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2010-06-30
|
BaFe2As2 Surface Domains and Domain Walls: Mirroring the Bulk Spin Structure
|
High-resolution scanning tunneling microscopy (STM) measurements on
BaFe2As2-one of the parent compounds of the iron-based superconductors-reveals
a (1x1) As-terminated unit cell on the (001) surface. However, there are
significant differences of the surface unit cell compared to the bulk: only one
of the two As atoms in the unit cell is imaged and domain walls between
different (1x1) regions display a C2 symmetry at the surface. It should have
been C2v if the STM image reflected the geometric structure of the surface or
the orthorhombic bulk. The inequivalent As atoms and the bias dependence of the
domain walls indicate that the origin of the STM image is primarily electronic
not geometric. We argue that the surface electronic topography mirrors the bulk
spin structure of BaFe2As2, via strong orbital-spin coupling.
|
1006.5907v1
|
2010-07-05
|
Impact of the Spin Density Wave Order on the Superconducting Gap of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$
|
We report a doping dependent electronic Raman scattering measurements on
iron-pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals. A
strongly anisotropic gap is found at optimal doping for x=0.065 with
$\Delta_{max}\sim 5\Delta_{min}$. Upon entering the coexistence region between
superconducting (SC) and spin-density-wave (SDW) orders, the effective pairing
energy scale is strongly reduced. Our results are interpreted in terms of a
competition between SC and SDW orders for electronic state at the Fermi level.
Our findings advocate for a strong connection between the SC and SDW gaps
anisotropies which are both linked to interband interactions.
|
1007.0720v2
|
2010-07-07
|
Magnetisation distribution in the tetragonal phase of BaFe2As2
|
We have determined the spatial distribution of the magnetisation induced by a
field of 9 T in the tetragonal phase of BaFe2As2 using polarised neutron
diffraction. Magnetic structure factors derived from the polarisation
dependence of the intensities of Bragg reflections were used to make a maximum
entropy reconstruction of the distribution projected on the 110 plane. The
reconstruction shows clearly that the magnetisation is confined to the region
around the iron atoms and that there is no significant magnetisation associated
with either the As or Ba atoms. The distribution of magnetisation around the Fe
atom is significantly non-spherical with a shape which is extended in the <111>
directions in the projection. These results show that the electrons which give
rise to the paramagnetic susceptibility are confined to the Fe atoms their
distribution suggests that they occupy 3d t_2g type orbitals with about 60% in
those of xy symmetry.
|
1007.1116v2
|
2010-07-12
|
Atomic data and spectral model for Fe III
|
We present new atomic data (radiative transitions rates and collision
strengths) from large scale calculations and a non-LTE spectral model for Fe
III. This model is in very good agreement with observed astronomical emission
spectra, in contrast with previous models that yield large discrepancies with
observations. The present atomic computations employ a combination of atomic
physics methods, e.g. relativistic Hatree-Fock, the Thomas-Fermi-Dirac
potential, and Dirac-Fock computation of A-values and R-matrix with
intermediate coupling frame transformation and Dirac R-matrix. We study the
advantages and shortcomings of each method. It is found that the Dirac R-matrix
collision strengths yield excellent agreement with observations, much improved
over previously available models. By contrast, the transformation of
LS-coupling R-matrix fails to yield accurate effective collision strengths at
around 10^4 K, despite using very large configuration expansions, due to the
limited treatment of spin-orbit effects in the near threshold resonances of the
collision strengths. The present work demonstrates that accurate atomic data
for low ionization iron-peak species is now within reach.
|
1007.1960v1
|
2010-07-13
|
Doping evolution of superconducting gaps and electronic densities of states in Ba(Fe1-xCox)2As2 iron pnictides
|
An extensive calorimetric study of the normal- and superconducting-state
properties of Ba(Fe1-xCox)2As2 is presented for 0 < x < 0.2. The normal-state
Sommerfeld coefficient increases (decreases) with Co doping for x < 0.06 (x >
0.06), which illustrates the strong competition between magnetism and
superconductivity to monopolize the Fermi surface in the underdoped region and
the filling of the hole bands for overdoped Ba(Fe1-xCox)2As2. All
superconducting samples exhibit a residual electronic density of states of
unknown origin in the zero-temperature limit, which is minimal at optimal
doping but increases to the normal-state value in the strongly under- and
over-doped regions. The remaining specific heat in the superconducting state is
well described using a two-band model with isotropic s-wave superconducting
gaps.
|
1007.2218v1
|
2010-07-14
|
Comparative study of the electronic structures of Fe3O4 and Fe2SiO4
|
The electronic properties of two spinels Fe$_3$O$_4$ and Fe$_2$SiO$_4$ are
studied by the density functional theory. The local Coulomb repulsion $U$ and
the Hund's exchange $J$ between the $3d$ electrons on iron are included. For
$U=0$, both spinels are half-metals, with the minority $t_{2g}$ states at the
Fermi level. Magnetite remains a metal in a cubic phase even at large values of
$U$. The metal-insulator transition is induced by the $X_3$ phonon, which
lowers the total energy and stabilizes the charge-orbital ordering.
Fe$_2$SiO$_4$ transforms to a Mott insulating state for $U>2$ eV with a gap
$\Delta_g\sim U$. The antiferromagnetic interactions induce the tetragonal
distortion, which releases the geometrical frustration and stabilizes the
long-range order. The differences of electronic structures in the high-symmetry
cubic phases and the distorted low-symmetry phases of both spinels are
discussed.
|
1007.2340v1
|
2010-07-17
|
Pressure-induced changes in the magnetic and valence state of EuFe2As2
|
We present the results of electrical resistivity, ac specific heat, magnetic
susceptibility, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular
dichroism (XMCD) of the ternary iron arsenide EuFe2As2 single crystal under
pressure. Applying pressure leads to a continuous suppression of the
antiferromagnetism associated with Fe moments and the antiferromagnetic
transition temperature becomes zero in the vicinity of a critical pressure Pc
~2.5-2.7 GPa. Pressure-induced re-entrant superconductivity, which is highly
sensitive to the homogeneity of the pressure, only appears in the narrow
pressure region in the vicinity of Pc due to the competition between
superconductivity and the antiferromagnetic ordering of Eu2+ moments. The
antiferromagnetic state of Eu2+ moments changes to the ferromagnetic state
above 6 GPa. We also found that the ferromagnetic order is suppressed with
further increasing pressure, which is connected with a valence change of Eu
ions.
|
1007.2889v2
|
2010-07-21
|
Electronic structure, spin state, and magnetism of the square-lattice Mott insulator La2Co2Se2O3 from first principles
|
Electronic and magnetic structures of the newly synthesized cobalt
oxyselenide La2Co2Se2O3 (structurally similar to the superconducting iron
pnictides) are studied through density functional calculations. The obtained
results show that this material is a Mott insulator, and that it has a very
stable Co2+ high-spin ground state with a t2g-like orbital ordering, which is
substantiated by the calculated crystal-field excitation energies. The square
lattice of the Co2+ spins is found to have a strong antiferro (a weak ferro)
magnetic coupling for the second nearest neighbors (2nn) via O (Se2) and an
intermediate antiferro one for the 1nn, with the strength ratio about 10:1:3.
The present results account for the available experimental data of magnetism,
and the prediction of a planar frustrated (2x2) antiferromagnetic structure
would motivate a new experiment.
|
1007.3697v1
|
2010-07-28
|
Nodes in the Order Parameter of Superconducting Iron Pnictides Observed by Infrared Spectroscopy
|
The temperature and frequency dependences of the conductivity are derived
from optical reflection and transmission measurements of electron doped
BaFe$_2$As$_2$ crystals and films. The data is consistent with gap nodes or
possibly a very small gap in the crossover region between these two
possibilities. This can arise when one of the several pockets known to exist in
these systems has extended s-wave gap symmetry with an anisotropic piece
canceling or nearly so the isotropic part in some momentum direction.
Alternatively, a node can be lifted by impurity scattering which reduces
anisotropy. We find that the smaller gap on the hole pocket at the $\Gamma$
point in the Brillouin zone is isotropic s-wave while the electron pocket at
the $M$ point has a larger gap which is anisotropic and falls in the crossover
region.
|
1007.5215v1
|
2010-07-29
|
BaFe_{1.8}Co_{0.2}As_2 thin film hybrid Josephson junctions
|
Josephson junctions with iron pnictides open the way for fundamental
experiments on superconductivity in these materials and their application in
superconducting devices. Here, we present hybrid Josephson junctions with a
BaFe_{1.8}Co_{0.2}As_2 thin film electrode, an Au barrier and a PbIn counter
electrode. The junctions show RSJ-like current-voltage characteristics up to
the critical temperature of the counter electrode of about 7.2K. The
temperature dependence of the critical current, IC, does not show an
Ambegaokar-Baratoff behavior. Well-pronounced Shapiro steps are observed at
microwave frequencies of 10-18GHz. Assuming an excess current, I_ex, of 200
{\mu}A at 4.2K we get an effective I_C R_N product of 6 {\mu}V.
|
1007.5252v2
|
2010-07-30
|
Reversible shift in the superconducting transition for La1.85Sr0.15CuO4 and BaFe1.8Co0.2As2 using piezoelectric substrates
|
The use of piezoelectric substrates enables a dynamic observation of strain
dependent properties of functional materials. Based on studies with
La1.85Sr0.15CuO4 we extended this approach to the iron arsenic superconductors
represented by BaFe2-xCoxAs2 to investigate strain driven changes in detail. We
demonstrate that epitaxial thin films can be prepared on
(001)Pb(Mg1/3Nb2/3)0.72Ti0.28O3 substrates using pulsed laser deposition. The
structural as well as the electric properties of the grown films were
characterized in detail. A reversible shift of the superconducting transition
of 0.44 K for La1.85Sr0.15CuO4 and 0.2 K for BaFe1.8Co0.2As2 was observed
applying a biaxial strain of 0.022% and 0.017% respectively.
|
1007.5454v1
|
2010-08-02
|
A Search for Defect Related Ferromagnetism in SrTiO$_3$
|
Room temperature ferromagnetic hysteresis is observed in commercial SrTiO$_3$
substrates purchased from a variety of suppliers. It is shown that the
ferromagnetic signal comes from the unpolished surfaces. Surface impurity
phases cannot be detected using either x-ray diffraction or energy dispersive
x-ray spectra on the unpolished surfaces. However, a possible correlation
between surface disorder (xray diffraction peak linewidth) and ferromagnetism
is observed. Ar ion bombardment (10keV-90 keV) can be used to produce surface
layer disorder but is not found to induce ferromagnetism. Annealing of the
substrates at temperatures ranging from 600 to 1100 $^\circ$C is found to alter
the hysteresis curves differently depending on whether the annealing is
performed in air or in vacuum. Identical annealing behaviour is observed if the
substrates are artificially spiked with iron. This suggests that the
ferromagnetic hysteresis of as purchased SrTiO$_3$ could be due to Fe
contamination of the unpolished surfaces. In addition, it is shown that no
ferromagnetism is observed in samples that contain $10^{19}-10^{21}$ cm$^{-3}$
of oxygen vacancies if all the faces are polished.
|
1008.0299v1
|
2010-08-02
|
Iron substitution in NdCoAsO: crystal structure and magnetic phase diagram
|
The effects of replacing small amounts of Co with Fe in NdCoAsO are reported.
