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2009-11-19
|
Comparison of Ab initio Low-Energy Models for LaFePO, LaFeAsO, BaFe2As2, LiFeAs, FeSe and FeTe: Electron Correlation and Covalency
|
Effective low-energy Hamiltonians for several different families of
iron-based superconductors are compared after deriving them from the
downfolding scheme based on first-principles calculations. Systematic
dependences of the derived model parameters on the families are elucidated,
many of which are understood from the systematic variation of the covalency
between Fe-3d and pnictogen-/chalcogen-p orbitals.
|
0911.3705v2
|
2009-11-19
|
Metallicity gradients in the Milky Way
|
Radial metallicity gradients are observed in the disks of the Milky Way and
in several other spiral galaxies. In the case of the Milky Way, many objects
can be used to determine the gradients, such as HII regions, B stars, Cepheids,
open clusters and planetary nebulae. Several elements can be studied, such as
oxygen, sulphur, neon, and argon in photoionized nebulae, and iron and other
elements in cepheids, open clusters and stars. As a consequence, the number of
observational characteristics inferred from the study of abundance gradients is
very large, so that in the past few years they have become one of the main
observational constraints of chemical evolution models. In this paper, we
present some recent observational evidences of abundance gradients based on
several classes of objects. We will focus on (i) the magnitude of the
gradients, (ii) the space variations, and (iii) the evidences of a time
variation of the abundance gradients. Some comments on recent theoretical
models are also given, in an effort to highlight their predictions concerning
abundance gradients and their variations.
|
0911.3763v1
|
2009-11-19
|
Magnetic Anistropy due to the Casimir Effect
|
We consider the Casimir interaction between a ferromagnetic and a
non-magnetic mirror, and show how the Casimir effect gives rise to a magnetic
anisotropy in the ferromagnetic layer. The anisotropy is out-of-plane if the
non-magnetic plate is optically isotropic. If the non-magnetic plate shows a
uniaxial optical anisotropy (with optical axis in the plate plane), we find an
in-plane magnetic anisotropy. In both cases, the energetically most favorable
magnetization orientation is given by the competition between polar,
longitudinal and transverse contributions to the magneto-optical Kerr effect,
and will therefore depend on the interplate distance. Numerical results will be
presented for a magnetic plate made out of iron, and non-magnetic plates of
gold (optically isotropic), quartz, calcite and barium titanate (all uniaxially
birefringent).
|
0911.3854v1
|
2009-11-19
|
In-plane electronic anisotropy in underdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ revealed by detwinning in a magnetic field
|
We present results of angle-dependent magnetoresistance measurements and
direct optical images of underdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ which reveal
partial detwinning by action of a 14T magnetic field. Driven by a substantial
magneto-elastic coupling, this result provides evidence for an electronic
origin of the lattice distortion in underdoped iron pnictides. The observed
anisotropy in these partially detwinned samples implies a substantial in-plane
electronic anisotropy in the broken symmetry state, with a smaller resistivity
along the antiferromagnetic ordering direction.
|
0911.3878v1
|
2009-11-21
|
Spectroscopic Study of $^{75}$As and $^{139}$La NMR on Layered Structure Ferromagnet LaCoAsO
|
$^{75}$As and $^{139}$La field-swept NMR spectra were obtained for the novel
weakly itinerant ferromagnet LaCoAsO with 2D layered structure above the Curie
temperature of 55 K. By analyzing NMR spectra, temperature dependences of
Knight shift $K$ and nuclear quadrupole resonance frequency $\nu_Q$ were
obtained successfully for each nucleus. We confirmed from the so-called
$K$-$\chi$ plots that the macroscopic magnetization of our {LaCoAsO} powder
sample is intrinsic and does not contain the contribution from impurity phases.
We estimated hyperfine coupling constants from the slope of $K$-$\chi$ plots
and compared to that of iron-arsenide superconductor.
|
0911.4166v2
|
2009-11-24
|
Incommensurate itinerant antiferromagnetic excitations and spin resonance in the FeTe$_{0.6}$Se$_{0.4}$ superconductor
|
We report on inelastic neutron scattering measurements that find
incommensurate itinerant like magnetic excitations in the normal state of
superconducting FeTe$_{0.6}$Se$_{0.4}$ (\Tc=14K) at wave-vector
$\mathbf{Q}_{inc}=(1/2\pm\epsilon,1/2\mp\epsilon)$ with $\epsilon$=0.09(1). In
the superconducting state only the lower energy part of the spectrum shows
significant changes by the formation of a gap and a magnetic resonance that
follows the dispersion of the normal state excitations. We use a four band
model to describe the Fermi surface topology of iron-based superconductors with
the extended $s(\pm)$ symmetry and find that it qualitatively captures the
salient features of these data.
|
0911.4713v1
|
2009-11-24
|
Stellar Parameters for a Sample of Stars with Planets
|
The study of chemical abundances in stars with planets is an important
ingredient for the models of formation and evolution of planetary systems. In
order to determine accurate abundances, it is crucial to have a reliable set of
atmospheric parameters. In this work, we describe the homogeneous determination
of effective temperatures, surface gravities and iron abundances for a large
sample of stars with planets as well as a control sample of stars without giant
planets. Our results indicate that the metallicity distribution of the stars
with planets is more metal rich by ~ 0.13 dex than the control sample stars.
|
0911.4723v1
|
2009-11-30
|
Disorder Induced Effects on the Critical Current Density of Iron Pnictide BaFe_1.8 Co_0.2 As_2 single crystals
|
Investigating the role of disorder in superconductors is an essential part of
characterizing the fundamental superconducting properties as well as assessing
potential applications of the material. In most cases, the information
available on the defect matrix is poor, making such studies difficult, but the
situation can be improved by introducing defects in a controlled way, as
provided by neutron irradiation. In this work, we analyze the effects of
neutron irradiation on a Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystal. We
mainly concentrate on the magnetic properties which were determined by
magnetometry. Introducing disorder by neutron irradiation leads to significant
effects on both the reversible and the irreversible magnetic properties, such
as the transition temperature, the upper critical field, the anisotropy, and
the critical current density. The results are discussed in detail by comparing
them with the properties in the unirradiated state.
|
0911.5582v1
|
2009-12-07
|
Direct observation of a nodeless superconducting energy gap in the optical conductivity of iron-pnictides
|
The temperature-dependent optical reflectivity and complex transmissivity of
an epitaxially grown Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ thin film were measured
and the optical conductivity and permittivity evaluated over a wide frequency
range. The opening of the superconducting gap $2\Delta_0 = 3.7$ meV below
$T_c\approx 20$ K is {\em directly} observed by a completely vanishing optical
conductivity. The temperature and frequency dependent electrodynamic properties
of Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ in the superconducting state agree well
with the BCS predictions with no nodes in the order parameter. The spectral
weight of the condensate $1.94\times 10^7 {\rm cm}^{-2}$ corresponds to a
London penetration depth $\lambda_L=3600$ \AA.
|
0912.1256v1
|
2009-12-08
|
The possible feature of the energy spectrum of the primary cosmic rays at ultra-high energies
|
The energies of the most energetic extensive air showers observed at the
Yakutsk array have been estimated with help of the all detectors readings
instead of using of the standard procedure with a parameter s(600). The energy
of the most energetic extensive air shower observed at the Yakutsk array
happened to be 200, 200, 180 and 165 EeV with the values of the Xi**2 function
per one degree of freedom 0.9, 1., 0.9 and 1.1 for the primary protons and
helium, oxygen and iron nuclei accordingly.
|
0912.1473v1
|
2009-12-10
|
Interplay between charge-order, ferroelectricity and ferroelasticity: tungsten bronze structures as a playground for multiferroicity
|
Large electron-electron Coulomb-interactions in correlated systems can lead
to a periodic arrangement of localized electrons, the so called "charge-order".
The latter is here proposed as a driving force behind ferroelectricity in iron
fluoride K0.6FeF3. By means of density functional theory, we propose different
non-centrosymmetric d5/d6 charge-ordering patterns, each giving rise to
polarization along different crystallographic axes and with different
magnitudes. Accordingly, we introduce the concept of "ferroelectric anistropy"
(peculiar to improper ferroelectrics with polarization induced by electronic
degrees of freedom), denoting the small energy difference between competing
charge-ordered states that might be stabilized upon electrical field-cooling.
Moreover, we suggest a novel type of charge-order-induced ferroelasticity:
first-principles simulations predict a monoclinic distortion to be driven by a
specific charge-ordering pattern, which, in turn, unambiguously determines the
direction of ferroelectric polarization. K0.6FeF3 therefore emerges as a
prototypical compound, in which the intimately coupled electronic and
structural degrees of freedom result in a manifest and peculiar
multiferroicity.
|
0912.1960v1
|
2009-12-10
|
Superconductivity at 31 K in 111 type iron arsenide superconductor NaxFeAs induced by pressure
|
The effect of pressure on superconductivity of 111 type NaxFeAs is
investigated through temperature dependent electrical resistance measurement in
a diamond anvil cell. The superconducting transition temperature (Tc) increases
from 26 K to a maximum 31 K as the pressure increases from ambient to 3 GPa.
Further increasing pressure suppresses Tc drastically. The behavior of pressure
tuned Tc in NaxFeAs is much different from that in LixFeAs, although they have
the same Cu2Sb type structure
|
0912.2025v1
|
2009-12-10
|
Towards a consistent picture for quasi-1D organic superconductors
|
The electrical resistivity of the quasi-1D organic superconductor (TMTSF)2PF6
was recently measured at low temperature from the critical pressure needed to
suppress the spin-density-wave state up to a pressure where superconductivity
has almost disappeared. This data revealed a direct correlation between the
onset of superconductivity at Tc and the strength of a non-Fermi-liquid linear
term in the normal-state resistivity, going as r(T) = r0 + AT + BT2 at low
temperature, so that A goes to 0 as Tc goes to 0. Here we show that the
contribution of low-frequency antiferromagnetic fluctuations to the
spin-lattice relaxation rate is also correlated with this non-Fermi-liquid term
AT in the resistivity. These correlations suggest that anomalous scattering and
pairing have a common origin, both rooted in the low-frequency
antiferromagnetic fluctuations measured by NMR. A similar situation may also
prevail in the recently-discovered iron-pnictide superconductors.
|
0912.2049v1
|
2009-12-12
|
Two types of s-wave pairing due to magnetic and orbital fluctuations in the two-dimensional 16-band d-p model for iron-based superconductors
|
We study superconductivity in the two-dimensional 16-band d-p model extracted
from a tight-binding fit to the band structure of LaFeAsO, using the random
phase approximation. When the intraorbital repulsion U is larger than the
interorbital one U', an extended s-wave (s+--wave) pairing with sign reversal
of order parameter is mediated by antiferromagnetic spin fluctuations, while
when U<U' another kind of s-wave (s++-wave) pairing without sign reversal is
mediated by ferro-orbital fluctuations. The s++-wave pairing is enhanced due to
the electron-phonon coupling and then can be expanded over the realistic
parameter region with U>U'.
