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2012-07-15 | Two-stage Hydrogen Compression Using Zr-based Metal Hydrides | Zr-based AB2-Laves phase type alloys containing the same type of A and B
metals, have been prepared from pure elements by melting and subsequent
re-melting under argon atmosphere by using a HF-induction levitation furnace.
Characterization of the alloys has resulted from powder X-Ray Diffraction (XRD)
measurements and SEM/EDX analyses. Systematic PCI
(Pressure-Composition-Isotherms) measurements have been recorded at 20 and 90
oC, using a high-pressure Sievert's type apparatus. The purpose of this study
is to find a series of alloys promptly forming metal hydrides (MH) with
suitable properties in order to build a MH-based hydrogen compressor, working
in the same way between 20 and ~100 oC. | 1207.3541v1 |
2012-08-30 | The anti-ordinary Hall effect in NiPt thin films | We study the anomalous Hall effect in binary alloys between the group-10
elements Ni and Pt. It is observed that the ordinary Hall effect is negative
(electron-like) at any composition of the alloy. The extraordinary Hall effect
is also negative except in the vicinity of the ferromagnetic quantum critical
point. Close to the critical point the sign of the extraordinary Hall effect
can be changed to positive (hole-like) by tuning either the temperature or the
composition of the alloy. We attribute such an "anti-ordinary" Hall effect with
opposite signs of the ordinary and the extraordinary contributions to a Berry
phase singularity, moving away from the Fermi energy with increasing the
ferromagnetic exchange energy. | 1208.6257v1 |
2012-10-02 | Microscopic Structural and Dynamical Properties of Amorphous Metallic Alloy $Ni_{33}Zr_{67}$ at the Temperature $T=300K$ | We study the structural properties and the collective microscopic dynamics of
atoms in the amorphous metallic alloy $Ni_{33}Zr_{67}$ at the temperature
$T=300K$ by molecular dynamics simulations. The calculated equilibrium
structural and dynamical characteristics are compared with the experimental
data on neutron diffraction and on inelastic X-ray scattering. We present the
interpretation of observed structural relaxation of the microscopic density
fluctuations of particles for amorphous metallic alloy in the framework of the
recurrent relation approach. The results of theoretical calculations of the
intensity of scattering $I(k,\omega)$ for amorphous $Ni_{33}Zr_{67}$ are in a
good agreement with the results of computer simulation as well as with the
experimental data on inelastic X-ray scattering. | 1210.0836v1 |
2012-12-31 | Effect of Ni on Cu precipitation kinetics in α-Fe by AKMC study | The kinetics of coherent Cu rich precipitation in Fe-Cu and Fe-Cu-Ni alloys
during thermal ageing have been modeled by Atomic Kinetic Monte Carlo method
(AKMC). The AKMC is parameterized by existing ab-inito data to treat vacancy
mediated diffusion which is depend on local atomic environment. A nonlinear
semi-empirical time adjusting method is proposed to rescaled the MC time. The
combining AKMC and time adjusting method give a good agreement with experiments
and other simulations, including advancement factor and the Cu cluster
mobility. Simulations of ternary alloys reveal Ni has a temporal delay effect
on Cu precipitation. This effect is caused by the decreasing of diffusion
coefficient of Cu clusters. And the reduction effect of diffusion coefficient
weakens with larger Cu cluster size. The simulation results can be used to
explain the experimental phenomenon that ternary Fe-Cu-Ni alloys have higher
cluster number density than corresponding binary during coarsening stage, which
is related to cluster mobility. | 1212.6900v3 |
2013-02-12 | Anomalous Surface Segregation Profiles in Ferritic FeCr Stainless Steel | The iron-chromium alloy and its derivatives are widely used for their
remarkable resistance to corrosion, which only occurs in a narrow concentration
range around 9 to 13 atomic percent chromium. Although known to be due to
chromium enrichment of a few atoms thick layer at the surfaces, the
understanding of its complex atomistic origin has been a remaining challenge.
We report an investigation of the thermodynamics of such surfaces at the atomic
scale by means of Monte Carlo simulations. We use a Hamiltonian which provides
a parameterization of previous ab initio results and successfully describes the
alloy's unusual thermodynamics. We report a strong enrichment in Cr of the
surfaces for low bulk concentrations, with a narrow optimum around 12 atomic
percent chromium, beyond which the surface composition decreases drastically.
This behavior is explained by a synergy between (i) the complex phase
separation in the bulk alloy, (ii) local phase transitions that tune the layers
closest to the surface to an iron-rich state and inhibit the bulk phase
separation in this region, and (iii) its compensation by a strong and
non-linear enrichment in Cr of the next few layers. Implications with respect
to the design of prospective nanomaterials are briefly discussed. | 1302.2838v1 |
2013-02-15 | Adsorption of silicon on Au(110): an ordered two dimensional surface alloy | We report on experimental evidence for the formation of a two dimensional
Si/Au(110) surface alloy. In this study, we have used a combination of scanning
tunneling microscopy, low energy electron diffraction, Auger electron
spectroscopy and ab initio calculations based on density functional theory. A
highly ordered and stable Si-Au surface alloy is observed subsequent to growth
of a sub-monolayer of silicon on an Au(110) substrate kept above the eutectic
temperature. | 1302.3703v1 |
2013-02-16 | Unipolar Vertical Transport in GaN/AlGaN/GaN Heterostructures | In this letter, we report on unipolar vertical transport characteristics in
c-plane GaN/AlGaN/GaN heterostructures. Vertical current in heterostructures
with random alloy barriers was found to be independent of dislocation density
and heterostructure barrier height, and significantly higher than theoretical
estimates. Percolation-based transport due to random alloy fluctuations in the
ternary AlGaN is suggested as the dominant transport mechanism, and confirmed
through experiments showing that non-random or digital AlGaN alloys and
polarization-engineered binary GaN barriers can eliminate percolation transport
and reduce leakage significantly. The understanding of vertical transport and
methods for effective control proposed here will greatly impact III-nitride
unipolar vertical devices. | 1302.3942v1 |
2013-03-20 | Disorder-induced significant enhancement in magnetization of ball-milled Fe2CrGa alloy | A new disordered atom configuration in Fe2CrGa alloy has been created by
ball-milling method. This leads to a significant enhancement of the magnetic
moment up to 3.2~3.9 {\mu}B and an increase of Curie temperature by about 200
K, compared with the arc-melt samples. Combination of first-principles
calculations and experimental results reveals that Fe2CrGa alloy should
crystallize in Hg2CuTi based structure with different atomic disorders for the
samples prepared by different methods. It is addressed that magnetic
interactions play a crucial role for the system to adopt such an atomic
configuration which disobeys the empirical rule. | 1303.4954v1 |
2013-04-04 | Surface parameters of ferritic iron-rich Fe-Cr alloy | Using first-principles density functional theory in the implementation of the
exact muffin-tin orbitals method and the coherent potential approximation, we
studied the surface energy and the surface stress of the thermodynamically most
stable surface facet (100) of the homogeneous disordered body-centred cubic
iron-chromium system in the concentration interval up to 20 at.% Cr. For the
low-index surface facets of Fe and Cr, the surface energy of Cr is slightly
larger than the one of Fe, while the surface stress of Cr is considerably
smaller than the one of Fe. We find that Cr addition to Fe generally increases
the surface energy of the Fe-Cr alloy, however, an increase of the bulk amount
of Cr also increases the surface stress. As a result of this unexpected trend,
the (100) surface of Fe-Cr becomes more stable against reconstruction with
increasing Cr concentration. We show that the observed trends are of magnetic
origin. In addition to the homogeneous alloy case, we also investigated the
impact of surface segregation on both surface parameters. | 1304.1278v2 |
2013-04-09 | Paramagnetic Fe_xTa_{1-x} alloys for engineering of perpendicularly magnetized tunnel junctions | Exchange coupling between two magnetic layers through an interlayer is of
broad interest for numerous recent applications of nano-magnetic systems. In
this Letter we study ferromagnetic exchange coupling through amorphous
paramagnetic Fe-Ta alloys. We show that the exchange coupling depends
exponentially on spacer thickness and scales with the Fe-Ta susceptibility,
which can be tuned via the alloy composition and/or temperature. Such materials
are of high interest for the engineering of perpendicularly magnetized
CoFeB-MgO based tunnel junctions as it enables ferromagnetic coupling of
magnetic layers with differing crystalline lattices, suppresses dead layers and
can act as an inter-diffusion barrier during annealing. | 1304.2763v1 |
2013-05-01 | Nanotube caps on Ni, Fe, and NiFe nano particles: A path to chirality selective growth | Carbon nanotubes have properties depending on the arrangement of carbon atoms
on the tube walls, called chirality. Also it has been tried to grow nanotubes
of only one chirality for more than a decade it is still not possible today. A
narrowing of the distribution of chiralities, however, which is a first step
towards chirality control, has been observed for the growth of nanotubes on
catalysts composed of nickel and iron. In this paper, we have calculated
carbon-metal bond energies, adhesion energies and charge distributions of
carbon nanotube caps on Ni, Fe and NiFe alloy clusters using density functional
theory. A growth model using the calculated energies was able to reproduce the
experimental data of the nanotube growth on the alloy catalysts. The electronic
charge was found to be redistributed from the catalyst particles to the edges
of the nanotube caps in dependence of the chiral angles of the caps increasing
the reactivity of the edge atoms. Our study develops an explanation for the
chirality enrichment in the carbon nanotube growth on alloy catalyst particles. | 1305.0145v1 |
2013-05-15 | The Dependence of Alloy Composition of InGaAs Inserts in GaAs Nanopillars on Selective-Area Pattern Geometry | GaAs nanopillars with 150 nm - 200 nm long axial InGaAs inserts are grown by
MOCVD via catalyst-free selective-area-epitaxy (SAE). The alloy composition of
the InGaAs region, as determined by room-temperature photoluminescence (PL),
depends critically on the pitch and diameter of the selective-area pattern
geometry. The PL emission varies based on pattern geometry from 1.0 \{mu}m to
1.25 \{mu}m corresponding to a In to Ga ratio from 0.15 to > 0.3. This In
enrichment is explained by a pattern dependent change in the incorporation rate
for In and Ga. Capture coefficients for Ga and In adatoms are calculated for
each pattern pitch. As the pitch decreases, these data reveal a contest between
a synergetic effect (related to nanopillar density) that increases the growth
rate and a competition for available material that limits the growth rate.
Gallium is more susceptible to both of these effects, causing the observed
changes in alloy composition. | 1305.3581v1 |
2013-06-06 | On the origin of the giant SHE in Cu(Bi) alloys | Two years after the prediction of a giant spin Hall effect for the dilute
Cu(Bi) alloy [Gradhand et al., Phys. Rev. B 81, 245109 (2010)], a comparably
strong effect was measured in thin films of Cu(Bi) alloys by Niimi et al.
[Phys. Rev. Lett. 109, 156602 (2012)]. Both theory and experiment consider the
skew-scattering mechanism to be responsible, however they obtain opposite sign
for the spin Hall angle. Based on a detailed analysis of existing theoretical
results, we explore differences between theory and experiment. | 1306.1471v1 |
2013-06-21 | Prediction of A2 to B2 Phase Transition in the High Entropy Alloy MoNbTaW | In this paper we show that an effective Hamiltonian fit with first principles
calculations predicts an order/disorder transition occurs in the high entropy
alloy MoNbTaW. Using the Alloy Theoretic Automated Toolset, we find T=0K
enthalpies of formation for all binaries containing Mo, Nb, Ta, and W, and in
particular we find the stable structures for binaries at equiatomic
concentrations are close in energy to the associated B2 structure, suggesting
that at intermediate temperatures a B2 phase is stabilized in MoNbTaW. Our
previously published hybrid Monte Carlo/molecular dynamics results for the
MoNbTaW system are analyzed to identify certain preferred chemical bonding
types. A mean field free energy model incorporating nearest neighbor bonds is
derived, allowing us to predict the mechanism of the order/disorder transition.
