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2011-11-16
|
Metamagnetic Behavior and Kondo Breakdown in Heavy-Fermion CeFePO
|
We report that nonmagnetic heavy-fermion (HF) iron oxypnictide CeFePO with
two-dimensional XY-type anisotropy shows a metamagnetic behavior at the
metamagnetic field H_M \simeq 4 T perpendicular to the c-axis and that a
critical behavior is observed around H_M. Although the magnetic character is
entirely different from that in other Ce-based HF metamagnets, H_M in these
metamagnets is linearly proportional to the inverse of the effective mass, or
to the temperature where the susceptibility shows a peak. This finding suggests
that H_M is a magnetic field breaking the local Kondo singlet, and the critical
behavior around H_M is driven by the Kondo breakdown accompanied by the
Fermi-surface instability.
|
1111.3749v3
|
2011-11-18
|
Magnetoelastic coupling in iron
|
Exchange interactions in {\alpha}- and {\gamma}-Fe are investigated within an
ab-initio spin spiral approach. We have performed total energy calculations for
different magnetic structures as a function of lattice distortions, related
with various cell volumes and the Bain tetragonal deformations. The effective
exchange parameters in {\gamma}-Fe are very sensitive to the lattice
distortions, leading to the ferromagnetic ground state for the tetragonal
deformation or increase of the volume cell. At the same time, the
magnetic-structure-independent part of the total energy changes very slowly
with the tetragonal deformations. The computational results demonstrate a
strong mutual dependence of crystal and magnetic structures in Fe and explain
the observable "anti-Invar" behavior of thermal expansion coefficient in
{\gamma}-Fe.
|
1111.4432v1
|
2011-11-21
|
Dissipative electro-elastic network model of protein electrostatics
|
We propose a dissipative electro-elastic network model (DENM) to describe the
dynamics and statistics of electrostatic fluctuations at active sites of
proteins. The model combines the harmonic network of residue beads with
overdamped dynamics of the normal modes of the network characterized by two
friction coefficients. The electrostatic component is introduced to the model
through atomic charges of the protein force field. The overall effect of the
electrostatic fluctuations of the network is recorded through the
frequency-dependent response functions of the electrostatic potential and
electric field at the active site. We also consider the dynamics of
displacements of individual residues in the network and the dynamics of
distances between pairs of residues. The model is tested against loss spectra
of residue displacements and the electrostatic potential and electric field at
the heme's iron from all-atom molecular dynamics simulations of three hydrated
globular proteins.
|
1111.4726v1
|
2011-11-21
|
Pairing Glue Activation in Cuprates within the Quantum Critical Regime
|
A grand challenge in many-body quantum physics is to explain the apparent
connection between quantum criticality and high-temperature superconductivity
in the cuprates and similar systems, such as the iron pnictides and
chalcogenides. Here we argue that the quantum-critical regime plays an
essential role in activating a strong-pairing mechanism: although pairing
bosons create a symmetry-breaking instability which suppresses pairing, the
combination of these broken-symmetry states within the critical regime can
restore this symmetry for the paired quasiparticles. This condition is shown to
be met within a large-U ansatz. A hidden quantum phase transition then arises
between a Fermi-liquid and a non-Fermi-liquid broken-symmetry striped state,
and a critical regime in which the broken-symmetry states are combined.
|
1111.5033v3
|
2011-11-17
|
About a possible path towards the reverse engineering of quantum mechanics
|
An out of the box intellectual path exploring the foundations of quantum
mechanics is discussed in some detail, in order to clarify why a possibly
different way to look at the relevant fundamental questions can be identified
and can support further research. Two key concepts arise. (1) Einstein critics
to quantum mechanics could be taken seriously, but ironically, in order to
really do so, one would have to take seriously also some of Lorentz critics to
special relativity - both in a possibly more modern way; such interconnection
possibly having been a blocking factor to openly discuss some of the cross
implications of alternative views about quantum mechanics to date. (2) The
probabilistic interpretation is a by-product of (a) quantum evolution
equations, (b) conservation laws for the combination of measuring system and
measured object and (c) persistency of calibration of the measuring system - as
such there is no intellectual conflict whatsoever between hidden variables
theories and probabilistic interpretation, provided we consider multicomponent
hidden variable models and we allow for the existence of an underlying network.
The implications of such concepts, in particular for the development of a
microscopic quantisation program, are heuristically discussed or preliminarily
explored.
|
1111.5215v1
|
2011-11-24
|
Interplay between superconductivity and antiferromagnetism in some iron-pnictide single crystals studied by $^{57}$Fe Mössbauer spectroscopy
|
We have performed detailed $^{57}$Fe M\"ossbauer spectroscopy measurements on
Ba$_{0.78}$K$_{0.22}$Fe$_2$As$_2$ and BaFe$_{2-x}$Ni$_x$As$_2$ single crystal
mosaics showing antiferromagnetic ordering below $T_N$ with superconductivity
below $T_C$. Analysis of the M\"ossbauer spectra shows a decrease in the
magnetic hyperfine (hf) field but no change in the magnetic volume fraction
below $T_C$. This clearly indicates the coexistence of magnetism and
superconductivity in these compounds. The decrease in the magnetic hf field
below $T_C$ depends on the difference between $T_N$ and $T_C$, being the
largest for $T_N$ close to $T_C$. Two different explanations for this
observation are given. We also find that the non-magnetic volume fraction below
$T_N$ correlates with the Ni doping $x$, being large for high $T_C$ and small
for high $T_N$.
|
1111.5853v1
|
2011-11-28
|
Phase diagram and oxygen annealing effect of FeTe1-xSex iron-based superconductor
|
Phase diagrams of as-grown and O2-annealed FeTe1-xSex determined from
magnetic susceptibility measurement were obtained. For as-grown samples, the
antiferromagnetic order was fully suppressed in the range region x \geq 0.15,
and weak superconductivity appeared when x \geq 0.1. Beginning at x = 0.5, weak
superconductivity was found to evolve into bulk superconductivity.
Interestingly, for O2-annealed samples, complete suppression of magnetic order
and the occurrence of bulk superconductivity were observed when x \geq 0.1. We
found that O2-annealing induces bulk superconductivity for FeTe1-xSex. Oxygen
probably plays a key role in the suppression of the magnetic order and the
appearance of bulk superconductivity.
|
1111.6472v3
|
2011-12-01
|
Detailed abundances for M giants in two inner bulge fields from Infrared Spectroscopy
|
We report abundance analysis for 30 M giant stars in two inner Galactic bulge
fields at (l,b)=(0,-1.75) deg and at (l,b)=(1,-2.65) deg, based on R=25,000
infrared spectroscopy from 1.5-1.8um using NIRSPEC at the Keck II telescope. We
find iron abundances of <[Fe/H]>=-0.16 +/- 0.03 dex with a 1-sigma dispersion
of 0.12 +/- 0.02 and <[Fe/H]>=-0.21 +/- 0.02 dex, with a 1-sigma dispersion of
0.09+/- 0.016 for the (l,b)=(0,-1.75) and (l,b)=(1,-2.65) deg fields,
respectively. In agreement with all prior studies, we find enhanced [alpha/Fe]
of +0.3 dex. We confirm the lack of any major vertical abundance or composition
gradient in the innermost ~600 pc between Baade's window and 150 pc from the
Galactic plane. We also confirm that the known enhancement of alpha elements
observed between 500 and 1000 pc from the nucleus is also present over the
volume of the inner bulge and may therefore be presumed to be a general
characteristic of bulge/bar stars within 1 kpc of the Galactic Center.
|
1112.0306v1
|
2011-12-08
|
Photovoltaic Effect on Molecule Coupled Ferromagnetic Films of a Magnetic Tunnel Junction
|
Economical solar energy conversion to electricity can be boosted by the
discovery of fundamentally new photovoltaic mechanism, and a suitable system to
realize it with commonly available materials like iron (Fe) and nickel (Ni).
This paper reports the observation of photovoltaic effect on a molecular
spintronics device, composed of magnetic tunnel junction (MTJ) and
organometallic molecular clusters (OMCs). A prefabricated MTJ with exposed side
edges, after enabling the bridging of OMC channels between its two
ferromagnetic films, exhibited following phenomenon (i) dramatic increase in
exchange coupling, (ii) 3-6 orders current suppression and (iii) photovoltaic
effect. This paper focuses on the photovoltaic effect. Control experiments on
isolated ferromagnetic films suggested that OMCs neither affected the magnetic
properties nor produced any photovoltaic effect; photovoltaic effect was only
observed on the ferromagnetic films serving as magnetic electrodes in a MTJ.
Present paper invites further investigation of the similar photovoltaic effect
on other combinations of MTJs and promising magnetic molecules, like single
molecular magnets, organometallic clusters and porphyrins. This research can
lead to mass producible and economical spin photovoltaic devices.
|
1112.1879v2
|
2011-12-10
|
Existence of Orbital Order and its Fluctuation in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Revealed by X-ray Absorption Spectroscopy
|
We performed temperature dependent X-ray linear dichroism (XLD) experiments
on an iron pnictide system, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with $x$=0.00 and
0.05, to experimentally verify existence of orbital ordering (OO). We observed
clear XLD in polarization dependent X-ray absorption spectra of Fe $L$ edges.
By exploiting the difference in the temperature dependent behaviors, we were
able to separate OO and structure contributions to XLD. The observed OO signal
indicates different occupation numbers for $d_{yz}$ and $d_{zx}$ orbitals and
supports existence of a ferro-OO. The results are also consistent with the
theoretical prediction. Moreover, we find substantial OO signal above the
transition temperature, which suggests that OO fluctuation exists well above
the transition temperature.
|
1112.2243v1
|
2011-12-20
|
A Constructive Approach to Reduced-Form Auctions with Applications to Multi-Item Mechanism Design
|
We provide a constructive proof of Border's theorem [Bor91, HR15a] and its
generalization to reduced-form auctions with asymmetric bidders [Bor07, MV10,
CKM13]. Given a reduced form, we identify a subset of Border constraints that
are necessary and sufficient to determine its feasibility. Importantly, the
number of these constraints is linear in the total number of bidder types. In
addition, we provide a characterization result showing that every feasible
reduced form can be induced by an ex-post allocation rule that is a
distribution over ironings of the same total ordering of the union of all
bidders' types.
We show how to leverage our results for single-item reduced forms to design
auctions with heterogeneous items and asymmetric bidders with valuations that
are additive over items. Appealing to our constructive Border's theorem, we
obtain polynomial-time algorithms for computing the revenue-optimal auction.