|
0912.2392v4
|
2009-12-15
|
Superconductivity at 23 K in Pt doped BaFe2As2 single crystals
|
We report superconductivity in single crystals of the new iron-pnictide
system BaFe1.9Pt0.1As2 grown by a self-flux solution method and characterized
via x-ray, transport, magnetic and thermodynamic measurements. The magnetic
ordering associated with a structural transition at 140 K present in BaFe2As2
is completely suppressed by substitution of 5% Fe with Pt and superconductivity
is induced at a critical temperature Tc=23 K. Full diamagnetic screening in the
magnetic susceptibility and a jump in the specific heat at Tc confirm the bulk
nature of the superconducting phase. All properties of the superconducting
state including transition temperature Tc, the lower critical field Hc1=200 mT,
upper critical field Hc2~65 T, and the slope dHc2/dT are comparable in value to
the those found in other transition-metal-substituted BaFe2As2 series,
indicating the robust nature of superconductivity induced by substitution of
Group VIII elements.
|
0912.2752v2
|
2009-12-15
|
Quadratic X-ray magneto-optical effect in reflection at the M-edges of 3d transition metals
|
We have observed a quadratic x-ray magneto-optical effect in near-normal
incidence reflection at the M-edges of iron. The effect appears as the
magnetically induced rotation of ca. 0.1$ degrees of the polarization plane of
linearly polarized x-ray radiation upon reflection. A comparison of the
measured rotation spectrum with results from x-ray magnetic linear dichroism
data demonstrates that this is the first observation of the Schaefer-Hubert
effect in the x-ray regime. Ab initio density-functional theory calculations
reveal that hybridization effects of the 3p core states necessarely need to be
considered when interpreting experimental data. The discovered magneto-x-ray
effect holds promise for future ultrafast and element-selective studies of
ferromagnetic as well as antiferromagnetic materials.
|
0912.2867v1
|
2009-12-16
|
Coupling of spin and orbital excitations in the iron-based superconductor FeSe(0.5)Te(0.5)
|
We present a combined analysis of neutron scattering and photoemission
measurements on superconducting FeSe(0.5)Te(0.5). The low-energy magnetic
excitations disperse only in the direction transverse to the characteristic
wave vector (1/2,0,0), whereas the electronic Fermi surface near (1/2,0,0)
appears to consist of four incommensurate pockets. While the spin resonance
occurs at an incommensurate wave vector compatible with nesting, neither
spin-wave nor Fermi-surface-nesting models can describe the magnetic
dispersion. We propose that a coupling of spin and orbital correlations is key
to explaining this behavior. If correct, it follows that these nematic
fluctuations are involved in the resonance and could be relevant to the pairing
mechanism.
|
0912.3205v2
|
2009-12-17
|
Droplet-like Fermi surfaces in the anti-ferromagnetic phase of EuFe$_2$As$_2$, an Fe-pnictide superconductor parent compound
|
Using angle resolved photoemission it is shown that the low lying electronic
states of the iron pnictide parent compound EuFe$_2$As$_2$ are strongly
modified in the magnetically ordered, low temperature, orthorhombic state
compared to the tetragonal, paramagnetic case above the spin density wave
transition temperature. Back-folded bands, reflected in the orthorhombic/
anti-ferromagnetic Brillouin zone boundary hybridize strongly with the
non-folded states, leading to the opening of energy gaps. As a direct
consequence, the large Fermi surfaces of the tetragonal phase fragment, the low
temperature Fermi surface being comprised of small droplets, built up of
electron and hole-like sections. These high resolution ARPES data are therefore
in keeping with quantum oscillation and optical data from other undoped
pnictide parent compounds.
|
0912.3434v1
|
2009-12-18
|
Elastic isotropy of hcp-Fe under Earth core conditions
|
Our first-principles calculations show that both the compressional and shear
waves of hcp-Fe become elastically isotropic under the high temperatures of
Earth inner core conditions, with the variation in sound velocities along
different angles from the c axis within 1%. We computed the thermoelasticity at
high pressures and temperatures from quasiharmonic linear response
linear-muffin-tin-orbital calculations in the generalized-gradient
approximation. The calculated anisotropic shape and magnitude in hcp-Fe at
ambient temperature agree well with previous first-principles predictions, and
the anisotropic effects show strong temperature dependences. This implies that
other mechanisms, rather than the preferential alignment of the hcp-Fe crystal
along the Earth rotation axis, account for the seismic P-wave travel time
anomalies. Either the inner core is not hcp iron, and/or the seismologically
observed anisotropy is caused by inhomogeneity, i.e. multiple phases.
|
0912.3587v1
|
2009-12-18
|
The crystal structure of FeSe0.44Te0.56
|
The crystal structure of the superconductor FeSe0.44Te0.56 was redetermined
by high-resolution X-ray single crystal diffraction at 173 K (anti-PbO-type,
P4/nmm, a=3.7996(2), c=5.9895(6) A, R1=0.022, wR2=0.041, 173 F^2).
Significantly different z-coordinates of tellurium and selenium at the 2c site
are clearly discernible and were refined to z_Te=0.2868(3) and z_Se=0.2468(7).
Thus the chalcogen heights differ by 0.24 A and the Fe-Se bonds are by 0.154 A
shorter than the Fe-Te bonds, while three independent (Te,Se)-Fe-(Te,Se) bond
angles occur. An elevated U33 displacement parameter of the iron atom is
suggestive of a slightly puckered Fe layer resulting from different
combinations of Se or Te neighbors. Such strong disorder underlines the
robustness of superconductivity against structural randomness and has not yet
been considered in theoretical studies of this system.
|
0912.3706v2
|
2009-12-21
|
Electronic Ferroelectricity and Frustration
|
Beyond a conventional classification of ferroelectricity, there is a class of
materials where electronic degrees of freedom and electronic interactions are
directly responsible for electric polarization and ferroelectric transition.
This is termed electronic ferroelectricity. In this article, we review
electronic ferroelectricity from a view point of frustration. Experimental and
theoretical examinations in spin driven ferroelectric materials, recently
termed multiferroics, are introduced. Spin frustration caused by competing
magnetic interactions is of prime importance for this type of ferroelectricity.
Charge driven ferroelectricity where electronic charge order induces electric
polarization is reviewed. In particular, exotic dielectric and
magneto-dielectric properties in layered iron oxides are focused on. Through a
number of recent experimental and theoretical researches, charge fluctuation
due to frustrated geometry plays essential roles on electronic ferroelectricity
in this compound.
|
0912.4083v1
|
2009-12-28
|
Volovik effect in the $\pm$s-wave state for the iron-based superconductors
|
We studied the field dependencies of specific heat coefficient $\gamma(H) =
\lim_{T \rightarrow 0} C(T,H)/T $ and thermal conductivity coefficient $\lim_{T
\rightarrow 0} \kappa(T ,H)/T$ of the $\pm$s-wave state in the mixed state. We
found that it is a generic feature of the two band s-wave state with the
unequal sizes of gaps, small $\Delta_S$ and large $\Delta_L$, that Doppler
shift of the quasiparticle excitations (Volovik effect) creates a finite
density of states, on the extended states outside of vortex cores, proportional
to $H$ in contrast to the $\sqrt{H}$ dependence of the d-wave state. Impurity
scattering effect on the $\pm$s-wave state, however, makes this generic
$H$-linear dependence sublinear approaching to the $\sqrt{H}$ behavior. Our
calculations of $\lim_{T \rightarrow 0} \kappa(T ,H)/T$ successfully fit the
experimental data of Ba(Fe$_{1-x}$Co$_x )_2$ As$_2$ with different Co-doping
$x$ by systematically varying the gap size ratio $R= |\Delta_S | / |\Delta_L
|$. We also resolve the dilemma of a substantial value of $\gamma(H \rightarrow
0)$ but almost zero value of $\lim_{T \rightarrow 0} \kappa(T ,H \rightarrow
0)/T$, observed in experiments.
|
0912.5049v2
|
2009-12-27
|
Observation of intertwined Fermi surface topology, orbital parity symmetries and electronic interactions in iron arsenide superconductors
|
We present a polarization and topology resolved study of the low energy band
structure in optimally doped superconducting Ba0.6K0.4Fe2As2 using angle
resolved photoemission spectroscopy. Polarization-contrasted measurements allow
us to identify and trace all low energy bands expected in models, revealing
unexpected symmetry breaking and a surprisingly intertwined Fermi surface
topology of hole-like bands near the Brillouin zone center. Band structure
correlations across the Gamma-M spin fluctuation wavevector are compared with
the superconducting gap anisotropy which suggest a partial scenario for
spin-mediated interband instability contributing to superconductivity in the
hole doped regime.
|
0912.5089v1
|
2009-12-29
|
The quite complex "Simple Stellar Populations" of Globular Clusters
|
There is compelling observational evidence that globular clusters (GCs) are
quite complex objects. A growing body of photometric results indicate that the
evolutionary sequences are not simply isochrones in the observational plane -as
believed until a few years ago- from the main sequence, to the subgiant, giant,
and horizontal branches. The strongest indication of complexity comes however
from the chemistry, from internal dispersion in iron abundance in a few cases,
and in light elements (C, N, O, Na, Mg, Al, etc.) in all GCs. This universality
means that the complexity is intrinsic to the GCs and is most probably related
to their formation mechanisms. The extent of the variations in light elements
abundances is dependent on the GC mass, but mass is not the only modulating
factor; metallicity, age, and possibly orbit can play a role. Finally, one of
the many consequences of this new way of looking at GCs is that their stars may
show different He contents.
|
0912.5280v1
|
2010-01-05
|
Magnetic frustration in an iron based Cairo pentagonal lattice
|
The Fe3+ lattice in the Bi2Fe4O9 compound is found to materialize the first
analogue of a magnetic pentagonal lattice. Due to its odd number of bonds per
elemental brick, this lattice, subject to first neighbor antiferromagnetic
interactions, is prone to geometric frustration. The Bi2Fe4O9 magnetic
properties have been investigated by macroscopic magnetic measurements and
neutron diffraction. The observed non-collinear magnetic arrangement is related
to the one stabilized on a perfect tiling as obtained from a mean field
analysis with direct space magnetic configurations calculations. The
peculiarity of this structure arises from the complex connectivity of the
pentagonal lattice, a novel feature compared to the well-known case of
triangle-based lattices.
|
1001.0710v1
|
2010-01-10
|
Chemical composition of stellar populations in Omega Centauri
|
We derive abundances of Fe, Na, O, and s-elements from GIRAFFE@VLT spectra
for more than 200 red giant stars in the Milky Way satellite Omega Centauri.
Our preliminary results are that: (i) we confirm that Omega Centauri exibiths
large star-to-star metallicity variations ($\sim$ 1.4 dex); (ii) the
metallicity distribution reveals the presence of at least five stellar
populations with different [Fe/H]; (iii) a clear Na-O anticorrelation is
clearly observed for the metal-poor and metal-intermediate populations while
apparently the anticorrelation disappears for the most metal-rich populations.
Interestingly the Na level grows with iron.
|
1001.1500v1
|
2010-01-11
|
Measurement of neutron star parameters: a review of methods for low-mass X-ray binaries
|
Measurement of at least three independent parameters, for example, mass,
radius and spin frequency, of a neutron star is probably the only way to
understand the nature of its supranuclear core matter. Such a measurement is
extremely difficult because of various systematic uncertainties. The lack of
knowledge of several system parameter values gives rise to such systematics.