We find the temperature evolution of the system is driven by strong Mo-Ta
bonding. Comparison of the free energy model and our hybrid Monte
Carlo/molecular dynamics results suggest the existence of additional
low-temperature phase transitions in the system likely ending with phase
segregation into binary phases. | 1306.5043v1 |
2013-07-01 | Growth and electrical characterization of Al0.24Ga0.76As/AlxGa1-xAs/Al0.24Ga0.76As modulation-doped quantum wells with extremely low x | We report on the growth and electrical characterization of modulation-doped
Al0.24Ga0.76As/AlxGa1-xAs/Al0.24Ga0.76As quantum wells with mole fractions as
low as x=0.00057. Such structures will permit detailed studies of the impact of
alloy disorder in the fractional quantum Hall regime. At zero magnetic field,
we extract an alloy scattering rate of 24 ns-1 per %Al. Additionally we find
that for x as low as 0.00057 in the quantum well, alloy scattering becomes the
dominant mobility-limiting scattering mechanism in ultra-high purity
two-dimensional electron gases typically used to study the fragile nu=5/2 and
nu=12/5 fractional quantum Hall states. | 1307.0385v1 |
2013-07-10 | Cluster expansion made easy with Bayesian compressive sensing | Long-standing challenges in cluster expansion (CE) construction include
choosing how to truncate the expansion and which crystal structures to use for
training. Compressive sensing (CS), which is emerging as a powerful tool for
model construction in physics, provides a mathematically rigorous framework for
addressing these challenges. A recently-developed Bayesian implementation of CS
(BCS) provides a parameterless framework, a vast speed up over current CE
construction techniques, and error estimates on model coefficients. Here, we
demonstrate the use of BCS to build cluster expansion models for several binary
alloy systems. The speed of the method and the accuracy of the resulting fits
are shown to be far superior than state-of-the-art evolutionary methods for all
alloy systems shown. When combined with high throughput first-principles
frameworks, the implications of BCS are that hundreds of lattice models can be
automatically constructed, paving the way to high throughput thermodynamic
modeling of alloys. | 1307.2938v1 |
2013-07-11 | Microstructure evolution and heterogeneous nucleation in ternary Al-Cu-Ni alloys | The simulations of the solidification of ternary Al-Cu-Ni alloys by means of
a general multi-phase-field model for an arbitrary number of phases reveal that
the real microstructure can be generated by coupling the real thermodynamic
parameters of the phases and the evolution equations. The stability
requirements on individual interfaces for model functions guarantee an absence
of "ghost" phases in a $n$-dimensional phase-field space. The special
constructed thermal noise terms disturb the stability and can produce the
heterogeneous nucleation of product phases in accordance to the energetic and
concentration conditions. Of particular interest is that in triple points the
nucleation of the forth phase occurs without additional noise. Another
observation is the growth of the eutectic-like or peritectic-like structure in
various alloys. | 1307.3198v2 |
2013-07-25 | Near-forward Raman scattering by bulk and surface phonon-polaritons in the model percolation-type ZnBeSe alloy | We study the bulk and surface phonon-polaritons of the Zn0.67Be0.33Se
zincblende alloy by near-forward Raman scattering. The short (Be-Se) bond
exhibits a distinct percolation doublet in the conventional backscattering
Raman spectra, corresponding to a three-mode behavior in total
[1(Zn-Se),2(Be-Se)] for Zn0.67Be0.33Se. This offers an opportunity to achieve a
refined understanding of the phonon-polariton modes of a zincblende alloy
beyond the current two-mode approximation, corresponding to a
[1(Zn-Se),1(Be-Se)] description in the present case. The discussion is
supported by contour modeling of the Raman signals of the multi-mode bulk and
surface phonon-polaritons within the formalism of the linear dielectric
response. | 1307.6671v1 |
2013-08-26 | Stable ordered structures of binary technetium alloys from first principles | Technetium, element 43, is the only radioactive transition metal. It occurs
naturally on earth in only trace amounts. Experimental investigation of its
possible compounds is thus inherently difficult and limited. Half of the
Tc-transition-metal systems (14 out of 28) are reported to be phase separating
or lack experimental data. Using high-throughput first-principles calculations,
we present a comprehensive investigation of the binary alloys of technetium
with the transition metals. The calculations predict stable, ordered structures
in nine of these 14 binary systems. They also predict additional compounds in
all nine known compound-forming systems and in two of the five systems reported
with disordered \c{hi} or {\sigma} phases. These results accentuate the
incompleteness of the current knowledge of technetium alloys and should guide
experimental studies to obtain accurate structural information on potential
compounds, obviating some of the difficulties associated with such work. | 1308.5721v1 |
2013-09-24 | Interatomic potential for the compound-forming Li-Pb liquid alloy | Atomistic simulations of liquid alloys face the challenge of correctly
modeling basic thermodynamic properties. In this work we present an interatomic
potential for the Li-Pb system, as well as a study of physical properties of
Li-Pb alloys. Despite the complexity due to Li-Pb being a compound forming
system where charge transfer is expected, we show here how the empirical EAM
formalism is able to satisfactorily describe several physical properties in a
wide range of Li concentration. Application of our potential to Li-Pb eutectic
allows us to correctly predict many physical properties observed experimentally
and calculated with ab initio techniques, providing in this way a potential
suitable for future studies in the context of tritium breeder blanket designs
in Fusion technology. | 1309.6152v1 |
2013-11-11 | Magneto-electric and thermo-magneto-electric effects in ferromagnetic transition-metal alloys from first-principles | The electric and thermo-magneto-electric transport of the prototypical
ferromagnetic transition-metal alloy system fcc-Co$_x$Pd$_{1-x}$ has been
investigated on the basis of Kubo's linear response formalism. The results for
the full electric conductivity tensor allow to discuss the spin-orbit induced
anisotropic magneto-resistance (AMR) and the anomalous Hall effect (AHE). These
are complemented by results for the corresponding thermo-magneto-electric
transport properties anisotropy of the Seebeck effect (ASE) and anomalous
Nernst effect (ANE). The relation between the respective response coefficients
is discussed with the underlying electronic structure calculated
relativistically within the Korringa-Kohn-Rostoker coherent potential
approximation (KKR-CPA) band structure method for disordered alloys. | 1311.2498v1 |
2013-11-14 | Electron effective mass in Al$_{0.72}$Ga$_{0.28}$N alloys determined by mid-infrared optical Hall effect | The effective electron mass parameter in Si-doped Al$_{0.72}$Ga$_{0.28}$N is
determined to be $m^\ast=(0.336\pm0.020)\,m_0$ from mid-infrared optical Hall
effect measurements. No significant anisotropy of the effective electron mass
parameter is found supporting theoretical predictions. Assuming a linear change
of the effective electron mass with the Al content in AlGaN alloys and
$m^\ast=0.232\,m_0$ for GaN, an average effective electron mass of
$m^\ast=0.376\,m_0$ can be extrapolated for AlN. The analysis of mid-infrared
spectroscopic ellipsometry measurements further confirms the two phonon mode
behavior of the E$_1$(TO) and one phonon mode behavior of the A$_1$(LO) phonon
mode in high-Al-content AlGaN alloys as seen in previous Raman scattering
studies. | 1311.3684v1 |
2013-12-12 | Niobium Silicon alloys for Kinetic Inductance Detectors | We are studying the properties of Niobium Silicon amorphous alloys as a
candidate material for the fabrication of highly sensitive Kinetic Inductance
Detectors (KID), optimized for very low optical loads. As in the case of other
composite materials, the NbSi properties can be changed by varying the relative
amounts of its components. Using a NbSi film with T_c around 1 K we have been
able to obtain the first NbSi resonators, observe an optical response and
acquire a spectrum in the band 50 to 300 GHz. The data taken show that this
material has very high kinetic inductance and normal state surface resistivity.
