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2019-11-18
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ONETEP + TOSCAM: uniting dynamical mean field theory and linear-scaling density functional theory
|
We introduce the unification of dynamical mean field theory (DMFT) and
linear-scaling density functional theory (DFT), as recently implemented in
ONETEP, a linear-scaling DFT package, and TOSCAM, a DMFT toolbox. This code can
account for strongly correlated electronic behavior while simultaneously
including the effects of the environment, making it ideally suited for studying
complex and heterogeneous systems containing transition metals and lanthanides,
such as metalloproteins. We systematically introduce the necessary formalism,
which must account for the non-orthogonal basis set used by ONETEP. In order to
demonstrate the capabilities of this code, we apply it to carbon
monoxide-ligated iron porphyrin and explore the distinctly quantum-mechanical
character of the iron $3d$ electrons during the process of photodissociation.
|
1911.07752v2
|
2019-11-21
|
First discovery of trans-iron elements in a DAO-type white dwarf (BD$-22{^\circ}3467$)
|
We have identified 484 lines of the trans-iron elements (TIEs) Zn, Ga, Ge,
Se, Br, Kr, Sr, Zr, Mo, In, Te, I, Xe, and Ba, for the first time in the
ultraviolet spectrum of a DAO-type WD, namely BD$-22{^\circ}3467$, surrounded
by the ionized nebula Abell 35. Our TIE abundance determination shows extremely
high overabundances of up to five dex -- a similar effect is already known from
hot, H-deficient (DO-type) white dwarfs. In contrast to these where a
pulse-driven convection zone has enriched the photosphere with TIEs during a
final thermal pulse and radiative levitation has established the extreme TIE
overabundances, {here the extreme TIE overabundances are exclusively driven by
radiative levitation on the initial stellar metallicity. The very low mass
($0.533^{+0.040}_{-0.025}\,M_\odot$) of BD$-22{^\circ}3467$ implies that a
third dredge-up with enrichment of s-process elements in the photosphere did
not occur in the AGB precursor.
|
1911.09573v1
|
2019-11-25
|
Local metallicity distribution function derived from Galactic large-scale radial iron pattern modelling
|
We develop an approach for fitting the results of modeling of wriggling
radial large scale iron pattern along the Galactic disk, derived over young
(high massive) Cepheids, with the metallicity distribution, obtained using low
mass long living dwarf stars in the close solar vicinity. For this, at the step
of computing of the theoretical abundance distribution over low mass stars in
the solar vicinity we propose to redefine the initial mass function so as the
resulting theoretical stellar distribution over masses would be close to the
distribution in the observed sample. By means of the above algorithm and
subsequent corrections of the theoretical metallicity distribution function,
described in literature, we have achieved fairly well agreement of the
theoretical and observed metallicity distribution functions for low mass stars
in the local solar vicinity.
|
1911.10842v1
|
2019-12-09
|
Pulsation in faint blue stars
|
Following the discovery of blue large-amplitude pulsators (BLAPs) by the OGLE
survey, additional hot, high-amplitude pulsating stars have been discovered by
the Zwicky Transient Facility. It has been proposed that all of these objects
are low-mass pre-white dwarfs and that their pulsations are driven by the
opacity of iron-group elements. With this expanded population of pulsating
objects, it was decided to compute a sequence of post-common-envelope stellar
models using the MESA stellar evolution code and to examine the pulsation
properties of low-mass pre-white dwarfs using non-adiabatic analysis with the
GYRE stellar oscillation code. By including the effects of atomic diffusion and
radiative levitation, it is shown that a large region of instability exists
from effective temperatures of 30,000 K up to temperatures of at least 50,000 K
and at a wide range of surface gravities. This encompasses both groups of
pulsator observed so far, and confirms that the driving mechanism is through
iron group element opacity. We make some conservative estimates about the range
of periods, masses, temperatures and gravities in which further such pulsators
might be observed.
|
1912.04129v1
|
2019-12-18
|
Giant enhancement of critical current density at high field in superconducting (Li,Fe)OHFeSe films by Mn doping
|
Critical current density (Jc) is one of the major limiting factors for high
field applications of iron-based superconductors. Here, we report that Mn-ions
are successfully incorporated into nontoxic superconducting (Li,Fe)OHFeSe
films. Remarkably, the Jc is significantly enhanced from 0.03 to 0.32 MA/cm^2
under 33 T, and the vortex pinning force density monotonically increases up to
106 GN/m^3, which is the highest record so far among all iron-based
superconductors. Our results demonstrate that Mn incorporation is an effective
method to optimize the performance of (Li,Fe)OHFeSe films, offering a promising
candidate for high-field applications.
|
1912.08414v1
|
2019-12-19
|
The influence of hydrogen on plasticity in pure iron-theory and experiment
|
Tensile stress relaxation is combined with transmission electron microscopy
to reveal dramatic changes in dislocation structure and sub structure in pure
alpha iron as a result of the effects of dissolved hydrogen. We find that
hydrogen charged specimens after plastic deformation display a very
characteristic pattern of trailing dipoles and prismatic loops which are absent
in uncharged pure metal. We explain these observations by use of a new self
consistent kinetic Monte Carlo model, which in fact was initially used to
predict the now observed microstructure. The results of this combined theory
and experimental study is to shed light on the fundamental mechanism of
hydrogen enhanced localised plasticity.
|
1912.09313v1
|
2019-12-22
|
Effects of momentum-dependent quasiparticle renormalization on the gap structure of iron-based superconductors
|
We discuss the influence of momentum-dependent correlations on the
superconducting gap structure in iron-based superconductors. Within the weak
coupling approach including self-energy effects at the one-loop
spin-fluctuation level, we construct a dimensionless pairing strength
functional which includes the effects of quasiparticle renormalization. The
stationary solution of this equation determines the gap function at $T_c$. The
resulting equations represent the simplest generalization of spin fluctuation
pairing theory to include the effects of an anisotropic quasiparticle weight.
We obtain good agreement with experimentally observed anisotropic gap
structures in LiFeAs, indicating that the inclusion of quasiparticle
renormalization effects in the existing weak-coupling theories can account for
the observed anomalies in the gap structure of Fe-based superconductors.
|
1912.10453v3
|
2019-12-23
|
Measuring transferability issues in machine-learning force fields: The example of Gold-Iron interactions with linearized potentials
|
Machine-learning force fields have been increasingly employed in order to
extend the possibility of current first-principles calculations. However, the
transferability of the obtained potential can not always be guaranteed in
situations that are outside the original database. To study such limitation, we
examined the very difficult case of the interactions in gold-iron
nanoparticles. For the machine-learning potential, we employed a linearized
formulation that is parameterized using a penalizing regression scheme which
allows us to control the complexity of the obtained potential. We showed that
while having a more complex potential allows for a better agreement with the
training database, it can also lead to overfitting issues and a lower accuracy
in untrained systems.
|
1912.10761v3
|
2020-01-08
|
Nucleosynthesis in early rotating massive stars and chemical composition of CEMP stars
|
The first massive stars triggered the onset of chemical evolution by
releasing the first metals (elements heavier than helium) in the Universe. The
nature of these stars and how the early chemical enrichment took place is still
largely unknown. Rotational-induced mixing in the stellar interior can impact
the nucleosynthesis during the stellar life of massive stars and lead to
stellar ejecta having various chemical compositions. We present low and
zero-metallicity 20, 25 and 40 $M_{\odot}$ stellar models with various initial
rotation rates and assumptions for the nuclear reactions rates. With increasing
initial rotation, the yields of light (from $\sim$ C to Al) and trans-iron
elements are boosted. The trans-iron elements (especially elements heavier than
Ba) are significantly affected by the nuclear reaction uncertainties. The
chemical composition of the observed CEMP (carbon-enhanced metal-poor) stars
CS29528-028 and HE0336+0113 are consistent with the chemical composition of the
material ejected by a fast rotating 40~$M_{\odot}$ model.
|
2001.02341v1
|
2020-01-17
|
Dissociated dislocation-mediated carbon transport and diffusion in austenitic iron
|
Dislocation-solute interaction plays fundamental roles in mechanical
properties of alloys. Here, we disclose the essential features of
dislocation-carbon interaction in austenitic Fe at the atomistic scale. We show
that passage of a Shockley partial dislocation in face-centered cubic iron is
able to move carbon atoms on the slip plane forward by one Burgers vector,
revealing a novel dissociated dislocation-mediated transport mechanism. This
mechanism is induced by shear, which is distinct from the normal thermally
activated diffusion process. Furthermore, we show that there exists a fast
diffusion channel with significantly reduced diffusion energy barrier in the
partial dislocation core, which is highly localized and directional. These
inherent geometrical features are crucial for understanding the dependence of
the diffusivity of dislocation pipe diffusion on the character of dislocations;
most importantly, they can result in unbalanced pinning effect on the leading
and trailing partials in a mixed dislocation, consequently facilitating
stacking fault formation and deformation twinning. This explains the
controversial effects of carbon on deformation twinning observed in various
alloys. Our findings pave the road to tune mechanical properties of materials
by manipulating dislocation-interstitial interaction.
|
2001.06196v1
|
2020-01-23
|
Testing general relativity with the stellar-mass black hole in LMC X-1 using the continuum-fitting method
|
The iron line and the continuum-fitting methods are currently the two leading
techniques for measuring black hole spins with electromagnetic radiation. They
can be naturally extended for probing the spacetime geometry around black holes
and testing general relativity in the strong field regime. In the past couple
of years, there has been significant work to use the iron line method to test
the nature of black holes. In this Letter, we use the continuum-fitting method
and we show its capability of constraining the spacetime geometry around black
holes by analyzing 17 RXTE data of the X-ray binary LMC X-1.
|
2001.08391v2
|
2020-01-24
|
Passivation mechanisms and pre-oxidation effects on model surfaces of FeCrNi austenitic stainless steel
|
Passivation mechanisms were investigated on (100)-oriented Fe-18Cr-13Ni
surfaces with direct transfer between surface preparation and analysis by X-ray
photoelectron spectroscopy and scanning tunneling microscopy and
electrochemical characterization. Starting from oxide-free surfaces,
pre-oxidation at saturation under ultra-low pressure (ULP) oxygen markedly
promotes the oxide film Cr(III) enrichment and hinders/delays subsequent iron
oxidation in water-containing environment. Exposure to sulfuric acid at open
circuit potential causes preferential dissolution of oxidized iron species.
Anodic passivation forces oxide film re-growth, Cr(III) dehydroxylation and
further enrichment. ULP pre-oxidation promotes Cr(III) hydroxide formation at
open circuit potential, compactness of the nanogranular oxide film and
corrosion protection.
|
2001.09025v1
|
2020-02-04
|
Magnetically controlled vector based on E coli Nissle 1917
|
In this paper, it is first discovered that EcN has natural magnetically
controlled properties, that is, E coli Nissle 1917 cells move in a gradient
magnetic field of permanent magnets without artificial magnetic labeling. It is
also shown in the paper that the cultivation of EcN in a medium enriched with
iron chelates and under the influence of an external magnetic field increases
several times the magnetophoretic mobility of E coli Nissle 1917 cells compared
to cultivation under standard conditions. Thus, the speed of movement of E coli
Nissle 1917 cells in a gradient magnetic field of laboratory magnets is
achieved of the order of several mm/s. It has also been shown for the first
time that the cultivation of E coli Nissle 1917 biomass is accelerated by
cultivation in an external magnetic field, while the change in the
concentration of iron chelates in the medium has no effect on the cultivation
dynamics of the E coli Nissle 1917 culture.
|
2002.01958v1
|
2020-02-10
|
Structural and magneto-transport studies of iron intercalated Bi2Se3 single crystals
|
A detailed investigation on the structural and magneto-transport properties
of iron intercalated Bi2Se3 single crystals have been presented. The x-ray
diffraction and Raman studies confirm the intercalation of Fe in the van der
Waals gaps between the layers. The electrical resistivity of the compounds
decreases upon intercalation, and Hall resistivity shows the enhancement of the
charge carriers upon intercalation. The magnetoresistance shows the
non-saturating linear behavior at higher magnetic field and low temperature.