Low-mass X-ray binaries, which contain neutron stars, provide a number of
methods to constrain the stellar parameters. Joint application of these methods
has a great potential to significantly reduce the systematic uncertainties, and
hence to measure three independent neutron star parameters accurately. Here we
review the methods based on (1) thermonuclear X-ray bursts; (2)
accretion-powered millisecond-period pulsations; (3) kilohertz quasi-periodic
oscillations; (4) broad relativistic iron lines; (5) quiescent emissions; and
(6) binary orbital motions.
|
1001.1642v1
|
2010-01-17
|
The Interior Dynamics of Water Planets
|
The ever-expanding catalog of detected super-Earths calls for theoretical
studies of their properties in the case of a substantial water layer. This work
considers such water planets with a range of masses and water mass fractions (2
to 5 M_Earth, 0.02% to 50% H2 O). First, we model the thermal and dynamical
structure of the near-surface for icy and oceanic surfaces, finding separate
regimes where the planet is expected to maintain a subsurface liquid ocean and
where it is expected to exhibit ice tectonics. Newly discovered exoplanets may
be placed into one of these regimes given estimates of surface temperature,
heat flux, and gravity. Second, we construct a parameterized convection model
for the underlying ice mantle of higher ice phases, finding that materials
released from the silicate iron core should traverse the ice mantle on the
timescale of 0.1 to 100 megayears. We present the dependence of the overturn
times of the ice mantle and the planetary radius on total mass and water mass
fraction. Finally, we discuss the implications of these internal processes on
atmospheric observables.
|
1001.2890v1
|
2010-01-18
|
The metal-insulator transition and lattice distortion in semiconductors
|
A relation between the energy of an elementary `insulating' excitation
corresponding to the metal-insulator transition and the bandgap width in a
semiconductor is obtained. An effect of atomic relaxation on the temperature
and pressure dependence of the bandgap width is considered. It is shown that
the metal-insulator transition in a semiconductor causes a weak rhombohedral or
monoclinic distortion in the case of a diamond and zincblende structure and a
weak tetragonal or orthorhombic distortion in the case of a rocksalt structure.
A change in the bandgap associated with a ferroelectric (antiferroelectric)
transition in a semiconductor is also obtained.
|
1001.3061v3
|
2010-01-19
|
Theory of Normal State Pseudogap Behavior in FeSe(1-x)$Te(x)
|
The normal state of the recently discovered Iron Selenide (FeSe)-based
superconductors shows a range of inexplicable features. Along with bad-metallic
resistivity, characteristic pseudogap features and proximity to insulating
states, reminiscent of the underdoped high-Tc cuprates, mark these systems as
strongly correlated non-Fermi Liquid metals. Here, using the first-principles
LDA+DMFT method, we show how strong multi-orbital correlation-induced
orbital-selective Mott-like physics leads to an orthogonality catastrophe
underpinning these inexplicable incoherent features. Excellent agreement with a
range of resistivity and Seebeck data strongly support our proposal. We discuss
pseudogap regime microscopically, along with implications for the nature of the
instability at lower T, and propose that related systems could be of use in
thermoelectric devices.
|
1001.3273v1
|
2010-01-20
|
Fermi surface and mass enhancement in KFe$_2$As$_2$ from de Haas-van Alphen effect measurements
|
We report on a band structure calculation and de Haas-van Alphen measurements
of KFe$_2$As$_2$. Three cylindrical Fermi surfaces are found. Effective masses
of electrons range from 6 to 18$m_e$, $m_e$ being the free electron mass.
Remarkable discrepancies between the calculated and observed Fermi surface
areas and the large mass enhancement ($\gtrsim 3$) highlight the importance of
electronic correlations in determining the electronic structures of iron
pnicitide superconductors.
|
1001.3441v2
|
2010-01-26
|
Gap structure in the electron-doped Iron-Arsenide Superconductor Ba(Fe0.92Co0.08)2As2: low-temperature specific heat study
|
We report the field and temperature dependence of the low-temperature
specific heat down to 400 mK and in magnetic fields up to 9 T of the
electron-doped Ba(Fe0.92Co0.08)2As2 superconductor. Using the phonon specific
heat obtained from pure BaFe2As2 we find the normal state Sommerfeld
coefficient to be 18 mJ/mol.K^2 and a condensation energy of 1.27 J/mol. The
temperature dependence of the electronic specific heat clearly indicate the
presence of the low-energy excitations in the system. The magnetic field
variation of field-induced specific heat cannot be described by single clean s-
or d-wave models. Rather, the data require an anisotropic gap scenario which
may or may not have nodes. We discuss the implications of these results.
|
1001.4564v1
|
2010-01-27
|
Coupled structural and magnetic properties of ferric fluoride nanostructures: part II, a Monte-Carlo Heisenberg study
|
We present a numerical study of the magnetic structure of nanostructured iron
fluoride, using the Monte-Carlo-Metropolis simulated annealing technique and a
classical Heisenberg Hamiltonian with a superexchange angle dependence. The
parameters are adjusted on experimental results, and the atomic structure and
topology taken from a previous atomistic model of grain boundaries in the same
system. We find perfect antiferromagnetic crystalline grains and a disordered
magnetic configuration (speromagnetic like) at the grain boundary, in agreement
with experimental findings. Both the lowest magnetic energy and the rate of
magnetic frustration are found to be dependent on the relative disorientation
of crystalline grains, i.e. on the cationic topology. By simulating hysteresis
loops, we find that the magnetization rotation is not spatially uniform. We
conclude on possible extensions of the model.
|
1001.4959v1
|
2010-01-27
|
Coupled structural and magnetic properties of ferric fluoride nanostructures part I: a Metropolis atomistic study
|
A modified Metropolis atomistic simulation is proposed to model the structure
of grain boundaries (GBs) and interfaces in ionic nanostructured systems and is
applied to the magnetically interesting case of iron trifluoride (FeF3). We
chose long-range interatomic potentials adjusted on experimental results, and
adapted a previously established Monte Carlo scheme consisting of various
modifications of the simulated annealing/ Metropolis algorithm. Atomic
structures of twisted and tilted GBs as a function of the relative
disorientation of the grains have been achieved yielding close to
experimentally measured properties. This approach takes into account the
structure of the grains far from the interface in order to constrain the
relative orientation of the grains, without any periodic boundary conditions.
One concludes that a long-range coulombic falloff of the interatomic potentials
is necessary to obtain GB structures presenting a correct local topology but
with a smooth transition from crystalline to amorphous states. The structural
features are finally discussed in terms of topological aspects and local
magnetic structure.
|
1001.4971v2
|
2010-01-27
|
A real-space effective c-axis lattice constant theory of superconductivity
|
Based on the recent developed real-space picture of superconductivity, we
study the stability of the superconducting vortex lattices in layered
superconductors. It is shown that the effective c-axis lattice constant play a
significant role in promoting the superconducting transition temperature in
these materials. An unified expression Tc(max)=10c*-28 can be applied to
estimate the highest possible Tc(max) for a given layered superconductor with
an effective c-axis lattice constant c*. For the newly discovered iron-based
superconductors, our results suggest that their Tc cannot exceed 60K, 50K and
40K for the 1111, 21311 and 122 series, respectively. In the case of
copper-based oxide superconductors, it seems that the highest Tc can reach
about 161K without applying of the external pressure. In our theoretical
framework, we could interpret the experimental results of the completely
different superconducting transition temperatures obtained in two very similar
cuprate superconductors (La{2-x}Ba{x}CuO{4} of 40K and
Sr{2-x}Ba{x}CuO{3+\delta} of 98K). In addition, the physical reason why the
superconductivity does not occur in noble metals (like gold, silver and copper)
is discussed. Finally, we argue that the metallic hydrogen cannot exhibit
superconductivity at room temperature, it even cannot be a superconductor at
any low temperature.
|
1001.5067v1
|
2010-01-28
|
EC 11481-2303 - A Peculiar Subdwarf OB Star Revisited
|
EC 11481-2303 is a peculiar, hot, high-gravity pre-white dwarf. Previous
optical spectroscopy revealed that it is a sdOB star with an effective
temperature (Teff) of 41790 K, a surface gravity log(g)= 5.84, and He/H = 0.014
by number. We present an on-going spectral analysis by means of non-LTE
model-atmosphere techniques based on high-resolution, high-S/N optical
(VLT-UVES) and ultraviolet (FUSE, IUE) observations. We are able to reproduce
the optical and UV observations simultaneously with a chemically homogeneous
NLTE model atmosphere with a significantly higher effective temperature and
lower He abundance (Teff = 55000 K, log (g) = 5.8, and He / H = 0.0025 by
number). While C, N, and O appear less than 0.15 times solar, the iron-group
abundance is strongly enhanced by at least a factor of ten.
|
1001.5115v1
|
2010-01-29
|
Strong correlation and massive spectral-weight redistribution induced spin density wave in a-Fe1.06Te
|
The electronic structure of a-Fe1.06Te is studied with angle-resolved
photoemission spectroscopy. We show that there is substantial spectral weight
around Gamma and X, and lineshapes are intrinsically incoherent in the
paramagnetic state. The magnetic transition is characterized by a massive
spectral-weight transfer over an energy range as large as the band width, which
even exhibits a hysteresis loop that marks the strong first order transition.
Coherent quasiparticles emerge in the magnetically ordered state due to
decreased spin fluctuations, which account for the change of transport
properties from insulating behavior to metallic behavior. Our observation
demonstrates that Fe1.06Te distinguishes itself from other iron-based systems
with more local characters and much stronger interactions among different
degrees of freedom, and how a spin density wave is formed in the presence of
strong correlation.
|
1001.5327v1
|
2010-02-01
|
Statistics of subgap states in $s_\pm$ superconductors
|
There is strong support in favor of an unusual $s_{\pm}$ superconducting
state in the new iron-based superconductors, in which the gap parameter has
opposite signs in different bands. In this case scattering between different
bands by impurities has a pair-breaking effect and introduces states inside the
gap. We studied the statistics of disorder-induced subgap states in $s_{\pm}$
superconductors due to collective effects of impurities. Numerically solving
the two-band Bogolyubov equations, we explored the behavior of the density of
states and localization length. We located the mobility edge separating the
localized and delocalized states for the 3D case and the crossover between the
weak and strong localization regimes for the 2D case. We found that the widely
used self-consistent T-matrix approximation is not very accurate in describing
subgap states.
|
1002.0363v1
|
2010-02-02
|
Structural and Magnetic Properties of Co1-xFexSr2YCu2O7 compounds
|
Here we study the structural and magnetic properties of the CoFeSr2YCu2O7
compound with x = 0.0 to 1.0. X-ray diffraction patterns and simulated data
obtained from Rietveld refinement of the same indicate that the iron ion
replacement in CoFeSr2YCu2O7 induces a change in crystal structure. The
orthorhombic Ima2 space group structure of Co-1212 changes to tetragonal P4/mmm
with increasing Fe ion. The XPS studies reveal that both Co and Fe ions are in
mixed states for the former and in case of later.Although none of the studied
as synthesized samples in CoFeSr2YCu2O7 are superconducting, the interesting
structural changes in terms of their crystallisation space groups and the weak
magnetism highlights the rich solid state chemistry of this class of materials.
|
1002.0402v2
|
2010-02-02
|
Comparative study on the thermoelectric effect of parent oxypnictides La$T$AsO ($T$ = Fe, Ni)
|
The thermopower and Nernst effect were investigated for undoped parent
compounds LaFeAsO and LaNiAsO. Both thermopower and Nernst signal in iron-based
LaFeAsO are significantly larger than those in nickel-based LaNiAsO.