These properties are ideal for the development of KID. More measurements are
planned to further characterize the NbSi alloy and fully investigate its
potential. | 1312.3588v1 |
2013-12-16 | Interface magnetic moments enhancement of FePt-L1$_0$/MgO(001): an ab-initio study | The interface between FePt--L1$_0$ and MgO(001) alloys has been studied using
density functional calculations. Because the stacking of the face-centered
tetragonal L1$_0$ phase is formed by alternating Fe and Pt planes, both the Fe-
and Pt-terminated contact layers were studied. Furthermore, due to the large
mismatch between the in-plane lattice constants of both systems, we have chosen
some common $a$ values for both alloys in order to explore in detail the
adsorption energy, the electronic structure and the interface magnetism. The
adsorption energy has been calculated by subtracting the energy of clean FePt
and MgO alloys from the total energy. The preferred adsorbed geometric sites
for Fe/Pt atoms are when they lie on $top$ of the O species, having a smaller
adsorption energy for the remaining positions. We found that expanding the MgO
lattice enhances the magnetic moment of the Fe species but the Pt moments
remain almost constant. | 1312.4588v1 |
2013-12-31 | Temperature and composition dependence of short-range order and entropy, and statistics of bond length: the semiconductor alloy (GaN)$_{1-x}$(ZnO)$_x$ | We present total energy and force calculations on the
(GaN)$_{1-x}$(ZnO)$_{x}$ alloy. Site-occupancy configurations are generated by
Monte Carlo (MC) simulations, based on a cluster expansion (CE) model proposed
in a previous study. Surprisingly large local atomic coordinate relaxations are
found by density-functional calculations using a 432-atom periodic supercell,
for three representative configurations at $x=0.5$. These are used to generate
bond length distributions. The configurationally averaged composition- and
temperature-dependent short-range order (SRO) parameters of the alloys are
discussed. Entropy is approximated in terms of pair distribution statistics and
thus related to SRO parameters. This approximate entropy is compared with
accurate numerical values from MC. An empirical model for the dependence of
bond length on local chemical environments is proposed. | 1401.0072v1 |
2014-02-10 | Probing atomic environments in alloys by electron spectroscopy | In alloys exhibiting substitutional disorder, the variety of atomic
environments manifests itself as a `disorder broadening' in their core level
binding energy spectra. Disorder broadening can be measured experimentally, and
in principle can be used to deduce information about specific atomic
environments within a sample. However, progress in this endeavor is hampered by
the lack of a model for this phenomenon which can treat complex systems. In
this work we describe such a model. The model is used to elucidate the
relationship between charge transfer, atomic environment, and disorder
broadening in complex systems, with a focus on the problem of characterizing
the interface quality of CuNi multilayers. We also validate the model against
the results of ab initio electronic structure calculations. Several
counterintuitive aspects of the disorder broadening phenomenon are uncovered,
an understanding of which is essential for the correct interpretation of
experimental results. For instance, it is shown that systems with inhomogeneous
concentration profiles can exhibit disorder broadenings significantly larger
than random alloys. Furthermore in some systems a `disorder narrowing' is even
possible. | 1402.2201v3 |
2014-02-13 | Enhanced spin Hall effect by electron correlations in CuBi alloys | A recent experiment in CuBi alloys obtained a large spin Hall angle (SHA) of
-0.24 (Niimi et al. Phys. Rev. Lett. 109, 156602 (2012)). We find that the SHA
can be dramatically enhanced by Bi impurities close to the Cu surface. The
mechanisms of this enhancement are two-fold. One is that the localized impurity
state on surface has a decreased hybridization and combined with Coulomb
correlation effect. The other comes from the low-dimensional state of
conduction electrons on surface, which results in a further enhancement of skew
scattering by impurities. Furthermore, we note that a discrepancy in sign of
SHA between the experiment and previous theories is simply caused by different
definitions of SHA. This re-establishes skew scattering as the essential
mechanism underlying the spin Hall effect in CuBi alloys. | 1402.3012v2 |
2014-02-23 | Effect of He-ion irradiation on short-range ordering in model (EFDA) Fe-Cr alloys | The effect of He-ion irradiation on a short-range ordering in model
Fe100-xCrx (x=5.8, 10.75, 15.15) was studied by means of conversion electrons
Mossbauer spectroscopy. The alloys were irradiated to the dose of 7.5 dpa with
ions of 0.25 and 2.0 MeV. The short-range order was expressed in terms of the
Warren-Cowley parameters: alpha1 (for the first nearest-neighbor shell), alpha2
(for the second nearest-neighbor shell) and alpha12 (for both shells). For all
alloys and both energies a clustering of Cr atoms was revealed, as far as
alpha1 and alpha2 are considered, yet its degree was found to be both
concentration and energy dependent. The average short-range order parameter,
alpha12, exhibits, however, a cross-over transition at x=10 from ordering into
clustering. The strongest effect of energy occurs at x=5.8 and the weakest one
at x=10.75. | 1402.5612v2 |
2014-03-25 | Energy scaling and branched microstructures in a model for shape-memory alloys with SO(2) invariance | Domain branching near the boundary appears in many singularly-perturbed
models for microstructure in materials and was first demonstrated
mathematically by Kohn and M\"uller for a scalar problem modeling the elastic
behavior of shape-memory alloys. We study here a model for shape-memory alloys
based on the full vectorial problem of nonlinear elasticity, including
invariance under rotations, in the case of two wells in two dimensions. We show
that, for two wells with two rank-one connections, the energy scales
proportional to the power $2/3$ of the surface energy, in agreement with the
scalar model. In a case where only one rank-one connection is present, we show
that the energy exhibits a different behavior, proportional to the power $4/5$
of the surface energy. This lower energy is achieved by a suitable interaction
of the two components of the deformations and hence cannot be reproduced by the
scalar model. Both scalings are proven by explicit constructions and matching
lower bounds. | 1403.6242v2 |
2014-03-27 | Experimental evidences of a large extrinsic spin Hall effect in AuW alloy | We report an experimental study of a gold-tungsten alloy (7% at. W
concentration in Au host) displaying remarkable properties for spintronics
applications using both magneto-transport in lateral spin valve devices and
spin-pumping with inverse spin Hall effect experiments. A very large spin Hall
angle of about 10% is consistently found using both techniques with the
reliable spin diffusion length of 2 nm estimated by the spin sink experiments
in the lateral spin valves. With its chemical stability, high resistivity and
small induced damping, this AuW alloy may find applications in the nearest
future. | 1403.7090v2 |
2014-05-16 | Unfolding the band structure of disordered solids: from bound states to high-mobility Kane fermions | Supercells are often used in ab initio calculations to model compound alloys,
surfaces and defects. One of the main challenges of supercell electronic
structure calculations is to recover the Bloch character of electronic
eigenstates perturbed by disorder. Here we apply the spectral weight approach
to unfolding the electronic structure of group III-V and II-VI semiconductor
solid solutions. The illustrative examples include: formation of donor-like
states in dilute Ga(PN) and associated enhancement of its optical activity,
direct observation of the valence band anticrossing in dilute GaAs:Bi, and a
topological band crossover in ternary (HgCd)Te alloy accompanied by emergence
of high-mobility Kane fermions. The analysis facilitates interpretation of
optical and transport characteristics of alloys that are otherwise ambiguous in
traditional first-principles supercell calculations. | 1405.4218v3 |
2014-05-29 | Sign change of the spin Hall effect due to electron correlation in nonmagnetic CuIr alloys | Recently a positive spin Hall angle (SHA) of 0.021, was observed
experimentally in nonmagnetic CuIr alloys [Niimi et al., Phys. Rev. Lett. 106,
126601 (2011)] and attributed predominantly to an extrinsic skew scattering
mechanism, while a negative SHA was obtained from ab initio calculations
[Fedorov et al., Phys. Rev. B 88, 085116 (2013)], using consistent definitions
of the SHA. We reconsider the SHA in CuIr alloys, with the effects of the local
electron correlation U in 5d orbitals of Ir impurities, included by the quantum
Monte Carlo method. We found that the SHA is negative if we ignore such local
electron correlation, but becomes positive once U approaches a realistic value.
This may open up a way to control the sign of the SHA by manipulating the
occupation number of impurities. | 1405.7449v3 |
2014-07-31 | Anomalous band-gap bowing of AlN(1-x)Px alloy | Electronic structure of zinc blende AlN(1-x)$Px alloy has been calculated
from first principles. Structural optimisation has been performed within the
framework of LDA and the band-gaps calculated with the modified Becke-Jonson
(MBJLDA) method. Two approaches have been examined: the virtual crystal
approximation (VCA) and the supercell-based calculations (SC). The composition
dependence of the lattice parameter obtained from the SC obeys Vegard's law
whereas the volume optimisation in the VCA leads to an anomalous bowing of the
lattice constant. A strong correlation between the band-gaps and the structural
parameter in the VCA method has been observed. On the other hand, in the SC
method the supercell size and atoms arrangement (clustered vs. uniform) appear
to have a great influence on the computed band-gaps. In particular, an
anomalously big band-gap bowing has been found in the case of a clustered
configuration with relaxed geometry. Based on the performed tests and obtained
results some general features of MBJLDA are discussed and its performance for
similar systems predicted. | 1407.8432v1 |
2014-08-04 | A Dual-Engine for Early Analysis of Critical Systems | This paper presents a framework for modeling, simulating, and checking
properties of critical systems based on the Alloy language -- a declarative,
first-order, relational logic with a built-in transitive closure operator. The
paper introduces a new dual-analysis engine that is capable of providing both
counterexamples and proofs. Counterexamples are found fully automatically using
an SMT solver, which provides a better support for numerical expressions than
the existing Alloy Analyzer. Proofs, however, cannot always be found
automatically since the Alloy language is undecidable. Our engine offers an
economical approach by first trying to prove properties using a
fully-automatic, SMT-based analysis, and switches to an interactive theorem
prover only if the first attempt fails. This paper also reports on applying our
framework to Microsoft's COM standard and the mark-and-sweep garbage collection
algorithm. | 1408.0707v1 |
2014-08-06 | Desarrollo de capas de AlN sobre aleaciones de aluminio como protección superficial contra la corrosión y el desgaste | Aluminum and their alloys, after careful treatments, can develop excellent
mechanical, tribological, electrical and chemical (high corrosion resistance)
properties. However, 7000 series, with Cu in the alloy, have a poor corrosion
resistance. On the other hand, aluminum nitride (AlN) has a wurtzitic phase and
good thermal stability, optical, electric, piezoelectric, mechanical,
tribological, and chemical properties, so the application range is huge. In
this paper, AlN deposition on Al 7075 was done using the reactive magnetron
sputtering technique in order to verify the aluminum nitride performance as an
Al alloy protective film under wear and corrosive processes. | 1408.1378v1 |
2014-08-28 | The Work Functions of Au/Mg Decorated Au(100), Mg(001), and AuMg Alloy Surfaces: A Theoretical Study | A plane-wave density functional theory is used to predict the work functions
of Au/Mg decorated Au(100), Mg(001), and stochiometric AuMg alloy surfaces. We
find, that irrespective of the details, all Au/Mg systems containing Mg on the
surface reveal the Mg-dominated work functions, i.e. significantly shifted
toward the work function of clean Mg(001) surface. The reported analyzes
suggest, that this general trend stems from a strong charge transfer from Mg to
Au and consequent enhancement of a surface dipole. The calculated properties of
the AuMg alloy well agree to the experiment. The reported results may readily
find application in Au/Mg/AuMg surface physics and technology of
metal/semiconductor contacts. | 1408.6636v1 |
2014-09-12 | Alloy inhomogeneity and carrier localization in AlGaN sections and AlGaN/AlN nanodisks in nanowires with 240-350 nm emission | The Al-Ga intermixing at Al(Ga)N/GaN interfaces in nanowires and the chemical
inhomogeneity in AlxGa1-xN/AlN nanodisks (NDs) are attributed to the strain
relaxation process. This interpretation is supported by the three-dimensional
strain distribution calculated by minimizing the elastic energy in the
structure. The alloy inhomogeneity increases with Al content, leading to
enhanced carrier localization signatures in their optical characteristics i.e.
red shift of the emission, s-shaped temperature dependence and linewidth
broadening. Despite these alloy fluctuations, the emission energy of AlGaN/AlN
NDs can be tuned in the 240-350 nm range with internal quantum efficiencies
around 30%. | 1409.3683v1 |
2014-09-19 | Magnetic anisotropy of Fe_1-yX_yPt-L10 [X=Cr,Mn,Co,Ni,Cu] bulk alloys | We demonstrate by means of fully relativistic first principles calculations
that, by substitution of Fe by Cr, Mn, Co, Ni or Cu in FePt-L10 bulk alloys,
with fixed Pt content, it is possible to tune the magnetocrystalline anisotropy
energy by adjusting the content of the non-magnetic species in the material.
The changes in the geometry due to the inclusion of each element induces
different values of the tetragonality and hence changes in the magnetic
anisotropy and in the net magnetic moment. The site resolved magnetic moments
of Fe increase with the X content whilst those of Pt and X are simultaneously
reduced. The calculations are in good quantitative agreement with experimental
data and demonstrate that models with fixed band structure but varying numbers
of electrons per unit cell are insufficient to describe the experimental data
for doped FePt-L10 alloys. | 1409.5806v1 |
2014-10-03 | Tuning the hydrogen desorption of Mg(BH$_4$)$_2$ through Zn alloying | We study the effect of Zn alloying on the hydrogen desorption properties of
Mg(BH$_4$)$_2$ using $\it{ab\ initio}$ simulations. In particular, we
investigate formation/reaction enthalpies/entropies for a number of compounds
and reactions at a wide range of temperatures and Zn concentrations in
Mg$_{1-x}$Zn$_x$(BH$_4$)$_2$. Our results show that the thermodynamic stability
of the resulting material can be significantly lowered through Zn alloying. We
find that e.g. the solid solution Mg$_{2/3}$Zn$_{1/3}$(BH$_4$)$_2$ has a
reaction enthalpy for the complete hydrogen desorption of only 25.3 kJ/mol
H$_2$$-$a lowering of 15 kJ/mol H$_2$ compared to the pure phase and a
corresponding lowering in critical temperature of 123 K. In addition, we find
that the enthalpy of mixing is rather small and show that the decrease in
reaction enthalpy with Zn concentration is approximately linear. | 1410.0858v1 |
2014-10-09 | TDDFT calculations for excitation spectra of III-V ternary alloys | We adopted the time-dependent density functional theory (TDDFT) within the
linear augmented Slater-type orbitals (LASTO) basis and the cluster averaging
method to compute the {\color{red}excitation} spectra of III-V ternary alloys
with arbitrary concentration $x$. The TDDFT was carried out with the use of
adiabatic meta-generalized gradient approximation (mGGA), which contains the
$1/q^2$ singularity in the dynamical exchange-correlation kernel
($f_{XC,00}(\mathbf{q})$) as $q\rightarrow 0$. We found that by using wave
functions obtained in local density approximation (LDA) while using mGGA to
compute self-energy correction to the band structures, we can get {\color{red}
good overall} agreement between theoretical results and experimental data for
the excitation spectra. Thus, our studies provide some insight into the
theoretical calculation of optical spectra of semiconductor alloys. | 1410.2527v2 |
2014-11-10 | Pressure studies of the quantum critical alloy Ce(0.93)Yb(0.07)CoIn5 | Here we present our experimental and theoretical study of the effects of
pressure on the transport properties of the heavy-fermion alloy
Ce(1-x)Yb(x)CoIn5 with x~0.07. We specifically choose this value of ytterbium
concentration because the magnetic-field-induced quantum critical point, which
separates the antiferromagnetic and paramagnetic states at zero temperature,
approaches zero, as has been established in previous studies. Our measurements
show that pressure further suppresses quantum fluctuations in this alloy, just
as it does in the parent compound CeCoIn5. In contrast, the square-root
temperature dependent part of resistivity remains insensitive to pressure,
indicating that the heavy-quasiparticles are not involved in the scattering
processes leading to such a temperature dependent resistivity. We demonstrate
that the growth of the coherence temperature with pressure, as well as the
decrease of the residual resistivity, can be accurately described by employing
the coherent potential approximation for a disordered Kondo lattice. | 1411.2524v1 |
2014-11-18 | Molybdenum-Rhenium alloy based high-$Q$ superconducting microwave resonators | Superconducting microwave resonators (SMR) with high quality factors have
become an important technology in a wide range of applications.