Polycrystalline materials with compositions NdCo1-xFexAsO (x = 0.05, 0.10,
0.15, and 0.20) are studied and the results compared to previous reports for
NdCoAsO. Rietveld analysis of powder x-ray diffraction data shows that as Fe
replaces Co on the transition metal (T) site, the T-As distance increases, and
the As tetrahedra surrounding the T-site become more regular. Electrical
resistivity and magnetization measurements indicate that the three magnetic
phase transitions in NdCoAsO are suppressed as Co is replaced by Fe, and these
transitions are not observed above 1.8 K for x = 0.20. Based on these results,
the magnetic phase diagram for the Co-rich side of the NdCoAsO-NdFeAsO system
is constructed.
|
1008.0304v2
|
2010-08-05
|
Reciprocal Relations Between Kinetic Curves
|
We study coupled irreversible processes. For linear or linearized kinetics
with microreversibility, $\dot{x}=Kx$, the kinetic operator $K$ is symmetric in
the entropic inner product. This form of Onsager's reciprocal relations implies
that the shift in time, $\exp (Kt)$, is also a symmetric operator. This
generates the reciprocity relations between the kinetic curves. For example,
for the Master equation, if we start the process from the $i$th pure state and
measure the probability $p_j(t)$ of the $j$th state ($j\neq i$), and,
similarly, measure $p_i(t)$ for the process, which starts at the $j$th pure
state, then the ratio of these two probabilities $p_j(t)/p_i(t)$ is constant in
time and coincides with the ratio of the equilibrium probabilities. We study
similar and more general reciprocal relations between the kinetic curves. The
experimental evidence provided as an example is from the reversible water gas
shift reaction over iron oxide catalyst. The experimental data are obtained
using Temporal Analysis of Products (TAP) pulse-response studies. These offer
excellent confirmation within the experimental error.
|
1008.1056v2
|
2010-08-09
|
Transport properties of iron-based FeTe0.5Se0.5 superconducting wire
|
FeTe0.5Se0.5 superconducting wires have been fabricated using ex-situ PIT
method with an Fe sheath. Among the other FeAs-based superconductor,
FeTe0.5Se0.5 has great advantage for application due to binary composition and
less toxic. Surprisingly, superconducting current was observed in the
as-fabricated wire without any heat treatments. Zero resistivity and transport
critical current density (Jc) were 3.2 K and 2.8 A/cm2, respectively. In
addition, by annealing at 200 degC for 2 h, critical temperature Tczero and Jc
were enhanced up to 9.1 K, and 64.1 A/cm2, respectively. This suggests that the
grain connectivity was improved by heat treatment, and superconducting property
of FeTe0.5Se0.5 wire was enhanced.
|
1008.1447v3
|
2010-08-13
|
'Genesis': A takeover from field-responsive matter?
|
Cairns-Smith (2008) has argued for a pre-Darwinian era, with a simpler basis
for life's functioning via primitive "crystal genes" (information transfer,
kinetic control on metabolic reactions). At the other extreme, guided by the
structural similarity of clusters in early-evolved enzymes to iron-sulphide
minerals like greigite, the hydrothermal mound scenario of Russell and
coworkers (1994) presents how non-equilibrium forces rooted in geochemistry
could be extrapolated to understand the metabolic functioning of living
systems. The informational vs metabolic aspects of life in these respective
scenarios can be linked together via a framboid-based theory of Sawlowicz
(2000), as these assemblies typically form in colloidal environments. In this
background, we consider the ramifications of a magnetic rock field on the mound
scenario, asking if soft matter assemblies are compatible with a coherent
order.
|
1008.2362v1
|
2010-08-16
|
Superconductivity at 28.3 and 17.1 K in (Ca4Al2O6-y)(Fe2Pn2) (Pn = As and P)
|
We have successfully synthesized (Ca4Al2O6-y)(Fe2Pn2) (Pn = As and P)
(Al-42622(Pn)) using high-pressure synthesis technique. Al-42622(Pn) exhibit
superconductivity for both Pn = As and P with the transition temperatures of
28.3 K and 17.1 K, respectively. The a-lattice parameters of Al-42622(Pn) (a =
3.713 {\AA} and 3.692 {\AA} for Pn = As and P, respectively) are smallest among
the iron-pnictide superconductors. Correspondingly, Al-42622(As) has the
smallest As-Fe-As bond angle (102.1 {\deg}) and the largest As distance from
the Fe planes (1.500 {\AA}).
|
1008.2586v3
|
2010-08-19
|
Nodeless two-gap superconductivity in stoichiometric iron pnictide LiFeAs
|
The variations of in- and inter- plane London penetration depths,
$\Delta\lambda(T)$, were measured using a tunnel diode resonator in single
crystals of the intrinsic pnictide superconductor LiFeAs. This compound appears
to be in the clean limit with a residual resistivity of 4 ($T\to0$) to 8
($T_c$) $\mu \Omega\cdot$cm and $RRR$ of 65 to 35, respectively. The superfluid
density, $\rho_s(T)=\lambda^2(0)/\lambda^2(T)$, is well described by the
self-consistent two-gap $\gamma-$model. Together with the previous data, our
results support the universal evolution of the superconducting gap from
nodeless to nodal upon departure from optimal doping. We also conclude that
pairbreaking scattering plays an important role in the deviation of the
low-temperature behavior of $\lambda(T)$ from exponential in Fe-based
compounds.
|
1008.3251v3
|
2010-08-19
|
The Lateral Distribution Function of Coherent Radio Emission from Extensive Air Showers; Determining the Chemical Composition of Cosmic Rays
|
The lateral distribution function (LDF) for coherent electromagnetic
radiation from air showers initiated by ultra-high-energy cosmic rays is
calculated using a macroscopic description. A new expression is derived to
calculate the coherent radio pulse at small distances from the observer. It is
shown that for small distances to the shower axis the shape of the electric
pulse is determined by the `pancake' function, describing the longitudinal
distribution of charged particles within the shower front, while for large
distances the pulse is determined by the shower profile. This reflects in a
different scaling of the LDF at small and at large distances. As a first
application we calculate the LDF for proton- and iron-induced showers and we
show that this offers a very sensitive measure to discriminate between these
two. We show that due to interference between the geo-magnetic and the
charge-excess contributions the intensity pattern of the radiation is not
circular symmetric.
|
1008.3308v1
|
2010-08-20
|
Two-band BCS superconductivity in Ba(Fe0.9Co0.1)2As2
|
The conductivity and permittivity optical spectra of iron-pnictide
Ba(Fe0.9Co0.1)2As2 film (Tc=20 K) are analyzed. In the superconducting state,
at all temperatures up to Tc the temperature dependences of the magnetic field
penetration depth and of the superconducting condensate density are well
described within the generalized two-band BCS model with intraband and
interband pairing interactions considered. It is shown that the smaller
superconducting energy gap 2{\Delta} = 3.7 meV develops in the electronic
subsystem while the larger gap 2{\Delta} >= 7 meV opens in the hole subsystem.
The normal state parameters (plasma frequencies and scattering rates) of
electron and hole conduction bands are determined. At all temperatures the
obtained data are consistent with the results of electronic photoemission
experiments on Ba(Fe1-x Cox)2As2.
|
1008.3473v3
|
2010-08-23
|
Testing the No-Hair Theorem with Observations of Black Holes in the Electromagnetic Spectrum
|
According to the no-hair theorem, astrophysical black holes are uniquely
described by their mass and spin. In this paper, we review a new framework for
testing the no-hair hypothesis with observations in the electromagnetic
spectrum. The approach is formulated in terms of a Kerr-like spacetime
containing a quadrupole moment that is independent of both mass and spin. If
the no-hair theorem is correct, then any deviation from the Kerr metric
quadrupole has to be zero. We show how upcoming VLBI imaging observations of
Sgr A* as well as spectroscopic observations of iron lines from accreting black
holes with IXO may lead to the first astrophysical test of the no-hair theorem.
|
1008.3902v2
|
2010-08-29
|
Fermiology of Sr4V2O6Fe2As2: Quasi-Nested Fe vs Mott-Insulating V Orbitals
|
We have performed an angle-resolved photoemission spectroscopy study of a new
iron-based superconductor Sr4V2O6Fe2As2. While V 3d orbitals are found to be in
a Mott insulator state and show an incoherent peak at ~ 1 eV below the Fermi
level, the dispersive Fe 3d bands form several hole- and electron-like Fermi
surfaces (FSs), some of which are quasi-nested by the (pi, 0) wave vector. This
differs from the local density approximation (LDA) calculations, which predict
non-nested FSs for this material. However, LDA+U with a large effective Hubbard
energy U on V 3d electrons can reproduce the experimental observation
reasonably well. The observed fermiology in superconducting Sr4V2O6Fe2As2
strongly supports that (pi, 0) interband scattering between quasi-nested FSs is
indispensable to superconductivity in pnictides.
|
1008.4905v1
|
2010-08-30
|
X-ray absorption by Broad Line Region Clouds in Mrk 766
|
We present a new analysis of a 9-day long XMM-Newton monitoring of the Narrow
Line Seyfert 1 galaxy Mrk 766. We show that the strong changes in spectral
shape which occurred during this observation can be interpreted as due to Broad
Line Region clouds crossing the line of sight to the X-ray source. Within the
occultation scenario, the spectral and temporal analysis of the eclipses
provides precise estimates of the geometrical structure, location and physical
properties of the absorbing clouds. In particular, we show that these clouds
have cores with column densities of at least a few 10^23 cm^-2 and velocities
in the plane of the sky of the order of thousands km/s. The three different
eclipses monitored by XMM-Newton suggest a broad range in cloud velocities (by
a factor ~4-5). Moreover, two iron absorption lines clearly associated with
each eclipse suggest the presence of highly ionized gas around the obscuring
clouds, and an outflow component of the velocity spanning from 3,000 to 15,000
km/s
|
1008.5067v1
|
2010-09-02
|
Association of oxygen vacancies with impurity metal ions in lead titanate
|
Thermodynamic, structural and electronic properties of isolated copper and
iron atoms as well as their complexes with oxygen vacancies in tetragonal lead
titanate are investigated by means of first principles calculations. Both
dopants exhibit a strong chemical driving force for the formation of M_Ti-V_O
(M = Cu, Fe) defect associates. The most stable configurations corresponds to a
local dipole aligned along the tetragonal axis parallel to the spontaneous
polarization. Local spin moments are obtained and the calculated spin densities
are discussed. The calculations provide a simple and consistent explanation for
the experimental findings. The results are discussed in the context of models
for degradation of ferroelectric materials.
|
1009.0552v1
|
2010-09-10
|
Nucleosynthesis in the Stellar Systems omega Cen and M22
|
The stellar system omega Centauri is well known for the large range in
abundance among its member stars. Recent work has indicated that the globular
cluster M22 (NGC 6656) also possesses an internal abundance range, albeit
substantially smaller than that in omega Cen. Here we compare, as a function of
[Fe/H], element-to-iron ratios in the two systems for a number of different
elements using data from abundance analyses of red giant branch stars. It
appears that the nucleosynthetic enrichment processes were very similar in
these two systems despite the substantial difference in total mass.