Furthermore, abrupt changes in both thermopower and Nernst effect are observed
below the structural phase transition temperature and spin-density wave (SDW)
type antiferromagnetic (AFM) order temperature in Fe-based LaFeAsO. On the
other hand, Nernst effect is very small in the Ni-based LaNiAsO and it is
weakly temperature-dependent, reminiscent of the case in normal metals. We
suggest that the effect of SDW order on the spin scattering rate should play an
important role in the anomalous temperature dependence of Hall effect and
Nernst effect in LaFeAsO. The contrast behavior between the LaFeAsO and LaNiAsO
systems implies that the LaFeAsO system is fundamentally different from the
LaNiAsO system and this may provide clues to the mechanism of high $T_c$
superconductivity in the Fe-based systems.
|
1002.0417v1
|
2010-02-03
|
Orbital Symmetry of Ba(Fe1-xCox)2As2 Superconductors Probed with X-ray Absorption Spectroscopy
|
The orbital symmetries of electron doped iron-arsenide superconductors
Ba(Fe1-xCox)2As2 have been measured with x-ray absorption spectroscopy. The
data reveal signatures of Fe d electron itinerancy, weak electronic
correlations, and a high degree of Fe-As hybridization related to the bonding
topology of the Fe dxz+yz states, which are found to contribute substantially
at the Fermi level. The energies and detailed orbital character of Fe and As
derived unoccupied s and d states are found to be in remarkably good agreement
with the predictions of standard density functional theory.
|
1002.0769v2
|
2010-02-04
|
Magnetic excitations in ferro-pnictide materials controlled by a quantum critical point into hidden order
|
The two-orbital J1-J2 model that describes a square lattice of frustrated
spin-1 iron atoms is analyzed within the linear spin-wave approximation and by
exact diagonalization over a 4x4 cluster. A quantum critical point (QCP) is
identified that separates hidden magnetic order at weak Hund's rule coupling
from a commensurate spins density wave (cSDW) at strong Hund's rule coupling.
Although the moment for cSDW order is small at the QCP, the critical linear
spin-wave spectrum shows strong low-energy excitations centered at the
wavenumbers that correspond to cSDW order. These disperse anisotropically. A
fit to the magnetic excitation spectrum of ferro-pnictide materials obtained
recently by inelastic neutron scattering measurements notably accounts for the
absence of softening at the wavenumber that corresponds to Neel order.
|
1002.0891v3
|
2010-02-05
|
Incommensurate spin-density wave and magnetic lock-in transition in CaFe4As3
|
The magnetic structure for the newly discovered iron-arsenide compound
\CaFeAs has been studied by neutron powder diffraction. Long-range magnetic
order is detected below 85K, with an incommensurate modulation described by the
propagation vector k=(0,$\delta$,0), $\delta\sim$ 0.39. Below $\sim$ 25K, our
measurements detect a first-order phase transition where $\delta$ locks into
the commensurate value 3/8. A model of the magnetic structure is proposed for
both temperature regimes, based on Rietveld refinements of the powder data and
symmetry considerations. The structures correspond to longitudinal
spin-density-waves with magnetic moments directed along the \textit{b}-axis. A
Landau analysis captures the change in thermodynamic quantities observed at the
two magnetic transitions, in particular the drop in resistivity at the lock-in
transition.
|
1002.1336v1
|
2010-02-07
|
Proposal to determine the Fermi-surface topology of a doped iron-based superconductor using bulk-sensitive Fourier-transform Compton scattering
|
We have carried out first-principles calculations of the Compton scattering
spectra to demonstrate that the filling of the hole Fermi surface in
LaO$_{1-x}$F$_{x}$FeAs produces a distinct signature in the Fourier transformed
Compton spectrum when the momentum transfer vector lies along the [100]
direction. We thus show how the critical concentration $x_c$, where hole Fermi
surface pieces are filled up and the superconductivity mediated by
antiferromagnetic spin fluctuations is expected to be suppressed, can be
obtained in a bulk-sensitive manner.
|
1002.1483v1
|
2010-02-08
|
A Triplet Resonance in Superconducting FeSe0.4Te0.6
|
From heavy fermions to cuprates and iron pnictides, a spin resonance is a
staple of nearly magnetic superconductors. Possible explanations include a
two-particle bound state or loss of magnon damping in the superconductor. While
both scenarios suggest a central role for magnetic fluctuations, distinguishing
them is important to identify the right theoretical framework and to guide the
search for higher temperature superconductors. Using inelastic neutron
scattering technique, we show that the spin resonance in optimally doped
Fe(Se,Te) splits into three peaks in a high magnetic field, a signature of a
two-particle triplet bound state
|
1002.1617v2
|
2010-02-08
|
Determination of the Nature of the Tetragonal to Orthorhombic Phase Transition in SrFe_2As_2 by Measurement of the Local Order Parameter
|
SrFe2As2 is the end-member for a series of iron-pnictide superconductors and
has a tetragonal-to-orthorhombic phase transition near 200 K. Previous
macroscopic measurements to determine the nature of the transition gave
seemingly inconsistent results so we use electron microscopy to monitor the
local order parameter showing that the transformation is first order and that
the orthorhombic phase grows as needle domains. This suggests the transition
occurs via the passage of transformation dislocations, explaining the apparent
inconsistencies. This mechanism may be common to similar transitions.
|
1002.1639v2
|
2010-02-09
|
High Resolution X-Ray Spectroscopy of SN 1987A: Monitoring with XMM-Newton
|
We report the results of our XMM-Newton monitoring of SN 1987A. The ongoing
propagation of the supernova blast wave through the inner circumstellar ring
caused a drastic increase in X-ray luminosity during the last years, enabling
detailed high resolution X-ray spectroscopy with the Reflection Grating
Spectrometer. The observations can be used to follow the detailed evolution of
the arising supernova remnant. The fluxes and broadening of the numerous
emission lines seen in the dispersed spectra provide information on the
evolution of the X-ray emitting plasma and its dynamics. These were analyzed in
combination with the EPIC-pn spectra, which allow a precise determination of
the higher temperature plasma. We modeled individual emission lines and fitted
plasma emission models. Especially from the observations between 2003 and 2007
we can see a significant evolution of the plasma parameters and a deceleration
of the radial velocity of the lower temperature plasma regions. We found an
indication (3-sigma-level) of an iron K feature in the co-added EPIC-pn
spectra. The comparison with Chandra grating observations in 2004 yields a
clear temporal coherence of the spectral evolution and the sudden deceleration
of the expansion velocity seen in X-ray images ~6100 days after the explosion.
|
1002.1865v1
|
2010-02-11
|
Numerical Forecast of the Melting and Thermal Histories of Particles Injected in a Plasma Jet
|
This work presents the numerical simulation of the melting process of a
particle injected in a plasma jet. The plasma process is nowadays applied to
produce thin coatings on metal mechanical components with the aim of improving
the surface resistance to different phenomena such as corrosion, temperature or
wear. In this work we studied the heat transfer including phase-change of a
bi-layer particle composed of a metallic iron core coated with ceramic alumina,
inside a plasma jet. The model accounted for the environmental conditions along
the particle path. The numerical simulation of this problem was performed via a
temperature-based phase-change finite element formulation. The results obtained
with this methodology satisfactorily described the melting process of the
particle. Particularly, the results of the present work illustrate the phase
change evolution in a bi-layer particle during its motion in the plasma jet.
Moreover, the numerical trends agreed with those previously reported in the
literature and computed with a finite volume enthalpy based formulation.
|
1002.2370v1
|
2010-02-12
|
Electron-phonon coupling and charge gap in spin-density-wave iron-pnictides from quasiparticle relaxation dynamics
|
We investigate the quasiparticle relaxation and low-energy electronic
structure in undoped SrFe_2As_2 exhibiting spin-density wave (SDW) ordering
using optical pump-probe femtosecond spectroscopy. A remarkable critical
slowing down of the quasiparticle relaxation dynamics at the SDW transition
temperature T_SDW = 200K is observed. From temperature dependence of the
transient reflectivity amplitude we determine the SDW-state charge gap
magnitude, 2Delta_SDW/k_BT_SDW=7.2+-1. The second moment of the Eliashberg
function, lambda<(hbar omega)^2>=110+-10meV^2, determined from the relaxation
time above T_SDW, is similar to SmFeAsO and BaFe_2As_2 indicating a rather
small electron phonon coupling constant unless the electron-phonon spectral
function (alpha^2F(omega) is strongly enhanced in the low-energy phonon region.
|
1002.2582v1
|
2010-02-17
|
Scaling relations in anisotropic superconductors with strong pair-breaking
|
Following the work of Abrikosov and Gor'kov on the pair-breaking effects, one
can derive the temperature dependencies of the electronic specific heat
$C_s/T=\gamma^\prime+\mu T^2$ (with the jump at the superconducting transition
$\Delta C \propto T_c^3$) for materials with zero Fermi surface average of the
order parameter $<\Delta>=0$ (e.g. d-wave) or for those with $<\Delta> \ll
\Delta_{max}$ (e.g., $\pm s$ of iron-pnictides) in the presence of strong
pair-breaking. Moreover, the London penetration depth satisfies
$\lambda^{-2}=\lambda_0^{-2}(1-T^2/T_c^2)$ (or $\lambda -\lambda_0=\beta T^2 $
at low temperatures) and the slope of the upper critical field near $T_c$ is
$H_{c2}^\prime \propto T_c$. Remarkably simple relations between these at first
sight unrelated quantities take place: $\mu \lambda_0^2
T_c^3/|H_{c2}^\prime|=3\phi_0/8\pi^2$ and $\Delta C \beta^2
T_c^4/|H_{c2,c}^\prime| = \phi_0/16\pi^2 $ are universal constants. The
prediction is checked on two samples of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ and on
CeCoIn$_5$ for which the data needed are available.
|
1002.3390v1
|
2010-02-19
|
Dielectronic recombination data for astrophysical applications: Plasma rate-coefficients for Fe^q+ (q=7-10, 13-22) and Ni^25+ ions from storage-ring experiments
|
This review summarizes the present status of an ongoing experimental effort
to provide reliable rate coefficients for dielectronic recombination of highly
charged iron ions for the modeling of astrophysical and other plasmas. The
experimental work has been carried out over more than a decade at the heavy-ion
storage-ring TSR of the Max-Planck-Institute for Nuclear Physics in Heidelberg,
Germany. The experimental and data reduction procedures are outlined. The role
of previously disregarded processes such as fine-structure core excitations and
trielectronic recombination is highlighted. Plasma rate coefficients for
dielectronic recombination of Fe^q+ ions (q=7-10, 13-22) and Ni^25+ are
presented graphically and in a simple parameterized form allowing for easy use
in plasma modeling codes. It is concluded that storage-ring experiments are
presently the only source for reliable low-temperature dielectronic
recombination rate-coefficients of complex ions.
|
1002.3678v1
|
2010-02-25
|
Signatures of pressure induced superconductivity in insulating Bi2212
|
We have performed several high pressure electrical resistance experiments on
Bi1.98Sr2.06Y0.68Cu2O8, an insulating parent compound of the high-Tc Bi2212
family of copper oxide superconductors. We find a resistive anomaly, a downturn
at low temperature, that onsets with applied pressure in the 20-40 kbar range.
