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2022-02-02 | An Investigation into the Kinetics and Mechanism of Phase Transitions in Optical Phase Change Ternary Alloy $Ge_2Sb_2Te_5$ | Ternary alloys of Ge-Sb-Te (GST) have been extensively studied due to their
unique ability display a reversible change in their phase upon stimulation by
optical pulses i.e., amorphous (a-GST) to crystalline (c-GST) and vice-versa.
The two phases exhibit remarkably different electrical and optical properties
like conductivity, reflectivity, refractive index and optical loss, this
coupled with their high phase switching speeds, low power phase switching,
large measurable optical and electrical contrast, and phase stability makes GST
alloys stand out from other phase change materials (PCM). GST alloys have
already found extensive use in optical disks and electronic memories due to
their non-volatility and zero static power consumption, but the precise
mechanism of the phase change is not clearly understood. The phase change
mechanism has usually been attributed to the optical pulse, usually a high
power short pulse laser, heating up the c-GST alloy to above its melting
temperature (Tm) after which, if it is cooled rapidly enough to below the glass
transition temperature (Tg), the atoms are fixed in place due to the drastic
reduction in their mobility, resulting in a phase which exhibits structure
similar to a frozen liquid and lacks long range order i.e. amorphous. When
heated above Tg with an intermediate power laser pulse for a significant amount
of time, then this favors the shift back to the energetically favorable
crystalline phase. With the growing interest in next generation data storage
technology for photonic and neuromorphic computing, the research into
understanding and improving the properties of GST alloys has also been
rekindled. In this term paper we hope to investigate and gain a deeper
understanding and appreciation of the kinetics and underlying atomistic
mechanism of this phase transition from c-GST to a-GST and vice-versa which is
only now becoming clearer. | 2202.01918v2 |
2018-04-10 | A mystery of "sluggish diffusion" in high-entropy alloys: the truth or a myth? | High entropy alloys (HEAs) are considered as a novel class of materials with
a large number of components available in nearly equatomic proportions. One of
the characteristic properties of HEAs was believed to be so-called "sluggish"
diffusion. The faith on this myth instead of rigorous experimental analysis
played such a dominant role that the first set of data on interdiffusion, in
fact based on an improper analysis, were cited in hundreds of articles to state
the presence of sluggishness of diffusion rates in high entropy alloys. In this
review, the recent data on atomic diffusion in HEAs are critically discussed.
The discussion is focused on tracer diffusion which is already measured
dominantly for polycrystalline, but in some cases for single crystalline
high-entropy alloys. Alternatively, a rigorous analysis of the interdiffuson
experiments, which provide the diffusion rates of chemical species, too,
becomes more and more sophisticated for three and more elements in an alloy and
it is challenging to derive physically sound quantities from a general
multicomponent diffusion experiment. Most promising in this case is the
diffusion couple technique, especially the so-called pseudo-binary approach.
This approach is analyzed with a focus on the applicability and the possible
errors induced if up-hill diffusion appears. It is shown that atomic diffusion
in HEAs cannot a priori be considered as sluggish and both atomic interactions
as well as correlation effects are responsible for the observed trends. Even if
estimated on the same homologous scale, the diffusion retardation induced by a
"high entropy" in FCC crystals is not simply proportional to the number of
alloying components and it is shown to be similar to that induced by the L12
ordering in a binary system. Furthermore, the importance of cross-correlations
in diffusion of different species in HEAs is highlighted. | 1804.03465v1 |
2021-09-05 | Pseudoelastic deformation in Mo-based refractory multi-principal element alloys | Phase diagrams supported by density functional theory methods can be crucial
for designing high-entropy alloys that are subset of multi-principal$-$element
alloys. We present phase and property analysis of quinary
(MoW)$_{x}$Zr$_{y}$(TaTi)$_{1-x-y}$ refractory high-entropy alloys from
combined Calculation of Phase Diagram (CALPHAD) and density-functional theory
results, supplemented by molecular dynamics simulations. Both CALPHAD and
density-functional theory analysis of phase stability indicates a Mo-W-rich
region of this quinary has a stable single-phase body-centered-cubic structure.
We report first quinary composition from Mo$-$W$-$Ta$-$Ti$-$Zr family of alloy
with pseudo-elastic behavior, i.e., hysteresis in stress$-$strain. Our analysis
shows that only Mo$-$W$-$rich compositions of Mo$-$W$-$Ta$-$Ti$-$Zr, i.e.,
Mo$+$W$\ge$ 85 at.%, show reproducible hysteresis in stress-strain responsible
for pseudo-elastic behavior. The (MoW)$_{85}$Zr$_{7.5}$(TaTi)$_{7.5}$ was
down-selected based on temperature-dependent phase diagram analysis and
molecular dynamics simulations predicted elastic behavior that reveals twinning
assisted pseudoelastic behavior. While mostly unexplored in body-centered-cubic
crystals, twinning is a fundamental deformation mechanism that competes against
dislocation slip in crystalline solids. This alloy shows identical cyclic
deformation characteristics during uniaxial $\lt$100$\gt$ loading, i.e., the
pseudoelasticity is isotropic in loading direction. Additionally, a temperature
increase from 77 to 1500 K enhances the elastic strain recovery in load-unload
cycles, offering possibly control to tune the pseudoelastic behavior. | 2109.02641v1 |
2021-09-18 | Microstructural engineering of medium entropy NiCo(CrAl) alloy for enhanced room and high-temperature mechanical properties | This work demonstrates the development of a strong and ductile medium entropy
alloy by employing conventional alloying and thermomechanical processing to
induce partial recrystallization (PR) and precipitation strengthening in the
microstructure. The combined usage of electron microscopy and atom probe
tomography reveals the sequence of microstructural evolution during the
process. First, the cold working of homogenized alloy resulted in a highly
deformed microstructure. On annealing at 700{\deg}C, B2 ordered precipitates
heterogeneously nucleate on the highly misoriented sites. These B2 promotes
particle stimulated nucleation (PSN) of new recrystallized strain-free grains.
The migration of recrystallized grain boundaries leads to discontinuous
precipitation of L12 ordered regions in highly dense lamellae structures.
Atomic-scale compositional analysis reveals a significant amount of Ni confined
to the GB regions between B2 and L12 precipitates, indicating Ni as a
rate-controlling element for coarsening the microstructure. On 20 hours of
annealing, the alloy comprises a composite microstructure of soft
recrystallized and hard non-recrystallized zones, B2 particles at the grain
boundaries (GBs), and coherent L12 precipitates inside the grains. The B2 pins
the GB movement during recrystallization while the latter provides high
strength. The microstructure results in a 0.2% yield stress (YS) value of 1030
MPa with 32% elongation at ambient temperature and retains up to 910 MPa at
670{\deg}C. Also, it shows exceptional microstructural stability at 700 {\deg}C
and resistance to deformation at high temperatures up to 770{\deg}C.
Examination of deformed microstructure reveals excessive twinning, formation of
stacking faults, shearing of L12 precipitates, and accumulation of dislocations
at around the B2 precipitates and GBs attributed to high strain hardening of
the alloy. | 2109.08894v3 |
2021-10-22 | Fully analytic valence force fields for the relaxation of group-IV semiconductor alloys: elastic properties of group-IV materials calculated from first principles | Si$_{y}$Ge$_{1-x-y}$(C,Sn,Pb)$_{x}$ alloys have attracted significant
attention as a route to achieve a direct-gap group-IV semiconductor. Using
density functional theory (DFT) - employing local density approximation and
hybrid Heyd-Scuzeria-Ernzerhof exchange-correlation functionals - we compute
the lattice parameters, relaxed and inner elastic constants, and internal
strain (Kleinman) parameters for elemental (diamond) group-IV materials and
zinc blende IV-IV compounds. Our DFT calculations support a little-known
experimental re-evaluation of the $\alpha$-Sn elastic constants, and contradict
a recent prediction of dynamic instability in selected IV-IV compounds.