Intercalation of Fe increases the onset of the linear magnetoresistance
behavior, indicating the reduction in quantum effects. The Kohler scaling
employed on the magnetoresistance data indicates single scattering process for
all these compounds in the measured temperature range of 3- 300 K.
|
2002.03609v2
|
2020-02-24
|
Local Structure of Mott Insulating Iron Oxychalcogenides La$_{2}$O$_{2}$Fe$_{2}$O$M$$_{2}$ ($M$ = S, Se)
|
We describe the local structural properties of the iron oxychalcogenides,
La$_2$O$_2$Fe$_2$O$M_2$ ($M$ = S, Se), by using pair distribution function
(PDF) analysis applied to total scattering data. Our results of neutron powder
diffraction show that $M$ = S and Se possess similar nuclear structure at low
and room temperatures. The local crystal structures were studied by
investigating deviations in atomic positions and the extent of the formation of
orthorhombicity. Analysis of the total scattering data suggests that buckling
of the Fe$_2$O plane occurs below 100 K. The buckling may occur concomitantly
with a change in octahedral height. Furthermore, within a typical range of 1-2
nm, we observed short-range orthorhombic-like structure suggestive of nematic
fluctuations in both of these materials.
|
2002.10305v2
|
2020-03-06
|
Adsorption property of fatty acid on iron surface with $Σ$3(111) grain boundary
|
Reducing the coefficient of boundary friction on steel surfaces is one of key
technologies to improve the efficiency of machines such as automotive engines.
It has been shown that the boundary friction on nanostructured steel surfaces
in the sliding test using hydrocarbon lubricant molecules is smaller than the
friction of normal steel surfaces. The main difference between the
nanostructured and a normal surfaces is the density of grain boundaries and
other surface defects. The surface defect can attract lubricant molecules and
enhance lubricating film formation on metal surfaces. This can be one of the
mechanisms that induce the friction reduction on the nanosructured steel
surface. In this work, using first principles calculations, the adsorbability
of a lubricant molecule, a fatty acid, on a defected iron surfaces has been
studied. Adsorbability of the Fe(110) surface with symmetrical tilt
{\Sigma}3(111) grain boundary was compared to that of the clean Fe(110)
surface. As a result, we found that the molecule is adsorbed on sites close to
the grain boundary more strongly (0.77 eV in average) than on the Fe surface
without grain boundary.
|
2003.03061v1
|
2020-03-12
|
Band Engineering of Dirac cones in Iron Chalcogenides
|
By band engineering the iron chalcogenide Fe(Se,Te) via ab-initio
calculations, we search for topological surface states and realizations of
Majorana bound states. Proposed topological states are expected to occur for
non-stoichiometric compositions on a surface Dirac cone where issues like
disorder scattering and charge transfer between relevant electronic states have
to be addressed. However, this surface Dirac cone is well above the
Fermi-level. Our goal is to theoretically design a substituted crystal in which
the surface Dirac cone is shifted towards the Fermi-level by modifying the bulk
material without disturbing the surface. Going beyond conventional density
functional theory (DFT), we apply the coherent potential approximation
(BEB-CPA) in a mixed basis pseudo-potential framework to scan the
substitutional phase-space of co-substitutions on the Se-sites. We have
identified iodine as a promising candidate for intrinsic doping. Our specific
proposal is that FeSe$_{0.325}$I$_{0.175}$Te$_{0.5}$ is a very likely candidate
to exhibit a Dirac cone right at the Fermi energy without inducing strong
disorder scattering.
|
2003.05732v2
|
2020-03-24
|
Momentum dependent $d_{xz/yz}$ band splitting in LaFeAsO
|
We performed angle-resolved photoemission spectroscopy (ARPES) studies of the
electronic structure of the nematic phase in LaFeAsO. Degeneracy breaking
between the dxz and dyz hole bands near the {\Gamma} and M point is observed in
the nematic phase. Different temperature dependent band splitting behaviors are
observed at the {\Gamma} and M points. The energy of the band splitting near
the M point decreases as the temperature decreases while it has little
temperature dependence near the {\Gamma} point. The nematic nature of the band
shift near the M point is confirmed through a detwin experiment using a piezo
device. Since a momentum dependent splitting behavior has been observed in
other iron based superconductors, our observation confirms that the behavior is
a universal one among iron based superconductors.
|
2003.10618v1
|
2020-03-26
|
Quasiclassical theory of $C_4$-symmetric magnetic order in disordered multiband metals
|
Recent experimental studies performed in the normal state of iron-based
superconductors have discovered the existence of the $C_4$-symmetric
(tetragonal) itinerant magnetic state. This state can be described as a spin
density wave with two distinct magnetic vectors ${\vec Q}_1$ and ${\vec Q}_2$.
Given an itinerant nature of magnetism in iron-pnictides, we develop a
quasiclassical theory of tetragonal magnetic order in disordered three-band
metal with anisotropic band structure. Within our model we find that the
$C_4$-symmetric magnetism competes with the $C_2$-symmetric state with a single
${\vec Q}$ magnetic structure vector. Our main results is that disorder
promotes tetragonal magnetic state which is in agreement with earlier
theoretical studies.
|
2003.12124v1
|
2020-03-30
|
Electronic correlation effects and local magnetic moments in L1$_0$ phase of FeNi
|
We study the electronic and magnetic properties of L1$_0$ phase of FeNi, a
perspective rare-earth-free permanent magnet, by using a combination of density
functional and dynamical mean-field theory. Although L1$_0$ FeNi has a slightly
tetragonally distorted fcc lattice, we find that magnetic properties of its
constituent Fe atoms resemble those in pure bcc Fe. In particular, our results
indicate the presence of well-localized magnetic moments on Fe sites, which are
formed due to Hund's exchange. At the same time, magnetism of Ni sites is much
more itinerant. Similarly to pure bcc Fe, the self-energy of Fe $3d$ states is
found to show the non-Fermi-liquid behavior. This can be explained by
peculiarities of density of Fe $3d$ states, which has pronounced peaks near the
Fermi level. Our study of local spin correlation function and momentum
dependence of particle-hole bubble suggests that the magnetic exchange in this
substance is expected to be of RKKY-type, with iron states providing
local-moment contribution, and the states corresponding to nickel sites
(including virtual hopping to iron sites) providing itinerant contribution.
|
2003.13347v2
|
2020-03-03
|
Novel Meta-Heuristic Model for Discrimination between Iron Deficiency Anemia and B-Thalassemia with CBC Indices Based on Dynamic Harmony Search
|
In recent decades, attention has been directed at anemia classification for
various medical purposes, such as thalassemia screening and predicting iron
deficiency anemia (IDA). In this study, a new method has been successfully
tested for discrimination between IDA and \b{eta}-thalassemia trait
(\b{eta}-TT). The method is based on a Dynamic Harmony Search (DHS). Complete
blood count (CBC), a fast and inexpensive laboratory test, is used as the input
of the system. Other models, such as a genetic programming method called
structured representation on genetic algorithm in non-linear function fitting
(STROGANOFF), an artificial neural network (ANN), an adaptive neuro-fuzzy
inference system (ANFIS), a support vector machine (SVM), k-nearest neighbor
(KNN), and certain traditional methods, are compared with the proposed method.
|
2004.00480v1
|
2020-04-03
|
Spin-Excitations Anisotropy in the Bilayer Iron-Based Superconductor CaKFe$_4$As$_4$
|
We use polarized inelastic neutron scattering to study the spin-excitations
anisotropy in the bilayer iron-based superconductor CaKFe$_4$As$_4$ ($T_c$ = 35
K). In the superconducting state, both odd and even $L-$modulations of spin
resonance have been observed in our previous unpolarized neutron scattering
experiments (T. Xie {\it et al.} Phys. Rev. Lett. {\bf 120}, 267003 (2018)).
Here we find that the high-energy even mode ($\sim 18$ meV) is isotropic in
spin space, but the low-energy odd modes consist of a $c-$axis polarized mode
around 9 meV along with another partially overlapped in-plane mode around 12
meV. We argue that such spin anisotropy is induced by the spin-orbit coupling
in the spin-vortex-type fluctuations of this unique compound. The spin
anisotropy is strongly affected by the superconductivity, where it is weak
below 6 meV in the normal state and then transferred to higher energy and
further enhanced in the odd mode of spin resonance below $T_c$.
|
2004.01405v1
|
2020-04-13
|
Topological magnetic line defects in Fe(Te,Se) high-temperature superconductors
|
Magnetic impurity chains on top of conventional superconductors are promising
platforms to realize Majorana modes. Iron-based high-temperature
superconductors are known in the vicinity of magnetic states due to the strong
Hund's coupling in iron atoms. Here we propose that the line defects with
missing Te/Se anions in Fe(Se,Te) superconductors provide the realization of
intrinsic antiferromagnetic(AFM) chains with Rashba spin-orbit coupling.
Against conventional wisdom, Majorana zero modes (MZMs) can be robustly
generated at these AFM chain ends. These results can consistently explain the
recent experimental observation of zero-energy end states in line defects of
monolayer Fe(Te,Se)/SrTiO$_3$ by scanning tunneling microscopy (STM)
measurements. Our research not only demonstrates an unprecedented interplay
among native line defect, emergent magnetism and topological superconductivity
but also explores a high-temperature platform for Majorana fermions.
|
2004.05848v1
|
2020-04-27
|
Permanent Magnet Penning Trap
|
The Penning trap has been investigated as the basis of a small nuclear fusion
reactor using a superconducting solenoid magnet. To extend this investigation,
we designed, constructed, and evaluated a permanent magnet Penning trap. The
device consists of a solenoid formed from an annular array of neodymium bar
magnets between two iron pole pieces designed to give a uniform magnetic field
in the central volume of the device. Critical to achieving the uniform
solenoidal field is an iron equatorial ring supported within the annular array
of magnets. A nonmagnetic titanium Penning trap with hyperbolic surfaces
designed to produce a spherical potential well was mounted inside the permanent
magnet assembly. The trap was fitted with a nonmagnetic hairpin filament
electron source and demonstrated to produce electron trapping at the
theoretically predicted magnetic fields and trap potentials. Trap potentials
achievable were limited by electrical breakdown within the trap operating in
constant potential mode. Efforts were made to extend the trap potentials using
pulsed anode voltages, but nuclear fusion in a Penning trap has not yet been
demonstrated. The design and construction of the permanent magnet solenoid and
nonmagnetic trap are presented here as potentially useful also in other
studies.
|
2004.13103v1
|
2020-05-13
|
Importance of Fermi surface and magnetic interactions for the superconducting dome in electron doped FeSe intercalates
|
The van-der-Waals gap of iron chalcogenide superconductors can be
intercalated with a variety of inorganic and organic compounds that modify the
electron doping level of the iron layers. In Lix(C3N2H10)0.37FeSe, a dome in
the superconducting transition temperature Tc has been reported to occur in the
doping range of x=0.06 to x=0.68. We use a combination of density functional
theory and spin fluctuation theory to capture the evolution of superconducting
transition temperatures theoretically. We clearly demonstrate how the changing
electronic structure supports an increasing superconducting Tc. The suppression
of Tc at high doping levels can, however, only be understood by analyzing the
magnetic tendencies, which evolve from stripe-type at low doping to bicollinear
at high doping.
