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2009-08-06
|
Track fitting by Kalman Filter method for a prototype cosmic ray muon detector
|
We have developed a track fitting procedure based on Kalman Filter technique
for an Iron Calorimeter (ICAL) prototype detector when the detector is flushed
with single muon tracks. The relevant track parameters i.e., momentum,
direction and charge are reconstructed and analyzed. This paper discusses the
design of the prototype detector, its geometry simulation by Geant4, and the
detector response with the cosmic ray muons. Finally we show the resolution of
reconstructed momenta and also the charge identification efficiency of $\mu^+$
and $\mu^-$ events in the magnetized ICAL.
|
0908.0851v1
|
2009-08-11
|
Large single crystal growth of BaFe1.87Co0.13As2 using a nucleation pole
|
Co-doped iron arsenic single crystal of BaFe1.87Co0.13As2 with dimension up
to 20 x 10 x 2 mm3 were grown by a nucleation pole: an alumina stick served as
nucleation center during growth. The high quality of crystalline was
illustrated by the measurements of neutron rocking curve and X-ray diffraction
pattern. A very sharp superconducting transition temperature Tc~25 K was
revealed by both resistivity and susceptibility measurements. A nearly 100%
shielding fraction and bulk nature of the superconductivity for the single
crystal were confirmed using magnetic susceptibility data.
|
0908.1486v2
|
2009-09-07
|
Evidence of Strong Electron Correlations in Gamma-Iron
|
Single-particle excitation spectra of gamma-Fe in the paramagnetic state have
been investigated by means of the first-principles dynamical coherent potential
approximation theory which has recently been developed. It is found that the
central peak in the density of states consisting of the t2g bands is destroyed
by electron correlations, and the Mott-Hubbard type correlated bands appear.
The results indicate that the gamma-Fe can behave as correlated electrons at
high temperatures.
|
0909.1122v1
|
2009-09-07
|
The theory of photorefractive resonance for localized beams in two-carrier photorefractive systems
|
This paper extends the existing theory of two carrier photorefractivity
resonance, which is generally applied to Iron doped Indium Phosphide (InP:Fe),
to the case of low non-harmonic illumination. The space charge field profile is
computed, and the variations of its amplitude, width and position are
determined as functions of the background intensity. The effect of
photorefractive resonance on these quantities is evidenced, contributing to the
understanding of published experimental results in InP:Fe.
|
0909.1217v1
|
2009-09-16
|
Intrinsic ferromagnetic impurity phases in SmFeAsO1-xFx detected by muSR
|
We report about muSR measurements on SmFeAsO1-xFx which helped us to identify
the signature of diluted ferromagnetic inclusions, ubiquitous in the iron
pnictides. These impurities are characterized by a Curie temperature close to
room temperature and they seem responsible for a non negligible magnetic
relaxation of the implanted muons, that should not be confused with intrinsic
pnictide properties.
|
0909.3004v1
|
2009-10-06
|
Superconductivity Near a Quantum Critical Point in Ba(Fe,Co)2As2
|
We will examine the possible link between spin fluctuations and the
superconducting mechanism in the iron-based high temperature superconductor
Ba(Fe,Co)2As2 based on NMR and high pressure transport measurements.
|
0910.1071v2
|
2009-10-08
|
Resonant inelastic x-ray scattering in single-crystal superconducting PrFeAsO0.7
|
Resonant inelastic x-ray scattering (RIXS) spectra at the Fe K-edge were
measured for a single crystal of the iron oxypnictide superconductor PrFeAsO0.7
(Tc=42 K). They disclose a weak, broad feature centered around 4.5 eV energy
loss, which is slightly resonantly enhanced when the incident energy is tuned
in the vicinity of the 4p white line. We tentatively ascribe it to the
charge-transfer excitation between As 4p and Fe 3d.
|
0910.1423v1
|
2009-10-08
|
The Puzzling Frequencies of CEMP and NEMP Stars
|
We present the results of binary population simulations of carbon- and
nitrogen-enhanced metal-poor (CEMP and NEMP) stars. We show that the observed
paucity of very nitrogen-rich stars puts strong constraints on possible
modifications of the initial mass function at low metallicity.
|
0910.1489v1
|
2009-10-19
|
Fe-peak element abundances in disk and halo stars
|
At present none of Galactic chemical evolution (GCE) models provides a
self-consistent description of observed trends for all iron-peak elements with
metallicity simultaneously. The question is whether the discrepancy is due to
deficiencies of GCE models, such as stellar yields, or due to erroneous
spectroscopically-determined abundances of these elements in metal-poor stars.
The present work aims at a critical reevaluation of the abundance trends for
several odd and even-Z Fe-peak elements, which are important for understanding
explosive nucleosynthesis in supernovae.
|
0910.3689v1
|
2009-10-20
|
$α$-FeSe as an orbital-selective incoherent metal: An LDA+DMFT study
|
$\alpha$-FeSe, a prototype iron-chalcogenide superconductor, shows clear
signatures of a strange incoherent normal state. Motivated thereby, we use
LDA+DMFT to show how strong multi-band correlations generate a low-energy
pseudogap in the normal state, giving an incoherent metal in very good
semi-quantitative agreement with observations. We interpret our results in
terms of $\alpha$-FeSe being close to Mottness. A wide range of anomalous
responses in the "normal" state are consistently explained, lending strong
support for this view. Implications for superconductivity arising from such an
anomalous state are touched upon.
|
0910.3828v1
|
2009-10-21
|
Surface-angle dependence of the tunneling spectroscopy in iron-based superconductors: sign-reversing s-wave scenarios
|
We discuss the surface Andreev bound states in Fe-based superconductors with
the use of an effective five-band model and investigate the surface-angle
dependence of the tunneling spectroscopy by a quasiclassical approach for an
isotropic and an anisotropic /pm s-wave gap superconductivity. We show that
information on the normal state is important for the Andreev bound state and
its peak positions do not depend on the gap amplitude anisotropy.
|
0910.4040v1
|
2009-12-02
|
Evolution of two-gap behavior of the superconductor FeSe_1-x
|
The superfluid density, \rho_s, of the iron chalcogenide superconductor,
FeSe_1-x, was studied as a function of pressure by means of muon-spin rotation.
The zero-temperature value of \rho_s increases with increasing transition
temperature T_c (increasing pressure) following the tendency observed for
various Fe-based and cuprate superconductors. The analysis of \rho_s(T) within
the two-gap scheme reveals that the effect on both, T_c and \rho_s(0), is
entirely determined by the band(s) where the large superconducting gap
develops, while the band(s) with the small gap become practically unaffected.
|
0912.0471v1
|
2009-12-10
|
Capillary force-induced structural instability in liquid infiltrated elastic circular tubes
|
The capillary-induced structural instability of an elastic circular tube
partially filled by a liquid is studied by combining theoretical analysis and
molecular dynamics simulations. The analysis shows that, associated with the
instability, there is a well-defined length scale (elasto-capillary length),
which exhibits a scaling relationship with the characteristic length of the
tube, regardless of the interaction details. We validate this scaling
relationship for a carbon nanotube partially filled by liquid iron. The
capillary-induced structural transformation could have potential applications
for nano-devices.
|
0912.1894v1
|
2009-12-22
|
Superconductivity Induced in Iron Telluride Films by Low Temperature Oxygen Incorporation
|
We report superconductivity induced in films of the non-superconducting,
antiferromagnetic parent material FeTe by low temperature oxygen incorporation
in a reversible manner. X-ray absorption shows that oxygen doping changes the
nominal Fe valence state from 2+ in the non-superconducting state to mainly 3+
in the superconducting state. Thus superconductivity in O doped FeTe occurs in
a quite different charge and strain state than the more common
FeTe$_{1-x}$Se$_x$. This work also suggests a convenient path for conducting
doping experiments in-situ with many measurement techniques.