Through both resistance and magnetoresistance measurements, we identify this
anomaly as a signature of induced superconductivity. Resistance to higher
pressures decreases Tc, giving a maximum of 10 K. The higher pressure
measurements exhibit a strong sensitivity to the hydrostaticity of the pressure
environment. We make comparisons to the pressure induced superconductivity now
ubiquitous in the iron arsenides.
|
1002.4672v1
|
2010-02-26
|
Significant reduction of electronic correlations upon isovalent Ru substitution of BaFe2As2
|
We present a detailed investigation of Ba(Fe0.65Ru0.35)2As2 by transport
measurements and Angle Resolved photoemission spectroscopy. We observe that Fe
and Ru orbitals hybridize to form a coherent electronic structure and that Ru
does not induce doping. The number of holes and electrons, deduced from the
area of the Fermi Surface pockets, are both about twice larger than in
BaFe2As2. The contribution of both carriers to the transport is evidenced by a
change of sign of the Hall coefficient with decreasing temperature. Fermi
velocities increase significantly with respect to BaFe2As2, suggesting a
significant reduction of correlation effects. This may be a key to understand
the appearance of superconductivity at the expense of magnetism in undoped iron
pnictides.
|
1002.4952v2
|
2010-02-28
|
Low-temperature synthesis of SmFeAsO0.7F0.3 wires with high transport critical current density
|
Ag-sheathed SmFeAsO0.7F0.3 (Sm-1111) superconducting wires were prepared by a
one-step solid state reaction at temperatures as low as 850~900C, instead of
commonly used temperatures of 1150~1250C. The X-ray diffraction pattern of the
as-sintered samples is well indexed on the basis of tetragonal ZrCuSiAs-type
structure. We characterized transport critical current density Jc of the
SmFeAsO0.7F0.3 wires in increasing and subsequently decreasing fields, by a
resistive four-probe method. A transport Jc as high as ~1300 A/cm^2 at 4.2 K
and self field has been observed for the first time in Sm-1111 type
polycrystalline superconductors. The Jc also shows a rapid depression in small
applied fields as well as a magnetic-history dependence, indicating weak-linked
grain boundaries. The low-temperature synthesis method can be very beneficial
to fabricating the RE-1111 iron oxynictides in a convenient and safe way.
|
1003.0153v1
|
2010-03-04
|
Medium Modifications of the Rho Meson in Nuclear Photoproduction
|
We extend our recent study of dilepton invariant-mass spectra from the decays
of rho mesons produced by photon reactions off nuclei. We specifically focus on
experimental spectra as recently measured by the CLAS Collaboration at the
Thomas Jefferson National Accelerator Facility using carbon and iron nuclei.
Building on our earlier work we broaden our description to a larger set of
observables in order to identify sensitivities to the medium effects predicted
by microscopic calculations of the rho spectral function. We compute mass
spectra for several target nuclei and study the spectral shape as a function of
the 3-momentum of the outgoing lepton pair. We also compute the so-called
nuclear transparency ratio which provides an alternative means (and thus
consistency check) of estimating the inelastic rho width in the cold nuclear
medium.
|
1003.0910v2
|
2010-03-04
|
Comparative study of transport and magnetic Jc in the polycrystalline iron-based superconductor NdFeAsO{0.88}F{0.12}
|
Polycrystalline NdFeAsO0.88F0.12 superconductors prepared by high pressure
(HP) and ambient pressure (AP) method were comparatively studied by
magnetization and transport measurements. Upper critical field (Hc2),
irreversibility field (Hirr) and the anisotropy parameter were estimated from
resistance transition curves. The broadening of transition width was observed,
and ascribed to both Hc2 anisotropy and superconductivity inhomogeneity of
samples. Magnetic hysteresis loops (MHLs) in low fields were measured to detect
the trace of weak-link behavior. The reclosed hysteresis loops in low fields
manifest that there are weak-links in both samples. Magnetization critical
current density Jcm were derived from MHLs. High-pressure synthesized sample
shows higher Jcm. However, by means of direct transport I-V measurements,
transport critical current density Jct was very low. The Jct values for two
samples are comparable. Large discrepancies between Jcm and Jct also indicate
that there are weak-links in both samples. The relative mechanism is discussed
in detail.
|
1003.0946v2
|
2010-03-11
|
Superconductivity and spin-density-waves in multi-band metals
|
We present a detailed description of two-band quasi-2D metals with s-wave
superconducting (SC) and antiferromagnetic spin-density wave (SDW)
correlations. We present a general approach and use it to investigate the
influence of the difference between the shapes and the areas of the two Fermi
surfaces on the phase diagram. In particular, we determine the conditions for
the co-existence of SC and SDW orders at different temperatures and dopings. We
argue that a conventional s-wave SC order co-exists with SDW order only at very
low $T$ and in a very tiny range of parameters. An extended s-wave
superconductivity, for which SC gap changes sign between the two bands,
co-exists with antiferromagnetic SDW over a much wider range of parameters and
temperatures, but even for this SC order the regions of SDW and SC can still be
separated by a first order transition. We show that the co-existence range
becomes larger if SDW order is incommensurate. We apply our results to
iron-based pnictide materials, in some of which co-existence of SDW and SC
orders has been detected.
|
1003.2389v2
|
2010-03-14
|
Site-dependent NMR Spin-lattice Relaxation in the Superconducting State of an Iron Pnictide Superconductor
|
In a conventional superconductor, the spin-lattice relaxation rate on all
nuclei should have the same temperature dependence below Tc. We performed
$^{23}$Na, $^{75}$As, and $^{59}$Co NMR studies on single crystals of
NaFe0.95Co0.05As, and found that spin-lattice relaxation rates show very
different temperature dependent power-law behavior on three sites. We propose
that such site-dependent behavior is due to the facts that the superconductor
has two gaps of very different sizes. The power-law exponent of each nucleus is
affected by the strength of the hyperfine coupling to the small gap. We also
found that the large superconducting gap on the cobalt site is smaller than on
other two sites. It suggests a local suppression of the superconducting gap on
the dopant site.
|
1003.2775v1
|
2010-03-19
|
Leaving the ISCO: the inner edge of a black-hole accretion disk at various luminosities
|
The "radiation inner edge" of an accretion disk is defined as the inner
boundary of the region from which most of the luminosity emerges. Similarly,
the "reflection edge" is the smallest radius capable of producing a significant
X-ray reflection of the fluorescent iron line. For black hole accretion disks
with very sub-Eddington luminosities these and all other "inner edges" locate
at ISCO. Thus, in this case, one may rightly consider ISCO as the unique inner
edge of the black hole accretion disk. However, even for moderate luminosities,
there is no such unique inner edge as differently defined edges locate at
different places. Several of them are significantly closer to the black hole
than ISCO. The differences grow with the increasing luminosity. For nearly
Eddington luminosities, they are so huge that the notion of the inner edge
losses all practical significance.
|
1003.3887v1
|
2010-03-24
|
Broad Balmer-line Absorption in SDSS J172341.10+555340.5
|
I present the discovery of Balmer-line absorption from H alpha to H9 in iron
low-ionizaton broad absorption line (FeLoBAL) quasar, SDSS~J172341.10+555340.5
by near-infrared spectroscopy with the Cooled Infrared Spectrograph and Camera
for OHS (CISCO) attached to the Subaru telescope. The redshift of the
Balmer-line absorption troughs is 2.0530 +/- 0.0003, and it is blueshifted by
5370 km s^{-1} from the Balmer emission lines. It is more than $4000$ km s^{-1}
blueshifted from the previously known UV absorption lines. I detect relatively
strong (EW_rest=20A) [O III] emission lines which are similar to those found in
other broad absorption line quasars with Balmer-line absorption. I derived a
column density of neutral hydrogen of 5.2x10^{17} cm^{-2} by using the curve of
growth and taking account of Ly alpha trapping. I searched for UV absorption
lines which have the same redshift with Balmer-line absorption. I found Al II}
and Fe III absorption lines at z=2.053 which correspond to previously
unidentified absorption lines, and the presence of other blended troughs that
were difficult to identify.
|
1003.4622v2
|
2010-03-26
|
Evolution of the optical spectrum with doping in iron pnictides Ba(Fe1-xCox)2As2
|
We investigated the optical spectrum of Ba(Fe1-xCox)2As2 single crystals with
various doping levels. It is found that the low-energy optical conductivity
spectrum of this system can be decomposed into two components: a sharp Drude
term and a broad "incoherent" term. For the compounds showing magnetic order, a
gap appears predominantly in the "incoherent" component, while an s-wave like
superconducting gap opens in both components for highly doped compounds. The
Drude weight steadily increases as doping proceeds, consistent with electron
doping in this system. On the other hand, the "incoherent" spectral weight is
almost doping independent, but its spectral feature is intimately connected
with the magnetism. We demonstrate that the presence of two distinct components
in the optical spectrum well explains the doping and temperature dependences of
the dc resistivity.
|
1003.5038v1
|
2010-03-27
|
Weak-Coupling Theory for Multiband Superconductivity Induced by Jahn-Teller Phonons
|
Emergence of superconductivity in a two-band system coupled with breathing
and Jahn-Teller phonons is discussed in a weak-coupling limit. With the use of
a standard quantum mechanical procedure, the phonon-mediated attraction is
derived. From the analysis of the model including such attraction, a BCS-like
formula for a superconducting transition temperature $T_{\rm c}$ is obtained.
When only the breathing phonon is considered, $T_{\rm c}$ is the same as that
of the one-band model. On the other hand, when Jahn-Teller phonons are active,
$T_{\rm c}$ is significantly enhanced by the interband attraction even within
the weak-coupling limit. Relevance of the present result to actual materials
such as iron pnictides is briefly commented.
|
1003.5274v1
|
2010-03-30
|
Influence of magnetism on the superconducting properties of iron-based superconductor NdFeAsO0.88F0.12
|
Both DC and AC magnetization measurements were performed on the
NdFeAsO0.88F0.12 superconductor to investigate the influence of magnetism on
the superconducting properties of this system. The crossover of the ZFC and FC
magnetic susceptibility curves under 7.5KOe was observed. The imaginary
component of the first harmonics of the AC magnetic susceptibility, increases
with the increasing DC field below 10K and shows frequency dependency under
7.5KOe at low temperature. The paramagnetism of Nd3+ ions tilts the magnetic
hysteresis loops and broadens the hysteresis width. After correction for the
paramagnetism, the field and temperature dependence of intrinsic Jcm was
obtained and compared with the experimentally obtained total Jcm. The origin of
the abnormal behavior of magnetization was investigated and attributed to a
magnetic background, which was speculated to be caused by the spin-glass state.
