements and with a resonant tunnel
diode oscillator circuit which is sensitive to the skin depth. We show that
initially T_c rises steeply with pressure, forming a superconducting dome with
a maximum T_c of ~44 K at 20 kbar. We discuss this observation in the context
of other electron-doped iron pnictide superconductors, and conclude that the
application of pressure offers an independent way to tune T_c in this system.
|
1107.0689v1
|
2011-07-05
|
Massless Leggett Mode in Three-band Superconductors with Time-Reversal-Symmetry Breaking
|
The Leggett mode associated with out-of-phase oscillations of superconducting
phase in multi-band superconductors usually is heavy due to the interband
coupling, which makes its excitation and detection difficult. We report on the
existence of a massless Leggett mode in three-band superconductors with
time-reversal-symmetry-breaking (TRSB). The mass of the Leggett mode is small
close to the TRSB transition and vanishes at the transition point, and thus
locates within the smallest superconducting energy gap, which makes it stable
and detectable. The mass of the Leggett mode can be measured by Raman
spectroscopy. The thermodynamic consequences of this massless mode and possible
realization in iron-based superconductors are also discussed.
|
1107.0814v2
|
2011-07-05
|
Evidence for filamentary superconductivity nucleated at antiphase domain walls in antiferromagnetic CaFe$_2$As$_2$
|
Resistivity, magnetization and microscopic $^{75}$As nuclear magnetic
resonance (NMR) measurements in the antiferromagnetically ordered state of the
iron-based superconductor parent material CaFe$_2$As$_2$ exhibit anomalous
features that are consistent with the collective freezing of domain walls.
Below $T^*\approx 10$ K, the resistivity exhibits a peak and downturn, the bulk
magnetization exhibits a sharp increase, and $^{75}$As NMR measurements reveal
the presence of slow fluctuations of the hyperfine field. These features in
both the charge and spin response are strongly field dependent, are fully
suppressed by $H^*\approx 15$ T, and suggest the presence of filamentary
superconductivity nucleated at the antiphase domain walls in this material.
|
1107.0904v2
|
2011-07-07
|
56Ni, Explosive Nucleosynthesis, and SNe Ia Diversity
|
The origin of the iron-group elements titanium to zinc in nature is
understood to occur under explosive burning conditions in both Type Ia
(thermonuclear) and Type II (core collapse) supernovae. In these dynamic
environments, the most abundant product is found to be 56Ni ({\tau} = 8.5 days)
that decays through 56Co ({\tau} = 111.5 days) to 56Fe. For the case of SNe Ia,
the peak luminosities are proportional to the mass ejected in the form of 56Ni.
It follows that the diversity of SNe Ia reflected in the range of peak
luminosity provides a direct measure of the mass of 56Ni ejected. In this
contribution, we identify and briefly discuss the factors that can influence
the 56Ni mass and use both observations and theory to quantify their impact. We
address specifically the variations in different stellar populations and
possible distinctions with respect to SNe Ia progenitors.
|
1107.1278v1
|
2011-07-07
|
Synthesis, crystal structure and magnetism of beta-Fe1.00(2)Se1.00(3) single crystals
|
Understanding iron based superconductors requires high quality impurity free
single crystals. So far they have been elusive for beta-FeSe and extraction of
intrinsic materials properties has been compromised by several magnetic
impurity phases. Herein we report synchrotron - clean beta-FeSe superconducting
single crystals grown via LiCl/CsCl flux method. Phase purity yields evidence
for a defect induced weak ferromagnetism that coexists with superconductivity
below Tc. In contrast to Fe1+yTe - based superconductors, our results reveal
that the interstitial Fe(2) site is not occupied and that all contribution to
density of states at the Fermi level must come from in-plane Fe(1).
|
1107.1496v1
|
2011-07-09
|
Pairing mechanism in multiband superconductors
|
We investigate pairing mechanism in multiband superconductors. To put our
feet on firm ground, unbiased renormalization group analysis is carried out for
iron-based superconductors. It is quite remarkable that, after integrating out
quantum fluctuations, the renormalization-group flows agree exceedingly well
with a mean-field Hamiltonian where interband pair hopping plays an essential
role. Through interband pair hopping, electrons can overcome the repulsive
interaction between them and form resonating Cooper pairs between different
bands. Unlike the conventional superconductors, the pairing mechanism in
multiband superconductors is resonating pair hopping between different bands,
just like the resonating chemical bonds in benzene. The effective mean-field
Hamiltonian spots a small parameter dictating the critical temperature and also
explains how interband pair hopping always enahnces spin fluctuations at the
nesting momentum connecting the Fermi surfaces. In short, no attractive glue is
needed and resonating interband pair hopping is the key to Cooper pair
formation in unconventional superconductors. Implications to cuprates and
related issues are also discussed at the end.
|
1107.1796v2
|
2011-07-11
|
Fermi-surface reconstruction involving two Van Hove singularities across the antiferromagnetic transition in BaFe2As2
|
We report an angle-resolved photoemission study of BaFe2As2, a parent
compound of iron-based superconductors. Low-energy tunable excitation photons
have allowed the first observation of a saddle-point singularity at the Z
point, as well as the Gamma point. With antiferromagnetic ordering, both of
these two van Hove singularities come down below the Fermi energy, leading to a
topological change in the innermost Fermi surface around the kz axis from
cylindrical to tear-shaped, as expected from first-principles calculation.
These singularities may provide an additional instability for the Fermi surface
of the superconductors derived from BaFe2As2.
|
1107.1960v2
|
2011-07-11
|
The need for a local source of UHE CR nuclei
|
Recent results of the Pierre Auger (Auger) fluorescence detectors indicate an
increasingly heavy composition of ultra-high energy (UHE) cosmic rays (CRs).
Assuming that this trend continues up to the highest energies observed by the
Auger surface detectors we derive the constraints this places on the local
source distribution of UHE CR nuclei. Utilizing an analytic description of UHE
CR propagation we derive the expected spectra and composition for a wide range
of source emission spectra. We find that sources of intermediate-to-heavy
nuclei are consistent with the observed spectra and composition data above the
ankle. This consistency requires the presence of nearby sources within 60 Mpc
and 80 Mpc for silicon and iron only sources, respectively. The necessity of
these local sources becomes even more compelling in the presence nano-Gauss
local extragalactic magnetic fields.
|
1107.2055v2
|
2011-07-11
|
New Indivisible Geoscience Paradigm
|
Earth's interior, I posit, is like one of the rare, oxygen-starved "enstatite
chondrite" meteorites (and unlike a more-oxidized "ordinary chondrite" as has
been believed for seventy years). Laboratory-analyzed enstatite-chondrite
samples are comparable to having-in-hand impossibleto- gather deep-Earth
samples. Enstatite-chondrite formation in oxygen-starved conditions caused
oxygen-loving elements to occur, in part, as non-oxides in their iron-alloy.