|
1009.1955v1
|
2010-09-10
|
Antiferromagnetic Critical Fluctuations in BaFe$_2$As$_2$
|
Magnetic correlations near the magneto-structural phase transition in the
bilayer iron pnictide parent compound, BaFe$_2$As$_2$, are measured. In close
proximity to the antiferromagnetic phase transition in BaFe$_2$As$_2$, a
crossover to three dimensional critical behavior is anticipated and has been
preliminarily observed. Here we report complementary measurements of
two-dimensional magnetic fluctuations over a broad temperature range about
T$_N$. The potential role of two-dimensional critical fluctuations in the
magnetic phase behavior of BaFe$_2$As$_2$ and their evolution near the
anticipated crossover to three dimensional critical behavior and long-range
order are discussed.
|
1009.2116v1
|
2010-09-17
|
Existence, character and origin of surface-related bands in the high temperature iron pnictide superconductor BaFe_{2-x}Co_{x}As_{2}
|
Low energy electron diffraction (LEED) experiments, LEED simulations and
finite slab density functional calculations are combined to study the cleavage
surface of Co doped BaFe_{2-x}Co_{x}As_{2} (x = 0.1, 0.17). We demonstrate that
the energy dependence of the LEED data can only be understood from a
terminating 1/2 Ba layer accompanied by distortions of the underlying
As-Fe_2-As block. As a result, surface related Fe 3d states are present in the
electronic structure, which we identify in angle resolved photoemission
experiments. The close proximity of the surface-related states to the bulk
bands inevitably leads to broadening of the ARPES signals, which excludes the
use of the BaFe_{2-x}Co_{x}As_{2} system for accurate determination of
self-energies using ARPES.
|
1009.3493v1
|
2010-09-20
|
Electronic Structure Calculation by First Principles for Strongly Correlated Electron Systems
|
Recent trends of ab initio studies and progress in methodologies for
electronic structure calculations of strongly correlated electron systems are
discussed. The interest for developing efficient methods is motivated by recent
discoveries and characterizations of strongly correlated electron materials and
by requirements for understanding mechanisms of intriguing phenomena beyond a
single-particle picture. A three-stage scheme is developed as renormalized
multi-scale solvers (RMS) utilizing the hierarchical electronic structure in
the energy space. It provides us with an ab initio downfolding of the global
band structure into low-energy effective models followed by low-energy solvers
for the models. The RMS method is illustrated with examples of several
materials. In particular, we overview cases such as dynamics of semiconductors,
transition metals and its compounds including iron-based superconductors and
perovskite oxides, as well as organic conductors of kappa-ET type.
|
1009.3851v1
|
2010-09-21
|
Single Crystal Growth and Characterization of the Iron-Based Superconductor KFe2As2 Synthesized by KAs Flux Method
|
Centimeter sized platelet single crystals of KFe2As2 were grown using a
self-flux method. An encapsulation technique using commercial stainless steel
container allowed the stable crystal growth lasting for more than 2 weeks.
Ternary K-Fe-As systems with various starting compositions were examined to
determine the optimal growth conditions. Employment of KAs flux led to the
growth of large single crystals with the typical size of as large as 15 mm x 10
mm x 0.4 mm. The grown crystals exhibit sharp superconducting transition at 3.4
K with the transition width 0.2 K, as well as the very large residual
resistivity ratio exceeding 450, evidencing the good sample quality.
|
1009.4002v1
|
2010-09-22
|
An ARPES view on the high-Tc problem: phonons vs spin-fluctuations
|
We review the search for a mediator of high-Tc superconductivity focusing on
ARPES experiment. In case of HTSC cuprates, we summarize and discuss a
consistent view of electronic interactions that provides natural explanation of
both the origin of the pseudogap state and the mechanism for high temperature
superconductivity. Within this scenario, the spin-fluctuations play a decisive
role in formation of the fermionic excitation spectrum in the normal state and
are sufficient to explain the high transition temperatures to the
superconducting state while the pseudogap phenomenon is a consequence of a
Peierls-type intrinsic instability of electronic system to formation of an
incommensurate density wave. On the other hand, a similar analysis being
applied to the iron pnictides reveals especially strong electron-phonon
coupling that suggests important role of phonons for high-Tc superconductivity
in pnictides.
|
1009.4336v1
|
2010-09-30
|
Mobility analysis of FeTe thin films
|
The Hall effect is investigated in detail for nonsuperconducting and
superconducting FeTe thin films. The Hall coefficient commonly exhibits a sign
reversal from positive in a high-temperature paramagnetic state to negative in
a low-temperature antiferromagnetic state. Phenomenological analysis by a
simple two-band Drude model indicates that hole mobility is significantly
suppressed in the antiferromagnetic state. When suppression of the hole
mobility is insufficient, superconductivity shows up in FeTe. This result
strongly suggests that the itinerancy in both hole and electron channels is the
essential factor for the occurrence of superconductivity in iron chalcogenide
superconductors.
|
1009.6035v2
|
2010-10-07
|
A past capture event at Sagittarius A* inferred from the fluorescent X-ray emission of Sagittarius B clouds
|
The fluorescent X-ray emission from neutral iron in the molecular clouds (Sgr
B) indicates that the clouds are being irradiated by an external X-ray source.
The source is probably associated with the Galactic central black hole (Sgr
A*), which triggered a bright outburst one hundred years ago. We suggest that
such an outburst could be due to a partial capture of a star by Sgr A*, during
which a jet was generated. By constraining the observed flux and the time
variability ($\sim$ 10 years) of the Sgr B's fluorescent emission, we find that
the shock produced by the interaction of the jet with the dense interstellar
medium represents a plausible candidate for the X-ray source emission.
|
1010.1312v1
|
2010-10-10
|
Electronic structure of BaNi$_2$As$_2$
|
BaNi$_2$As$_2$, with a first order phase transition around 131 K, is studied
by the angle-resolved photoemission spectroscopy. The measured electronic
structure is compared to the local density approximation calculations,
revealing similar large electronlike bands around $\bar{M}$ and differences
along $\bar{\Gamma}$-$\bar{X}$. We further show that the electronic structure
of BaNi$_2$As$_2$ is distinct from that of the sibling iron pnictides.
Particularly, there is no signature of band folding, indicating no collinear
SDW related magnetic ordering. Moreover, across the strong first order phase
transition, the band shift exhibits a hysteresis, which is directly related to
the significant lattice distortion in BaNi$_2$As$_2$.
|
1010.1905v2
|
2010-10-14
|
Macroscopic quantum tunneling in multigap superconducting Josephson junctions: Escape rate enhancement via quantum fluctuations of Josephson-Leggett mode
|
We theoretically study the macroscopic quantum tunneling (MQT) in a hetero
Josephson junction formed by a conventional single-gap superconductor and a
multi-gap one such as ${MgB}_{2}$ and iron-based superconductors. In such a
Josephson junction more than one phase differences are defined. We clarify the
quantum dynamics of the phase differences and construct a theory for the MQT in
the multi-gap Josephson junctions. The dynamics of the phase differences are
strongly affected by the Josephson-Leggett mode, which is the out-of-phase
oscillation mode of the phase differences. The escape rate is calculated in
terms of the effective action renormalized by the Josephson-Leggett mode. It is
shown that the escape rate is drastically enhanced when the frequency of the
Josephson-Leggett mode is less than the Josephson-plasma frequency.
|
1010.2804v2
|
2010-10-12
|
Revealing the degree of magnetic frustration by non-magnetic impurities
|
Imaging the magnetic fields around a non-magnetic impurity can provide a
clear benchmark for quantifying the degree of magnetic frustration. Focusing on
the strongly frustrated $J_1$-$J_2$ model and the spatially anisotropic
$J_{1a}$-$J_{1b}$-$J_2$ model, very distinct low energy behaviors reflect
different levels of magnetic frustration. In the $J_1$-$J_2$ model, bound
magnons appear trapped near the impurity in the ground state and strongly
reduce the ordered moments for sites proximal to the impurity. In contrast,
local moments in the $J_{1a}$-$J_{1b}$-$J_2$ model are enhanced on the impurity
neighboring sites. These theoretical predictions can be probed by experiments
such as nuclear magnetic resonance and scanning tunneling microscopy, and the
results can elucidate the role of frustration in antiferromagnets and help
narrow the possible models to understand magnetism in the iron pnictdies.
|
1010.2917v2
|
2010-10-14
|
Theory of Orbital Nematicity in Underdoped Iron Arsenides
|
Recent finding of an {\it unusual} in-plane resistivity anisotropy in the
underdoped 122-family at high temperature ($T$) suggests an orbital nematic
(ON) order, posing a challenge to extant theories. The {\it sign} of the
anisotropy contradicts expectations from weakly correlated as well as pure
spin-only nematic views. Here, we show how such an ON order with accompanying
structural distortion arises from {\it residual}, intersite and inter-orbital
two-body interactions in an incoherent "bad metal" close to Mottness.
Enhancement of orbital-selective incoherence is shown to be necessary for
understanding transport anisotropy. Our results suggest that ON order, with
subsequent antiferromagnetic order might be the {\it primary} competitor to
superconductivity in Fe arsenides.
|
1010.2940v1
|
2010-10-19
|
Pressure versus concentration tuning of the superconductivity in Ba(Fe(1-x)Cox)2As2
|
In the iron arsenide compound BaFe2As2, superconductivity can be induced
either by a variation of its chemical composition, e.g., by replacing Fe with
Co, or by a reduction of the unit-cell volume through the application of
hydrostatic pressure p. In contrast to chemical substitutions, pressure is
expected to introduce no additional disorder into the lattice. We compare the
two routes to superconductivity by measuring the p dependence of the
superconducting transition temperature Tc of Ba(Fe(1-x)Cox)2As2 single crystals
with different Co content x. We find that Tc(p) of underdoped and overdoped
samples increases and decreases, respectively, tracking quantitatively the
Tc(x) dependence. To clarify to which extent the superconductivity relies on
distinct structural features we analyze the crystal structure as a function of
x and compare the results with that of BaFe2As2 under pressure.
|
1010.3863v1
|
2010-10-22
|
Double resonance Raman study of disorder in CVD-grown single-walled carbon nanotubes
|
Single-walled carbon nanotubes (SWNTs) with varying degrees of disorder were
investigated using multiple-excitation Raman spectroscopy. The lattice disorder
was imparted into the nanotubes by the addition of varying amounts of sulfur to
the iron catalyst in a thermal chemical vapor deposition process. Changes in
the intensities of peaks occurring due to a double resonance Raman process were
studied. The intensity of the disorder-induced D band increased with a decrease
in the sulfur content. Upon post-synthesis heat treatment, the double resonance
process got quenched due to defect healing. The second order G' band and iTOLA
bands exhibited a two-peak structure, of which one of the peaks is relatively
more sensitive to defects and decreased in intensity with heat treatment.
|
1010.4714v2
|
2010-10-25
|
Macroscopic Model of Geomagnetic-Radiation from Air Showers, II
|
The generic properties of the emission of coherent radiation from a moving
charge distribution are discussed. The general structure of the charge and
current distributions in an extensive air shower are derived. These are
subsequently used to develop a very intuitive picture for the properties of the
emitted radio pulse. Using this picture can be seen that the structure of the
pulse is a direct reflection of the shower profile. At higher frequencies the
emission is suppressed because the wavelength is shorter than the important
length scale in the shower. It is shown that radio emission can be used to
distinguish proton and iron induced air showers.