Molybdenum-Rhenium (MoRe) is a disordered superconducting alloy with a noble
surface chemistry and a relatively high transition temperature. These
properties make it attractive for SMR applications, but characterization of
MoRe SMR has not yet been reported. Here, we present the fabrication and
characterization of SMR fabricated with a MoRe 60-40 alloy. At low drive
powers, we observe internal quality-factors as high as 700,000. Temperature and
power dependence of the internal quality-factors suggest the presence of the
two level systems from the dielectric substrate dominating the internal loss at
low temperatures. We further test the compatibility of these resonators with
high temperature processes such as for carbon nanotube CVD growth, and their
performance in the magnetic field, an important characterization for hybrid
systems. | 1411.4815v1 |
2014-12-01 | Microwave pinning modes near Landau filling $ν=1$ in two-dimensional electron systems with alloy disorder | We report measurements of microwave spectra of two-dimensional electron
systems hosted in dilute Al alloy, Al$_x$Ga$_{1-x}$As, for a range of Landau
level fillings, $\nu$, around 1. For $\nu>0.8$ or $\nu<1.2$, the samples
exhibit a microwave resonance whose frequency decreases as $\nu$ moves away
from 1. A resonance with this behavior is the signature of solids of
quasiparticles or -holes in the partially occupied Landau level, which was
previously seen in ultralow disorder samples. For $\nu<0.8$ down to as low as
$\nu=0.54$, a resonance in the spectra is still present in the Al
alloy-disordered samples, though it is partially or completely suppressed at
$\nu=3/5$ and $1/2$, and is strongly damped over much of this $\nu$ range. The
resonance also shows a striking enhancement in peak frequency for $\nu$ just
below 3/4. We discuss possible explanations of the resonance behavior for
$\nu<0.8$ in terms of the composite fermion picture. | 1412.0689v1 |
2014-12-03 | Low In solubility and band offsets in the small-$x$ $β$-Ga$_2$O$_3$/(Ga$_{1-x}$In$_x$)$_2$O$_3$ system | Based on first-principles calculations, we show that the maximum reachable
concentration $x$ in the (Ga$_{1-x}$In$_x$)$_2$O$_3$ alloy in the low-$x$
regime (i.e. In solubility in $\beta$-Ga$_2$O$_3$) is around 10%. We then
calculate the band alignment at the (100) interface between $\beta$-Ga$_2$O$_3$
and (Ga$_{1-x}$In$_x$)$_2$O$_3$ at 12%, the nearest computationally treatable
concentration. The alignment is strongly strain-dependent: it is of type-B
staggered when the alloy is epitaxial on Ga$_2$O$_3$, and type-A straddling in
a free-standing superlattice. Our results suggest a limited range of
applicability of low-In-content GaInO alloys. | 1412.1323v1 |
2014-12-05 | Calculating linear response functions for finite temperatures on the basis of the alloy analogy model | A scheme is presented that is based on the alloy analogy model and allows to
account for thermal lattice vibrations as well as spin fluctuations when
calculating response quantities in solids. Various models to deal with spin
fluctuations are discussed concerning their impact on the resulting temperature
dependent magnetic moment, longitudinal conductivity and Gilbert damping
parameter. It is demonstrated that using the Monte Carlo (MC) spin
configuration as an input, the alloy analogy model is capable to reproduce
results of MC simulations on the average magnetic moment within all spin
fluctuation models under discussion. On the other hand, response quantities are
much more sensitive to the spin fluctuation model. Separate calculations
accounting for either the thermal effect due to lattice vibrations or spin
fluctuations show their comparable contributions to the electrical conductivity
and Gilbert damping. However, comparison to results accounting for both thermal
effects demonstrate violation of Matthiessen's rule, showing the non-additive
effect of lattice vibrations and spin fluctuations. The results obtained for
bcc Fe and fcc Ni are compared with the experimental data, showing rather good
agreement for the temperature dependent electrical conductivity and Gilbert
damping parameter. | 1412.1988v1 |
2014-12-09 | Plastic yielding in nanocrystalline Pd-Au alloys mimics universal behavior of metallic glasses | We studied solid solution effects on the mechanical properties of
nanocrystalline (NC) $\mathrm{Pd}_{\mathrm{100-x}} \mathrm{Au}_{\mathrm{x}}$
alloys ($0 \leq \mathrm{x} < 50 \mathrm{at.\%}$) at the low end of the
nanoscale. Concentration has been used as control parameter to tune material
properties (elastic moduli, Burgers vector, stacking fault energies) at
basically unaltered microstructure (grain size $D\approx 10\mathrm{ nm}$). In
stark contrast to coarse grained fcc alloys, we observe solid solution
softening for increasing Au-content. The available predictions from models and
theories taking explicitly into account the effect of the nanoscale
microstructure on the concentration-dependent shear strength have been
disproved without exception. As a consequence, it is implied that dislocation
activity contributes only marginally to strength. In fact, we find a linear
correlation between shear strength and shear modulus which quantitatively
agrees with the universal behavior of metallic glasses discovered by Johnson
and Samwer [W.L. Johnson and K. Samwer, PRL 95, 195501 (2005)]. | 1412.3044v2 |
2014-12-29 | Anomalous and normal dependence of the sound velocity in the liquid Bi-Sb system | The sound velocity in selected liquid alloys of the isomorphous Bi-Sb system
was measured as a function of temperature to a high accuracy of 0.2%. The sound
velocity temperature coefficient, dlnc/dT, at the liquidus is found to vary
non-monotonously as a function of alloy composition, with the transition from
normal to anomalous temperature dependence occurring at a composition of
approximately Bi35Sb65. Beyond this composition up to approximately Bi10Sb90
the sound velocity is found to be temperature independent over a wide range.
The deviation of the sound velocity from that expected in an ideal solution is
found to be dominated by a sub-regular interaction. The present measurements
allow the determination of the pressure dependence of the sub-regular solution
interaction parameters and are found to be consistent with high pressure
studies of the phase diagram in this system. The sound velocity is shown to be
an effective measure of the pressure dependence of the alloy interactions. | 1412.8309v1 |
2015-02-04 | Thermodynamic stability of Mg-Y-Zn ternary alloys through first-principles | In order to clarify thermodynamic stability of Mg-based long-period stacking
ordered (LPSO) structure, we systematically study energetic preference for
alloys on multiple stacking with different composition for random mixing of
constituent elements, Mg, Y, and Zn based on special quasirandom structure
(SQS). Through calculation of formation free energy of SQS, Mg-Y-Zn alloy
exhibits phase separation into Mg- and Y-Zn rich phase, which is consistent
with previous theoretical studies. Bulk modulus of SQSs for multiple
compositions, stacking sequences, and atomic configuratons ranges around 35
GPa, i.e., they do not show significant dependence of Mg concentration, which
therefore means that the effects of phonon do not play significant role on LPSO
phase stability. Introducing stacking fault to hcp stacking gains "negative"
energy, which indicates profound relationship between introducing stacking
faults and the formation of long-period stacking ordering. | 1502.01138v5 |
2015-03-16 | Dzyaloshinskii-Moriya interaction and Hall effects in the skyrmion phase of MnFeGe alloys | We carry out density functional theory calculations which demonstrate that
the electron dynamics in the skyrmion phase of Fe-rich Mn$_{1-x}$Fe$_x$Ge
alloys is governed by Berry phase physics. We observe that the magnitude of the
Dzyaloshinskii-Moriya interaction, directly related to the mixed space-momentum
Berry phases, changes sign and magnitude with concentration $x$ in direct
correlation with the data of Shibata {\it et al.}, Nature Nanotech. {\bf 8},
723 (2013). The computed anomalous and topological Hall effects in FeGe are
also in good agreement with available experiments. We further develop a simple
tight-binding model able to explain these findings. Finally, we show that the
adiabatic Berry phase picture is violated in the Mn-rich limit of the alloys. | 1503.04842v1 |
2015-03-26 | Dodecagonal quasicrystal in Mn-based quaternary alloys containing Cr, Ni and Si | A dodecagonal quasicrystal showing 12-fold symmetry forms in Mn-rich
quaternary alloys containing 5.5 or 7.5 at.% Cr, 5.0 at.% Ni and 17.5 at.% Si.
After annealing at 700 C for 130 h, the quasicrystal precipitated in a matrix
of \b{eta}-Mn type crystalline phase. The shape of the quasicrystal is
needle-like having a length of several tens of micrometers. Electron
diffraction as well as powder X-ray diffraction experiments have revealed the
following characteristics of the quasicrystal; diffraction symmetry 12/mmm,
presence of systematic extinction for h1h2h2h1h5 type reflections with odd h5
index, and then five-dimensional space group P126/mmc. Indexing of the
reflections indicated that the dimension of the common edge in the equilateral
triangle-square tiling is 4.560 {\AA}, and the periodicity is 4.626 {\AA} along
the 12-fold axis. This is the first example of the dodecagonal quasicrystal
synthesized by ordinary metallurgical method in 3-d transition-metal alloys. | 1503.07602v1 |
2015-04-09 | Positron annihilation spectroscopy for heat-treated Fe-Ga alloys and their defect structure | We present the results of lifetime positron annihilation spectroscopy (PALS)
for the alloys Fe21Ga and Fe22.4Ga, whose defect structure is connected with
annealing from 20 to 1000C along the phase diagram of Fe21Ga and Fe22.4Ga, at
the L12-A2 transitions and in the sub-lattice of the D03 phase. In frames of
the standard trapping model, we estimate the positron annihilation parameters
for the bulk metal and for the different thermal vacancies herein. Also we
define concentrations of these vacancy defects, which may be helpful for
explanation of the physical properties of Ge-Ga alloys, including giant
softening and extremal $\lambda_{100}$ at the given inter-metallic composition. | 1504.02330v3 |
2015-04-21 | The question of intrinsic origin of the metal-insulator transition in i-AlPdRe quasicrystal | The icosahedral (i-) AlPdRe is the most resistive quasicrystalline alloy
discovered so far. Resistivities ($\rho$) of $1\Omega cm$ at 4K and correlated
resistance ratios ($RRR = \rho_{4K}/\rho_{300K}$) of more than 200 are observed
in polycrystalline samples. These values are two orders of magnitude larger
than for the isomorphous i-AlPdMn phase. We discuss here the controversial
microscopic origin of the i-AlPdRe alloy electrical specificity. It has been
proposed that the high resistivity values are due to extrinsic parameters, such
as secondary phases or oxygen contamination. From comprehensive measurements
and data from the literature including electronic transport correlated with
micro structural and micro chemical analysis, we show that on the contrary
there is mounting evidence in support of an origin intrinsic to the i-phase.
