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2013-04-10 | Large room-temperature magnetoresistance in lateral organic spin valves fabricated by in-situ shadow evaporation | We report the successful fabrication of lateral organic spin valves with a
channel length in the sub $100\,nm$ regime. The fabication process is based on
in-situ shadow evaporation under UHV conditions and therefore yields clean and
oxygen-free interfaces between the ferromagnetic metallic electrodes and the
organic semiconductor. The spin valve devices consist of Nickel and Cobalt-iron
electrodes and the high mobility \emph{n}-type organic semiconductor
$N,N'$-bis(heptafluorobutyl)-$3,4:9,10$-perylene diimide. Our studies comprise
fundamental investigations of the process' and materials' suitability for the
fabrication of lateral spin valve devices as well as magnetotransport
measurements at room temperature. The best devices exhibit a magnetoresistance
of up to $50\,%$, the largest value for room temperature reported so far. | 1304.2911v2 |
2013-04-10 | Finite temperature and pressure molecular dynamics for BaFe2As2 | We study the temperature and pressure dependence of the structural and
electronic properties of the iron pnictide superconductor BaFe2As2. We use
density functional theory based Born-Oppenheimer molecular dynamics simulations
to investigate the system at temperatures from T=5 K to 150 K and pressures
from P=0 GPa to 30 GPa. When increasing the pressure at low temperature, we
find the two transitions from an orthorhombic to a tetragonal and to a
collapsed tetragonal structure that are also observed in zero temperature
structure relaxations and in experiment. However, these transitions are
considerably smeared out at finite temperature, whereas the critical pressure
for the first transition increases with temperature. We also analyze the
electronic structure of BaFe2As2 at finite temperature and work out differences
between the time averaged band structure and Fermi surface at finite
temperature compared to the known zero temperature results. Our results should
be helpful for resolving some open issues in experimental reports for BaFe2As2
under high pressure. | 1304.3039v1 |
2013-04-12 | Effect of Vacuum Annealing on Superconductivity in Fe(Se,Te) Single Crystals | The effect of vacuum annealing on superconductivity is investigated in
Fe(Se,Te) single crystals. It is found that superconductivity is not enhanced
by annealing under high vacuum (~ 10^(-3) Pa) or by annealing in a sealed
evacuated quartz tube. In a moderate vacuum atmosphere (~ 1 Pa), iron oxide
layers are found to show up on sample surfaces, which would draw excess Fe out
of the crystal. Thus, it is suggested that remanent oxygen effectively works to
remove excess Fe from the matrix of Fe(Se,Te) crystals, resulting improvement
of superconducting transition temperature. Our transport measurements suggest
that the excess Fe scatters the carriers on electron- and hole-type channels in
a different manner. We discuss how the mobility of two types of carriers
correlate with superconductivity. Since both the electron and hole bands are
important for the occurrence of superconductivity, excess Fe would suppress
superconductivity mainly due to strong scattering of electrons. | 1304.3590v1 |
2013-04-14 | Peculiarities of performance of the spin valve for the superconducting current | The spin valve effect for the superconducting current based on the
superconductor/ferromagnet proximity effect has been studied for a
CoO_x/Fe1/Cu/Fe2/Cu/Pb multilayer. The magnitude of the effect $\Delta T_c$ =
T_c^{AP} - T_c^{P}, where T_c^{P} and T_c^{AP} are the superconducting
transition temperatures for the parallel (P) and antiparallel (AP) orientation
of magnetizations, respectively, has been measured for different thicknesses of
the Fe1 layer d_{Fe1}. The obtained dependence of the effect on d_{Fe1} reveals
that $\Delta T_c$ can be increased in comparison with the case of a
half-infinite Fe1 layer considered by the previous theory. A maximum of the
spin valve effect occurs at d_{Fe1} ~ d_{Fe2}. At the optimal value of d_{Fe1},
almost full switching from the normal to the superconducting state when
changing the mutual orientation of magnetizations of the iron layers Fe1 and
Fe2 from P to AP is demonstrated. | 1304.3939v1 |
2013-04-23 | Low-energy bound states at interfaces between superconducting and block antiferromagnet regions in KxFe{2-y}Se2 | The high-Tc alkali doped iron selenide superconductors KxFe{2-y}Se2 have been
recently shown to be intrinsically phase separated into Fe vacancy ordered
block antiferromagnetic regions and superconducting regions at low
temperatures. In this work, we use a microscopic five orbital Hubbard model to
obtain the electronic low-energy states near the interfaces between block
antiferromagnets and superconductors. It is found that abundant low-energy
in-gap bound states exist near such interfaces irrespective of whether the
superconductor has d- or s-wave pairing symmetry. By contrast, it is shown how
nonmagnetic scattering planes can provide a natural means to distinguish
between these two leading pairing instabilities of the KxFe{2-y}Se2 materials. | 1304.6424v1 |
2013-04-24 | Thermodynamic Observation of a Vortex Melting Transition in the Fe-based Superconductor Ba0.5K0.5Fe2As2 | In cuprate high-temperature superconductors the small coherence lengths and
high transition termperatures result in strong thermal fluctuations, which
render the superconducting transition in applied magnetic fields into a wide
continuous crossover. A state with zero resistance is found only below the
vortex melting transition, which occurs well below the onset of superconducting
correlations. Here we investigate the vortex phase diagram of the novel
Fe-based superconductor in form of a high-quality single crystal of
Ba0.5K0.5Fe2As2, using three different experimental probes (specific heat,
thermal expansion and magnetization). We find clear thermodynamic signatures of
a vortex melting transition, which shows that the thermal fluctuations in
applied magnetic fields also have a considerable impact on the superconducting
properties of iron-based superconductors. | 1304.6464v1 |
2013-04-26 | Direct microscopic calculations of nuclear level densities in the shell model Monte Carlo approach | Nuclear level densities are crucial for estimating statistical nuclear
reaction rates. The shell model Monte Carlo method is a powerful approach for
microscopic calculation of state densities in very large model spaces. However,
these state densities include the spin degeneracy of each energy level, whereas
experiments often measure level densities in which each level is counted just
once. To enable the direct comparison of theory with experiments, we introduce
a method to calculate directly the level density in the shell model Monte Carlo
approach. The method employs a projection on the minimal absolute value of the
magnetic quantum number. We apply the method to nuclei in the iron region as
well as the strongly deformed rare-earth nucleus $^{162}$Dy. We find very good
agreement with experimental data including level counting at low energies,
charged particle spectra and Oslo method at intermediate energies, neutron and
proton resonance data, and Ericson's fluctuation analysis at higher excitation
energies. | 1304.7258v1 |
2013-04-28 | Temperature dependent local atomic displacements in Ru substituted SmFe_{1-x}Ru_{x}AsO_{0.85}F_{0.15} superconductors | Local structure of SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ ($x$ = 0.0, 0.05,
0.25 and 0.5) superconductors has been investigated by temperature dependent As
$K$-edge extended x-ray absorption fine structure. The effect of Ru
substitution remains confined to the iron-arsenide layer but neither the static
disorder nor the Fe-As bond strength suffers any change for $x \le$ 0.25. With
further Ru substitution the static disorder increases while the Fe-As bond
strength remains unchanged. Also, the Ru-As distance ($\sim$2.42 \AA),
different from the Fe-As distance ($\sim$2.39 \AA), does not show any change in
its force constant with the Ru substitution. These observations suggest that
the SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ system breaks down to coexisting
local electronic phases on isoelectric substitution in the active FeAs layer. | 1304.7416v1 |
2013-05-01 | Nanotube caps on Ni, Fe, and NiFe nano particles: A path to chirality selective growth | Carbon nanotubes have properties depending on the arrangement of carbon atoms
on the tube walls, called chirality. Also it has been tried to grow nanotubes
of only one chirality for more than a decade it is still not possible today. A
narrowing of the distribution of chiralities, however, which is a first step
towards chirality control, has been observed for the growth of nanotubes on
catalysts composed of nickel and iron. In this paper, we have calculated
carbon-metal bond energies, adhesion energies and charge distributions of
carbon nanotube caps on Ni, Fe and NiFe alloy clusters using density functional
theory. A growth model using the calculated energies was able to reproduce the
experimental data of the nanotube growth on the alloy catalysts. The electronic
charge was found to be redistributed from the catalyst particles to the edges
of the nanotube caps in dependence of the chiral angles of the caps increasing
the reactivity of the edge atoms. Our study develops an explanation for the
chirality enrichment in the carbon nanotube growth on alloy catalyst particles. | 1305.0145v1 |
2013-05-06 | Dzyaloshinsky-Moriya driven helical-butterfly structure in Ba3NbFe3Si2O14 | We have used soft x-ray magnetic diffraction at the Fe3+ L2,3 edges to
examine to what extent the Dzyaloshinsky-Moriya interaction in Ba3NbFe3Si2O14
influences its low temperature magnetic structure. A modulated component of the
moments along the c-axis is present, adding to the previously proposed helical
magnetic configuration of co-planar moments in the a,b-plane. This leads to a
helical-butterfly structure and suggests that both the multi-axial in-plane and
the uniform out-of-plane Dzyaloshinsky-Moriya vectors are relevant. A non zero
orbital magnetic signal is also observed at the oxygen K edge, which reflects
the surprisingly strong hybridization between iron 3d and oxygen 2p states,
given the nominal spherical symmetry of the Fe3+ half filled shell. | 1305.1160v1 |
2013-05-06 | Analysis of a carbon dimer bound to a vacancy in iron using density functional theory and a new tight binding model | Recent density functional theory (DFT) calculations by Foerst et al. have
predicted that vacancies in both low and high carbon steels have a carbon dimer
bound to them. This is likely to change the thinking of metallurgists in the
kinetics of the development of microstructures. While the notion of a C2
molecule bound to a vacancy in Fe will potentially assume a central importance
in the atomistic modeling of steels, neither a recent tight binding (TB) model
nor existing classical interatomic potentials can account for it. Here we
present a new TB model for C in Fe, based on our earlier work for H in Fe,
which correctly predicts the structure and energetics of the carbon dimer at a
vacancy in Fe. Moreover the model is capable of dealing with both concentrated
and dilute limits of carbon in both bcc-Fe and fcc-Fe as comparisons with DFT
show. We use both DFT and TB to make a detailed analysis of the dimer and to
come to an understanding as to what governs the choice of its curious
orientation within the vacancy. | 1305.1171v1 |
2013-05-08 | Nematic State of the Pnictides Stabilized by the Interplay Between Spin, Orbital, and Lattice Degrees of Freedom | The nematic state of the iron-based superconductors is studied in the undoped
limit of the three-orbital ($xz$, $yz$, $xy$) spin-fermion model via the
introduction of lattice degrees of freedom. Monte Carlo simulations show that
in order to stabilize the experimentally observed lattice distortion and
nematic order, and to reproduce photoemission experiments, {\it both} the
spin-lattice and orbital-lattice couplings are needed. The interplay between
their respective coupling strengths regulates the separation between the
structural and N\'eel transition temperatures. Experimental results for the
temperature dependence of the resistivity anisotropy and the angle-resolved
photoemission (ARPES) orbital spectral weight are reproduced by the present
numerical simulations. | 1305.1879v1 |
2013-05-10 | Theory of superconductivity in a three-orbital model of Sr$_2$RuO$_4$ | In conventional and high transition temperature copper oxide and iron
pnictide superconductors, the Cooper pairs all have even parity. As a rare
exception, Sr$_2$RuO$_4$ is the first prime candidate for topological chiral
p-wave superconductivity, which has time-reversal breaking odd-parity Cooper
pairs known to exist before only in the neutral superfluid $^3$He. However,
there are several key unresolved issues hampering the microscopic description
of the unconventional superconductivity. Spin fluctuations at both large and
small wavevectors are present in experiments, but how they arise and drive
superconductivity is not yet clear. Spontaneous edge current is expected but
not observed conclusively. Specific experiments point to highly band- and/or
momentum-dependent energy gaps for quasiparticle excitations in the
superconducting state. Here, by comprehensive functional renormalization group
calculations with all relevant bands, we disentangle the various competing
possibilities. In particular we show the small wavevector spin fluctuations,
driven by a single two-dimensional band, trigger p-wave superconductivity with
quasi-nodal energy gaps. | 1305.2317v2 |
2013-05-10 | Implementation of non-equilibrium vertex corrections in KKR: transport through disordered layers | The theoretical description of modern nanoelectronic devices requires a
quantum mechanical treatment and often involves disorder, e.g. form alloys.