Appealing to our characterization of feasible reduced forms, we characterize
feasible multi-item allocation rules.
|
1112.4572v2
|
2011-12-20
|
Observation of Multiple Superconducting Gaps in the Infrared Reflectivity Spectra of Ba(Fe0.9Co0.1)2As2
|
The results of infrared reflectivity measurements for the iron-based
high-temperature superconductor Ba(Fe0.9Co0.1)2As2 are reported. The
reflectivity is found to be close to unity at frequencies w lower than 2Delta/h
(2Delta is the superconducting gap and h is Planck's constant). This is
evidence for the s+/- or s+/+ symmetry of the superconducting order parameter
in the studied compound. The infrared reflectivity spectra of
Ba(Fe0.9Co0.1)2As2 manifest opening of several superconducting gaps at
temperatures lower than critical Tc.
|
1112.4636v2
|
2011-12-21
|
Optical properties and electronic structure of multiferroic hexagonal orthoferrites RFeO3 (R=Ho, Er, Lu)
|
We report on optical studies of the thin films of multiferroic hexagonal
(P.G. 6mm) rare-earth orthoferrites RFeO3 (R=Ho, Er, Lu) grown epitaxially on a
(111)-surface of ZrO2(Y2O3) substrate. The optical absorption study in the
range of 0.6-5.6 eV shows that the films are transparent below 1.9 eV; above
this energy four broad intense absorption bands are distinguished. The
absorption spectra are analyzed taking into account the unusual fivefold
coordination of the Fe(3+) ion. Temperature dependence of the optical
absorption at 4.9 eV shows anomaly at 124 K, which we attribute to magnetic
ordering of iron sublattices.
|
1112.5102v3
|
2011-12-26
|
Surface properties of the clean and Au/Pd covered Fe$_3$O$_4$(111): a DFT and DFT+$U$ study
|
The spin-density functional theory (DFT) and DFT+$U$ with Hubbard $U$ term
accounting for on-site Coulomb interactions were applied to investigate
structure, stability, and electronic properties of different terminations of
the Fe$_3$O$_4$(111) surface. All terminations of the ferrimagnetic
Fe$_3$O$_4$(111) surface exhibit very large (up to 90%) relaxations of the
first four interlayer distances, decreasing with the oxide layer depth. Our
calculations predict the iron terminated surface to be most stable in a wide
range of the accessible values of the oxygen chemical potential. The adsorption
of Au and Pd on two stable Fe- and O-terminated surfaces is studied. Our
results show that Pd binds stronger than Au both to the Fe- and O-terminated
surface. DFT+$U$ gives stronger bonding than DFT. The bonding of both
adsorbates to the O-terminated magnetite surface is by 1.5-2.5 eV stronger than
to the Fe-terminated surface.
|
1112.5827v1
|
2011-12-28
|
Possibility of Horizontal Line Node in KFe$_2$As$_2$ Probed by Muon Spin Rotation
|
The anisotropy and temperature dependence of the magnetic field penetration
depth in the iron-based superconductor KFe$_2$As$_2$ have been measured down to
20 mK by means of muon spin rotation. We have observed that the flux-line
lattice forms triangular symmetry when we applied the field parallel to c-axis,
suggesting the interaction between vortices should be isotropic. The
temperature dependence of the penetration depth observd with an applied field
parallel and perpendicular to c-axis are different. These results can be
accounted for by a superconducting gap function with a horizontal line node in
the basal plane.
|
1112.6078v1
|
2011-12-29
|
Phenomenology of Neutrino Oscillations at the Neutrino Factory
|
We consider the prospects for a neutrino factory to measure mixing angles,
the CP violating phase and mass-squared differences by detecting wrong-charged
muons arising from the chain \mu^+ to \nu_e to \nu_\mu\ to \mu^- and the
right-charged muons coming from the chain \mu^+ to \bar{\nu}_\mu\ to
\bar{\nu}_\mu\ to \mu^+ (similar to \mu^- chains), where \nu_e to \nu_\mu\ and
\bar{\nu}_\mu\ to \bar{\nu}_\mu\ are neutrino oscillation channels through a
long baseline. First, we perform the baseline and energy optimization of the
neutrino factory including the latest simulation results from the magnetized
iron neutrino detector (MIND). Second, we study physics with near detectors and
consider the treatment of systematic errors including cross section errors,
flux errors, and background uncertainties. Third, the effects of one additional
massive sterile neutrino are investigated in the context of near and far
detector combinations.
|
1112.6259v1
|
2012-01-04
|
KxFe2-ySe2 single crystals: Floating-zone growth, Transport and Structural properties
|
Single crystals of superconducting KxFe2-ySe2 have been grown with the
optical floating-zone technique under application of 8 bar of argon pressure.
We found that large and high quality single crystals with dimensions of
~\varnothing6 \times 10 mm could be obtained at the termination of the grown
ingot through quenching, while the remaining part of the ingot decomposed.
As-grown single crystals commonly represent an intergrowth of two sets of the
c-axis characterized by slightly different lattice constants. Single crystal of
K0.80Fe1.81Se2 shows a superconducting transition at Tc = 31.6 K, leading to a
near 100% expulsion of the external magnetic field in magnetization
measurements. On the other hand, neutron-diffraction data indicate that
superconductivity in the sample coexists with a iron-vacancy superstructure and
static antiferromagnetic order. The anisotropic ratio of the upper critical
field Hc2 for both H//c and H//ab configurations is \sim3.46.
|
1201.0902v1
|
2012-01-04
|
A new candidate for probing Population III nucleosynthesis with carbon-enhanced damped Lyman-alpha systems
|
We report the identification of a very metal-poor damped Lyman-alpha system
(DLA) at z_abs = 3.067295 that is modestly carbon-enhanced, with an iron
abundance of ~1/700 solar ([Fe/H] = -2.84) and [C,O/Fe] ~ +0.6. Such an
abundance pattern is likely to be the result of nucleosynthesis by massive
stars. On the basis of 17 metal absorption lines, we derive a 2 sigma upper
limit on the DLA's kinetic temperature of T_DLA <= 4700 K, which is broadly
consistent with the range of spin temperature estimates for DLAs at this
redshift and metallicity. While the best-fitting abundance pattern shows the
expected hallmarks of Population III nucleosynthesis, models of high-mass
Population II stars can match the abundance pattern almost as well. We discuss
current limitations in distinguishing between these two scenarios and the
marked improvement in identifying the remnants of Population III stars expected
from the forthcoming generation of 30-metre class telescopes.
|
1201.1004v2
|
2012-01-05
|
Metallicities for 6 nearby open clusters from high-resolution spectra of giant stars. [Fe/H] values for a planet search sample
|
We present a study of the stellar parameters and iron abundances of 18 giant
stars in 6 open clusters. The analysis was based on high-resolution and
high-S/N spectra obtained with the UVES spectrograph (VLT-UT2). The results
complement our previous study where 13 clusters were already analyzed. The
total sample of 18 clusters is part of a program to search for planets around
giant stars. The results show that the 18 clusters cover a metallicity range
between -0.23 and +0.23 dex. Together with the derivation of the stellar
masses, these metallicities will allow the metallicity and mass effects to be
disentangled when analyzing the frequency of planets as a function of these
stellar parameters.
|
1201.1108v1
|
2012-01-09
|
A General Mechanism for Orbital Selective Phase Transitions
|
Based on the analysis of a two-orbital Hubbard model within a mean-field
approach, we propose a mechanism for an orbital selective phase transition
(OSPT) where coexistence of localized and itinerant electrons can be realized.
We show that this OSPT exists both at and near half filling even in the absence
of crystal field splittings or when bandwidths, orbital degeneracies and
magnetic states are equal for both orbitals provided the orbitals have
different band dispersions. Such conditions should be generally satisfied in
many materials. We find that this OSPT is not sensitive to the strength of
Hund's rule coupling and that heavy doping favors the collinear
antiferromagnetic state over the OSPT. We discuss our results in relation to
the iron pnictides.
|
1201.1689v2
|
2012-01-19
|
Impact of microstructure, temperature and strain ratio on energy-based low- cycle fatigue life prediction models for TiAl alloys
|
In this paper, two fatigue lifetime prediction models are tested on TiAl
intermetallic using results from uniaxial low-cycle fatigue tests. Both
assessments are based on dissipated energy but one of them considers a
hydrostatic pressure correction. This work allows to confirm, on this kind of
material, the linear nature, already noticed on silicon molybdenum cast iron,
TiNi shape memory alloy and 304L stainless steel, of dissipated energy,
corrected or not with hydrostatic pressure, according to the number of cycles
to failure. This study also highlights that, firstly, the dissipated energy
model is here more adequate to estimate low-cycle fatigue life and that,
secondly, intrinsic parameters like microstructure as well as extrinsic
parameters like temperature or strain ratio have an impact on prediction
results.
|
1201.4084v1
|
2012-01-20
|
Magnetic origin of chemical balance in alloyed Fe-Cr stainless steels: first-principles and Ising model study
|
Iron-chromium forms the basis of most of the stainless steel grades in the
markets. Recently new insights into the physical and chemical properties of
Fe-Cr based alloys have been obtained. Some of the new results are quite
unexpected and call for further investigations. The present study addresses the
magnetic contribution in the atomic driving forces behind the chemical
composition in Fe-Cr alloyed with Al, Ti, V, Mn, Co, Ni, and Mo. Using the ab
initio exact muffin-tin orbitals method and an Ising-type spin model, it is
found that the magnetic moment of the solute atom combined with the induced
changes in the magnetic moments of the host atoms form the main framework in
determining the mixing energy and chemical potentials of low-Cr Fe-Cr based
alloys. The results obtained in the present work are related to tuning of the
microstructure and corrosion protection of low-Cr steels.
|
1201.4336v1
|
2012-01-23
|
Influence of the magnetoelectric coupling on the electric field induced magnetization reversal in a composite non-strained multiferroic chain
|
We study theoretically the multiferroic dynamics in a composite
one-dimensional system consisting of BaTiO3 multiferroically coupled to an iron
chain. The method treats the magnetization and the polarization as
thermodynamic quantities describable via a combination of the Landau-Lifshits-
Gilbert and the Ginzburg-Landau dynamics coupled via an additional term in the
total free energy density. This term stems from the multiferroic interaction at
the interface. For a wide range of strengths of this coupling we predict the
possibility of obtaining a well-developed hysteresis in the ferromagnetic part
of the system induced by an external electric field. The dependence of the
reversal modes on the electric field frequency is also investigated and we
predict a considerable stability of the magnetization reversal for frequencies
in the range of 0.5 - 12 [GHz].
|
1201.4740v1
|
2012-01-24
|
Electronic Orders in the Verwey Structure of Magnetite
|
Electronic structure calculations of the Verwey ground state of magnetite,
Fe3O4, using density functional theory with treatment of on-site Coulomb
interactions (DFT+U scheme) are reported. These calculations use the
recently-published experimental crystal structure coordinates for magnetite in
the monoclinic space group Cc. The computed density distribution for minority
spin electron states close to the Fermi level demonstrates that charge order
and Fe2+-orbital order are present at the B-type lattice sites to a
first-approximation. However, Fe2+/Fe3+ charge differences are diminished
through weak bonding interactions of the Fe2+-states to specific pairs of
neighboring iron sites that create linear, three-B-atom trimeron units that may
be regarded as 'orbital molecules'. Trimerons are ordered evenly along most Fe
atom chains in the Verwey structure, but more complex interactions are observed
within one chain.
|
1201.5106v2
|
2012-01-25
|
Effect of impurities on growth and morphology of cementite nanowires
|
Effects of doping on morphology of iron carbide (cementite) nanowires have
been explored by first principle electronic structure calculations. We examined
the role of several realistic impurities (Si, Mn, V, P and S) in the formation
energies of the cementite nanowires with different sizes and morphologies. It
is shown that the presence of the impurities decreases the formation energy and
can switch the preferable axis of the cementite nanowire growth. The conditions
of the formation and decomposition of cementite nanowires in steels are also
discussed.
|
1201.5327v2
|
2012-01-26
|
Infrared emissivity spectroscopy of a soda-lime silicate glass up to the melt
|
The short-range structure of an iron doped soda-lime glass was investigated
by infrared emissivity spectroscopy from room temperature up to the melt.