Similarly to the other quasicrystalline alloys, the electrical resistivity of
the i-AlPdRe samples depends critically on minute changes in the structural
quality and chemical composition. The low resistivity in i-AlPdRe single-grains
compared to polycrystaline samples can be explained by difference in chemical
composition, heterogeneity and thermal treatment. | 1504.05464v1 |
2015-06-04 | Thermal conductivity of bulk and nanoscaled Si/Ge alloys from the Kinetic Collective Model | Several hitherto unexplained features of thermal conductivity in group IV
materials, such as the change in the slope as a function of sample size for
pure vs. alloyed samples and the fast decay in thermal conductivity for low
impurity concentration, are described in terms of a transition from a
collective to kinetic regime in phonon transport. We show that thermal
transport in pure bulk silicon samples is mainly collective, and that
impurity/alloy and boundary scattering are responsible for the destruction of
this regime with an associated strong reduction in thermal conductivity,
leaving kinetic transport as the only one allowed when those resistive
scattering mechanisms are dominant. | 1506.01522v2 |
2015-06-22 | What determines the sign of the spin Hall effects in Cu alloys doped with 5d elements? | We perform a systematical analysis of the spin Hall effect (SHE) in the Cu
alloys doped with a series of 5d elements, by the combined approach of density
functional theory and Hartree-Fock approximation. We find that not only the
spin orbit interactions (SOI) in both the 5d and 6p orbitals, but also the
local correlations in the 5d orbitals of the impurities, are decisive on the
sign of the spin Hall angle (SHA). Including all of these three factors
properly, we predict the SHA for each alloy in the series. The signs of CuIr
and CuPt are sensitive to perturbation of the local correlations. This
observation is favorable for controlling the sign of the transverse spin Hall
voltage. | 1506.06437v1 |
2015-08-14 | High-throughput exploration of alloying as design strategy for thermoelectrics | We explore a material design strategy to optimize the thermoelectric power
factor. The approach is based on screening the band structure changes upon a
controlled volume change. The methodology is applied to the binary silicides
and germanides. We first confirm the effect in antifluorite Mg2Si and Mg2Ge
where an increased power factor by alloying with Mg2Sn is experimentally
established. Within a high-throughput formalism we identify six previously
unreported binaries that exhibit an improvement in their transport properties
with volume. Among these, hexagonal MoSi2 and orthorhombic Ca2Si and Ca2Ge have
the highest increment in zT with volume. We then perform super-cell
calculations on special quasi-random structures to investigate the possibility
of obtaining thermodynamically stable alloy systems which would produce the
necessary volume changes. We find that for Ca2Si and Ca2Ge the solid solutions
with the isostructural Ca2Sn readily forms even at low temperatures. | 1508.03477v1 |
2015-09-12 | AC magnetic susceptibility study of a sigma-phase Fe65.9V34.1 alloy | A sigma-phase Fe65.9V34.1 alloy was investigated with the AC magnetic
susceptibility as a function of temperature, frequency and external magnetic
field. An evidence was found that its magnetism shows features characteristic
of a reentrant behavior viz. two transitions: first at TC ca.312K from the
paramagnetic state into the collinear ferromagnetic one, and second at Tf
ca.302K to a mixed state (sometimes termed as a ferromagnetic re-entrant spin
glass) which, finally, at a lower temperature (TRSG ca.60K) transforms to a
state where replica symmetry is broken. The frequency dependence of Tf is lower
than that of canonical spin glasses, a feature, that in the light of a high
concentration of magnetic carriers, can be understood in terms of a weak
coupling between magnetic clusters. | 1509.03719v1 |
2015-09-23 | Ferroelectric thermal phase transition and polarization precursor dynamics in CaxBa1-xNb2O6(CBN) tungsten bronze type oxides | Polycrystals of CaxBa1-xNb2O6(CBN) tungsten bronze type oxides have been
prepared and their structural, dielectric, and thermal properties have been
investigated. It was found that CBN alloys with ferroelectric tetragonal
tungsten bronze structure were only available in a composition range of 0.19 <=
x <= 0.32. It was also showed that CBN can be classified as a ferroelectric
with a first-order thermal phase transition showing polarization precursor
dynamics before transition into the ferroelectric phase, in sharp contrast to
an isostructural alloy SrxBa1-xNb2O6(SBN) that shows typical relaxor behaviors.
The local polarizations were found to grow exponentially within the
paraelectric mother phase in a large temperature range of Tc<T<Tc+ 88~140 K on
cooling. Furthermore, a phase diagram was established for CBN ferroelectric
alloys. These findings may get an insight into the true nature of ferroelectric
phase transition in this potential electro-optic material. | 1509.06852v1 |
2015-10-02 | Mössbauer spectroscopic study of a sigma-Fe65.9V34.1 alloy: Curie and Debye temperatures | Sigma-phase Fe65.9V34.1 alloy was investigated with the M\"ossbauer
spectroscopy. M\"ossbauer spectra were recorded in the temperature interval of
80-300 K. Their analysis in terms of the hyperfine distribution protocol
yielded the average hyperfine field, <B>, the average center shift, <CS>, and
the spectral area, A. The magnetic ordering temperature, T_C=312.5(5) K was
determined from the temperature dependence of <B>, and the Debye temperature,
T_D, from the temperature dependence of <CS> and the relative spectral area.
The value obtained from the former was 403(17) K and that from the latter
374(2) K. The value of the force constant was determined. The lattice dynamics
of Fe atoms was described in terms of the kinetic, E_k, and the potential
energy, E_p. | 1510.00520v1 |
2015-10-13 | Thin-film based phase plates for transmission electron microscopy fabricated from metallic glasses | Thin-film based phase plates are meanwhile a widespread tool to enhance the
contrast of weak-phase objects in transmission electron microscopy (TEM). The
thin film usually consists of amorphous carbon, which suffers from quick
degeneration under the intense electron-beam illumination. Recent
investigations have focused on the search for alternative materials with an
improved material stability. This work presents thin-film based phase plates
fabricated from metallic glass alloys, which are characterized by a high
electrical conductivity and an amorphous structure. Thin films of the
zirconium-based alloy Zr65.0Al7.5Cu27.5 (ZAC) are prepared and their
phase-shifting properties are tested. The ZAC-alloy film is investigated by
different TEM techniques, which reveal a range of beneficial characteristics.
Particularly favorable is the small probability for inelastic plasmon
scattering, which is promising to improve the performance of thin-film based
phase plates in phase-contrast TEM. | 1510.03587v1 |
2015-10-30 | Bandgap narrowing in Mn doped GaAs probed by room-temperature photoluminescence | The electronic band structure of the (Ga,Mn)As system has been one of the
most intriguing problems in solid state physics over the past two decades.
Determination of the band structure evolution with increasing Mn concentration
is a key issue to understand the origin of ferromagnetism. Here we present room
temperature photoluminescence and ellipsometry measurements of
Ga_{100%-x}Mn_{x}As alloy. The up-shift of the valence-band is proven by the
red shift of the room temperature near band gap emission from the
Ga_{100%-x}Mn_{x}As alloy with increasing Mn content. It is shown that even a
doping by 0.02 at.% of Mn affects the valence-band edge and it merges with the
impurity band for a Mn concentration as low as 0.6 at.%. Both X-ray diffraction
pattern and high resolution cross-sectional TEM images confirmed full
recrystallization of the implanted layer and GaMnAs alloy formation. | 1510.09017v1 |
2015-11-05 | Structural, Magnetic and Magneto-caloric studies of Ni50Mn30Sn20Shape Memory Alloy | We have synthesized a nominal composition of Ni50Mn30Sn20 alloy using arc
melting technique. Rietveld refinement confirms the austenite L21 structure in
Fm-3m space group. Electrical resistivity has been found to clearly exhibiting
two different phenomena viz. a magnetic transition from paramagnetic to
ferromagnetic and a structural transition from austenite to martensitic phase.
Thermo-magnetization measurements M(T) confirms ferromagnetic transition
temperature TC at 222 K and martensitic transition starting at 127 K(MS).
Magnetization measurement M(H) at 10 K confirms the ferromagnetic state.
Frequency dependence of ac susceptibility \c{hi}' at low temperature suggests
spin glass behavior in the system. The isothermal magnetic entropy change
values have been found to be 1.14 J/Kg.K, 2.69 J/Kg.K and 3.9 J/Kg.K, with
refrigeration capacities of 19.6 J/kg, 37.8 J/kg and 54.6 J/kg for the field
change of 1, 2 and 3 Tesla respectively at 227 K. | 1511.01632v1 |
2015-11-09 | Properties of (Ga$_{1-x}$In$_x$)$_2$O$_3$ over the whole $x$ range | Using density-functional ab initio theoretical techniques, we study
(Ga$_{1-x}$In$_x$)$_2$O$_3$ in both its equilibrium structures (monoclinic
$\beta$ and bixbyite) and over the whole range of composition. We establish
that the alloy exhibits a large and temperature-independent miscibility gap. On
the low-$x$ side, the favored phase is isostructural with $\beta$-Ga$_2$O$_3$;
on the high-$x$ side, it is isostructural with bixbyite In$_2$O$_3$. The
miscibility gap opens between approximately 15\% and 55\% In content for the
bixbyite alloy grown epitaxially on In$_2$O$_3$, and 15\% and 85\% In content
for the free-standing bixbyite alloy. The gap, volume and band offsets to the
parent compound also exhibit anomalies as function of $x$. Specifically, the
offsets in epitaxial conditions are predominantly type-B staggered, but have
opposite signs in the two end-of-range phases. | 1511.02606v1 |
2015-12-21 | Possible "Magnéli" phases and self-alloying in the superconducting sulfur hydride | We theoretically give an infinite number of metastable crystal structures for
the superconducting sulfur hydride H$_{x}$S under pressure. It has been thought
that theoretically predicted structures of H$_{2}$S and H$_{3}$S exhibit low
and high $T_{\rm c}$ in the experiment, respectively. The newly found
structures are long-period modulated crystals where slab-like H$_{2}$S and
H$_{3}$S regions intergrow in a microscopic scale. The extremely small
formation enthalpy for the H$_{2}$S--H$_{3}$S boundary indicated with the
first-principles calculations suggests possible alloying of these phases
through formation of local H$_{3}$S regions. The modulated structures and
gradual alloying transformations between them not only explain the peculiar
pressure dependence of $T_{\rm c}$ in sulfur hydride observed experimentally,
but also could prevail in the experimental samples under various compression
schemes. | 1512.06680v2 |
2016-01-21 | The mechanism of solute-enriched clusters formation in neutron-irradiated pressure vessel steels: the case of Fe-Cu model alloys | Mechanism of solute-enriched clusters formation in neutron-irradiated
pressure vessel steels is proposed and developed in case of Fe-Cu model alloys.