Therefore, the ab initio theory of transport using non-equilibrium Green's
functions is extended to the case of disorder described by the coherent
potential approximation. This requires the calculation of non-equilibrium
vertex corrections. We implement the vertex corrections in a
Korringa-Kohn-Rostoker multiple scattering scheme. In order to verify our
implementation and to demonstrate the accuracy and applicability we investigate
a system of an iron-cobalt alloy layer embedded in copper. The results obtained
with the coherent potential approximation are compared to supercell
calculations. It turns out that vertex corrections play an important role for
this system. | 1305.2399v1 |
2013-05-14 | Spectral matching for abundances of 848 stars of the giant branches of the globular cluster ω Centauri | We present the effective temperatures, surface gravities and abundances of
iron, carbon and barium of 848 giant branch stars, of which 557 also have
well-defined nitrogen abundances, of the globular cluster {\omega} Centauri.
This work used photometric sources and lower resolution spectra for this
abundance analysis. Spectral indices were used to estimate the oxygen abundance
of the stars, leading to a determination of whether a particular star was
oxygen-rich or oxygen-poor.
The 557-star subset was analyzed in the context of evolutionary groups, with
four broad groups identified. These groups suggest that there were at least
four main four periods of star formation in the cluster. The exact order of
these star formation events is not yet understood.
These results compare well with those found at higher resolution and show the
value of more extensive lower resolution spectral surveys. They also highlight
the need for large samples of stars when working with a complex object like
{\omega} Cen. | 1305.3059v2 |
2013-05-15 | Design and Performance Analysis of a 2.5 MW-Class HTS Synchronous Motor for Ship Propulsion | The development of cryogenic technology and high temperature superconducting
(HTS) materials has seen continued interest worldwide in the development of HTS
machines since the late 1980s. In this paper, the authors present a conceptual
design of a 2.5 MW class synchronous motor. The structure of the motor is
specified and the motor performance is analyzed via a three-dimensional model
using the finite element method (FEM). Rotor optimization is carried out to
decrease the harmonic components in the air gap field generated by HTS tapes.
Based on the results of this 3D simulation, the determination of the operating
conditions and load angle is discussed with consideration to the HTS material
properties. The economic viability of air-core and iron-core designs is
compared. The results show that this type of HTS machine has the potential to
achieve an economic, efficient and effective machine design, which operates at
a low load angle, and this design process provides a practical way to simulate
and analyze the performance of such machines. | 1305.3590v3 |
2013-05-18 | Estimation of Lateral Distribution Function in Extensive Air Showers by Using AIRES Simulation System | In this work the estimation of the lateral distribution function in Extensive
Air showers was performed by using a system for air shower simulations which is
called AIRES version 2.6 for different hadronic models like (QGSJET99, SIBYLL
and SIBYLL1.6). The simulation was fulfilled in the high energy range
(10^15-10^19 eV) for different primary particles like (gamma, protons and iron
nuclei) for vertical showers. This simulation can be used to reconstruct the
type and energy of the particle that generated Extensive Air showers for
charged particles that registered with different arrays. | 1305.4628v2 |
2013-05-30 | Guided self-assembly of magnetic beads for biomedical applications | Micromagnetic beads are widely used in biomedical applications for cell
separation, drug delivery, and hypothermia cancer treatment. Here we propose to
use self-organized magnetic bead structures which accumulate on fixed magnetic
seeding points to isolate circulating tumor cells. The analysis of circulating
tumor cells is an emerging tool for cancer biology research and clinical cancer
management including the detection, diagnosis and monitoring of cancer.
Microfluidic chips for isolating circulating tumor cells use either affinity,
size or density capturing methods. We combine multiphysics simulation
techniques to understand the microscopic behavior of magnetic beads interacting
with Nickel accumulation points used in lab-on-chip technologies. Our proposed
chip technology offers the possibility to combine affinity and size capturing
with special antibody-coated bead arrangements using a magnetic gradient field
created by Neodymium Iron Boron permanent magnets. The multiscale simulation
environment combines magnetic field computation, fluid dynamics and discrete
particle dynamics. | 1305.7072v1 |
2013-06-03 | Review on Superconducting Materials | Short review of the topical comprehension of the superconductor materials
classes Cuprate High-Temperature Superconductors, other oxide superconductors,
Iron-based Superconductors, Heavy-Fermion Superconductors, Nitride
Superconductors, Organic and other Carbon-based Superconductors and Boride and
Borocarbide Superconductors, featuring their present theoretical understanding
and their aspects with respect to technical applications. | 1306.0429v2 |
2013-06-05 | Magnetic chains: From self-buckling to self-assembly | Spherical neodymium-iron-boron magnets are perman-ent magnets that can be
assembled into a variety of structures due to their high magnetic strength. A
one-dimensional chain of these magnets responds to mechanical loadings in a
manner reminiscent of an elastic rod. We investigate the macroscopic mechanical
properties of assemblies of ferromagnetic spheres by considering chains, rings,
and chiral cylinders of magnets. Based on energy estimates and simple
experiments, we introduce an effective magnetic bending stiffness for a chain
of magnets and show that, used in conjunction with classic results for elastic
rods, it provides excellent estimates for the buckling and vibration dynamics
of magnetic chains. We then use this estimate to understand the dynamic
self-assembly of a cylinder from an initially straight chain of magnets. | 1306.1014v2 |
2013-06-07 | Negative Magnetoresistance and Spin Filtering of Spin-Coupled Diiron-Oxo Clusters | Spin dependent transport has been investigated for an {\it open shell
singlet} diiron-oxo cluster. Currents and magnetoresistances have been studied,
as a function of spin state, within the non-equilibrium Green's function
approach. The applied bias can be used for tuning the sign of the observed
magnetoresistance. A colossal magnetoresistance ratio has been determined, on
the order of to 6000$%$, for hydrogen anchoring. Applied biases lower than 0.3
V, in conjunction with sulfur anchoring, induce a negative magnetoresistance
due to lowering of the anchor-scatterer tunneling barrier. In addition, the
diiron-oxo cluster displays nearly perfect spin filtering for parallel
alignment of the iron magnetic moments due to energetic proximity, relative to
the Fermi level, of its highest occupied molecular orbitals. | 1306.1588v2 |
2013-06-07 | Free energy generalization of the Peierls potential in iron | In body-centered cubic (bcc) crystals, ${1}{2}111$ screw dislocations exhibit
high intrinsic lattice friction as a consequence of their non-planar core
structure, which results in a periodic energy landscape known as the Peierls
potential, $U_P$. The main features determining plastic flow, including its
stress and temperature dependences, can be derived directly from this
potential, hence its importance. In this Letter, we use thermodynamic
integration to provide a full thermodynamic extension of $U_P$ for bcc Fe. We
compute the Peierls free energy path as a function of stress and temperature
and show that the critical stress vanishes at 700K, supplying the qualitative
elements that explain plastic behavior in the athermal limit. | 1306.1633v4 |
2013-06-11 | Discovery of high-frequency iron K lags in Ark 564 and Mrk 335 | We use archival XMM-Newton observations of Ark 564 and Mrk 335 to calculate
the frequency dependent time-lags for these two well-studied sources. We
discover high-frequency Fe K lags in both sources, indicating that the red wing
of the line precedes the rest frame energy by roughly 100 s and 150 s for Ark
564 and Mrk 335, respectively. Including these two new sources, Fe K
reverberation lags have been observed in seven Seyfert galaxies. We examine the
low-frequency lag-energy spectrum, which is smooth, and shows no feature of
reverberation, as would be expected if the low-frequency lags were produced by
distant reflection off circumnuclear material. The clear differences in the low
and high frequency lag-energy spectra indicate that the lags are produced by
two distinct physical processes. Finally, we find that the amplitude of the Fe
K lag scales with black hole mass for these seven sources, consistent with a
relativistic reflection model where the lag is the light travel delay
associated with reflection of continuum photons off the inner disc. | 1306.2551v1 |
2013-06-11 | Effect of pressure cycling on Iron: Signatures of an electronic instability and unconventional superconductivity | High pressure electrical resistivity and x-ray diffraction experiments have
been performed on Fe single crystals. The crystallographic investigation
provides direct evidence that in the martensitic $bcc \rightarrow hcp$
transition at 14 GPa the $\lbrace 110\rbrace_{bcc}$ become the $\lbrace
002\rbrace_{hcp}$ directions. During a pressure cycle, resistivity shows a
broad hysteresis of 6.5 GPa, whereas superconductivity, observed between 13 and
31 GPa, remains unaffected. Upon increasing pressure an electronic instability,
probably a quantum critical point, is observed at around 19 GPa and, close to
this pressure, the superconducting $T_{c}$ and the isothermal resistivity
($0<T<300\,$K) attain maximum values. In the superconducting pressure domain,
the exponent $n = 5/3$ of the temperature power law of resistivity and its
prefactor, which mimics $T_{c}$, indicate that ferromagnetic fluctuations may
provide the glue for the Cooper pairs, yielding unconventional
superconductivity. | 1306.2587v1 |
2013-06-11 | Nature of the bad metallic behavior of Fe_{1.06}Te inferred from its evolution in the magnetic state | We investigate with angle resolved photoelectron spectroscopy the change of
the Fermi Surface (FS) and the main bands from the paramagnetic (PM) state to
the antiferromagnetic (AFM) occurring below 72 K in Fe_{1.06}Te. The evolution
is completely different from that observed in iron-pnictides as nesting is
absent. The AFM state is a rather good metal, in agreement with our magnetic
band structure calculation. On the other hand, the PM state is very anomalous
with a large pseudogap on the electron pocket that closes in the AFM state. We
discuss this behavior in connection with spin fluctuations existing above the
magnetic transition and the correlations predicted in the spin-freezing regime
of the incoherent metallic state. | 1306.2603v2 |
2013-06-12 | Hybrid deterministic/stochastic algorithm for large sets of rate equations | We propose a hybrid algorithm for the time integration of large sets of rate
equations coupled by a relatively small number of degrees of freedom. A subset
containing fast degrees of freedom evolves deterministically, while the rest of
the variables evolves stochastically. The emphasis is put on the coupling
between the two subsets, in order to achieve both accuracy and efficiency. The
algorithm is tested on the problem of nucleation, growth and coarsening of
clusters of defects in iron, treated by the formalism of cluster dynamics. We
show that it is possible to obtain results indistinguishable from fully
deterministic and fully stochastic calculations, while speeding up
significantly the computations with respect to these two cases. | 1306.2817v1 |
2013-06-18 | Of Substitution and Doping: Spatial and Electronic Structure in Fe Pnictides | A highly intriguing aspect in iron-pnictide superconductors is the
composition-dependent electronic structure, in particular the question if and
how charge carriers are introduced to the system upon substitution of Ba by
alkali metals or of Fe by other transition metals, TM. We report on a
systematic study of spatial structure and electronic states by x-ray
diffraction and x-ray absorption on a large number of compositions in the
(Ba,K)(Fe,TM)2As2 family. The coherent combination of detailed structural
information with an in-depth analysis of the electronic structure allows us to
sensitively disentangle (charge-carrier) "doping" effects from "substitutional"
effects. Results include a doping character that is site-decoupled, as well as
TM 3d energy-level schemes that exhibit non-standard level sequences and even
t2-e level crossings. Our study indicates that doping per se seems to play a
lesser role than expected for pnictide superconductivity and magnetism. | 1306.4222v2 |
2013-06-18 | Electronic Raman scattering from orbital nematic fluctuations | We compute Raman scattering intensities via the lowest-order coupling to the
bosonic propagator associated with orbital nematic fluctuations in a minimal
model for iron pnictides. The model consists of two bands on a square lattice
exhibiting four Fermi pockets and a transition from the normal to a nematic
state. It is shown that the orbital fluctuations produce in the B1g channel
strong quasi-elastic light scattering around the nematic critical temperature
Tn, both above and below Tn. This holds for the A1g symmetry only below Tn
whereas no low-energy scattering from orbital fluctuations is found in the B2g
symmetry. Due to the nematic distortion the electron pocket at the X-point may
disappear at low temperatures. Such a Lifshitz transition causes in the B2g
spectrum a large upward shift of spectral weight in the high energy region
whereas no effect is seen in the other symmetries. | 1306.4267v1 |
2013-06-18 | Complex state induced by impurities in multiband superconductors | We study the role of impurities in a two-band superconductor, and elucidate
the nature of the recently predicted transition from s_{+-} state to s_{++}
state induced by interband impurity scattering. Using a Ginzburg-Landau theory,
derived from microscopic equations, we demonstrate that close to T_c this
transition is necessarily a direct one, but deeper in the superconducting state
an intermediate complex state appears. This state has a distinct order
parameter, which breaks the time-reversal symmetry, and is separated from the
s_{+-} and s_{++} states by phase transitions. Based on our results, we suggest
a phase diagram for systems with weak repulsive interband pairing, and discuss
its relevance to iron-based superconductors. | 1306.4268v2 |
2013-06-18 | Interplay of the electronic and lattice degrees of freedom in A_{1-x}Fe_{2-y}Se_{2} superconductors under pressure | The local structure and electronic properties of Rb$_{1-x}$Fe$_{2-y}$Se$_2$
are investigated by means of site selective polarized x-ray absorption
spectroscopy at the iron and selenium K-edges as a function of pressure. A
combination of dispersive geometry and novel nanodiamond anvil pressure-cell
has permitted to reveal a step-like decrease in the Fe-Se bond distance at
$p\simeq11$ GPa. The position of the Fe K-edge pre-peak, which is directly
related to the position of the chemical potential, remains nearly constant
until $\sim6$ GPa, followed by an increase until $p\simeq 11$ GPa. Here, as in
the local structure, a step-like decrease of the chemical potential is seen.