DFT-calculated structural and elastic properties are used in conjunction with a
recently derived analytical parametrisation of a harmonic valence force field
(VFF) [Phys. Rev. B 100, 094112 (2019)] to obtain a complete set of VFF
potentials for Si$_{y}$Ge$_{1-x-y}$(C,Sn,Pb)$_{x}$ and Si$_{x}$Ge$_{1-x}$
alloys. The analytical parametrisation exactly reproduces the relaxed elastic
constants and Kleinman parameter without recourse to numerical fitting,
allowing for accurate and computationally inexpensive lattice relaxation. The
accuracy of the VFF potentials is demonstrated via comparison to the results of
DFT supercell relaxation for (i) ordered Si (Ge) alloy supercells containing a
substitutional C, Ge (Si), Sn or Pb impurity, where comparison is also made to
a model analytical VFF, and (ii) disordered Si$_{x}$Ge$_{1-x}$ alloy
supercells. The VFF potentials we present enable accurate and computationally
inexpensive relaxation of large-scale supercells representing bulk-like
group-IV alloys or group-IV heterostructures, providing input to first
principles or empirical electronic structure calculations, and enabling
structural analysis and calculation of strain fields in heterostructures for
device applications. | 2110.11888v1 |
2021-12-01 | Raman spectroscopy of group-IV Ge$_{1-x}$Sn$_{x}$ alloys: theory and experiment | Ge$_{1-x}$Sn$_{x}$ alloys are a promising candidate material to realise
direct-gap group-IV semiconductors for applications in Si-compatible electronic
and photonic devices. Here, we present a combined theoretical and experimental
analysis of Raman spectroscopy in Ge$_{1-x}$Sn$_{x}$ alloys. We describe
liquid-vapour-solid growth and structural characterisation of
Ge$_{1-x}$Sn$_{x}$ ($x \leq 8$%) nanowires displaying high crystalline quality,
and investigate the structural and vibrational properties of the nanowires
using Raman spectroscopy. Our theoretical analysis is based on a fully analytic
anharmonic valence force field (VFF) potential, which describes exactly - i.e.
without recourse to numerical fitting - the second-order elastic constants,
third-order bulk modulus, selected second- and third-order inner elastic
constants and, as a consequence, the zone-centre transverse optical phonon mode
frequency and its hydrostatic and axial strain dependence. We compute bulk
elastic properties via density functional theory to parametrise the VFF
potential for Ge$_{1-x}$Sn$_{x}$ alloys, and apply the VFF potential to
explicitly compute the Raman spectra of realistic, disordered
Ge$_{1-x}$Sn$_{x}$ alloy supercells. Our atomistic theoretical calculations
quantitatively capture: (i) the evolution of the measured Raman spectra with Sn
composition $x$, (ii) demonstrate explicitly that the presence of short-range
alloy disorder can significantly impact the shift coefficients $a$ and $b$ that
respectively describe the dependence of the Raman shift on Sn composition and
pseudomorphic strain, (iii) elucidate the origin of the so-called "disorder
activated" mode identified in previous experimental investigations, and (iv)
allow for detailed atomic-scale interpretation of measured Raman spectra.
Overall, our analysis provides insight relevant to the characterisation of this
emerging material system. | 2112.00523v1 |
2023-02-21 | Predicting the Early Stages of Solid-State Precipitation in Al-rich Al-Pt Alloys | The high strength of structural aluminium alloys depends strongly on the
controlled precipitation of specific intermetallic phases, whose identity and
crystal structure can be difficult to predict. Here, we investigate the Al-Pt
system, which shows some similarity to the Al-Cu system as one of their main
intermetallic phases, $Al_2Pt$, is nearly isostructural with $\theta^{\prime}
(Al_2Cu)$, the metastable phase responsible for the high-strength of Al-Cu
alloys. However, phases in Al-Pt alloys are complex and have not been studied
in detail. Using a combination of density-functional theory (DFT) calculations
and classical nucleation theory (CNT) applied to the Al-Pt system, we design a
workflow to predict the thermodynamics of solid solution, intermediate phases
such as GP zones, stable and metastable precipitates, and their precipitation
sequence. This workflow can be applied to an arbitrary binary alloying system.
We confirm the known stable phases $Al_4Pt$, $Al_{21}Pt_8$, $Al_2Pt$,
$Al_3Pt_2$, $AlPt (\alpha \& \beta)$, $Al_3Pt_5$, $AlPt_2 (\alpha \& \beta)$
and $AlPt_3 (\alpha \& \beta)$. We also reveal the possible existence of two
phases of chemical formulae $Al_5Pt$ and $Al_3Pt$. This large number of
intermetallic phases is due to the strong bonding between Al and Pt, which also
leads to significant favourable Pt solute formation energy in the Al matrix.
Our findings are compared with the known precipitation characteristics of the
binary Al-Cu and Al-Au systems. We find that the $\theta^{\prime}$-like
$Al_2Pt$ precipitate phase has a lower coherent interfacial energy than
$\theta^{\prime}$. Our calculations strongly suggest that $Al_2Pt$ will
precipitate first in Al-rich Al-Pt alloys and will form bulk-like interfaces
similar to $\eta (Al_2Au)$ rather than like $\theta^{\prime}(Al_2Cu)$. | 2302.10399v1 |
2023-04-08 | An Automated Fully-Computational Framework to Construct Printability Maps for Additively Manufactured Metal Alloys | In additive manufacturing, the optimal processing conditions need to be
determined to fabricate porosity-free parts. For this purpose, the design space
for an arbitrary alloy needs to be scoped and analyzed to identify the areas of
defects for different laser power-scan speed combinations and can be visualized
using a printability map. Constructing printability maps is typically a costly
process due to the involvement of experiments, which restricts their
application in high-throughput product design. To reduce the cost and effort of
constructing printability maps, a fully computational framework is introduced
in this work. The framework combines CALPHAD models and a reduced-order model
to predict material properties. THen, an analytical thermal model, known as the
Eagar-Tsai model, utilizes some of these materials' properties to calculate the
melt pool geometry during the AM processes. In the end, printability maps are
constructed using material properties, melt pool dimensions, and commonly used
criteria for lack of fusion, balling, and keyholing defects. To validate the
framework and its general application to laser powder-bed fusion alloys, five
common additive manufacturing alloys are analyzed. Furthermore, NiTi-based
alloys at three different compositions are evaluated to show the further
extension of the framework to alloy systems at different compositions. The
defect regions in these printability maps are validated with corresponding
experimental observations to compare and benchmark the defect criteria and find
the optimal criterion set with the maximum accuracy for each unique material
composition. Furthermore, printability maps for NiTi that are obtained from our
framework are used in conjunction with process maps resulting from a
multi-model framework to guide the fabrication of defect-free additive
manufactured parts with tailorable properties and performance. | 2304.04113v1 |
2023-09-12 | Investigating the "Cocoon Effect" in Niobium-Copper Alloy: Metallic Nano-Precipitate Distribution and Niobium Migration | We report the observation of the metallic niobium migration within the molten
Cu-Nb alloy mass on the synthesis of nano-granular Cuxwt%Nb evolution, we
prepared a series of granular samples by rapidly cooling a molten mixture of
Cuxwt$%$Nb, where the niobium concentration varied (x=3,5,15,20). Our main goal
in this work was not only to establish a systematic, innovative and robust
method to obtaining good quality samples, but also provide a clear recipe for
obtaining similar systems to the investigations of their interesting physical
properties. Beyond the understanding of the "Cocoon Effect" in Niobium-Copper
alloys, we include a wide complementary elsewhere investigation into the very
interesting and rich superconducting properties exhibited by the Niobium-Copper
alloy. By employing a robust synthesis method, we successfully obtained samples
characterized by well-defined spherical nano-precipitates of niobium, featuring
regular sizes and grain spacing. Our study contributes not only to our
understanding of the Niobium-Copper molten phase separation, micro-structure
and the Cocoon Effect in these metallic alloys, but also sheds light on the
intricate and important implications for the development and optimization of
good quality granular metallic alloys for various applications. From our work,
we obtained very impressive micro structural results, such as: $d_{m}$ = 1.2
$\mu$m, $D_{m}$ $\le$ 2.2 $\mu$m and $\rho$ =1.785 $\mu$$m^{2}$, where $d_{m}$
is the distance between Niobium grains, $D_{m}$ is the mean diameter of Niobium
grains and $\rho$ is the Niobium grain mean density in the Copper matrix. | 2309.06385v2 |
1993-10-15 | Dynamics of Ordering in Alloys with Modulated Phases | This paper presents a theoretical model for studying the dynamics of ordering
in alloys which exhibit modulated phases. The model is different from the
standard time-dependent Ginzburg-Landau description of the evolution of a
non-conserved order parameter and resembles the Swift-Hohenberg model. The
early-stage growth kinetics is analyzed and compared to the Cahn-Hilliard
theory of continuous ordering. The effects of non-linearities on the growth
kinetics are discussed qualitatively and it is shown that the presence of an
underlying elastic lattice introduces qualitatively new effects. A lattice
Hamiltonian capable of describing these effects and suitable for carrying out
simulations of the growth kinetics is also constructed. | 9310039v1 |
1993-10-26 | The relationship between perturbation theory and direct calculations of rare-earth transition intensities | We use a simplified calculation to demonstrate the equivalence between three
different methods for calculating transition line strengths [B.R. Judd and D.