|
1010.5268v1
|
2010-10-31
|
On the Progenitors of Core-Collapse Supernovae
|
Theory holds that a star born with an initial mass between about 8 and 140
times the mass of the Sun will end its life through the catastrophic
gravitational collapse of its iron core to a neutron star or black hole. This
core collapse process is thought to usually be accompanied by the ejection of
the star's envelope as a supernova. This established theory is now being tested
observationally, with over three dozen core-collapse supernovae having had the
properties of their progenitor stars directly measured through the examination
of high-resolution images taken prior to the explosion. Here I review what has
been learned from these studies and briefly examine the potential impact on
stellar evolution theory, the existence of "failed supernovae", and our
understanding of the core-collapse explosion mechanism.
|
1011.0203v1
|
2010-11-03
|
Importance of Fermi surface topology for high temperature superconductivity in electron-doped iron arsenic superconductors
|
We used angle resolved photoemission spectroscopy and thermoelectric power to
study the poorly explored, highly overdoped side of the phase diagram of
Ba(Fe1-xCox)2As2 high temperature superconductor. Our data demonstrate that
several Lifshitz transitions - topological changes of the Fermi surface - occur
for large x. Tc starts to decrease with doping when the cylindrical, central
hole pocket changes to ellipsoids centering at the Z point, and goes to zero
before these ellipsoids disappear around x = 0.2. Changes in thermoelectric
power occur at similar x-values. Beyond this doping level the central pocket
changes to electron-like and superconductivity does not exist. Our observations
reveal the crucial importance of the underlying Fermiology in this class of
materials. A necessary condition for superconductivity is the presence of the
central hole pockets rather than perfect nesting between central and corner
pockets.
|
1011.0980v2
|
2010-11-04
|
Electronic Structure Studies of Detwinned BaFe$_2$As$_2$ by Photoemission
|
We performed angle resolved photoelectron spectroscopy (ARPES) studies on
mechanically detwinned BaFe2As2. We observe clear band dispersions and the
shapes and characters of the Fermi surfaces are identified. Shapes of the two
hole pockets around the {\Gamma}-point are found to be consistent with the
Fermi surface topology predicted in the orbital ordered states. Dirac-cone like
band dispersions near the {\Gamma}-point are clearly identified as
theoretically predicted. At the X-point, split bands remain intact in spite of
detwinning, barring twinning origin of the bands. The observed band dispersions
are compared with calculated band structures. With a magnetic moment of 0.2 ?B
per iron atom, there is a good agreement between the calculation and
experiment.
|
1011.1112v2
|
2010-11-06
|
The Elemental Abundance Distributions of Milky Way Satellite Galaxies
|
The chemical compositions of the stars in Milky Way (MW) satellite galaxies
reveals the history of gas flows and star formation (SF) intensity. This talk
presented a Keck/DEIMOS spectroscopic survey of the Fe, Mg, Si, Ca, and Ti
abundances of nearly 3000 red giants in eight MW dwarf satellites. The
metallicity and alpha-to-iron ratio distributions obey the following trends:
(1) The more luminous galaxies are more metal-rich, indicating that they
retained gas more efficiently than the less luminous galaxies. (2) The shapes
of the metallicity distributions of the more luminous galaxies require gas
infall during their SF lifetimes. (3) At [Fe/H] < -1.5, [alpha/Fe] falls
monotonically with increasing [Fe/H] in all MW satellites. One interpretation
of these trends is that the SF timescale in any MW satellite is long enough
that Type Ia supernovae exploded for nearly the entire SF lifetime.
|
1011.1530v1
|
2010-11-08
|
Consistent model of magnetism in ferropnictides
|
The discovery of superconductivity in LaFeAsO introduced the ferropnictides
as a major new class of superconducting compounds with critical temperatures
second only to cuprates. The presence of magnetic iron makes ferropnictides
radically different from cuprates. Antiferromagnetism of the parent compounds
strongly suggests that superconductivity and magnetism are closely related.
However, the character of magnetic interactions and spin fluctuations in
ferropnictides, in spite of vigorous efforts, has until now resisted
understanding within any conventional model of magnetism. Here we show that the
most puzzling features can be naturally reconciled within a rather simple
effective spin model with biquadratic interactions, which is consistent with
electronic structure calculations. By going beyond the Heisenberg model, this
description explains numerous experimentally observed properties, including the
peculiarities of the spin wave spectrum, thin domain walls, crossover from
first to second order phase transition under doping in some compounds, and
offers new insight in the occurrence of the nematic phase above the
antiferromagnetic phase transition.
|
1011.1715v2
|
2010-11-11
|
Evidence for electromagnetic granularity in polycrystalline Sm1111 iron-pnictides with enhanced phase purity
|
We prepared polycrystalline SmFeAsO1-xFx (Sm1111) bulk samples by sintering
and hot isostatic pressing (HIP) in order to study the effects of phase purity
and relative density on the intergranular current density. Sintered and HIPped
Sm1111 samples are denser with fewer impurity phases, such as SmOF and the
grain boundary wetting phase, FeAs. We found quite complex magnetization
behavior due to variations of both the inter and intragranular current
densities. Removing porosity and reducing second phase content enhanced the
intergranular current density, but HIPping reduced Tc and the intragranular
current density, due to loss of fluorine and reduction of Tc. We believe that
the HIPped samples are amongst the purest polycrystalline 1111 samples yet
made. However, their intergranular current densities are still small, providing
further evidence that polycrystalline pnictides, like polycrystalline cuprates,
are intrinsically granular.
|
1011.2547v1
|
2010-11-11
|
Baryonic Bound State of Vortices in Multicomponent Superconductors
|
We construct a bound state of three 1/3-quantized Josephson coupled vortices
in three-component superconductors with intrinsic Josephson couplings, which
may be relevant with regard to iron-based superconductors. We find a Y-shaped
junction of three domain walls connecting the three vortices, resembling the
baryonic bound state of three quarks in QCD. The appearance of the Y-junction
(but not a Delta-junction) implies that in both cases of superconductors and
QCD, the bound state is described by a genuine three-body interaction (but not
by the sum of two-body interactions). We also discuss a
confinement/deconfinement phase transition.
|
1011.2552v2
|
2010-11-11
|
Origin of the Ultrahigh-Energy Cosmic Rays and their Spectral Break
|
The energy spectrum, composition and arrival directions of ultrahigh energy
cosmic rays (UHECRs) with energy above the cosmic ray ankle, measured by the
Pierre Auger Observatory, are inconsistent if their origin is assumed to be
extragalactic. Their observed properties, however, are those expected from
UHECRs accelerated by the highly relativistic jets emitted in Galactic gamma
ray bursts, most of which are beamed away from Earth. If this alternative
interpretation is correct, the observed break in the energy spectrum of UHECRs
around 50 EeV is not the Greisen-Zatsepin-Kuzmin cutoff but the energy
threshold for free escape of ultrahigh energy iron cosmic rays from the Galaxy
and above their respective free-escape threshold-energies, UHECR nuclei should
point back to their Galactic sources or their remnants rather than to active
galactic nuclei (AGN) within the GZK horizon.
|
1011.2672v2
|
2010-11-16
|
Itinerant Spin Excitations in SrFe2As2 Measured by Inelastic Neutron Scattering
|
We report inelastic neutron scattering measurements of the magnetic
excitations in SrFe2As2, the parent of a family of iron-based superconductors.
The data extend throughout the Brillouin zone and up to energies of ~260meV. An
analysis with the local-moment J_1-J2 model implies very different in-plane
nearest-neighbor exchange parameters along the $a$ and $b$ directions, both in
the orthorhombic and tetragonal phases. However, the spectrum calculated from
the J1-J2 model deviates significantly from our data. We show that the
qualitative features that cannot be described by the J1-J2 model are readily
explained by calculations from a 5-band itinerant mean-field model.
|
1011.3831v2
|
2010-11-17
|
A Ray-Tracing Algorithm for Spinning Compact Object Spacetimes with Arbitrary Quadrupole Moments. I. Quasi-Kerr Black Holes
|
We describe a new numerical algorithm for ray tracing in the external
spacetimes of spinning compact objects characterized by arbitrary quadrupole
moments. Such spacetimes describe non-Kerr vacuum solutions that can be used to
test the no-hair theorem in conjunction with observations of accreting black
holes. They are also appropriate for neutron stars with spin frequencies in the
300-600 Hz range, which are typical of the bursting sources in low-mass X-ray
binaries. We use our algorithm to show that allowing for the quadrupole moment
of the spacetime to take arbitrary values leads to observable effects in the
profiles of relativistic broadened fluorescent iron lines from geometrically
thin accretion disks.
|
1011.4078v1
|
2010-11-19
|
Spin canting as a result of the competition between stripes and spirals in cuprates
|
Based on the extended Hubbard model we calculate the energy of stripe and
spiral ground states. We find that uniform spirals get favored by a large
$t'/t$ ratio but are unstable at small doping towards stripes and checkerboard
textures with spin canting. The structure of these inhomogeneities also depends
on t'/t and the associated spin currents may induce a small lattice distortion
associated with local dipole moments. We discuss a new kind of stripe which
appears as a domain wall of the antiferromagnetic (AF) order parameter with a
fractional change of the phase of the AF order. For large |t'/t| spirals can be
stabilized under certain conditions in the overdoped regime which may explain
the elastic incommensurate magnetic response recently observed in iron-codoped
Bi2201 materials.
|
1011.4447v1
|
2010-11-19
|
The AGB population of NGC 6822
|
The metallicity gradient and the stellar distribution within the Local Group
dwarf galaxy NGC 6822 has been studied photometrically using asymptotic branch
stars (AGB). In order to study the stellar and metallicity distribution, the
carbon- and oxygen-rich AGB stars have been isolated using deep high-quality
near-infrared UKIRT photometry. The ratio between them, the C/M ratio, has been
used to derive the [Fe/H] abundance within the galaxy. The [Fe/H] abundance and
stellar distribution were analysed as a function of galactic radius. A mean C/M
ratio of 0.288 +/- 0.014 has been found which corresponds to an iron abundance
of [Fe/H] =-1.14 +/-0.08 dex, with variations in the north and south, as well
as at larger galactocentric distances. Variations in the magnitude of the tip
of the red giant branch has also been detected.
|
1011.4464v1
|
2010-11-22
|
Orbital occupation and magnetic moments of tetrahedrally coordinated iron in CaBaFe4O7
|
CaBaFe4O7 is a mixed-valent transition metal oxide having both Fe2+ and Fe3+
ions in tetrahedral coordination. Here we characterize its magnetic properties
by magnetization measurements and investigate its local electronic structure
using soft x-ray absorption spectroscopy at the Fe L2,3 edges, in combination
with multiplet cluster and spin-resolved band structure calculations. We found
that the Fe2+ ion in the unusual tetrahedral coordination is Jahn-Teller active
with the high-spin e^2 (up) t2^3 (up) e^1 (down) configuration having a
x^2-y^2-like electron for the minority spin. We deduce that there is an
appreciable orbital moment of about L_z=0.36 caused by multiplet interactions,
thereby explaining the observed magnetic anisotropy. CaBaFe4O7, a member of the
'114' oxide family, offers new opportunities to explore charge, orbital and
spin physics in transition metal oxides.