|
2005.06215v1
|
2020-06-07
|
Kinetics of segregation formation in elastic field of edge dislocation in bcc iron
|
We study the kinetics of the redistribution of impurity atoms in the elastic
fields of dislocations by computer simulation methods. A work consists of
several stages. The first is the simulation of a dislocation core structure
with a Burgers vector along [100] direction by using the modified molecular
static method. The second is obtaining the coefficients that determine the
influence of the components of the strain tensor on the diagonal elements of
the matrix of diffusion coefficients and in the equations for fluxes of carbon
atoms in bcc iron. The third stage is associated with modeling the diffusion
characteristics of carbon atoms by the method of molecular dynamics in a
crystal without defects. The fourth and final stage uses the data about atomic
structure that we obtained at first stage, as well as the characteristics
calculated in the second and third, is a simulation of the formation of
segregations of interstitial atoms, which is based on solving diffusion
equations that take into account the elastic deformations created by the
dislocation. The complex of developed models is used to analyze the kinetics of
segregation formation. 3D graphs illustrate the distribution of interstitial
atoms in the vicinity of a dislocation for different times at certain
temperatures.
|
2006.04036v1
|
2020-06-10
|
Quantum-Critical Spin-Density Waves in Iron-Selenide High-Tc Superconductors
|
Hidden spin-density waves (hSDW) with Neel ordering vector (pi,pi) have been
proposed recently as parent groundstates to electron-doped iron-selenide
superconductors. Doping such groundstates can result in visible electron-type
Fermi surface pockets and faint hole-type Fermi surface pockets at the corner
of the folded Brillouin zone. A Cooper pair instability that alternates in sign
between the electron-type and the hole-type Fermi surfaces has recently been
predicted. The previous is due to the interaction of electrons and holes with
hidden spin fluctuations connected with hSDW order that is near a
quantum-critical point. Quantum criticality is tuned in by increasing the
strength of Hund's Rule from the hSDW state. We find that the exchange of
hidden spin fluctuations by electrons/holes in the critical hSDW state results
in asymptotic freedom. In particular, the strength of spin-flip interactions
becomes weaker and weaker on length scales that are shorter and shorter
compared to the range of hSDW order. We then argue that string states that
connect well-separated particle/hole excitations in the hSDW are robust. This
suggests a picture where the hole degrees of freedom mentioned previously are
confined.
|
2006.05680v1
|
2020-06-11
|
Low thermal conductivity of iron-silicon alloys at Earth core conditions with implications for the geodynamo
|
Earth core is composed of iron (Fe) alloyed with light elements, e.g.,
silicon (Si). Its thermal conductivity critically affects Earth thermal
structure, evolution, and dynamics, as it controls the magnitude of thermal and
compositional sources required to sustain a geodynamo over Earth history. Here
we directly measured thermal conductivities of solid Fe and Fe-Si alloys up to
144 GPa and 3300 K. 15 at% Si alloyed in Fe substantially reduces its
conductivity by about 2 folds at 132 GPa and 3000 K. An outer core with 15 at%
Si would have a conductivity of about 20 W m-1 K-1, lower than pure Fe at
similar pressure-temperature conditions. This suggests a lower minimum heat
flow, around 3 TW, across the core-mantle boundary than previously expected,
and thus less thermal energy needed to operate the geodynamo. Our results
provide key constraints on inner core age that could be older than two
billion-years.
|
2006.06271v2
|
2020-06-11
|
Double single-channel Kondo coupling in graphene with Fe molecules
|
We study the interaction between graphene and a single-molecule-magnet,
[Fe4(L)2(dpm)6]. Focusing on the closest Iron ion in a hollow position with
respect to the graphene sheet, we derive a channel selective tunneling
Hamiltonian, that couples different d orbitals of the Iron atom to precise
independent combinations of sublattice and valley degrees of freedom of the
electrons in graphene. When looking at the spin-spin interaction between the
molecule and the graphene electrons, close to the Dirac point the channel
selectivity results in a channel decoupling of the Kondo interaction, with two
almost independent Kondo systems weakly interacting among themselves. The
formation of magnetic moments and the development of a full Kondo effect
depends on the charge state of the graphene layer.
|
2006.06723v1
|
2020-06-24
|
Iron line reverberation mapping in Ghasemi-Nodehi-Bambi background
|
The reverberation associated with the iron line is the time lag between
direct photons from the corona and the photons reflected from the disk. The
resulting line spectrum is called the 2D transfer function. The shape of the 2D
transfer function is determined by the geometry of spacetime and the properties
of BH. In a paper (Ghasemi-Nodehi and Bambi in EPJC 76:290, 2016), the authors
have proposed a parametrization. This parametrization is aimed to test the Kerr
nature of astrophysical black hole candidates. In this paper, I provide a
reverberation mapping of the Ghasemi-Nodehi-Bambi metric in order to constrain
the parameter of spacetime. All parameters can be constrained with the
exception of b11. The parameter b4 is harder to constrain too.
|
2006.13640v1
|
2020-07-13
|
Effect of interactions and non-uniform magnetic states on the magnetization reversal of iron nanowire arrays
|
Ordered ferromagnetic nanowire arrays are widely studied due to the diversity
of possible applications. However, there is still no complete understanding of
the relation between the array's parameters and its magnetic behavior. The
effect of vortex states on the magnetization reversal of large-diameter
nanowires is of particular interest. Here, we compare analytical and
micromagnetic models with experimental results for three arrays of iron
nanowires with diameters of 33, 52 and 70 nm in order to find the balance
between the number of approximations and resources used for the calculations.
The influence of the vortex states and the effect of interwire interactions on
the remagnetization curves is discussed. It has been found that 7 nanowires
treated by a mean field model are able to reproduce well the reversal behaviour
of the whole array in the case of large diameter nanowires. Vortex states tend
to decrease the influence of the structural inhomogeneities on reversal process
and thus lead to the increased predictability of the system.
|
2007.06499v1
|
2020-07-19
|
Isostructural spin-density-wave and superconducting gap anisotropies in iron-arsenide superconductors
|
When passing through a phase transition, electronic system saves energy by
opening energy gaps at the Fermi level. Delineating the energy gap anisotropy
provides insights into the origin of the interactions that drive the phase
transition. Here, we report the angle-resolved photoemission spectroscopy
(ARPES) study on the detailed gap anisotropies in both the tetragonal magnetic
and superconducting phases in Sr$_{1-x}$Na$_x$Fe$_2$As$_2$. First, we found
that the spin-density-wave (SDW) gap is strongly anisotropic in the tetragonal
magnetic phase. The gap magnitude correlates with the orbital character of
Fermi surface closely. Second, we found that the SDW gap anisotropy is
isostructural to the superconducting gap anisotropy regarding to the angular
dependence, gap minima locations, and relative gap magnitudes. Our results
indicate that the superconducting pairing interaction and magnetic interaction
share the same origin. The intra-orbital scattering plays an important role in
constructing these interactions resulting in the orbital-selective magnetism
and superconductivity in iron-based superconductors.
|
2007.09572v1
|
2020-07-15
|
Structure and Properties of Thermoresponsive Diblock Copolymers Embedded with Metal Oxide Nanoparticles
|
Nanostructured polymer-metal oxide composites are a current research area of
great importance due to its highlight applications in sensors, optics,
catalysts and drug delivery. Particularly the use of thermoresponsive polymers
gives more flexibilities and possibilities in the design and construction of
polymer templates. In the present investigation, the structure and magnetic
properties of hybrid metal oxide/DBC films composed of two kinds of
polystyrene-block-poly (N-isopropylacrylamide)(PS-b-PNIPAM) diblock copolymers
(DBCs) with PS and PNIPAM as the major polymer domains respectively, and iron
oxide were investigated. The thermoresponsive PNIPAM has a lower critical
solution temperature (LCST) in aqueous solution at 32{\deg}C, which enables the
controllable volume ratio of PS and PNIPAM in the structure of PS-b-PNIPAM
diblock copolymers (DBCs). Thus, a temperature and humidity controlling cell
was designed and built for precisely tuning the block structure of PS-b-PNIPAM
DBCs, which was investigated by in-situ small-angle X-ray scattering (SAXS) and
grazing-incidence small-angle X-ray scattering (GISAXS) measurements. The
superparamagnetic behavior of the heat-treated hybrid iron oxide/PS-b-PNIPAM
DBC films was investigated using a superconducting quantum interference device
(SQUID) magnetometer.
|
2007.10111v1
|
2020-08-09
|
The Iron Line Profile from Warped Black Hole Accretion Disks
|
The profile of the fluorescent iron line from black hole accretion disks is a
powerful diagnostic of black hole properties, such as spin and inclination. The
state-of-the-art, however, considers an accretion disk whose angular momentum
is aligned with that of the black hole; this is a very constraining assumption
which is unlikely to apply to many or even most astrophysical systems. Here, we
present the first simulation of the reflection spectrum from warped accretion
disks using a realistic model of the reflected emission based on the xillver
code. We present the effects that the radial location of the warp and the tilt
angle have on the line profile, and highlight that the results are highly
dependent on the azimuth position of the observer relative to the tilt angle.
We fit these profiles in XSpec with the standard relxill lamppost model to
quantify the effect that neglecting the disk warps has on the inferred black
hole spins and inclinations. We show that fits with two-component relxill can
be used to derive more accurate spin and inclination estimates.
|
2008.03829v2
|
2020-08-13
|
Detection of a possible multiphase ultra-fast outflow in IRAS 13349+2438 with NuSTAR and XMM-Newton
|
We present joint NuSTAR and XMM-Newton observations of the bright, variable
quasar IRAS 13349+2438. This combined dataset shows two clear iron absorption
lines at 8 and 9 keV, which are most likely associated with two layers of
mildly relativistic blueshifted absorption, with velocities of 0.14c and 0.27c.
We also find strong evidence for a series of Ly$\alpha$ absorption lines at
intermediate energies in a stacked XMM-Newton EPIC-pn spectrum, at the same
blueshift as the lower velocity iron feature. This is consistent with a
scenario where an outflowing wind is radially stratified, so faster, higher
ionization material is observed closer to the black hole, and cooler, slower
material is seen from streamlines at larger radii.
|
2008.05965v1
|
2020-08-18
|
Thickness-dependent electron momentum relaxation times in iron films
|
Terahertz time-domain conductivity measurements in 2 to 100 nm thick iron
films resolve the femtosecond time delay between applied electric fields and
resulting currents. This current response time decreases from 29 fs for
thickest films to 7 fs for the thinnest films. The macroscopic response time is
not strictly proportional to the conductivity. This excludes the existence of a
single relaxation time universal for all conduction electrons. We must assume a
distribution of microscopic momentum relaxation times. The macroscopic response
time depends on average and variation of this distribution; the observed
deviation between response time and conductivity scaling corresponds to the
scaling of the variation. The variation of microscopic relaxation times depends
on film thickness because electrons with different relaxation times are
affected differently by the confinement since they have different mean free
paths.
|
2008.07852v1
|
2020-09-14
|
BaCuS2: a superconductor with moderate electron-electron correlation
|
We show that the layered-structure BaCuS$_2$ is a moderately correlated
electron system in which the electronic structure of the CuS layer bears a
resemblance to those in both cuprates and iron-based superconductors.