Observations, consistent with solar abundance and behavior of chemical
elements, lead me to a new interpretation of: (1) Earth's early formation as a
Jupiter-like gas-giant, (2) its decompressionpowered surface geology, (3)
Earth's internal composition, and (4) a natural, planetocentric nuclear-fission
reactor as source of both the geomagnetic field and energy channeled to surface
"hot-spots". I present a unified vision of Earth formation and concomitant
dynamics that explains in a logical and causally related way: (1) fluid
Earth-core formation without wholeplanet melting, and (2) the myriad
measurements and observations, previously attributed to "plate tectonics", but
without necessitating mantle convection.
|
1107.2149v1
|
2011-07-13
|
The MOLDY short-range molecular dynamics package
|
We describe a parallelised version of the MOLDY molecular dynamics program.
This Fortran code is aimed at systems which may be described by short-range
potentials and specifically those which may be addressed with the embedded atom
method. This includes a wide range of transition metals and alloys. MOLDY
provides a range of options in terms of the molecular dynamics ensemble used
and the boundary conditions which may be applied. A number of standard
potentials are provided, and the modular structure of the code allows new
potentials to be added easily. The code is parallelised using OpenMP and can
therefore be run on shared memory systems, including modern multicore
processors. Particular attention is paid to the updates required in the main
force loop, where synchronisation is often required in OpenMP implementations
of molecular dynamics. We examine the performance of the parallel code in
detail and give some examples of applications to realistic problems, including
the dynamic compression of copper and carbon migration in an iron-carbon alloy.
|
1107.2619v1
|
2011-07-14
|
Successive transition from superconducting to antiferromagnetic phase in (Ca_6(Al, Ti)_4O_y)Fe_2As_2 studied via ^{75}As and ^{27}Al NMR
|
An unusual successive phase transition from superconducting (SC) to
antiferromagnetic (AF) phases was discovered via ^{75}As and ^{27}Al nuclear
magnetic resonance (NMR) in (Fe_2As_2)(Ca_6(Al, Ti)_4O_y) with four (Al, Ti)O
layers intercalated between FeAs planes. Although the spatially-uniform AF
ordering is clearly visible from ^{27}Al spectra, the ordered moments are very
small and the low-frequency fluctuation is much suppressed, contrary to
existing pnictides with localized magnetic elements. Furthermore, the
temperature (T) dependence of the fluctuation at both nuclei is very similar
throughout the entire temperature range. These facts suggest that some
hybridization between Ti and Fe orbitals induces a uniform electronic state
within FeAs and (Al, Ti)O layers accompanied by the SC and AF transitions. The
iron-based pnictide with Ti-doped blocking layers is the first high-T_c
compound having metallic blocking layers.
|
1107.2765v2
|
2011-07-15
|
Satellites and large doping- and temperature-dependence of electronic properties in hole-doped BaFe2As2
|
Over the last years, superconductivity has been discovered in several
families of iron-based compounds. Despite intense research, even basic
electronic properties of these materials, such as Fermi surfaces, effective
electron masses, or orbital characters are still subject to debate. Here, we
address an issue that has not been considered before, namely the consequences
of dynamical screening of the Coulomb interactions among Fe-d electrons. We
demonstrate its importance not only for correlation satellites seen in
photoemission spectroscopy, but also for the low-energy electronic structure.
From our analysis of the normal phase of BaFe2As2 emerges the picture of a
strongly correlated compound with strongly doping- and temperature-dependent
properties. In the hole overdoped regime, an incoherent metal is found, while
Fermi-liquid behavior is recovered in the undoped compound. At optimal doping,
the self-energy exhibits an unusual square-root energy dependence which leads
to strong band renormalizations near the Fermi level.
|
1107.3128v1
|
2011-07-21
|
A de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs
|
We report a de Haas-van Alphen (dHvA) oscillation study of the 111 iron
pnictide superconductors LiFeAs with T_c ~18K and LiFeP with T_c~5K. We find
that for both compounds the Fermi surface topology is in good agreement with
density functional band-structure calculations and shows quasi-nested electron
and hole bands. The effective masses generally show significant enhancement, up
to ~3 for LiFeP and ~5 for LiFeAs. However, one hole Fermi surface in LiFeP
shows a very small enhancement, as compared with its other sheets. This
difference probably results from k-dependent coupling to spin fluctuations and
may be the origin of the different nodal and nodeless superconducting gap
structures in LiFeP and LiFeAs respectively.
|
1107.4375v2
|
2011-07-22
|
The Chemical and Dynamical Evolution of Isolated Dwarf Galaxies
|
Using a suite of simulations (Governato et al. 2010) which successfully
produce bulgeless (dwarf) disk galaxies, we provide an analysis of their
associated cold interstellar media (ISM) and stellar chemical abundance
patterns. A preliminary comparison with observations is undertaken, in order to
assess whether the properties of the cold gas and chemistry of the stellar
components are recovered successfully. To this end, we have extracted the
radial and vertical gas density profiles, neutral hydrogen velocity dispersion,
and the power spectrum of structure within the ISM. We complement this analysis
of the cold gas with a brief examination of the simulations' metallicity
distribution functions and the distribution of alpha-elements-to-iron.
|
1107.4538v1
|
2011-07-25
|
Uncertainties in modeling low-energy neutrino induced reactions on iron group nuclei
|
Charged-current neutrino-nucleus cross sections for 54,56Fe and 58,60Ni are
calculated and compared using frameworks based on relativistic and Skyrme
energy density functionals, and the shell model. The current theoretical
uncertainties in modeling neutrino-nucleus cross sections are assessed in
relation to the predicted Gamow-Teller transition strength and available data,
multipole decomposition of the cross sections, and cross sections averaged over
the Michel flux and Fermi-Dirac distribution. Employing different microscopic
approaches and models, the DAR neutrino-56Fe cross section and its theoretical
uncertainty are estimated: <sigma>_th=(258+-57) 10^{-42} cm^2, in very good
agreement with the experimental value: <sigma>_exp=(256+-108+-43) 10^{-42}
cm^2.
|
1107.4872v2
|
2011-08-01
|
Propagation of Ultrahigh Energy Nuclei in the Magnetic Field of our Galaxy
|
In this work, we present detailed simulations for propagation of ultra-high
energy (UHE) heavy nuclei, with E > 60 EeV, within recent Galactic Magnetic
Field (GMF) models. We investigate the impacts of the regular and turbulent
components of the GMF. We show that with UHE heavy nuclei, there is no
one-to-one correspondence between the arrival directions of cosmic rays (CR)
measured at Earth and the direction of their extragalactic sources. Sources can
have several distorted images on the sky. We compute images of galaxy clusters
and of the supergalactic plane in recent GMF models and show the challenges,
and possibilities, of "UHECR astronomy" with heavy nuclei. Finally, we present
a quantitative study of the impact of the GMF on the (de-)magnification of
source fluxes, due to magnetic lensing effects. We find that for 60 EeV iron
nuclei, sources located in up to about one fifth of the sky would have their
fluxes so strongly demagnified that they would not be detectable at Earth, even
by the next generation of UHECR experiments.