|
0912.4539v2
|
2010-01-07
|
Genesis of coexisting itinerant and localized electrons in Iron Pnictides
|
We show how the general features of the electronic structure of the Fe-based
high-Tc superconductors are a natural setting for a selective localization of
the conduction electrons to arise. Slave-spin and Dynamical mean-field
calculations support this picture and allow for a comparison of the magnetic
properties with experiments.
|
1001.1098v1
|
2010-01-07
|
Calculated Cleavage Behavior and Surface States of LaOFeAs
|
The layered structure of the iron based superconductors gives rise to a more
or less pronounced two-dimensionality of their electronic structure, most
pronounced in LaOFeAs. A consequence are distinct surface states to be expected
to influence any surface sensitive experimental probe. In this work a detailed
density functional analysis of the cleavage behavior and the surface electronic
structure of LaOFeAs is presented. The surface states are obtained to form
two-dimensional bands with their own Fermi surfaces markedly different from the
bulk electronic structure.
|
1001.1127v1
|
2010-01-19
|
Superconductivity at 33 K in "111" single crystals at ambient pressure
|
We have successfully grown single crystalline NaFeAs with cobalt or phosphor
doping. Bulk superconductivity occurs in NaFe$_{0.95}$Co$_{0.05}$As at 19 K
while much higher transition temperature is observed in
NaFeAs$_{0.8}$P$_{0.2}$, in which the Tc of 33 K is even higher than the
highest value realized by applying pressure in NaFeAs and all other
isoelectronic element substituted samples. We discuss these behaviours by
comparison with 122 and 1111 systems. We hope that our findings will help
improve our understanding of iron-based superconductivity.
|
1001.3311v2
|
2010-02-05
|
High resolution Compton scattering as a Probe of the Fermi surface in the Iron-based superconductor $LaO_{1-x}F_xFeAs$
|
We have carried out first principles all-electron calculations of the
(001)-projected 2D electron momentum density and the directional Compton
profiles along the [100], [001] and [110] directions in the Fe-based
superconductor LaOFeAs within the framework of the local density approximation.
We identify Fermi surface features in the 2D electron momentum density and the
directional Compton profiles, and discuss issues related to the observation of
these features via Compton scattering experiments.
|
1002.1072v1
|
2010-02-10
|
The heavily polluted atmosphere of the DAZ white dwarf GALEX J193156.8+011745
|
We report on the discovery of a new heavily polluted white dwarf. The DAZ
white dwarf GALEX J193156.8+011745 was identified in a joint GALEX/GSC survey
of ultraviolet-excess objects. Optical spectra obtained at ESO NTT show strong
absorption lines of magnesium and silicon and a detailed abundance analysis
based on VLT-Kueyen UVES spectra reveal super-solar abundances of silicon and
magnesium, and near-solar abundances of oxygen, calcium, and iron. The overall
abundance pattern bears the signature of ongoing accretion onto the white dwarf
atmosphere. The infrared spectral energy distribution shows an excess in the H
and K bands likely associated with the accretion source.
|
1002.2069v1
|
2010-02-12
|
Iron isotope effect on the superconducting transition temperature and the crystal structure of FeSe_1-x
|
The Fe isotope effect (Fe-IE) on the transition temperature T_c and the
crystal structure was studied in the Fe chalcogenide superconductor FeSe_1-x by
means of magnetization and neutron powder diffraction (NPD). The substitution
of natural Fe (containing \simeq 92% of ^{56}Fe) by its lighter ^{54}Fe isotope
leads to a shift of T_c of 0.22(5)K corresponding to an Fe-IE exponent of
\alpha_Fe=0.81(15). Simultaneously, a small structural change with isotope
substitution is observed by NDP which may contribute to the total Fe isotope
shift of T_c.
|
1002.2510v2
|
2010-02-26
|
Comment on "Nonmagnetic Impurity Resonances as a Signature of Sign-Reversal Pairing in Fe-As-Based Superconductors"
|
In a recent Letter [PRL 103, 186402 (2009)], the energy band structure of
Fe-As-based superconductors is fitted with a tight-binding model with two Fe
ions per unit cell and two degenerate xz and yz orbitals per Fe ion. The author
claims that the proposed model, which differs markedly from a model previously
used by other authors for the same two orbitals in the same compounds,
possesses the symmetry required to describe the Fe-As planes in iron-pnictide
superconductors. In this comment we argue that this is not the case.
|
1002.4939v1
|
2010-03-03
|
Temperature - pressure phase diagram of the superconducting iron pnictide LiFeP
|
Electrical-resistivity and magnetic-susceptibility measurements under
hydrostatic pressure up to p = 2.75 GPa have been performed on superconducting
LiFeP. A broad superconducting (SC) region exists in the temperature - pressure
(T-p) phase diagram. No indications for a spin-density-wave transition have
been found, but an enhanced resistivity coefficient at low pressures hints at
the presence of magnetic fluctuations. Our results show that the
superconducting state in LiFeP is more robust than in the isostructural and
isoelectronic LiFeAs. We suggest that this finding is related to the nearly
regular [FeP_4] tetrahedron in LiFeP.
|
1003.0785v1
|
2010-03-05
|
Theoretical evidence for strong correlations and incoherent metallic state in FeSe
|
The role of electronic Coulomb correlations in iron-based superconductors is
an important open question. We provide theoretical evidence for strong
correlation effects in the FeSe compound, based on dynamical mean field
calculations. A lower Hubbard band is found in the spectral properties.
Moreover, together with significant orbital-dependent mass enhancements, we
find that the normal state is a bad metal over an extended temperature range,
implying a non-Fermi liquid. Predictions for angle-resolved photoemission
spectroscopy are made.
|
1003.1286v2
|
2010-03-09
|
Implementation of the SU(2) Hamiltonian Symmetry for the DMRG Algorithm
|
In the Density Matrix Renormalization Group (DMRG) algorithm, Hamiltonian
symmetries play an important role. Using symmetries, the matrix representation
of the Hamiltonian can be blocked. Diagonalizing each matrix block is more
efficient than diagonalizing the original matrix. This paper explains how the
the DMRG++ code has been extended to handle the non-local SU(2) symmetry in a
model independent way. Improvements in CPU times compared to runs with only
local symmetries are discussed for the one-orbital Hubbard model, and for a
two-orbital Hubbard model for iron-based superconductors. The computational
bottleneck of the algorithm and the use of shared memory parallelization are
also addressed.
|
1003.1919v1
|
2010-03-19
|
Non-equilibrium current and noise in inelastic tunneling through a magnetic atom
|
In a recent experiment, Hirjibehedin {\it et al.} [Science {\bf 317}, 1199
(2007)] performed inelastic tunneling spectroscopy of a single iron atom
absorbed on a nonmagnetic substrate. The observed steps in the differential
conductance marked the spin excitation energies. In this Letter, we explain
observed nonmonotonicities in the differential conductance by a nonequilibrium
population of the atom spin states. Furthermore, we predict super-Poissonian
current noise due to this nonequilibrium situation. By introducing an
anisotropic relaxation channel we are also able to explain the remarkable
absence of nonequilibrium features at certain conductance steps.
|
1003.3794v2
|
2010-04-05
|
Magnetic vortex as a ground state for micron-scale antiferromagnetic samples
|
Here we consider micron-sized samples with any axisymmetric body shape and
made with a canted antiferromagnet, like hematite or iron borate. We find that
its ground state can be a magnetic vortex with a topologically non-trivial
distribution of the sublattice magnetization $\vec{l}$ and planar coreless
vortex-like structure for the net magnetization $\vec{M}$. For
antiferromagnetic samples in the vortex state, in addition to low-frequency
modes, we find high-frequency modes with frequencies over the range of hundreds
of gigahertz, including a mode localized in a region of radius $\sim$ 30--40 nm
near the vortex core.