Quantitative information on the distribution of the Q^n tetrahedral units was
obtained by fitting the emissivity spectra using a dielectric function model
(DFM). The DFM is based on causal Gaussian bands, associated with the
stretching motions of the silicate tetrahedra. The changes in the absorption
modes are related to the activation of a dynamical disorder that continuously
increases with temperature. The obtained Q^n speciation at room temperature is
in good agreement with the magic-angle spinning nuclear magnetic resonance (MAS
NMR) study. The distribution of the tetrahedral species undergoes slight
changes with temperature, except during glass crystallization where Q^4 units
increases, with a slight augmentation of Q^2 and Q^4 units in the melt. These
results demonstrate the potentiality of infrared spectroscopy in the
quantitative analysis of the polymerization degree of glasses and melts.
|
1201.5676v1
|
2012-01-27
|
The effects of Vanadium on the strength of a bcc Fe Σ3(111)[1-10] grain boundary
|
The effects of micro-alloying element, vanadium, on a bcc Fe \Sigma
3(111)[1-10] symmetric tilt grain boundary strength are studied using density
functional theory calculations. The lowest energy configuration of the grain
boundary structure are obtained from the first-principles calculations. The
substitutional and interstitial point defect formation energies of vanadium in
the grain boundary are compared. The substitutional defect is prefered to
interstitial one. The segregation energies of vanadium onto the grain boundary
and its fractured surfaces are computed. The cohesive energy calculation of the
grain boundary with and without vanadium show that vanadium strengthen the bcc
iron \Sigma 3(111)[1-10] grain boundary.
|
1201.5915v1
|
2012-02-01
|
Theory of High-Tc Superconductivity: Transition Temperature
|
It is demonstrated that the transition temperature (Tc) of high-Tc
superconductors is determined by their layered crystal structure, bond lengths,
valency properties of the ions, and Coulomb coupling between electronic bands
in adjacent, spatially separated layers. Analysis of 31 high-Tc materials
(cuprates, ruthenates, rutheno-cuprates, iron pnictides, organics) yields the
universal relationship for optimal compounds, kBTc0 = {\beta}/\ell{\zeta},
where \ell is related to the mean spacing between interacting charges in the
layers, {\zeta} is the distance between interacting electronic layers, {\beta}
is a universal constant and Tc0 is the optimal transition temperature
(determined to within an uncertainty of +/- 1.4 K by this relationship).
Non-optimum compounds, in which sample degradation is evident, e.g. by
broadened superconducting transitions and diminished Meissner fractions,
typically exhibit reduced Tc < Tc0. It is shown that Tc0 may be obtained from
an average of Coulomb interaction forces between the two layers.
|
1202.0306v1
|
2012-02-03
|
Experimental and theoretical electronic structure of EuRh2As2
|
The Fermi surfaces (FS's) and band dispersions of EuRh2As2 have been
investigated using angle-resolved photoemission spectroscopy. The results in
the high-temperature paramagnetic state are in good agreement with the full
potential linearized augmented plane wave calculations, especially in the
context of the shape of the two-dimensional FS's and band dispersion around the
Gamma (0,0) and X (pi,pi) points. Interesting changes in band folding are
predicted by the theoretical calculations below the magnetic transition
temperature Tn=47K. However, by comparing the FS's measured at 60K and 40K, we
did not observe any signature of this transition at the Fermi energy indicating
a very weak coupling of the electrons to the ordered magnetic moments or strong
fluctuations. Furthermore, the FS does not change across the temperature (~
25K) where changes are observed in the Hall coefficient. Notably, the Fermi
surface deviates drastically from the usual FS of the superconducting
iron-based AFe2As2 parent compounds, including the absence of nesting between
the Gamma and X FS pockets.
|
1202.0744v1
|
2012-02-12
|
Theory of High-Tc Superconductivity: Accurate Predictions of Tc
|
The superconducting transition temperatures of high-Tc compounds based on
copper, iron, ruthenium and certain organic molecules are discovered to be
dependent on bond lengths, ionic valences, and Coulomb coupling between
electronic bands in adjacent, spatially separated layers [1]. Optimal
transition temperature, denoted as T_c0, is given by the universal expression
$k_BT_c0 = e^2 \Lambda / \ell\zeta$; $\ell$ is the spacing between interacting
charges within the layers, \zeta is the distance between interacting layers and
\Lambda is a universal constant, equal to about twice the reduced electron
Compton wavelength (suggesting that Compton scattering plays a role in
pairing). Non-optimum compounds in which sample degradation is evident
typically exhibit Tc < T_c0. For the 31+ optimum compounds tested, the
theoretical and experimental T_c0 agree statistically to within +/- 1.4 K. The
elemental high Tc building block comprises two adjacent and spatially separated
charge layers; the factor e^2/\zeta arises from Coulomb forces between them.
The theoretical charge structure representing a room-temperature superconductor
is also presented.
|
1202.2480v2
|
2012-02-16
|
Orbital Selective Magnetism in the Spin-Ladder Iron Selenides Ba$_{1-x}$K$_{x}$Fe$_2$Se$_3$
|
Here we show that the 2.80(8) {\mu}B/Fe block antiferromagnetic order of
BaFe2Se3 transforms into stripe antiferromagnetic order in KFe2Se3 with a
decrease in moment to 2.1(1) {\mu}B/Fe. This reduction is larger than expected
from the change in electron count from Ba$^{2+}$ to K$^{+}$, and occurs with
the loss of the displacements of Fe atoms from ideal positions in the ladders,
as found by neutron pair distribution function analysis. Intermediate
compositions remain insulating, and magnetic susceptibility measurements show a
suppression of magnetic order and probable formation of a spin-glass. Together,
these results imply an orbital-dependent selection of magnetic versus bonded
behavior, driven by relative bandwidths and fillings.
|
1202.3676v2
|
2012-02-19
|
High $T_c$ superconductivity at the FeSe/SrTiO$_3$ Interface
|
In a recent experiment the superconducting gap of a single unit cell thick
FeSe film on SrTiO$_3$ substrate is observed by scanning tunneling spectroscopy
and angle-resolved photoemission spectroscopy. The value of the superconducting
gap is much larger than that of the bulk FeSe under ambient pressure. In this
paper we study the effects of screening due to the ferroelectric phonons on
Cooper pairing. We conclude it can significantly enhance the energy scale of
Cooper pairing and even change the pairing symmetry. Our results also raise
some concerns on whether phonons can be completely ignored for bulk iron-based
superconductors.
|
1202.4141v2
|
2012-02-20
|
Observation of superconductivity at 30 K~46 K in AxFe2Se2 (A = Li, Na, Ba, Sr, Ca, Yb, and Eu)
|
New iron selenide superconductors by intercalating smaller-sized alkali
metals (Li, Na) and alkaline earths using high-temperature routes have been
pursued ever since the discovery of superconductivity at about 30 K in KFe2Se2,
but all have failed so far. Here we demonstrate that a series of
superconductors with enhanced Tc=30~46 K can be obtained by intercalating
metals, Li, Na, Ba, Sr, Ca, Yb, and Eu in between FeSe layers by the
ammonothermal method at room temperature. Analysis on their powder X-ray
diffraction patterns reveals that all the main phases can be indexed based on
body-centered tetragonal lattices with a~3.755-3.831 {\AA} while c~15.99-20.54
{\AA}. Resistivities show the corresponding sharp transitions at 45 K and 39 K
for NaFe2Se2 and Ba0.8Fe2Se2, respectively, confirming their bulk
superconductivity. These findings provide a new starting point for studying the
properties of these superconductors and an effective synthetic route for the
exploration of new superconductors as well.
|
1202.4340v2
|
2012-02-20
|
Antiferromagnetic Domain Wall Engineering in Chromium Films
|
We have engineered an antiferromagnetic domain wall by utilizing a magnetic
frustration effect of a thin iron cap layer deposited on a chromium film.
Through lithography and wet etching we selectively remove areas of the Fe cap
layer to form a patterned ferromagnetic mask over the Cr film. Removing the Fe
locally removes magnetic frustration in user-defined regions of the Cr film. We
present x-ray microdiffraction microscopy results confirming the formation of a
90{\deg} spin-density wave propagation domain wall in Cr. This domain wall
nucleates at the boundary defined by our Fe mask.
|
1202.4401v1
|
2012-02-21
|
A first-principles investigation on the effects of magnetism on the Bain transformation of $α$-phase FeNi systems
|
The effects of magnetism on the Bain transformation of $\alpha$-phase FeNi
systems are investigated by using the full potential linearized augmented plane
wave (FLAPW) method based on the generalized gradient approximation (GGA). We
found that Ni impurity in bcc Fe increases the lattice constant in
ferromagnetic (FM) states, but not in the nonmagnetic (NM) states. The shear
modulus $G$ and Young's modulus $E$ of bcc Fe are also increased by raising the
concentration of nickel. All the compositions considered show high shear
anisotropy and the ratio of the bulk to shear modulus is greater than 1.75
implying ductility. The mean sound velocities in the $[100]$ directions are
greater than in the $[110]$ directions. The Bain transformation, which is a
component of martensitic transformation, has also been studied to reveal that
Ni$_{x}$Fe$_{1-x}$ alloys are elastically unstable in the NM states, but not so
in the FM states. The electronic structures explain these results in terms of
the density of states at the Fermi level. It is evident that magnetism cannot
be neglected when dealing with the Bain transformation in iron and its alloys.
|
1202.4726v1
|
2012-02-23
|
Tuning magnetism in FeAs-based materials via tetrahedral structure
|
Resistivity, magnetic susceptibility, neutron scattering and x-ray
crystallography measurements were used to study the evolution of magnetic order
and crystallographic structure in single-crystal samples of the Ba1-xSrxFe2As2
and Sr1-yCayFe2As2 series. A non-monotonic dependence of the magnetic ordering
temperature T0 on chemical pressure is compared to the progression of the
antiferromagnetic staggered moment, characteristics of the ordering transition
and structural parameters to reveal a distinct relationship between the
magnetic energy scale and the tetrahedral bond angle, even far above T0. In
Sr1-yCayFe2As2, an abrupt drop in T0 precisely at the Ca concentration where
the tetrahedral structure approaches the ideal geometry indicates a strong
coupling between the orbital bonding structure and the stabilization of
magnetic order, providing strong constraints on the nature of magnetism in the
iron-arsenide superconducting parent compounds.
|
1202.5324v2
|
2012-02-24
|
Block magnetism coupled with local distortion in the iron-based spin-ladder compound BaFe2Se3
|
Magnetism in the insulating BaFe$_2$Se$_3$ was examined through
susceptibility, specific heat, resistivity and neutron diffraction
measurements. After formation of a short-range magnetic correlation, a
long-range ordering was observed below $T_{\rm N}\sim 255$ K. The transition is
obscured by bulk properties. Magnetic moments ($\parallel a$) are arranged to
form a Fe$_4$ ferromagnetic unit, and each Fe$_4$ stacks antiferromagnetically.