|
1011.4818v1
|
2010-12-03
|
Magnetic and electrical properties and carrier doping effects on the Fe-based host compound Sr4Sc2Fe2As2O6
|
Additional charge carriers were introduced to the iron oxyarsenide
Sr4Sc2Fe2As2O6 under a high-pressure condition, followed by measurements of
electrical resistivity, Hall coefficient, and magnetic susceptibility. The host
compound Sr4Sc2Fe2As2O6 shows metallic conductivity down to ~200 K and turns to
show a semiconducting-like conductivity accompanied by a positive
magneto-resistance (22% at 70 kOe). Although the carrier density is comparable
at 300 K (5.9x1021 cm-3) with that of the other Fe-based superconductors, no
superconductivity appears down to 2 K. This is primarily because the net
carrier density decreases over 3 orders of magnitude on cooling and
additionally a possible magnetic order at ~120 K prevents carriers from
pairing. The properties were altered largely by introducing the additional
carriers.
|
1012.0616v1
|
2010-12-10
|
Density functional study of orbital-selective magnetism in FeAs-based superconductors
|
We performed spin-polarized density functional calculations of
lanthanide-series (Ln) iron oxypnictides LnFeAsO (Ln=La, Ce, Pr, Nd, Sm, and
Gd) with constrained Fe magnetic moments, finding that in-plane dxy and
out-of-plane dyz orbital characters are preferred for small Fe magnetic
moments. Comparison of LnFeAsO compounds shows that the antiferromagnetism
(AFM) from the Fe dxy orbital is itinerantly driven by orbital-dependent
Fermi-surface nesting while AFM from the Fe dyz orbital is driven by
superexchange mechanism. The Fe magnetic moments of the two orbital characters
show different coupling strengths to Fermi-surface electrons
orbital-selectively, suggesting that they may play different roles in
superconductivity and in AFM, and making d orbital characters of the magnetic
moment resolvable by measuring the electronic structures.
|
1012.2224v2
|
2010-12-10
|
Defect driven ferroelectricity and magnetism in nanocrystalline KTaO3
|
Nominally pure nanocrystalline KTaO3 was thoroughly investigated by
micro-Raman and magnetic resonance spectroscopic techniques. In all samples the
defect driven ferroelectricity and magnetism are registered. Both ordering
states are suggested to appear due to the iron atoms and oxygen vacancies. The
concentration of defects was estimated to be 0.04 and 0.06-0.1 mole %,
respectively. Note that undoped single crystals of KTaO3 are nonmagnetic and
have never exhibited ferromagnetic properties. The results enable us to refer a
nanosized KTa(Fe)O3 to the class of multiferroics and assume that it could
perform the magnetoelectric effect at T<29 K. It was also established that the
critical concentration of impurity defects necessary to provoke the appearance
of the new phase states in the material strongly correlates with the size of
the particle; as the size of the particle decreases, the critical concentration
decreases as well.
|
1012.2295v1
|
2010-12-10
|
The Curie temperature and exchange energy between two sublattices in half-metallic greigite Fe3S4
|
High-temperature magnetic measurements have been carried out in
hydrothermally synthesized greigite (Fe3S4). We show that the Curie temperature
of greigite is significantly lower than that for its iron oxide counterpart
Fe3O4. The lower TC value (about 677 K) of greigite is in quantitative
agreement with that calculated using the exchange energy (3.25 meV) and the
spin values of the two sublattices, which are inferred from the neutron and
magnetization data of high-quality pure greigite samples. We further show that,
with an effective on-site Hubbard energy Ueff = 1.16 eV, the lattice constant
and two sublattice spins predicted from ab initio density-function theory are
in nearly perfect agreement with the measured values. The parameter Ueff = 1.16
eV ensures Fe3S4 to be an excellent half-metallic material for spintronic
applications.
|
1012.2364v1
|
2010-12-12
|
Thermal effects on neutrino-nucleus inelastic scattering in stellar environments
|
Thermal effects for inelastic neutrino-nucleus scattering off even-even
nuclei in the iron region are studied. Allowed and first-forbidden
contributions to the cross sections are calculated within the quasiparticle
random phase approximation, extended to finite temperatures within the
Thermo-Field-Dynamics formalism. The GT$_0$ strength distribution at finite
temperatures is calculated for the sample nucleus $^{54}$Fe. The
neutral-current neutrino-nucleus inelastic cross section is calculated for
relevant temperatures during the supernova core collapse. The thermal
population of the excited states significantly enhances the cross section at
low neutrino energies. In agreement with studies using a large scale
shell-model approach the enhancement is mainly due to neutrino up-scattering at
finite temperatures.
|
1012.2543v1
|
2010-12-13
|
Distinct electronic nematicities between electron and hole underdoped iron pnictides
|
We systematically investigated the in-plane resistivity anisotropy of
electron-underdoped $EuFe_{2-x}Co_xAs_2$ and $BaFe_{2-x}Co_xAs_2$, and
hole-underdoped $Ba_{1-x}K_xFe_2As_2$. Large in-plane resistivity anisotropy
was found in the former samples, while {\it tiny} in-plane resistivity
anisotropy was detected in the latter ones. When it is detected, the anisotropy
starts above the structural transition temperature and increases smoothly
through it. As the temperature is lowered further, the anisotropy takes a
dramatic enhancement through the magnetic transition temperature. We found that
the anisotropy is universally tied to the presence of non-Fermi liquid T-linear
behavior of resistivity. Our results demonstrate that the nematic state is
caused by electronic degrees of freedom, and the microscopic orbital
involvement in magnetically ordered state must be fundamentally different
between the hole and electron doped materials.
|
1012.2731v1
|
2010-12-14
|
A detailed spectroscopic analysis of the open cluster NGC 5460
|
Within the context of a large project aimed at studying early F-, A- and late
B-type stars we present the abundance analysis of the photospheres of 21
members of the open cluster NGC 5460, an intermediate age cluster (logt ~ 8.2)
previously unstudied with spectroscopy. Our study is based on medium and high
resolution spectra obtained with the FLAMES instrument of the ESO/VLT. We show
that cluster members have a nearly solar metallicity, and that there is
evidence that the abundances of magnesium and iron are correlated with the
effective temperature, exhibiting a maximum around Teff=10500 K. No
correlations are found between abundances and projected equatorial velocity,
except for marginal evidence of barium being more abundant in slower than in
faster rotating stars. We discovered two He-weak stars, and a binary system
where the hotter component is a HgMn star. We provide new estimates for the
cluster distance (720 +/- 50 pc), age (logt = 8.2 +/- 0.1), and mean radial
velocity (-17.9 +/- 5.2 km/s).
|
1012.3050v1
|
2010-12-16
|
Fabrication and some properties of biaxially aligned Sr0.6K0.4Fe2As2 superconductors by processing in high magnetic field
|
We fabricated the c axis and ab-plane biaxially aligned Sr0.6K0.4Fe2As2
superconductor using a two-step magnetic field procedure. The effect of
magnetic fields on the structure and superconducting properties of
Sr0.6K0.4Fe2As2 has been investigated by using X-ray diffraction and magnetic
measurements. The degree of orientation of the samples was about 0.39 for the c
axis and 0.51 for ab-plane orientation, as evaluated from the Lotgering factor
of X-ray diffraction. This technology might be useful in a variety of potential
applications, including preparing iron based superconducting bulks and wires
with high critical currents.
|
1012.3572v1
|
2010-12-16
|
Synthesis and crystal growth of Cs0.8(FeSe0.98)2: a new iron-based superconductor with Tc=27K
|
We report on the synthesis of large single crystals of a new FeSe-layer
superconductor Cs0.8(FeSe0.98)2. X-ray powder diffraction, neutron
powder-diffraction and magnetization measurements have been used to compare the
crystal structure and the magnetic properties of Cs0.8(FeSe0.98)2 with those of
the recently discovered potassium intercalated system KxFe2Se2. The new
compound Cs0.8(FeSe0.98)2 shows a slightly lower superconducting transition
temperature (Tc=27.4 K) in comparison to 29.5 in K0.8(FeSe0.98)2). The volume
of the crystal unit cell increases by replacing K by Cs - the c-parameter grows
from 14.1353(13) {\AA} to 15.2846(11) {\AA}. For the so far known alkali metal
intercalated layered compounds (K0.8Fe2Se2 and Cs0.8(FeSe0.98)2) the Tc
dependence on the anion height (distance between Fe-layers and Se-layers) was
found to be analogous to those reported for As-containing Fe-superconductors
and Fe(Se1-xChx), where Ch=Te, S.
|
1012.3637v1
|
2010-12-22
|
Explosive nucleosynthesis in core-collapse supernovae
|
The specific mechanism and astrophysical site for the production of half of
the elements heavier than iron via rapid neutron capture (r-process) remains to
be found. In order to reproduce the abundances of the solar system and of the
old halo stars, at least two components are required: the heavy r-process
nuclei (A>130) and the weak r-process which correspond to the lighter heavy
nuclei (A<130). In this work, we present nucleosynthesis studies based on
trajectories of hydrodynamical simulations for core-collapse supernovae and
their subsequent neutrino-driven winds. We show that the weak r-process
elements can be produced in neutrino-driven winds and we relate their
abundances to the neutrino emission from the nascent neutron star. Based on the
latest hydrodynamical simulations, heavy r-process elements cannot be
synthesized in the neutrino-driven winds. However, by artificially increasing
the wind entropy, elements up to A=195 can be made. In this way one can mimic
the general behavior of an ejecta where the r-process occurs. We use this to
study the impact of the nuclear physics input (nuclear masses, neutron capture
cross sections, and beta-delayed neutron emission) and of the long-time
dynamical evolution on the final abundances.
|
1012.4917v1
|
2010-12-23
|
Non-collinear spin-density wave antiferromagnetism in FeAs
|
We present the magnetic structure of the itinerant monoarsenide, FeAs, with
the B31 structure. Powder neutron diffraction confirms incommensurate modulated
magnetism with wavevector $\mathbf{q} = (0.395\pm0.001)\mathbf{c}^*$ at 4 K,
but cannot distinguish between a simple spiral and a collinear spin-density
wave structure. Polarized single crystal diffraction confirms that the
structure is best described as a non-collinear spin-density wave arising from a
combination of itinerant and localized behavior with spin amplitude along the
b-axis direction being (15 $\pm$ 5)% larger than in the a-direction.
Furthermore, the propagation vector is temperature dependence, and the
magnetization near the critical point indicates a two-dimensional Heisenberg
system. The nature of the magnetism in the simplest iron arsenide is of
fundamental importance in understanding the interplay between localized and
itinerant magnetism and superconductivity.
|
1012.5311v2
|
2010-12-27
|
Electronic Structure of KFe$_2$Se$_2$ from First Principles Calculations
|
Electronic structure and magnetic properties for iron-selenide KFe$_2$Se$_2$
are studied by first-principles calculations. The ground state is stripe-like
antiferromagnetic with calculated 2.26 $\mu_B$ magnetic moment on Fe atoms; and
the $J_1$, $J_2$ coupling strengths are calculated to be 0.038 eV and 0.029 eV.