|
1108.0362v2
|
2011-08-01
|
Critical fields, thermally-activated transport and critical current density of beta-FeSe single crystals
|
We present critical fields, thermally-activated flux flow (TAFF) and critical
current density of tetragonal phase beta-FeSe single crystals. The upper
critical fields Hc2(T) for H||(101) and H\bot(101) are nearly isotropic and are
likely governed by Pauli limiting process. The obtained large Ginzburg-Landau
parameter k \sim 72.3(2) indicates that beta-FeSe is a type-II superconductor
with smaller penetration depth than in Fe(Te,Se). The resistivity below Tc
follows Arrhenius TAFF behavior. For both field directions below 30 kOe single
vortex pinning is dominant whereas collective creep becomes important above 30
kOe. The critical current density Jc from M-H loops for H||(101) is about five
times larger than for H\bot(101), yet much smaller than in other iron-based
superconductors.
|
1108.0407v1
|
2011-08-02
|
Spin fluctuations and unconventional pairing in KFe$_2$As$_2$
|
We study the relation between the spin fluctuation and superconductivity in
an heavily hole doped end material KFe$_2$As$_2$. We construct a five orbital
model by approximately unfolding the Brillouin zone of the three dimensional
ten orbital model obtained from first principles calculation. By applying the
random phase approximation, we obtain the spin susceptibility and solve the
linearized Eliashberg equation. The incommensurate spin fluctuation observed
experimentally is understood as originating from interband interactions, where
the multiorbital nature of the band structure results in an electron-hole
asymmetry of the incommensurability in the whole iron-based superconductor
family. As for superconductivity, s-wave and d-wave pairings are found to be in
close competition, where the sign change in the gap function in the former is
driven by the incommensurate spin fluctuations. We raise several possible
explanations for the nodes in the superconducting gap of KFe$_2$As$_2$ observed
experimentally.
|
1108.0657v2
|
2011-08-02
|
What controls the phase diagram and superconductivity in Ru substituted BaFe$_2$As$_2$?
|
We use high resolution angle-resolved photoemission to study the electronic
structure of the iron based high-temperature superconductors
Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ as a function of Ru concentration. We find that
substitution of Ru for Fe is isoelectronic, i. e., it does not change the value
of the chemical potential. More interestingly, there are no measured,
significant changes in the shape of the Fermi surface or in the Fermi velocity
over a wide range of substitution levels ($0<x<0.55$). Given that the
suppression of the antiferromagnetic and structural phase has been associated
with the emergence of the superconducting state, Ru substitution must achieve
this via a mechanism that does not involve changes of the Fermi surface. We
speculate that this mechanism relies on magnetic dilution which leads to the
reduction of the effective Stoner enhancement.
|
1108.0711v2
|
2011-08-03
|
Planar hybrid superconductor-normal metal-superconductor thin film junctions based on BaFe1.8Co0.2As2
|
To investigate the transport properties of iron based superconductors, we
prepared planar hybrid superconductor-normal metal-superconductor (SNS') thin
film junctions with BaFe1.8Co0.2As2 as base electrode. As counter electrode we
used a lead indium alloy, while the normal metal layer was formed by thin gold
films. Temperature dependent measurements of the electrical conductivity were
strongly influenced by the properties of the electrodes. We developed a
junction structure that allows us to characterize the electrodes, too,
including the behavior of their normal state resistance in order to correct
their influences on the conduction spectra. The corrected conductivity of the
junction was described within an extended BTK-model and shows a behavior
dominated by Andreev reflexion.
|
1108.0851v2
|
2011-08-04
|
$β$-decay of key titanium isotopes in stellar environment
|
Amongst iron regime nuclei, $\beta$-decay rates on titanium isotopes are
considered to be important during the late phases of evolution of massive
stars. The key $\beta$-decay isotopes during presupernova evolution were
searched from available literature and a microscopic calculation of the decay
rates were performed using the proton-neutron quasiparticle random phase
approximation (pn-QRPA) theory. As per earlier simulation results electron
capture and $\beta$-decay on certain isotopes of titanium are considered to be
important for the presupernova evolution of massive stars. Earlier the stellar
electron capture rates and neutrino energy loss rates due to relevant titanium
isotopes were presented. In this paper we finally present the $\beta$-decay
rates of key titanium isotopes in stellar environment. The results are also
compared against previous calculations. The pn-QRPA $\beta$-decay rates are
bigger at high stellar temperatures and smaller at high stellar densities
compared to the large scale shell model results. This study can prove useful
for the core-collapse simulators.
|
1108.1026v1
|
2011-08-08
|
Conceptual design of 20 T dipoles for high-energy LHC
|
Availability of 20 T operational field dipole magnets would open the way for
a 16.5 TeV beam energy accelerator in the LHC tunnel. Here we discuss the main
issues related to the magnet design of this extremely challenging dipole: main
constraints, superconductor choice, coil lay-out, iron, forces and stresses,
and field quality. A tentative cost estimate is also given. The present
technology, based on Nb-Ti and now near to be extended to Nb3Sn superconductor,
would allow reaching 15 T operational field. To reach 20 T, HTS conductors
capable to carry 400 A/mm2 at 15-20 T under transverse stress of 150-200 MPa
are an essential element.
|
1108.1619v1
|
2011-08-08
|
Synthesis, crystal and magnetic structure of iron selenide BaFe2Se3 with possible superconductivity at Tc=11K
|
We report on synthesis of single crystals of BaFe2Se3 and study of their
crystal and magnetic structures by means of synchrotron single crystal X-ray
and neutron powder diffraction. The crystal structure has orthorhombic symmetry
and consists of double chains of FeSe4 edge connected tetrahedra intercalated
by barium. Below 240 K long range block-spin checkerboard antiferromagnetic
(AFM) order is developed. The magnetic structure is similar to one observed in
A0.8Fe1.6Se2 (A=K, Rb or Cs) superconductors. The crystals exhibit a transition
to the diamagnetic state with an onset transition temperature of Tc ~11 K.
Though we observe FeSe as an impurity phase (<0.8% mass fraction) the
diamagnetism unlikely can be attributed to the FeSe-superconductor which has
Tc\approx8.5K.
|
1108.1670v3
|
2011-08-11
|
Chemical abundances and kinematics of a sample of metal-rich barium stars
|
We determined the atmospheric parameters and abundance pattern for a sample
of metal-rich barium stars. We used high-resolution optical spectroscopy.