|
1004.0606v1
|
2010-05-11
|
Comment on "Nematic Electronic Structure in the "Parent" State of the Iron-Based Superconductor Ca(Fe1-xCox)2As2"
|
Chuang et al (Reports, 8th of January 2010, p. 181)1 report quasiparticle
interference (QPI) imaging that shows pronounced C2 asymmetry. They interpreted
this result as indication of an electronic nematic phase with an electron band
dispersive only along the b-axis of this orthorhombic material. We argue that
this asymmetry is consistent with the underlying long range magnetic order and
that LDA electronic structure provides a better description of the QPI images
than the 1D band structure conjectured by Chuang et al.
|
1005.1761v1
|
2010-06-09
|
Interaction of iron clusters (Fe_n; n <= 6) with a divacancy in graphene
|
In this work, we have studied the chemical and magnetic interactions of Fe_n;
n <= 6, clusters with a divacancy site in a graphene sheet by ab-initio density
functional calculations. Our results show significant chemical interactions
between the cluster and graphene. As a result, a complex distribution of
magnetic moments appear on the distorted Fe clusters in presence of graphene
and results in a lower average magnetic moments compared to the free clusters.
The presence of cluster also prevents the formation of 5-8-5 ringed structure
known to exhibit in a graphene sheet having a divacancy defect. The clusters
induce electronic states primarily of d-character near the Fermi level.
|
1006.1910v1
|
2010-06-24
|
Magnetic Properties of Ab initio Model for Iron-Based Superconductors LaFeAsO
|
By using variational Monte Carlo method, we examine an effective low-energy
model for LaFeAsO derived from an ab initio downfolding scheme. We show that
quantum and many-body fluctuations near a quantum critical point largely reduce
the antiferromagnetic (AF) ordered moment and the model not only quantitatively
reproduces the small ordered moment in LaFeAsO, but also explains the diverse
dependence on LaFePO, BaFe2As2 and FeTe. We also find that LaFeAsO is under
large orbital fluctuations, sandwiched by the AF Mott insulator and weakly
correlated metals. The orbital fluctuations and Dirac-cone dispersion hold keys
for the diverse magnetic properties.
|
1006.4812v1
|
2010-07-09
|
UHECRs from magnetic reconnection in relativistic jets
|
Ultra-high energy cosmic rays (UHECRs) may be produced in active galactic
nuclei (AGN) or gamma-ray burst (GRB) jets. I argue that magnetic reconnection
in jets can accelerate UHECRs rather independently of physical processes in the
magnetic dissipation region. First order Fermi acceleration can efficiently
take place in the region where the unreconnected (upstream) magnetized fluid
converges into the reconnection layer. I find that protons can reach energies
up to E~10^{20} eV in GRB and powerful AGN jets while iron nuclei can reach
similar energies in AGN jets of more moderate luminosity.
|
1007.1522v2
|
2010-07-20
|
First-principles studies on electrical resistivity of iron under pressure
|
We investigate the temperature and pressure dependences of the electrical
resistivity for bcc and hcp Fe using the low-order variational approximation
and theoretical transport spectral functions calculated from first principles
linear response linear-muffin-tin-orbital method in the generalized-gradient
approximation. The calculated values are in close agreement with available
experimental data, and show strong increase with temperature and decrease with
pressure. We also discuss the behavior of the electrical resistivity for the
bcc -> hcp phase transition.
|
1007.3423v1
|
2010-07-26
|
Ab initio Low-Dimensional Physics Opened Up by Dimensional Downfolding: Application to LaFeAsO
|
An ab initio downfolding method is formulated to construct low-dimensional
models for correlated electrons. In addition to the band downfolding by
constrained random phase approximation formulated for 3D models, screening away
from the target layer (chain) is further involved. Eliminating the off-target
degrees of freedom, namely, dimensional downfolding yields ab initio
low-dimensional models. The method is applied to derive a 2D model for a
layered superconductor LaFeAsO, where the interlayer screening crucially makes
the effective interaction short ranged and reduces the onsite Coulomb
interactions by 10-20 \% from the 3D model for the 5 iron-3d orbitals.
|
1007.4429v1
|
2010-08-07
|
Superconductivity in SrFe2As2 with Pt Doping
|
We have synthesized polycrystalline samples of Pt-substituted SrFe2As2 and
measured the temperature dependence of magnetization and electrical
resistivity. We have observed the superconducting transition at Tc = 17 K with
the maximum shielding volume fraction at x = 0.125 in Sr(Fe1-xPtx)2As2. It is
found that the maximum Tc depends on the substituted element, so it is
important to substitute various elements to explore new iron-based
superconductors with higher Tc.
|
1008.1297v1
|
2010-08-16
|
Quasiparticle interference of C2-symmetric surface states in LaOFeAs parent compound
|
We present scanning tunneling microscopy studies of the LaOFeAs parent
compound of iron pnictide superconductors. Topographic imaging reveals two
types of atomically flat surfaces, corresponding to the exposed LaO layer and
FeAs layer respectively. On one type of surface, we observe strong standing
wave patterns induced by quasiparticle interference of two-dimensional surface
states. The distribution of scattering wavevectors exhibits pronounced two-fold
symmetry, consistent with the nematic electronic structure found in the
Ca(Fe1-xCox)2As2 parent state.
|
1008.2642v1
|
2010-08-23
|
Vortex states in hole-doped iron-pnictide superconductors
|
Based on a phenomenological model with competing spin-density-wave (SDW) and
extended $s-$wave superconductivity, the vortex states in
Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ are investigated by solving Bogoliubov-de
Gennes equations. Our result for the optimally doped compound without induced
SDW is in qualitative agreement with recent scanning tunneling microscopy
experiment. We also propose that the main effect of the SDW on the vortex
states is to reduce the intensity of the in-gap peak in the local density of
states and transfer the spectral weight to form additional peaks outside the
gap.
|
1008.3885v1
|
2010-08-31
|
Proximity of Iron Pnictide Superconductors to a Quantum Tricritical Point
|
We determine the nature of the magnetic quantum critical point in the doped
LaFeAsO using a set of constrained density functional calculations that provide
ab initio coefficients for a Landau order parameter analysis. The system turns
out to be remarkably close to a quantum tricritical point, where the nature of
the phase transition changes from first to second order. We compare with the
effective field theory and discuss the experimental consequences.
|
1009.0009v1
|
2010-09-16
|
Consistency of neutrino DIS and the present parton distribution functions
|
We study the nuclear effects in the neutrino\anti-neutrino-nucleon deep
inelastic scattering (DIS) by comparing the NuTeV, CDHSW, and CHORUS
cross-sections from Iron and Lead targets to the predictions derived from the
latest parton distribution functions (PDFs). The nuclear modifications found
seem to display agreement with those in charged lepton DIS. Our study thus
lends support to the consistency of employing neutrino data in global fits of
PDFs.
|
1009.3143v1
|
2010-09-26
|
Spin dependent electron-phonon interaction in SmFeAsO by low-temperature Raman spectroscopy
|
The interplay between spin dynamics and lattice vibration has been suggested
as an important part of the puzzle of high-temperature superconductivity. Here
we report the strong interaction between spin fluctuation and phonon in
SmFeAsO, a parent compound of the iron arsenide family of superconductors,
revealed by low-temperature Raman spectroscopy. Anomalous zone-boundary-phonon
Raman scattering from spin superstructure was observed at temperatures below
the antiferromagnetic ordering point, which offers compelling evidence on spin
dependent electron-phonon coupling in pnictides.
|
1009.5050v2
|
2010-10-06
|
The Relation Between Charged Particles and Muons With Threshold Energy 1 GeV in Extensive Air Showers Registered at the Yakutsk EAS Array
|
Characteristics of the muon component in EAS are analyzed together with their
fluctuations. The aim of this analysis -- a comparison of experimental data
with computational results obtained within frameworks of various hadron
interaction models for protons and iron nuclei and an estimation of cosmic ray
mass composition in the ultra-high energy region.