R. Pooler, J. Phys. C 15 (1982) 591; G. W. Burdick and M. F. Reid, Phys. Rev.
Lett. 70 (1993) 2491; G. W. Burdick, H. J. Kooy, and M. F. Reid, J. Phys.:
Condens. Matter 5 (1993) L323]. These calculations demonstrate the complex
interplay between spin-orbit and correlation contributions to two-photon
transitions in rare-earth ions. | 9310057v1 |
1995-03-09 | Microscopic Modeling of the Growth of Order in an Alloy: Nucleated and Continuous Ordering | We study the early-stages of ordering in $Cu_3 Au$ using a model Hamiltonian
derived from the effective medium theory of cohesion in metals: an approach
providing a microscopic description of interatomic interactions in alloys. Our
simulations show a crossover from a nucleated growth regime to a region where
the ordering does not follow any simple growth laws. This mirrors the
experimental observations in $Cu_3 Au$. The kinetics of growth, obtained from
the simulations, is in semi-quantitative agreement with experiments. The
real-space structures observed in our simulations offer some insight into the
nature of early-stage kinetics | 9503050v1 |
1996-10-01 | A Model for Growth of Binary Alloys with Fast Surface Equilibration | We study a simple growth model for (d+1)-dimensional films of binary alloys
in which atoms are allowed to interact and equilibrate at the surface, but are
frozen in the bulk. The resulting crystal is highly anisotropic: Correlations
perpendicular to the growth direction are identical to a d-dimensional
two-layer system in equilibrium, while parallel correlations generally reflect
the (Glauber) dynamics of such a system. For stronger in-plane interactions,
the correlation volumes change from oblate to highly prolate shapes near a
critical demixing or ordering transition. In d=1, the critical exponent z
relating the scaling of the two correlation lengths varies continuously with
the chemical interactions. | 9610014v1 |
1997-04-26 | Wetting phenomena in bcc binary alloys | We study the influence of the surface orientation on the wetting behavior of
bcc binary alloys, using a semi-infinite lattice model equivalent to a
nearest-neighbor Ising antiferromagnet in an external field. The salient
feature of the model is the generation of an ``effective'' ordering surface
field for symmetry-breaking surface orientations like (100). Such a field
couples to the local order parameter at the surface and leads to the occurrence
of wetting phenomena below the critical temperature T_c. Utilizing a
Ginzburg-Landau continuum model which has been derived earlier from the lattice
mean-field theory, the wetting phase diagram is calculated. | 9704215v2 |
1997-08-30 | Delocalization in Continuous Disordered Systems | Continuous One-dimensional models supporting extended states are studied.
These delocalized statesoccur at well defined values of the energy and are
consequences of simple statistical correlation rules. We explicitly study
alloys of delta-barrier potentials as well as alloys and liquids of quantum
well as.The divergence of the localization length is studied and a critical
exponent 2/3 is found for the delta-barrier case, whereas for the quantum wells
we find an exponent of 2 or 2/3 depending on the well's parameters. These
results support the idea that correlations between random scattering sequences
break Anderson localization. We further calculate the conductance of disordered
superlattices. At the peak transmission the relative fluctuations of the
transmission coefficient are vanishing. | 9709003v1 |
1998-01-11 | Origin of the anomaly in diffuse scattering from disordered Pt-V alloys | An explanation of the anomalous concentration dependence of diffuse
scattering from the Pt-V alloy system (splitting of the (100) short-range order
intensity peak with increasing Pt content) is proposed. The effect is
attributed to the competition between the interaction and self-energy
curvatures. A similar temperature behaviour is predicted. | 9801090v2 |
1998-09-03 | Fermi Surface as the Driving Mechanism for Helical Antiferromagnetic Ordering in Gd-Y Alloys | The first direct experimental evidence for the Fermi surface (FS) driving the
helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported.
The presence of a FS sheet capable of nesting is revealed, and the nesting
vector associated with the sheet is found to be in excellent agreement with the
periodicity of the helical ordering. | 9809063v1 |
1998-09-09 | Magnetic Properties Of Ni-Mo Single Crystal Alloys: Theory and Experiment | The magnetization of Ni_{1-x}Mo_{x} single crystals with x=4,6,8 and 10 % by
weight have been measured at 4.2K using a vibrating sample magnetometer and a
Superconducting Quantum Interference Device (SQUID). The magnetization of the
alloy at these low concentrations and at 0 K have been theoretically determined
by using the tight-binding linearized muffin-tin orbital method coupled with
augmented space recursion. The theoretical data are compared with the
experiment. | 9809137v1 |
1999-06-29 | Quantitative Model of Large Magnetostrain Effect in Ferromagnetic Shape Memory Alloys | A quantitative model describing large magnetostrain effect observed in
several ferromagnetic shape memory alloys such as Ni2MnGa is briefly
reported.The paper contains an exact thermodynamic consideration of the
mechanical and magnetic properties for a similar type materials. As a result,
the basic mechanical state equation including magnetic field effect is directly
derived from a general Poisson's rule. It is shown that the magnetic field
induced deformation effect is directly connected with the strain dependence of
magnetization. A simple model of magnetization and its dependence on the strain
is considered and applied to explain the results of experimental study of large
magnetostrain effects in Ni2MnGa. | 9906433v1 |
1999-08-03 | Oscillatory Exchange Coupling across Cr$_{(1-x)}$V$_x$ Alloy spacers | We have identified the pieces of the Fermi surface responsible for the long
period oscillations of magnetic coupling across Cr and Cr$_{(1-x)}$V$_x$ alloy
spacers in metallic multilayers. Analysing experiments and results of KKR-CPA
calculations we find that the periods are determined by the extremal
wave~vectors of the hole pockets centered on the N-point in the Brillouin zone. | 9908049v1 |
1999-09-10 | Magnetic Properties of disordered CoCu alloys:A first principles approach | Crystalline Co$_{x}$Cu$_{1-x}$ alloys show interesting magnetic behavior over
the entire concentration regime. We here present a fully self- consistent first
principles electronic structure studies of the electronic structure and
magnetic properties of the system.We present results for the variation of
density of states, magnetic moment,spin susceptibility and Curie temperature. | 9909145v1 |
1999-09-13 | Impurity correlations in dilute Kondo alloys | The single impurity Kondo model is often used to describe metals with dilute
concentrations (n_i) of magnetic impurities. Here we examine how dilute the
impurities must be for this to be valid by developing a virial expansion in
impurity density. The O(n_i^2) term is determined from results on the
2-impurity Kondo problem by averaging over the RKKY coupling. The non-trivial
fixed point of the 2-impurity problem could produce novel singularities in the
heat capacity of dilute alloys at O(n_i^2). | 9909186v1 |
1999-10-26 | Composition Patterning in Systems Driven by Competing Dynamics | We study an alloy system where short-ranged, thermally-driven diffusion
competes with externally imposed, finite-ranged, athermal atomic exchanges, as
is the case in alloys under irradiation. Using a Cahn-Hilliard-type approach,
we show that when the range of these exchanges exceeds a critical value,
labyrinthine concentration patterns at a mesoscopic scale can be stabilized.