Atmospheric parameters and abundances were determined using the local
thermodynamic equilibrium atmosphere models of Kurucz and the spectral analysis
code MOOG. We show that the stars have enhancement factors, [s/Fe], from 0.25
to 1.16. Their abundance pattern of the Na, Al, alpha-elements, and iron group
elements as well as their kinematical properties are similar to the
characteristics of the other metal-rich and super metal-rich stars already
analyzed. We conclude that metal-rich barium stars do not belong to the bulge
population. We also show that metal-rich barium stars are useful targets for
probing the s-process enrichment in high-metallicity environments.
|
1108.2500v1
|
2011-08-12
|
Nuclear medium effects in $ν/\barν$-A DIS
|
Nuclear medium effects in the weak structure functions $F_2(x,Q^2)$ and
$F_3(x,Q^2)$ have been studied for deep inelastic neutrino/antineutrino
reactions in iron nucleus by taking into account Fermi motion, binding, pion
and rho meson cloud contributions, target mass correction, shadowing and
anti-shadowing corrections. The calculations have been performed in a local
density approximation using relativistic nuclear spectral functions which
include nucleon correlations. Using these structure functions we have obtained
the ratio $R_{F2,F3}^A(x,Q^2)= \frac{2F_{2,3}^A(x,Q^2)}{AF_{2,3}^D(x,Q^2)}$,
the differential scattering cross section $\frac{1}{E}\frac{d^2\sigma}{dxdy}$
and the total scattering cross section $\sigma$. The results of our numerical
calculations in $^{56}Fe$ are compared with the experimental results of NuTeV
and CDHSW collaborations.
|
1108.2564v1
|
2011-08-17
|
A minimum single-band model for low-energy excitations in superconducting K$_x$Fe$_2$Se$_2$
|
We propose a minimum single-band model for the newly discovered iron-based
superconducting K$_x$Fe$_2$Se$_2$. Our model is found to be numerically
consistent with the five-orbital model at low energies. Based on our model and
the random phase approximation, we study the spin fluctuation and the pairing
symmetry of superconducting gap function. The $(\pi/2,\pi/2)$ spin excitation
and the $d_{x^2-y^2}$ pairing symmetry are revealed. All of the results can
well be understood in terms of the interplay between the Fermi surface topology
and the local spin interaction, providing a sound picture to explain why the
superconducting transition temperature is as high as to be comparable to those
in pnictides and some cuprates. A common origin of superconductivity is
elucidated for this compound and other high-T$_c$ materials.
|
1108.3491v1
|
2011-08-17
|
Proximity fingerprint of s+- superconductivity
|
We suggest a straightforward and unambiguous test to identify possible
opposite signs of superconducting order parameter in different bands proposed
for iron-based superconductors (s+- state). We consider proximity effect in a
weakly coupled sandwich composed of a s+- superconductor and thin layer of
s-wave superconductor. In such system the s-wave order parameter is coupled
differently with different s+- gaps and it typically aligns with one of these
gaps. This forces the other s+- gap to be anti-aligned with the s-wave gap. In
such situation the aligned band induces a peak in the s-wave density of states
(DoS), while the anti-aligned band induces a dip. Observation of such
contact-induced negative feature in the s-wave DoS would provide a definite
proof for s+- superconductivity.
|
1108.3515v3
|
2011-08-22
|
Block Spin Magnetic Phase Transition of A$_y$Fe$_{1.6}$Se$_2$ Under High Pressure
|
We predict an unconventional magnetic ground state in A$_y$Fe$_{1.6}$Se$_2$
with $\sqrt{5}\times\sqrt{5}$ Fe-vacancy superstructure under hydraulic
external pressure based on first-principles simulations. While the Fe-vacancy
ordering persists up to at least $\sim $ 12GPa, the magnetic ground state goes
at $\sim$10GPa from the BS-AFM phase to a N{\'e}el-FM phase, a ferromagnetic
arrangement of a "{\it{N{\'e}el cluster}}". The new magnetic phase is metallic,
and the BS-AFM to N{\'e}el-FM phase transition is accompanied by a sizable
structural change. The two distinct magnetic phases can be understood within
the extended $J_1$-$J_2$ Heisenberg model by assuming a pressure-tuned
competition between the intrablock and interblock nearest-neighbor couplings of
iron moments.
|
1108.4322v2
|
2011-08-23
|
Unconventional multiband superconductivity with nodes in single-crystalline SrFe2(As_0.65P_0.35)2 as seen via 31P-NMR and specific heat
|
We report 31P-NMR and specific heat measurements on an iron (Fe)-based
superconductor SrFe2(As0.65P0.35)2 with Tc=26 K, which have revealed the
development of antiferromagnetic correlations in the normal state and the
unconventional superconductivity(SC) with nodal gap dominated by the gapless
low-lying quasiparticle excitations. The results are consistently argued with
an unconventional multiband SC state with the gap-size ratio of different bands
being significantly large; the large full gaps in s\pm-wave state keep Tc high,
whereas a small gap with a nodal-structure causes gapless feature under
magnetic field. The present results will develop an insight into the strong
material dependence of SC-gap structure in Fe-based superconductors.
|
1108.4480v2
|
2011-08-24
|
In vitro toxicity and uptake of magnetic nanorods
|
In this paper we investigate the internalization and cytotoxicity of
nanostructured materials having the form of elongated rods, with diameter of
200 nm and lengths 1 - 10 {\mu}m. The rods were made from the controlled
aggregation of sub-10 nm iron oxide nanoparticles. Recently, we have shown that
the nanorods inherited the superparamagnetic property of the particles. These
rods can actually be moved by the application of an external magnetic field.
Here we evaluate the in vitro toxicity of the magnetic nanorods by using MTT
assays on NIH/3T3 mouse fibroblasts. The toxicity assays revealed that the
nanorods are biocompatible, as exposed cells remained 100% viable relative to
controls over a period of a few days. Optical microscopy allow to visualize the
rods inside the cells and to determine their number per cell. Roughly 1/3 of
the total incubated rods were uptaken by the fibroblasts.
|
1108.4773v1
|
2011-08-25
|
XHIP: An Extended Hipparcos Compilation
|
We present the Extended Hipparcos Compilation (XHIP), a database of all stars
in the New Reduction of the Hipparcos Catalog extensively cross-referenced with
data from a broad survey of presently available sources. The resulting
collection uniquely assigns 116,096 spectral classifications, 46,392 radial
velocities, and 18,549 homogenized iron abundances [Fe/H] to Hipparcos stars.
Stellar classifications from SIMBAD, indications of multiplicity from CCDM or
WDS, stellar ages from the Geneva-Copenhagen Survey III, supplemental
photometry from 2MASS and SIMBAD, and identifications of exoplanet host stars
are also included. Parameters for solar encounters and Galactic orbits are
calculated for a kinematically complete subset. Kinetic bias is found to be
minimal. Our compilation is available through the Centre de Donn\'ees
astronomiques de Strasbourg (CDS) as Catalog V/137B.
|
1108.4971v2
|
2011-08-30
|
X-ray evidence for the accretion disc-outflow connection in 3C 111
|
We present the spectral analysis of three Suzaku XIS observations of 3C 111
requested to monitor the predicted variability of its ultra-fast outflow on ~7
days time-scales. We detect an ionized iron emission line in the first
observation and a blue-shifted absorption line in the second, when the flux is
~30% higher. The location of the material is constrained at <0.006pc from the
variability. Detailed modelling support an identification with ionized
reflection off the accretion disc at ~20-100r_g from the black hole and a
highly ionized and massive ultra-fast outflow with velocity ~0.1c,
respectively. The outflow is most probably accelerated by radiation pressure,
but additional magnetic thrust can not be excluded. The measured high outflow
rate and mechanical energy support the claims that disc outflows may have a
significant feedback role. This work provides the first direct evidence for an
accretion disc-outflow connection in a radio-loud AGN, possibly linked also to
the jet activity.