|
1010.1182v1
|
2010-10-19
|
Coexistence of Superconductivity and Charge Density Wave in SrPt2As2
|
SrPt2As2 is a novel arsenide superconductor, which crystallizes in the
CaBe2Ge2-type structure as a different polymorphic form of the ThCr2Si2-type
structure. SrPt2As2 exhibits a charge-density-wave (CDW) ordering at about 470
K and enters into a superconducting state at Tc = 5.2 K. The coexistence of
superconductivity and CDW refers to Peierls instability with a moderately
strong electron-phonon interaction. Thus SrPt2As2 can be viewed as a
nonmagnetic analog of iron-based superconductors, such as doped BaFe2As2, in
which superconductivity emerges in close proximity to spin-density-wave
ordering.
|
1010.3950v2
|
2010-11-03
|
The role of correlations in the high-pressure phase of FeSe
|
We present a systematic study of the high-pressure FeSe phase performed by
means of the first-principle electronic structure calculations. Basing on
available experimental information about the unit cell geometry we calculate
the band structure and characterize the related properties during their
pressure driven evolution. The electronic structure including the hybrid
functional B3LYP or the Hubbard parameter U for the iron d states lead to the
correct semiconducting ground state for the hexagonal stoichiometric FeSe
within the broad pressure range (up to 30 GPa).
|
1011.0848v1
|
2010-11-11
|
Half-metallic Antiferromagnet Sheets in Sr4M2O6CrFeAs2 (M=Sc, Cr) and Their Bulk Form
|
We reveal by first-principles calculations that in iron pnictides
Sr4M2O6CrFeAs2 (M=Sc, Cr) the two-dimensional CrFeAs2 layers exhibit a robust
band structure of half-metallic antiferromagnet (HMAFM). Due to the thick
blocking layer, the interlayer coupling is vanishingly small and thus the
conductive channels in individual layers may take the alternative spin
direction randomly. We show that, since the spin magnetizations of Fe and Cr
are different in a ferromagnetic state, applying a strong magnetic field and
ramping it down gradually can align the spin direction of conductive channel of
all HMAFM layers, which restores the bulk HMAFM.
|
1011.2576v2
|
2010-11-21
|
Magnetoelectric studies on polycrystalline FeVO4
|
We report the magnetic, dielectric and ferroelectric properties of
polycrystalline iron vanadate(FeVO4), which has been recently found to exhibit
multiferroicity in low temperature with noncollinear magnetic orderings. The
influence of external magnetic field up to 9T on these properties is
systematically investigated. Besides the suppressing effect on the original
ferroelectric transition, the strong magnetic field seems to induce a secondary
ferroelectric transition at a slight lower temperature. And the corresponding
magnetization measurement reveals a field-induced metamagnetic transition at
low temperatures. Our results will help to clarify the complex magnetic
structure and microscopic mechanism of multiferroicity in FeVO4.
|
1011.4677v2
|
2010-11-30
|
Enhanced superconducting properties in FeCr$_x$Se
|
We report an enhancement of superconducting transition temperature ($T_{\rm
c}$) when Chromium (Cr) is substituted in excess at the Iron (Fe) site
(FeCr$_x$Se, $x=$0.01, 0.02 and 0.03). There is a corresponding increase in the
superconducting volume fraction with $T_{\rm c}$ attaining a value of 11.2 K on
2 $%$ Cr substitution when compared to a $T_{\rm c}$ of 8.5 K for the
conventional tetragonal Fe-excess sample Fe$_{1+x}$Se. The results point to the
role of chemical pressure (introduced via ionic size variation at the Fe site
upon Cr substitution in excess) on superconducting properties.
|
1011.6519v1
|
2010-12-13
|
Abundance analysis of DAZ white dwarfs
|
We present an abundance analysis of a sample of 33 hydrogen-rich (DA) white
dwarfs. We have used archival high-resolution spectra to measure abundances of
calcium, magnesium and iron in a set of 30 objects. In addition, we present
preliminary calcium abundances in three new white dwarfs based on
low-dispersion spectra. We investigate some abundance ratios (Mg/Ca, Fe/Ca)
that may help uncover the composition of the accretion source.
|
1012.2639v1
|
2010-12-14
|
Superconductivity in the iron selenide KxFe2Se2 (0 <= x <= 1)
|
We report the superconductivity at above 30 K in a new FeSe-layer compound
K0.8Fe2Se2 (nominal composition) achieved by metal K intercalating in between
FeSe layers. It is isostructural to BaFe2As2 and possesses the highest Tc for
FeSe-layer materials so far under ambient pressure. Hall effect indicates the
carriers are dominated by electron in this superconductor. We confirm that the
observed superconductivity at above 30 K is due to this new FeSe-based 122
phase. Our results demonstrate that FeSe-layer materials are really remarkable
superconductors via structure and carrier modulation.
|
1012.2924v1
|
2011-01-05
|
Variety of c-axis collective excitations in layered multigap superconductors
|
We present a dynamical theory for the phase differences along a stacked
direction of intrinsic Josephson junctions (IJJ's) in layered multigap
superconductors, motivated by the discovery of highly-anisotropic iron-based
superconductors with thick perovskite-type blocking layers. The dynamical
equations describing AC and DC intrinsic Josephson effects peculiar to multigap
IJJ's are derived, and collective Leggett mode excitations in addition to the
Josephson plasma established in single-gap IJJ's are predicted. The dispersion
relations of their collective modes are explicitly displayed, and the
remarkable peculiarity of the Leggett mode is demonstrated.
|
1101.0886v2
|
2011-01-17
|
Upper critical field of the 122-type iron pnictide superconductors
|
The upper critical fields ($H_{c2}$) of the single crystals
$\rm(Sr,Na)Fe_2As_2$ and $\rm Ba_{0.55}K_{0.45}Fe_2As_2$ were determined by
means of measuring the electrical resistivity, $ \rho_{xx}(\mu_0H)$, using the
facilities of pulsed magnetic field at Los Alamos. In general, these compounds
possess a very large upper critical field ($H_{c2}(0)$) with a weak anisotropic
effect. The detailed curvature of $H_{c2}(T_c)$ may depend on the magnetic
field orientation and the sample compositions. We argue that such a difference
mainly results from the multi-band effect, which might be modified via doping.
|
1101.3150v1
|
2011-01-20
|
Ab initio Random Structure Searching
|
It is essential to know the arrangement of the atoms in a material in order
to compute and understand its properties. Searching for stable structures of
materials using first-principles electronic structure methods, such as density
functional theory (DFT), is a rapidly growing field. Here we describe our
simple, elegant and powerful approach to searching for structures with DFT
which we call ab initio random structure searching (AIRSS). Applications to
discovering structures of solids, point defects, surfaces, and clusters are
reviewed. New results for iron clusters on graphene, silicon clusters,
polymeric nitrogen, hydrogen-rich lithium hydrides, and boron are presented.
|
1101.3987v1
|
2011-02-04
|
Anisotropy in transport and magnetic properties of K0.64Fe1.44Se2
|
We report a study of anisotropy in transport and magnetic properties of
K0.64Fe1.44Se2.00 single crystals. The anisotropy in resistivity is up to one
order of magnitude between 1.8 K and 300 K. Magnetic susceptibility exhibits
weak temperature dependence in the normal state with decrease in temperature
with no significant anomalies. The lower critical fields Hc1 of
K0.64Fe1.44Se2.00 are only about 3 Oe and the anisotropy of Hc1,c/Hc1,ab is
about 1. The critical currents for H||ab and H||c are about 10-10^3 A/cm2,
smaller than in iron pnictides and in FeTe_{1-x}Se_{x} and nearly isotropic.
|
1102.1010v3
|
2011-02-05
|
Simulations of decomposition kinetics of Fe-Cr solid solutions during thermal aging
|
The decomposition of Fe-Cr solid solutions during thermal aging is modeled by
Atomistic Kinetic Monte Carlo (AKMC) simulations, using a rigid lattice
approximation with composition dependant pair interactions that can reproduce
the change of sign of the mixing energy with the alloy composition. The
interactions are fitted on ab initio mixing energies and on the experimental
phase diagram, as well as on the migration barriers in iron and chromium rich
phases. Simulated kinetics is compared with 3D atom probe and neutron
scattering experiments.
|
1102.1091v1
|
2011-02-08
|
Coupling of replicate order-parameters in incommensurate multiferroics
|
The specific properties of incommensurate multiferroic phases resulting from
the coupling of order-parameter replicates are worked out using the
illustrative example of iron vanadate. The dephasing between the
order-parameter copies induces an additional broken symmetry phase
corresponding to the lowest symmetry of the system and varies critically at the
transition to the multiferroic phase. It reflects the temperature dependence of
the angle between paired spins in the antiferromagnetic spiral structure.