This block magnetism is of the third type among magnetic structures of ferrous
materials. The magnetic ordering drives unusually large distortion via
magnetoelastic coupling.
|
1202.5371v1
|
2012-02-24
|
Nanoscale layering of antiferromagnetic and superconducting phases in Rb2Fe4Se5
|
We studied phase separation in a single-crystalline antiferromagnetic
superconductor Rb2Fe4Se5 (RFS) using a combination of scattering-type scanning
near-field optical microscopy (s-SNOM) and low-energy muon spin rotation
(LE-\mu SR). We demonstrate that the antiferromagnetic and superconducting
phases segregate into nanometer-thick layers perpendicular to the iron-selenide
planes, while the characteristic in-plane size of the metallic domains reaches
10 \mu m. By means of LE-\mu SR we further show that in a 40-nm thick surface
layer the ordered antiferromagnetic moment is drastically reduced, while the
volume fraction of the paramagnetic phase is significantly enhanced over its
bulk value. Self-organization into a quasiregular heterostructure indicates an
intimate connection between the modulated superconducting and antiferromagnetic
phases.
|
1202.5446v2
|
2012-02-28
|
Conductance of non-ballistic point contacts in hybrid systems "normal metal/superconductor" Cu/Mo-C and Cu/LaOFFeAs
|
We consider the shape of the curves of "Andreev" conductance of non-ballistic
point contact NS heterosystems depending on the bias voltage at the contact.
The obtained shape of those curves is caused by the contribution from the
mechanism of coherent scattering by impurities which doubles the scattering
cross section. The behavior of generalized and differential conductance is
compared for ballistic and non-ballistic transport regimes. The criteria are
considered allowing one to discriminate between those regimes with the
corresponding conduction curves similar in appearance. The analysis is extended
to the case of non-ballistic transport in NS point contacts with exotic
superconductors, molybdenum carbide Mo-C and oxypnictide
La[O$_{1-x}$F$_{x}$]FeAs from a group of iron-based superconductors.
|
1202.6173v1
|
2012-03-01
|
Macroscopic phase segregation in superconducting K0.73Fe1.67Se2 as seen by muon spin rotation and infrared spectroscopy
|
Using muon spin rotation (\muSR) and infrared spectroscopy we investigated
the recently discovered superconductor K0.73Fe1.67Se2 with Tc = 32 K. We show
that the combined data can be consistently described in terms of a
macroscopically phase segregated state with a matrix of ~88% volume fraction
that is insulating and strongly magnetic and inclusions with a ~12% volume
fraction which are metallic, superconducting and non-magnetic. The electronic
properties of the latter, in terms of the normal state plasma frequency and the
superconducting condensate density, appear to be similar as in other iron
selenide or arsenide superconductors.
|
1203.0286v2
|
2012-03-03
|
Extension of the Wu-Jing equation of state (EOS) for highly porous materials: calculations to validate and compare the thermoelectron model
|
In order to verify and validate the newly developed thermoelectron equation
of state (EOS) model that is based on the Wu-Jing (W-J) EOS, calculations of
shock compression behavior have been made on five different porous metals-iron,
copper, lead, tungsten, and aluminum-which are commonly used as standards. The
model was used to calculate the Hugoniot, shock temperature, sound velocity,
and unloading isentrope for these materials and comparisons were made to
previous calculations and available data. Based on these comparisons, it is
felt that the model provides information in good agreement with the
corresponding experimental and theoretical data published previously. This
suggests that the new model can satisfactorily describe the properties of
shocked porous materials over a wide range of pressure and porosity.
|
1203.0619v1
|
2012-03-07
|
The interplay of electron doping and chemical pressure in Ba(Fe(1-y)Coy)2(As(1-x)Px)2
|
The effects of internal chemical pressure on electron doped iron arsenide
superconductors are studied in the series Ba(Fe(1-y)Coy)2(As(1-x)Px)2.
Combinations of both dopants induce superconductivity also in such areas where
only one would not suffice, and can likewise move the system into an overdoped
state, while no higher critical temperature than 31 K in BaFe2(As(1-x)Px)2 was
found. The phase diagram gives no evidence of holes in BaFe2(As(1-x)Px)2 as
suggested by recent photoemission experiments. Chemical and physical pressure
act similarly in Ba(Fe(1-y)Coy)2(As(1-x)Px)2, but our data reveal that the most
important control parameter is the length of the Fe-As bond and not the unit
cell volume. This emphasizes that differences between chemical and physical
pressure which manifest oneself as the non-linear reduction of the Fe-As
distance in BaFe2(As(1-x)Px)2 are strongly linked to the superconducting
properties also in the Co doped compounds.
|
1203.1459v1
|
2012-03-12
|
Suppression of Superconductivity by Twin Boundaries in FeSe
|
Low-temperature scanning tunneling microscopy and spectroscopy are employed
to investigate twin boundaries in stoichiometric FeSe films grown by molecular
beam epitaxy. Twin boundaries can be unambiguously identified by imaging the
90{\deg} change in the orientation of local electronic dimers from Fe site
impurities on either side. Twin boundaries run at approximately 45{\deg} to the
Fe-Fe bond directions, and noticeably suppress the superconducting gap, in
contrast with the recent experimental and theoretical findings in other iron
pnictides. Furthermore, vortices appear to accumulate on twin boundaries,
consistent with the degraded superconductivity there. The variation in
superconductivity is likely caused by the increased Se height in the vicinity
of twin boundaries, providing the first local evidence for the importance of
this height to the mechanism of superconductivity.
|
1203.2629v2
|
2012-03-13
|
Coexistence and competition of the short-range incommensurate antiferromagnetic order with superconductivity in BaFe2-xNixAs2
|
Superconductivity in the iron pnictides develops near antiferromagnetism, and
the antiferromagnetic (AF) phase appears to overlap with the superconducting
phase in some materials such as BaFe2-xTxAs2 (where T = Co or Ni). Here we use
neutron scattering to demonstrate that genuine long-range AF order and
superconductivity do not coexist in BaFe2-xNixAs2 near optimal
superconductivity. In addition, we find a first-order-like AF to
superconductivity phase transition with no evidence for a magnetic quantum
critical point. Instead, the data reveal that incommensurate short-range AF
order coexists and competes with superconductivity, where the AF spin
correlation length is comparable to the superconducting coherence length.
|
1203.2759v2
|
2012-03-17
|
Magnetic Phase Transition and Relaxation Effects in LiFePO4
|
We report the observation of para - antiferromagnetic transition at ~ 50 K in
lithium iron phosphate, LiFePO4 through DC magnetization and M\"ossbauer
spectroscopy. The Ferrous ion Fe2+ (3d6, 5D) in LiFePO4 exhibits relaxation
effects with a relaxation frequency ~1.076 \times 10(rise to 7) s-1 at 300 K.
The temperature dependence of the frequency suggests the origin of the
relaxation is spin-lattice type. The quadrupole splitting at low temperatures
indicates the excited orbital states mix strongly to the orbital doublet ground
state via spin-orbit coupling. Modified molecular field model analysis yields a
saturation value for hyperfine field ~125 kOe. The anomaly in magnetization and
M\"ossbauer parameters below 27 K may be ascribed to contribution of orbital
angular momentum. The high value of the asymmetry parameter ({\eta} ~ 0.8) of
the electric field gradient obtained in the antiferromagnetic regime indicates
a strongly distorted octahedral oxygen neighbourhood for the ferrous sites.
|
1203.3863v2
|
2012-03-19
|
Antiferromagnetic Order and Superconductivity in Sr4(Mg0.5-xTi0.5+x)2O6Fe2As2 with Electron Doping: 75As-NMR Study
|
We report an 75As-NMR study on iron (Fe)-based superconductors with thick
perovskitetype blocking layers Sr4(Mg0.5-xTi0.5+x)2O6Fe2As2 with x=0 and 0.2.
We have found that antiferromagnetic (AFM) order takes place when x=0, and
superconductivity (SC) emerges below Tc=36 K when x=0.2. These results reveal
that the Fe-pnictides with thick perovskitetype blocks also undergo an
evolution from the AFM order to the SC by doping electron carriers into FeAs
planes through the chemical substitution of Ti+4 ions for Mg+2 ions, analogous
to the F-substitution in LaFeAsO compound. The reason why the Tc=36 K when
x=0.2 being higher than the optimally electron-doped LaFeAsO with Tc=27 K
relates to the fact that the local tetrahedron structure of FeAs4 is optimized
for the onset of SC.
|
1203.4190v1
|
2012-03-20
|
Phase soliton and pairing symmetry of a two-band superconductor: Role of the proximity effect
|
We suggest a mechanism which promotes the existence of a phase soliton --
topological defect formed in the relative phase of superconducting gaps of a
two-band superconductor with s+- type of pairing. This mechanism exploits the
proximity effect with a conventional s-wave superconductor which favors the
alignment of the phases of the two-band superconductor which, in the case of
s+- pairing, are pi-shifted in the absence of proximity. In the case of a
strong proximity such effect can be used to reduce soliton's energy below the
energy of a soliton-free state thus making the soliton thermodynamically
stable. Based on this observation we consider an experimental setup, applicable
both for stable and metastable solitons, which can be used to distinguish
between s+- and s++ types of pairing in the iron-based multiband
superconductors.
|
1203.4554v3
|
2012-03-25
|
First Principles Calculation of Helical Spin Order in Iron Perovskite SrFeO3 and BaFeO3
|
Motivated by recent discovery of ferromagnetism in cubic perovskite BaFeO3
under small magnetic field, we investigate spin order in BaFeO3 and
isostructual SrFeO3 by the first principles calculation. The on-site Coulomb
and exchange interactions are necessary for the helical spin order consistent
with experiments. SrFeO3 exhibits stable G-type helical order, while A- and
G-type helical orders in BaFeO3 are almost degenerate at short propagating
vector with tiny energetic barrier with respect to ferromagnetic spin order,
explaining ferromagnetism under small field. The results are consistent with
model calculation where negative charge-transfer energy is explicitly taken
into account.
|
1203.5470v1
|
2012-03-26
|
Probing the role of Co substitution in the electronic structure of iron-pnictides
|
The role of Co substitution in the low-energy electronic structure of
Ca(Fe$_{0.944}$Co$_{0.056}$)$_2$As$_2$ is investigated by resonant
photoemission spectroscopy and density functional theory. The Co 3d-state
center-of-mass is observed at 250 meV higher binding energy than Fe's,
indicating that Co posses one extra valence electron, and that Fe and Co are in
the same 2+ oxidation state. Yet, significant Co character is detected for the
Bloch wavefunctions at the chemical potential, revealing that the Co 3d
electrons are part of the Fermi sea determining the Fermi surface. This
establishes the complex role of Co substitution in CaFe2As2, and the inadequacy
of a rigid-band shift description.
|
1203.5814v1
|
2012-03-28
|
Opening of the superconducting gap in the hole pockets of Ba(Fe(1-x)Co(x))2As2 as seen via Angle-Resolved PhotoElectron Spectroscopy
|
We present an Angle-Resolved PhotoElectron Spectroscopy study of the changes
in the electronic structure of electron doped Ba(Fe(1-x)Co(x))2As2 across the
superconducting phase transition. By changing the polarization of the incoming
light, we were able to observe the opening of the gap for the inner hole pocket
alpha, and to compare its behavior with the outer hole-like band beta.