|
1108.6095v1
|
2011-09-07
|
Probing the Order Parameter of Superconducting LiFeAs using Pb/LiFeAs and Au/LiFeAs Point-Contact Spectroscopy
|
We have fabricated c-axis point contact junctions between high-quality LiFeAs
single crystals and Pb or Au tips in order to study the nature of the
superconducting order parameter of LiFeAs, one of the few stoichiometric
iron-based superconductors. The observation of the Josephson current in c-axis
junctions with a conventional s-wave superconductor as the counterelectrode
indicates that the pairing symmetry in LiFeAs is not pure d-wave or pure
spin-triplet p-wave. A superconducting gap is clearly observed in point contact
Andreev reflection measurements performed on both Pb/LiFeAs and Au/LiFeAs
junctions. The conductance spectra can be well described by the
Blonder-Tinkham-Klapwijk model with a lifetime broadening term, resulting in a
gap value of \approx 1.6 meV (2{\Delta}/kBTC \approx 2.2).
|
1109.1537v2
|
2011-09-09
|
Microscopically derived Ginzburg-Landau theory for magnetic order in the iron pnictides
|
We examine the competition of the observed stripe spin density wave (SDW)
with other commensurate and incommensurate SDW phases in a two-band model of
the pnictides. Starting from this microscopic model, we rigorously derive an
expansion of the free energy in terms of the different order parameters at the
mean-field level. We show that three distinct commensurate SDW states are
possible and study their appearance as a function of the doping and the
electronic structure. We show that the stripe phase is generally present, but
its extent in the phase diagram depends strongly upon the number of hole Fermi
pockets that are nested with the electron Fermi pockets. Electron pockets
competing for the same portion of a hole pocket play a crucial role. We discuss
the relevance of our results for the antiferromagnetism of the pnictides.
|
1109.2071v2
|
2011-09-09
|
Competing superconducting, magnetic and charge orderings in the AF Heisenberg-Kondo lattice with Dirac electrons
|
Many recently discovered advanced materials, such as high-Tc cuprates, iron
pnictides and several heavy-fermions, exhibit a rich phase diagram suggesting
the presence of different competing interactions that would lead to various
types of ordering. Nevertheless, there is not yet a clear unifying picture
allowing the understanding of the detailed mechanisms that generate such
competing interactions. Having such a picture, however, could quite well be at
the very roots of the requirements for understanding high-Tc superconductivity
in cuprates and pnictides, for instance. In this work we consider the
antiferromagnetic (AF) Heisenberg-Kondo lattice, consisting of localized spins
with AF exchange interactions between nearest neighbors on a square lattice and
itinerant electrons, which undergo a magnetic Kondo interaction with the
localized spins, but are otherwise non-interacting. Using the Schwinger-boson
(CP$^1$) formalism and assuming the electrons are Dirac-like, we integrate on
the localized degrees of freedom thereby obtaining the effective interaction
among the itinerant electrons. This contains a BCS-like superconducting term, a
Nambu-Jona-Lasinio-like, charge gap term and a Ising and Heisenberg-like
magnetic terms. All these four competing interactions, therefore are generated
by the original Kondo magnetic interaction.
|
1109.2151v1
|
2011-09-12
|
Central stars of planetary nebulae: The white dwarf connection
|
This paper is focused on the transition phase between central stars and white
dwarfs, i.e. objects in the effective temperature range 100,000 - 200,000 K. We
confine our review to hydrogen-deficient stars because the common H-rich
objects are subject of the paper by Ziegler et al. in these proceedings. We
address the claimed iron-deficiency in PG1159 stars and [WC] central stars. The
discovery of new Ne VII and Ne VIII lines in PG1159 stars suggests that the
identification of O VII and O VIII lines that are used for spectral
classification of [WCE] stars is wrong. We then present evidence for two
distinct post-AGB evolutionary sequences for H-deficient stars based on
abundance analyses of the He-dominated O(He) stars and the hot DO white dwarf
KPD0005+5106. Finally, we report on evidence for an H-deficient post-super AGB
evolution sequence represented by the hottest known, carbon/oxygen-atmosphere
white dwarf H1504+65 and the recently discovered carbon-atmosphere "hot DQ"
white dwarfs.
|
1109.2391v1
|
2011-09-12
|
Spatial and Temporal Variations of the Diffuse Iron 6.4 keV Line in the Galactic Center Region
|
We analyze the diffuse Fe I K-alpha line generated in the diffuse
interstellar molecular hydrogen by primary photons or subrelativistic protons
injected by Sagittarius (Sgr) A*. We showed that unlike emission from compact
molecular clouds, this emission can be permanently observed in the directions
of the Galactic center. We conclude that the diffuse emission of 6.4 keV line
observed at present is probably due to Fe I K-alpha vacancy production by
primary photons if the X-ray luminosity of Sgr A* was about Lx ~ 10^39-10^40
erg/s. In principle these data can also be described in the framework of the
model when the 6.4 keV line emission is generated by subrelativistic protons
generated by accretion onto the central black hole but in this case extreme
parameters of this model are necessary.
|
1109.2614v1
|
2011-09-14
|
Long- to short-range magnetic order in fluorine-doped CeFeAsO
|
The evolution of the antiferromagnetic order parameter in CeFeAsO_{1-x}F_{x}
as a function of the fluorine content x was investigated primarily via
zero-field muon-spin spectroscopy. The long-range magnetic order observed in
the undoped compound gradually turns into a short-range order at x=0.04,
seemingly accompanied/induced by a drastic reduction of the magnetic moment of
the iron ions. Superconductivity appears upon a further increase in doping
(x>0.04) when, unlike in the cuprates, the Fe magnetic moments become even
weaker. The resulting phase diagram evidences the presence of a crossover
region, where the superconducting and the magnetic order parameters coexist on
a nanoscopic range.
|
1109.3170v1
|
2011-09-19
|
Mechanism of Cooper-pairing in layered high temperature superconductors
|
In this study, the pairing mechanism for layered HTS materials based on
attraction between electrons from adjacent layers is proposed. Initially, each
layer has expanded Fermi sphere owing to ridged geometry. When the two layers
are close enough for tunneling, it becomes energetically advantageous to form
correlated quantum states (CQS), reducing the Fermi sphere volume. Cooper
pairs, comprising inter-tunneling electrons, occupy the CQS. The image force is
responsible for the electron-electron attraction. Pair-binding energy and the
corresponding effective mass vary in a wide range. At T>0, some heavy pairs do
not condense. Such pairs are responsible for pseudogap. Light pairs get Bose
condensed and are responsible for superconductivity. The proposed mechanism
provides clarification of superconductivity in cuprates, iron based
superconductors and LSCO/LCO interfaces. It provides explanation of two energy
gaps and two characteristic temperatures in layered superconducting materials.