Furthermore, these steady-state patterns appear only for a window of the
frequency of forced exchanges. Our results suggest that ion beams may provide a
novel route to stabilize and tune the size of nanoscale structural features in
materials. | 9910435v1 |
2000-01-19 | Electron states in a one-dimensional random binary alloy | We present a model for alloys of compound semiconductors by introducing a
one-dimensional binary random system where impurities are placed in one
sublattice while host atoms lie on the other sublattice. The source of disorder
is the stochastic fluctuation of the impurity energy from site to site.
Although the system is one-dimensional and random, we demonstrate analytical
and numerically the existence of extended states in the neighborhood of a given
resonant energy, which match that of the host atoms. | 0001266v1 |
2000-02-10 | Order and Disorder Phenomena at Surfaces of Binary Alloys | We present recent Monte Carlo results on surfaces of bcc-structured binary
alloys which undergo an order-disorder phase transformation in the bulk. In
particular, we discuss surface order and surface induced disorder at the bulk
transition between the ordered (DO3) phase and the disordered (A2) phase. An
intricate interplay between different ordering and segregation phenomena leads
to a complex surface behavior, which depends on the orientation of the surface
under consideration. abstract | 0002157v1 |
2000-04-13 | Large Magnetic-Field-Induced Strains in Ni-Mn-Ga Alloys due to Redistribution of Martensite Variants | A polycrystalline and single-crystal samples of near-stoichiometric Ni2MnGa
alloys have been investigated. It was confirmed that martensite short
crystallographic axis (c-axis) is the easy axis of magnetization. The
reversible reorientation of the easy magnetization direction of martensite
samples after 5% compression is found. The same reversible reorientation of the
easy magnetization direction under applied magnetic field occurs in the
single-crystal samples. The field-induced reorientation of the easy
magnetization direction is accompanied with more than 4% of sample's dimension
change. About the same value of the field-induced strains can be obtained for
polycrystalline samples after appropriate thermo-mechanical treatment. | 0004211v1 |
2000-04-18 | Kondo effect in a magnetic field and the magnetoresistivity of Kondo alloys | The effect of a magnetic field on the spectral density of a $\rm{S=1/2}$
Kondo impurity is investigated at zero and finite temperatures by using
Wilson's numerical renormalization group method. A splitting of the total
spectral density is found for fields larger than a critical value
$H_{c}(T=0)\approx 0.5 T_{K}$, where $T_{K}$ is the Kondo scale. The splitting
correlates with a peak in the magnetoresistivity of dilute magnetic alloys
which we calculate and compare with the experiments on
$\rm{Ce_{x}La_{1-x}Al_{2}}, x=0.0063$. The linear magnetoconductance of quantum
dots exhibiting the Kondo effect is also calculated. | 0004302v2 |
2000-05-22 | CVM studies on the atomic ordering in complex perovskite alloys | The atomic ordering in complex perovskite alloys is investigated by the
cluster variation method (CVM). For the 1/3\{111\}-type ordered structure, the
order-disorder phase transition is the first order, and the order parameter of
the 1:2 complex perovskite reaches its maximum near x=0.25. For the
1/2\{111\}-type ordered structure, the ordering transition is the second order.
Phase diagrams for both ordered structures are obtained. The order-disorder
line obeys the linear law. | 0005341v1 |
2000-08-01 | An elastic model for the In-In correlations in In(x)Ga(1-x)As semiconductor alloys | Deviations from randomicity in In(x)Ga(1-x) As semiconductor alloys induced
by elastic effects are investigated within the Keating potential. Our model is
based on Monte Carlo simulations on large (4096 atoms) supercells, performed
with two types of boundary conditions: Fully periodic boundary conditions
represent the bulk, while periodic boundary conditions along the x and y
directions and a free surface in the z direction simulate the epitaxial growth
environment. We show that In-In correlations identified in the bulk tend to be
enhanced in the epitaxially grown samples. | 0008020v1 |
2000-10-23 | First order metamagnetic transition in CeFe$_2$ based pseudobinary alloys | We present results of dc magnetisation study showing that the low temperature
antiferromagnetic state in various CeFe$_2$-based pseudobinary alloys can be
transformed into ferromagnetic state through a magnetic field induced phase
transition. We highlight the presence of hysteresis and phase coexistence
across this metamagnetic transition and argue that the observed phase
transition is of first order in nature. | 0010341v1 |
2000-11-09 | "Cold Melting" of Invar Alloys | An anomalously strong volume magnetostriction in Invars may lead to a
situation when at low temperatures the dislocation free energy becomes negative
and a multiple generation of dislocations becomes possible. This generation
induces a first order phase transition from the FCC crystalline to an amorphous
state, and may be called "cold melting". The possibility of the cold melting in
Invars is connected with the fact that the exchange energy contribution into
the dislocation self energy in Invars is strongly enhanced, as compared to
conventional ferromagnetics, due to anomalously strong volume magnetostriction.
The possible candidate, where this effect can be observed, is a FePt disordered
Invar alloy in which the volume magnetostriction is especially large. | 0011158v2 |
2001-10-01 | Ab-initio design of perovskite alloys with predetermined properties: The case of Pb(Sc_{0.5} Nb_{0.5})O_{3} | A first-principles derived approach is combined with the inverse Monte Carlo
technique to determine the atomic orderings leading to prefixed properties in
Pb(Sc_{0.5}Nb_{0.5})O_{3} perovskite alloy. We find that some arrangements
between Sc and Nb atoms result in drastic changes with respect to the
disordered material, including ground states of new symmetries, large
enhancement of electromechanical responses, and considerable shift of the Curie
temperature. We discuss the microscopic mechanisms responsible for these
unusual effects. | 0110044v1 |
2002-05-17 | Tight-binding parameters from the full-potential linear muffin-tin orbital method: A feasibility study on NiAl | We have examined a method of direct extraction of accurate tight-binding
parameters from an ab-initio band-structure calculation. The linear muffin-tin
potential method, in its full-potential implementation, has been used to
provide the hamiltonian and overlap matrix elements in the momentum space.
These matrix elements are Fourier transformed to real space to produce the
tight-binding parameters. The feasibility of this method has been tested on the
intermetallic alloy NiAl, using spd orbitals for each atom. The parameters
generated for this alloy have been used as input to a real-space calculation of
the local density of states using the recursion method. | 0205368v1 |
2002-07-27 | KP-approach for non-symmetric short-range defects: resonant states and alloy bandstructure | The short-range defect with reduced symmetry is studied in the framework of
KP-approach taking into account a matrix structure of potential energy in the
equations for envelope functions. The case of the narrow-gap semiconductor,
with defects which are non-symmetric along the [001], [110], or [111]
directions, is considered. Resonant state at a single defect is analyzed within
the Koster-Slater approximation. The bandstructure modification of the alloy,
formed by non-symmetric impurities, is discussed and a generalized virtual
crystal approximation is introduced. | 0207661v2 |
2002-09-13 | Disorder induced critical phenomena in magnetically glassy Cu-Al-Mn alloys | Measurements of magnetic hysteresis loops in Cu-Al-Mn alloys of different Mn
content at low temperatures are presented. The loops are smooth and continuous
above a certain temperature, but exhibit a magnetization discontinuity below
that temperature. Scaling analysis suggest that this system displays a disorder
induced phase transition line. Measurements allow to determine the critical
exponents $\beta=0.03\pm 0.01$ and $\beta \delta = 0.4 \pm 0.1$ in agreement
with those reported recently [Berger et al., Phys. Rev. Lett. {\bf 85}, 4176
(2000)] | 0209323v1 |
2002-10-08 | Coulomb Correlations and Magnetic Anisotropy in ordered $L1_0$ CoPt and FePt alloys | We present results of the magneto-crystalline anisotropy energy (MAE)
calculations for chemically ordered $L1_0$ CoPt and FePt alloys taking into
account the effects of strong electronic correlations and spin-orbit coupling.