The states around $E_F$ are dominated by the Fe-3d orbitals which hybridize
noticeably to the Se-4p orbitals. While the band structure of KFe$_2$Se$_2$ is
similar to a heavily electron-doped BaFe$_2$As$_2$ or FeSe system, the Fermi
surface of KFe$_2$Se$_2$ is much closer to \fs11 system since the electron
sheets around $M$ is symmetric with respect to $x$-$y$ exchange. These
features, as well as the absence of Fermi surface nesting, suggest that the
parental KFe$_2$Se$_2$ could be regarded as an electron over-doped 11 system
with possible local moment magnetism.
|
1012.5621v2
|
2010-12-29
|
Electronic structures and magnetic orders of Fe-vacancies ordered ternary iron selenides TlFe$_{1.5}$Se$_2$ and AFe$_{1.5}$Se$_2$ (A=K, Rb, or Cs)
|
By the first-principles electronic structure calculations, we find that the
ground state of the Fe-vacancies ordered TlFe$_{1.5}$Se$_2$ is a
quasi-two-dimensional collinear antiferromagnetic semiconductor with an energy
gap of 94 meV, in agreement with experimental measurements. This
antiferromagnetic order is driven by the Se-bridged antiferromagnetic
superexchange interactions between Fe moments. Similarly, we find that crystals
AFe$_{1.5}$Se$_2$ (A=K, Rb, or Cs) are also antiferromagnetic semiconductors
but with a zero-gap semiconducting state or semimetallic state nearly
degenerated with the ground states. Thus rich physical properties and phase
diagrams are expected.
|
1012.6015v2
|
2011-01-10
|
Monopolelike probes for quantitative magnetic force microscopy: calibration and application
|
A local magnetization measurement was performed with a Magnetic Force
Microscope (MFM) to determine magnetization in domains of an exchange coupled
[Co/Pt]/Co/Ru multilayer with predominant perpendicular anisotropy. The
quantitative MFM measurements were conducted with an iron filled carbon
nanotube tip, which is shown to behave like a monopole. As a result we
determined an additional in-plane magnetization component of the multilayer,
which is explained by estimating the effective permeability of the sample
within the \mu*-method.
|
1101.1804v1
|
2011-01-10
|
Latest results from the Pierre Auger Observatory
|
Recent results obtained with the Pierre Auger Observatory are described.
These include measurements of the spectrum, anisotropies and composition of
ultra-high energy cosmic rays. The ankle of the spectrum is measured at
$4\times 10^{18}$~eV and a suppression above $3\times 10^{19}$~eV consistent
with the GZK effect is observed. At energies above $5.5\times 10^{19}$~eV a
correlation with the distribution of nearby extragalactic objects is found,
including an excess around the direction of Centaurus~A, the nearest radio loud
active galaxy. Measurements of the depth of shower maximum and its fluctuations
suggest a gradual change in the average mass of the primary cosmic rays (under
standard extrapolations of hadronic interaction models), being the results
consistent with a light composition consisting mostly of protons at few$\times
10^{18}$~eV and approaching the expectations from iron nuclei at $4\times
10^{19}$~eV. Upper bounds on the photon fraction and the neutrino fluxes are
also obtained.
|
1101.1825v1
|
2011-01-10
|
Coexistence of Magnetism and Superconductivity in the Iron-based Compound Cs_0.8(FeSe_0.98)_2
|
We report on muon-spin rotation/relaxation (\muSR), electrical resistivity,
magnetization and differential scanning calorimetry measurements performed on a
high-quality single crystal of Cs_0.8(FeSe_0.98)_2. Whereas our transport and
magnetization data confirm the bulk character of the superconducting state
below T_c = 28.5(2)K, the \muSR data indicate that the system is {\it magnetic}
below T_N = 478.5(3)K, where a first-order transition occurs. The first-order
character of the magnetic transition is confirmed by differential scanning
calorimetry data. Taken all together, these data indicate in
Cs_0.8(FeSe_0.98)_2 a microscopic coexistence between the superconducting phase
and a strong magnetic phase. The observed ratio between T_N and T_c is the
highest one reported to date for a magnetic superconductor.
|
1101.1873v3
|
2011-01-12
|
[O/Fe] Estimates for Carbon-Enhanced Metal-Poor Stars from Near-IR Spectroscopy
|
We report on oxygen abundances determined from medium-resolution near-IR
spectroscopy for a sample of 57 carbon-enhanced metal-poor (CEMP) stars
selected from the Hamburg/ESO survey. The majority of our program stars exhibit
oxygen-to-iron ratios in the range +0.5 < [O/Fe]< +2.0. The [O/Fe] values for
this sample are statistically compared to available high-resolution estimates
for known CEMP stars, as well as to high-resolution estimates for a set of
carbon-normal metal-poor stars. Carbon, nitrogen, and oxygen abundance patterns
for a sub-sample of these stars are compared to yield predictions for very
metal-poor asymptotic giant-branch abundances in the recent literature. We find
that the majority of our sample exhibit patterns that are consistent with
previously studied CEMP stars having s-process-element enhancements, and thus
have very likely been polluted by carbon- and oxygen-enhanced material
transferred from a metal-poor asymptotic giant-branch companion.
|
1101.2260v1
|
2011-01-21
|
A biotic cosmos demystified?
|
Oceanic planets formed by type Ia supernovae become spectacularly abundant as
stars cease to shine. However, the timing may not be altogether inappropriate.
Neutrino annihilation might thermally regulate iron-cored water-worlds,
sustaining habitable subglacial oceans. If dark matter and dark energy decay to
neutrinos, the universe could support life for ~ 10^23 years. Civilisations
surmounting the arduous process of hereditary genetics soon discern the biotic
nature of the cosmos and accept their role within it. An infrastructure guards
against the spread of rogue colonists. Recruited colonists could harness the
available energy for the benefit of life with stupendous efficiency, providing
unmistakeable evidence of cosmological optimisation. The anthropic
coincidences, inhospitable aspects of the current universe and Fermi's paradox
would all be illuminated. Semiconductors sensitive to a neutrinoelectric effect
offer a laboratory test of the planetary heating mechanism.
|
1101.4172v3
|
2011-01-26
|
A Near-Infrared Search for Silicates in Jovian Trojan Asteroids
|
We obtained near-infrared (0.8 - 2.5 micron) spectra of seven Jovian Trojan
asteroids that have been formerly reported to show silicate-like absorption
features near 1 micron. Our sample includes the Trojan (1172) Aneas, which is
one of three Trojans known to possess a comet-like 10 micron emission feature,
indicative of fine-grained silicates. Our observations show that all seven
Trojans appear featureless in high signal-to-noise ratio spectra. The
simultaneous absence of the 1 micron band and presence of the 10 micron
emission can be understood if the silicates on (1172) Aneas are iron-poor.
In addition, we present near infrared observations of five optically grey
Trojans, including three objects from the collisionally produced Eurybates
family. The five grey Trojans appear featureless in the near infrared with no
diagnostic absorption features. The near infrared spectrum of Eurybates can be
best fitted with the spectrum of a CM2 carbonaceous chondrite, which hints that
the C-type Eurybates family members may have experienced aqueous alteration.
|
1101.5167v1
|
2011-01-30
|
Controlling electrostatic co-assembly using ion-containing copolymers : from surfactants to nanoparticles
|
In this review, we address the issue of the electrostatic complexation
between charged-neutral diblock copolymers and oppositely charged nanocolloids.
We show that nanocolloids such as surfactant micelles and iron oxide magnetic
nanoparticles share similar properties when mixed with charged-neutral
diblocks. Above a critical charge ratio, core-shell hierarchical structures
form spontaneously under direct mixing conditions. The core-shell structures
are identified by a combination of small-angle scattering techniques and
transmission electron microscopy. The formation of multi-level objects is
driven by the electrostatic attraction between opposite charges and by the
release of the condensed counterions. Alternative mixing processes inspired
from molecular biology are also described. The protocols applied here consist
in screening the electrostatic interactions of the mixed dispersions, and then
removing the salt progressively as example by dialysis. With these techniques,
the oppositely charged species are intimately mixed before they can interact,
and their association is monitored by the desalting kinetics. As a result,
sphere- and rod-like aggregates with remarkable superparamagnetic and stability
properties are obtained. These findings are discussed in the light of a new
paradigm which deals with the possibility to use inorganic nanoparticles as
building blocks for the design and fabrication of supracolloidal assemblies
with enhanced functionalities.
|
1101.5821v1
|
2011-01-31
|
Template assisted self-assembly of individual and clusters of magnetic nanoparticles
|
The deliberate control over the spatial arrangement of nanostructures is the
desired goal for many applications as e.g. in data storage, plasmonics or
sensor arrays. Here we present a novel method to assist the self-assembly
process of magnetic nanoparticles. The method makes use of nanostructured
aluminum templates obtained after anodization of aluminum disks and the
subsequent growth and removal of the newly formed alumina layer, resulting in a
regular honeycomb type array of hexagonally shaped valleys. The iron oxide
nanoparticles, 20 nm in diameter, are spin coated onto the nanostructured
templates. Depending on the size, each hexagon site can host up to 30
nanoparticles. These nanoparticles form clusters of different arrangements
within the valleys, such as collars, chains, and hexagonally closed islands.
Ultimately, it is possible to isolate individual nanoparticles. The strengths
of magnetic interaction between particles in a cluster is probed using the
memory effect known from the coupled state in superspin glass systems.
|
1101.5899v1
|
2011-01-31
|
Metastable $π$-junction between an s$_\pm$-wave and an s-wave superconductor
|
We examine a contact between a superconductor whose order parameter changes
sign across the Brillioun zone, and an ordinary, uniform-sign superconductor.
Within a Ginzburg-Landau type model, we find that if the the barrier between
the two superconductors is not too high, the frustration of the Josephson
coupling between different portions of the Fermi surface across the contact can
lead to surprising consequences. These include time-reversal symmetry breaking
at the interface and unusual energy-phase relations with multiple local minima.
We propose this mechanism as a possible explanation for the half-integer flux
quantum transitions in composite niobium--iron pnictide superconducting loops,
which were discovered in a recent experiment [Chen et al., Nature Phys.
\textbf{6},260 (2010)].
|
1102.0042v3
|
2011-02-01
|
Ab initio study of the modification of elastic properties of alpha-iron by hydrostatic strain and by hydrogen interstitials
|
The effect of hydrostatic strain and of interstitial hydrogen on the elastic
properties of $\alpha$-iron is investigated using \textit{ab initio}
density-functional theory calculations. We find that the cubic elastic
constants and the polycrystalline elastic moduli to a good approximation
decrease linearly with increasing hydrogen concentration. This net strength
reduction can be partitioned into a strengthening electronic effect which is
overcome by a softening volumetric effect. The calculated hydrogen-dependent
elastic constants are used to determine the polycrystalline elastic moduli and
anisotropic elastic shear moduli. For the key slip planes in $\alpha$-iron,
$[1\bar{1}0]$ and $[11\bar{2}]$, we find a shear modulus reduction of
approximately 1.6% per at.% H.
|
1102.0187v1
|
2011-02-02
|
Hall effect of FeTe and Fe(Se_1-x_Te_x_) thin films
|
The Hall effect is investigated in thin-film samples of iron-chalcogenide
superconductors in detail. The Hall coefficient (RH) of FeTe and Fe(Se1-xTex)
exhibits a similar positive value around 300 K, indicating that the
high-temperature normal state is dominated by hole-channel transport. FeTe
exhibits a sign reversal from positive to negative across the transition to the
low-temperature antiferromagnetic state, indicating the occurrence of drastic
reconstruction in the band structure. The mobility analysis using the carrier
density theoretically calculated reveals that the mobility of holes is strongly
suppressed to zero, and hence the electric transport looks to be dominated by
electrons. The Se substitution to Te suppresses the antiferromagnetic
long-range order and induces superconductivity instead. The similar mobility
analysis for Fe(Se0.4Te0.6) and Fe(Se0.5Te0.5) thin films shows that the
mobility of electrons increases with decreasing temperature even in the
paramagnetic state, and keeps sufficiently high values down to the
superconducting transition temperature. From the comparison between FeTe and
Fe(Se1-xTex), it is suggested that the coexistence of 'itinerant' carriers both
in electron and hole channels is indispensable for the occurrence of
superconductivity.
|
1102.0332v1
|
2011-02-07
|
Nanoscale phase separation of antiferromagnetic order and superconductivity in K$_{0.75}$Fe$_{1.75}$Se$_2$
|
We report an in-plane optical spectroscopy study on the iron-selenide
superconductor K$_{0.75}$Fe$_{1.75}$Se$_2$. The measurement revealed the
development of a sharp reflectance edge below T$_c$ at frequency much smaller
than the superconducting energy gap on a relatively incoherent electronic
background, a phenomenon which was not seen in any other Fe-based
superconductors so far investigated. Furthermore, the feature could be
noticeably suppressed and shifted to lower frequency by a moderate magnetic
field. Our analysis indicates that this edge structure arises from the
development of a Josephson-coupling plasmon in the superconducting condensate.