Measurements along high symmetry directions show that the behavior of beta is
consistent with an isotropic gap opening, while slight anisotropies are
detected for the inner band alpha. The implications of these results for the
s+/- symmetry of the superconducting order parameter are discussed, in relation
to the nature of the different iron orbitals contributing to the electronic
structure of this multiband system.
|
1203.6194v2
|
2012-04-02
|
Abrupt change in the energy gap of superconducting Ba1-xKxFe2As2 single crystals with hole doping
|
We performed a Laser angle-resolved photoemission spectroscopy (ARPES) study
on a wide doping range of Ba1-xKxFe2As2 (BaK) and precisely determined the
doping evolution of the superconducting (SC) gaps in this compound. The gap
size of the outer hole Fermi surface (FS) sheet around the Brillioun zone (BZ)
center shows an abrupt drop with overdoping (for x > 0.6) while the inner and
middle FS gaps roughly scale with Tc. This is accompanied by the simultaneous
disappearance of the electron FS sheet with similar orbital character at the BZ
corner. These results browse the different contributions of X2-Y2 and XZ/YZ
orbitals to superconductivity in BaK and can be hardly completely reproduced by
the available theories on iron-based superconductors.
|
1204.0326v1
|
2012-04-02
|
Quantum critical point for stripe order: An organizing principle of cuprate superconductivity
|
A spin density-wave quantum critical point (QCP) is the central organizing
principle of organic, iron-pnictide, heavy-fermion and electron-doped cuprate
superconductors. It accounts for the superconducting Tc dome, the
non-Fermi-liquid resistivity, and the Fermi-surface reconstruction. Outside the
magnetically ordered phase above the QCP, scattering and pairing decrease in
parallel as the system moves away from the QCP. Here we argue that a similar
scenario, based on a stripe-order QCP, is a central organizing principle of
hole-doped cuprate superconductors. Key properties of Eu-LSCO, Nd-LSCO and YBCO
are naturally unified, including stripe order itself, its QCP, Fermi-surface
reconstruction, the linear-T resistivity, and the nematic character of the
pseudogap phase.
|
1204.0490v1
|
2012-04-03
|
A photonic crystal cavity-optical fiber tip nanoparticle sensor for biomedical applications
|
We present a sensor capable of detecting solution-based nanoparticles using
an optical fiber tip functionalized with a photonic crystal cavity. When sensor
tips are retracted from a nanoparticle solution after being submerged, we find
that a combination of convective fluid forces and optically-induced trapping
cause an aggregation of nanoparticles to form directly on cavity surfaces. A
simple readout of quantum dot photoluminescence coupled to the optical fiber
shows that nanoparticle presence and concentration can be detected through
modified cavity properties. Our sensor can detect both gold and iron oxide
nanoparticles and can be utilized for molecular sensing applications in
biomedicine.
|
1204.0818v1
|
2012-04-05
|
Gaseous Material Orbiting the Polluted, Dusty White Dwarf HE1349-2305
|
We present new spectroscopic observations of the polluted, dusty,
helium-dominated atmosphere white dwarf star HE1349-2305. Optical spectroscopy
reveals weak CaII infrared triplet emission indicating that metallic gas debris
orbits and is accreted by the white dwarf. Atmospheric abundances are measured
for magnesium and silicon while upper limits for iron and oxygen are derived
from the available optical spectroscopy. HE1349-2305 is the first gas
disk-hosting white dwarf star identified amongst previously known polluted
white dwarfs. Further characterization of the parent body polluting this star
will require ultraviolet spectroscopy.
|
1204.1132v1
|
2012-04-04
|
Rare earth substitution in lattice-tuned Sr0.3Ca0.7Fe2As2 solid solutions
|
The effects of aliovalent rare earth substitution on the physical properties
of Sr0.3Ca0.7Fe2As2 solid solutions are explored. Electrical transport,
magnetic susceptibility and structural characterization data as a function of
La substitution into (Sr_1-y_Ca_y)_1-x_La_x_Fe2As2 single crystals confirm the
ability to suppress the antiferromagnetic ordering temperature from 200 K in
the undoped compound down to 100 K approaching the solubility limit of La.
Despite up to ~30% La substitution, the persistence of magnetic order and lack
of any signature of superconductivity above 10 K present a contrasting phase
diagram to that of Ca1-xLaxFe2As2, indicating that the suppression of magnetic
order is necessary to induce the high-temperature superconducting phase
observed in Ca1-xLaxFe2As2.
|
1204.1335v2
|
2012-04-08
|
Two successive field-induced spin-flop transitions in single-crystalline CaCo$_{2}$As$_{2}$
|
CaCo$_{2}$As$_{2}$, a ThCr$_{2}$Si$_{2}$-structure compound, undergoes an
antiferromagnetic transition at \emph{T$_{N}$}=76K with the magnetic moments
being aligned parallel to the \emph{c} axis. Electronic transport measurement
reveals that the coupling between conducting carriers and magnetic order in
CaCo$_{2}$As$_{2}$ is much weaker comparing to the parent compounds of iron
pnictide. Applying magnetic field along \emph{c} axis induces two successive
spin-flop transitions in its magnetic state. The magnetization saturation
behaviors with \emph{\textbf{H}$\parallel$c} and \emph{\textbf{H}$\parallel$ab}
at 10K indicate that the antiferromagnetic coupling along \emph{c} direction is
very weak. The interlayer antiferromagntic coupling constant \emph{J$_{c}$} is
estimated to be about 2 meV.
|
1204.1736v1
|
2012-04-09
|
Measurement of The Broad Line Region Size in a Luminous MACHO Quasar
|
We measure the broad emission line region (BLR) size of a luminous, L~1E47
erg/s, high-z quasar using broadband photometric reverberation mapping. To this
end, we analyze ~7.5 years of photometric data for MACHO 13.6805.324 (z~1.72)
in the B and R MACHO bands and find a time delay of 180+/-40 days in the rest
frame of the object. Given the spectral-variability properties of high-z
quasars, we associate this lag with the rest-UV iron emission blends. Our
findings are consistent with a simple extrapolation of the BLR size-luminosity
relation in local active galactic nuclei to the more luminous, high-z quasar
population. Long-term spectroscopic monitoring of MACHO 13.6805.324 may be able
to directly measure the line-to-continuum time-delay and test our findings.
|
1204.1862v1
|
2012-04-19
|
Plasma Pressure Driven Asymmetric Supernovae and Highly Collimated Gamma-Ray Bursts
|
During the process of collapse of a massive star, a cavity is generated
between the central iron core and an outer stellar envelope. The dynamics of
this cavity, filled with plasma and magnetic field of the rapidly rotating
proto-magnetar's magnetosphere, is believed to be very relevant in
understanding supernovae and gamma-ray bursts. The interactions of the
pressurized conducting plasma and the magnetic fields are described by a set of
magnetohydrodynamic (MHD) equations with poloidal and toroidal plasma flows not
aligned with magnetic fields. A sequence of MHD equilibria in response to the
increasing plasma pressure in the cavity, by continuous filling from the
rotating magnetosphere, is solved to account for asymmetric supernovae, highly
collimated gamma-ray burst jets, and also active galactic nucleus plasma torus.
It is shown that the magnetosphere of the central compact star is likely the
central engine of supernova and gamma-ray burst by feeding them plasma,
magnetic energy, and rotational energy.
|
1204.4360v2
|
2012-04-23
|
Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition
|
We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic
topological transition at pressures of about 40 GPa. This topological change of
the Fermi surface manifests itself through anomalous behavior of the Debye
sound velocity, c/a lattice parameter ratio and M\"ossbauer center shift
observed in our experiments. First-principles simulations within the dynamic
mean field approach demonstrate that the transition is induced by many-electron
effects. It is absent in one-electron calculations and represents a clear
signature of correlation effects in hcp Fe.
|
1204.5130v1
|
2012-05-03
|
Generalized joint density of states and its application to exploring the pairing symmetry of superconductors
|
We introduce a generalized joint density of states (GJDOS), which
incorporates the coherent factor into the JDOS, to study quasiparticle
interference (QPI) in superconductors. The intimate relation between the
Fourier-transformed local density of states and GJDOS is revealed: they corre-
spond respectively to the real and imaginary parts of a generalized
impurity-response function, and particularly share the same angular factors and
singular boundaries, as seen from our approximate analytic results for d-wave
superconductors. Remarkably, our numerical GJDOS analysis agrees well with the
QPI patten of d-wave cuprates and s\pm-wave iron-based superconductors.
Moreover, we illustrate that the present GJDOS scenario can uncover the sign
features of the superconducting gap and thus can be used to explore the
possible pairing symmetry of the KxFe2-ySe2 superconductors.
|
1205.0598v1
|
2012-05-03
|
Quenched Fe Moment in the Collapsed Tetragonal Phase of Ca$_{1-x}$Pr$_{x}$Fe$_2$As$_2$
|
We report $^{75}$As NMR studies on single crystals of rare-earth doped iron
pnictides superconductor Ca$_{1-x}$Pr$_{x}$Fe$_{2}$As$_{2}$ ($x$=0.075 and
0.15). The $^{75}$As spectra show a chemical pressure effect with doping and a
first order structure transition to the collapsed tetragonal phase upon
cooling. A sharp drop of the Knight shift is seen below the structural
transition, whereas $1/T_1$ is strongly enhanced at low-temperatures. These
evidences indicate quenching of Fe local magnetism and short-range ordering of
Pr$^{3+}$ moment in the collapsed tetragonal phase. The quenched Fe moment
through structure collapse suggests a strong interplay of structure and
magnetism, which is important for understanding the nature of the collapsed
tetragonal phase.
|
1205.0604v1
|
2012-05-09
|
Pseudo-potentials based first-principles approach to the magneto-optical Kerr effect: from metals to the inclusion of local fields and excitonic effects
|
We propose a first-principles scheme for the description of the
magneto-optical kerr effect within density functional theory (DFT). Though the
computation of Kerr parameters is often done within DFT, starting from the
conductivity or the dielectric tensor, there is no formal justification to this
choice.
As a first steps, using as reference materials iron, cobalt and nickel we
show that pseudo-potential based calculations give accurate predictions. Then
we derive a formal expression for the full dielectric tensor in terms of the
density-density correlation function. The derived equation is exact in systems
with an electronic gap, with the possible exception of Chern insulators, and
whenever the time reversal symmetry holds and can be used as a starting point
for the inclusion of local fields and excitonic effects within time-dependent
DFT for such systems.
In case of metals instead we show that, starting from the density-density
correlation function, the term which describes the anomalous Hall effect is
neglected giving a wrong conductivity.
|
1205.1994v3
|
2012-05-14
|
Isotope shifts and hyperfine structure of the Fe I 373.7 nm resonance line
|
We report measurements of the isotope shifts of the 3d6 4s2 a 5D3 - 3d6 4s4p
z 5F4o FeI resonance line at 373.7 nm between all four stable isotopes 54Fe,
56Fe, 57Fe and 58Fe, as well as the complete hyperfine structure of that line
for 57Fe, the only stable isotope having a non-zero nuclear spin. The field and
specific mass shift coefficients of the transition have been derived from the
data, as well as the experimental value for the hyperfine structure magnetic
dipole coupling constant A of the excited state of the transition in 57Fe :
A(3d6 4s4p z 5F4o) = 68.21(69) MHz. The measurements were carried out by means
of high-resolution Doppler-free laser saturated absorption spectroscopy in a
Fe-Ar hollow cathode discharge cell using both natural and enriched iron
samples. The measured isotope shifts and hyperfine constants are reported with
uncertainties at the percent level.
|
1205.3146v1
|
2012-05-17
|
Interstellar Dust Close to the Sun
|
The low density interstellar medium (ISM) close to the Sun and inside of the
heliosphere provides a unique laboratory for studying interstellar dust grains.