We show that the obtained results are in a good agreement with available
experimental data on the parameters of clusters enriched with the alloying
elements. The suggested solute-drag mechanism is analogous to the well-known
zone-refining process. Our model explains why the formation of solute-enriched
clusters does not happen in austenitic stainless steels with fcc lattice
structure. It also allows to quantify the method of evaluation of neutron
irradiation dose for the process of RPV steels hardening. | 1601.05634v1 |
2016-05-05 | First-principles evaluation of intrinsic, side-jump, and skew-scattering parts of anomalous Hall conductivities in disordered alloys | We develop a first-principles procedure for the individual evaluation of the
intrinsic, side-jump, and skew-scattering contributions to the anomalous Hall
conductivity $\sigma_{xy}$. This method is based on the different microscopic
conductive processes of each origin of $\sigma_{xy}$ in the Kubo--Bastin
formula. We also present an approach for implementing this scheme in the
tight-binding linear muffin-tin orbital (TB-LMTO) method with the coherent
potential approximation (CPA). The validity of this calculation method is
demonstrated for disordered FePt and FePd alloys. We find that the estimated
value of each origin of $\sigma_{xy}$ exhibits reasonable dependencies on the
electron scattering in these disordered alloys. | 1605.01495v2 |
2016-06-12 | Phase separation of metastable CoCrFeNi high entropy alloy at intermediate temperatures | The CoCrFeNi alloy is widely accepted as an exemplary stable base for high
entropy alloys (HEAs). Although various investigations prove it to be stable
solid solution, its phase stability is still suspicious. Here, we identified
that the CoCrFeNi HEA was thermally metastable at intermediate temperatures,
and composition decomposition occurred after annealed at 750oC for 800 hrs. The
increased lattice distortion induced by minor addition of Al into the CoCrFeNi
base accelerated the composition decomposition and a second fcc phase with a
different lattice constant occurred in the long time annealed CoCrFeNiAl0.1
HEA. A Cr-rich {\sigma} phase also precipitated from the CoCrFeNiAl0.1 HEA. The
Al element can induce the instability of CoCrFeNi HEA. The revealed metastable
CoCrFeNi at intermediate temperatures will greatly change the way of HEAs
development. | 1606.03693v1 |
2016-06-17 | Ordering effects in 2D hexagonal systems of binary and ternary BCN alloys | We present theoretical study of ordering phenomena in binary $C_{1-x}B_{x}$,
$C_{1-x}N_{x}$ and ternary $B_{x}C_{1-x-y}N_{y}$ alloys forming
two-dimensional, graphene-like systems. For calculating energy of big systems
(20 000 atoms in the supercell with periodic boundary conditions assumed)
empirical Tersoff potential was employed. In order to find equilibrium
distribution of different species corresponding to minima of the energy, we use
Monte Carlo approach in Metropolis regime. We take into consideration wide
range of concentrations (1-50%) and temperatures (70-1500 K), to provide more
complete picture. For quantitative description of order, we determine
Warren-Cowley Short Range Order (SRO) parameters for the first coordination
shell. This procedure allows us to determine energetically favorable
compositions of all alloys, and characterize resulting types of order for both
binary and ternary systems. | 1606.05548v2 |
2016-07-06 | Solubility and partitioning of impurities in Be alloys | The most energetically favourable accommodation processes for common
impurities and alloying elements in Be metal and Be-Fe-Al intermetallics were
investigated using atomic scale simulations. Fe additions, combined with
suitable heat treatments, may scavange Al and Si through their incorporation
into the FeBe5 intermetallic. In the absence of Fe, Al and Si will not be
associated with Be metal. Li and Mg are also not soluble, but may react with
other impurities if present (such as Al or H). Mg may also form the MgBe13
intermetallic phase under certain conditions. He and H exhibit negligible
solubility in all phases investigated and whilst He will tend to form bubbles,
H can precipitate as BeH2. Similarly, C additions will form the stable compound
Be2C. Finally, oxygen exhibits a strong affinity to Be, exhibiting both some
degree of solubility in all phases considered here (though especially metallic
Be) and a highly favourable energy of formation for BeO. | 1607.01502v1 |
2016-07-15 | Distribution of Formation and migration energies of point defects in concentrated solid-solution alloys: Ni_{0.5}Co_{0.5}, Ni_{0.5}Fe_{0.5}, Ni_{0.8}Fe_{0.2} and Ni_{0.8}Cr_{0.2} | Using ab initio calculations and special quasirandom structures, we have
characterized the distribution of defect formation energy and migration barrier
in Ni-based solid-solution alloys: Ni_{0.5}Co_{0.5}, Ni_{0.5}Fe_{0.5},
Ni_{0.8}Fe_{0.2} and Ni_{0.8}Cr_{0.2}. As defect formation energies depend
sensitively on elemental chemical potential, we have developed a
computationally efficient method for determining it which takes into account
the global composition and local short-range order. We find that Fe has the
biggest alloy effects for Ni among these four elements. Our results show that
the distribution of migration energies for vacancies and interstitial have a
region of overlap, which will facilitate the recombination between them. | 1607.04667v2 |
2016-07-20 | Energy of mixing and entropy of mixing for Cu$_{x}$Al$_{1-x}$ liquid binary alloys | The free energy of mixing and the entropy of mixing for Cu$_{x}$Al$_{1-x}$
liquid binary alloys have been systematically investigated by using the
electronic theory of metals along with the perturbation approach at a
thermodynamic state $T=1373$ K. The interionic interaction and a reference
liquid are the fundamental components of the theory. The interionic interaction
is described by a local pseudopotential. A liquid of hard spheres (HS) of two
different effective diametres is used to describe the reference system for
alloys. The results of the calculations for energy of mixing agree well with
the available experimental data. Calculation of entropy of mixing is parameter
free and, the agreement with experiment, in this case, is found to be fairly
good. | 1607.05827v1 |
2016-09-01 | First-principles Predictions of Electronic Properties of GaAs1-x-yPyBix and GaAs1-x-yPyBix-based Heterojunctions | Significant efficiency droop is a major concern for light-emitting diodes and
laser diodes operating at high current density. Recent study has suggested that
heavily Bi-alloyed GaAs can decrease the non-radiative Auger recombination and
therefore alleviate the efficiency droop. Using density functional theory, we
studied a newly fabricated quaternary alloy, GaAs1-x-yPyBix, which can host
significant amounts of Bi, through calculations of its band gap, spin-orbit
splitting, and band offsets with GaAs. We found that the band gap changes of
GaAs1-x-yPyBix relative to GaAs are determined mainly by the local structural
changes around P and Bi atoms rather than their electronic structure
differences. To obtain alloy with lower Auger recombination than GaAs bulk, we
identified the necessary constraints on the compositions of P and Bi. Finally,
we demonstrated that GaAs/GaAs1-x-yPyBix heterojunctions with potentially low
Auger recombination can exhibit small lattice mismatch and large enough band
offsets for strong carrier confinement. This work shows that the electronic
properties of GaAs1-x-yPyBix are potentially suitable for high-energy infrared
light-emitting diodes and laser diodes with improved efficiency. | 1609.00335v2 |
2016-09-20 | Tunable Magnetic Properties of Carbon-shielded NiPt Nanoalloys | Spherical NiPt@C nanoalloys encapsulated in carbon shells are synthesized by
means of high-pressure chemical vapour deposition. Upon variation of the
synthesis parameters, both the alloy core composition and the particle size of
the resulting spherical NixPt1-x@C nanocapsules can be controlled. The
sublimation temperatures of the Ni- and Pt-precursors are found key to control
the alloy composition and diameter. Depending on the synthesis parameters, the
diameters of the cores are tuneable in the range of 3 - 15 nm while the carbon
coatings for all conditions are 1 - 2 nm. The core particle size decreases
linearly upon increasing either of the sublimation temperatures while the Ni
and Pt content, respectively, are linearly increasing with the related
precursor temperature. Accordingly, the magnetic properties of the nanoalloys,
i.e. magnetization, remanent magnetization and critical field, are well
controlled by the two sublimation temperatures. As compared to bulk NiPt, our
data show an increase of Stoner enhancement by nanoscaling as ferromagnetism
appears in the Pt rich NiPt nanoalloy which is not observed for the related
bulk alloys. The magneto-crystalline anisotropy constants K are in the range of
0.3 - 4 x 105 J/m3 underlining that NiPt@C is a stable, highly magnetic
functional nanomaterial. | 1609.06110v1 |
2016-09-23 | Simulated Cu-Zr glassy alloys: the impact of composition on icosahedral order | The structural properties of the simulated $\rm Cu_{\alpha}Zr_{1-\alpha}$
glassy alloys are studied in the wide range of the copper concentration
$\alpha$ to clarify the impact of the composition on the number density of the
icosahedral clusters. Both bond orientational order parameters and Voronoi
tessellation methods are used to identify these clusters. Our analysis shows
that abundance of the icosahedral clusters and the chemical composition of
these clusters are essentially non monotonic versus $\alpha$ and demonstrate
local extrema. That qualitatively explains the existence of pinpoint
compositions of high glass-forming ability observing in Cu-Zr alloys. Finally,
it has been shown that Voronoi method overestimates drastically the abundance
of the icosahedral clusters in comparison with the bond orientational order
parameters one. | 1609.07457v1 |
2016-09-30 | First-principles Study of Rashba Effect in Ultra-thin Bismuth Surface Alloys | We performed density functional calculations for ultra-thin bismuth surface
alloys: surface alloys of bismuth and face-centered cubic metals
Bi/$M$(111)-$(\sqrt{3}\times\sqrt{3})R30\deg$ ($M$=Cu, Ag, Au, Ni, Co, and Fe).