The local spin density + Hubbard U approximation (LSDA+U) is shown to provide a
consistent picture of the magnetic ground state properties when intra-atomic
Coulomb correlations are included for both 3$d$ and 5$d$ elements. Our results
demonstrate significant and complex contribution of correlation effects to
large MAE of these material. | 0210157v2 |
2002-11-18 | Study of Phase Stability in NiPt Systems | We have studied the problem of phase stability in NiPt alloy system. We have
used the augmented space recursion based on the TB-LMTO as the method for
studying the electronic structure of the alloys. In particular, we have used
the relativistic generalization of our earlier technique. We note that, in
order to predict the proper ground state structures and energetics, in addition
to relativistic effects, we have to take into account charge transfer effects
with precision. | 0211378v3 |
2003-01-09 | Soft X-ray Magnetic Circular Dichroism of c(2x2) CuMn Ordered Surface Alloy | Mn 2p soft X-ray absorption (XAS) spectroscopy excited with circularly
polarized synchrotron radiation has been applied to a new class of material,
c(2x2)CuMn/Cu(001) two-dimensional ordered surface alloy. A significant X-ray
magnetic circular dichroism (XMCD) signal has been clearly observed at T=25K,
indicating the existence of the ferromagnetic state under the external magnetic
field of 1.4 Tesla. The lineshape analyses of the XAS and XMCD spectra clearly
show that the Mn 3d state is rather localized and has a high spin magnetic
moment due to its half-filled character. | 0301114v1 |
2003-03-04 | Monte Carlo Study of Ordering and Domain Growth in a Class of fcc-Alloy Models | Ordering processes in fcc-alloys with composition A_3B (like Cu_3Au, Cu_3Pd,
CoPt_3 etc.) are investigated by Monte Carlo simulation within a class of
lattice models based on nearest-neighbor (NN) and second-neighbor (NNN)
interactions. Using an atom-vacancy exchange algorithm, we study the growth of
ordered domains following a temperature quench below the ordering spinodal. For
zero NNN-interactions we observe an anomalously slow growth of the domain size
L(t) \sim t^\alpha, where \alpha \sim 1/4 within our accessible timescales.
With increasing NNN-interactions domain growth becomes faster and \alpha
gradually approaches the value 1/2 as predicted by the conventional
Lifshitz-Allen-Cahn theory. | 0303052v1 |
2003-07-01 | Magnetic Properties of Fe75Cu4Ni4Mo2B15 Alloy | The structural and magnetic properties of ball-milled Fe75Cu4Ni4Mo2B15 alloy
have been investigated through powder X-ray diffraction studies, magnetization
and magnetoimpedance measurements. The particle size decreased rapidly with
ball-milling up to 165 hours and then remained as a constant with further hours
of milling while, the lattice expanded with milling time up to 165 hours and
contracted back with further milling. Saturation magnetization is found to
decrease rapidly up to 165 hours of milling and then started decreasing slowly
with further milling time. Magnetoimpedance studies were carried out in pellets
and thin films of Fe75Cu4Ni4Mo2B15. Magnetoimpedance in pellet is observed to
be ~2.5% at 5Hz whereas, in thin films, it is observed to be ~1.5% at 3 MHz. | 0307019v1 |
2003-07-24 | First-principles thermodynamics of transition metals and alloys: W, NiAl, PdTi | We apply the pseudopotential density functional perturbation theory approach
along with the quasiharmonic approximation to calculate the thermal expansion
of tungsten and two important metallic alloys, NiAl and PdTi. We derive the
theory for anisotropic crystal structures and test the approximation that the
anisotropic effects of thermal expansion are equivalent to negative pressure -
this simplifies the calculation enormously for complex structures. Throughout,
we find excellent agreement with experimental results. | 0307621v1 |
2003-09-09 | Relaxation under Supercooling of the Disordered Vortex State in Doped CeRu2 Alloy | Relaxation in magnetisation of metastable states is reported. The states are
prepared by supercooling a Ce(Ru, 5% Nd)2 alloy sample to different extents
through similar paths in the H-T space, and then by applying a field jerk of 20
Oe of either sign. Based on standard theory, a possible explanation is put
forward. | 0309222v1 |
2003-12-12 | Chiral susceptibility in canonical spin glass and reentrant alloys from Hall effect measurements | The extraordinary Hall effect coefficients $R_s(T)$ of the canonical spin
glass alloys ${\bf Au}Fe 8 at%$ and ${\bf Au}Mn 8 at%$ and of an archetype
reentrant system ${\bf Au}Fe 18 at%$ were measured as functions of temperature.
The data show a critical cusp-like term superimposed on a smooth background for
the spin glasses, and change of sign with temperature for the reentrant. The
results can be interpreted consistently by invoking a chiral Hall effect
contribution as proposed by Kawamura. | 0312311v1 |
2004-02-13 | Order-disorder transition in the Cd-Ca cubic approximant | Recent experiments discovered an order-disorder transition occuring at low
temperatures in large unit 1/1 cell cubic approximants of the stable Cd-based
binary alloy quasicrystals. The transition is related to correlations among
orientational degrees of freedom whose separations are around 12 \AA. We
analyze the interactions between the degrees of freedom using {\em ab-initio}
calculations for Cd-Ca alloys and derive an equivalent antiferromagnetic Ising
model which shows a similar phase transition. However, the calculated
transition temperature is higher than observed experimentally, indicating that
the actual structure and its order-disorder transition are more complex than
originally proposed. A side-benefit of our study is the discovery of a
canonical-cell decoration model for the Cd-Ca icosahedral phase. | 0402370v1 |
2004-02-13 | Quasicrystal approximants with novel compositions and structures | We identify several new quasicrystal approximants in alloy systems in which
quasicrystals have not been previously reported. Some occur in alloys with
large size contrast between the constituent elements, either containing small
Boron atoms, or large Ca/Eu atoms, leading to quasicrystal structures quite
different from currently known systems where the size contrast is smaller.
Another group of the approximants are layered Frank--Kasper structures,
demonstrating competition between decagonal and dodecagonal ordering within
this family of structures. | 0402371v1 |
2004-03-18 | Optical properties of random alloys : A formulation | We present here a formulation for the calculation of the
configuration-averaged optical conductivity in random alloys. Our formulation
is based on the augmented-space theorem introduced by one of us [A. Mookerjee,
J. Phys. C: Solid State Phys. 6, 1340 (1973)]. We show that disorder scattering
renormalizes the electron and hole propagators as well as the transition
amplitude. The corrections to the transition amplitude have been shown to be
related to the self-energy of the propagators and vertex corrections. | 0403456v2 |
2004-05-13 | Precessional dynamics of elemental moments in a ferromagnetic alloy | We demonstrate an element-specific measurement of magnetization precession in
a metallic ferromagnetic alloy, separating Ni and Fe moment motion in Ni81Fe19.