Grain characteristics in the nearby ISM are obtained from observations of
interstellar gas and dust inside of the heliosphere and the interstellar gas
towards nearby stars. Comparison between the gas composition and solar
abundances suggests that grains are dominated by olivines and possibly some
form of iron oxide. Measurements of the interstellar Ne/O ratio by the
Interstellar Boundary Explorer spacecraft indicate that a high fraction of
interstellar oxygen in the ISM must be depleted onto dust grains. Local
interstellar abundances are consistent with grain destruction in ~150 km/s
interstellar shocks, provided that the carbonaceous component is hydrogenated
amorphous carbon and carbon abundances are correct. Variations in relative
abundances of refractories in gas suggest variations in the history of grain
destruction in nearby ISM. The large observed grains, > 1 micron, may indicate
a nearby reservoir of denser ISM. Theoretical three-dimensional models of the
interaction between interstellar dust grains and the solar wind predict that
plumes of about 0.18 micron dust grains form around the heliosphere.
|
1205.4017v1
|
2012-05-19
|
Iron spin-reorientation transition in NdFeAsO
|
The low-temperature magnetic structure of NdFeAsO has been revisited using
neutron powder diffraction and symmetry analysis using the Sarah
representational analysis program. Four magnetic models with one magnetic
variable for each of the Nd and Fe sublattices were tested. The best fit was
obtained using a model with Fe moments pointing along the c-direction, and Nd
moments along the a-direction. This signals a significant interplay between
rare-earth and transition metal magnetism, which results in a
spin-reorientation of the Fe sublattice upon ordering of the Nd moments. All
models that fit the data well, including collinear models with more than one
magnetic variable per sublattice, were found to have an Fe moment of 0.5 BM and
a Nd moment of 0.9 BM, demonstrating that the low-temperature Fe moment is not
substantially enhanced compared to the spin-density wave (SDW) state.
|
1205.4312v1
|
2012-05-22
|
Theory of nonequilibrium dynamics of multiband superconductors
|
We study the nonequilibrium dynamics of multiband BCS superconductors
subjected to ultrashort pump pulses. Using density-matrix theory, the time
evolution of the Bogoliubov quasiparticle densities and the superconducting
order parameters are computed as a function of pump pulse frequency, duration,
and intensity. Focusing on two-band superconductors, we consider two different
model systems. The first one, relevant for iron-based superconductors,
describes two-band superconductors with a repulsive interband interaction
$V_{12}$ which is much larger than the intraband pairing terms. The second
model, relevant for MgB$_2$, deals with the opposite limit where the intraband
interactions are dominant and the interband pair scattering $V_{12}$ is weak
but attractive. For ultrashort pump pulses, both of these models exhibit a
nonadiabatic behavior which is characterized by oscillations of the
superconducting order parameters. We find that for nonvanishing $V_{12}$, the
superconducting gap on each band exhibits two oscillatory frequencies which are
determined by the long-time asymptotic values of the gaps. The relative
strength of these two frequency components depends sensitively on the magnitude
of the interband interaction $V_{12}$.
|
1205.4861v2
|
2012-05-23
|
Bulk electronic structure of superconducting LaRu2P2 single crystals measured by soft x-ray angle-resolved photoemission spectroscopy
|
We present a soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES)
study of the stoichiometric pnictide superconductor LaRu2P2. The observed
electronic structure is in good agreement with density functional theory (DFT)
calculations. However, it is significantly different from its counterpart in
high-temperature superconducting Fe-pnictides. In particular the bandwidth
renormalization present in the Fe-pnictides (~2 - 3) is negligible in LaRu2P2
even though the mass enhancement is similar in both systems. Our results
suggest that the superconductivity in LaRu2P2 has a different origin with
respect to the iron pnictides. Finally we demonstrate that the increased
probing depth of SX-ARPES, compared to the widely used ultraviolet ARPES, is
essential in determining the bulk electronic structure in the experiment.
|
1205.5216v1
|
2012-05-23
|
Electronic phase diagram of disordered Co doped BaFe2As2
|
Superconducting and normal state transport properties in iron pnictides are
sensitive to disorder and impurity scattering. By investigation of
Ba(Fe1-xCox)2As2 thin films with varying Co concentration, we demonstrate that
in the dirty limit the superconducting dome in the electronic phase diagram of
Ba(Fe1-xCox)2As2 shifts towards lower doping concentrations, which differs
significantly from observations in single crystals. We show that especially in
the underdoped regime superconducting transition temperatures higher than 27 K
are possible.
|
1205.5219v3
|
2012-05-25
|
Superconductivity at Tc = 44 K in LixFe2Se2(NH3)y
|
Following a recent proposal by Burrard-Lucas et al. [unpublished, arXiv:
1203.5046] we intercalated FeSe by Li in liquid ammonia. We report on the
synthesis of new LixFe2Se2(NH3)y phases as well as on their magnetic and
superconducting properties. We suggest that the superconducting properties of
these new hybride materials appear not to be influenced by the presence of
electronically-innocent Li(NH2) salt moieties. Indeed, high onset temperatures
of 44 K and shielding fractions of almost 80% were only obtained in samples
containing exclusively Lix(NH3)y moieties acting simultaneously as electron
donors and spacer units. The c-axis of the new intercalated phases is strongly
enhanced when compared to the alkali-metal intercalated iron selenides
A1-xFe2-ySe2 with A = K, Rb, Cs, Tl with T c = 32 K.
|
1205.5731v1
|
2012-05-31
|
Superconducting Fluctuations and Anomalous Phonon Renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2
|
Raman studies on Ca4Al2O5.7Fe2As2 superconductor in the temperature range of
5 K to 300 K, covering the superconducting transition temperature Tc ~ 28.3 K,
reveal that the Raman mode at ~ 230 cm-1 shows a sharp jump in frequency by ~ 2
% and linewidth increases by ~ 175 % at To ~ 60 K. Below To, anomalous
softening of the mode frequency and a large decrease by ~ 10 cm-1 in the
linewidth is observed. These precursor effects at T0 (~ 2Tc) are attributed to
significant superconducting fluctuations, possibly enhanced due to reduced
dimensionality arising from weaked coupling between the well separated (~ 15
{\AA}) Fe-As layers in the unit cell. A large blue-shift of the mode frequency
between 300 K to 60 K (~7%) indicates strong spin-phonon coupling in this
superconductor.
|
1205.7033v1
|
2012-06-03
|
Importance of many body effects in the kernel of hemoglobin for ligand binding
|
We propose a mechanism for binding of diatomic ligands to heme based on a
dynamical orbital selection process. This scenario may be described as bonding
determined by local valence fluctuations. We support this model using
linear-scaling first-principles calculations, in combination with dynamical
mean-field theory, applied to heme, the kernel of the hemoglobin metalloprotein
central to human respiration. We find that variations in Hund's exchange
coupling induce a reduction of the iron 3d density, with a concomitant increase
of valence fluctuations. We discuss the comparison between our computed optical
absorption spectra and experimental data, our picture accounting for the
observation of optical transitions in the infrared regime, and how the Hund's
coupling reduces, by a factor of five, the strong imbalance in the binding
energies of heme with CO and O_2 ligands.
|
1206.0412v2
|
2012-06-08
|
Substituent Effects on the Spin-Transition Temperature in Complexes with Tris(pyrazolyl) Ligands
|
Iron (II) complexes with substituted tris(pyrazolyl) ligands, which exhibit a
thermally driven transition from a low-spin state at low temperatures to a
high-spin state at elevated temperatures, have been studied by M\"ossbauer
spectroscopy and magnetic susceptibility measurements. From the observed
spectra the molar high-spin fraction and the transition temperature have been
extracted. All substituents, except for bromine, lead to a decrease of the
transition temperature. Density functional calculations have been carried out
to compare the experimentally observed shifts of the transition temperature
with those derived from theory.
|
1206.1883v1
|
2012-06-10
|
Anomalous impurity effects in the iron-based superconductor KFe$_2$As$_2$
|
High-quality K(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals have been grown by
using KAs flux method. Instead of increasing the superconducting transition
temperature $T_{\rm c}$ through electron doping, we find that Co impurities
rapidly suppress $T_{\rm c}$ down to zero at only $x \approx$ 0.04. Such an
effective suppression of $T_{\rm c}$ by impurities is quite different from that
observed in Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ with multiple nodeless
superconducting gaps. Thermal conductivity measurements in zero field show that
the residual linear term $\kappa_0/T$ only change slightly with $3.4\%$ Co
doping, despite the sharp increase of scattering rate. The implications of
these anomalous impurity effects are discussed.
|
1206.2030v3
|
2012-06-11
|
Diffusion of hydrogen within idealised grains of bcc-Fe: A kinetic Monte Carlo study
|
Structural defects in materials such as vacancies, grain boundaries, and
dislocations may trap hydrogen and a local accumulation of hydrogen at these
defects can lead to the degradation of the materials properties. An important
aspect in obtaining insight into hydrogen induced embrittlement on the
atomistic level is to understand the diffusion of hydrogen in these materials.
In our study we employ kinetic Monte Carlo (kMC) simulations to investigate
hydrogen diffusion in bcc iron within different microstructures. All input data
to the kMC model, such as available sites, solution energies, and diffusion
barriers are obtained from first-principles calculations. We find that hydrogen
mainly diffuses within the interface region with an overall diffusivity that is
lower than in pure bcc-Fe bulk. The concentration dependence of the diffusion
coefficient is strongly non-linear and the diffusion coefficient may even
decrease with increasing hydrogen concentration. To describe the macroscopic
diffusion coefficient we derive an analytic expression as a function of
hydrogen concentration and temperature which is in excellent agreement with our
numerical results for idealised microstructures.
|
1206.2314v1
|
2012-06-12
|
Electric field control of magnetic exchange coupling
|
Electric control of magnetism is a vision which drives intense research on
magnetic semiconductors and multiferroics. Recently, also ultrathin metallic
films were reported to show magnetoelectric effects at room temperature. Here
we demonstrate much stronger effects by exploiting reduction/oxidation
reactions in a naturally grown oxide layer exchange coupled to an underlying
ferromagnet. For the exemplarily studied FePt/iron oxide composite in an
electrolyte, a large and reversible change of magnetization and anisotropy is
obtained. The principle can be transferred to various metal/oxide combinations.