Theoretical calculations reveal that the in-plane $d$-$p$ $\sigma^*$-bonding
bands are isolated near the Fermi level. As the energy separation between the
$d$ and $p$ orbitals are much smaller than those in cuprates and iron-based
superconductors, BaCuS$_2$ is expected to be moderately correlated. We suggest
that this material is an ideal system to study the competitive/collaborative
nature between two distinct superconducting pairing mechanisms, namely the
conventional BCS electron-phonon interaction and the electron-electron
correlation, which may be helpful to establish the elusive mechanism of
unconventional high-temperature superconductivity.
|
2009.06230v1
|
2020-09-29
|
On the possibility of through passage of asteroid bodies across the Earth's atmosphere
|
We have studied the conditions of through passage of asteroids with diameters
200, 100 and 50 m, consisting of three types of materials -- iron, stone and
water ice across the Earth's atmosphere with the minimum trajectory altitude
10--15 km. The conditions of this passage with subsequent exit into outer space
with the preservation of a substantial fraction of the initial mass have been
found. The results obtained support our idea explaining one of the
long-standing problems of astronomy -- the Tunguska phenomenon which has not
received reasonable and comprehensive interpretations to date. We argue that
Tunguska event was caused by an iron asteroid body, which passed through the
Earth's atmosphere and continued to the near-solar orbit.
|
2009.14234v1
|
2020-10-02
|
Effect of the surface shape of a large space body on its fragmentation in a planetary atmosphere
|
Employing the finite element and computational fluid dynamics methods, we
have determined the conditions for the fragmentation of space bodies or
preservation of their integrity when they penetrate into the Earth's
atmosphere. The origin of forces contributing to the fragmentation of space
iron bodies during the passage through the dense layers of the planetary
atmosphere has been studied. It was shown that the irregular shape of the
surface can produce transverse aerodynamic forces capable of causing
deformation stress in the body exceeding the tensile strength threshold of
iron.
|
2010.01095v1
|
2020-10-09
|
Pressure-induced magnetism in iron-based superconductors $A$Fe$_2$As$_2$ ($A=$ K, Cs, Rb)
|
The magnetic properties of iron-based superconductors $A$Fe$_2$As$_2$ ($A=$K,
Cs, and Rb), which are characterized by the V-shaped dependence of the critical
temperature ($T_{\rm c}$) on pressure ($P$) were studied by means of the muon
spin rotation/relaxation technique. In all three systems studied the magnetism
was found to appear for pressures slightly below the critical one ($P_{\rm
c}$), i.e. at pressure where $T_{\rm c}(P)$ changes the slope. Rather than
competing, magnetism and superconductivity in $A$Fe$_2$As$_2$ are coexisting at
$P\gtrsim P_{\rm c}$ pressure region. Our results support the scenario of a
transition from one pairing state to another, with different symmetries on
either side of $P_{\rm c}$.
|
2010.04448v1
|
2020-11-24
|
Short-range nematic fluctuations in Sr1-xNaxFe2As2 superconductors
|
Interactions between nematic fluctuations, magnetic order and
superconductivity are central to the physics of iron-based superconductors.
Here we report on in-plane transverse acoustic phonons in hole-doped
Sr$_{1-x}$Na$_x$Fe$_2$As$_2$ measured via inelastic X-ray scattering, and
extract both the nematic susceptibility and the nematic correlation length. By
a self-contained method of analysis, for the underdoped ($x=0.36$) sample,
which harbors a magnetically-ordered tetragonal phase, we find it hosts a short
nematic correlation length $\xi$ ~ 10 $\AA$ and a large nematic susceptibility
$\chi_{\rm nem}$. The optimal-doped ($x=0.55$) sample exhibits weaker phonon
softening effects, indicative of both reduced $\xi$ and $\chi_{\rm nem}$. Our
results suggest short-range nematic fluctuations may favor superconductivity,
placing emphasis on the nematic correlation length for understanding the
iron-based superconductors.
|
2011.12444v1
|
2020-12-07
|
Magnetic anisotropy from linear defect structures in correlated electron systems
|
Correlated electron systems, particularly iron-based superconductors, are
extremely sensitive to strain, which inevitably occurs in the crystal growth
process. Built-in strain of this type has been proposed as a possible
explanation for experiments where nematic order has been observed at high
temperatures corresponding to the nominally tetragonal phase of iron-based
superconductors. Strain is assumed to produce linear defect structures, e.g.
dislocations, which are quite similar to O vacancy chainlets in the underdoped
cuprate superconductor YBCO. Here we investigate a simple microscopic model of
dislocations in the presence of electronic correlations, which create defect
states that can drive magnetic anisotropy of this kind, if spin orbit
interaction is present. We estimate the contribution of these dislocations to
magnetic anisotropy as detected by current torque magnetometry experiments in
both cuprates and Fe-based systems.
|
2012.03824v2
|
2020-12-08
|
Mean field theory and Monte Carlo simulation of Phase transitions and Magnetic Properties of a tridimensional Fe7S8 Compound
|
The structural, electronic and magnetic properties of Fe7S8 material have
been studied within the framework of the ab-initio calculations, the mean field
approximation (MFA) and Monte Carlo simulation (MCS). Our study shows that two
forms of the iron atoms, Fe2+ with spin S=2, and Fe3+ with spin {\sigma}=5/2
are the most probable configurations. A mixed Ising model with ferromagnetic
spin coupling between Fe2+ and Fe3+ ions and between Fe3+ and Fe3+ ions, and
with antiferromagnetic spin coupling between Fe2+ ions of adjacent layers has
been used to study the magnetic properties of this compound. We demonstrated
that the magnetic phase transition can be either of the first or of the second
order, depending on the value of the exchange interaction and crystal field.
The presence of vacancies in every second iron layer leads to incomplete
cancellation of magnetic moments, hence to the emergence of the ferrimagnetism.
Anomalies in the magnetization behavior have been found and compared with the
experimental results.
|
2012.04306v1
|
2020-12-13
|
Under an Iron Sky: On the Entropy at the Start of the Universe
|
Curiously, our Universe was born in a low entropy state, with abundant free
energy to power stars and life. The form that this free energy takes is usually
thought to be gravitational: the Universe is almost perfectly smooth, and so
can produce sources of energy as matter collapses under gravity. It has
recently been argued that a more important source of low-entropy energy is
nuclear: the Universe expands too fast to remain in nuclear statistical
equilibrium (NSE), effectively shutting off nucleosynthesis in the first few
minutes, providing leftover hydrogen as fuel for stars. Here, we fill in the
astrophysical details of this scenario, and seek the conditions under which a
Universe will emerge from early nucleosynthesis as almost-purely iron. In so
doing, we identify a hitherto-overlooked character in the story of the origin
of the second law: matter-antimatter asymmetry.
|
2012.06975v2
|
2020-12-24
|
Intrinsic Time-reversal-invariant Topological Superconductivity in Thin Films of Iron-based Superconductors
|
We establish quasi-two-dimensional thin films of iron-based superconductors
(FeSCs) as a new high-temperature platform for hosting intrinsic
time-reversal-invariant helical topological superconductivity (TSC). Based on
the combination of Dirac surface state and bulk extended $s$-wave pairing, our
theory should be directly applicable to a large class of experimentally
established FeSCs, opening a new TSC paradigm. In particular, an applied
electric field serves as a "topological switch" for helical Majorana edge modes
in FeSC thin films, allowing for an experimentally feasible design of
gate-controlled helical Majorana circuits. Applying an in-plane magnetic field
drives the helical TSC phase into a higher-order TSC carrying corner-localized
Majorana zero modes. Our proposal should enable the experimental realization of
helical Majorana fermions.
|
2012.13411v2
|
2021-01-14
|
Thermal response of the iron-based Ba122 superconductor to in situ and ex situ processes
|
The thermal properties are one of the key parameters to control phase purity
and microstructure of polycrystalline materials. The melting point of the
iron-based BaFe2As2 superconductor (Ba122), which foresees high-field
applications, remains controversial. In this work,
thermogravimetry-differential scanning calorimetry measurements (TG-DSC) of
undoped and Co-doped Ba122 were carried out. Mixtures of elemental metals and
pre-reacted Ba122 powders were prepared to investigate the thermal responses
during in situ and ex situ synthesis routes, respectively. In addition, the
phases and microstructures of the quenched samples were evaluated to elucidate
the observed exothermic/endothermic peaks. Our results suggest that the melting
point of Ba122 is ~1300{\deg}C.
|
2101.05502v1
|
2021-01-19
|
Role of interface morphology on the martensitic transformation in pure Fe
|
Using classical molecular dynamics simulations, we study austenite to ferrite
phase transformation in iron, focusing on the role of interface morphology. We
compare two different morphologies; a \textit{flat} interface in which the two
phases are joined according to Nishiyama-Wasserman orientation relationship vs.
a \textit{ledged} one, having steps similar to the vicinal surface. We identify
the atomic displacements along a misfit dislocation network at the interface
leading to the phase transformation. In case of \textit{ledged} interface,
stacking faults are nucleated at the steps, which hinder the interface motion,
leading to a lower mobility of the inter-phase boundary, than that of flat
interface. Interestingly, we also find the temperature dependence of the
interface mobility to show opposite trends in case of \textit{flat} vs.
\textit{ledged} boundary. We believe that our study is going to present a
unified and comprehensive view of martensitic transformation in iron with
different interface morphology, which is lacking at present, as \textit{flat}
and \textit{ledged} interfaces are treated separately in the existing
literature.
|
2101.07468v1
|
2021-02-19
|
Superconducting FeSe monolayer with milli-electron volt Fermi energy
|
Iron selenide (FeSe) is an iron-based superconductor which shows unique
properties, including strongly anisotropic superconducting gap, paramagnetism
in undoped compound and extremely small Fermi pocket size. In this work, we
demonstrate that the sizes of electron and hole pockets in FeSe monolayer
become much smaller than those in bulk. The Fermi energy is in the order of a
few meV and can be fine-tuned by the thickness of graphene layers underneath.
Despite the low carrier density, the FeSe monolayers grown on trilayer or
multi-layer graphene are superconducting. The superconducting gap size is
sensitive to the Fermi energy of the hole band. Remarkably, the FeSe monolayer
provides the opportunity to study the physics in the crossover regime where the
Fermi energy and superconducting gap are comparable to each other.
|
2102.09792v1
|
2021-02-19
|
Revealing the intrinsic superconducting gap anisotropy in surface-neutralized BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$
|
Alkaline-earth iron arsenide (122) is one of the most studied families of
iron-based superconductors, especially for angle-resolved photoemission
spectroscopy. While extensive photoemission results have been obtained, the
surface complexity of 122 caused by its charge-non-neutral surface is rarely
considered. Here, we show that the surface of 122 can be neutralized by
potassium deposition. In potassium-coated BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$,
the surface-induced spectral broadening is strongly suppressed, and hence the
coherent spectra that reflect the intrinsic bulk electronic state recover. This
enables the measuring of superconducting gap with unpreceded precision. The
result shows the existence of two pairing channels. While the gap anisotropy on
the outer hole/electron pockets can be well fitted using an s$_\pm$ gap
function, the gap anisotropy on the inner hole/electron shows a clear
deviation. Our results provide quantitative constraints for refining
theoretical models and also demonstrate an experimental method for revealing
the intrinsic electronic properties of 122 in future studies.
|
2102.09919v1
|
2021-02-22
|
Influence of the Fermi surface geometry on a Josephson effect between an iron-pnictide and conventional superconductors
|
We study Josephson junctions between a multi-band iron-pnictide
Ba1-xNaxFe2As2 and conventional s-wave superconductors Nb and Cu/Nb bilayer. We
observe that junctions with a Cu interlayer exhibit much larger IcRn, despite a
weaker proximity-induced superconductivity. This counterintuitive result is
attributed to the difference in Fermi surface geometries of Nb and Cu, which
leads to a selective one-band tunneling from Cu and a non-selective multi-band
tunnelng from Nb. The latter leads to a mutual cancellation of supercurrents
due to the sign-reversal s+- symmetry of the order parameter in the pnictide.