It also provides clarification on the Fermi surface pockets, anisotropy of
charge transport in pseudogap state, and other properties of HTS materials. The
pseudogap, estimated within the model, fits the experimental values for the
two-layer cuprates, such as YBCO, Bi2212, Tl2212, and Hg1212.
|
1109.3978v1
|
2011-09-19
|
Magnetic nanocomposites: new methodology for micromagnetic modeling and SANS experiments
|
A new methodology for micromagnetic simulations of magnetic nanocomposites is
presented. The methodology is especially suitable for simulations of two-phase
composites consisting of magnetically hard inclusions in a soft magnetic matrix
phase. The proposed technique allows to avoid the unnecessary discretization of
the 'hard' inclusions (these are normally in a single-domain state), but
enables an arbitrary fine discretization of the 'soft' phase. The method is
applied to the determination of the equilibrium magnetization state of an
iron-based nanocomposite from the Nanoperm (FeZrBCu) family of alloys and to
the calculation of the corresponding small-angle neutron scattering (SANS)
cross-section. For this highly interesting material, the results of our
simulations exhibit a remarkable agreement with the nontrivial 'clover-leaf'
SANS cross-sections observed experimentally (A. Michels et al., Phys. Rev. B,
v. 74, 134407 (2006)).
|
1109.4075v1
|
2011-09-20
|
Fundamental Parameters and Chemical Composition of Arcturus
|
We derive a self-consistent set of atmospheric parameters and abundances of
17 elements for the red giant star Arcturus: Teff = 4286+/-30 K, logg =
1.66+/-0.05, and [Fe/H] = -0.52+/-0.04. The effective temperature was
determined using model atmosphere fits to the observed spectral energy
distribution from the blue to the mid-infrared (0.44 to 10 um). The surface
gravity was calculated using the trigonometric parallax of the star and stellar
evolution models. A differential abundance analysis relative to the solar
spectrum allowed us to derive iron abundances from equivalent width
measurements of 37 FeI and 9 FeII lines, unblended in the spectra of both
Arcturus and the Sun; the [Fe/H] value adopted is derived from FeI lines. We
also determine the mass, radius, and age of Arcturus: M = 1.08+/-0.06 Msun, R =
25.4+/-0.2 Rsun, and t = 7.1(+1.5/-1.2) Gyr. Finally, abundances of the
following elements are measured from an equivalent width analysis of atomic
features: C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, and Zn. We
find the chemical composition of Arcturus typical of that of a local thick-disk
star, consistent with its kinematics.
|
1109.4425v1
|
2011-09-21
|
RKKY interaction in SDW phase of iron-based superconductors
|
Using the multiband model we analyze the Ruderman-Kittel-Kasuya-Yosida (RKKY)
interaction between the magnetic impurities in layered ferropnictide
superconductors. In the normal state the interaction is spin isotropic and is
dominated by the nesting features of the electron and hole bands separated by
the antiferromagnetic momentum, ${\bf Q}_{AF}$. In the AF state the RKKY
interaction maps into an effective anisotropic XXZ-type Heisenberg exchange
model. The anisotropy originates from the breaking of the spin-rotational
symmetry induced by the AF order and its strength depends on the size of the AF
gap and the structure of the folded Fermi surface. We discuss our results in
connection to the recent experiments.
|
1109.4643v1
|
2011-09-23
|
Friedel-like Oscillations from Interstitial Iron in Superconducting Fe1+yTe0.62Se0.38
|
Using polarized and unpolarized neutron scattering we show that interstitial
Fe in superconducting Fe_{1+y}Te_{1-x}Se_x induces a magnetic Friedel-like
oscillation that diffracts at Q_(in-plane)=(1/2,0) and involves >50 neighboring
Fe sites. The interstitial >2 mu_B moment is surrounded by compensating
ferromagnetic four spin clusters that may seed double stripe ordering in
Fe_{1+y}Te. A semi-metallic 5-band model with (1/2,1/2) Fermi surface nesting
and four fold symmetric super-exchange between interstitial Fe and two in-plane
nearest neighbors largely accounts for the observed diffraction.
|
1109.5196v1
|
2011-10-06
|
Energy and Xmax reconstruction of hadron-initiated showers in surface arrays
|
The current methods to determine the primary energy in surface arrays are
different when dealing with hadron or photon initiated showers. In this work,
we adapt a method previously developed for photon-initiated showers to hadron
primaries. We determine the Monte Carlo parametrizations that relate the
surface energy estimator and the maximum of shower development for both, proton
and Iron primaries. Using for each primary their own set of calibration curves,
which is of course impossible in practice, we show that the energy could be
inferred with a negligible bias and 12% resolution. However, we show that a
mixed calibration could also be performed, including both type of primaries,
such that the bias still remains low and the achieved resolution is around 15%.
In addition, the method allows the simultaneous determination of Xmax in pure
surface arrays with resolution better than 7%.
|
1110.1179v1
|
2011-10-07
|
Direct observation of the influence of the FeAs4 tetrahedron on superconductivity and antiferromagnetic correlations in Sr2VO3FeAs
|
We measure the pressure dependence of the electrical resistivity and the
crystal structure of iron superconductor Sr2VO3FeAs. Below ~10 GPa the
structure compresses but remains undeformed, with regular FeAs4 tetrahedrons,
and a constant Tc. Beyond 10GPa, the tetrahedron strongly distorts, while Tc
goes gradually to zero. Band structure calculations of the undistorted
structure show multiple nesting features that hinder the development of an
antiferromagnetic ground state (AF), allowing the appearance of
superconductivity. The deformation of the tetrahedra that breaks band
degeneracy degrades multiple nesting, thus favouring one particular AF state at
the expense of Tc.
|
1110.1559v1
|
2011-10-11
|
Spin Glass and Semiconducting Behavior in 1D BaFe2-δSe3 Crystals
|
We investigate the physical properties and electronic structure of
BaFe2-{\delta}Se3 crystals, which were grown out of tellurium flux. The crystal
structure of the compound, an iron-deficient derivative of the ThCr2Si2-type,
is built upon edge-shared FeSe4 tetrahedra fused into double chains. The
semiconducting BaFe2-{\delta}Se3 with {\delta} \approx 0.2 ({\rho}295K = 0.18
{\Omega}\cdotcm and Eg = 0.30 eV) does not order magnetically, however there is
evidence for short-range magnetic correlations of spin glass type (Tf \approx
50 K) in magnetization, heat capacity and neutron diffraction results. A
one-third substitution of selenium with sulfur leads to a slightly higher
electrical conductivity ({\rho}295K = 0.11 {\Omega}\cdotcm and Eg = 0.22 eV)
and a lower spin glass freezing temperature (Tf \approx 15 K), corroborating
with higher electrical conductivity reported for BaFe2S3. According to the
electronic structure calculations, BaFe2Se3 can be considered as a
one-dimensional ladder structure with a weak interchain coupling.