Our calculated Rashba parameters for the Bi/Ag are consistent with the previous
experimental and theoretical results. We predicted a trend in the Rashba
coefficients $\alpha_R$ of bands around the Fermi energy for noble metals as
follows: Bi/Ag > Bi/Cu > Bi/Au. As for the transition metals, there is a trend
in $\alpha_R$: Bi/Ni > Bi/Co > Bi/Fe. Our finding may lead to design efficient
spin-charge conversion materials. | 1609.09782v1 |
2016-10-03 | First-Principles Study on Preferential Energetics of Mg-based Ternary Alloys Revisited by Short-Range Order in Disordered Phases | To investigate the formation of Mg-based long-period stacking ordered (LPSO)
structure, we systematically study the preference of the short-range order
(SRO) in metastable disordered phases of Mg-RE-Zn (RE = Y, La, Er, Ho, Dy, Tb)
and Mg-Gd-Al ternary alloy systems through first-principles calculation. RE-Zn
(RE = Y, Er, Ho, Dy, Tb) and Gd-Al pair clusters' probability showed the
tendency of increasing. In contrast, La-Zn pair clusters' probability is not
increasing, whose system dose not form LPSO structure. This preference of SRO
in disordered phases of Mg-based ternary alloys certainly indicates that
peculiar L12-type ordering in LPSO as well as possibility of LPSO formation
should have strong correlation with SRO tendency of energetically competitive
disordered phases. | 1610.00786v2 |
2016-10-07 | Thermal Conductivity of GaAs/Ge Nanostructures | Superlattices are promising low-dimensional nanomaterials for thermoelectric
technology that is capable of directly converting low-grade heat energy to
useful electrical power. In this work, the thermal conductivities of GaAs/Ge
superlattice nanostructures were investigated systematically in relation to
their morphologies and interfaces. Thermal conductivities were measured using
ultrafast time-domain thermoreflectance and were found to decrease with
increasing interface densities, consistent with our understanding of
microscopic phonon transport in the particle regime. Lower thermal
conductivities were observed in (GaAs)0.77(Ge2)0.23 alloys; transmission
electron microscopy study reveals phase separation in the alloys. These alloys
can be interpreted as fine nanostructures, with length scales comparable to the
periods of very thin superlattices. Our experimental findings help gain
fundamental insight into nanoscale thermal transport in superlattices and are
also useful for future improvement of thermoelectric performance using
superlattice nanostructures. | 1610.02102v1 |
2016-11-04 | Structure-Related Optical Fingerprints in the Absorption Spectra of Colloidal Quantum Dots: Random Alloy vs. Core/Shell Systems | We argue that the experimentally easily accessible optical absorption
spectrum can often be used to distinguish between a random alloy phase and a
stoichiometrically equivalent core/shell realization of ensembles of
monodisperse colloidal semiconductor quantum dots without the need for more
advanced structural characterization tools. Our proof-of-concept is performed
by conceptually straightforward exact-disorder tight-binding calculations. The
underlying stochastical tight-binding scheme only parametrizes bulk band
structure properties and does not employ additional free parameters to
calculate the optical absorption spectrum, which is an easily accessible
experimental property. The method is applied to selected realizations of type-I
Cd(Se,S) and type-II (Zn,Cd)(Se,S) alloyed quantum dots with an underlying
zincblende crystal structure and the corresponding core/shell counterparts. | 1611.01293v2 |
2016-11-04 | Phase separation of a supersaturated nanocrystalline Cu Co alloy and its influence on thermal stability | The thermal decomposition behavior, the microstructural evolution and its
influence on the mechanical properties of a supersaturated Cu Co solid solution
with ~100 nm average grain size prepared by severe plastic deformation is
investigated under non-isothermal and isothermal annealing conditions. Pure
fine grained Cu and Co exhibit substantial grain growth upon annealing, whereas
the Cu Co alloy is thermally stable at the same annealing temperatures. The
annealed microstructures are studied by independent characterization methods,
including scanning electron microscopy, electron energy loss spectroscopy and
atom probe tomography. The phase separation process in the Cu Co alloy proceeds
by the same mechanism, but on different length scales: a fine scaled spinodal
type decomposition is observed in the grain interior, simultaneously Co and Cu
regions with a larger scale are formed near the grain boundary regions.
Subsequent grain growth at higher annealing temperatures results in a
microstructure consisting of the pure equilibrium phases. Such mechanisms can
be used to tailor nano structures to optimize certain properties. | 1611.01363v1 |
2017-01-09 | A nexus between 3D atomistic data hybrids derived from atom probe microscopy and computational materials science: a new analysis of solute clustering in Al-alloys | Solute clusters affect the physical properties of alloys. Knowledge of the
atomic structure of solute clusters is a prerequisite for material
optimisation. In this study, solute clusters in a rapid-hardening Al-Cu-Mg
alloy were characterised by a combination of atom probe tomography and density
functional theory, making use of a hybrid data type that combines lattice
rectification and data completion to directly input experimental data into
atomistic simulations. The clusters input to the atomistic simulations are thus
observed experimentally, reducing the number of possible configurations. Our
results show that spheroidal, compact clusters are more energetically
favourable and more abundant. | 1701.02329v1 |
2017-01-10 | Magnetic properties in ultra-thin 3d transition metal alloys II: Experimental verification of quantitative theories of damping and spin-pumping | A systematic experimental study of Gilbert damping is performed via
ferromagnetic resonance for the disordered crystalline binary 3d transition
metal alloys Ni-Co, Ni-Fe and Co-Fe over the full range of alloy compositions.
After accounting for inhomogeneous linewidth broadening, the damping shows
clear evidence of both interfacial damping enhancement (by spin pumping) and
radiative damping. We quantify these two extrinsic contributions and thereby
determine the intrinsic damping. The comparison of the intrinsic damping to
multiple theoretical calculations yields good qualitative and quantitative
agreement in most cases. Furthermore, the values of the damping obtained in
this study are in good agreement with a wide range of published experimental
and theoretical values. Additionally, we find a compositional dependence of the
spin mixing conductance. | 1701.02475v1 |
2017-01-14 | Strong perpendicular magnetic anisotropy energy density at Fe alloy/HfO2 interfaces | We report on the perpendicular magnetic anisotropy (PMA) behavior of heavy
metal (HM)/ Fe alloy/MgO thin film heterostructures after an ultrathin HfO2
passivation layer is inserted between the Fe alloy and the MgO. This is
accomplished by depositing one to two atomic layers of Hf onto the Fe alloy
before the subsequent rf sputter deposition of the MgO layer. This Hf layer is
fully oxidized during the subsequent deposition of the MgO layer, as confirmed
by X-ray photoelectron spectroscopy measurements. As the result a strong
interfacial perpendicular anisotropy energy density can be achieved without any
post-fabrication annealing treatment, for example 1.7 erg/cm^2 for the
Ta/Fe60Co20B20/HfO2/MgO heterostructure. Depending on the HM, further
enhancements of the PMA can be realized by thermal annealing to at least 400C.
We show that ultra-thin HfO2 layers offer a range of options for enhancing the
magnetic properties of magnetic heterostructures for spintronics applications. | 1701.03933v1 |
2017-03-27 | Machining of Spherical Component Fabricated by Selected Laser Melting, Part II: Application of Ti in Biomedical | Ti and Ti-Based alloys have unique properties such as high strength, low
density and excellent corrosion resistance. These properties are essential for
the manufacture of lightweight and high strength components for biomedical
applications. In this paper, Ti properties such as metallurgy, mechanical
properties, surface modification, corrosion resistance, biocompatibility and
osseointegration in biomedical applications have been discussed. This paper
also analyses the advantages and disadvantages of various Ti manufacturing
processes for biomedical applications such as casting, powder metallurgy, cold
and hot working, machining, laser engineering net shaping, superplastic
forming, forging and ring rolling. The contributions of this research are
twofold, firstly scrutinizing the behaviour of Ti and Ti-Based alloys in-vivo
and in-vitro experiments in biomedical applications to determine the factors
leading to failure, and secondly strategies to achieve desired properties
essential to improving the quality of patient outcomes after receiving surgical
implants. Future research will be directed toward manufacturing of Ti for
medical applications by improving the production process, for example using
optimal design approaches in additive manufacturing and investigating alloys
containing other materials in order to obtain better medical and mechanical
characteristics. | 1703.10045v1 |
2017-07-12 | Radioactive isotopes reveal a non sluggish kinetics of grain boundary diffusion in high entropy alloys | High entropy alloys (HEAs) have emerged as a new class of multicomponent
materials, which have potential for high temperature applications. Phase
stability and creep deformation, two key selection criteria for high
temperature materials, are predominantly influenced by the diffusion of
constituent elements along the grain boundaries (GBs). For the first time, GB
diffusion of Ni in chemically homogeneous CoCrFeNi and CoCrFeMnNi HEAs is
measured by radiotracer analysis using the $^{63}$Ni isotope. Atom probe
tomography confirmed the absence of elemental segregation at GBs that allowed
reliable estimation of the GB width to be about 0.5 nm. Our GB diffusion
measurements prove that a mere increase in number of constituent elements does
not lower the diffusion rates in HEAs, but the nature of added constituents
plays a more decisive role. The GB energies in both HEAs are estimated at about
0.8-0.9 J/m$^2$, they are found to increase significantly with temperature and
the effect is more pronounced for the CoCrFeMnNi alloy. | 1707.03919v1 |
2017-08-01 | Ab initio based analysis of grain boundary segregation in Al-Mg and Al-Zn binary alloys | Based on ab-initio simulations, we report on the nature of principally
different mechanisms for interaction of Mg and Zn atoms with grain boundaries
in Al alloys leading to different morphology of segregation. The Mg atoms
segregate in relatively wide GB region with heterogeneous agglomerations due to
the deformation mechanism of solute-GB interaction. In contrast, in the case of
Zn atoms an electronic mechanism associated with the formation of directional
bonding is dominating in the solute-GB interaction. As a result, for Zn atoms
it is energetically beneficial to occupy interstitial positions at the very GB
and to be arranged into thin layers along the GBs. The results obtained show
the essential role of elements chemistry in segregation formation and explain
the qualitative features in morphology of GB segregation observed in Al-Mg and
Al-Zn alloys with ultrafine grains. | 1708.00211v1 |
2017-08-02 | Fabrication of tetragonal FeSe - FeS alloy films with high sulfur contents by alternate deposition | We report the synthesis of tetragonal $\mathrm{FeS}_x\mathrm{Se}_{1-x}$ films
($x \leq 0.78$) by pulsed-laser deposition. To fabricate the tetragonal alloy
films with tetragonal FeSe and hexagonal FeS targets, we adopted an alternate
deposition technique with FeSe buffer layer on MgO(001). The overall film
composition is controlled by the thickness ratio of FeS / FeSe layers. The
out-of-plane lattice parameter of the films follows Vegard's law, demonstrating
homogeneous alloying by inter-diffusion. The sulfur solid solubility reaches $x
= 0.78$ in the $\mathrm{FeS}_x\mathrm{Se}_{1-x}$ films, which is by far larger
than $x \approx 0.40$ in bulk governed by the tetragonal phase instability. | 1708.00572v1 |
2017-08-10 | Zero resistance from one atmosphere to the pressure of earth's outer core in a superconducting high entropy alloy | We report the observation of extraordinarily robust zero-resistance
superconductivity in the pressurized (TaNb)0.67(HfZrTi)0.33 high entropy alloy
- a new kind of material with a body-centered cubic crystal structure made from
five randomly distributed transition metal elements. The transition to
superconductivity (TC) increases from an initial temperature of 7.7 K at
ambient pressure to 10 K at ~ 60 GPa, and then slowly decreases to 9 K by 190.6
GPa, a pressure that falls within that of the outer core of the earth. We infer
that the continuous existence of the zero-resistance superconductivity from one
atmosphere up to such a high pressure requires a special combination of
electronic and mechanical characteristics. This high entropy alloy
superconductor thus may have a bright future for applications under extreme
conditions, and also poses a challenge for understanding the underlying quantum
physics. | 1708.03146v2 |
2017-08-26 | Scaling of Memories and Crossover in Glassy Magnets | Glassiness is ubiquitous and diverse in characteristics in nature.
Understanding their differences and classification remains a major scientific
challenge. Here, we show that scaling of magnetic memories with time can be
used to classify magnetic glassy materials into two distinct classes. The
systems studied are high-temperature superconductor-related materials,
spin-orbit Mott insulators, frustrated magnets, and dilute magnetic alloys. Our
bulk magnetization measurements reveal that most densely populated magnets
exhibit similar memory behavior characterized by a relaxation exponent of 1-n ~
0.6(1). This exponent is different from 1-n ~ 1/3 of dilute magnetic alloys
that was ascribed to their hierarchical and fractal energy landscape and is
also different from 1-n=1 of the conventional Debye relaxation expected for a
spin solid, a state with long range order. Furthermore, our systematic study on
dilute magnetic alloys with varying magnetic concentration exhibits crossovers
among the two glassy states and spin solid. | 1708.07974v1 |
2017-08-30 | Large enhancement of the spin Hall effect in Au by scattering with side-jump on Ta impurities | We present measurements of the Spin Hall Effect (SHE) in AuW and AuTa alloys
for a large range of W or Ta concentrations by combining experiments on lateral
spin valves and Ferromagnetic-Resonance/spin pumping technique. The main result
is the identification of a large enhancement of the Spin Hall Angle (SHA) by
the side-jump mechanism on Ta impurities, with a SHA as high as + 0.5 (i.e
$50\%$) for about 10\% of Ta. In contrast the SHA in AuW does not exceed + 0.15
and can be explained by intrinsic SHE of the alloy without significant
extrinsic contribution from skew or side-jump scattering by W impurities. The
AuTa alloys, as they combine a very large SHA with a moderate resistivity
(smaller than $85\,\mu\Omega.cm$), are promising for spintronic devices
exploiting the SHE. | 1708.09214v1 |
2017-09-26 | Phase stability, ordering tendencies, and magnetism in single-phase fcc Au-Fe nanoalloys | Bulk Au-Fe alloys separate into Au-based fcc and Fe-based bcc phases, but
L1$_0$ and L1$_2$ orderings were reported in single-phase Au-Fe nanoparticles.