Thus, the present results provide compelling evidence that the origin of the
reemerging superconductivity in $A_{1-x}$Fe$_{2-y}$Se$_2$ in vicinity of a
quantum critical transition is caused mainly by the changes in the electronic
structure. | 1306.4305v1 |
2013-06-18 | Measurement of the elastoresistivity coefficients of the underdoped iron-arsenide Ba(Fe$_{0.975}$Co$_{0.025}$)$_2$As$_2$ | A new method is presented for measuring terms in the elastoresistivity tensor
$m_{ij}$ of single crystal samples with tetragonal symmetry. The technique is
applied to a representative underdoped Fe-arsenide,
Ba(Fe$_{0.975}$Co$_{0.025}$)$_2$As$_2$, revealing an anomalously large and
anisotropic elastoresistance in comparison to simple metals. The $m_{66}$
coefficient follows a Curie-Weiss temperature dependence, providing direct
evidence that the tetragonal-to-orthorhombic structural phase transition that
occurs at $T_s$ = 97.5 K in this material is not the result of a true-proper
ferro-elastic transition. Rather, the material suffers a pseudo-proper
transition for which the lattice strain is not the primary order parameter. | 1306.4377v2 |
2013-06-20 | Chiral properties of hematite (α-Fe2O3) inferred from resonant Bragg diffraction using circularly polarized x-rays | Chiral properties of the two phases - collinear motif (below Morin transition
temperature, TM=250 K) and canted motif (above TM) - of magnetically ordered
hematite ({\alpha}-Fe2O3) have been identified in single crystal resonant x-ray
Bragg diffraction, using circular polarized incident x-rays tuned near the iron
K-edge. Magneto-electric multipoles, including an anapole, fully characterize
the high-temperature canted phase, whereas the low-temperature collinear phase
supports both parity-odd and parity-even multipoles that are time-odd. Orbital
angular momentum accompanies the collinear motif, while it is conspicuously
absent with the canted motif. Intensities have been successfully confronted
with analytic expressions derived from an atomic model fully compliant with
chemical and magnetic structures. Values of Fe atomic multipoles previously
derived from independent experimental data, are shown to be completely
trustworthy. | 1306.4923v1 |
2013-06-24 | Pseudogap Formation and Quantum Phase Transition in Strongly-Correlated Electron Systems | Pseudogap formation is an ubiquitous phenomena in strongly-correlated
superconductors, for example cuprates, heavy-fermion superconductors, and iron
pnictides. As the system is cooled, an energy gap opens in the excitation
spectrum before entering the superconducting phase. The origin of formation and
the relevancy to the superconductivity remains unclear, which is the most
challenging problem in condensed matter physics. Here, using the cuprate as a
model, we demonstrate that the formation of pseudogap is due to a massive gauge
interaction between electrons, where the mass of the gauge boson, determining
the interaction length scale, is the consequence of the remnant
antiferromagnetic fluctuation inherited from the parent compounds. Extracting
from experimental data, we predict that there is a quantum phase transition
belonging to the 2D XY universality class at the critical doping where
pseudogap transition vanishes. | 1306.5498v4 |
2013-06-25 | Orbital-Selective Mottness in KxFe2-ySe2 Superconductors Revealed by Pump-Probe Spectroscopy | We report transient optical signatures of the orbital-selective Mottness in
superconducting KxFe2-ySe2 crystals by using dual-color pump-probe
spectroscopy. Besides multi-exponential decay recovery dynamics of
photo-induced quasiparticles, a damped oscillatory component due to coherent
acoustic phonons emerges when the superconducting phase is suppressed by
increasing the temperature or excitation power. The oscillatory component
diminishes with significant enhancement of a slow decay component upon raising
temperature to 150-160 K. These results are in consistence with the picture of
orbital-selective Mott phase transition, indicating a vital role played by
electron correlation in the iron-based superconductors. | 1306.5817v1 |
2013-06-25 | Collision-Induced Dissociation Studies on Fe(O2)n+ (n=1-6) Clusters: Application of a New Technique Based on Crossed Molecular Beams | Gas-phase oxygen-rich iron oxide clusters Fe(O2)n+ (n=1-6), are produced in a
molecular beam apparatus. Their stability and structure are investigated by
measuring the fragmentation cross sections from collision-induced-dissociation
experiments. For this purpose, two different techniques have been employed. The
first one relies on the measurement of the fragments resulting after
collisional activation and subsequent dissociation of mass selected cluster
ions in a molecular beam passing through a cell filled with noble gas atoms.
The second one is a new approach that we introduce and is based on crossed
molecular beams to measure the fragmentation cross sections, in a more
efficient manner without mass selection of the individual clusters. The cross
sections obtained with the different techniques are compared with each other as
well as with theoretical ones resulting from the application of a simple
geometrical projection model. Finally, the general trends observed are compared
with results for other Fe-molecule clusters available in the literature. | 1306.5902v1 |
2013-06-25 | The influence of oxygen and hydrogen adsorption on the magnetic structure of the ultrathin iron film on the Ir(001) surface | We present a detailed ab initio study of the electronic structure and
magnetic order of an Fe monolayer on the Ir(001) surface covered by adsorbed
oxygen and hydrogen. The results are compared to the clean Fe/Ir(001) system,
where recent intensive studies indicated a strong tendency towards an
antiferromagnetic order and complex magnetic structures. The adsorption of an
oxygen overlayer significantly increases interlayer distance between the Fe
layer and the Ir substrate, while the effect of hydrogen is much weaker. We
show that the adsorption of oxygen (and also of hydrogen) leads to a p(2$\times
$1) antiferromagnetic order of the Fe moments, which is also supported by an
investigation based on a disordered local moment state. Simulated scanning
tunneling images using the simple Tersoff-Hamann model hint that the proposed
p(2$\times $1) antiferromagnetic order could be detected even by non-magnetic
tips. | 1306.5925v1 |
2013-06-25 | The Effect of Hydrogen Adsorption on the Magnetic Properties of a Surface Nanocluster of Iron | The effect of hydrogen adsorption on the magnetic properties of an Fe$_3$
cluster immersed in a Cu(111) surface has been calculated using densifty
functional theory and the results used to parametrize an Alexander-Anderson
model which takes into account the interaction of d-electrons with itinerant
electrons. A number of adatom configurations containing one to seven H-atoms
were analyzed. The sequential addition of hydrogen atoms is found to
monotonically reduce the total magnetic moment of the cluster with the effect
being strongest when the H-atoms sit at low coordinated sites. Decomposition of
the charge density indicates a transfer of 0.4 electrons to each of the H-atoms
from both the Fe-atoms and from the copper substrate, irrespective of
adsorption site and coverage. The magnetic moment of only the nearest neighbor
Fe-atoms is reduced and mainly due to increased population of minority spin
d-states. This can be modeled by increased indirect coupling of d-states via
the conduction s-band in the Alexander-Anderson model. | 1306.5999v1 |
2013-07-04 | Prediction of phonon-mediated high temperature superconductivity in stoichiometric Li$_2$B$_3$C | The discovery of superconductivity in Magnesium Diborate (MgB$_2$) has
stimulated great interest in the search of new superconductors with similar
lattice structures. Unlike cuprate or iron-based superconductors, MgB$_2$ is
indisputably a phonon-mediated high temperature superconductor. The emergence
of high temperature superconductivity in this material results from the strong
coupling between the boron $\sigma$-bonding electrons around the Fermi level
and the bond-stretching optical phonon modes. Here we show, based on the
first-principles calculations, that Li$_2$B$_3$C is such a good candidate of
superconductor whose superconducting transition temperature (T$_c$) might be
even higher than MgB$_2$. Li$_2$B$_3$C consists of alternating graphene-like
boron-carbon layers and boron-boron layers with intercalated lithium atoms
between them. Similar to MgB$_2$, Li$_2$B$_3$C is inherently metallic and
possesses two $\sigma$- and two $\pi$-electron bands around the Fermi energy.