|
1110.2439v1
|
2011-10-17
|
MINERvA Status and Event Reconstruction
|
MINERvA (Main INjector ExpeRiment v-A) is a few-GeV neutrino scattering
experiment that began taking data in the NuMI beam at Fermilab (FNAL) in the
Fall of 2009. MINERvA employs a fine-grained detector, with an eight ton active
target region composed of plastic scintillator. It also uses nuclear targets
composed of carbon, iron, and lead placed upstream of the active region to
measure v-A dependence. The experiment will provide important inputs for
neutrino oscillation experiments and a pure weak probe of nuclear structure. We
offer a set of initial kinematic distributions of interest and provide a
summary of current operations and event reconstruction status. Contribution to
NUFACT 11, XIIIth International Workshop on Neutrino Factories, Super beams and
Beta beams, 1-6 August 2011, CERN and University of Geneva .
|
1110.3727v1
|
2011-10-21
|
Magnetic properties of spin diluted iron pnictides from muSR and NMR in LaFe1-xRuxAsO
|
The effect of isoelectronic substitutions on the microscopic properties of
LaFe1-xRuxAsO, for 0< x< 0.8, has been investigated by means of muSR and 139La
NMR. It was found that Ru substitution causes a progressive reduction of the
N\`eel temperature (T_N) and of the magnetic order parameter without leading to
the onset of superconductivity. The temperature dependence of 139La nuclear
spin-lattice relaxation rate 1/T_1 can be suitably described within a two-band
model. One band giving rise to the spin density wave ground-state, while the
other one is characterized by weakly correlated electrons. Fe for Ru
substitution yields to a progressive decrease of the density of states at the
Fermi level close to the one derived from band structure calculations. The
reduction of T_N with doping follows the predictions of the J_1-J_2 model on a
square lattice, which appears to be an effective framework to describe the
magnetic properties of the spin density wave ground-state.
|
1110.4812v1
|
2011-10-25
|
Optical conductivity of superconducting K{0.8}Fe{2-y}Se2 single crystals: Evidence for a Josephson-coupled phase
|
The optical properties of the iron-chalcogenide superconductor
K{0.8}Fe{2-y}Se2, with a critical temperature Tc = 31 K, have been measured
over a wide frequency range in the a-b planes above and below Tc. The
conductivity is incoherent at room temperature, but becomes coherent
(Drude-like) at T ~ Tc; however, R_\Box ~ 64 k-Ohm, well above the threshold
for the superconductor-insulator transition at R_\Box = h/4e^2 ~ 6.9 k-Ohm.
Below Tc, the superfluid density \rho_{s0} ~ 48 x 10^3 cm^{-2} places this
material on the scaling line \rho_{s0}/8 ~ 4.4 \sigma_{dc} Tc, but in a region
associated with Josephson coupling, suggesting this material is inhomogeneous
and constitutes a Josephson phase.
|
1110.5529v2
|
2011-10-25
|
Ba{1-x}KxMn2As2: An Antiferromagnetic Local-Moment Metal
|
The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a
local-moment antiferromagnetic insulator with a Neel temperature TN = 625 K and
a large ordered moment mu = 3.9 mu_B/Mn. We demonstrate that this compound can
be driven metallic by partial substitution of Ba by K, while retaining the same
crystal and antiferromagnetic structures together with nearly the same high TN
and large mu. Ba_{1-x}K_xMn2As2 is thus the first metallic ThCr2Si2-type
MAs-based system containing local 3d transition metal M magnetic moments, with
consequences for the ongoing debate about the local moment versus itinerant
pictures of the FeAs-based superconductors and parent compounds. The
Ba_{1-x}K_xMn2As2 class of compounds also forms a bridge between the layered
iron pnictides and cuprates and may be useful to test theories of high Tc
superconductivity.
|
1110.5546v2
|
2011-10-26
|
Correlation-enhanced electron-phonon coupling: Applications of GW and screened hybrid functional to bismuthates, chloronitrides, and other high Tc superconductors
|
We show that the electron-phonon coupling (EPC) in many materials can be
significantly underestimated by the standard density functional theory (DFT) in
the local density approximation (LDA) due to large non-local correlation
effects. We present a simple yet efficient methodology to evaluate the
realistic EPC going beyond LDA by using more advanced and accurate GW and
screened hybrid functional DFT approaches. The corrections we propose explain
the extraordinarily high superconducting temperatures that are observed in two
distinct classes of compounds-the bismuthates and the transition metal
chloronitrides, thus solving a thirty-year-old puzzle. Our work calls for the
critically reevaluation of the EPC of certain phonon modes in many other
materials such as cuprates and iron-based superconductors. The proposed
methodology can be used to design new correlation-enhanced high temperature
superconductors and other functional materials involving electron-phonon
interaction.
|
1110.5751v2
|
2011-10-26
|
High transport critical current densities in textured Fe-sheathed Sr1-xKxFe2As2+Sn superconducting tapes
|
We report the realization of grain alignment in Sn-added Sr1-xKxFe2As2
superconducting tapes prepared by ex-situ powder-in-tube method. At 4.2 K, high
transport critical current densities Jc of 2.5x10^4 A/cm^2 (Ic = 180 A) in
self-field and 3.5x10^3 A/cm^2 (Ic = 25.5 A) in 10 T have been measured. These
values are the highest ever reported so far for Fe-based superconducting wires
and tapes. We believe the superior Jc in our tape samples are due to well
textured grains and strengthened intergrain coupling achieved by Sn addition.
Our results demonstrated an encouraging prospect for application of iron based
superconductors.
|
1110.5784v2
|
2011-11-03
|
Effect of tensile stress on the in-plane resistivity anisotropy in BaFe2As2
|
The effect of uniaxial tensile stress and the resultant strain on the
structural/magnetic transition in the parent compound of the iron arsenide
superconductor, BaFe$_2$As$_2$, is characterized by temperature-dependent
electrical resistivity, x-ray diffraction and quantitative polarized light
imaging. We show that strain induces a measurable uniaxial structural
distortion above the first-order magnetic transition and significantly smears
the structural transition. This response is different from that found in
another parent compound, SrFe$_2$As$_2$, where the coupled structural and
magnetic transitions are strongly first order. This difference in the
structural responses explains the in-plain resistivity anisotropy above the
transition in BaFe$_2$As$_2$. This conclusion is supported by the
Ginzburg-Landau - type phenomenological model for the effect of the uniaxial
strain on the resistivity anisotropy.
|
1111.0997v1
|
2011-11-03
|
Doping - dependent superconducting gap anisotropy in the two-dimensional 10-3-8 pnictide Ca$_{10}$(Pt$_3$As$_8$)[(Fe$_{1-x}$Pt$_{x}$)$_2$As$_2$]$_5$
|
The characteristic features of
Ca$_{10}$(Pt$_3$As$_8$)[(Fe$_{1-x}$Pt$_x$)$_2$As$_2$]$_5$ ("10-3-8")
superconductor are relatively high anisotropy and a clear separation of
superconductivity and structural/magnetic transitions, which allows studying
the superconducting gap without complications due to the coexisting order
parameters. The London penetration depth, measured in underdoped single
crystals of 10-3-8 ($x =$ 0.028, 0.041, 0.042, and 0.097), shows behavior
remarkably similar to other Fe-based superconductors, exhibiting robust
power-law, $\Delta \lambda(T) = A T^n$. The exponent $n$ decreases from 2.36
($x =$ 0.097, close to optimal doping) to 1.7 ($x =$ 0.028, a heavily
underdoped composition), suggesting that the superconducting gap becomes more
anisotropic at the dome edge. A similar trend is found in low-anisotropy
superconductors based on BaFe$_2$As$_2$ ("122"), implying that it is an
intrinsic property of superconductivity in iron pnictides, unrelated to the
coexistence of magnetic order and superconductivity or the anisotropy of the
normal state. Overall this doping dependence is consistent with $s_{\pm}$
pairing competing with intra-band repulsion.