Expressing the transition order-parameters in terms of spin-density waves
allows showing that isotropic exchange interactions contribute to the
stabilization of the ferroelectric phase.
|
1102.1575v1
|
2011-02-10
|
Thermoelectric studies of KxFe2-ySe2: weakly correlated superconductor
|
We report thermal transport measurement of KxFe2-ySe2 superconducting single
crystal. Significant peak anomaly in thermal conductivity is observed at nearly
TC/2 indicating a large phonon mean-free-path in the superconducting state. The
zero-temperature extrapolated thermoelectric power is smaller than the value in
typical strongly correlated superconductors, implying large normalized Fermi
temperature. In contrast to other iron superconductors, thermoelectric power in
our sample does not exhibit significant anomalies. These findings indicate that
KxFe2-ySe2 is a weakly or intermediately correlated superconductor without
significant Fermi surface nesting.
|
1102.2217v2
|
2011-02-17
|
Superconductivity on the verge of Mott localization in ternary iron sulfide
|
We report the results of electrical and magnetic properties on two new
compounds, K0.8Fe1.7S2 and K0.8Fe1.7SeS, both having similar structures to
newly discovered superconducting K0.8Fe1.7Se2. K0.8Fe1.7S2 exhibits a
semiconductor-like electrical property and undergoes an anti-ferromagnetic
transition at about 260 K. Upon replacing half of S with Se, K0.8Fe1.7SSe
becomes a superconductor at 25 K, implying the superconductivity evolves from a
Mott AFM state in Fe-Se based superconductors.
|
1102.3505v1
|
2011-02-17
|
Quenching of superconductivity by Co doping in K0.8Fe2Se2
|
We synthesized a series of K0.8Fe2-xCoxSe2 samples with nominal compositions
0\leq x\leq 0.035 and investigated their physical properties. The results show
that the superconductivity in K0.8Fe2-xCoxSe2 is quenched down to 5 K by 0.5
at. % Co doping, the fastest quenching rate ever-reported. The role played here
by Co is in contrast with the one in FeAs based superconductors where Co
usually induces superconductivity from parent compounds. Such a rapid quenching
favors a localized 3d model against the itinerant one for iron pnictide
superconductors.
|
1102.3506v1
|
2011-03-29
|
Spin waves in the block checkerboard antiferromagnetic phase
|
Motivated by the discovery of new family 122 iron-based superconductors, we
present the theoretical results on the ground state phase diagram, spin wave
and dynamic structure factor of the extended $J_{1}-J_{2}$ Heisenberg model. In
the reasonable physical parameter region of $K_{2}Fe_{4}Se_{5}$, we fi{}nd the
block checkerboard antiferromagnetic order phase is stable. There are two
acoustic branches and six optical branches spin wave in the block checkerboard
antiferromagnetic phase, which has analytic expression in the high symmetry
points. To compare the further neutron scattering experiments, we discuss the
saddlepoint structure in the magnetic excitation spectrum and calculate the
predicted inelastic neutron scattering pattern based on linear spin wave
theory.
|
1103.5521v1
|
2011-04-08
|
Materials and mechanisms of hole superconductivity
|
The theory of hole superconductivity proposes that there is a single
mechanism of superconductivity that applies to all superconducting materials.
This paper discusses several material families where superconductivity occurs
and how they can be understood within this theory. Materials discussed include
the elements, transition metal alloys, high $T_c$ cuprates both hole-doped and
electron-doped, $MgB_2$, iron pnictides, doped semiconductors, and elements
under high pressure.
|
1104.1624v2
|
2011-05-02
|
Impeding effect of cerium on the growth of helium bubble in iron
|
Our first-principles density functional theory calculations suggest that the
rare earth element, Ce, has a stronger attraction (-1.31eV) to He than He-He
(-1.18eV) in bcc Fe. Consequently, the mobile He atoms could be pinned to Ce,
and hence a reduced merging of He clusters. Moreover, we find that the
segregated Ce layer at the He bubble surface presents an energy barrier of 0.33
eV to the upcoming He atom and thus slows down the bubble growth.
|
1105.0458v1
|
2011-05-09
|
Status Update for the MINER$ν$A Experiment
|
MINER$\nu$A (Main INjEctoR $\nu$-A) is a few-GeV neutrino cross section
experiment that began taking data in the FNAL NuMI beam-line in the fall of
2009. MINER$\nu$A employs a fine-grained detector capable of complete kinematic
characterization of neutrino interactions. The detector consists of an
approximately 6.5 ton active target region composed of plastic scintillator
with additional carbon, iron, and lead targets upstream of the active region.
The experiment will provide important inputs for neutrino oscillation searches
and a pure weak probe of nuclear structure. Here we offer a set of initial
kinematic distributions of interest and provide a general status update.
|
1105.1698v1
|
2011-06-16
|
Evidence of defect-induced ferromagnetism in ZnFe$_{2}$O$_{4}$ thin films
|
X-ray absorption near-edge and grazing incidence X-ray fluorescence
spectroscopy are employed to investigate the electronic structure of
ZnFe$_{2}$O$_{4}$ thin films. The spectroscopy techniques are used to determine
the non-equilibrium cation site occupancy as a function of depth and oxygen
pressure during deposition and its effects on the magnetic properties. It is
found that low deposition pressures below 10$^{-3}$ mbar cause iron
superoccupation of tetrahedral sites without Zn$^{2+}$ inversion, resulting in
an ordered magnetic phase with high room temperature magnetic moment.
|
1106.3128v1
|
2011-06-27
|
Charge redistribution at the antiferromagnetic phase transition in SrFeAsF compound
|
The relationship between spin, electron, and crystal structure has been one
of the foremost issues in understanding the superconducting mechanism since the
discovery of iron-based high temperature superconductors. Here, we report
M\"ossbauer and first-principles calculations studies of the parent compound
SrFeAsF with the largest temperature gap ($\sim$50\,K) between the structural
and antiferromagnetic (AFM) transitions. Our results reveal that the structural
transition has little effect on the electronic structure of the compound
SrFeAsF while the development of the AFM order induces a redistribution of the
charges near the Fermi level.
|
1106.5370v2
|
2011-07-18
|
Recent Measurements of Neutrino-Nucleus Quasi-Elastic Scattering
|
We present recent measurements of neutrino charged current quasi-elastic (CC
QE) scattering, nu_mu + p -> mu- + n. Measurements of CC QE on carbon near 1
GeV by MiniBooNE and SciBooNE, as well as measurements on iron at 3 GeV by
MINOS, disagree with current interaction models, while measurements at higher
energies on carbon by NOMAD show excellent agreement with those same models.