The superconducting pairs are glued predominately by the strong interaction
between boron $\sigma$-bonding electrons and various optical phonon modes. | 1307.1323v1 |
2013-07-04 | "Anti-glitches" in the Quark-Nova model for AXPs | In the Quark-Nova model, AXPs are quark stars surrounded by a degenerate
iron-rich Keplerian ring (a few stellar radii away). AXP bursts are caused by
accretion of chunks from the inner edge of the ring following magnetic field
penetration. For bright bursts, the inner disk is prone to radiation induced
warping which can tilt it into counter-rotation (i.e. retrograde). For AXP
1E2259+586, the 2002 burst satisfies the condition for the formation of a
retrograde inner ring. We hypothesize the 2002 burst reversed the inner ring
setting the scene for the 2012 outburst and "anti-glitch" when the retrograde
inner ring was suddenly accreted leading to the basic observed properties of
the 2012 event. | 1307.1386v2 |
2013-07-07 | Andreev Reflection Like Enhancement Above Bulk $T_c$ in Electron Underdoped Iron Arsenides | We use point contact spectroscopy (PCS) to probe the superconducting
properties of electron doped $\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ ($\rm{x = 0.05,
0.055, 0.07, 0.08}$) and hole doped $\rm{Ba_{0.8}K_{0.2}Fe_2As_2}$. PCS
directly probes the low energy density of states via Andreev reflection,
revealing two distinct superconducting gaps in both compound families. Apart
from the electron underdoped $\rm{Ba(Fe_{1-x}Co_{x})_2As_2}$, the excess
current due to Andreev reflection for the compounds follows the typical BCS
temperature dependence. For underdoped $\rm{Ba(Fe_{1-x}Co_{x})_2As_2}$, the
temperature dependence of the excess current deviates from that of BCS,
developing a tail at higher temperatures and surviving above bulk $T_c$.
Possible explanations for this anomalous behavior are explored. | 1307.1908v1 |
2013-07-10 | Kondo-like mass enhancement of Dirac fermion in iron pnictides Ba(Fe$_{1-x}$Mn$_x$As)$_2$ | The effect of Mn substitution, acting as a magnetic impurity for Fe, on the
Dirac cone was investigated in Ba(Fe$_{1-x}$Mn$_x$As)$_2$. Both
magnetoresistance and Hall resistivity studies clearly indicate that the
cyclotron effective mass ($m^{\ast}$) of the Dirac cone is anomalously enhanced
at low temperatures by the impurity, although its evolution as a function of
carrier number proceeds in a conventional manner at higher temperatures.
Kondo-like band renormalization induced by the magnetic impurity scattering is
suggested as an explanation for this, and the anomalous mass enhancement of the
Dirac fermions is discussed. | 1307.2813v3 |
2013-07-11 | Progressive slowing down of spin fluctuations in underdoped LaFeAsO$_{1-x}$F$_x$ | The evolution of low-energy spin dynamics in the iron-based superconductor
LaFeAsO$_{1-x}$F$_x$ was studied over a broad doping, temperature, and magnetic
field range (x = 0 - 0.15, T up to 480K, H up to 30T) by means of As nuclear
magnetic resonance (NMR). An enhanced spin-lattice relaxation rate divided by
temperature, 1/T1T, in underdoped superconducting samples (x = 0.045, 0.05 and
0.075) suggests the presence of antiferromagnetic spin fluctuations, which are
strongly reduced in optimally-doped (x = 0.10) and completely absent in
overdoped (x = 0.15) samples. In contrast to previous analysis, Curie-Weiss
fits are shown to be insufficient to describe the data over the whole
temperature range. Instead, a BPP-type model is used to describe the occurrence
of a peak in 1/T1T clearly above the superconducting transition, reflecting a
progressive slowing down of the spin fluctuations down to the superconducting
phase transition. | 1307.3138v1 |
2013-07-12 | $B_{1g}$-like pairing states in two-leg ladder iron superconductors | Motivated by the recent report of superconductivity in Fe-based ladder
materials, we study the pairing state of a multi-orbital t-J model defined on
two-leg ladders using the standard mean-field theory. We find that the
superconducting order parameters change sign between the $d_{xz}$ and $d_{yz}$
orbitals in most of the phase diagram. By analogy with the two-dimensional Fe
planes, we conclude that the leading pairing channel of this state belongs to
the $B_{1g}$ symmetry class, which is distinct from the common $s_{\pm}$ gap
with the $A_{1g}$ symmetry. By smoothly interpolating from planes into ladders,
we show that a first-order transition occurs between these two competing phases
when the dimension of the system is reduced. | 1307.3309v2 |
2013-07-12 | Electron-capture supernovae as sources of 60Fe | We investigate the nucleosynthesis of the radionuclide 60Fe in
electron-capture supernovae (ECSNe). The nucleosynthetic results are based on a
self-consistent, two-dimensional simulation of an ECSN as well as models in
which the densities are systematically increased by some factors (low-entropy
models). 60Fe is found to be appreciably made in neutron-rich ejecta during the
nuclear quasi-equilibrium phase with greater amounts being produced in the
lower-entropy models. Our results, combining them with the yields of
core-collapse supernovae (CCSNe) in the literature, suggest that ECSNe account
for at least 4-30% of live 60Fe in the Milky Way. ECSNe co-produce neutron-rich
isotopes, 48Ca, 50Ti, 54Cr, some light trans-iron elements, and possibly weak
r-process elements including some radionuclides such as 93Zr, 99Tc, and 107Pd,
whose association with 60Fe might have been imprinted in primitive meteorites
or in the deep ocean crust on the Earth. | 1307.3319v2 |
2013-07-12 | Electronic effects in high-energy radiation damage in iron | Electronic effects are believed to be important in high--energy radiation
damage processes where high electronic temperature is expected, yet their
effects are not currently understood. Here, we perform molecular dynamics
simulations of high-energy collision cascades in $\alpha$-iron using the
coupled two-temperature molecular dynamics (2T-MD) model that incorporates both
effects of electronic stopping and electron-ion interaction. We subsequently
compare it with the model employing the electronic stopping only, and find
several interesting novel insights. The 2T-MD results in both decreased damage
production in the thermal spike and faster relaxation of the damage at short
times. Notably, the 2T-MD model gives a similar amount of the final damage at
longer times, which we interpret to be the result of two competing effects:
smaller amount of short-time damage and shorter time available for damage
recovery. | 1307.3385v2 |
2013-07-12 | Artificially engineered superlattices of pnictide superconductor | Significant progress has been achieved in fabricating high quality bulk and
thinfilm iron-based superconductors. In particular, artificial layered pnictide
superlattices offer the possibility of tailoring the superconducting properties
and understanding the mechanism of the superconductivity itself. For high field
applications, large critical current densities (Jc) and irreversibility fields
(Hirr) are indispensable along all crystal directions. On the other hand, the
development of superconducting devices such as tunnel junctions requires
multilayered heterostructures. Here we show that artificially engineered
undoped Ba-122 / Co doped Ba-122 compositionally modulated superlattices
produce ab-aligned nanoparticle arrays. These layer and self-assemble along
c-axis aligned defects, and combine to produce very large Jc and Hirr
enhancements over a wide angular range. We also demonstrate a structurally
modulated SrTiO3 (STO) / Co doped Ba-122 superlattice with sharp interfaces.
Success in superlattice fabrication involving pnictides will aid the progress
of heterostructured systems exhibiting novel interfacial phenomena and device
applications. | 1307.3576v1 |
2013-07-17 | Growth modes of nanoparticle superlattice thin films | We report about the fabrication and characterization of iron oxide
nanoparticle thin film superlattices. The formation into different film
morphologies is controlled by tuning the particle plus solvent-to-substrate
interaction. It turns out that the wetting vs. dewetting properties of the
solvent before the self-assembly process during solvent evaporation plays a
major role to determine the resulting film morphology. In addition to layerwise
growth also three-dimensional mesocrystalline growth is evidenced. The
understanding of the mechanisms ruling nanoparticle self-assembly represents an
important step toward the fabrication of novel materials with tailored optical,
magnetic or electrical transport properties. | 1307.4693v4 |
2013-07-19 | Microstructure and Structural Phase Transitions in Iron-Based Superconductors | Crystal structures and microstructural features, such as structural phase
transitions, defect structures, chemical and structural inhomogeneities, are
known to have profound effects on the physical properties of superconducting
materials. Recently, a large amount of works on the structural properties of
Fe-based high-Tc superconductors have been published. This review article will
mainly focus on typical microstructural features in samples that have been well
characterized by physical measurements. (a) Certain common structural features
are discussed. In particular, crystal structural features for different
superconducting families, local structural distortions in the Fe2Pn2 (Pn=P, As,
Sb) or Fe2Ch2 (Ch=S, Se, Te) blocks, and structural transformations in the 122
system. (b) In FeTe(Se) (11-family), the superconductivity, chemical and
structural inhomogeneities are investigated and discussed in correlation with
superconductivity. (c) In K0.8Fe1.6+xSe2 system, we focus on typical compounds
with emphasis on Fe-vacancy order and phase separations. The microstructural
features in other superconducting materials are also briefly discussed. | 1307.5112v1 |
2013-07-24 | Role of Dirac cones in magnetotransport properties of REFeAsO (RE=rare earth) oxypnictides | In this work we study the effect of the rare earth element in iron
oxypnictides of composition REFeAsO (RE=rare earth). On one hand we carry out
Density Functional Theory calculations of the band structure, which evidence
the multiband character of these compounds and the presence of Dirac cones
along the Y-{\Gamma} and Z-R directions of the reciprocal space. On the other
hand, we explore transport behavior by means of resistivity, Hall resistance
and magnetoresistance measurements, which confirm the dominant role of Dirac
cones. By combining our theoretical and experimental approaches, we extract
information on effective masses, scattering rates and Fermi velocities for
different rare earth elements. | 1307.6352v1 |
2013-07-26 | N-Body Simulation of the Formation of the Earth-Moon System from a Single Giant Impact | The giant impact hypothesis is the dominant theory of how the Earth-Moon
system was formed. Models have been created that can produce a disk of debris
with the proper mass and composition to create our Moon. Models have also been
created which start with a disk of debris that eventually coalesces into a
Moon. To date, no model has been created that produces a stable Earth-Moon
system in a single simulation. Here we combine two recently published ideas in
this field, along with a new gravity-centered model, and generate such a
simulation. In addition, we show how the method can produce a heterogeneous,
iron-deficient Moon made of mantle material from both colliding bodies, and a
resultant Earth whose equatorial plane is significantly tilted off the ecliptic
plane. The accuracy of the simulation adds credence to the theory that our Moon
was born from the violent union of two heavenly bodies. | 1307.7062v1 |
2013-07-26 | Ferro-orbital ordering transition in iron telluride Fe$_{1+y}$Te | Fe$_{1+y}$Te with $y \lesssim 0.05$ exhibits a first-order phase transition
on cooling to a state with a lowered structural symmetry, bicollinear
antiferromagnetic order, and metallic conductivity, $d\rho/dT > 0$. Here, we
study samples with $y = 0.09(1)$, where the frustration effects of the
interstitial Fe decouple different orders, leading to a sequence of
transitions. While the lattice distortion is closely followed by
\emph{incommensurate} magnetic order, the development of \emph{bicollinear}
order and metallic electronic coherence is uniquely associated with a separate
hysteretic first-order transition, at a markedly lower temperature, to a phase
with dramatically enhanced bond-order wave (BOW) order. The BOW state suggests
ferro-orbital ordering, where electronic delocalization in ferromagnetic zigzag
chains decreases local spin and results in metallic transport. | 1307.7162v2 |
2013-07-26 | Linear magnetoconductivity in multiband spin-density-wave metals with nonideal nesting | In several parent iron-pnictide compounds the resistivity has an extended
range of linear magnetic field dependence. We argue that there is a simple and
natural explanation of this behavior. Spin density wave transition leads to
Fermi-surface reconstruction corresponding to strong modification of the
electronic spectrum near the nesting points. It is difficult for quasiparticles
to pass through these points during their orbital motion in magnetic field,
because they must turn sharply. As the area of the Fermi surface affected by
the nesting points increases proportionally to magnetic field, this mechanism
leads to the linear magnetoresistance. The crossover between the quadratic and
linear regimes takes place at the field scale set by the SDW gap and scattering
rate. | 1307.7184v3 |
2013-07-29 | Simple concentration-dependent pair interaction model for large-scale simulations of Fe-Cr alloys | This work is motivated by the need for large-scale simulations to extract
physical information on the iron-chromium system that is a binary model alloy
for ferritic steels used or proposed in many nuclear applications. From
first-principles calculations and the experimental critical temperature we
build a new energetic rigid lattice model based on pair interactions with
concentration and temperature dependence. Density functional theory
calculations in both norm-conserving and projector augmented-wave approaches
have been performed. A thorough comparison of these two different ab initio
techniques leads to a robust parametrization of the Fe-Cr Hamiltonian.