|
1111.1003v1
|
2011-11-07
|
Effects of correlation in LiFeAs
|
We discuss the role of electronic correlations in the iron-based
superconductor LiFeAs by studying the effects on band structure, mass
enhancements, and Fermi surface in the framework of density functional theory
combined with dynamical mean field theory calculations. We conclude that LiFeAs
shows characteristics of a moderately correlated metal and that the strength of
correlations is mainly controlled by the value of the Hund's rule coupling J.
The hole pockets of the Fermi surface show a distinctive change in form and
size with implications for the nesting properties. Our calculations are in good
agreement with recent angle-resolved photoemission spectroscopy and de Haas-van
Alphen experiments.
|
1111.1620v2
|
2011-11-08
|
Superconducting proximity effect to the block antiferromagnetism in K$_{y}$Fe$_{2-x}$Se$_{2}$
|
Recent discovery of superconducting (SC) ternary iron selenides has block
antiferromagentic (AFM) long range order. Many experiments show possible
mesoscopic phase separation of the superconductivity and antiferromagnetism,
while the neutron experiment reveals a sizable suppression of magnetic moment
due to the superconductivity indicating a possible phase coexistence. Here we
propose that the observed suppression of the magnetic moment may be explained
due to the proximity effect within a phase separation scenario. We use a
two-orbital model to study the proximity effect on a layer of block AFM state
induced by neighboring SC layers via an interlayer tunneling mechanism. We
argue that the proximity effect in ternary Fe-selenides should be large because
of the large interlayer coupling and weak electron correlation. The result of
our mean field theory is compared with the neutron experiments
semi-quantitatively. The suppression of the magnetic moment due to the SC
proximity effect is found to be more pronounced in the d-wave superconductivity
and may be enhanced by the frustrated structure of the block AFM state.
|
1111.1860v1
|
2011-11-08
|
A Bayesian approach to magnetic moment determination using muSR
|
A significant challenge in zero-field muSR experiments arises from the
uncertainty in the muon site. It is possible to calculate the dipole field (and
hence precession frequency nu) at any particular site given the magnetic moment
mu and magnetic structure. One can also evaluate f(nu), the probability
distribution function of nu assuming that the muon site can be anywhere within
the unit cell with equal probability, excluding physically forbidden sites.
Since nu is obtained from experiment, what we would like to know is g(mu|nu),
the probability density function of mu given the observed nu. This can be
obtained from our calculated f(nu/mu) using Bayes' theorem. We describe an
approach to this problem which we have used to extract information about real
systems including a low-moment osmate compound, a family of molecular magnets,
and an iron-arsenide compound.
|
1111.1980v1
|
2011-11-09
|
Effect of uniaxial strain on the structural and magnetic phase transitions in BaFe$_2$As$_2$
|
We report neutron scattering experiments probing the influence of uniaxial
strain on both the magnetic and structural order parameters in the parent iron
pnictide compound, BaFe$_2$As$_2$. Our data show that modest strain fields
along the in-plane orthorhombic b-axis can affect significant changes in phase
behavior simultaneous to the removal of structural twinning effects. As a
result, we demonstrate in BaFe$_2$As$_2$ samples detwinned via uniaxial strain
that the in-plane C$_4$ symmetry is broken by \textit{both} the structural
lattice distortion \textit{and} long-range spin ordering at temperatures far
above the nominal (strain-free), phase transition temperatures. Surprising
changes in the magnetic order parameter of this system under relatively small
strain fields also suggest the inherent presence of magnetic domains
fluctuating above the strain-free ordering temperature in this material.
|
1111.2326v2
|
2011-11-14
|
VLT/X-shooter observations and the chemical composition of cool white dwarfs
|
We present a model atmosphere analysis of cool hydrogen-rich white dwarfs
observed at the Very Large Telescope (VLT) with the X-shooter spectrograph. The
intermediate-dispersion and high signal-to-noise ratio of the spectra allowed
us to conduct a detailed analysis of hydrogen and heavy element line profiles.
In particular, we tested various prescriptions for hydrogen Balmer line
broadening parameters and determined the effective temperature and surface
gravity of each star. Three objects (NLTT 1675, 6390 and 11393) show the
presence of heavy elements (Mg, Al, Ca, or Fe). Our abundance analysis revealed
a relatively high iron to calcium ratio in NLTT 1675 and NLTT 6390. We also
present an analysis of spectropolarimetric data obtained at the VLT using the
focal reducer and low dispersion spectrograph (FORS) and we established strict
upper limits on the magnetic field strengths in three of the DAZ white dwarfs
and determined the longitudinal magnetic field strength in the DAZ NLTT 10480.
The class of DAZ white dwarfs comprises objects that are possibly accreting
material from their immediate circumstellar environment and the present study
contributes in establishing class properties.
|
1111.3250v1
|
2011-11-14
|
Quantised orbital angular momentum transfer and magnetic dichroism in the interaction of electron vortices with matter
|
Following the very recent experimental realisation of electron vortices, we
consider their interaction with matter, in particular the transfer of orbital
angular momentum in the context of electron energy loss spectroscopy, and the
recently observed dichroism in thin film magnetised iron samples. We show here
that orbital angular momentum exchange does indeed occur between electron
vortices and the internal electronic-type motion, as well as center of mass
motion of atoms in the electric dipole approximation. This contrasts with the
case of optical vortices where such transfer only occurs in transitions
involving multipoles higher than the dipole. The physical basis of the observed
dichroism is explained.
|
1111.3259v2
|
2011-11-15
|
Unconventional Anisotropic s-Wave Superconducting Gaps of LiFeAs Iron-Pnictide Superconductor
|
We have performed high-resolution angle-resolved photoemission spectroscopy
on Fe-based superconductor LiFeAs (Tc = 18 K). We reveal multiple nodeless
superconducting (SC) gaps with 2D/kBTc ratios varying from 2.8 to 6.4,
depending on the Fermi surface (FS). We also succeeded in directly observing a
gap anisotropy along the FS with magnitude up to ~30 %. The anisotropy is
four-fold symmetric with an antiphase between the hole and electron FSs,
suggesting complex anisotropic interactions for the SC pairing. The observed
momentum dependence of the SC gap offers an excellent opportunity to
investigate the underlying pairing mechanism.
|
1111.3496v2
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.