Our results indicate that Fermi surface geometries play a crucial role for
pnictide-based junctions. This provides a new tool for phase sensitive studies
and paves a way to a conscious engineering of such junctions.
|
2102.10972v1
|
2021-02-24
|
Intermediate mass and heavy Galactic cosmic-ray nuclei: the case of new AMS-02 measurements
|
The recent measurement of the spectra of intermediate mass nuclei and iron
nuclei carried out with the AMS-02 experiment provided us with the most
complete set of data on cosmic ray fluxes to date, and allowed us to test the
standard model for the transport of these particles through the Galaxy to the
finest details. We show that the parameters derived from lighter primary and
secondary elements in the cosmic radiation also lead to a good description of
the data on heavier nuclei, with no need to invoke different injection spectra
for such nuclei, provided the whole chain of fragmentation is properly
accounted for. The only exception to this finding is represented by iron
nuclei, which show a very unusual trend at rigidity $\lesssim 100$ GV. This
trend reflects in a Fe/O ratio that is at odds with the results of the standard
model of cosmic ray transport, and is in contradiction with data collected by
HEAO, ACE-CRIS and Voyager at lower energy. We speculate on possible origins of
such findings.
|
2102.12576v2
|
2021-03-06
|
Autonomous detection of molecular configurations in microscopic images based on deep convolutional neural network
|
In an effort to explore high-throughput processing of microscopic image data,
a method based on deep convolutional neural network is proposed. The
state-of-the-art computer vision algorithm, Faster R-CNN, was trained for the
detection of iron (II) phthalocyanines on Se-terminated Au(111) platform
resolved by scanning probe microscopy. The construction of the feature pyramid
enables the multi-scale molecule detection in images of different scales from
10 nm to 50 nm. After the detection, the orientation of each molecule is
measured by a following program. Based on the statistical distribution of the
orientation angles, the preferred adsorption configurations of iron (II)
phthalocyanine on the platform are revealed. This method yields high accuracy
and recall with F1 score close to 1 after optimization of hyperparameters and
training. It is expected to be a feasible solution in the scenarios where
autonomous and high-throughput processing of microscopic image data is needed.
|
2103.04213v1
|
2021-03-11
|
Stimuli-responsive assembly of iron oxide nanoparticles into magnetic flexible filaments
|
The combination of multiple functionalities in a single material is an
appealing strategy for the de-velopment of smart materials with unique
features. In this work, we present the preparation of thermoresponsive magnetic
nanoparticles and their one-dimensional assembly into transient
micro-filaments. The material is based on 9.4 nm iron oxide nanoparticles
grafted with poly(N-n-propylacrylamide) via multiphosphonic acid anchoring
sites. The hybrid nanoparticles present a low critical solution temperature
(LCST) transition between 21 {\deg}C and 28 {\deg}C, depending on the pH and
the ionic strength. When heated above the LCST in defined conditions, the
nanoparticles ag-gregate and respond to an external magnetic field. An
intrinsic characteristic of the thermorespon-sive particles is an asymmetric
transition between cooling and heating cycles, that was favorably exploited to
build one-dimensional permanent microstructures, such as magnetic
microfilaments and cilia. In summary, we present the development of a
nanoplatform responsive to multiple stimu-li, including temperature, magnetic
field, pH and ionic strength and its transformation into magneti-cally active
microfilaments that could find potential applications in remotely controlled
devices.
|
2103.06837v1
|
2021-03-24
|
Proximity Effect of Epitaxial Iron Phthalocyanine Molecules on High-Quality Graphene Devices
|
Depositing magnetic insulators on graphene has been a promising route to
introduce magnetism via exchange proximity interaction in graphene for future
spintronics applications. Molecule-based magnets may offer unique opportunities
because of their synthesis versatility. Here, we investigated the magnetic
proximity effect of epitaxial iron phthalocyanine (FePc) molecules on
high-quality monolayer and bilayer graphene devices on hexagonal boron nitride
substrate by probing the local and non-local transport. Although the FePc
molecules introduce large hole doping effects combined with mobility
degradation, the magnetic proximity gives rise to a canted antiferromagnetic
state under a magnetic field in the monolayer graphene. On bilayer graphene and
FePc heterostructure devices, the non-local transport reveals a pronounced
Zeeman spin-Hall effect. Further analysis of the scattering mechanism in the
bilayer shows a dominated long-range scattering. Our findings in
graphene/organic magnetic insulator heterostructure provide a new insight for
the use of molecule-based magnets in two-dimensional spintronic devices.
|
2103.12974v1
|
2021-03-29
|
Fluorescence intensity correlation imaging with high resolution and elemental contrast using intense x-ray pulses
|
We theoretically investigate the fluorescence intensity correlation (FIC) of
Ar clusters and Mo-doped iron oxide nanoparticles subjected to intense,
femtosecond and sub-femtosecond XFEL pulses for high-resolution and elemental
contrast imaging. We present the FIC of {\Ka} and {\Kah} emission in Ar
clusters and discuss the impact of sample damage on retrieving high-resolution
structural information and compare the obtained structural information with
those from the coherent difractive imaging (CDI) approach. We found that, while
sub-femtosecond pulses will substantially benefit the CDI approach,
few-femtosecond pulses may be sufficient for achieving high-resolution
information with FIC. Furthermore, we show that the fluorescence intensity
correlation computed from the fluorescence of Mo atoms in Mo-doped iron oxide
nanoparticles can be used to image dopant distributions.
|
2103.15872v1
|
2021-03-31
|
Slip band interactions and GND latent hardening in a galling resistant stainless steel
|
Slip activation, slip band interactions, and GND densities in iron-base,
galling resistant alloy Nitronic 60 have been characterised at the grain length
scale using small-scale mechanical testing with high resolution digital image
correlation and high-angular resolution electron backscatter diffraction. By
correlating the two measurement techniques, new insight into slip band
interactions, the generation of lattice curvature and the corresponding
accumulation of geometrically necessary dislocations (GNDs) is provided.
Multiple discrete slip bands are typically active within single grains,
resulting in significant slip band interactions. Crossing slip bands were found
to generate accumulations of GNDs. Regions where slip bands block other slip
bands were associated with the highest GND densities, in excess of three time
the densities of crossing slip bands. Representative crystal plasticity
modelling investigations have demonstrated that discrete slip blocking events
are responsible for locally elevated GND density. This behaviour is
rationalised in terms of lattice curvature associated with the differing levels
of constraint provided by the crossing or blocking-type behaviours. Ferrite
grains are also found to contribute to the generation of GNDs. Together, these
two effects provide significant work hardening mechanisms, likely to be key to
the development of future iron-base hard facing alloys.
|
2103.16864v1
|
2021-01-21
|
Time-Dependent Density Functional Theory Applied to Average Atom Opacity
|
We focus on studying the opacity of iron, chromium, and nickel plasmas at
conditions relevant to experiments carried out at Sandia National Laboratories
[J. E. Bailey et al., Nature 517, 56 (2015)]. We calculate the photo-absorption
cross-sections and subsequent opacity for plasmas using linear response
time-dependent density functional theory (TD-DFT). Our results indicate that
the physics of channel mixing accounted for in linear response TD-DFT leads to
an increase in the opacity in the bound-free quasi-continuum, where the Sandia
experiments indicate that models under-predict iron opacity. However, the
increase seen in our calculations is only in the range of 5-10%. Further, we do
not see any change in this trend for chromium and nickel. This behavior
indicates that channel mixing effects do not explain the trends in opacity
observed in the Sandia experiments.
|
2104.00551v1
|
2021-04-15
|
Unconventional iron-magnesium compounds at terapascal pressures
|
Being a lithophile element at ambient pressure, magnesium is long believed to
be immiscible with iron. A recent study by Gao et al. [1] showed that pressure
turns magnesium into a siderophile element and can produce unconventional Fe-Mg
compounds. Here, we extend the investigation to exoplanetary pressure
conditions using an adaptive genetic algorithm-based variable-composition
structural prediction approach. We identify several Fe-Mg phases up to 3 TPa.
Our cluster alignment analysis reveals that most of the predicted Fe-Mg
compounds prefer a BCC packing motif at terapascal pressures. This study
provides a more comprehensive structure database to support future
investigations of the high-pressure structural behavior of Fe-Mg and ternary,
quaternary, etc. compounds involving these elements.
|
2104.07700v1
|
2021-04-19
|
Infrared phonon spectroscopy on the Cairo pentagonal antiferromagnet Bi2Fe4O9: a study through the pressure induced structural transition
|
Magnetic and crystallographic transitions in the Cairo pentagonal magnet
Bi2Fe4O9 are investigated by means of infrared synchrotron-based spectroscopy
as a function of temperature (20 - 300 K) and pressure (0 - 15.5 GPa). One of
the phonon modes is shown to exhibit an anomalous softening as a function of
temperature in the antiferromagnetic phase below 240 K, highlighting
spin-lattice coupling. Moreover, under applied pressure at 40 K, an even larger
softening is observed through the pressure induced structural transition.
Lattice dynamical calculations reveal that this mode is indeed very peculiar as
it involves a minimal bending of the strongest superexchange path in the
pentagonal planes, as well as a decrease of the distances between second
neighbor irons. The latter confirms the hypothesis made by Friedrich et al.,1
about an increase in the oxygen coordination of irons being at the origin of
the pressure-induced structural transition. As a consequence, one expects a new
magnetic superexchange path that may alter the magnetic structure under
pressure.
|
2104.09384v1
|
2021-04-21
|
The effect of core formation on surface composition and planetary habitability
|
The melt productivity of a differentiated planet's mantle is primarily
controlled by its iron content, which is itself approximated by the planet's
core mass fraction (CMF). Here we show that estimates of an exo-planet's CMF
allows robust predictions of the thickness, composition and mineralogy of the
derivative crust. These predicted crustal compositions allow constraints to be
placed on volatile cycling between surface and the deep planetary interior,
with implications for the evolution of habitable planetary surfaces. Planets
with large, terrestrial-like, CMFs ($\geq$0.32) will exhibit thin crusts that
are inefficient at transporting surface water and other volatiles into the
underlying mantle. By contrast, rocky planets with smaller CMFs ($\leq$0.24)
and higher, Mars-like, mantle iron contents will develop thick crusts capable
of stabilizing hydrous minerals, which can effectively sequester volatiles into
planetary interiors and act to remove surface water over timescales relevant to
evolution. The extent of core formation has profound consequences for the
subsequent planetary surface environment and may provide additional constraints
in the hunt for habitable, Earth-like exo-planets.
|
2104.10612v1
|
2021-04-22
|
Hund's metal crossover and superconductivity in the 111 family of iron-based superconductors
|
We study LiFeP, LiFeAs and NaFeAs in their paramagnetic metallic phase
including dynamical electronic correlations within a density functional theory
+ slave-spin mean-field framework. The three compounds are found to lie next to
the crossover between a normal and a Hund's metal, where a region of enhanced
electronic compressibility that may boost superconductivity is systematically
present in this type of systems. We find that LiFeP lies in the normal metallic
regime, LiFeAs at the crossover, and NaFeAs is in the Hund's metal regime,
which possibly explains the different experimental trends for the pressure- and
doping-dependence of superconductivity in these compounds. Our picture captures
the orbitally-resolved mass renormalizations measured in these materials, while
an analysis of the Sommerfeld specific-heat coefficient highlights some
limitations of currently used implementations of density-functional theory for
the correct prediction of the details of band structures in the iron-based
superconductors.
|
2104.11018v1
|
2021-05-03
|
A Bayesian Method for Estimating Uncertainty in Excavated Material
|
This paper proposes a method to probabilistically quantify the moments (mean
and variance) of excavated material during excavation by aggregating the prior
moments of the grade blocks around the given bucket dig location. By modelling
the moments as random probability density functions (pdf) at sampled locations,
a formulation of the sums of Gaussian based uncertainty estimation is presented
that jointly estimates the location pdfs, as well as the prior values for
uncertainty coming from ore body knowledge (obk) sub block models. The moments
calculated at each random location is a single Gaussian and they are the
components of Gaussian mixture distribution. The overall uncertainty of the
excavated material at the given bucket location is represented by the Gaussian
Mixture Model (GMM) and therefore moment matching method is proposed to
estimate the moments of the reduced GMM. The method was tested in a region at a
Pilbara iron ore deposit situated in the Brockman Iron Formation of the
Hamersley Province, Western Australia, and suggests a frame work to quantify
the uncertainty in the excavated material that hasn't been studied anywhere in
the literature yet.
|
2105.00600v1
|
2021-05-19
|
Thermal conductivity of iron and nickel during melting: Implication to Planetary liquid outer core
|
We report the measurements of the thermal conductivity ($\kappa$) of iron
(Fe) and nickel (Ni) at high pressures and high temperatures. $\kappa$ values
are estimated from the temperature measurements across the sample surface in a
laser heated diamond anvil cell (LHDAC) and using the COMSOL software.