|
1107.3400v1
|
2011-07-26
|
Nuclear structure in strong magnetic fields: nuclei in the crust of a magnetar
|
Covariant density functional theory is used to study the effect of strong
magnetic fields, up to the limit predicted for neutron stars (for magnetars $B
\approx10^{18}$G), on nuclear structure. All new terms in the equation of
motion resulting from time reversal symmetry breaking by the magnetic field and
the induced currents, as well as axial deformation, are taken into account in a
self-consistent fashion. For nuclei in the iron region of the nuclear chart it
is found that fields in the order of magnitude of $10^{17}$G significantly
affect bulk properties like masses and radii.
|
1107.5243v1
|
2011-07-29
|
Neutrino nucleus reactions at high energies within the GiBUU model
|
The GiBUU model, which implements all reaction channels relevant at medium
neutrino energy, is used to investigate the neutrino and antineutrino
scattering on iron. Results for integrated cross sections are compared with
NOMAD and MINOS data. It is shown, that final state interaction can noticeably
change the spectra of the outgoing hadrons. Predictions for the Miner$\nu$a
experiment are made for pion spectra, averaged over NuMI neutrino and
antineutrino fluxes.
|
1107.5944v1
|
2011-08-04
|
Storage-ring ionization and recombination experiments with multiply charged ions relevant to astrophysical and fusion plasmas
|
Past and ongoing research activities at the Heidelberg heavy-ion storage-ring
TSR are reviewed which aim at providing accurate absolute rate coefficients and
cross sections of atomic collision processes for applications in astrophysics
and magnetically confined fusion. In particular, dielectronic recombination and
electron impact ionization of iron ions are discussed as well as dielectronic
recombination of tungsten ions.
|
1108.1073v1
|
2011-08-09
|
Magnetic resonance from the interplay of frustration and superconductivity
|
Motivated by the iron-based superconductors, we develop a self-consistent
electronic theory for the itinerant spin excitations in the regime of
coexistence of the antiferromagnetic stripe order with wavevector ${\bf Q}_{1}
= (\pi,0)$ and $s^{+-}$ superconductivity. The onset of superconductivity leads
to the appearance of a {\em magnetic} resonance near the wavevector ${\bf
Q}_{2} = (0,\pi)$ where magnetic order is absent. This resonance is isotropic
in spin space, unlike the excitations near ${\bf Q}_{1}$ where the magnetic
Goldstone mode resides. We discuss several features which can be observed
experimentally.
|
1108.2046v1
|
2011-08-17
|
The hyperfine properties of a hydrogenated Fe/V superlattice
|
: We study the effect of hydrogen on the electronic, magnetic and hyperfine
structures of an iron-vanadium superlattice consisting of three Fe monolayers
and nine V monolayers. The contact charge density ({\rho}), the contact
hyperfine field (Bhf) and the electronic field gradient (EFG) at the Fe sites
for different H locations and H fillings are calculated using the first
principle full-potential linear-augmented-plane-wave (FP-LAPW) method . It is
found that sizeable changes in the hyperfine properties are obtained only when
H is in the interface region.
|
1108.3388v1
|
2011-08-22
|
Pressure effects on strained FeSe0.5Te0.5 thin films
|
The pressure effect on the resistivity and superconducting Tc of prestrained
thin films of the iron chalcogenide superconductor FeSe0.5Te0.5 is studied.
Films with different anion heights above the Fe layer showing different values
of ambient pressure Tc's are compressed up to a pressure of 1.7 GPa. All films
exhibit a significant increase of Tc with pressure. The results cannot solely
be explained by a pressure-induced decrease of the anion height but other
parameters have to be considered to explain the data for all films.
|
1108.4399v1
|
2011-08-23
|
Superconductivity and local non-centrosymmetricity in crystal lattices
|
Symmetry of the crystal lattice can be a determining factor for the structure
of Cooper pairs in unconventional superconductors. In this study we extend the
discussion of superconductivity in non-centrosymmetric materials to the case
when inversion symmetry is missing locally, but is present on a global level.
Concretely, we investigate the staggered non-centrosymmetricity within a
regular sublattice structure, in some analogy to the discussion of
superconductivity in antiferromagnetic systems. Three crystal structures are
analyzed in detail as illustrative examples for the extended classification of
Cooper-pairing channels. One of the cases may be relevant for the class of
iron-pnictide superconductors.
|
1108.4694v1
|
2011-09-01
|
High-energy pseudogap in degenerate Hubbard model induced via Hund coupling
|
Hund coupling in the degenerate five-band Hubbard model near n=6 occupancy is
shown to give rise to a significant depletion of spectral weight above the
Fermi level. Calculations within dynamical mean field theory combined with
exact diagonalization reveal that this pseudogap is associated with a
collective mode in the self-energy caused by spin fluctuations. The pseudogap
is remarkably stable over a wide range of Coulomb and exchange energies, but
disappears for weak Hund coupling. The implications of this phenomenon for
optical spectra of iron pnictides are discussed.
|
1109.0162v1
|
2011-09-05
|
Gutzwiller theory of band magnetism in LaOFeAs
|
We use the Gutzwiller variational theory to calculate the ground-state phase
diagram and quasi-particle bands of LaOFeAs. The Fe3d--As4p Wannier-orbital
basis obtained from density-functional theory defines the band part of our
eight-band Hubbard model. The full atomic interaction between the electrons in
the iron orbitals is parameterized by the Hubbard interaction U and an average
Hund's-rule interaction J. We reproduce the experimentally observed small
ordered magnetic moment over a large region of (U,J) parameter space. The
magnetically ordered phase is a stripe spin-density wave of quasi-particles.
|
1109.0929v1
|
2011-09-09
|
Theoretical Investigation of the Magnetic Order in FeAs
|
The magnetic structure of the iron monoarsenide FeAs is studied using
first-principles calculations. We consider the collinear and non-collinear
(spin-spiral wave) magnetic ordering and magnetic anisotropy. It is
analitically shown that a magnetic triaxial anisotropy results in a sum of two
spin-spiral waves with opposite directions of wave vectors and different spin
amplitudes, so that the magnetic moments in two perpendicular directions do not
equal each other.
|
1109.1924v1
|
2011-09-12
|
Spin-transfer mechanism for magnon-drag thermopower
|
We point out a relation between the dissipative spin-transfer-torque
parameter $\beta$ and the contribution of magnon drag to the thermoelectric
power in conducting ferromagnets. Using this result we estimate $\beta$ in iron
at low temperatures, where magnon drag is believed to be the dominant
contribution to the thermopower. Our results may be used to determine $\beta$
from magnon-drag-thermopower experiments, or, conversely, to infer the strength
of magnon drag via experiments on spin transfer.
|
1109.2414v1
|
2011-09-13
|
Instability of vortex-antivortex interface in optimally-doped Ba(Fe_{1-x}Co_{x})_{2}As_{2}
|
We explore the flux front patterns during virgin penetration and after
remagnetization in the iron-based superconductor Ba(Fe_{1-x}Co_{x})_{2}As_{2}.
After remagnetization we observe an instability characterized by turbulent
dynamics at the vortex-antivortex boundary. Associated with the turbulent flux
boundary a band of excess current density is observed. The turbulence is
contrasted with the instability observed in the cuprate superconductors. Our
results suggest turbulent instability at the vortex-antivortex interface may be
a common feature in all remagnetized type-II superconductors.
|
1109.2703v1
|
2011-09-22
|
The interaction of Fe thin layers between MgO(100)-MgO and MgO(100)-Ag surfaces
|
The atomic interaction and magnetic properties of ultrathin Fe films grown on
cleaved and polished MgO(100) surfaces were studied by conversion electron
M\"{o}ssbauer spectroscopy (CEMS) in broad temperature range. Fe with different
layer thickness was deposited on MgO substrates. The layers were formed on
polished and cleaved substrate surfaces at RT. The analysis of the spectra
showed no Fe-O$^{2-}$ interaction in MgO/Fe interface. Iron layers showed
different magnetic anisotropy depending on their thickness.
|
1109.4811v1
|
2011-09-22
|
Ironsilicide formation by high temperature codeposition of Fe-2Si with different thicknesses on Si (111)
|
Fe and 2Si were co-deposited on Si (111) surface at 853 K. The formation of
silicides was investigated by Mossbauer spectroscopy and electron microscopy.