Together with the transmission electron microscopy analysis, our study yields
compelling evidence for the presence of nanoscale phase separation between
superconductivity and magnetism. The results also enable us to understand
various seemingly controversial experimental data probed from different
techniques.
|
1102.1381v4
|
2011-02-08
|
Neutron Repulsion
|
Earth is connected gravitationally, magnetically and electrically to its heat
source - a neutron star that is obscured from view by waste products in the
photosphere. Neutron repulsion is like the hot filament in an incandescent
light bulb. Excited neutrons are emitted from the solar core and decay into
hydrogen that glows in the photosphere like a frosted light bulb. Neutron
repulsion was recognized in nuclear rest mass data in 2000 as the overlooked
source of energy, the keystone of an arch that locked together these puzzling
space-age observations: 1.) Excess 136Xe accompanied primordial helium in the
stellar debris that formed the solar system (Fig. 1); 2.) The Sun formed on the
supernova core (Fig. 2); 3.) Waste products from the core pass through an
iron-rich mantle, selectively carrying lighter elements and lighter isotopes of
each element into the photosphere (Figs. 3-4); and 4.) Neutron repulsion powers
the Sun and sustains life (Figs. 5-7). Together these findings offer a
framework for understanding how: a.) The Sun generates and releases neutrinos,
energy and solar-wind hydrogen and helium; b.) An inhabitable planet formed and
life evolved around an ordinary-looking star; c.) Continuous climate change -
induced by cyclic changes in gravitational interactions of the Sun's energetic
core with planets - has favored survival by adaptation.
|
1102.1499v1
|
2011-02-09
|
Quasiparticle interference in antiferromagnetic parent compounds of Fe-based superconductors
|
Recently reported quasiparticle interference imaging in underdoped
Ca(Fe{1-x}Co{x})_2As{2} shows pronounced C{2} asymmetry that is interpreted as
an indication of an electronic nematic phase with a unidirectional electron
band, dispersive predominantly along the $b$-axis of this orthorhombic
material. On the other hand, even more recent transport measurements on
untwinned samples show near isotropy of the resistivity in the $ab$ plane, with
slightly larger conductivity along a (and not b). We show that in fact both
sets of data are consistent with the calculated ab initio Fermi surfaces, which
has a decisively broken C_{4}, and yet similar Fermi velocity in both
directions. This reconciles completely the apparent contradiction between the
conclusions of the STM and the transport experiments.
|
1102.1930v1
|
2011-02-11
|
Towards an ideal graphene-based spin-filter
|
The Fe$_3$O$_4$(111)/graphene/Ni(111) trilayer is proposed to be used as an
ideal spin-filtering sandwich where the half-metallic properties of magnetite
are used. Thin magnetite layers on graphene/Ni(111) were prepared via
successive oxidation of thin iron layer predeposited on graphene/Ni(111) and
formed system was investigated by means of low-energy electron diffraction
(LEED) and photoelectron spectroscopy (PES) of core levels as well as valence
band. The electronic structure and structural quality of the graphene film
sandwiched between two ferromagnetic layers remains unchanged upon magnetite
formation as confirmed by LEED and PES data.
|
1102.2367v1
|
2011-02-11
|
Pressure cycle of superconducting Cs0.8Fe2Se2: a transport study
|
We report measurements of the temperature and pressure dependence of the
electrical resistivity of single crystalline iron-based chalcogenide
Cs0.8Fe2Se2. In this material superconductivity Tc~30K develops from a normal
state with extremely large resistivity. At ambient pressure a large "hump" in
the resistivity is observed around 200K. Under pressure, the resistivity
decreases by two orders of magnitude, concomitant with a sudden Tc suppression
around p~8GPa. Even at 9GPa a metallic resistivity state is not recovered, and
the {\rho}(T) "hump" is still detected. A comparison of the data measured upon
increasing and decreasing the external pressure leads us to suggest that
superconductivity is not related to this hump.
|
1102.2464v2
|
2011-02-14
|
Common Crystalline and Magnetic Structure of superconducting A2Fe4Se5
|
Single crystal neutron diffraction study has been performed on new
superconductor A2Fe4Se5, where A = Cs, K, (Tl,Rb) and (Tl,K), and the same Fe
vacancy ordered crystal structure and the same block checkerboard
antiferromagne tic order recently discovered in the K2Fe4Se5 superconductor are
observed. Thus, all known alkali metal and/or Tl intercalated iron selenide
superconductors share common crystalline and magnetic structure. Investigation
on electronic process in these new superconductors, for which a different
electronic structure from those in previous Fe-based high-Tc superconductors is
emerging, now has a valid structural and magnetic framework.
|
1102.2882v2
|
2011-02-16
|
Elastic properties and inter-atomic bonding in new superconductor KFe2Se2 from first principles calculations
|
Very recently (November, 2010, PRB, 82, 180520R) the first 122-like ternary
superconductor KxFe2Se2 with enhanced TC ~ 31K has been discovered. This
finding has stimulated much activity in search of related materials and
triggered the intense studies of their properties. Indeed already in 2010-2011
the superconductivity (TC ~ 27-33K) was also found in the series of new
synthesized 122 phases such as CsxFe2Se2, RbxFe2Se2, (TlK)xFeySe2 etc. which
have formed today the new family of superconducting iron-based materials
without toxic As. Here, using the ab initio FLAPW-GGA method we have predicted
for the first time the elastic properties for KFe2Se2 and discussed their
interplay with inter-atomic bonding for this system. Our data reveal that the
examined phase is relatively soft material. In addition, this system is
mechanically stable, adopts considerable elastic anisotropy, and demonstrates
brittleness. These conclusions agree with the bonding picture for KFe2Se2,
where the inter-atomic bonding is highly anisotropic and includes ionic,
covalent and metallic contributions.
|
1102.3248v1
|
2011-02-17
|
Anomalous optical phonons in FeTe pnictides: spin state, magnetic order, and lattice anharmonicity
|
Polarized Raman-scattering spectra of non-superconducting, single-crystalline
FeTe are investigated as function of temperature. We have found a relation
between the magnitude of ordered magnetic moments and the linewidth of A1g
phonons at low temperatures. This relation is attributed to the intermediate
spin state (S=1) and the orbital degeneracy of the Fe ions. Spin-phonon
coupling constants have been estimated based on microscopic modeling using
density-functional theory and analysis of the local spin density. Our
observations show the importance of orbital degrees of freedom for the Fe-based
superconductors with large ordered magnetic moments, while small magnetic
moment of Fe ions in some iron pnictides reflects the low spin state of Fe ions
in those systems.
|
1102.3688v1
|
2011-02-23
|
Absence of Tunneling Character in c-axis Transport of SmFeAsO0.85 Single Crystals
|
We made electrical transport measurements along the c-axis of SmFeAsO0.85
single crystals, in both three- and four-terminal configurations, focusing on
examining the possible formation of Josephson coupling between FeAs
superconducting layers. Anisotropic bulk superconductivity was observed along
the c-axis, but without Josephson coupling, as confirmed by the absence of both
the hysteresis in current-voltage curves and the modulation of the critical
current by the in-plane magnetic fields. The variation of the critical currents
for different magnetic-field directions gives the anisotropy ratio of 5. This
three-dimensional electronic structure of our iron pnictide superconductors is
in clear contrast to the two-dimensional one observed in the cuprate
superconductors, another stacked superconducting system.
|
1102.4645v1
|
2011-02-23
|
On the nature of spectral line broadening in solar coronal dimmings
|
We analyze the profiles of iron emission lines observed in solar coronal
dimmings associated with coronal mass ejections, using the EUV Imaging
Spectrometer on board Hinode. We quantify line profile distortions with
empirical coefficients (asymmetry and peakedness) that compare the fitted
Gaussian to the data. We find that the apparent line broadenings reported in
previous studies are likely to be caused by inhomogeneities of flow velocities
along the line of sight, or at scales smaller than the resolution scale, or by
velocity fluctuations during the exposure time. The increase in the amplitude
of Alfv\'en waves cannot, alone, explain the observed features. A
double-Gaussian fit of the line profiles shows that, both for dimmings and
active region loops, one component is nearly at rest while the second component
presents a larger Doppler shift than that derived from a single-Gaussian fit.
|
1102.4745v1
|
2011-02-24
|
Angle-resolved photoemission spectroscopy study of PrFeAsO_{0.7}: Dependence of the electronic structure on the pnictogen height
|
We have performed an angle-resolved photoemission spectroscopy (ARPES) study
of the iron-based superconductor PrFeAsO_{0.7} and examined the Fermi surfaces
and band dispersions near the Fermi level. Heavily hole-doped electronic states
have been observed due to the polar nature of the cleaved surfaces.
Nevertheless, we have found that the ARPES spectra basically agree with band
dispersions calculated in the local density approximation (LDA) if the
bandwidth is reduced by a factor of ~2.5 and then the chemical potential is
lowered by ~70 meV. Comparison with previous ARPES results on LaFePO reveals
that the energy positions of the d_{3z^2-r^2}- and d_{yz,zx}-derived bands are
considerably different between the two materials, which we attribute to the
different pnictogen height as predicted by the LDA calculation.
|
1102.4907v1
|
2011-02-25
|
Thermal evolution of Pt-rich FePt/Fe3O4 heterodimers studied using x-ray absorption near edge spectroscopy
|
FePt/Fe3O4 nanoparticles can be used as building blocks to obtain, upon
thermal annealing, magnetic nanocomposites with combined magnetic properties.