Mean-field approximations and Monte Carlo calculations are then used to account
for temperature effects. The predictions of the model are in agreement with the
most recent phase diagram at all temperatures and compositions. The solubility
of Cr in Fe below 700 K remains in the range of about 6 to 12%. It reproduces
the transition between the ordering and demixing tendency and the spinodal
decomposition limits are also in agreement with the values given in the
literature. | 1307.7561v1 |
2013-08-02 | Effect of heavy-ion irradiation on London penetration depth in over-doped Ba(Fe,Co)2As2 | Irradiation with 1.4 GeV $^{208}$Pb ions was used to induce artificial
disorder in single crystals of iron-arsenide superconductor
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and to study its effect on the
temperature-dependent London penetration depth and transport properties. Study
was undertaken on overdoped single crystals with $x$=0.108 and $x$=0.127
characterized by notable modulation of the superconducting gap. Irradiation
with doses 2.22$\times10^{11}$$\textit{d}$/cm$^2$ and
2.4$\times10^{11}$$\textit{d}$/cm$^2$, corresponding to the matching fields of
$B_{\phi} = $6 T and 6.5 T, respectively, suppresses the superconducting $T_c$
by approximately 0.3 to 1 K. The variation of the low-temperature penetration
depth in both pristine and irradiated samples is well described by the
power-law, $\Delta \lambda (T)=AT^n$. Irradiation increases the magnitude of
the pre-factor $A$ and decreases the exponent $n$, similar to the effect of
irradiation in optimally doped samples. This finding supports universal
$s_{\pm}$ pairing in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ compounds for the whole Co
doping range. | 1308.0537v1 |
2013-08-10 | Impurity effects on electronic transport in ferropnictide superconductors | Effects of impurities and disorder on transport properties by electronic
quasiparticles in superconducting iron pnictides are theoretically considered.
The most prominent new features compared to the case of pure material should
appear at high enough impurity concentration when a specific narrow band of
conducting quasiparticle states can develop within the superconducting gap,
around the position of localized impurity level by a single impurity center.
The predicted specific threshold effects in the frequency dependent optical
conductivity and temperature dependent thermal conductivity and also in Seebeck
and Peltier coefficients can have interesting potentialities for practical
applications. | 1308.2351v3 |
2013-08-13 | Strain effects on the electronic structure of the FeSe0.5Te0.5 superconductor | The electronic structure of the strained FeSe0.5Te0.5 superconductor has been
investigated from first principles. Our calculation results indicate that the
influence of hydrostatic, biaxial or uniaxial compressive stress on the density
of states at the Fermi level is insignificant. The overall shape of the
Fermi-surface (FS) nesting function for FeSe0.5Te0.5 at ambient pressure
resembles that of its parent compound, FeSe, but under the ab-plane compressive
strain. In these two systems, changes of their FSs under various stress
conditions are qualitatively almost the same. However, in FeSe0.5Te0.5 the
intensity of the perfect Q=(0.5,0.5)*(2\pi/a) nesting vector is more
diminished. These findings are in good agreement with former experimental data
and support the idea of spin-fluctuation mediated superconductivity in iron
chalcogenides. | 1308.2949v1 |
2013-08-15 | Magnetic Phase Diagram of a Five-Orbital Hubbard Model in the Real-Space Hartree Fock Approximation Varying the Electronic Density | Using the real-space Hartree Fock approximation, the magnetic phase diagram
of a five-orbital Hubbard model for the iron-based superconductors is studied
varying the electronic density $n$ in the range from 5 to 7 electrons per
transition metal atom. The Hubbard interaction $U$ is also varied, at a fixed
Hund coupling $J/U=0.25$. Several qualitative trends and a variety of competing
magnetic states are observed. At $n$=5, a robust G-type antiferromagnetic
insulator is found, in agreement with experimental results for BaMn$_2$As$_2$.
As $n$ increases away from 5, magnetic states with an increasing number of
nearest-neighbors ferromagnetic links become energetically stable. This
includes the well-known C-type antiferromagnetic state at $n$=6, the E-phase
known to exist in FeTe, and also a variety of novel states not found yet
experimentally, some of them involving blocks of ferromagnetically oriented
spins. Regions of phase separation, as in Mn-oxides, have also been detected.
Comparison with previous theoretical investigations indicate that these
qualitative trends may be generic characteristics of phase diagrams of
multiorbital Hubbard models. | 1308.3426v1 |
2013-08-16 | Quantum Criticality in Electron-doped BaFe_{2-x}Ni_xAs_2 | A quantum critical point (QCP) is a point in a system's phase diagram at
which an order is completely suppressed at absolute zero temperature (T). The
presence of a quantum critical point manifests itself in the finite-T physical
properties, and often gives rise to new states of matter. Superconductivity in
the cuprates and in heavy fermion materials is believed by many to be mediated
by fluctuations associated with a quantum critical point. In the
recently-discovered iron-pnictide high temperature superconductors, it is
unknown whether a QCP exists or not in a carrier-doped system. Here we report
transport and nuclear magnetic resonance (NMR) measurements on
BaFe_{2-x}Ni_xAs_2 (0 =< x =< 0.17). We find two critical points at x_{c1} =
0.10 and x_{c2} = 0.14. The electrical resistivity follows \rho = \rho_0 +
A*T^n, with n = 1 around x_{c1} and another minimal n = 1.1 at x_{c2}. By NMR
measurements, we identity x_{c1} to be a magnetic QCP and suggest that x_{c2}
is a new type of QCP associated with a nematic structural phase transition. Our
results suggest that the superconductivity in carrier-doped pnictides is
closely linked to the quantum criticality. | 1308.3539v1 |
2013-08-18 | Spin excitation anisotropy as a probe of orbital ordering in the paramagnetic tetragonal phase of superconducting BaFe1.904Ni0.096As2 | We use polarized neutron scattering to demonstrate that in-plane spin
excitations in electron doped superconducting BaFe1.904Ni0.096As2 (Tc=19.8 K)
change from isotropic to anisotropic in the tetragonal phase well above the
antiferromagnetic (AF) ordering and tetragonal-to-orthorhombic lattice
distortion temperatures (Tn=Ts=33 K) without an uniaxial pressure. While the
anisotropic spin excitations are not sensitive to the AF order and
tetragonal-to-orthorhombic lattice distortion, superconductivity induces
further anisotropy for spin excitations along the [1,1,0] and [1,-1,0]
directions. These results indicate that the spin excitation anisotropy is a
probe of the electronic anisotropy or orbital ordering in the tetragonal phase
of iron pnictides. | 1308.3858v1 |
2013-08-19 | The Origin of HVS17, an Unbound Main Sequence B Star at 50 kpc | We analyze Keck ESI spectroscopy of HVS17, a B-type star traveling with a
Galactic rest frame radial velocity of +445 km/s in the outer halo of the Milky
Way. HVS17 has the projected rotation of a main sequence B star and is
chemically peculiar, with solar iron abundance and sub-solar alpha abundance.
Comparing measured T_eff and logg with stellar evolution tracks implies that
HVS17 is a 3.91 +-0.09 Msun, 153 +-9 Myr old star at a Galactocentric distance
of r=48.5 +-4.6 kpc. The time between its formation and ejection significantly
exceeds 10 Myr and thus is difficult to reconcile with any Galactic disk
runaway scenario involving massive stars. The observations are consistent, on
the other hand, with a hypervelocity star ejection from the Galactic center. We
show that Gaia proper motion measurements will easily discriminate between a
disk and Galactic center origin, thus allowing us to use HVS17 as a test
particle to probe the shape of the Milky Way's dark matter halo. | 1308.4181v1 |
2013-08-20 | Spatial Inhomogeneity of the Superconducting Gap and Order Parameter in FeSe_{0.4}Te_{0.6} | We have performed a low temperature scanning tunneling microscopy and
spectroscopy study of the iron chalcogenide superconductor FeSe_{0.4}Te_{0.6}
with T_{C}~14 K. Spatially resolved measurements of the superconducting gap
reveal substantial inhomogeneity on a nanometer length scale. Analysis of the
structure of the gap seen in tunneling spectra by comparison with calculated
spectra for different superconducting order parameters (s-wave, d-wave, and
anisotropic s-wave) yields the best agreement for an order parameter with
anisotropic s-wave symmetry with an anisotropy of ~40%. The temperature
dependence of the superconducting gap observed in places with large and small
gap size indicates that it is indeed the superconducting transition temperature
which is inhomogeneous. The temperature dependence of the gap size is
substantially larger than would be expected from BCS theory. An analysis of the
local gap size in relation with the local chemical composition shows almost no
correlation with the local concentration of Se-/Te-atoms at the surface. | 1308.4339v1 |
2013-08-22 | A Precision Measurement of Charm Dimuon Production in Neutrino Interactions from the NOMAD Experiment | We present our new measurement of the cross-section for charm dimuon
production in neutrino-iron interactions based upon the full statistics
collected by the NOMAD experiment. After background subtraction we observe
15,344 charm dimuon events, providing the largest sample currently available.
The analysis exploits the large inclusive charged current sample - about
$9\times 10^6$ events after all analysis cuts - and the high resolution NOMAD
detector to constrain the total systematic uncertainty on the ratio of charm
dimuon to inclusive Charged Current (CC) cross-sections to $\sim 2%$. We also
perform a fit to the NOMAD data to extract the charm production parameters and
the strange quark sea content of the nucleon within the NLO QCD approximation.
We obtain a value of $m_c(m_c)=1.159\pm0.075$ GeV/c$^2$ for the running mass of
the charm quark in the $\bar{\rm MS}$ scheme and a strange quark sea
suppression factor of $\kappa_s = 0.591 \pm 0.019$ at $Q^2=20$ GeV$^2$/c$^2$. | 1308.4750v1 |
2013-08-27 | Origin of Dust around V1309 Sco | The origin of dust grains in the interstellar medium is still open problem.