Depending on the thickness of the deposited films different phases were formed.
At low thickness, stable $\epsilon$-FeSi (B20) and metastable
[CsCl]-Fe$_{1-x}$Si (B2) phases were observed. In the latter case, because of
the presence of Fe vacancies in the lattice the local symmetry around the iron
components was lower than cubic. At larger (12 nm) deposited thickness, stable
$\beta$-FeSi$_2$ has been formed.
|
1109.4825v1
|
2011-09-22
|
Predicting dislocation climb: Classical modeling versus atomistic simulations
|
The classical modeling of dislocation climb based on a continuous description
of vacancy diffusion is compared to recent atomistic simulations of dislocation
climb in body-centered cubic iron under vacancy supersaturation [Phys. Rev.
Lett. 105 095501 (2010)]. A quantitative agreement is obtained, showing the
ability of the classical approach to describe dislocation climb. The analytical
model is then used to extrapolate dislocation climb velocities to lower
dislocation densities, in the range corresponding to experiments. This allows
testing of the validity of the pure climb creep model proposed by Kabir et al.
[Phys. Rev. Lett. 105 095501 (2010)].
|
1109.4905v1
|
2011-09-30
|
Stellar Parameters and Chemical Abundances of G Giants
|
We present basic stellar parameters of 99 late-type G giants based on high
resolution spectra obtained by the High Dispersion Spectrograph attached to
Subaru Telescope. These stars are targets of a Doppler survey program searching
for extra-solar planets among evolved stars, with a metallicity of
-0.8<[Fe/H]<+0.2. We also derived their abundances of 15 chemical elements,
including four $\alpha$-elements (Mg, Si, Ca, Ti), three odd-Z light elements
(Al, K, Sc), four iron peak elements (V, Cr, Fe, Ni), and four neutron-capture
elements (Y, Ba, La, Eu). Kinematic properties reveal that most of the program
stars belong to the thin disk.
|
1109.6742v1
|
2011-10-04
|
Neutrino nucleus reactions within the GiBUU model
|
The GiBUU model, which implements all reaction channels relevant at medium
neutrino energy, is used to investigate the neutrino and antineutrino
scattering on iron. Results for integrated cross sections are compared with
NOMAD and MINOS data. It is shown, that final state interaction can noticeably
change the spectra of the outgoing hadrons. Predictions for the Miner$\nu$a
experiment are made for pion spectra, averaged over NuMI neutrino and
antineutrino fluxes.
|
1110.0674v1
|
2011-11-01
|
Electronic band structure of ferro-pnictide superconductors from ARPES experiment
|
ARPES experiments on iron based superconductors show that the differences
between the measured and calculated electronic band structures look
insignificant but can be crucial for understanding of the mechanism of high
temperature superconductivity. Here we focus on those differences for 111 and
122 compounds and discuss the observed correlation of the experimental band
structure with the superconductivity.
|
1111.0288v1
|
2011-11-02
|
Electrochemical synthesis of iron-based superconductor FeSe films
|
The superconducting FeSe films were successfully fabricated using the
electrochemical synthesis. The composition ratio of Fe and Se can be controlled
by the electric potential and pH value. We found that the FeSe films deposited
at the electric potential -1.75 V and pH 2.3 show the superconducting
transition at 3.5 K. The establishment of this electrochemical synthesis
technique will provide many advantages for application.
|
1111.0611v1
|
2011-11-18
|
Stripes and Superconductivity in Cuprates
|
Holes doped into the CuO2 planes of cuprate parent compounds frustrate the
antiferromagnetic order. The development of spin and charge stripes provides a
compromise between the competing magnetic and kinetic energies. Static stripe
order has been observed only in certain particular compounds, but there are
signatures which suggest that dynamic stripe correlations are common in the
cuprates. Though stripe order is bad for superconducting phase coherence,
stripes are compatible with strong pairing. Ironically, magnetic-field-induced
stripe order appears to enhance the stability of superconducting order within
the planes.
|
1111.4268v1
|
2012-01-13
|
Observation of multiple superconducting gaps in Fe1+yTe1-xSex via a nano-scale approach to point-contact spectroscopy
|
We report a distinct experimental approach to point-contact Andreev
reflection spectroscopy with diagnostic capability via a unique design of
nano-scale normal metal/superconductor devices with excellent thermo-mechanical
stability, and have employed this method to unveil the existence of two
superconducting energy gaps in iron chalcogenide Fe1+yTe1-xSex which is crucial
for understanding its pairing mechanism. This work opens up new opportunities
to study gap structures in superconductors and elemental excitations in solids.
|
1201.2881v2
|
2012-01-16
|
Odd-particle systems in the shell model Monte Carlo: circumventing a sign problem
|
We introduce a novel method within the shell model Monte Carlo approach to
calculate the ground-state energy of a finite-size system with an odd number of
particles by using the asymptotic behavior of the imaginary-time
single-particle Green's functions. The method circumvents the sign problem that
originates from the projection on an odd number of particles and has hampered
direct application of the shell model Monte Carlo method to odd-particle
systems. We apply this method to calculate pairing gaps of nuclei in the iron
region. Our results are in good agreement with experimental pairing gaps.
|
1201.3341v1
|
2012-02-08
|
Role of spin-orbit coupling on the electronic structure and properties of SrPtAs
|
The effect of spin-orbit coupling on the electronic structure of the layered
iron-free pnictide superconductor, SrPtAs, has been studied using the full
potential linearized augmented plane wave method. The anisotropy in Fermi
velocity, conductivity and plasma frequency stemming from the layered structure
are found to be enhanced by spin-orbit coupling. The relationship between
spin-orbit interaction and the lack of two-dimensional inversion in the PtAs
layers is analyzed within a tight-binding Hamiltonian based on the
first-principles calculations. Finally, the band structure suggests that
electron doping could increase $T_c$.
|
1202.1604v1
|
2012-02-16
|
Comparison of EPOS and QGSJET-II in EAS Simulation using CORSIKA
|
In this work we compare the predictions of two representative hadronic
interaction models, EPOS 1.99, and QGSJET II-03 with several extensive air
showers (EAS) parameters for proton and iron primaries in the energy range
$10^{17}$ - $10^{19} eV$ using CORSIKA-6990. The EAS parameters depth of shower
maximum, shower size, size of muon shower, muon number distribution, electron
number distribution,size of hadron shower, hadron energy sum, electron muon
correlations, and, hadron energy spectra are studied in this paper.
|
1202.3661v4
|
2012-02-28
|
Finite Elements Magnetic Analysis of the CLIC MDI Region
|
Considering the current CLIC SiD detector design and the machine parameter
L*, the final focus quadrupole QD0 will be placed inside the experiment itself.