It represents a novel approach towards multifunctionality.
|
1206.2467v3
|
2012-06-13
|
Mn local moments prevent superconductivity in iron-pnictides Ba(Fe 1-x Mn x)2As2
|
75As nuclear magnetic resonance (NMR) experiments were performed on
Ba(Fe1-xMnx)2As2 (xMn = 2.5%, 5% and 12%) single crystals. The Fe layer
magnetic susceptibility far from Mn atoms is probed by the75As NMR line shift
and is found similar to that of BaFe2As2, implying that Mn does not induce
charge doping. A satellite line associated with the Mn nearest neighbours
(n.n.) of 75As displays a Curie-Weiss shift which demonstrates that Mn carries
a local magnetic moment. This is confirmed by the main line broadening typical
of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to
the localization of the additional Mn hole. These findings explain why Mn does
not induce superconductivity in the pnictides contrary to other dopants such as
Co, Ni, Ru or K.
|
1206.2741v2
|
2012-06-15
|
Pressure-induced changes of the vibrational modes of spin-crossover complexes studied by nuclear resonance scattering of synchrotron radiation
|
Nuclear inelastic scattering (NIS) spectra were recorded for the
spin-crossover complexes STP and ETP (STP =
[Fe(1,1,1-trisf[N-(2-pyridylmethyl)-N-methylamino]methylg- ethane)](ClO4)2 and
ETP =
[Fe(1,1,1-trisf[N-(2-pyridylmethyl)-N-methylamino]methylg-butane)](ClO4)2) at
30 K and at room temperature and also at ambient pressure and applied pressure
(up to 2.6 GPa). Spin transition from the high-spin (HS) to the low-spin (LS)
state was observed by lowering temperature and also by applying pressure at
room temperature and has been assigned to the hardening of iron-bond stretching
modes due to the smaller volume in the LS isomer.
|
1206.3451v1
|
2012-06-20
|
Evolution of band structure from optimally doped to heavily overdoped Co-substituted NaFeAs
|
Using angle-resolved photoemission spectroscopy, we studied the electronic
structure of NaFe$_{1-x}$Co$_x$As from an optimally doped superconducting
compound ($x=0.028$) to a heavily overdoped non-superconducting one
($x=0.109$). Similar to the case of "122" type iron pnictides, our data suggest
that Co dopant in NaFe$_{1-x}$Co$_x$As supplies extra charge carriers and
shifts the Fermi level accordingly. In the $x=0.109$ compound, the hole-like
bands around the zone center $\Gamma$ move to deeper binding energies and an
electron pocket appears instead. The overall band renormalization remains
basically the same throughout the doping range we studied, suggesting that the
local magnetic/electronic correlations are not affected by carrier doping. We
speculate that a balance between itinerant properties of mobile carriers and
local interactions may play an important role for the superconductivity.
|
1206.4402v1
|
2012-06-27
|
BaFe2Se2O as an Iron-Based Mott Insulator with Antiferromagnetic Order
|
A new compound with a quasi-two-dimensional array of FeSe3O tetrahedra and an
orthorombic structure, namely BaFe2Se2O, has been successfully fabricated.
Experimental results show that this compound is an insulator and has an
antiferromagnetic (AF) transition at 240 K. Band structure calculation reveals
the narrowing of Fe 3d bands near the Fermi energy, which leads to the
localization of magnetism and the Mott insulating behavior. The large distances
between the Fe atoms perhaps are responsible for the characters. Linear
response calculation further indicates a strong in-plane AF exchange $J$, this
can account for the enhanced magnetic susceptibility (which has a maximum at
about 450 K) above the Neel temperature.
|
1206.6154v3
|
2012-06-27
|
Effect of Ru susbstitution on atomic displacements in the layered SmFe_{1-x}Ru_xAsO_{0.85}F_{0.15} superconductor
|
The effect of Ru substitution on the local structure of layered
SmFe$_{1-x}$Ru$_x$AsO$_{0.85}$F$_{0.15}$ superconductor has been studied by As
$K$- and Sm $L_3$ - edges x-ray-absorption spectroscopy. The extended
x-ray-absorption fine-structure measurements reveal distinct Fe-As and Ru-As
bondlengths in the Ru substituted samples with the latter being $\sim$0.03 \AA\
longer. Local disorder induced by the Ru substitution is mainly confined to the
FeAs layer while the SmO spacer layer sustains a relative order, consistent
with the x-ray-absorption near-edge structure spectra. The results suggest
that, in addition to the order/disorder in the active active iron-arsenide
layer, its coupling to the rare-earth\textminus oxygen spacer layer needs to be
considered for describing the electronic properties of these layered
superconductors.
|
1206.6526v1
|
2012-06-29
|
Ferromagnetism and superconductivity in P-doped CeFeAsO
|
We report on superconductivity in CeFeAs1-xPxO and the possible coexistence
with Ce- ferromagnetism (FM) in a small homogeneity range around x = 30% with
ordering temperatures of T_SC = T_C = 4K. The antiferromagnetic (AFM) ordering
temperature of Fe at this critical concentration is suppressed to T^N_Fe ~ 40K
and does not shift to lower temperatures with further increase of the P
concentration. Therefore, a quantum-critical-point scenario with T^N_Fe -> 0K
which is widely discussed for the iron based superconductors can be excluded
for this alloy series. Surprisingly, thermal expansion and X-ray powder
diffraction indicate the absence of an orthorhombic distortion despite clear
evidence for short range AFM Fe-ordering from muon-spin-rotation measurements.
Furthermore, we discovered the formation of a sharp electron spin resonance
signal unambiguously connected with the emergence of FM ordering.
|
1206.7088v2
|
2012-07-02
|
Germanium, Arsenic, and Selenium Abundances in Metal-Poor Stars
|
The elements germanium (Ge, Z=32), arsenic (As, Z=33), and selenium (Se,
Z=34) span the transition from charged-particle or explosive synthesis of the
iron-group elements to neutron-capture synthesis of heavier elements. Among
these three elements, only the chemical evolution of germanium has been studied
previously. Here we use archive observations made with the Space Telescope
Imaging Spectrograph on board the Hubble Space Telescope and observations from
several ground-based facilities to study the chemical enrichment histories of
seven stars with metallicities -2.6 < [Fe/H] < -0.4. We perform a standard
abundance analysis of germanium, arsenic, selenium, and several other elements
produced by neutron-capture reactions. When combined with previous derivations
of germanium abundances in metal-poor stars, our sample reveals an increase in
the [Ge/Fe] ratios at higher metallicities. This could mark the onset of the
weak s-process contribution to germanium. In contrast, the [As/Fe] and [Se/Fe]
ratios remain roughly constant. These data do not directly indicate the origin
of germanium, arsenic, and selenium at low metallicity, but they suggest that
the weak and main components of the s-process are not likely sources.
|
1207.0518v1
|
2012-07-03
|
Updated Atomic Data and Calculations for X-ray Spectroscopy
|
We describe the latest release of AtomDB, version 2.0.2, a database of atomic
data and a plasma modeling code with a focus on X-ray astronomy. This release
includes several major updates to the fundamental atomic structure and process
data held within AtomDB, incorporating new ionization balance data,
state-selective recombination data, and updated collisional excitation data for
many ions, including the iron L-shell ions from Fe$^{+16}$ to Fe$^{+23}$ and
all of the hydrogen- and helium-like sequences. We also describe some of the
effects that these changes have on calculated emission and diagnostic line
ratios, such as changes in the temperature implied by the He-like G-ratios of
up to a factor of 2.
|
1207.0576v1
|
2012-07-05
|
Spin reorientation in TlFe1.6Se2 with complete vacancy ordering
|
The relationship between vacancy ordering and magnetism in TlFe1.6Se2 has
been investigated via single crystal neutron diffraction, nuclear forward
scattering, and transmission electron microscopy. The examination of chemically
and structurally homogenous crystals allows the true ground state to be
revealed, which is characterized by Fe moments lying in the ab-plane below
100K. This is in sharp contrast to crystals containing regions of order and
disorder, where a competition between c-axis and ab-plane orientations of the
moments is observed. The properties of partially-disordered TlFe1.6Se2 are
therefore not associated with solely the ordered or disordered regions. This
contrasts the viewpoint that phase separation results in independent physical
properties in intercalated iron selenides, suggesting a coupling between
ordered and disordered regions may play an important role in the
superconducting analogues.
|
1207.1318v2
|
2012-07-06
|
Two-band superconductivity featuring different anisotropies in the ternary iron silicide Lu$_{2}$Fe$_{3}$Si$_{5}$
|
We report detailed studies of the upper critical field and low-temperature
specific heat in the two-gap superconductor Lu$_{2}$Fe$_{3}$Si$_{5}$. The
anisotropy of the upper critical field suggests that the active band is
quasi-one-dimensional. Low-temperature specific heat in magnetic fields reveals
that the virtual $H_{c2}$ in the passive band is almost isotropic. These
results strongly indicate that the two bands have two different anisotropies,
similar to the typical two-gap superconductor MgB$_{2}$, and their interplay
may be essential to the two-gap superconductivity in Lu$_{2}$Fe$_{3}$Si$_{5}$.
|
1207.1502v1
|
2012-07-08
|
Emergent Phases of Nodeless and Nodal Superconductivity Separated by Antiferromagnetic Order in Iron-based Superconductor (Ca4Al2O6)Fe2(As1-xPx)2: 75As- and 31P-NMR Studies
|
We report $^{31}$P- and $^{75}$As-NMR studies on
(Ca$_4$Al$_2$O$_{6}$)Fe$_2$(As$_{1-x}$P$_x$)$_2$ with an isovalent substitution
of P for As. We present the novel evolution of emergent phases that the
nodeless superconductivity (SC) in 0$\le x \le$0.4 and the nodal one around
$x$=1 are intimately separated by the onset of a commensurate stripe-type
antiferromagnetic (AFM) order in 0.5$\le x \le$ 0.95, as an isovalent
substitution of P for As decreases a pnictogen height $h_{Pn}$ measured from
the Fe plane. It is demonstrated that the AFM order takes place under a
condition of 1.32\AA$\le h_{Pn} \le$1.42\AA, which is also the case for other
Fe-pnictides with the Fe$^{2+}$ state in (Fe$Pn$)$^{-}$ layers. This novel
phase evolution with the variation in $h_{Pn}$ points to the importance of
electron correlation for the emergence of SC as well as AFM order.
|
1207.1920v2
|
2012-07-09
|
Time-Dependent Density Diagnostics of Solar Flare Plasmas Using SDO/EVE
|
Temporally-resolved electron density measurements of solar flare plasmas are
presented using data from the EUV Variability Experiment (EVE) onboard the
Solar Dynamics Observatory (SDO). The EVE spectral range contains emission
lines formed between 10^4-10^7 K, including transitions from highly ionized
iron (>10 MK). Using three density-sensitive Fe XXI ratios, peak electron
densities of 10^(11.2)-10^(12.1) cm^(-3) were found during four X-class flares.