However, this magnetic background does not affect the flux pinning properties
in this sample. The related mechanism was discussed.
|
1003.5807v2
|
2010-03-31
|
Evidence for superconducting gap nodes in the zone-centered hole bands of KFe2As2 from magnetic penetration-depth measurements
|
Among the iron-based pnictide superconductors the material KFe$_2$As$_2$ is
unusual in that its Fermi surface does not consist of quasi-nested electron and
hole pockets. Here we report measurements of the temperature dependent London
penetration depth of very clean crystals of this compound with residual
resistivity ratio $>1200$. We show that the superfluid density at low
temperatures exhibits a strong linear-in-temperature dependence which implies
that there are line nodes in the energy gap on the large zone-centered hole
sheets. The results indicate that KFe$_2$As$_2$ is an unconventional
superconductor with strong electron correlations.
|
1003.6022v2
|
2010-04-02
|
Performance of the MIND detector at a Neutrino Factory using realistic muon reconstruction
|
A Neutrino Factory producing an intense beam composed of nu_e(nubar_e) and
nubar_mu(nu_mu) from muon decays has been shown to have the greatest
sensitivity to the two currently unmeasured neutrino mixing parameters,
theta_13 and delta_CP . Using the `wrong-sign muon' signal to measure nu_e to
nu_mu(nubar_e to nubar_mu) oscillations in a 50 ktonne Magnetised Iron Neutrino
Detector (MIND) sensitivity to delta_CP could be maintained down to small
values of theta_13. However, the detector efficiencies used in previous studies
were calculated assuming perfect pattern recognition. In this paper, MIND is
re-assessed taking into account, for the first time, a realistic pattern
recognition for the muon candidate. Reoptimisation of the analysis utilises a
combination of methods, including a multivariate analysis similar to the one
used in MINOS, to maintain high efficiency while suppressing backgrounds,
ensuring that the signal selection efficiency and the background levels are
comparable or better than the ones in previous analyses.
|
1004.0358v1
|
2010-04-09
|
No anomalous spin fluctuation in optimally doped and overdoped iron-based oxypnictide superconductors LaFeAsO$_{1-x}$F$_x$ probed by muon spin relaxation
|
Zero field and longitudinal field muon spin relaxation measurements have been
performed in optimally doped and overdoped superconductors LaFeAsO$_{1-x}$F$_x$
in order to investigate the magnetic fluctuation over a wide range of
temperature and longitudinal field. We have observed no sign of magnetic
fluctuation against temperature in the muons' time window (10$^{-5}\sim$
10$^{-9}$s). Considering the current results and other results, i.e., spin
fluctuation observed by neutron scattering, pseudogap-like behaviors by NMR and
photoemission spectroscopy, it is suggested that not only the spin fluctuation
but also the multiband character with several different orbital contributions
at the Fermi surface may play an important role in the superconducting pairing
mechanism of LaFeAsO$_{1-x}$F$_x$.
|
1004.1589v1
|
2010-04-12
|
Density of Phonon States in Superconducting FeSe as a Function of Temperature and Pressure
|
The temperature and pressure dependence of the partial density of phonon
states of iron atoms in superconducting Fe1.01Se was studied by 57Fe nuclear
inelastic scattering (NIS). The high energy resolution allows for a detailed
observation of spectral properties. A sharpening of the optical phonon modes
and shift of all spectral features towards higher energies by ~4% with
decreasing temperature from 296 K to 10 K was found. However, no detectable
change at the tetragonal - orthorhombic phase transition around 100 K was
observed. Application of a pressure of 6.7 GPa, connected with an increase of
the superconducting temperature from 8 K to 34 K, results in an increase of the
optical phonon mode energies at 296 K by ~12%, and an even more pronounced
increase for the lowest-lying transversal acoustic mode. Despite these strong
pressure-induced modifications of the phonon-DOS we conclude that the
pronounced increase of Tc in Fe1.01Se with pressure cannot be described in the
framework of classical electron-phonon coupling. This result suggests the
importance of spin fluctuations to the observed superconductivity.
|
1004.2007v1
|
2010-04-13
|
Dissipationless Anomalous Hall Current in $Fe_{100-x}(SiO_2)_x$ Films
|
The observation of dissipationless anomalous Hall current is one of the
experimental evidences to confirm the intrinsic origin of anomalous Hall
effect. To study the origin of anomalous Hall effect in iron,
Fe$_{100-x}$(SiO$_{2}$)$_{x}$ granular films with volume fraction of SiO$_{2}$
0\le x \le 40.51 were fabricated using co-sputtering. Hall and longitudinal
resistivities were measured in the temperature range 5 to 350 K with magnetic
fields up to 5 Tesla. As x increased from 0 to 40.51, the anomalous Hall
resistivity and longitudinal resistivity increased about 4 and 3 orders in
magnitude, respectively. Analysis of the results revealed that the normalized
anomalous Hall conductivity is a constant for all the samples, the evidence of
dissipationless anomalous Hall current in Fe.
|
1004.2105v1
|
2010-04-13
|
Heat treatment effects on the superconducting properties of Ag-doped SrKFeAs compound
|
The superconducting properties of polycrystalline Sr0.6K0.4Fe2As2 were
strongly influenced by Ag doping (Supercond. Sci. Technol. 23 (2010) 025027).
Ag addition is mainly dominated by silver diffusing, so the annealing process
is one of the essential factors to achieve high quality Ag doped
Sr0.6K0.4Fe2As2. In this paper, the optimal annealing conditions were studied
for Ag doped Sr0.6K0.4Fe2As2 bulks prepared by a one-step solid reaction
method. It is found that the annealing temperature has a strong influence on
the superconducting properties, especially on the critical current density Jc.
As a result, higher heat treatment temperature (~900C) is helpful in diffusing
Ag and reducing the impurity phase gathered together to improve the grain
connectivity. In contrast, low-temperature sintering is counterproductive for
Ag doped samples. These results clearly suggest that annealing at ~900C is
necessary for obtaining high Jc Ag-doped samples.
|
1004.2137v2
|
2010-04-17
|
Structural and superconducting properties in LaFeAs1-xSbxO1-yFy
|
We report the antimony (Sb) doping effect in a prototype system of iron-based
supercon-ductors LaFeAsO1-yFy (y=0, 0.1, 0.15). X-ray powder diffraction
indicates that the lattice pa-rameters increase with Sb content within the
doping limit. Rietveld structural refinements show that, with the partial
substitution of Sb for As, while the thickness of the Fe2As2 layers increases
significantly, that of the La2O2 layers shrinks simultaneously. So a negative
chemical pressure is indeed "applied" to the superconducting-active Fe2As2
layers, in con-trast to the effect of positive chemical pressure by the
phosphorus doping. Electrical resis-tance and magnetic susceptibility
measurements indicate that, while the Sb doping hardly influences the SDW
anomaly in LaFeAsO, it recovers SDW order for the optimally-doped sample of
y=0.1. In the meantime, the superconducting transition temperature can be
raised up to 30 K in LaFeAs1-xSbxO1-yFy with x=0.1 and y=0.15. The Sb doping
effects are discussed in term of both J1-J2 model and Fermi Surface (FS)
nesting scenario.
|
1004.2940v1
|
2010-04-21
|
Precise determination of muon and electromagnetic shower contents from shower universality property
|
We consider two new aspects of Extensive Air Shower development universality
allowing to make accurate estimation of muon and electromagnetic (EM) shower
contents in two independent ways. In the first case, to get muon (or EM) signal
in water Cherenkov tanks or in scintillator detectors it is enough to know the
vertical depth of shower maximum and the total signal in the ground detector.
In the second case, the EM signal can be calculated from the primary particle
energy and the zenith angle. In both cases the parametrizations of muon and EM
signals are almost independent on primary particle nature, energy and zenith
angle. Implications of the considered properties for mass composition and
hadronic interaction studies are briefly discussed. The present study is
performed on 28000 of proton, oxygen and iron showers, generated with CORSIKA
6.735 for $E^{-1}$ spectrum in the energy range log(E/eV)=18.5-20.0 and
uniformly distributed in cos^2(theta) in zenith angle interval theta=0-65
degrees for QGSJET II/Fluka interaction models.
|
1004.3673v1
|
2010-04-21
|
A minimal Beta Beam with high-Q ions to address CP violation in the leptonic sector
|
In this paper we consider a Beta Beam setup that tries to leverage at most
existing European facilities: i.e. a setup that takes advantage of facilities
at CERN to boost high-Q ions (8Li and 8B) aiming at a far detector located at L
= 732 Km in the Gran Sasso Underground Laboratory. The average neutrino energy
for 8Li and 8B ions boosted at \gamma ~ 100 is in the range E_\nu = [1,2] GeV,
high enough to use a large iron detector of the MINOS type at the far site. We
perform, then, a study of the neutrino and antineutrino fluxes needed to
measure a CP-violating phase delta in a significant part of the parameter
space. In particular, for theta_13 > 3 deg, if an antineutrino flux of 3 10^19
useful 8Li decays per year is achievable, we find that delta can be measured in
60% of the parameter space with 6 10^18 useful 8B decays per year.
|
1004.3773v2
|
2010-04-27
|
Orbital-weight redistribution triggered by spin order in the pnictides
|
The one-particle spectral function and its orbital composition are
investigated in a three-orbital model for the undoped parent compounds of the
iron-based superconductors. In the realistic parameter regime, where results
best fit experimental data, it is observed that the magnetization in the xz and
yz orbitals are markedly different and the Fermi surface presents mostly xz
character, as recently observed in photoemission experiments [T. Shimojima et
al., Phys. Rev. Lett. 104, 057002 (2010)]. Since the ferro-orbital order in
this regime is at most a few percent, these results are mainly driven by the
magnetic order. An analogous analysis for a five-orbital model leads to similar
conclusions.
|
1004.4803v2
|
2010-05-03
|
Collective spin mode in a multi-component system of coupled itinerant and localized electrons
|
We study collective spin excitations of a magnetically ordered state in a
multi-component system composed of both itinerant electrons and local moments.
Here the induced spin-density-wave (SDW) ordering of itinerant electrons and
the collinear antiferromangetic (AF) ordering of local moments are locked
together via a Hund's rule coupling. We show that the Goldstone theorem still
holds at the RPA level with the gapless spin wave protected inside the small
SDW gap of itinerant electrons, which, however, is fragile in the presence of
ion-anisotropy. A gapped out-of-phase spin mode extending over a much wider
energy scale above the SDW gap is found to be more robust against the
ion-anisotropy, which is mainly contributed by the local moment fluctuations.
While the scattering between the Goldstone mode and itinerant electrons
diminishes within the SDW gap, the out-of-phase mode will strongly interact
with itinerant electrons and thus dominate the spin and charge dynamics in such
an ordered phase. Possible relevance of such a model to the iron-pnictides will
be also discussed.
|
1005.0421v1
|
2010-05-04
|
Competition between Ferrimagnetism and Magnetic Frustration in Zinc Substituted YBaFe4O7
|
The substitution of zinc for iron in YBaFe4O7 has allowed the oxide series
YBaFe4-xZnxO7, with 0.40 < x < 1.50, belonging to the "114" structural family
to be synthesized. These oxides crystallize in the hexagonal symmetry (P63mc),
as opposed to the cubic symmetry (F-43m) of YBaFe4O7. Importantly, the d.c.
magnetization shows that the zinc substitution induces ferrimagnetism, in
contrast to the spin glass behaviour of YBaFe4O7. Moreover, a.c. susceptibility
measurements demonstrate that concomitantly these oxides exhibit a spin glass
or a cluster glass behaviour, which increases at the expense of ferrimagnetism,
as the zinc content is increased. This competition between ferrimagnetism and
magnetic frustration is interpreted in terms of lifting of the geometric
frustration, inducing the magnetic ordering, and of cationic disordering, which
favours the glassy state.
|
1005.0480v1
|
2010-05-04
|
Magnetic ordering and dense Kondo behaviour in EuFe$_{2}$P$_{2}$
|
Ternary iron phosphide EuFe$_2$P$_2$ with ThCr$_2$Si$_2$-type structure has
been systematically studied by the measurements of crystal structure,
magnetization, M\"{o}ssbauer effect, transport properties and specific heat.