Pump-probe X-ray magnetic circular dichroism (XMCD), synchronized with short
magnetic field pulses, is used to measure free magnetization oscillations up to
2.6 GHz with elemental specificity and a rotational resolution of < 2 deg.
Magnetic moments residing on Ni sites and Fe sites in a Ni81Fe19(50nm) thin
film are found to precess together at all frequencies, coupled in phase within
instrumental resolution of 90 ps. | 0405295v1 |
2004-08-06 | Scaling Behavior of the Portevin-Le Chatelier Effect in an Al-2.5%Mg Alloy | The scaling behavior of the Portevin-Le Chatelier (PLC) effect was studied by
deforming Al-2.5%Mg alloy for a wide range of strain rates. To reveal the exact
scaling nature, the time series data of true stress vs. time, obtained during
deformation, were analyzed by two complementary methods: the finite variance
scaling method and the diffusion entropy analysis. From these analyses we could
establish that in the entire span of strain rates, PLC effect showed Levy walk
property. | 0408132v3 |
2004-11-08 | Improved Hc2 in Bulk-Form Magnesium Diboride by Mechanical Alloying With Carbon | High energy milling of MgB2 pre-reacted powder renders the material largely
amorphous through extreme mechanical deformation and is suitable for
mechanically alloying MgB2 with dopants including carbon. Bulk samples of
milled C and MgB2 powders subjected to hot isostatic pressing and Mg vapor
annealing have achieved critical fields in excess of 32T and critical current
density approaching 10^6 A/cm^2. | 0411199v2 |
2005-02-22 | High-throughput ab initio analysis of the Bi-In, Bi-Mg, Bi-Sb, In-Mg, In-Sb, and Mg-Sb systems | Prediction and characterization of crystal structures of alloys are a key
problem in materials research. Using high-throughput ab initio calculations we
explore the low-temperature phase diagrams for the following systems: {Bi-In,
Bi-Mg, Bi-Sb, In-Mg, In-Sb, and Mg-Sb}. For the experimentally observed phases
in these systems we provide information about their stability at low
temperatures. Keywords: Binary Alloys, Ab initio, Intermetallics, Transition
Metals, Structure Prediction, Phase Stability, Magnesium, Indium, Bismuth,
Antimony. | 0502535v1 |
2005-04-04 | Transition from Ferromagnetism to Antiferromagnetism in Ga$_{1-x}$Mn$_x$N | Using density functional theory, we study the magnetic stability of the
Ga$_{1-x}$Mn$_x$N alloy system. We show that unlike Ga$_{1-x}$Mn$_x$As, which
shows only ferromagnetic (FM) phase, Ga$_{1-x}$Mn$_x$N can be stable in either
FM or antiferromagnetic phases depending on the alloy concentration. The
magnetic order can also be altered by applying pressure or with charge
compensation. A unified model is used to explain these behaviors. | 0504083v1 |
2005-05-25 | Sharp magnetization step across the ferromagnetic to antiferromagnetic transition in doped-CeFe$_2$ alloys | Very sharp magnetization step is observed across the field induced
antiferromagnetic to ferromagnetic transition in various doped-CeFe$_2$ alloys,
when the measurement is performed below 5K. In the higher temperature regime
(T$>$5K) this transition is quite smooth in nature. Comparing with the recently
observed similar behaviour in manganites showing colossal magnetoresistance and
magnetocaloric material Gd$_5$Ge$_4$ we argue that such magnetization step is a
generalized feature of a disorder influenced first order phase transition. | 0505609v1 |
2005-11-07 | Observation of enhanced fluctuation diamagnetism in lanthanum superconductors with dilute magnetic impurities | The fluctuation-induced diamagnetism (FD), associated with the presence of
precursor Cooper pairs in the normal state, has been measured in lanthanum with
dilute magnetic (Pr) and nonmagnetic (Lu) impurities. It is found that while
for pure La and La-Lu alloys the FD agrees, as expected, with the theoretical
predictions, it is much larger for La-Pr alloys (around a factor 5 for
La-2at.%Pr). These results suggest the existence of an indirect contribution to
the FD arising from the interaction between fluctuating Cooper pairs and
magnetic impurities. | 0511175v1 |
2005-12-14 | Optical conductivity in disordered alloys : an approach via the augmented space recursion | We present here a calculation of the configuration averaged optical
conductivity of random binary alloys CuAu and AgAu. Our formulation is based on
the augmented space formalism proposed by Mookerjee [J. Phys. C : Solid State
Phys. {\bf 6} 1340 (1973)] and the optical conductivity is obtained directly
through a recursive procedure suggested by Viswanath and M\"uller ["The user
friendly recursion method", Troisieme Cycle de la Physique, en Suisse Romande
(1993)]. | 0512300v1 |
2005-12-22 | Point defect concentrations in metastable Fe-C alloys | Point defect species and concentrations in metastable Fe-C alloys are
determined using density functional theory and a constrained free-energy
functional. Carbon interstitials dominate unless iron vacancies are in
significant excess, whereas excess carbon causes greatly enhances vacancy
concentration. Our predictions are amenable to experimental verification; they
provide a baseline for rationalizing complex microstructures known in hardened
and tempered steels, and by extension other technological materials created by
or subjected to extreme environments. | 0512598v2 |
2006-01-24 | Anomalous electronic correlations in ground state momentum density of Al$_{97}$Li$_3$ | We report high resolution Compton scattering measurements on an
Al$_{97}$Li$_3$ disordered alloy single crystal for momentum transfer along the
[100], [110] and [111] symmetry directions. The results are interpreted via
corresponding KKR-CPA (Korringa-Kohn-Rostoker coherent potential approximation)
first principles computations. By comparing spectra for Al$_{97}$Li$_3$ and Al,
we show that the momentum density in the alloy differs significantly from the
predictions of the conventional Fermi liquid picture and that the ground state
of Al is modified anomalously by the addition of Li. | 0601530v1 |
2006-01-30 | Is Delta_{pi}-gap-only superconductivity possible in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2} alloys? | Using density-functional-based method, we study the k-resolved sigma- and
pi-band holes in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2} alloys. We find
that the calculated profiles of the loss of sigma- and pi-band holes in these
two systems as a function of impurity concentration are in qualitative
agreement with experiments, as expected. We also describe its implications
vis-a-vis superconductivity in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2}. | 0601675v1 |
2006-03-01 | Lattice dynamics of the high temperature shape memory alloy Nb-Ru | Nb-Ru is a high temperature shape memory alloy that undergoes a Martensitic
transformation from a parent cubic b-phase into a tetragonal b' phase at TM 900
C. Measurements of the phonon dispersion curves show that the [110]-TA2 phonon
branch, corresponding in the q=0 limit to the elastic constant C'=1/2(C11-C12)
has an anomalous temperature dependence. Nearly the entire branch softens with
decreasing temperature as TM is approached. The temperature dependence of the
low-q phonon energies suggests that the elastic constants would approach 0 as T
approaches TM, indicating a second order transition. No additional lattice
modulation is observed in the cubic phase. | 0603031v1 |
2006-07-03 | First principles molecular dynamics study of amorphous Si\sub{1-x}Ge\sub{x}:H alloys | We study the structural, dynamical and electronic properties of amorphous
Si\sub{1-x}Ge\sub{x}:H alloys using first principles local basis molecular
dynamics simulation. The network topology and defects in the amorphous network
have been analyzed. Structural changes and an increase in number of defects
have been found as the Ge atomic percentage increases from x=0.1 to x=0.5. The
electronic density of states exhibits a decreasing band-gap and increased
mid-gap and band-tail defect states as Ge concentration increases.
Investigation of the band tails of the density of states show an exponential
(Urbach) behavior. The mobility gap is estimated as a function of Ge
concentration. | 0607058v1 |
2006-07-17 | Density Functional Theory of Freezing and Phase Field Crystal Modeling | In this paper the relationship between the density functional theory of
freezing and phase field modeling is examined. More specifically a connection
is made between the correlation functions that enter density functional theory
and the free energy functionals used in phase field crystal modeling and
standard models of binary alloys (i.e., regular solution model). To demonstrate
the properties of the phase field crystal formalism a simple model of binary
alloy crystallization is derived and shown to simultaneously model
solidification, phase segregation, grain growth, elastic and plastic
deformations in anisotropic systems with multiple crystal orientations on
diffusive time scales. | 0607419v1 |
2006-08-30 | Self - Organized Si Dots On Ge Substrates | The epitaxial growth conditions for silicon on germanium substrates were
investigated as a function of growth temperature and monolayer coverage. Island
formation was observed for the hole studied temperature range, although strong
alloying with the substrate occurred for the highest temperatures. Carbon
pre-deposition offers suitable nucleation centers for the Si island and
reduction of alloying. pre-structured Ge substrates were prepared to enhance
islanding and to achieve ordering. | 0608660v1 |
2006-09-08 | Magneto-optical characterization of MnxGe1-x alloys obtained by ion implantation | Magneto-optical Kerr effect hysteresis loops at various wavelengths in the
visible/near-infrared range have been used to characterize the magnetic
properties of alloys obtained by implanting Mn ions at fixed energy in a Ge
matrix. The details of the hysteresis loops reveal the presence of multiple
magnetic contributions. They may be attributed to the inhomogeneous
distribution of the magnetic atoms and, in particular, to the known coexistence
of diluted Mn in the Ge matrix and metallic Mn-rich nanoparticles embedded in
it [Phys. Rev. B 73, 195207(2006)]. | 0609194v1 |
2006-10-08 | Nonlocal spectral properties of disordered alloys | A general method is proposed for calculating a fully k-dependent, continuous,
and causal spectral function A(k,E) within the recently introduced nonlocal
version of the coherent-potential approximation (NLCPA). The method involves
the combination of both periodic and anti-periodic solutions to the associated
cluster problem and also leads to correct bulk quantities for small cluster
sizes. We illustrate the method by investigating the Fermi surface of a
two-dimensional alloy. Dramatically, we find a smeared electronic topological
transition not predicted by the conventional CPA. | 0610208v1 |
2006-10-11 | 0-pi transition in SFS junctions with strongly spin-dependent scattering | We develop theory of proximity effect in a superconductor - GMR alloy -
superconductor trilayers, which takes into account strong spin dependence of
electron scattering of compositional disorder in a diluted ferromagnetic alloy.