While previous measurements of densities at such high temperatures were made at
only one point during a flaring event, EVE now allows the temporal evolution of
these high-temperature densities to be determined at 10 s cadence. A comparison
with GOES data revealed that the peak of the density time profiles for each
line ratio correlated well with that of the emission measure time profile for
each of the events studied.
|
1207.1990v1
|
2012-07-10
|
Magnetism in parent Fe-chalcogenides: quantum fluctuations select a plaquette order
|
We analyze magnetic order in iron-chalcogenide Fe$_{1+y}$Te -- the parent
compound of high-temperature superconductor Fe$_{1+y}$Te$_{1-x}$Se$_x$. Neutron
scattering experiments show that magnetic order in this material contains
components with momentum $Q_1=(\pi/2, \pi/2)$ and $Q_2 =(\pi/2, -\pi/2)$ in
Fe-only Brillouin zone. The actual spin order depends on the interplay between
these two components. Previous works argued that spin order is a single-$Q$
state (either $Q_1$ or $Q_2$). Such an order breaks rotational $C_4$ symmetry
and order spins into a double diagonal stripe. We show that quantum
fluctuations actually select another order -- a double $Q$ plaquette state with
equal weight of $Q_1$ and $Q_2$ components, which preserves $C_4$ symmetry but
breaks $Z_4$ translational symmetry. We argue that the plaquette state is
consistent with recent neutron scattering experiments on Fe$_{1+y}$Te.
|
1207.2201v2
|
2012-07-16
|
CVD Synthesis of Small-Diameter Single Walled Carbon Nanotubes on Silicon
|
A simple process for chemical vapor deposition of ultra SD single wall carbon
nanotubes has been developed. In this process, an iron nitrate nonahydrate
solution in isopropyl alcohol with a concentration of 400 ug/mlit was used to
catalyze nanoparticles formation on an oxidized silicon wafer. The oxide on the
substrate was made of a thick layer of wet oxide sandwiched between tow thin
layers of dry oxide. The process results in semiconducting single-walled carbon
nanotubes (SWNTs) with diameter of less than 0.7nm and more than 1ev band gap
energy, which are amongst the smallest diameters of SWNTs ever reported.
|
1207.3555v1
|
2012-07-16
|
Infrared Measurement of the Pseudogap in P-Doped and Co-Doped BaFe2As2 High-Temperature Superconductors
|
We report on infrared studies of charge dynamics in a prototypical pnictide
system: the BaFe2As2 family. Our experiments have identified hallmarks of the
pseudogap state in the BaFe2As2 system that mirror the spectroscopic
manifestations of the pseudogap in the cuprates. The magnitude of the infrared
pseudogap is in accord with that of the spin-density-wave gap of the parent
compound. By monitoring the superconducting gap of both P- and Co-doped
compounds, we find that the infrared pseudogap is unrelated to
superconductivity. The appearance of the pseudogap is found to correlate with
the evolution of the antiferromagnetic fluctuations associated with the
spin-density-wave instability. The strong-coupling analysis of infrared data
further reveals the interdependence between the magnetism and the pseudogap in
the iron pnictides.
|
1207.3567v1
|
2012-07-17
|
Magneto-elastically coupled structural, magnetic and superconducting order parameters in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$
|
We measure the transport properties of mechanically strained single crystals
of BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ over a wide range of $x$. The N\'eel
transition is extremely sensitive to stress and this sensitivity increases as
optimal doping is approached, even though the transition itself is strongly
suppressed. Furthermore, we observe significant changes in the superconducting
transition temperature with applied strain, which mirror changes as a function
of the composition $x$. These experiments are a direct illustration of the
intimate coupling between different degrees of freedom in iron-based
superconductors, revealing the importance of magneto-elastic coupling to the
magnetic and superconducting transition temperatures.
|
1207.3858v1
|
2012-07-17
|
Non-Fermi Liquid behavior at the Orbital Ordering Quantum Critical Point in the Two-Orbital Model
|
The critical behavior of a two-orbital model with degenerate $d_{xz}$ and
$d_{yz}$ orbitals is investigated by multidimensional bosonization. We find
that the corresponding bosonic theory has an overdamped collective mode with
dynamical exponent $z=3$, which appears to be a general feature of a
two-orbital model and becomes the dominant fluctuation in the vicinity of the
orbital-ordering quantum critical point. Since the very existence of this $z=3$
overdamped collective mode induces non-Fermi liquid behavior near the quantum
critical point, we conclude that a two-orbital model generally has a sizable
area in the phase diagram showing non-Fermi liquid behavior. Furthermore, we
show that the bosonic theory resembles the continuous model near the d-wave
Pomeranchuk instability, suggesting that orbital order in a two-orbital model
is identical to nematic order in a continuous model. Our results can be applied
to systems with degenerate $d_{xz}$ and $d_{yz}$ orbitals such as iron-based
superconductors and bilayer strontium ruthenates Sr$_3$Ru$_2$O$_7$.
|
1207.4206v2
|
2012-07-18
|
Bound States of Defects in Superconducting LiFeAs Studied by Scanning Tunneling Spectroscopy
|
Defects in LiFeAs are studied by scanning tunneling microscopy (STM) and
spectroscopy (STS). Topographic images of the five predominant defects allow
the identification of their position within the lattice. The most commonly
observed defect is associated with an Fe site and does not break the local
lattice symmetry, exhibiting a bound state near the edge of the smaller gap in
this multi-gap superconductor. Three other common defects, including one also
on an Fe site, are observed to break local lattice symmetry and are
pair-breaking indicated by clear in-gap bound states, in addition to states
near the smaller gap edge. STS maps reveal complex, extended real-space bound
state patterns, including one with a chiral distribution of the local density
of states (LDOS). The multiple bound state resonances observed within the gaps
and at the inner gap edge are consistent with theoretical predictions for
s$^{\pm}$ gap symmetry proposed for LiFeAs and other iron pnictides.
|
1207.4249v2
|
2012-07-18
|
Detection of Iron Kα Emission from a Complete Sample of Submillimeter Galaxies
|
We present an X-ray stacking analysis of a sample of 38 submillimeter
galaxies with <z>=2.6 discovered at >4{\sigma} significance in the Lockman Hole
North with the MAMBO array. We find a 5{\sigma} detection in the stacked soft
band (0.5-2.0 keV) image, and no significant detection in the hard band (2.0-8
keV). We also perform rest-frame spectral stacking based on spectroscopic and
photometric redshifts and find a ~4{\sigma} detection of Fe K{\alpha} emission
with an equivalent width of EW>1 keV. The centroid of the Fe K{\alpha} emission
lies near 6.7 keV, indicating a possible contribution from highly ionized Fe
XXV or Fe XXVI; there is also a slight indication that the line emission is
more spatially extended than the X-ray continuum. This is the first X-ray
analysis of a complete, flux-limited sample of SMGs with statistically robust
radio counterparts.
|
1207.4486v1
|
2012-07-27
|
Evidence for a cos(4\varphi) Modulation of the Superconducting Energy Gap of Optimally Doped FeTe_{0.6}Se_{0.4} Single Crystals Using Laser Angle-Resolved Photoemission Spectroscopy
|
We study the superconducting(SC)-gap anisotropy of the \Gamma-centered hole
Fermi surface in optimally doped FeTe_{0.6}Se_{0.4} (T_c = 14.5 K), using
laser-excited angle-resolved photoemission spectroscopy (ARPES). We observe
sharp superconducting coherence peaks at T = 2.5 K. In contrast to earlier
ARPES studies but consistent with thermodynamic results, the momentum
dependence shows a \cos(4\varphi) modulation of the SC-gap anisotropy. The
observed SC-gap anisotropy strongly indicates that the pairing interaction is
not a conventional phonon-mediated isotropic one. Instead, the results suggest
the importance of second-nearest-neighbor electronic interactions between the
iron sites in the framework of s_\pm-wave superconductivity.
|
1207.6571v3
|
2012-07-28
|
Self-doping effect and possible antiferromagnetism at titanium-layers in the iron-based superconductor Ba$_2$Ti$_2$Fe$_2$As$_4$O
|
The electronic structure of Ba$_2$Ti$_2$Fe$_2$As$_4$O, a newly discovered
superconductor, is investigated using first-principles calculations based on
local density approximations. Multiple Fermi surface sheets originating from
Ti-3$d$ and Fe-3$d$ states are present corresponding to the conducting
Ti$_2$As$_2$O and Fe$_2$As$_2$ layers respectively. Compared with
BaFe$_2$As$_2$, sizeable changes in the related Fermi surface sheets indicate
significant electron transfer (about 0.12$e$) from Ti to Fe, which suppresses
the stripe-like antiferromagnetism at the Fe sites and simultaneously induces
superconductivity. Our calculations also suggest that an additional
N\'{e}el-type antiferromagnetic instability at the Ti sites is relatively
robust against the electron transfer, which accounts for the anomaly at 125 K
in the superconducting Ba$_2$Ti$_2$Fe$_2$As$_4$O.
|
1207.6705v1
|
2012-07-29
|
Dynamic approach to finite-temperature magnetic phase transitions in the extended J1- J2 model with vacancy order
|
The recently discovered iron-based superconductors A$_{y}$Fe$_{2-x}$Se$_{2}$
($A$=K, Rb, Cs, Tl) show a long-range antiferromagnetic order with an
unexpected high transition temperature $T_N \sim 550$ K and a unique $\sqrt{5}
\times \sqrt{5}$ vacancy order. Taking the extended $J_1$-$J_2$ model as a
minimal model, we investigate the finite-temperature magnetic phase transitions
in a square lattice with a $\sqrt{5} \times \sqrt{5}$ vacancy superstructure by
using large-scale Monte Carlo simulations. By the parallel tempering technique,
the block spin checkerboard and stripe antiferromagnetic states are detected to
be the groundstates for three representative sets of model parameters. The
short-time dynamic approach is applied to accurately determine the critical
temperature as well as the static and dynamic exponents. Our results indicate
that the dramatic enhancement of the critical temperature as observed in
experiments should be mainly due to a combination effect of the vacancy order
and the block lattice contraction.
|
1207.6740v3
|
2012-07-29
|
Quantum Criticality Stabilizes High T_c Superconductivity Against Competing Symmetry-Breaking Instabilities
|
The occurrence of high-T_c superconductivity in systems including the
cuprates and the iron-based superconductors, is known to coincide with the
existence of anomalous normal-state properties which have been associated with
quantum criticality. We argue here that this observation results from the fact
that quantum criticality can allow the occurrence of very-strong-coupling
superconductivity by preventing its suppression due to competing
symmetry-breaking instabilities. Treating the electrons through a large-U
ansatz yields their separation into boson quasiparticles which are directly
involved in the formation of these instabilities, represented as their Bose
condensates, and charge-carrying fermion quasiparticles which are affected by
them indirectly. Within the critical regime, condensates corresponding to the
different broken-symmetry states are combined; consequently their negative
effect on the pairing of the fermions is strongly diminished, enabling high-T_c
superconductivity to occur. The observed phase diagram of the hole-doped
cuprates then derives from a hidden T=0 quantum phase transition between a
Fermi-liquid and a non-Fermi-liquid broken-symmetry striped state. The
pseudogap range within this diagram is found to include two distinct regimes,
with partial pairing occurring in one of them.
|
1207.6789v1
|
2012-08-06
|
Spin-state Crossover Model for the Magnetism of Iron Pnictides
|
We propose a minimal model describing magnetic behavior of Fe-based
superconductors. The key ingredient of the model is a dynamical mixing of
quasi-degenerate spin states of Fe2+ ion by intersite electron hoppings,
resulting in an effective local spin Seff. The moments Seff tend to form
singlet pairs, and may condense into a spin nematic phase due to the emergent
biquadratic exchange couplings. The long-range ordered part m of Seff varies
widely, 0<=m<=Seff, but magnon spectra are universal and scale with Seff,
resolving the puzzle of large but fluctuating Fe-moments. Unusual temperature
dependences of a local moment and spin susceptibility are also explained.
|
1208.1197v3
|
2012-08-07
|
Anisotropic electrical resistivity of LaFeAsO: evidence for electronic nematicity
|
Single crystals of LaFeAsO were successfully grown out of KI flux.
Temperature dependent electrical resistivity was measured with current flow
along the basal plane, \rho_perpend(T), as well as with current flow along the
crystallographic c-axis, \rho_parallel(T), the latter one utilizing electron
beam lithography and argon ion beam milling. The anisotropy ratio was found to
lie between \rho_parallel/\rho_perpend = 20 - 200. The measurement of
\rho_perpend(T) was performed with current flow along the tetragonal [1 0 0]
direction and along the [1 1 0] direction and revealed a clear in-plane
anisotropy already at T \leq 175 K. This is significantly above the
orthorhombic distortion at T_0 = 147 K and indicates the formation of an
electron nematic phase. Magnetic susceptibility and electrical resistivity give
evidence for a change of the magnetic structure of the iron atoms from
antiferromagnetic to ferromagnetic arrangement along the c-axis at T^\ast = 11
K.
|
1208.1480v3
|
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