The structural refinement result confirms no direct P-P covalent bonding. The
M\"{o}ssbauer spectra indicate no magnetic moment for the Fe atoms and, that
the Eu ions are divalent in the whole temperatures, carrying local moments of
$S$=7/2. The Eu$^{2+}$ spins order ferromagnetically at $T_C$=29.5 K, followed
by a possible helimagnetic ordering below $T_{HM}$=26 K, where the Eu$^{2+}$
moments tilt a little from the c-axis. External magnetic field increases $T_C$
gradually, but suppresses $T_{HM}$ rapidly. (Magneto)resistivity data indicate
characteristic dense Kondo behaviour above the Curie temperature. The result is
discussed in terms of the interplay between intersite RKKY and intrasite Kondo
interactions.
|
1005.0516v2
|
2010-05-06
|
Chemical composition of the old globular clusters NGC 1786, NGC 2210 and NGC 2257 in the Large Magellanic Cloud
|
This paper presents the chemical abundance analysis of a sample of 18 giant
stars in 3 old globular clusters in the Large Magellanic Cloud, namely NGC
1786, NGC 2210 and NGC 2257. The derived iron content is [Fe/H]= --1.75+-0.01
dex (sigma= 0.02 dex), --1.65+-0.02 dex (sigma= 0.04 dex) and --1.95+-0.02 dex
(sigma= 0.04 dex) for NGC 1786, NGC 2210 and NGC 2257, respectively. All the
clusters exhibit similar abundance ratios, with enhanced values (+0.30 dex) of
[alpha/Fe], consistent with the Galactic Halo stars, thus indicating that these
clusters have formed from a gas enriched by Type II SNe. We also found evidence
that r-process are the main channel of production of the measured neutron
capture elements (Y, Ba, La, Nd, Ce and Eu). In particular the quite large
enhancement of [Eu/Fe] (+0.70 dex) found in these old clusters clearly
indicates a relevant efficiency of the r-process mechanism in the LMC
environment.
|
1005.0964v1
|
2010-05-10
|
23Na and 75As NMR Studies on NaFeAs0.8P0.2 Single Crystals
|
We report our $^{23}$Na and $^{75}$As NMR studies on isovalent
phosphorus-doped NaFeAs$_{0.8}$P$_{0.2}$ ($T_c=$ 33 K) single crystals. Our
data suggest a dramatic enhancement of the electron density of states on the
Fermi surface, and a large residual electron density of state below $T_c$.
However, evidence of antiferromagnetic spin fluctuations is shown by the
spin-lattice relaxation rate. The penetration depth, revealed from $^{23}$Na
NMR linewidth, follows the universal Uemura relation in iron pnictides. These
observations suggest that the correlation effects are still important for the
superconductivity in the phosphorus doped sample, although the Fermi surface is
probably significantly changed upon doping.
|
1005.1448v2
|
2010-05-12
|
Transfer of optical spectral weight in magnetically ordered superconductors
|
We show that, in antiferromagnetic superconductors, the optical spectral
weight transferred to low frequencies below the superconducting transition
temperature originates from energies that can be much larger than twice the
superconducting gap $\Delta$. This contrasts to non-magnetic superconductors,
where the optical spectrum is suppressed only for frequencies below $2\Delta$.
In particular, we demonstrate that the superfluid condensate of the
magnetically ordered superconductor is not only due to states of the
magnetically reconstructed Fermi surface, but is enhanced by transfer of
spectral weight from the mid infrared peak generated by the spin density wave
gap. We apply our results to the iron arsenide superconductors, addressing the
decrease of the zero-temperature superfluid density in the doping regime where
magnetism coexists with unconventional superconductivity.
|
1005.2174v3
|
2010-05-17
|
Cooperative effects of Coulomb and electron-phonon interactions in the two-dimensional 16-band d-p model for iron-based superconductors
|
We study the electronic states and the superconductivity in the
two-dimensional 16-band d-p model coupled with A1g, B1g and Eg local phonons
and obtain the rich phase diagram including the magnetic, charge and orbital
ordered phases on the parameter plane of the Coulomb and electron-phonon
interactions. When the electron-phonon interaction is dominant, the charge
fluctuations induce the s++ wave superconductivity, while when the Coulomb
interaction is dominant, the magnetic fluctuations induce the s+- wave
superconductivity. Remarkably, the orbital fluctuations are enhanced due to the
cooperative effects of the Coulomb and electron-phonon interactions and induce
the s++ wave and the nodal s+- wave superconductivities.
|
1005.2814v2
|
2010-05-17
|
Electron-Hole Asymmetry in Superconductivity of Pnictides Originated from the Observed Rigid Chemical Potential Shift
|
We have performed a systematic photoemission study of the chemical potential
shift as a function of carrier doping in a pnictide system based on
BaFe$_2$As$_2$. The experimentally determined chemical potential shift is
consistent with the prediction of a rigid band shift picture by the
renormalized first-principle band calculations. This leads to an electron-hole
asymmetry (EHA) in the Fermi surface (FS) nesting condition due to different
effective masses for different FS sheets, which can be calculated from the
Lindhard function of susceptibility. This built-in EHA from the band structure,
which matches well with observed asymmetric superconducting domes in the phase
diagram, strongly supports FS near-nesting driven superconductivity in the iron
pnictides.
|
1005.2966v1
|
2010-05-18
|
Superconductivity induced by doping Rh in CaFe2-xRhxAs2
|
In this paper we report the synthesis of iron-based superconductors
CaFe2-xRhxAs2 using one-step solid state reaction method, which crystallizes in
the ThCr2Si2-type structure with a space group I4/mmm. The systematic evolution
of the lattice constants demonstrates that the Fe ions are successfully
replaced by the Rh. By increasing the doping content of Rh, the
spin-density-wave (SDW) transition in the parent compound is suppressed and
superconductivity emerges. The maximum superconducting transition temperature
is found at 18.5 K with the doping level of x = 0.15. The temperature
dependence of DC magnetization confirms superconducting transitions at around
15 K. The general phase diagram was obtained and found to be similar to the
case of Rh-doping Sr122 system. Our results explicitly demonstrate the
feasibility of inducing superconductivity in Ca122 compounds by higher
d-orbital electrons doping, however, different Rh-doping effect between FeAs122
compounds and FeAs1111 systems still remains an open question.
|
1005.3172v2
|
2010-05-20
|
Distinct high-T transitions in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$
|
In contrast to the simultaneous structural and magnetic first order phase
transition $T_{0}$ previously reported, our detailed investigation on an
underdoped Ba$_{0.84}$K$_{0.16}$Fe$_{2}$As$_{2}$ single crystal unambiguously
revealed that the transitions are not concomitant. The tetragonal ($\tau$:
I4/mmm) - orthorhombic ($\vartheta$: Fmmm) structural transition occurs at
$T_{S}\simeq$ 110 K, followed by an adjacent antiferromagnetic (AFM) transition
at $T_{N}\simeq$ 102 K. Hysteresis and coexistence of the $\tau$ and
$\vartheta$ phases over a finite temperature range observed in our NMR
experiments confirm the first order character of the structural transition and
provide evidence that both $T_{S}$ and $T_{N}$ are strongly correlated. Our
data also show that superconductivity (SC) develops in the $\vartheta$ phase
below $T_{c}$ = 20 K and coexists with long range AFM. This new observation,
$T_{S}\neq T_{N}$, firmly establishes another similarity between the hole-doped
BaFe$_{2}$As$_{2}$ via K substitution and the electron-doped iron-arsenide
superconductors.
|
1005.3718v1
|
2010-05-21
|
Microscopic Study of the Superconducting State of the Iron Pnictide RbFe_2As_2
|
A study of the temperature and field dependence of the penetration depth
\lambda of the superconductor RbFe_2As_2 (T_c=2.52 K) was carried out by means
of muon-spin rotation measurements. In addition to the zero temperature value
of the penetration depth \lambda(0)=267(5) nm, a determination of the upper
critical field B_c2(0)=2.6(2) T was obtained. The temperature dependence of the
superconducting carrier concentration is discussed within the framework of a
multi-gap scenario. Compared to the other "122" systems which exhibit much
higher Fermi level, a strong reduction of the large gap BCS ratio 2\Delta/k_B
T_c is observed. This is interpreted as a consequence of the absence of
interband processes. Indications of possible pair-breaking effect are also
discussed.
|
1005.3989v3
|
2010-05-23
|
Inelastic x-ray scattering investigations of lattice dynamics in SmFeAsO$_{1-x}$F$_y$ superconductors
|
We report measurements of the phonon density of states as measured with
inelastic x-ray scattering in SmFeAsO$_{1-x}$F$_y$ powders. An unexpected
strong renormalization of phonon branches around 23 meV is observed as fluorine
is substituted for oxygen. Phonon dispersion measurements on
SmFeAsO$_{1-x}$F$_y$ single crystals allow us to identify the 21 meV A$_{1g}$
in-phase (Sm,As) and the 26 meV B$_{1g}$ (Fe,O) modes to be responsible for
this renormalization, and may reveal unusual electron-phonon coupling through
the spin channel in iron-based superconductors.
|
1005.4165v1
|
2010-05-24
|
The Puzzle of High Temperature Superconductivity in Layered Iron Pnictides and Chalcogenides
|
The response of the worldwide scientific community to the discovery in 2008
of superconductivity at Tc = 26 K in the Fe-based compound LaFeAsO_{1-x}F_x has
been very enthusiastic. In short order, other Fe-based superconductors with the
same or related crystal structures were discovered with Tc up to 56 K. Many
experiments were carried out and theories formulated to try to understand the
basic properties of these new materials and the mechanism for Tc. In this
selective critical review of the experimental literature, we distill some of
this extensive body of work, and discuss relationships between different types
of experiments on these materials with reference to theoretical concepts and
models. The experimental normal-state properties are emphasized, and within
these the electronic and magnetic properties because of the likelihood of an
electronic/magnetic mechanism for superconductivity in these materials.
|
1005.4392v2
|
2010-05-26
|
RXTE observations of the 1A 1118-61 in an outburst, and the discovery of a cyclotron line
|
We present the analysis of RXTE monitoring data obtained during the January
2009 outburst of the hard X-ray transient 1A 1118-61. Using these observations
the broadband (3.5-120 keV) spectrum of the source was measured for the first
time ever. We have found that the broadband continuum spectrum of the source is
similar to other accreting pulsars and is well described by several
conventionally used phenomenological models. We have discovered that regardless
of the applied continuum model, a prominent broad absorption feature at ~55 keV
is observed. We interpret this feature as a cyclotron resonance scattering
feature (CRSF). The observed CRSF energy is one of the highest known and
corresponds to a magnetic field of B~4.8 x 10^12 G in the scattering region.