Near-isothermal $\kappa$'s are observed to increase with pressure in both the
metals due to the increase of density of the pressed metals. In both metals
$\kappa$'s are observed to follow a sharp fall during melting at different
pressure points and are consistence with the other multi-anvil measurements.
Constant values of $\kappa$ in these metals during melting at different
pressures reveal the loss of long range order, which creates independent
movement of atomic metals. The melting temperature measured in these metals
from the sudden drop of $\kappa$-values are in a good agreement with the other
melting measurements in LHDAC. The results obtained in this study is expected
to provide an insight to the studies on the planets Mercury and Mars and their
interior.
|
2105.08962v1
|
2021-05-19
|
Origin of insulating ferromagnetism in iron oxychalcogenide Ce$_2$O$_2$FeSe$_2$
|
An insulating ferromagnetic (FM) phase exists in the quasi-one-dimensional
iron chalcogenide Ce$_2$O$_2$FeSe$_2$ but its origin is unknown. To understand
the FM mechanism, here a systematic investigation of this material is provided,
analyzing the competition between ferromagnetic and antiferromagnetic
tendencies and the interplay of hoppings, Coulomb interactions, Hund's
coupling, and crystal-field splittings. Our intuitive analysis based on
second-order perturbation theory shows that large entanglements between
doubly-occupied and half-filled orbitals play a key role in stabilizing the FM
order in Ce$_2$O$_2$FeSe$_2$. In addition, via many-body computational
techniques applied to a multi-orbital Hubbard model, the phase diagram confirms
the proposed FM mechanism, in agreement with experiments.
|
2105.09239v2
|
2021-05-24
|
Unveiling non-Abelian statistics of vortex Majorana bound states in iron-based superconductors using fermionic modes
|
Motivated by the recent experiments that reported the discovery of vortex
Majorana bound states (vMBSs) in iron-based superconductors, we establish a
portable scheme to unveil the non-Abelian statistics of vMBSs using normal
fermionic modes. The unique non-Abelian statistics of vMBSs is characterized by
the charge flip signal of the fermions that can be easily read out through the
charge sensing measurement. In particular, the charge flip signal will be
significantly suppressed for strong hybridized vMBSs or trivial vortex modes,
which efficiently identifies genuine vMBSs. To eliminate the error induced by
the unnecessary dynamical evolution of the fermionic modes, we further propose
a correction strategy by continually reversing the energy of the fermions,
reminiscent of the quantum Zeno effect. Finally, we establish a feasible
protocol to perform non-Abelian braiding operations on vMBSs.
|
2105.11199v1
|
2021-05-24
|
Redox hysteresis of super-Earth exoplanets from magma ocean circulation
|
Internal redox reactions may irreversibly alter the mantle composition and
volatile inventory of terrestrial and super-Earth exoplanets and affect the
prospects for atmospheric observations. The global efficacy of these
mechanisms, however, hinges on the transfer of reduced iron from the molten
silicate mantle to the metal core. Scaling analysis indicates that turbulent
diffusion in the internal magma oceans of sub-Neptunes can kinetically entrain
liquid iron droplets and quench core formation. This suggests that the chemical
equilibration between core, mantle, and atmosphere may be energetically limited
by convective overturn in the magma flow. Hence, molten super-Earths possibly
retain a compositional memory of their accretion path. Redox control by magma
ocean circulation is positively correlated with planetary heat flow, internal
gravity, and planet size. The presence and speciation of remanent atmospheres,
surface mineralogy, and core mass fraction of atmosphere-stripped exoplanets
may thus constrain magma ocean dynamics.
|
2105.11208v1
|
2021-05-27
|
Iron phthalocyanine on Au(111) is a "non-Landau" Fermi liquid
|
The paradigm of Landau's Fermi liquid theory has been challenged with the
finding of a strongly interacting Fermi liquid that cannot be adiabatically
connected to a non-interacting system. A spin-1 two-channel Kondo impurity with
anisotropy D has a quantum phase transition between two topologically different
Fermi liquids with a peak (dip) in the Fermi level for D < Dc (D > Dc).
Extending this theory to general multi-orbital problems with finite magnetic
field, we reinterpret in a unified and consistent fashion several experimental
studies of iron phthalocyanine molecules on Au(111) that were previously
described in disconnected and conflicting ways. The differential conductance
shows a zero-bias dip that widens when the molecule is lifted from the surface
(reducing the Kondo couplings) and is transformed continuously into a peak
under an applied magnetic field. We reproduce all features and propose an
experiment to induce the topological transition.
|
2105.13248v2
|
2021-06-04
|
Finite element stress analysis of a combined stacker-reclaimer machine: A design audit report
|
Design audit or design verification is an important step in engineering of
heavy mobile materials handling equipment. Usually, the costumers employ third
parties for audition of contractors engineering. Here a part of design audit of
a combined stacker-reclaimer machine is reported. This equipment is designed
and constructed by a local supplier in Iran for the iron ore pelletizing plants
at GOHARZAMIN Iron Ore Company. The structure plays an important role in mobile
material handling machines such as Stackers and Reclaimers and its failure and
damage may cause considerable financial and human life losses. In this report
the undercarriage of stacker-reclaimer machine including gantry and traveling
system are numerically analyzed. The Finite Element Method is used for stress
prediction under the critical operating loads according to the design
standards. The critical areas of the undercarriage are identified and it is
observed that, the maximum stress is in the safe range.
|
2106.02291v1
|
2021-06-16
|
Random iron-nickel alloys: From first principles to dynamic spin fluctuation theory
|
We provide a systematic analysis of finite-temperature magnetic properties of
random alloys Fe$_x$Ni$_{1-x}$ with the face-centered-cubic structure over a
broad concentration range $x$. By means of the spin-polarized relativistic
Korringa-Kohn-Rostoker method we calculate the electronic structure of
disordered iron-nickel alloys and discuss how a composition change affects
magnetic moments of Fe and Ni and the density of states. We investigate how the
Curie temperature depends on Fe concentration using conventional approaches,
such as mean-field approximation or Monte Carlo simulations, and dynamic
spin-fluctuation theory. Being devised to account for spin fluctuations
explicitly, the latter method shows the best fit to experimental results.
|
2106.08765v2
|
2021-06-22
|
Quantum-confined charge transfer that enhances magnetic anisotropy in lanthanum M-type hexaferrites
|
Iron-based hexaferrites are critical-element-free permanent magnet components
of magnetic devices. Of particular interest is electron-doped M-type
hexaferrite i.e., LaFe$_{12}$O$_{19}$ (LaM) in which extra electrons introduced
by lanthanum substitution of barium/strontium play a key role in uplifting the
magnetocrystalline anisotropy. We investigate the electronic structure of
lanthanum hexaferrite using a \textit{localized} density functional theory
which reproduces semiconducting behavior and identifies the origin of the very
large magnetocrystalline anisotropy. Localized charge transfer from lanthanum
to the iron at the crystal's $2a$ site produces a narrow $3d_{z^2}$ valence
band strongly locking the magnetization along the $c$ axis. The calculated
uniaxial magnetic anisotropy energies from fully self-consistent calculations
are nearly double the single-shot values, and agree well with available
experiments. The chemical similarity of lanthanum to other rare earths suggests
that LaM can host for other rare earths possessing non-trivial $4f$ electronic
states for, \textit{e.g.,} microwave-optical quantum transduction.
|
2106.11947v1
|
2021-07-02
|
Breakdown of the Hund's Rule in CuFeAs
|
The ground-state properties of CuFeAs were investigated by applying density
functional theory calculations within generalized gradient approximation (GGA)
and GGA+U. We find that the bicollinear antiferromagnetic state with
antiparallel orbital magnetic moments on each iron which violates the Hund's
rule is favored by the on-site Coulomb interaction, which is further stabilized
by Cu vacancy. The magnetic ground state can be used to understand weak
antiferromagnetism in CuFeAs observed experimentally. We argue that breakdown
of the Hund's rule may be the possible origin for reduced magnetism in iron
pnictides, rather than magnetic fluctuations induced by electronic
correlations.
|
2107.01014v1
|
2021-07-06
|
The role of orbital nesting in the superconductivity of Iron-based Superconductors
|
We analyze the magnetic excitations and the spin-mediated superconductivity
in iron-based superconductors within a low-energy model that operates in the
band basis but fully incorporates the orbital character of the spin
excitations. We show how the orbital selectivity, encoded in our low-energy
description, simplifies substantially the analysis and allows for analytical
treatments, while retaining all the main features of both spin-excitations and
gap functions computed using multiorbital models. Importantly, our analysis
unveils the orbital matching between the hole and electron pockets as the key
parameter to determine the momentum-dependence and the hierarchy of the
superconducting gaps, instead of the Fermi surface matching as in the common
nesting scenario.
|
2107.02825v2
|
2021-07-14
|
Phase-manipulation-induced Majorana Mode and Braiding Realization in Iron-based Superconductor Fe(Te,Se)
|
Recent experiment reported the evidence of dispersing one-dimensional
Majorana mode trapped by the crystalline domain walls in
FeSe$_{0.45}$Te$_{0.55}$. Here, we perform the first-principles calculations to
show that iron atoms in the domain wall spontaneously form the ferromagnetic
order in line with orientation of the wall. The ferromagnetism can impose a
$\pi$ phase difference between the domain-wall-separated surface
superconducting regimes under the appropriate width and magnetization of the
wall. Accordingly, the topological surface superconducting state of
FeSe$_{0.45}$Te$_{0.55}$ can give rise to one-dimensional Majorana modes
trapped by the wall. More interestingly, we further propose a surface junction
in the form of FeSe$_{0.45}$Te$_{0.55}$/ferromagnet/FeSe$_{0.45}$Te$_{0.55}$,
which can be adopted to create and fuse the Majorana zero modes through
controlling the width or magnetization of the interior ferromagnetic barrier.