This configuration is very challenging from an integration point of view. Among
several other aspects, the iron-dominated QD0 will need an active magnetic
shielding to avoid undesired interactions with the magnetic field generated by
the main solenoid of the detector. This shielding will be provided by a
superconducting anti-solenoid, and this paper aims to describe the method used
to design such device, the results obtained and the issues still to be solved.
|
1202.6334v1
|
2012-03-01
|
Magnetic and transport properties of iron-platinum arsenide Ca10(Pt4-δAs8)(Fe2-xPtxAs2)5 single crystal
|
We report superconducting properties of single crystalline
Ca10(Pt4-{\delta}As8)(Fe2-xPtxAs2)5 by X-ray diffraction, magnetization,
resistivity, and magneto-optical imaging measurements. The magnetization
measurements reveal fish-tail hysteresis loop and relatively high critical
current density Jc ~ 0.8\times105 A/cm2 at low temperatures. The exponential
temperature dependence of Jc, which arises from nonlinear effective flux-creep
activation energy, has been observed. Upper critical field determined by
resistive transition shows a relatively large anisotropy. The magneto-optical
images reveal homogenous current flow within the crystal.
|
1203.0099v1
|
2012-03-05
|
Breakdown of Chemical Scaling for Pt-Doped CaFe2As2
|
We report a breakdown of the scaling of the antiferromagnetic transition
temperature T_N on chemical doping for Pt-doped CaFe2As2. The substitution of
Pt is ineffective in the reduction of antiferromagnetic ordering as well as in
inducing superconductivity in Ca(Fe1-xPtx)2As2. The Pt-doped CaFe2As2 that does
not exhibit superconductivity will, however, provide us an ideal opportunity to
elucidate the role of chemical doping in the occurrence of superconductivity in
iron-based materials.
|
1203.0822v1
|
2012-03-07
|
Structural distortion as prerequisite for superconductivity in LiFeAs
|
The nonadiabatic Heisenberg model predicts a structural distortion in LiFeAs
below a temperature higher than (or at least equal to) the superconducting
transition temperature. Within this group-theoretical model, the reduction of
the symmetry caused by the distortion is a prerequisite for the superconducting
state in this compound and can be realized by a mere displacement of the iron
atoms from their positions in the space group P4/nmm.
|
1203.1543v1
|
2012-03-29
|
Field-Induced Magnetostructural Transitions in Antiferromagnetic Fe1+yTe1-xSx
|
The transport and structural properties of Fe1+yTe1-xSx (x=0, 0.05, and 0.10)
crystals were studied in pulsed magnetic fields up to 65 T. The application of
high magnetic fields results in positive magnetoresistance effect with
prominent hystereses in the antiferromagnetic state. Polarizing microscope
images obtained at high magnetic fields showed simultaneous occurrence of
structural transitions. These results indicate that magnetoelastic coupling is
the origin of the bicollinear magnetic order in iron chalcogenides.
|
1203.6482v1
|
2012-04-12
|
Effective doping and suppression of Fermi surface reconstruction via Fe vacancy disorder in KxFe2-ySe2
|
We investigate the effect of disordered vacancies on the normal-state
electronic structure of the newly discovered alkali-intercalated iron selenide
superconductors. To this end we use a recently developed Wannier function based
method to calculate from first principles the configuration-averaged spectral
function <A(k,w)> of K0.8Fe1.6Se2 with disordered Fe and K vacancies. We find
that the disorder can suppress the expected Fermi surface reconstruction
without completely destroying the Fermi surface. More interestingly, the
disorder effect raises the chemical potential significantly, giving enlarged
electron pockets almost identical to highly doped KFe2Se2, without adding
carriers to the system.
|
1204.2849v1
|
2012-05-04
|
Thermodynamic evidence for pressure-induced bulk superconductivity in the Fe-As pnictide superconductor CaFe2As2
|
We report specific-heat and resistivity experiments performed in parallel in
a Bridgman-type of pressure cell in order to investigate the nature of
pressure-induced superconductivity in the iron pnictide compound CaFe2As2. The
presence of a pronounced specific-heat anomaly at Tc reveals a bulk nature of
the superconducting state. The thermodynamic transition temperature differs
dramatically from the onset of the resistive transition. Our data indicates
that superconductivity occurs in the vicinity of a crystallographic phase
transition. We discuss the discrepancy between the two methods as caused by
strain-induced superconducting precursors formed above the bulk thermodynamic
transition due to the vicinity of the structural instability.
|
1205.0874v1
|
2012-06-04
|
Microscopic origin of the linear temperature increase of the magnetic susceptibility of BaFe$_{2}$As$_{2}$
|
Employing a combination of \emph{ab initio} band structure theory and
dynamical mean-field theory we explain the experimentally observed linear
temperature increase of the magnetic susceptibility of the iron pnictide
material BaFe$_{2}$As$_{2}$. The microscopic origin of this anomalous behaviour
is traced to a sharp peak in the spectral function located approximately 100
meV below the Fermi level. This peak is due to the weak dispersion of
two-dimensional bands associated with the layered crystal structure of
pnictides.
|
1206.0569v2
|
2012-06-06
|
Modulations of the local pairing interaction near magnetic impurities in d-wave superconductors
|
The spin-fluctuation based pairing mechanism has proven successful in
explaining the pairing symmetries due to Fermi surface nesting of both cuprates
and iron-based materials. In this work, we study signatures of a
spin-fluctuation mediated pairing at the local scale. Specifically, we focus on
magnetic impurities and calculate both the local antiferromagnetism and the
resulting modulated pairing interaction. The latter gives rise to distinct
local enhancements of the superconducting gap in the immediate vicinity of the
impurities. Our results show that Coulomb-driven pairing naturally leads to
unusual superconducting gap modulations near disorder potentials.
|
1206.1129v1
|
2012-06-14
|
DFT Calculations as a Tool to Analyse Quadrupole Splittings of Spin Crossover Fe(II) complexes
|
Density functional methods have been applied to calculate the quadrupole
splitting of a series of iron(II) spin crossover complexes. Experimental and
calculated values are in reasonable agreement. In one case spin-orbit coupling
is necessary to explain the very small quadrupole splitting value of 0.77 mm/s
at 293 K for a high-spin isomer.
|
1206.3036v1
|
2012-06-18
|
Interpretation of Mössbauer spectra in the energy and time domain with neural networks
|
An artificial neural network for extracting reasonable and fast estimates of
hyperfine parameters from M\"ossbauer spectra in the energy or time domain is
outlined. First promising results for determining the asymmetry of the electric
field gradient at the nucleus of a diamagnetic iron center as derived with
different types of neural networks are reported.
|
1206.3892v1
|
2012-06-20
|
Fermi surface topology of LaFePO and LiFeP
|
We perform charge self-consistent LDA+DMFT (density functional theory
combined with dynamical mean field theory) calculations to study correlation
effects on the Fermi surfaces of the iron pnictide superconductors LaFePO and
LiFeP. We find a distinctive change in the topology of the Fermi surface in
both compounds where a hole pocket with Fe dz^2 orbital character changes its
geometry from a closed shape in LDA to an open shape upon inclusion of
correlations.
|
1206.4551v3
|
2012-06-22
|
Evolution of the neutron resonances in AFe2Se2
|
Recent experiments on the alkali-intercalated iron selenides have raised
questions about the symmetry of the superconducting phase. Random phase
approximation calculations of the leading pairing eigenstate for a tight-
binding 5-orbital Hubbard-Hund model of AFe2Se2 find that a d-wave (B1g) state
evolves into an extended s{\pm} (A1g) state as the system is hole-doped.
However, over a range of doping these two states are nearly degenerate. Here,
we calculate the imaginary part of the magnetic spin susceptibility
\chi"(q,{\omega}) for these gaps and discuss how the evolution of neutron
scattering resonances can distinguish between them.
|
1206.5235v1
|
2012-06-27
|
Electronic, magnetic properties and correlation effects in the layered quaternary iron oxyselenide Na2Fe2Se2O from first principles
|
By means of the first-principle calculations, we have investigated
electronic, magnetic properties and correlation effects for the newly
discovered layered oxyselenide Na2Fe2Se2O. Our results reveal that the electron
correlations in the Fe 3d bands promote a transition of Na2Fe2Se2O from
magnetic metallic or half-metallic states to the antiferromagnetic
Mott-insulating state. In addition, the bonding picture in Na2Fe2Se2O is
described as an anisotropic mixture of ionic and covalent contributions.
|
1206.6182v1
|
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