We show that in such a system the critical current oscillations as the function
of the thickness of the ferromagnetic layer, with the period of $v_{F}/2I$,
decay exponentially with the characteristic length of the order of the mean
free path. | 0610299v3 |
2006-10-18 | Ab initio Calculations of the Vibrational Modes of MnAs and Ga1-xMnxAs | In this work, we present our theoretical results for the equation of state
and the phonon dispersions of MnAs, as well as the Mn concentration dependence
of both the lattice parameter and the phonon frequencies of the cubic GaMnAs
alloys. The results are in good agreement with the experimental results
whenever this comparison is possible. Based on the obtained results, the
lattice constants and the phonon frequencies of the alloys do not obey the
Vegard rule. | 0610493v1 |
2006-12-07 | Lanthanum-Cerium Based Bulk Metallic Glasses with Superior Glass-Forming Ability | A quinary (La0.5Ce0.5)65Al10(Co0.6Cu0.4)25 alloy with superior glass-forming
ability (GFA), identified by the formation of fully glassy rod of 32 mm in
diameter by tilt-pour casting, was reported. By comparing with the GFA of
quarternary (La0.5Ce0.5)65Al10TM25 and ternary Ln65Al10TM25 alloys (Ln = La or
Ce; TM = Co or Cu), we suggest that the strong frustration of crystallization
by utilizing the coexistence of La-Ce and Co-Cu to complicate competing
crystalline phases is helpful to construct BMG component with superior GFA. | 0612184v1 |
2006-12-19 | First-principles prediction of high Curie temperature for ferromagnetic bcc-Co and bcc-FeCo alloys and its relevance to tunneling magnetoresistance | We determine from first-principles the Curie temperature Tc for bulk Co in
the hcp, fcc, bcc, and tetragonalized bct phases, for FeCo alloys, and for bcc
and bct Fe. For bcc-Co, Tc=1420 K is predicted. This would be the highest Curie
temperature among the Co phases, suggesting that bcc-Co/MgO/bcc-Co tunnel
junctions offer high magnetoresistance ratios even at room temperature. The
Curie temperatures are calculated by mapping ab initio results to a Heisenberg
model, which is solved by a Monte Carlo method. | 0612497v1 |
2007-01-12 | An experimental method for the in-situ observation of eutectic growth patterns in bulk samples of transparent alloys | We present an experimental method for the in-situ observation of
directional-solidification fronts in bulk samples of transparent eutectic
alloys. The growth front is observed obliquely in dark field through the liquid
and a glass wall of the container with a long-distance microscope. We show that
a focused image of the whole growth front can be obtained at a certain tilt
angle of the microscope. At this tilt angle, eutectic fibers of about 3.5\mic
in diameter can be clearly seen over the whole growth front in 400-\mic thick
samples. | 0701267v1 |
2007-01-29 | Alloying mechanisms for epitaxial nanocrystals | The different mechanisms involved in the alloying of epitaxial nanocrystals
are reported in this letter. Intermixing during growth, surface diffusion and
intra-island diffusion were investigated by varying the growth conditions and
annealing environments during chemical vapor deposition. The relative
importance of each mechanism was evaluated in determining a particular
composition profile for dome-shaped Ge:Si (001) islands. For samples grown at a
faster rate, intermixing during growth was reduced. Si surface diffusion
dominates during H$_2$ annealing whereas Ge surface diffusion and intra-island
diffusion prevail during annealing in a PH$_3$ environment. | 0701710v2 |
2007-02-10 | Geometrical Frustration in Liquid Fe and Fe-Based Metallic Glass | We investigate short rane order in liquid and supercooled liquid Fe and
Fe-based metallic glass using ab-initio simulation methods. We analyze the data
to quantify the degree of local icosahedral and polytetrahedral order and to
understand the role of alloying in controlling the degree of geometric
frustration . Comparing elemental Fe to Cu we find that the degree of
icosahedral order is greater in Fe than in Cu, possibly because icosahedral
disclination line defects are more easily incorporated into BCC environments
than FCC. In Fe-based metallic glass-forming alloys (FeB and FeZrB) we find
that introducing small concentrations of small B atoms and large Zr atoms
controls the frustration of local icosahedral order. | 0702263v1 |
2007-03-06 | Mean-field heat capacity of dilute magnetic alloys | Using an asymptotic solution of the M-impurity thermodynamics of a dilute s-d
system, the impurity energy and impurity heat capacity DeltaC(T) are derived
for dilute magnetic alloys with spin 1/2 and spin 3/2 impurities. The
parameters which enter DeltaC are adjusted to fit experimental data on impurity
heat capacity of CuCr and LaCeAl_2. Agreement is satisfactory for CuCr, at
temperatures below 1K, and good for LaCeAl_2. The magnitude of theoretical
DeltaC(T) agrees with experiment and does not require scaling as in previous
s-d theories. Nonlinear dependence of DeltaC(T) on impurity concentration has
been accounted for the first time. | 0703146v1 |
2002-04-15 | Existence of the density of states for one-dimensional alloy-type potentials with small support | We study spectral properties of Schr\"odinger operators with random
potentials of alloy type on $L^2(\RR)$ and their restrictions to finite
intervals. A Wegner estimate for non-negative single site potentials with small
support is proven. It implies the existence and local uniform boundedness of
the density of states. Our estimate is valid for all bounded energy intervals.