The braiding and readout of Majorana zero modes can be realized by the designed
device. Such surface junction has the potential application in the
superconducting topological quantum computation.
|
2107.06558v3
|
2021-07-15
|
Multipolar nematic state of nonmagnetic FeSe based on the DFT+$U$
|
Clarifying the origin of nematic state in FeSe is one of urgent problems in
the field of iron-based superconductivity. Motivated by the discovery of a
nematic solution in the density-functional theory implemented by on-site
Coulomb interaction (DFT+$U$) [npj Quantum Mater. \textbf{5}, 50 (2020)], we
reexamine the $U$ dependence of electronic states in the nonmagnetic normal
state of FeSe and perform full multipolar analyses for the nematic state. We
find that with increasing $U$ the normal state experiences a topological change
in the Fermi surfaces before the emergence of a nematic ground state. The
resulting nematic ground state is a multipolar state having both
antiferrohexadecapoles in the $E$ representation and ferromultipoles in the
$B_2$ representation on each Fe site. Cooperative coupling between the $E$ and
the $B_2$ multipoles in the local coordinate with the $D_{2d}$ point group will
play an important role in the formation of the $d_{xz},~d_{yz}$
orbital-splitting nematic state not only in FeSe, but also in other iron
pnictides.
|
2107.07244v3
|
2021-07-16
|
Discovery and implications of hidden atomic-scale structure in a metallic meteorite
|
Iron and its alloys have made modern civilisation possible, with metallic
meteorites providing one of the human's earliest sources of usable iron as well
as providing a window into our solar system's billion-year history. Here
highest-resolution tools reveal the existence of a previously hidden FeNi
nanophase within the extremely slowly cooled metallic meteorite NWA 6259. This
new nanophase exists alongside Ni-poor and Ni-rich nanoprecipitates within a
matrix of tetrataenite, the uniaxial, chemically ordered form of FeNi. The
ferromagnetic nature of the nanoprecipitates combined with the
antiferromagnetic character of the FeNi nanophases give rise to a complex
magnetic state that evolves dramatically with temperature. These observations
extend and possibly alter our understanding of celestial metallurgy, provide
new knowledge concerning the archetypal Fe-Ni phase diagram and supply new
information for the development of new types of sustainable, technologically
critical high-energy magnets.
|
2107.07909v1
|
2021-07-24
|
Controllable Majorana vortex states in iron-based superconducting nanowires
|
There has been experimental evidence for the Majorana zero modes (MZMs) in
solid state systems, which are building blocks for potential topological
quantum computing. It is important to design devices, in which MZMs are easy to
manipulate and possess a broad topological non-trivial parameter space for
fusion and braiding. Here, we propose that the Majorana vortex states in
iron-based superconducting nanowires fulfill these desirable conditions. This
system has a radius-induced topological phase transition, giving a lower limit
to the radius of the nanowire. In the topological phase, there is only one pair
of MZMs in the nanowire over a wide range of radius, chemical potential, and
external magnetic field. The wavefunction of the MZM has a sizable distribution
at the side edge of the nanowire. This property enables one to control the
interaction of the MZMs in neighboring vortex nanowires, and paves the way for
Majorana fusion and braiding.
|
2107.11562v1
|
2021-08-07
|
Strong Momentum-Dependent Electron-Magnon Renormalization of a Surface Resonance on Iron
|
The coupling of fermionic quasiparticles to magnons is essential for a wide
range of processes, from ultrafast magnetization dynamics in ferromagnets to
Cooper pairing in superconductors. Although magnon energies are generally much
larger than phonon energies, up to now their electronic band renormalization
effect in ferromagnetic metals suggests a significantly weaker quasiparticle
interaction. Here, using spin- and angle-resolved photoemission, we show an
extraordinarily strong renormalization leading to replica-band formation of an
iron surface resonance at ~200 meV. Its strong magnetic linear dichroism
unveils the magnetic nature and momentum dependence of the energy
renormalization. By determining the frequency- and momentum-dependent
self-energy due to generic electron-boson interaction to compute the resultant
electron spectral function, we show that the surface-state replica formation is
consistent with strong coupling to an optical spin wave in a Fe thin film.
|
2108.03421v1
|
2021-08-13
|
Discovery of mesoscopic nematicity wave in iron-based superconductors
|
Nematicity is ubiquitous in electronic phases of high transition temperature
superconductors, particularly in iron-based superconductors (IBSCs). Order
parameter that characterizes the nematic phase has been investigated in
momentum space, but its real-space arrangement remains largely unclear. We use
linear dichroism (LD) in low-temperature laser-photoemission electron
microscope to map out the nematic order parameter of nonmagentic FeSe and
antiferromagnetic BaFe2(As0.87P0.13)2. In contrast to the structural domains
that have atomic-scale domain walls, the LD patterns in both materials show
peculiar sinusoidal waves of electronic nematicity with mesoscopic wavelength.
The analysis reveals that the nematic order has an extremely long coherence
length, more than 1000 times longer than the unit cell. Our direct
visualization of electronic spatial variation uncovers a new fundamental aspect
of quantum liquid crystalline states of correlated electrons in IBSCs.
|
2108.06122v3
|
2021-08-17
|
Importance of the many-body effects for structural properties of the novel iron oxide: Fe$_2$O
|
The importance of many-body effects on electronic and magnetic properties and
stability of different structural phases was studied in novel iron oxide -
Fe$_2$O. It was found that while Hubbard repulsion hardly affects the
electronic spectrum of this material ($m^*/m \sim 1.2$), but it strongly
changes its phase diagram shifting critical pressures of structural transitions
to much lower values. Moreover, one of the previously obtained in the density
functional theory (DFT) structures (P$\bar 3$m1) becomes energetically unstable
if many-body effects are taken into consideration. It is shown that this is an
account of magnetic moment fluctuations in the DFT+DMFT approach, which
strongly contributes to modification of the phase diagram of Fe$_2$O.
|
2108.07645v1
|
2021-08-26
|
Magnetic design study of coil-dominated superconducting quadrupole magnets based on racetrack coils
|
Several coil structures have been used in accelerator superconducting
quadrupole magnets, and cos2{\theta} quadrupole magnets are the most mature in
theoretical research and engineering applications. However, the cos2{\theta}
quadrupole magnet has a complicated coil structure, especially at the end of
the coil, which makes it difficult to apply strain-sensitive high-temperature
superconductors. Racetrack quadrupole magnets are friendly to high-temperature
superconductors. Field strength of iron-dominated racetrack magnets is limited
by the magnetic saturation of the iron poles. Therefore, coil-dominated
racetrack quadrupole magnets with simple geometry have become the focus of our
research. In this paper, analytical expressions of the magnetic field harmonics
related to racetrack quadrupole coil parameters are obtained. These expressions
are used to find the solution of coil geometry parameters with field harmonics
on the order of 10-4. Then, examples are given to build ideal quadrupole model
and verify the theoretical formulas. Next, the design and optimization of
example racetrack quadrupole magnets are completed in ROXIE. Finally, the
advantages and disadvantages of the racetrack coils and the cos2{\theta} coils
are compared and discussed.
|
2108.11643v1
|
2021-09-13
|
Pseudospin-Triplet Pairing in Iron-Chalcogenide Superconductors
|
We study superconductivity of electron systems with both spin and
pseudospin-1/2 degrees of freedom. By solving linearized gap equations, we
derive a weak coupling criterion for the even-parity spin-singlet
pseudospin-triplet pairing. It can generally mix with the on-site s-wave
pairing since both of them belong to the same symmetry representation
($A_{1g}$) and their mixture could naturally give rise to anisotropic
intra-band pairing gap functions with or without nodes. This may directly
explain why some of the iron-chalcogenide superconductors are fully gapped
(e.g. FeSe thin film) and some have nodes (e.g. LaFePO and LiFeP). We also find
that the anisotropy of gap functions can be enhanced when the principal
rotation symmetry is spontaneously broken in the normal state such as
nematicity, and the energetic stabilization of pseudospin-triplet pairings
indicates the coexistence of nematicity and superconductivity. This could be
potentially applied to bulk FeSe, where gap anisotropy has been experimentally
observed.
|
2109.06039v4
|
2021-09-15
|
Exploring DFT$+U$ parameter space with a Bayesian calibration assisted by Markov chain Monte Carlo sampling
|
Density-functional theory is widely used to predict the physical properties
of materials. However, it usually fails for strongly correlated materials. A
popular solution is to use the Hubbard corrections to treat strongly correlated
electronic states. Unfortunately, the exact values of the Hubbard $U$ and $J$
parameters are initially unknown, and they can vary from one material to
another. In this semi-empirical study, we explore the $U$ and $J$ parameter
space of a group of iron-based compounds to simultaneously improve the
prediction of physical properties (volume, magnetic moment, and bandgap). We
used a Bayesian calibration assisted by Markov chain Monte Carlo sampling for
three different exchange-correlation functionals (LDA, PBE, and PBEsol). We
found that LDA requires the largest $U$ correction. PBE has the smallest
standard deviation and its $U$ and $J$ parameters are the most transferable to
other iron-based compounds. Lastly, PBE predicts lattice parameters reasonably
well without the Hubbard correction.
|
2109.07617v1
|
2021-09-16
|
Zero-energy Andreev bound states in iron-based superconductor Fe(Te,Se)
|
Majorana bound states have been predicted to exist in vortices of topological
superconductors (SC). A realization of the Fu-Kane model, based on a
three-dimensional topological insulator brought into proximity to an $s$-wave
SC, in iron-based SC Fe(Te,Se) has attracted strong interest after pronounced
zero-energy bias peaks were observed in several experiments. Here, we show
that, by taking into account inhomogeneities of the chemical potential or the
presence of potential impurities on the surface of Fe(Te,Se), the emergence of
these zero-energy bias peaks can be explained by trivial Andreev bound states
(ABSs) whose energies are close to zero. Our numerical simulations reveal that
the ABSs behave similarly to Majorana bound states. ABSs are localized only on
the, say, top surface and cannot be distinguished from their topological
counterparts in transport experiments performed with STM tips. Thus, such ABSs
deserve a careful investigation of their own.
|
2109.08200v1
|
2021-09-29
|
High-performance Ba1-xKxFe2As2 superconducting joints for persistent current operation
|
Superconducting joints are one of the key technologies to make Ba1-xKxFe2As2
(Ba-122) superconducting wires or tapes for high-field applications. Herein,
superconducting joints were fabricated by a simple cold-pressing method, and
the joint resistance of the iron-based superconducting joint was estimated for
the first time. The superconducting properties, microstructures, and elements
distribution in the joint regions were investigated. At 4.2 K and 10 T, a
transport critical current Ic of 105 A for the joint was obtained, and the
critical current ratio (CCR= Ic-joint/Ic-tape) of the joint was 94.6%. On the
other hand, the joint show very low joint resistance of 2.7x10^-13 ohm in
self-field at 4.2 K. Among iron-based superconductors (IBS), this work is the
first to successfully realize a superconducting joint with such high CCR and
low joint resistance. This work shows great potential to apply Ba-122 in a
range of practical applications, where superconducting joints are essential.
|
2109.14300v1
|
2021-10-06
|
A New Weakly Supervised Learning Approach for Real-time Iron Ore Feed Load Estimation
|
Iron ore feed load control is one of the most critical settings in a mineral
grinding process, directly impacting the quality of final products. The setting
of the feed load is mainly determined by the characteristics of the ore
pellets. However, the characterisation of ore is challenging to acquire in many
production environments, leading to poor feed load settings and inefficient
production processes. This paper presents our work using deep learning models
for direct ore feed load estimation from ore pellet images. To address the
challenges caused by the large size of a full ore pellets image and the
shortage of accurately annotated data, we treat the whole modelling process as
a weakly supervised learning problem. A two-stage model training algorithm and
two neural network architectures are proposed. The experiment results show
competitive model performance, and the trained models can be used for real-time
feed load estimation for grind process optimisation.
|
2110.04063v1
|
2021-10-13
|
Oxygen adsorption induced superconductivity in ultrathin FeTe film on SrTiO3(001)
|
The phenomenon of oxygen incorporation induced superconductivity in iron
telluride (Fe1+yTe, with antiferromagnetic (AFM) orders) is intriguing and
quite different from the case of FeSe. Until now, the microscopic origin of the
induced superconductivity and the role of oxygen are far from clear. Here, by
combining in-situ scanning tunneling microscopy/spectroscopy (STM/STS) and
x-ray photoemission spectroscopy (XPS) on oxygenated FeTe, we found physically
adsorbed O2 molecules crystallized into c(2/3x2) structure as an oxygen
overlayer at low temperature, which was vital for superconductivity. The O2
overlayer were not epitaxial on the FeTe lattice, which implied weak O2-FeTe
interaction but strong molecular interactions. Energy shift observed in the STS
and XPS measurements indicated hole doping effect from the O2 overlayer to the
FeTe layer, leading to a superconducting gap of 4.5 meV opened across the Fermi
level. Our direct microscopic probe clarified the role of oxygen on FeTe and
emphasized the importance of charge transfer effect to induce superconductivity
in iron-chalcogenide thin films.
|
2110.06417v1
|
2021-10-20
|
High-entropy ejecta plumes in Cassiopeia A from neutrino-driven convection
|
Recent multi-dimensional simulations suggest that high-entropy buoyant plumes
help massive stars to explode. Outwardly protruding iron-rich fingers in the
galactic supernova remnant Cassiopeia A are uniquely suggestive of this
picture. Detecting signatures of specific elements synthesized in the
high-entropy nuclear burning regime (i.e., $\alpha$-rich freeze out) would be
among the strongest substantiating evidence. Here we report the discovery of
such elements, stable Ti and Cr, at a confidence level greater than 5$\sigma$
in the shocked high-velocity iron-rich ejecta of Cassiopeia A. We found the
observed Ti/Fe and Cr/Fe mass ratios require $\alpha$-rich freeze out,
providing the first observational demonstration for the existence of
high-entropy ejecta plumes that boosted the shock wave at explosion. The metal
composition of the plumes agrees well with predictions for strongly
neutrino-processed proton-rich ejecta. These results support the operation of
the convective supernova engine via neutrino heating in the supernova that
produced Cassiopeia A.
|
2110.10384v1
|
2021-10-26
|
Tunable discontinuous shear thickening in capillary flow of MR suspensions
|
Very concentrated suspensions of iron particles in water or ethylene glycol
can be obtained thanks to the use of superplasticizer molecules used in cement
industry. At high volume fractions, these suspensions show a discontinuous
shear thickening which was thoroughly characterized in rotational geometries.