Our data also indicate the presence of an iron emission line presence that has
not been previously reported for 1A 1118-61. Timing properties of the source,
including a strong spin-up, were found to be similar to those observed by
CGRO/BATSE during the previous outburst, but the broadband capabilities of RXTE
reveal a more complicated energy dependency of the pulse-profile.
|
1005.4782v1
|
2010-05-26
|
Probing CPT violation in neutrino oscillation: A three flavor analysis
|
We have studied $CPT$ violation in neutrino oscillation considering three
flavor framework with matter effect. We have constructed a new way to find the
oscillation probability incorporating $CPT$ violating terms without any
approximation. Then $CPT$ violation with atmospheric neutrinos for a magnetized
iron calorimeter detector considering the muons (directly measurable with high
resolution) of the charge current events has been studied for zero and nonzero
$\theta_{13}$ values. It is found that a potential bound of $\delta b_{32}
\lapp 6\times 10^{-24}$ GeV at 99% CL can be obtained with 1 Mton.year exposure
of this detector; and unlike neutrino beam experiments, there is no possibility
to generate `fake' $CPT$ violation due to matter effect with atmospheric
neutrinos. The advantages of atmospheric neutrinos to discriminate $CPT$
violation from CP violation and nonstandard interactions are also discussed.
|
1005.4851v3
|
2010-05-27
|
Milne-Eddington inversion of the Fe I line pair at 630~nm
|
The iron lines at 630.15 and 630.25 nm are often used to determine the
physical conditions of the solar photosphere. A common approach is to invert
them simultaneously under the Milne-Eddington approximation. The same
thermodynamic parameters are employed for the two lines, except for their
opacities, which are assumed to have a constant ratio. We aim at investigating
the validity of this assumption, since the two lines are not exactly the same.
We use magnetohydrodynamic simulations of the quiet Sun to examine the behavior
of the ME thermodynamic parameters and their influence on the retrieval of
vector magnetic fields and flow velocities. Our analysis shows that the two
lines can be coupled and inverted simultaneously using the same thermodynamic
parameters and a constant opacity ratio. The inversion of two lines is
significantly more accurate than single-line inversions because of the larger
number of observables.
|
1005.5013v2
|
2010-05-29
|
{\it Ab initio} calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers
|
The magnetization distribution, its energetic characterization by the
interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt
multilayers are investigated using density functional theory combined with the
direct method to determine phonon frequencies. It is found that ferromagnetic
order between consecutive Fe layers is favoured, with the enhanced magnetic
moments at the interface. The bilinear and biquadratic coupling coefficients
between Fe layers are shown to saturate fast with increasing thickness of
nonmagnetic Pt layers which separate them. The phonon calculations demonstrate
a rather strong dependence of partial iron phonon densities of states on the
actual position of Fe monolayer in the multilayer structure.
|
1005.5454v1
|
2010-06-02
|
X-ray reflected spectra from accretion disk models. I. Constant density atmospheres
|
We present new models for illuminated accretion disks, their structure and
reprocessed emission. We consider the effects of incident X-rays on the surface
of an accretion disk by solving simultaneously the equations of radiative
transfer, energy balance and ionization equilibrium over a large range of
column densities. We assume plane-parallel geometry and azimuthal symmetry,
such that each calculation corresponds to a ring at a given distance from the
central object. Our models include recent and complete atomic data for K-shell
processes of the iron and oxygen isonuclear sequences. We examine the effect on
the spectrum of fluorescent K$\alpha$ line emission and absorption in the
emitted spectrum. We also explore the dependence of the spectrum on the
strength of the incident X-rays and other input parameters, and discuss the
importance of Comptonization on the emitted spectrum.
|
1006.0485v1
|
2010-06-03
|
Unusual Nernst effect and spin density wave precursors in superconducting $\rm\bf LaFeAsO_{1-x}F_x$
|
The Nernst effect has recently proven as a sensitive probe for detecting
unusual normal state properties of unconventional superconductors. Here we
present a systematic study of the Nernst effect of the iron pnictide
superconductor \laf with a particular focus on its evolution upon doping. For
the parent compound we observe a huge negative Nernst coefficient accompanied
with a severe violation of the Sondheimer cancellation in the spin density wave
(SDW) ordered state. Surprisingly, an unusual and enhanced Nernst signal is
also found at underdoping ($x=0.05$) despite the presence of bulk
superconductivity and the absence of static magnetic order, strongly suggestive
of SDW precursors at $T\lesssim150$ K. A more conventional normal state Nernst
response is observed at optimal doping ($x=0.1$) where it is rather featureless
with a more complete Sondheimer cancellation.
|
1006.0715v1
|
2010-06-04
|
Surface-driven electronic structure in LaFeAsO studied by angle resolved photoemission spectroscopy
|
We measured the electronic structure of an iron arsenic parent compound
LaFeAsO using angle resolved photoemission spectroscopy (ARPES). By comparing
with a full-potential Linear Augmented PlaneWave calculation we show that the
extra large Gamma hole pocket measured via ARPES comes from electronic
structure at the sample surface. Based on this we discuss the strong
polarization dependence of the band structure and a temperature-dependent
hole-like band around the M point. The two phenomena give additional evidences
for the existence of the surface-driven electronic structure.
|
1006.0929v1
|
2010-06-09
|
Comment on "Symmetries and Interaction Coefficients of Kelvin waves" [arXiv:1005.4575] by Lebedev and L'vov
|
We comment on the claim by Lebedev and L'vov [arXiv:1005.4575] that the
symmetry with respect to a tilt of a quantized vortex line does not yet
prohibit coupling between Kelvin waves and the large-scale slope of the line.
Ironically, the counterexample of an effective scattering vertex in the local
induction approximation (LIA) attempted by Lebedev and L'vov invalidates their
logic all by itself being a notoriously known example of how symmetries impose
stringent constraints on kelvon kinetics---not only the coupling in question
but the kinetics in general are absent within LIA. We further explain that the
mistake arises from confusing symmetry properties of a specific mathematical
representation in terms of the canonical vortex position field w(z) = x(z) +
iy(z), which explicitly breaks the tilt symmetry due to an arbitrary choice of
the z-axis, with those of the real physical system recovered in final
expressions.
|
1006.1789v2
|
2010-06-11
|
Thermal nuclear pairing within the self-consistent quasiparticle RPA
|
The self-consistent quasiparticle RPA (SCQRPA) is constructed to study the
effects of fluctuations on pairing properties in nuclei at finite temperature
and z-projection M of angular momentum. Particle-number projection (PNP) is
taken into account within the Lipkin-Nogami method. Several issues such as the
smoothing of superfluid-normal phase transition, thermally assisted pairing in
hot rotating nuclei, extraction of the nuclear pairing gap using an improved
odd-even mass difference are discussed. A novel approach of embedding the PNP
SCQRPA eigenvalues in the canonical and microcanonical ensembles is proposed
and applied to describe the recent empirical thermodynamic quantities for iron,
molybdenum, dysprosium, and ytterbium isotopes.
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1006.2201v1
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2010-06-15
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Angular resolved specific heat in iron-based superconductors: the case for nodeless extended $s$-wave gap
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We consider the variation of the field-induced component of the specific heat
$C({\bf H})$ with the direction of the applied field in $Fe-$pnictides within
quasi-classical Doppler-shift approximation, with special emphasis to recent
experiments on FeSe$_{0.4}$Te$_{0.6}$ [Zheng et al., arXiv:1004.2236]. We show
that for extended $s-$wave gap with no nodes, $C({\bf H})$ has $\cos 4 \phi$
component, where $\phi$ is the angle between ${\bf H}$ and the direction
between hole and electron Fermi surfaces. The maxima of $C({\bf H})$ are at
$\pi/4$, $3\pi/4$, etc. if the applied field is smaller than $H_0 \leq 1T$, and
at $\phi =0, \pi/2$, etc. if the applied field is larger than $H_0$. The
angle-dependence of $C({\bf H})$, the positions of the maxima, and the relative
magnitude of the oscillating component are consistent with the experiments
performed in the field of $9T >> H_0$. We show that the observed $\cos 4 \phi$
variation does not hold if the $s-$wave gap has accidental nodes along the two
electron Fermi surfaces.
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1006.3091v1
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2010-06-17
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Homogeneous Cu-Fe super saturated solid solutions prepared by severe plastic deformation
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A Cu-Fe nanocomposite containing 50 nm thick iron filaments dispersed in a
copper matrix was processed by torsion under high pressure at various strain
rates and temperatures. The resulting nanostructures were characterized by
transmission electron microscopy, atom probe tomography and M\"ossbauer
spectrometry. It is shown that alpha-Fe filaments are dissolved during severe
plastic deformation leading to the formation of a homogeneous supersaturated
solid solution of about 12 at.% Fe in fcc Cu. The dissolution rate is
proportional to the total plastic strain but is not very sensitive to the
strain rate. Similar results were found for samples processed at liquid
nitrogen temperature. APT data revealed asymmetric composition gradients
resulting from the deformation induced intermixing. On the basis of these
experimental data, the formation of the supersaturated solid solutions is
discussed
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1006.3433v1
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2010-06-17
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Competition of local-moment ferromagnetism and superconductivity in Co-substituted EuFe2As2
|
In contrast to SrFe2As2, where only the iron possesses a magnetic moment, in
EuFe2As2 an additional large, local magnetic moment is carried by Eu2+. Like
SrFe2As2, EuFe2As2 exhibits a spin-density wave transition at high
temperatures, but in addition the magnetic moments of the Eu2+ order at around
20 K. The interplay of pressure-induced superconductivity and the Eu2+ order
leads to a behavior which is reminiscent of re-entrant superconductivity as it
was observed, for example, in the ternary Chevrel phases or in the rare-earth
nickel borocarbides. Here, we study the delicate interplay of the ordering of
the Eu2+ moments and superconductivity in EuFe1.9Co0.1As2, where application of
external pressure makes it possible to sensitively tune the ratio of the
magnetic (T_C) and the superconducting (T_{c,onset}) critical temperatures. We
find that superconductivity disappears once T_C > T_{c,onset}.
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1006.3471v1
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2010-06-17
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Ultrafast transient response and electron-phonon coupling in the iron-pnictide superconductor Ba(Fe(1-x)Co(x))2As2
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The transient response of Ba(Fe(1-x)Co(x))2As2, x=0.08 was studied by
pump-probe optical reflectivity. After ultrafast photoexcitation, hot electrons
were found to relax with two different characteristic times, indicating the
presence of two distinct decay channels: a faster one, of less than 1 ps in the
considered pump fluence range, and a slower one, corresponding to lattice
thermalization and lasting ~6 ps. Our analysis indicates that the fast
relaxation should be attributed to preferential scattering of the electrons
with only a subset of the lattice vibration modes, with a second moment of the
Eliashberg function lambda(omega^2)~64 meV^2. The simultaneous excitation of a
strong fully symmetric A(1g) optical phonon corroborates this conclusion and
makes it possible to deduce the value of lambda~0.12. This small value for the
electron-phonon coupling confirms that a phonon mediated process cannot be the
only mechanism leading to the formation of superconducting pairs in this family
of pnictides.
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1006.3490v2
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2010-06-20
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Out-of-plane momentum and symmetry dependent superconducting gap in Ba0.6K0.4Fe2As2
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The three-dimensional band structure and superconducting gap of
Ba0.6K0.4Fe2As2 are studied with high-resolution angle-resolved photoemission
spectroscopy. In contrast to previous results, we have identified three
hole-like Fermi surfaces near the zone center with sizable out-of-plane or kz
dispersion. The superconducting gap on certain Fermi surface shows significant
kz-dependence. Moreover, we found that the superconducting gap sizes are
different at the same Fermi momentum for two bands with different spatial
symmetries (one odd, one even). Our results provide further information on the
rich superconducting gap structure in iron pnictides, and a distinct test for
theories.
|
1006.3936v1
|
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