\cite{Nicholls2013} found the presence of a significant amount of dust around
V1309 Sco which maybe originate from the merger of a contact binary. We
investigate the origin of dust around V1309 Sco, and suggest that these dust
grains are efficiently produced in the binary-merger ejecta. By means of
\emph{AGBDUST} code, we estimate that $\sim 5.2\times10^{-4} M_\odot$ of dust
grains are produced, and their radii are $\sim 10^{-5}$ cm. These dust grains
mainly are composed of silicate and iron grains. Because the mass of the
binary-merger ejecta is very small, the contribution of dust produced by
binary-merger ejecta to the overall dust production in the interstellar medium
is negligible. However, it is the most important that the discovery of a
significant amount of dust around V1309 Sco offers a direct support for the
idea---common-envelope ejecta provides an ideal environment for dust formation
and growth. Therefore, we confirm that common-envelope ejecta can be important
source of cosmic dust. | 1308.5735v1 |
2013-08-27 | Calculating Time Lags From Unevenly-Sampled Light Curves | Timing techniques offer powerful tools to study dynamical astrophysical
phenomena. In the X-ray band, they offer the potential of probing accretion
physics down to the event horizon. Recent work has used frequency and
energy-dependent time lags as a tool for studying relativistic reverberation
around the black holes in several Seyfert galaxies. This was achieved thanks to
the evenly-sampled light curves obtained using XMM-Newton. Continuous-sampled
data is however not always available and standard Fourier techniques are not
applicable. Here, building on the work of Miller et al. (2010), we discuss and
use a maximum likelihood method to obtain frequency-dependent lags that takes
into account light curve gaps. Instead of calculating the lag directly, the
method estimates the most likely lag values at a particular frequency given two
observed light curves. We use Monte Carlo simulations to assess the method's
applicability, and use it to obtain lag-energy spectra from Suzaku data for two
objects, NGC 4151 and MCG-5-23-16, that had previously shown signatures of iron
K reverberation. The lags obtained are consistent with those calculated using
standard methods using XMM-Newton data. | 1308.5852v1 |
2013-08-28 | Tunneling spectroscopy for probing orbital anisotropy in iron pnictides | Using realistic multi-orbital tight-binding Hamiltonians and the T-matrix
formalism, we explore the effects of a non-magnetic impurity on the local
density of states in Fe-based compounds. We show that scanning tunneling
spectroscopy (STS) has very specific anisotropic signatures that track the
evolution of orbital splitting (OS) and antiferromagnetic gaps. Both
anisotropies exhibit two patterns that split in energy with decreasing
temperature, but for OS these two patterns map onto each other under 90 degree
rotation. STS experiments that observe these signatures should expose the
underlying magnetic and orbital order as a function of temperature across
various phase transitions. | 1308.6248v2 |
2013-08-30 | Why is the superconducting Tc so high in rare-earth-doped CaFe2As2? | In rare-earth doped single crystalline CaFe2As2, the mysterious small volume
fraction which superconducts up to 49 K, much higher than the bulk Tc ~ 30s K,
has prompted a long search for a hidden variable that could enhance the Tc by
more than 30% in iron-based superconductors of the same structure. Here we
report a chemical, structural, and magnetic study of CaFe2As2 systematically
doped with La, Ce, Pr, and Nd. Coincident with the high Tc phase, we find
extreme magnetic anisotropy, accompanied by an unexpected doping-independent Tc
and equally unexpected superparamagnetic clusters associated with As vacancies.
These observations lead us to conjecture that the tantalizing Tc enhancement
may be associated with naturally occurring chemical interfaces and may thus
provide a new paradigm in the search for superconductors with higher Tc. | 1309.0034v2 |
2013-09-08 | Evidence of unconventional low-frequency dynamics in the normal phase of Ba(Fe1-xRhx)2As2 iron-based supercondutors | This work presents 75As NMR spin echo decay rate (1/T2) measurements in
Ba(Fe1-xRhx)2As2 superconductors, for 0.041 < x < 0.094. It is shown that 1/T2
increases upon cooling, in the normal phase, suggesting the onset of an
unconventional very low-frequency activated dynamic. The correlation times of
the fluctuations and their energy barriers are derived. The motion is favored
at large Rh content, while it is hindered by the application of a magnetic
field perpendicular to the FeAs layers. The same dynamic is observed in the
spin-lattice relaxation rate, in a quantitatively consistent manner. These
results are discussed in the light of nematic fluctuations involving domain
wall motion. The analogies with the behaviour observed in the cuprates are also
outlined. | 1309.1936v1 |
2013-09-11 | Concepts relating magnetic interactions, intertwined electronic orders and strongly correlated superconductivity | Unconventional superconductivity (SC) is said to occur when Cooper pair
formation is dominated by repulsive electron-electron interactions, so that the
symmetry of the pair wavefunction is other than isotropic s-wave. The strong,
on-site, repulsive electron-electron interactions that are the proximate cause
of such superconductivity are more typically drivers of commensurate magnetism.
Indeed, it is the suppression of commensurate antiferromagnetism (AF) that
usually allows this type of unconventional superconductivity to emerge.
Importantly, however, intervening between these AF and SC phases, intertwined
electronic ordered phases of an unexpected nature are frequently discovered.
For this reason, it has been extremely difficult to distinguish the microscopic
essence of the correlated superconductivity from the often spectacular
phenomenology of the intertwined phases. Here we introduce a model conceptual
framework within which to understand the relationship between antiferromagnetic
electron-electron interactions, intertwined ordered phases and correlated
superconductivity. We demonstrate its effectiveness in simultaneously
explaining the consequences of antiferromagnetic interactions for the
copper-based, iron-based and heavy-fermion superconductors, as well as for
their quite distinct intertwined phases. | 1309.2719v1 |
2013-09-12 | Low-energy interband transitions in the infrared response of Ba(Fe1-xCox)2As2 | We studied the doping and temperature (T) dependence of the infrared (IR)
response of Ba(Fe1-xCox)2As2 single crystals. We show that a weak band around
1000 cm-1, that was previously interpreted in terms of interaction of the
charge carriers with magnetic excitations or of a pseudogap, is rather related
to low-energy interband transitions. Specifically, we show that this band
exhibits a similar doping and T-dependence as the hole pockets seen by angle
resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes
as a function of doping near the critical point where superconductivity is
suppressed in the overdoped regime. Our IR data thus provide bulk specific
information (complementary to the surface sensitive ARPES) for a Lifshitz
transition. Our IR data also reveal a second low-energy band around 2300 cm-1
which further emphasizes the necessity to consider the multiband nature of
these iron arsenides in the analysis of the optical response. | 1309.3198v1 |
2013-09-13 | The lateral shower age parameter as an estimator of chemical composition | We explore the feasibility of estimating primary cosmic ray composition at
ultra high energies from the study of lateral age parameter of Extensive Air
Showers (EAS) at ground level. Using different types of lateral distribution
functions, we fit the particle density of simulated EAS to find the lateral age
parameter. We discuss the chemical composition calculating the merit factor for
each parameter distribution. The analysis considers three different primary
particles (proton, iron and gamma), four different zenith angles (0{\deg},
15{\deg}, 30{\deg} and 45{\deg}) and three primary energies (10^{17.25} eV,
10^{17.50} eV and 10^{17.75} eV). | 1309.3536v1 |
2013-09-17 | Strongly Correlated Materials | Strongly correlated materials are profoundly affected by the repulsive
electron-electron interaction. This stands in contrast to many commonly used
materials such as silicon and aluminum, whose properties are comparatively
unaffected by the Coulomb repulsion. Correlated materials often have remarkable
properties and transitions between distinct, competing phases with dramatically
different electronic and magnetic orders. These rich phenomena are fascinating
from the basic science perspective and offer possibilities for technological
applications. This article looks at these materials through the lens of
research performed at Rice University. Topics examined include: Quantum phase
transitions and quantum criticality in "heavy fermion" materials and the iron
pnictide high temperature superconductors; computational ab initio methods to
examine strongly correlated materials and their interface with analytical
theory techniques; layered dichalcogenides as example correlated materials with
rich phases (charge density waves, superconductivity, hard ferromagnetism) that
may be tuned by composition, pressure, and magnetic field; and nanostructure
methods applied to the correlated oxides VO2 and Fe3O4, where metal-insulator
transitions can be manipulated by doping at the nanoscale or driving the system
out of equilibrium. We conclude with a discussion of the exciting prospects for
this class of materials. | 1309.4473v1 |
2013-09-23 | Interactions of Cosmic Rays in the Atmosphere: Growth Curves Revisited | Measurements of cosmic-ray abundances on balloons are affected by
interactions in the residual atmosphere above the balloon. Corrections for such
interactions are particularly important for observations of rare secondary
particles such as boron, antiprotons and positrons. These corrections can
either be calculated if the relevant cross sections in the atmosphere are
known, or may be empirically determined by extrapolation of the "growth
curves", i. e. the individual particle intensities as functions of atmospheric
depth. The growth-curve technique is particularly attractive for long-duration
balloon flights where the periodic daily altitude variations permit rather
precise determinations of the corresponding particle intensity variations. We
determine growth curves for nuclei from boron (Z=5) to iron (Z=26), using data
from the 2006 Arctic balloon flight of the TRACER detector for cosmic-ray
nuclei, and we compare the growth curves with predictions from published cross
section values. In general, good agreement is observed. We then study the
boron/carbon abundance ratio and derive a simple and energy-independent
correction term for this ratio. We emphasize that the growth-curve technique
can be developed further to provide highly accurate tests of published
interaction cross section values. | 1309.5739v1 |
2013-09-26 | The polluted atmosphere of the white dwarf NLTT 25792 and the diversity of circumstellar environments | We present an analysis of X-Shooter spectra of the polluted, hydrogen-rich
white dwarf NLTT 25792. The spectra show strong lines of calcium (Ca H&K,
near-infrared calcium triplet, and Ca I 4226 and numerous lines of iron along
with magnesium and aluminum lines from which we draw the abundance pattern.
Moreover, the photospheric Ca H&K lines are possibly blended with a
circumstellar component shifted by -20 km/s relative to the photosphere. A
comparison with a sample of four white dwarfs with similar parameters show
considerable variations in their abundance patterns, particularly in the
calcium to magnesium abundance ratio that varies by a factor of five within
this sample. The observed variations, even after accounting for diffusion
effects, imply similar variations in the putative accretion source. Also, we
find that silicon and sodium are significantly underabundant in the atmosphere
of NLTT 25792, a fact that may offer some clues on the nature of the accretion
source. | 1309.6804v1 |
2013-09-27 | Unusual pressure effects on the superconductivity of indirectly electron-doped (Ba1-xLax)Fe2As2 epitaxial films | Applying an external pressure to indirectly electron-doped 122-type
(Ba1-xLax)Fe2As2 epitaxial films enhances the superconducting critical
temperature (Tc) up to 30.3 K. Different from the other family compounds, the
Tc is enhanced not only in the under-doped region but also in the optimally
doped and over-doped regions. Narrowing of the superconducting transition width
and an increase in the carrier density take place simultaneously in the whole
doping region, except at the heavily over-doped limit. This characteristic is
unique to and observed only in (Ba1-xLax)Fe2As2, in which the La doping is
stabilized via non-equilibrium growth of the vapor phase epitaxy, among the
122-type iron-based superconductors, AFe2As2 (A = Ba, Sr, and Ca). | 1309.7100v2 |
2013-09-27 | Beating the superparamagnetic size limit of nanoparticles on a ferroelectric substrate | When decreasing the size of nanoscale magnetic particles their magnetization
becomes vulnerable to thermal fluctuations as approaching the superparamgnetic
limit, hindering thus applications relying on a stable magnetization. Here, we
show theoretically that a magnetoelectric coupling to a ferroelectric substrate
renders possible the realization of substantially smaller nano clusters with
thermally stable magnetization. For an estimate of cluster size we perform
calculations with realistic material parameters for iron nano particles on
ferroelectric BaTiO3 substrate. We find, steering the polarization of BaTiO3
with electric fields affects the magnetism of the deposited magnetic clusters.