We will show that the jamming transition is also present in a capillary flow,
and that it manifests through the formation of a non-consolidated porous medium
at the constriction between the barrel and the capillary. In suspension of iron
particles, the dynamics of formation of this porous medium, and so the
pressure, can be controlled by a low magnetic field and is reversible for a
constant volume flow rate, opening potential new applications in the domain of
dampers and force control devices.
|
2110.13759v1
|
2021-12-04
|
Quantitative relationship between structural orthorhombicity, shear modulus and heat capacity anomaly of the nematic transition in iron-based superconductors
|
Electronic nematicity in iron pnictide materials has been extensively studied
by various experimental techniques, yet its heat capacity anomaly at the phase
transition has not been examined quantitatively. In this work we review the
thermodynamic description of nematicity in $Ba(Fe_{1-x}Co_{x})_{2}As_{2}$ using
the Landau free energy, which defines the behavior of three thermodynamic
quantities: the structural orthorhombicity that develops below the nematic
transition, the softening shear modulus above the transition, and the
discontinuous heat capacity at the transition. We derive a quantitative
relationship between these three quantities, which is found to hold for a range
of dopings. This result shows that the nematic transition is exceedingly well
described by a mean-field model in the underdoped regime of the phase diagram.
|
2112.02239v2
|
2021-12-23
|
The Three-Dimensional Collapse of a Rapidly Rotating 16 $M_{\odot}$ Star
|
We report on the three-dimensional (3D) hydrodynamic evolution to iron
core-collapse of a rapidly rotating 16 $M_{\odot}$ star. For the first time, we
follow the 3D evolution of the angular momentum (AM) distribution in the iron
core and convective shell burning regions for the final 10 minutes up to and
including gravitational instability and core-collapse. In 3D, we find that
convective regions show efficient AM transport that leads to an AM profile that
differs in shape and magnitude from $\texttt{MESA}$ within a few shell
convective turnover timescales. For different progenitor models, such as those
with tightly coupled Si/O convective shells, efficient AM transport in 3D
simulations could lead to a significantly different AM distribution in the
stellar interior affecting estimates of the natal neutron star or black hole
spin. Our results suggest that 3D AM transport in convective and rotating shell
burning regions are critical components in models of massive stars and could
qualitatively alter the explosion outcome and inferred compact remnant
properties.
|
2112.12800v1
|
2021-12-28
|
Cluster structure of superconducting phase and the nature of peaks in the doping dependences of the London penetration depth in iron pnictides
|
The mechanism of formation of dome-like phase diagrams and the features of
magnetic field penetration in iron pnictides with hetero- and isovalent doping
are considered within the framework of the previously proposed model, which
assumes the local character of doping and the cluster structure of resulted
superconducting phase. It is shown that the proposed model, despite its
simplicity and neglect of the features of the electronic structure, makes it
possible not only to accurately calculate the positions of superconducting
domes on the phase diagrams of specific HTSC compounds, but also to explain the
nature and position of sharp peaks in the London penetration depth depending on
the doping level.
|
2112.14167v2
|
2022-01-18
|
The Wrought Iron Beauty of Poncelet Loci
|
We've built a web-based tool for the real-time interaction with loci of
Poncelet triangle families. Our initial goals were to facilitate exploratory
detection of geometric properties of such families. During frequent walks in my
neighborhood, it appeared to me Poncelet loci shared a palette of motifs with
those found in wrought iron gates at the entrance of many a residential
building. As a result, I started to look at Poncelet loci aesthetically, a kind
of generative art. Features were gradually added to the tool with the sole
purpose of beautifying the output. Hundreds of interesting loci were
subsequently collected into an online "gallery", with some further enhanced by
a graphic designer. We will tour some of these byproducts here. An interesting
question is if Poncelet loci could serve as the basis for future metalwork
and/or architectural designs.
|
2201.06960v1
|
2022-01-18
|
In-situ alignment of anisotropic hard magnets of 3D printed magnets
|
Within this work, we demonstrate in-situ easy-axis alignment of
single-crystal magnetic particles inside a polymer matrix using fused filament
fabrication. Two different magnetic materials are investigated: (i) Strontium
hexaferrite inside a PA6 matrix, fill grade: 49 vol% and (ii) Samarium iron
nitride inside a PA12 matrix, fill grade: 44 vol%. In the presence of the
external alignment field, the strontium hexaferrite particles inside the PA6
matrix can be well aligned with a ratio of remanent magnetization to saturation
magnetization of 0.7. No significant alignment for samarium iron nitride could
be achieved. The results show the feasibility to fabricate magnets with
arbitrary and locally defined easy axis using fused filament fabrication since
the permanent magnets used for the alignment (or alternatively an
electromagnet) can be mounted on a rotatable platform.
|
2201.07111v1
|
2022-01-19
|
Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2
|
The 12442 compounds are a recently discovered family of iron-based
superconductors, that share several features with the cuprates due to their
strongly anisotropic structure, but are so far poorly understood. Here, we
report on the gap structure and anisotropy of RbCa2(Fe1-xNix)4As4F2 single
crystals, investigated by a combination of directional point-contact
Andreev-reflection spectroscopy and coplanar waveguide resonator measurements.
Two gaps were identified, with clear signatures of d-wave-like nodal structures
which persist upon Ni doping, well described by a two-band d-d state with
symmetry-imposed nodes. A large London penetration depth anisotropy was
revealed, weakly dependent on temperature and fully compatible with the d-d
model.
|
2201.07593v1
|
2022-01-24
|
Hund's coupling and electronic anisotropy in the spin-density wave state of iron pnictides
|
In the multiband systems, Hund's coupling ($J$) plays a significant role in
the spin and charge excitations. We study the dependence of electronic
anisotropy on $J$ in terms of Drude-weight along different directions as well
as the orbital order in the four-fold symmetry broken spin-density wave state
of iron pnictides. A robust behavior of the Drude-weight anisotropy within a
small window around $J \sim 0.25U$ with $U$ as intraorbital Coulomb interaction
is described in terms of orbital-weight distribution along the reconstructed
Fermi surfaces. We also find that the ferro-orbital order increases with $J$ in
the widely accepted regime for the latter, which is explained as a consequence
of rising exchange field with an increase in magnetization.
|
2201.09808v1
|
2022-01-24
|
Low-energy physics for an iron phthalocyanine molecule on Au(111)
|
The system of an iron phthalocyanine molecule on the Au(111) surface, has
been studied recently due to its peculiar properties. In particular, several
surprising results of scanning tunneling spectroscopy changing the position of
the molecule and applying magnetic field can be explained by the {\it
non-Landau} Fermi liquid state of a 2-channel spin-1 Kondo model with
anisotropy. The localized orbitals near the Fermi level are three, one of
symmetry $z^2$ and two (nearly) degenerate $\pi$ orbitals of symmetry $xz$ and
$yz$. Previous studies using the numerical renormalization group neglected one
of these orbitals to render the problem tractable. Here we investigate, using a
slave-boson mean-field approximation, if the splitting $S$ between $\pi$
orbitals caused by spin-orbit coupling (SOC) justifies this approximation. We
obtain an abrupt transition from a 3-band regime to a 2-band one at a value of
$S$ which is about 1/3 of the atomic SOC for Fe, justifying the 2-band model
for the system.
|
2201.10010v2
|
2022-01-25
|
Multiscale machine-learning interatomic potentials for ferromagnetic and liquid iron
|
We develop and compare four interatomic potentials for iron: a simple
machine-learned embedded atom method (EAM) potential, a potential with
machine-learned two- and three-body-dependent terms, a potential with
machine-learned EAM and three-body terms, and a Gaussian approximation
potential with the SOAP descriptor. All potentials are trained to the same
diverse database of body-centered cubic and liquid structures computed with
density functional theory. The four presented potentials represent different
levels of complexity and span three orders of magnitude in computational cost.
The first three potentials are tabulated and evaluated efficiently using cubic
spline interpolations, while the fourth one is implemented without additional
optimization. We compare and discuss the advantages of each implementation,
transferability and applicability in terms of the balance between required
accuracy versus computational cost.
|
2201.10237v1
|
2022-01-31
|
Mathematical analysis of a thermodynamically consistent reduced model for iron corrosion
|
We are interested in a reduced model for corrosion of iron, in which ferric
cations and electrons evolve in a fixed oxide layer subject to a
self-consistent electrostatic potential. Reactions at the boundaries are
modeled thanks to Butler-Volmer formulas, whereas the boundary conditions on
the electrostatic potential model capacitors located at the interfaces between
the materials. Our model takes inspiration in existing papers, to which we
bring slight modifications in order to make it consistent with thermodynamics
and its second principle. Building on a free energy estimate, we establish the
global in time existence of a solution to the problem without any restriction
on the physical parameters, in opposition to previous works. The proof further
relies on uniform estimates on the chemical potentials that are obtained thanks
to Moser iterations. Numerical illustrations are finally provided to highlight
the similarities and the differences between our new model and the one
previously studied in the literature.
|
2201.13193v1
|
2022-05-10
|
A Machine-Learned Spin-Lattice Potential for Dynamic Simulations of Defective Magnetic Iron
|
A machine-learned spin-lattice interatomic potential (MSLP) for magnetic iron
is developed and applied to mesoscopic scale defects. It is achieved by
augmenting a spin-lattice Hamiltonian with a neural network term trained to
descriptors representing a mix of local atomic configuration and magnetic
environments. It reproduces the cohesive energy of BCC and FCC phases with
various magnetic states. It predicts the formation energy and complex magnetic
structure of point defects in quantitative agreement with density functional
theory (DFT) including the reversal and quenching of magnetic moments near the
core of defects. The Curie temperature is calculated through spin-lattice
dynamics showing good computational stability at high temperature. The
potential is applied to study magnetic fluctuations near sizable dislocation
loops. The MSLP transcends current treatments using DFT and molecular dynamics,
and surpasses other spin-lattice potentials that only treat near-perfect
crystal cases.
|
2205.04732v1
|
2022-05-26
|
Elastoresistivity of heavily hole doped 122 iron pnictides superconductors
|
Nematicity in the heavily hole-doped iron pnictide superconductors remains
controversial. Sizeable nematic fluctuations and even nematic orders far from a
magnetic instability were declared in RbFe$_2$As$_2$ and its sister compounds.
Here we report a systematic elastoresistance study of series of isovalent- and
electron-doped KFe$_2$As$_2$ crystals. We found divergent elastoresistance upon
cooling for all the crystals along their [110] direction. The amplitude of
elastoresistivity diverges if K is substituted with larger ions or if the
system is driven towards a Lifshitz transition. However, we conclude none of
them necessarily indicates an independent nematic critical point. Instead, the
increased nematicity can be associated with another electronic criticality. In
particular, we propose a mechanism how elastoresistivity is enhanced at a
Lifshitz transition.
|
2205.13663v1
|
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