Motivated by these observations, we study the structural and ordering
energetics in this alloy by combining density functional theory (DFT)
calculations with effective Hamiltonian techniques: a cluster expansion with
structural filters, and the configuration-dependent lattice deformation model.
The phase separation tendency in Au-Fe persists even if the fcc-bcc
decomposition is suppressed. The relative stability of disordered bcc and fcc
phases observed in nanoparticles is reproduced, but the fully ordered L1$_0$
AuFe, L1$_2$ Au$_3$Fe, and L1$_2$ AuFe$_3$ structures are unstable in DFT.
However, a tendency to form concentration waves at the corresponding [001]
ordering vector is revealed in nearly-random alloys in a certain range of
concentrations. This incipient ordering requires enrichment by Fe relative to
the equiatomic composition, which may occur in the core of a nanoparticle due
to the segregation of Au to the surface. Effects of magnetism on the chemical
ordering are also discussed. | 1709.09245v1 |
2017-11-09 | Study on planar segregation of solute atoms in Mg-Al-Gd alloy system | Solute segregation plays an important role in formation of long-period
stacking ordered (LPSO) structure in Mg-M-RE (M: Zn, Ni etc., RE: Y, Gd, etc.)
alloy systems. In this work, the planar segregation in Mg-Al-Gd alloy is
characterized by high angle annular dark field (HAADF) scanning transmission
electron microscopy (STEM) and three-dimensional atom probe (3DAP). It is found
there is no planar fault accompanying the segregation, and the spatial
distribution of segregation may resemble the periodicity of LPSO structure. The
segregation is further quantified by 3DAP, and it mainly enriches with Gd
atoms. The segregation behaviour is rationalized by First-Principles
calculation. | 1711.03262v2 |
2017-11-14 | Solid solution decomposition and Guinier-Preston zone formation in Al-Cu alloys: A kinetic theory with anisotropic interactions | Using methods of statistical kinetic theory parametrized with
first-principles interatomic interactions that include chemical and strain
contributions, we investigated the kinetics of decomposition and microstructure
formation in Al-Cu alloys as a function of temperature and alloy concentration.
We show that the decomposition of the solid solution forming platelets of
copper, known as Guinier-Preston (GP) zones, includes several stages and that
the transition from GP1 to GP2 zones is determined mainly by kinetic factors.
With increasing temperature, the model predicts a gradual transition from
platelet-like precipitates to equiaxial ones and at intermediate temperatures
both precipitate morphologies may coexist. | 1711.04984v1 |
2017-11-23 | Non-linear alloying and strain effects on trivial-topological and semimetal-semiconductor transitions in Bi$_{1-x}$Sb$_x$ | Applying the approximate DFT-1/2 quasiparticle scheme, band structure
unfolding, and generalized quasichemical approximation to describe chemical and
structural disorder, we investigate the electronic structure of
Bi$_{1-x}$Sb$_x$ alloys from first principles. We calculate the important
energy levels near the Fermi energy versus the Sb concentration $x$ where the
trivial-topological (TT) and semimetal-semiconductor (SMSC) transitions occur.
We demonstrate that the energy variation of the relevant states deviates
significantly from linear behavior and that the bowings are important to
correctly describe the critical compositions. The influence of strain on the
energy levels is briefly discussed. It is concluded that the type or sign of
strain applied on antimony atoms during the growth of the alloy should be
heavily dependent on its composition. | 1711.08667v1 |
2017-12-04 | A self-contained algorithm for determination of solid-liquid equilibria in an alloy system | We describe a self-contained procedure to evaluate the free energy of liquid
and solid phases of an alloy system. The free energy of a single-element solid
phase is calculated with thermodynamic integration using the Einstein crystal
as the reference system. Then, free energy difference between the solid and
liquid phases is calculated by Gibbs-Duhem integration. The central part of our
method is the construction of a reversible alchemical path connecting a pure
liquid and a liquid alloy to calculate the mixing enthalpy and entropy. We have
applied the method to calculate the free energy of solid and liquid phases in
the Al-Sm system. The driving force for fcc-Al nucleation in Al-Sm liquid and
the melting curve for fcc-Al and Al3Sm are also calculated. | 1712.01339v1 |
2017-12-12 | Tunable Band Gaps of In$_x$Ga$_{1-x}$N Alloys: From Bulk to Two-Dimensional Limit | Using first-principles calculations combined with a semi-empirical van der
Waals dispersion correction, we have investigated structural parameters, mixing
enthalpies, and band gaps of buckled and planar few-layer In$_x$Ga$_{1-x}$N
alloys. We predict that the free-standing buckled phases are less stable than
the planar ones. However, with hydrogen passivation, the buckled
In$_x$Ga$_{1-x}$N alloys become more favorable. Their band gaps can be tuned
from 6 eV to 1 eV with preservation of direct band gap and well-defined Bloch
character, making them promising candidate materials for future light-emitting
applications. Unlike their bulk counterparts, the phase separation could be
suppressed in these two-dimensional systems due to reduced geometrical
constraints. In contrast, the disordered planar thin films undergo severe
lattice distortion, nearly losing the Bloch character for valence bands;
whereas the ordered planar ones maintain the Bloch character yet with the
highest mixing enthalpies. | 1712.04147v1 |
2017-12-15 | Giant Enhancement of Intrinsic Spin Hall Conductivity in $β$ Tungsten via Substitutional Doping | A key challenge in manipulating the magnetization in
heavy-metal/ferromagnetic bilayers via the spin-orbit torque is to identify
materials that exhibit an efficient charge-to-spin current conversion. Ab
initio electronic structure calculations reveal that the intrinsic spin Hall
conductivity (SHC) for pristine $\beta$-W is about sixty percent larger than
that of $\alpha$-W. More importantly, we demonstrate that the SHC of $\beta$-W
can be enhanced via Ta alloying. This is corroborated by spin Berry curvature
calculations of W$_{1-x}$Ta$_x$ ($x$ $\sim$ 12.5%) alloys which show a giant
enhancement of spin Hall angle of up to $\approx$ $-0.5$. The underlying
mechanism is the synergistic behavior of the SHC and longitudinal conductivity
with Fermi level position. These findings, not only pave the way for enhancing
the intrinsic spin Hall effect in $\beta$-W, but also provide new guidelines to
exploit substitutional alloying to tailor the spin Hall effect in various
materials. | 1712.05516v1 |
2018-01-08 | Large Spin Hall Effect in an Amorphous Binary Alloy | We investigate the spin Hall effect of W-Hf thin films, which exhibit a phase
transition from a segregated phase mixture to an amorphous alloy below 70% W.
The spin Hall angle was determined with a planar harmonic Hall voltage
technique. Due to the accompanying jump in resistivity, the spin Hall angle
shows a pronounced maximum at the composition of the phase transition. The spin
Hall conductivity does, however, reduce from W to Hf with a weak discontinouity
across the phase transition. The maximum spin Hall angle of $\theta_\mathrm{SH}
= -0.25$ is obtained for amorphous W$_{0.7}$Hf$_{0.3}$. A detailed comparison
with spin Hall conductivities calculated from first principles for hcp, fcc,
and bcc solid solutions provides valuable insight into the alloying physics of
this binary system. | 1801.02524v1 |
2018-01-23 | Pure spin current transport in a SiGe alloy | Using four-terminal nonlocal magnetoresistance measurements in lateral
spin-valve devices with Si$_{\rm 0.1}$Ge$_{\rm 0.9}$, we study pure spin
current transport in a degenerate SiGe alloy ($n \sim$ 5.0 $\times$ 10$^{18}$
cm$^{-3}$). Clear nonlocal spin-valve signals and Hanle-effect curves,
indicating generation, manipulation, and detection of pure spin currents, are
observed. The spin diffusion length and spin lifetime of the Si$_{\rm
0.1}$Ge$_{\rm 0.9}$ layer at low temperatures are reliably estimated to be
$\sim$ 0.5 $\mu$m and $\sim$ 0.2 ns, respectively. This study demonstrates the
possibility of exploring physics and developing spintronic applications using
SiGe alloys. | 1801.07450v3 |
2018-03-08 | Design of a nickel-base superalloy using a neural network | A new computational tool has been developed to model, discover, and optimize
new alloys that simultaneously satisfy up to eleven physical criteria. An
artificial neural network is trained from pre-existing materials data that
enables the prediction of individual material properties both as a function of
composition and heat treatment routine, which allows it to optimize the
material properties to search for the material with properties most likely to
exceed a target criteria. We design a new polycrystalline nickel-base
superalloy with the optimal combination of cost, density, gamma' phase content
and solvus, phase stability, fatigue life, yield stress, ultimate tensile
strength, stress rupture, oxidation resistance, and tensile elongation.
Experimental data demonstrates that the proposed alloy fulfills the
computational predictions, possessing multiple physical properties,
particularly oxidation resistance and yield stress, that exceed existing
commercially available alloys. | 1803.03039v1 |
2018-03-15 | Low Temperature Ageing Behaviour of U-Nb $γ^{o}$ Phase Alloys | Ageing mechanisms of the U-7\%wtNb alloy have been investigated on samples
exposed to temperatures of 150$^{o}$C for up to 5000\,hours. A variety of
surface and bulk analytic techniques have been used to investigate
microstructural, chemical and crystallographic changes. Characterisation of the
alloy's evolving behaviour was carried out through secondary electron
microscopy, energy dispersive x-ray spectroscopy, electron backscatter
diffraction, transmission electron microscopy and x-ray diffraction. Vickers
hardness testing showed evidence of a strong thermal hardening relationship
with ageing. The mechanism responsible for these changes is thought to be a
stress-induced isothermal martensitic transformation; a displacive reaction, in
which correlated shuffling of atoms creates a phase change from $\gamma^{o}$ to
$\alpha''$ without chemical species redistribution. | 1803.05700v1 |
2018-03-20 | Single crystal growth of TIMETAL LCB titanium alloy by a floating zone method | The methodology of single crystal growth of metastable $\beta$-Ti alloy
TIMETAL LCB in an optical floating zone furnace is presented in this paper.
Chemical compositions of both precursor material and single crystals were
checked. It was found that the concentration of base alloying elements did not
change significantly during the growth process, while the concentrations of
interstitial elements O and N increased. DSC measurement determined that this
concentration shift has a slight impact on ongoing phase transformations, as in
the single-crystalline material peak associated with $\alpha$ phase
precipitation moves by a few degrees to a lower temperature and peak attributed
to diffusion controlled growth of $\omega$ particles shifts to a higher
temperature. X-ray reciprocal space maps were measured and their simulation
showed that the single crystal has a mosaic structure with mean size of mosaic
blocks of approximately 60 nm. | 1803.07412v1 |
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