These findings point to a qualitatively new class of superparamagnetic
composites. | 1309.7136v1 |
2013-09-27 | Inelastic neutron scattering study of crystal field excitations of Nd3+ in NdFeAsO | Inelastic neutron scattering experiments were performed to investigate the
crystalline electric field (CEF) excitations of Nd3+ (J = 9/2) in the iron
pnictide NdFeAsO. The crystal field level structures for both the
high-temperature paramagnetic phase and the low-temperature antiferromagnetic
phase of NdFeAsO are constructed. The variation of CEF excitations of Nd3+
reflects not only the change of local symmetry but also the change of magnetic
ordered state of the Fe sublattice. By analyzing the crystal field interaction
with a crystal field Hamiltonian, the crystal field parameters are obtained. It
was found that the sign of the fourth and sixth-order crystal field parameters
change upon the magnetic phase transition at 140 K, which may be due to the
variation of exchange interactions between the 4f and conduction electrons. | 1309.7323v1 |
2013-09-29 | Coupled magnetic and ferroelectric excitations in PbFe_{1/2}Nb_{1/2}O_{3} | A neutron scattering investigation of the magnetoelectric coupling in
PbFe_{1/2}Nb_{1/2}O_{3} (PFN) has been undertaken. Ferroelectric order occurs
below 400 K, as evidenced by the softening with temperature and subsequent
recovery of the zone center transverse optic phonon mode energy (\hbar
\Omega_{0}). Over the same temperature range, magnetic correlations become
resolution limited on a terahertz energy scale. In contrast to the behavior of
nonmagnetic disordered ferroelectrics (namely Pb(Mg,Zn)_{1/3}Nb_{2/3}O_{3}), we
report the observation of a strong deviation from linearity in the temperature
dependence of (\hbar \Omega_{0})^{2}. This deviation is compensated by a
corresponding change in the energy scale of the magnetic excitations, as probed
through the first moment of the inelastic response. The coupling between the
short-range ferroelectric and antiferromagnetic correlations is consistent with
calculations showing that the ferroelectricity is driven by the displacement of
the body centered iron site, illustrating the multiferroic nature of magnetic
lead based relaxors in the dynamical regime. | 1309.7587v1 |
2013-09-29 | Strongly enhanced current densities in Sr0.6K0.4Fe2As2 superconducting tapes | Improving transport current has been the primary issue for practical
application of superconducting wires and tapes. However, the porous nature of
powder-in-tube (PIT) processed iron-based superconducting tapes is thought to
be one of the important reasons for the low Jc values. In this work, the
superconducting core density of ex-situ Sr0.6K0.4Fe2As2 tapes was significantly
improved by employing a simple hot pressing as an alternative route for final
sintering. At 4.2 K, the transport Jc values showed excellent values of
5.1x10^4 A/cm^2 at 10 T and 4.3x10^4 A/cm^2 at 14 T, respectively, which attain
the Jc level desired for practical applications. Moreover, the Jc values
exhibited extremely weak magnetic field dependence. These results clearly
demonstrate that PIT pnictide wire conductors are very promising for high field
magnet applications. | 1309.7618v2 |
2013-10-02 | Chemical Abundances of RR Lyrae Type C Star AS162158 | We report the first extensive model atmosphere and detailed chemical
abundance study of eight RR Lyrae variable stars of c subclass throughout their
pulsation cycles. Atmospheric parameters effective temperature, surface
gravity, microturbulent velocity, and metallicity have been derived. Spectra
for this abundace analysis have been obtained with the echelle spectrograph of
100-inch du Pont telescope at Las Campanas Observatory. We have found
metallicities and element abundance ratios to be constant within observational
uncertainties at all phases of all stars. Moreover, the $\alpha$-element and
Fe-group abundance ratios with respect to iron are consistent with other
horizontal-branch members (RRab, blue and red non-variables). The [Fe/H] values
of these eight RRc stars have been used to anchor the metallicity scale of a
much larger sample of RRc stars obtained with low S/N "snapshot" spectra. | 1310.0580v1 |
2013-10-04 | In-plane spin excitation anisotropy in the paramagnetic phase of NaFeAs | We use unpolarized and polarized inelastic neutron scattering to study
low-energy spin excitations in NaFeAs, which exhibits a
tetragonal-to-orthorhombic lattice distortion at $T_s\approx 58$ K followed by
a collinear antiferromagnetic (AF) order below $T_N\approx 45$ K. In the AF
ordered state ($T<T_N$), spin waves are entirely c-axis polarized below
$\sim$10 meV, exhibiting a gap of $\sim4$ meV at the AF zone center and
disperse to $\sim$7 meV near the c-axis AF zone boundary. On warming to the
paramagnetic state with orthorhombic lattice distortion ($T_N<T<T_s$), spin
excitations become anisotropic within the FeAs plane. Upon further warming to
the paramagnetic tetragonal state ($T>T_s$), spin excitations become more
isotropic. Since similar magnetic anisotropy is also observed in the
paramagnetic tetragonal phase of superconducting
BaFe$_{1.904}$Ni$_{0.096}$As$_2$, our results suggest that the spin excitation
anisotropy in superconducting iron pnictides originates from similar anisotropy
already present in their parent compounds. | 1310.1143v1 |
2013-10-09 | Spin resonance in AFe2Se2 with s-wave pairing symmetry | We study spin resonance in the superconducting state of recently discovered
alkali-intercalated iron selenide materials A_xFe_(2-y)Se_2 (A=K,Rb,Cs) in
which the Fermi surface has only electron pockets. Recent angle-resolved
photoemission spectroscopy (ARPES) studies [M. Xu et al., Phys. Rev. B 85,
220504(R) (2012)] were interpreted as strong evidence for s-wave gap in these
materials, while the observation of the resonance peak in neutron scattering
measurements [G. Friemel et al., Phys. Rev. B 85, 140511 (2012)] suggests that
the gap must have different signs at Fermi surface points connected by the
momentum at which the resonance has been observed. We consider recently
proposed unconventional s+- superconducting state of A_xFe_(2-y)Se_2 with
superconducting gap changing sign between the hybridized electron pockets. We
argue that such a state supports a spin resonance. We compute the dynamical
structure factor and show that it is consistent with the results of inelastic
neutron scattering. | 1310.2334v2 |
2013-10-10 | Strange Inter-layer Properties of $Ba(Fe_{1-x}Co_x)2As_2$ Appearing in Ultrasonic Measurements | We have investigated the elastic constant C33 of Ba(Fe1-xCox)2As2 with eight
different Co concentrations by ultrasonic measurement. We found remarkable
elastic anomalies near the quantum critical point. We have studied them by
measuring the electrical resistivity, heat capacity, and ultrasonic attenuation
in addition to the elastic constant. These results have revealed that the
inter-layer three-dimensional properties appearing in C33 to be possibly
originated from the magnetic character of these materials. Our data about the
elastic constant C33 highlight the importance of controlling the c-axis length
in the emergence of superconductivity in iron-based superconductors. | 1310.2681v1 |
2013-10-11 | Enhanced superconductivity and evidence for novel pairing in single-layer FeSe on SrTiO3 thin film under large tensile strain | Single-layer FeSe films with extremely expanded in-plane lattice constant of
3.99A are fabricated by epitaxially growing FeSe/Nb:SrTiO3/KTaO3
heterostructures, and studied by in situ angle-resolved photoemission
spectroscopy. Two elliptical electron pockets at the Brillion zone corner are
resolved with negligible hybridization between them, indicating the symmetry of
the low energy electronic structure remains intact as a free-standing
single-layer FeSe, although it is on a substrate. The superconducting gap
closes at a record high temperature of 70K for the iron based superconductors.
Intriguingly, the superconducting gap distribution is anisotropic but nodeless
around the electron pockets, with minima at the crossings of the two pockets.
Our results put strong constraints on the current theories, and support the
coexistence of both even and odd parity spin-singlet pairing channels as
classified by the lattice symmetry. | 1310.3060v1 |
2013-10-20 | Impurity effects in Cu$_2$O | The doping of wide gap semiconductors is an interesting problem both from the
scientific and technological point of view. A well known example of this
problem is the doping of Cu$_2$O. The only element which has produced an order
of magnitude increase in the conductivity of Cu$_2$O bulk samples is chlorine,
as previously reported by us and others. However the solar cells produced with
this material do not show any improvement in performances because of the
reduction in the minority carrier diffusion length. In this paper we
investigate the effect of other impurities in order to check their possible use
as dopants and to assess their effects on the minority carrier diffusion
length. Seven impurities have been introduced by evaporation on the starting
copper sheet before the oxidation used to produce Cu$_2$O: chromium (Cr), iron
(Fe), silver (Ag), silicon (Si), sodium (Na), sulfur (S) and phosphorus (P).