Although the pre- and post-annealed samples are usually well characterized, a
detailed investigation during annealing is necessary to reveal the role of
intermediate processes to produce a desirable composite. We present an
alternative method using in situ XANES to investigate the thermal evolution of
oleic acid and oleylamine coated Pt-rich FePt/Fe3O4 heterodimers. As the
temperature increases, a progressive reduction of Fe3O4 to FeO occurs helped by
the thermolysis of the surfactants; while above 550 K Fe3Pt starts to be
formed. At 840 K an abrupt increase of FeO further drives the phase
transformation to stabilize the iron platinum soft phase. Thus, the Fe3O4
reduction acts as catalyst that promotes the Fe and Pt interdiffusion between
the Pt-rich FePt and Fe3O4/FeO to form Fe3Pt instead of exchange coupled
FePt/Fe3O4 with hard magnetic properties. In addition, the role of the
interface of the heterodimer ends is discussed. The pre- and post-annealed
samples were also characterized by TEM, XRD, EXAFS, magnetometry and
M\"ossbauer spectroscopy.
|
1102.5321v1
|
2011-03-01
|
Time-Reversal-Symmetry-Broken Superconductivity Induced by Frustrated Inter-Component Couplings
|
Superconductivity is associated with spontaneously broken gauge symmetry. In
some exotic superconductors the time-reversal symmetry is broken as well,
accompanied with internal magnetic field. A time-reversal symmetry broken
(TRSB) superconductivity without internal magnetic field involved can be
induced by frustrated inter-component couplings, which becomes a realistic
issue recently due to the discovery of iron-pnictide superconductors. Here we
derive stability condition for this novel TRSB state using the Ginzburg-Landau
(GL) theory. We find that there are multiple divergent coherence lengths, and
that this novel superconductivity cannot be categorized by the GL number into
type I or type II. We reveal that the critical Josephson current of a
constriction junction between two bulk superconductors of different chiralities
is suppressed significantly from that for same chirality. This effect provides
a unique way to verify experimentally this brand new superconductivity.
|
1103.0123v2
|
2011-03-02
|
Fresnel aperture diffraction: a phase-sensitive probe for superconducting pairing symmetry
|
Fresnel single aperture diffraction (FSAD) is proposed as a phase-sensitive
probe for pairing symmetry and Fermi surface of a superconductor. We consider
electrons injected, through a small aperture, into a thin superconducting (SC)
layer. It is shown that in case of SC gap symmetry
$\Delta(-k_x,\mathbf{k}_\parallel)=\Delta(k_x,\mathbf{k}_\parallel)$ with $k_x$
and $\mathbf{k}_\parallel$ respectively the normal and parallel component of
electron Fermi wavevector, quasiparticle FSAD pattern developed at the image
plane is zeroth-order minimum if $k_x x=n\pi$ ($n$ is an integer and $x$ is SC
layer thickness). In contrast, if
$\Delta(-k_x,\mathbf{k}_\parallel)=-\Delta(k_x, \mathbf{k}_\parallel)$, the
corresponding FSAD pattern is zeroth-order maximum. Observable consequences are
discussed for iron-based superconductors of complex multi-band pairings.
|
1103.0424v1
|
2011-03-04
|
Nodal gap structure of BaFe_2(As_{1-x}P_x)_2 from angle-resolved thermal conductivity in a magnetic field
|
The structure of the superconducting order parameter in the iron-pnictide
superconductor BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$ ($T_c=31$\,K) with line
nodes is studied by the angle-resolved thermal conductivity measurements in a
magnetic field rotated within the basal plane. We find that the thermal
conductivity displays distinct fourfold oscillations with minima when the field
is directed at $\pm45^\circ$ with respect to the tetragonal a-axis. We discuss
possible gap structures that can account for the data, and conclude that the
observed results are most consistent with the closed nodal loops located at the
flat parts of the electron Fermi surface with high Fermi velocity.
|
1103.0885v2
|
2011-03-06
|
Basic design and engineering of normal-conducting, iron-dominated electromagnets
|
The intention of this course is to provide guidance and tools necessary to
carry out an analytical design of a simple accelerator magnet. Basic concepts
and magnet types will be explained as well as important aspects which should be
considered before starting the actual design phase. The central part of this
course is dedicated to describing how to develop a basic magnet design.
Subjects like the layout of the magnetic circuit, the excitation coils, and the
cooling circuits will be discussed. A short introduction to materials for the
yoke and coil construction and a brief summary about cost estimates for magnets
will complete this topic.
|
1103.1119v1
|
2011-03-07
|
Stable magnetostructural coupling with tunable magnetoresponsive effects in hexagonal phase-transition ferromagnets
|
The magnetostructural coupling between the structural and the magnetic
transition plays a crucial role in magnetoresponsive effects in a
martensitic-transition system. A combination of various magnetoresponsive
effects based on this coupling may facilitate the multifunctional applications
of a host material. Here, we demonstrate a possibility to obtain a stable
magnetostructural coupling in a broad temperature window from 350 to 70 K,
showing tunable magnetoresponsive effects, based on simultaneous manipulation
of the phase stability and the magnetic structure by suitable chemical
substitution of iron in MnNiGe. The resultant MnNiGe:Fe exhibits a
magnetic-field-induced martensitic transition from paramagnetic austenite to
ferromagnetic martensite, featuring (i) a large volume increase, (ii) a
distinct magnetization change, (iii) small thermal hysteresis and (iv) a giant
negative magnetocaloric effect. The results indicate that stable
magnetostructural coupling is accessible in hexagonal phase-transition systems
to attain the magnetoresponsive effects with broad tunability.
|
1103.1313v2
|
2011-03-12
|
Varying Eu$^{2+}$ magnetic order in EuFe$_2$As$_2$ by chemical pressure
|
Among iron 122 pnictide superconductors, the EuFe$_2$As$_2$ series draws
particular interest because, in addition to superconductivity or the long-range
spin-density-wave order in the Fe subsystem, the localized Eu$^{2+}$ magnetic
moments order at low temperatures. Here we present a novel scheme of how the
spins align in the Eu compounds when pressure varies the coupling; we explain
magnetization measurements on EuFe$_2$(As$_{1-x}$P$_x$)$_2$ single crystals as
well as other observations of the Eu$^{2+}$ ordering previously reported in
literature. The magnetic moments of the Eu$^{2+}$ ions are slightly canted even
in the parent compound EuFe$_2$As$_2$, yielding a ferromagnetic contribution
along the $c$-direction that becomes stronger with pressure. Reducing the
interlayer distance even further, the antiferromagnetic coupling of the $ab$
planes finally turns ferromagnetic.
|
1103.2446v1
|
2011-03-15
|
Oxygen hyperstoichiometric hexagonal ferrite CaBaFe4O7+δ(δ \approx 0.14) : coexistence of ferrimagnetism and spin glass behavior
|
An oxygen hyperstoichiometric ferrite CaBaFe4O7+\delta (\delta \approx 0.14)
has been synthesized using "soft" reduction of CaBaFe4O8. Like the oxygen
stoichiometric ferrimagnet CaBaFe4O7, this oxide also keeps the hexagonal
symmetry (space group: P63mc), and exhibits the same high Curie temperature of
270 K. However, the introduction of extra oxygen into the system weakens the
ferrimagnetic interaction significantly at the cost of increased magnetic
frustration at low temperature. Moreover, this canonical spin glass (Tg ~ 166
K) exhibits an intriguing cross-over from de Almeida-Thouless type to
Gabay-Toulouse type critical line in the field temperature plane above a
certain field strength, which can be identified as the anisotropy field. Domain
wall pinning is also observed below 110 K. These results are interpreted on the
basis of cationic disordering on the iron sites.
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1103.2939v1
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2011-03-15
|
Calorimetric Evidence of Strong-Coupling Multiband Superconductivity in Fe(Te0.57Se0.43) Single Crystal
|
We have investigated the specific heat of optimally-doped iron chalcogenide
superconductor Fe(Te0.57Se0.43) with a high-quality single crystal sample. The
electronic specific heat Ce of this sample has been successfully separated from
the phonon contribution using the specific heat of a non-superconducting sample
(Fe0.90Cu0.10)(Te0.57Se0.43) as a reference. The normal state Sommerfeld
coefficient gamma_n of the superconducting sample is found to be ~ 26.6 mJ/mol
K^2, indicating intermediate electronic correlation. The temperature dependence
of Ce in the superconducting state can be best fitted using a double-gap model
with 2Delta_s(0)/kBTc = 3.92 and 2Delta_l(0)/kBTc = 5.84. The large gap
magnitudes derived from fitting, as well as the large specific heat jump of
Delta_Ce(Tc)/gamma_n*Tc ~ 2.11, indicate strong-coupling superconductivity.
Furthermore, the magnetic field dependence of specific heat shows strong
evidence for multiband superconductivity.
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1103.2961v1
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2011-03-16
|
Point-contact study of ReFeAs(1-x)Fx (Re=La, Sm) superconducting films
|
Point-contact (PC) Andreev-reflection (AR) measurements of the
superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have
been carried out. The value of the gap is distributed in the range 2\Delta
\simeq 5-10 meV (for Re=Sm) with a maximum in the distribution around 6 meV.
Temperature dependence of the gap \Delta(T) can be fitted well by BCS curve
giving reduced gap ratio 2\Delta /kT_c^*\simeq 3.5 (here T_c^* is the critical
temperature from the BCS fit). At the same time, an expected second larger gap
feature was difficult to resolve distinctly on the AR spectra making
determination reliability of the second gap detection questionable. Possible
reasons for this and the origin of other features like clear-cut asymmetry in
the AR spectra and current regime in PCs are discussed.
|
1103.3157v1
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2011-03-18
|
Composition of Transiting and Transiting-only super-Earths
|
The relatively recent detections of the first three transiting super-Earths
mark the beginning of a subfield within exoplanets that is both fruitful and
challenging. The first step into characterizing these objects is to infer their
composition given the degenerate character of the problem. The calculations
show that Kepler-10b has a composition between an Earth-like and a Mercury-like
(enriched in iron) composition. In contrast, GJ 1214b is too large to be solid,
and has to have a volatile envelope. Lastly, while three of the four reported
mass estimates of CoRoT-7b allow for a rocky composition, one forbids it and
can only be reconciled with significant amounts of water vapor. In addition to
these three transiting low-mass planets, there are now more than one thousand
Kepler planets with only measured radius. Even without a mass measurement
("transiting-only") it is still possible to place constraints on the amount of
volatile content of the highly-irradiated planets, as their envelopes, if
present, are flared. Using Kepler-9d as an example, we estimate its water
vapor, or hydrogen and helium content to be less than 50% or 0.1% by mass
respectively.
|
1103.3725v1
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2011-03-25
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A temporal study of oxygen-rich pulsating variable AGB star, T Cep: Investigation on dust formation and dust properties
|
Pulsation is believed to be the leading cause of dusty mass loss from
Asymptotic Giant Branch (AGB) stars. We present a temporal study of T Cep, a
long-period Mira variable, using seven ISO SWS spectra, covering a 16-month
period over a single pulsation cycle. The observed spectral dust features
change over the pulsation cycle of this Mira. In general, the overall apparent
changes in spectral features can be attributed to changes in the dust
temperature, resulting from the intrinsic pulsation cycle of the central star.
However, not all feature changes are so easily explained. Based on direct
comparison with laboratory spectra of several potential minerals, the dust is
best explained by crystalline iron-rich silicates. These findings contradict
the currently favored dust formation hypotheses.
|
1103.5040v1
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2011-03-27
|
Spin-filtering and Disorder Induced Giant Magnetoresistance in Carbon Nanotubes: Ab Initio Calculations
|
Nitrogen-doped carbon nanotubes can provide reactive sites on the
porphyrin-like defects. It's well known that many porphyrins have transition
metal atoms, and we have explored transition metal atoms bonded to those
porphyrin-like defects in N-doped carbon nanotubes. The electronic structure
and transport are analyzed by means of a combination of density functional
theory and recursive Green's functions methods. The results determined the Heme
B-like defect (an iron atom bonded to four nitrogens) as the most stable and
with a higher polarization current for a single defect. With randomly
positioned Heme B-defects in a few hundred nanometers long nanotubes the
polarization reaches near 100% meaning an effective spin filter. A disorder
induced magnetoresistance effect is also observed in those long nanotubes,
values as high as 20000% are calculated with non-magnectic eletrodes.
|
1103.5177v2
|
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