The experiments show that a 20 ppm of concentration of these dopants does not
give any relevant effect neither on the resistivity, nor on the mobility. The
effect on minority carrier diffusion length is also negligible except for
sodium which produces a slight degradation of the samples. | 1310.5341v1 |
2013-10-22 | Point defect modeling in materials: coupling ab initio and elasticity approaches | Modeling point defects at an atomic scale requires careful treatment of the
long-range atomic relaxations. This elastic field can strongly affect point
defect properties calculated in atomistic simulations because of the finite
size of the system under study. This is an important restriction for ab initio
methods which are limited to a few hundred atoms. We propose an original
approach coupling ab initio calculations and linear elasticity theory to obtain
the properties of an isolated point defect for reduced supercell sizes. The
reliability and benefit of our approach are demonstrated for three problematic
cases: the self-interstitial in zirconium, clusters of self-interstitials in
iron, and the neutral vacancy in silicon. | 1310.5799v1 |
2013-10-22 | Nucleosynthesis inside accretion disks and outflows formed during core collapse of massive stars | We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion
disks and in the outflows launched from these disks mainly in the context of
Type II collapsars. We report the synthesis of several unusual nuclei like 31P,
39K, 43Sc, 35Cl and various isotopes of titanium, vanadium, chromium, manganese
and copper in the disk. We also confirm the presence of iron-group and
alpha-elements in the disk, as shown by previous authors. Much of these heavy
elements thus synthesized are ejected from the disk and survive in the
outflows. While emission lines of several of these elements have been observed
in the X-ray afterglows of GRBs by BeppoSAX, Chandra, XMM-Newton etc., Swift
seems to have not found these lines yet. | 1310.5911v1 |
2013-10-22 | Large low-temperature magnetoresistance in SrFe2As2 single crystals | We present the first report on a large low-temperature magnetoresistance (MR)
of more than 1600% in a SrFe2As2 single crystal and 1300% in a low-energy Ca
ion-implanted SrFe2As2 single crystal that occurs before the emergence of
crystallographic strain-induced bulk superconductivity arising from a sample
aging effect. In accordance to band structure calculations from literature,
which consitently show more than 2 bands are involved in the transport, we have
modeled this large MR at high fields using a 3-carrier scenario rather than
solely on quantum linear MR model generally used to explain the MR in
iron-pnictides. At and below 20 K the large MR may be due to 3-carrier
transport in an inhomogeneous state where there are superconducting and
metallic regions. | 1310.6071v1 |
2013-10-28 | Principle of Maximum Entanglement Entropy and Local Physics of Correlated many-body Electron-Systems | We argue that, because of the quantum-entanglement, the local physics of the
strongly-correlated materials at zero temperature is described in very good
approximation by a simple generalized Gibbs distribution, which depends on a
relatively small number local quantum thermodynamical potentials. We
demonstrate that our statement is exact in certain limits, and we perform
numerical calculations of the iron compounds FeSe and FeTe and of the elemental
cerium by employing the Gutzwiller Approximation (GA) that strongly support our
theory in general. | 1310.7520v2 |
2013-10-28 | Enhanced Superconductivity on the Tetragonal Lattice in FeSe under Hydrostatic Pressure | Superconductivity under pressure in FeSe ($T_{\rm c}$$\sim$7.5 K) has been
investigated using single-crystal specimens through the measurements of DC
magnetization and electrical resistivity. A characteristic three-step increase
in $T_{\rm c}$ has been found under hydrostatic pressure up to $\sim$34 K above
7 GPa. The structural transition from a tetragonal phase to an orthorhombic
phase ($T_{\rm s}$$\sim$87 K) is found to disappear at $P$$\sim$2.3 GPa, above
which $T_{\rm c}$ increases rapidly, suggesting that the superconductivity is
enhanced by the tetragonal environment. Under non-hydrostatic pressure, the
increase in $T_{\rm c}$ is suppressed and the superconductive volume fraction
is considerably reduced above 2 GPa, probably owing to the breaking of the
tetragonal lattice symmetry by the uniaxial stress. The intimate correlation
between the enhanced (suppressed) superconductivity and the tetragonality
(orthorhombicity) in the phase diagram is a common feature of FeSe and other
iron-pnictide superconductors. | 1310.7546v3 |
2013-11-01 | New Li-Ethylenediamine-Intercalated Superconductor Li$_x$(C$_2$H$_8$N$_2$)$_y$Fe$_{2-z}$Se$_2$ with $T_c$ = 45 K | A new iron-based superconductor Li$_x$(C$_2$H$_8$N$_2$)$_y$Fe$_{2-z}$Se$_2$
with $T_c$ = 45 K has successfully been synthesized via intercalation of
dissolved lithium metal in ethylenediamine. The distance between neighboring Fe
layers is 10.37 {\AA} and much larger than those of FeSe with $T_c$ = 8 K and
K$_x$Fe$_2$Se$_2$ with $T_c$ ~ 30 K. It seems that the high-$T_c$ of
Li$_x$(C$_2$H$_8$N$_2$)$_y$Fe$_{2-z}$Se$_2$ is caused by the possible
two-dimensional electronic structure due to the large $c$-axis length. | 1311.0141v1 |
2013-11-03 | s-wave pairing in the optimally-doped LaO0.5F0.5BiS2 superconductor | We report on the magnetic and superconducting properties of LaO0.5F0.5BiS2 by
means of zero- (ZF) and transverse-field (TF) muon-spin spectroscopy
measurements (uSR). Contrary to previous results on iron-based superconductors,
measurements in zero field demonstrate the absence of magnetically ordered
phases. TF-uSR data give access to the superfluid density, which shows a marked
2D character with a dominant s-wave temperature behavior. The field dependence
of the magnetic penetration depth confirms this finding and further suggests
the presence of an anisotropic superconducting gap. | 1311.0457v2 |
2013-11-03 | Magnetization distribution and orbital moment in the non-Superconducting Chalcogenide Compound K0.8Fe1.6Se2 | We have used polarized and unpolarized neutron diffraction to determine the
spatial distribution of the magnetization density induced by a magnetic field
of 9 T in the tetragonal phase of K0.8Fe1.6Se2. The maximum entropy
reconstruction shows clearly that most of the magnetization is confined to the
region around the iron atoms whereas there is no significant magnetization
associated with either Se or K atoms. The distribution of magnetization around
the Fe atom is slightly nonspherical with a shape which is extended along the
[0 0 1] direction in the projection. Multipolar refinement results show that
the electrons which give rise to the paramagnetic susceptibility are confined
to the Fe atoms and their distribution suggests that they occupy 3d t2g-type
orbitals with around 66% in those of xz/yz symmetry. Detail modeling of the
magnetic form factor indicates the presence of an orbital moment to the total
paramagnetic moment of Fe2+ | 1311.0492v1 |
2013-11-04 | An extended Heitler-Matthews model for the full hadronic cascade in cosmic air showers | The Heitler-Matthews model for hadronic air showers will be extended to all
the generations of electromagnetic subshowers in the hadronic cascade. The
analysis is outlined in detail for showers initiated by primary protons. For
showers initiated by iron primaries the part of the analysis is given for as
far as it differs from the analysis for a primary proton. Predictions for
shower sizes and the depth of maximum shower size are compared with results of
Monte Carlo simulations. The depth of maximum as it follows from the
extrapolation of the Heitler-Matthews model restricted to the first generation
of electromagnetic subshowers is too small with respect to Monte Carlo
predictions. It is shown that the inclusion of all the generations of
electromagnetic subshowers leads to smaller predictions for the depth of
maximum and to smaller predictions for the elongation rate. The discrepancy
between discrete model predictions and Monte Carlo predictions for the depth of
maximum can therefore not be explained from the number of generations that is
taken into consideration. An alternative explanation will be proposed. | 1311.0642v3 |
2013-11-06 | On the Sodium versus Iron Correlation in Late B-Type Stars | With an aim to study whether the close correlation between [Na/H] and [Fe/H]
recently found in A-type stars further persists in the regime of B-type stars,
the abundances of Na were determined for 30 selected sharp-lined late B-type
stars (10000K < T_eff < 14000K) from the Na I 5890/5896 doublet. These Na
abundances were then compared with the O and Fe abundances (derived from the O
I 6156-8 and Fe II 6147/6149 lines) showing anti-correlated peculiarities. It
turned out that, unlike the case of A-type stars, [Na/H] is roughly constant at
a slightly subsolar level ([Na/H] ~ -0.2 (+/-0.2)) without any significant
correlation with [Fe/H] which shows considerable dispersion ranging from ~ -0.6
to ~ +1.0. This may serve as an important observational constraint for
understanding the abundance peculiarities along with the physical mechanism of
atomic diffusion in upper main-sequence stars of late A through late B-type
including Am and HgMn stars. | 1311.1260v1 |
2013-11-06 | Enhanced Superconductivity up to 43 K by P/Sb Doping of Ca1-xLaxFeAs2 | The effects of isovalent doping on the superconducting transition temperature
Tc in novel 112-type iron arsenide Ca1-xLaxFeAs2 were studied by conducting
measurements of electrical resistivity rho and magnetization M. P or Sb doping
strongly enhances Tc: P-doped Ca0.84La0.16FeAs2 and Sb-doped Ca0.85La0.15FeAs2
exhibited Tc of 41 and 43 K, respectively, while Ca0.85La0.15FeAs2 without P/Sb
exhibited Tc = 35 K. These observations will give a hint for further enhancing
Tc of rare-earth doped Ca-Fe-As systems. | 1311.1269v2 |
2013-11-06 | Superconductivity in Ca10(Ir4As8)(Fe2As2)5 with Square-Planar Coordination of Iridium | We report the unprecedented square-planar coordination of iridium in the iron
iridium arsenide Ca10(Ir4As8)(Fe2As2)5. This material experiences
superconductivity at 16 K. X-ray photoemission spectroscopy and first-
principles band calculation suggest Ir(II) oxidation state, which yields
electrically conductive Ir4As8 layers. Such metallic spacer layers are thought
to enhance the interlayer coupling of Fe2As2, in which superconductivity
emerges, thus offering a way to control the superconducting transition
temperature. | 1311.1280v1 |
2013-11-07 | The Terzan 5 puzzle: discovery of a third, metal-poor component | We report on the discovery of 3 metal-poor giant stars in Terzan 5, a complex
stellar system in the the Galactic bulge, known to have two populations at
[Fe/H]=-0.25 and +0.3. For these 3 stars we present new echelle spectra
obtained with NIRSPEC at Keck II, which confirm their radial velocity
membership and provide average [Fe/H]=-0.79 dex iron abundance and
[alpha/Fe]=+0.36 dex enhancement. This new population extends the metallicity
range of Terzan~5 0.5 dex more metal poor, and it has properties consistent
with having formed from a gas polluted by core collapse supernovae. | 1311.1706v1 |
2013-11-09 | Formation of Nanofoam carbon and re-emergence of Superconductivity in compressed CaC6 | Pressure can tune material's electronic properties and control its quantum
state, making some systems present disconnected superconducting region as
observed in iron chalcogenides and heavy fermion CeCu2Si2. For CaC6
superconductor (Tc of 11.5 K), applying pressure first Tc increases and then
suppresses and the superconductivity of this compound is eventually disappeared
at about 18 GPa. Here, we report a theoretical finding of the re-emergence of
superconductivity in heavily compressed CaC6. The predicted phase III (space
group Pmmn) with formation of carbon nanofoam is found to be stable at wide
pressure range with a Tc up to 14.7 K at 78 GPa. Diamond-like carbon structure
is adhered to the phase IV (Cmcm) for compressed CaC6 after 126 GPa, which has
bad metallic behavior, indicating again departure from superconductivity.
Re-emerged superconductivity in compressed CaC6 paves a new way to design
new-type superconductor by inserting metal into nanoporous host lattice. | 1311.2184v2 |
2013-11-12 | Unconventional sign-changing superconductivity near quantum criticality in YFe$_2$Ge$_2$ | I present the results of first principles calculations of the electronic
structure and magnetic interactions for the recently discovered superconductor
YFe$_2$Ge$_2$ and use them to identify the nature of superconductivity and
quantum criticality in this compound. I find that the Fe $3d$ derived states
near the Fermi level show a rich structure with the presence of both linearly
dispersive and heavy bands. The Fermi surface exhibits nesting between hole and
electron sheets that manifests as a peak in the susceptibility at $(1/2,1/2)$.
I propose that the superconductivity in this compound is mediated by
antiferromagnetic spin fluctuations associated with this peak resulting in a
$s_\pm$ state similar to the previously discovered iron-based superconductors.
I also find that various magnetic orderings are almost degenerate in energy,
which indicates that the proximity to quantum criticality is due to competing
magnetic interactions. | 1311.2922v1 |
2013-11-20 | Polarization-Dependent Three-Dimensional Angle-Resolved Photoemission Spectroscopy of BaFe$_{1.8}$Co$_{0.2}$As$_{2}$ | We performed polarization- and photon-energy-dependent angle-resolved
photoemission spectroscopy of a slightly overdoped iron pnictide
superconductor, BaFe$_{1.8}$Co$_{0.2}$As$_{2}$, to clarify the
three-dimensional electronic structure including its orbital characters at the
Brillouin zone center. Two hole Fermi surfaces (FSs) with $d_{xz/yz}$ and
$d_{xy/x^2-y^2}$ orbitals were observed but $d_{z^2}$ hole FS, which has nodes
according to a theory of the spin-fluctuation superconductivity mechanism, did
not appear. These results suggest that no node will appear at hole FSs at the
zone center. | 1311.4990v1 |