Wegner estimates play a key role in an existence proof of pure point spectrum. | 0204030v1 |
2002-04-15 | Existence of the density of states for some alloy type models with single site potentials of changing sign | We study spectral properties of ergodic random Schr\"odinger operators on
$L^2 (\RR^d)$. The density of states is shown to exist for a certain class of
alloy type potentials with single site potentials of changing sign. The Wegner
estimate we prove implies Anderson localization under certain additional
assumptions. For some examples we discuss briefly some properties of the common
and conditional densities of the random coupling constants used in the proof of
the Wegner estimate. | 0204031v1 |
2005-10-17 | Wegner estimate and the density of states of some indefinite alloy type Schroedinger Operators | We study Schroedinger operators with a random potential of alloy type. The
single site potentials are allowed to change sign. For a certain class of them
we prove a Wegner estimate. This is a key ingredient in an existence proof of
pure point spectrum of the considered random Schroedinger operators. Our
estimate is valid for all bounded energy intervals and all space dimensions and
implies the existence of the density of states. | 0510062v1 |
2006-06-01 | Similarity Solution of the 3-phase Stefan Problem for Alloys with Arbitrary Temperature Dependent Properties | In this paper a 3-phase Stefan problem solution method for 1D semi-infinity
alloy is developed. The problem is first solved for full enthalpy of the system
and then the thermal diffusivity has been eliminated from the divergence
operator by Kirchoff transformation. Moreover, we introduce a similarity
independent variable $\eta=x^{2}/\tau$ and original problem transforms to
ordinary differential equations (ODE) for each phase separately. These ODEs
have Dirichlet's type boundary conditions. | 0606001v1 |
2005-05-24 | Calculation of single-beam two-photon absorption transition rate of rare-earth ions using effective operator and diagrammatic representation | Effective operators needed in single-beam two-photon transition calculations
have been represented with modified Goldstone diagrams similar to the type
suggested by Duan and co-workers [J. Chem. Phys. 121, 5071 (2004) ]. The rules
to evaluate these diagrams are different from those for effective Hamiltonian
and one-photon transition operators. It is verified that the perturbation terms
considered contain only connected diagrams and the evaluation rules are
simplified and given explicitly. | 0505162v1 |
2005-11-03 | A small sealed Ta crucible for thermal analysis of volatile metallic samples | Differential thermal analysis on metallic alloys containing volatile elements
can be highly problematic. Here we show how measurements can be performed in
commercial, small-sample, equipment without modification. This is achieved by
using a sealed Ta crucible, easily fabricated from Ta tubing and sealed in a
standard arc furnace. The crucible performance is demonstrated by measurements
on a mixture of Mg and MgB$_2$, after heating up to 1470$^{\circ}{\rm C}$. We
also show data, measured on an alloy with composition Gd$_{40}$Mg$_{60}$, that
clearly shows both the liquidus and a peritectic, and is consistent with
published phase diagram data. | 0511022v1 |
2007-05-31 | Lowest-energy structures of 13-atom binary clusters: Do icosahedral clusters exist in binary liquid alloys? | Although the existence of 13-atom icosahedral clusters in one-component
close-packed undercooled liquids was predicted more than half a century ago by
Frank, the existence of such icosahedral clusters is less clear in liquid
alloys. We study the lowest-energy structures of 13-atom AxB13-x Lennard-Jones
binary clusters using the modified space-fixed genetic algorithm and the
artificial Lennard-Jones potential designed by Kob and Andersen. Curiously, the
lowest-energy structures are non-icosahedral for almost all compositions. The
role played by the icosahedral cluster in a binary glass is questionable. | 0705.4555v1 |
2007-07-10 | The modulus of continuity of Wegner estimates for random Schrödinger operators on metric graphs | We consider an alloy type potential on an infinite metric graph. We assume a
covering condition on the single site potentials. For random Schr\"odingers
operator associated with the alloy type potential restricted to finite volume
subgraphs we prove a Wegner estimate which reproduces the modulus of continuity
of the single site distribution measure. The Wegner constant is independent of
the energy. | 0707.1486v1 |
2007-08-29 | Relation of Curie temperature and conductivity: (Ga,Mn)As alloy as a case study | Experimental investigations of diluted magnetic semiconductors indicate a
strong relation between Curie temperature and conductivity. Both quantities
depend non trivially on the concentration of magnetic impurities, the carrier
density, and the presence of compensating defects. We calculate both Curie
temperature and conductivity of (Ga,Mn)As alloys in a selfconsistent manner
based on the same first principles Hamiltonian in which the presence of
compensating defects is taken into account. The effect of As-antisites and
Mn-interstitials is determined separately and a good agreement between theory
and experiment exists only in the case where the dominating mechanism of is due
to the Mn-interstitials. | 0708.3921v1 |
2007-09-06 | The formation and ordering of local magnetic moments in Fe-Al alloys | With density functional theory, studied are the local magnetic moments in
Fe-Al alloys depending on concentration and Fe nearest environment. At zero
temperature, the system can be in different states: ferromagnetic,
antiferromagnetic and spin-spiral waves (SSW) which has a minimum energy. Both
SSW and negative moment of Fe atoms with many Al atoms around them agree with
experiments. Magnetization curves taken from literature are analysed.
Assumption on percolation character of size distribution of magnetic clusters
describes well the experimental superparamagnetic behaviour above 150 K. | 0709.0793v1 |
2007-10-05 | The Debye-Waller factor of stabilized delta-Pu | The Debye-Waller factor has been calculated for stabilized delta-phase
plutonium with 5% aluminum. A quasi-harmonic Born-von Karman force model with
temperature dependent phonon frequencies was used to calculate the mean-square
thermal atomic displacement from absolute zero to 800 K. Implementation of the
observed anomalous softening of the long wavelength phonons with increasing
temperature cannot account for the softening of the measured thermal parameter
at high temperatures nor for its rather high value at low temperatures. The
implications for diffraction measurements on delta-phase stabilized plutonium
alloys are discussed. | 0710.1241v1 |
2007-10-15 | Ferromagnetic resonance study of polycrystalline Fe_{1-x}V_x alloy thin films | Ferromagnetic resonance has been used to study the magnetic properties and
magnetization dynamics of polycrystalline Fe$_{1-x}$V$_{x}$ alloy films with
$0\leq x < 0.7$. Films were produced by co-sputtering from separate Fe and V
targets, leading to a composition gradient across a Si substrate. FMR studies
were conducted at room temperature with a broadband coplanar waveguide at
frequencies up to 50 GHz using the flip-chip method. The effective
demagnetization field $4 \pi M_{\mathrm{eff}}$ and the Gilbert damping
parameter $\alpha$ have been determined as a function of V concentration. The
results are compared to those of epitaxial FeV films. | 0710.2826v2 |
2007-11-28 | Evidence for Two Current Conduction in Iron | Measurements of resistivities of dilute iron based alloys show strong
deviations from Matthiessen's rule. These deviations can be explained by a
model in which spin up and spin down electrons conduct in parallel. The results
are consistent with the theory of impurity shielding in these alloys. [This
1967 paper provides the first experimental demonstration of two current
conduction at low temperatures in a ferromagnetic metal. One direct consequence
of this property is the Giant Magnetoresistance discovered in 1988 by the
groups of Albert Fert and of Peter Gr\"unberg]. | 0711.4478v1 |
2007-12-14 | Theory of electronic transport in random alloys with short-range order: Korringa-Kohn-Rostoker non-local coherent potential approximation | We present an ab-initio formalism for the calculation of transport properties
in compositionally disordered systems within the framework of the
Korringa-Kohn-Rostoker non-local coherent potential approximation. Our
formalism is based upon the single-particle Kubo-Greenwood linear response and
provides a natural means of incorporating the effects of short-range order upon
the transport properties. We demonstrate the efficacy of the formalism by
examining the effects of short-range order and clustering upon the transport
properties of disordered $AgPd$ and $CuZn$ alloys. | 0712.2388v1 |
2008-01-25 | Ferromagnetism and Kondo Insulator Behavior in the Disordered Periodic Anderson Model | The effect of binary alloy disorder on the ferromagnetic phases of f-electron
materials is studied within the periodic Anderson model. We find that disorder
in the conduction band can drastically enhance the Curie temperature due to an
increase of the local f-moment. The effect may be explained qualitatively and
even quantitatively by a simple theoretical ansatz. The emergence of an alloy
Kondo insulator at non-integer filling is also pointed out. | 0801.3934v1 |
2008-02-20 | Empirical oscillating potentials for alloys from ab-initio fits | By fitting to a database of ab-initio forces and energies, we can extract
pair potentials for alloys, with a simple six-parameter analytic form including
Friedel oscillations, which give a remarkably faithful account of many complex
intermetallic compounds. As examples we show results for (crystal or
quasicrystal) structure prediction and phonon spectrum for three systems:
Fe--B, Al--Mg--Zn, and Al--Cu--Fe. | 0802.2926v2 |
2008-03-07 | Orbital magnetism in transition-metal systems: The role of local correlation effects | The influence of correlation effects on the orbital moments for transition
metals and their alloys is studied by first-principle relativistic Density
Functional Theory in combination with the Dynamical Mean-Field Theory. In
contrast to the previous studies based on the orbital polarization corrections
we obtain an improved description of the orbital moments for wide range of
studied systems as bulk Fe, Co and Ni, Fe-Co disordered alloys and 3$d$
impurities in Au. The proposed scheme can give simultaneously a correct
dynamical description of the spectral function as well as static magnetic
properties of correlated disordered metals. | 0803.1000v1 |
2008-03-07 | First-Principles Theory of Competing Order Types, Phase Separation, and Phonon Scattering in Thermoelectric Pb-Sb-Ag-Te Alloys | Using a first-principles cluster expansion, we shed light on the solid-state
phase diagram and structure of the recently discovered high-performance
thermoelectrics, Pb-Ag-Sb-Te alloys. The observed nanoscale inhomogeneities are
shown to be coherent precipitates of ordered (Ag)m(Sb)n(Te)m+n phases, such as
AgSbTe2, all immiscible with rocksalt PbTe. The solubility is high for PbTe in
AgSbTe2 and low for (Ag,Sb)Te in PbTe (8% vs. 0.6% at 850 K). The differences
in the phonon spectra of PbTe and AgSbTe2 confirm that the inhomogeneities
enhance the thermoelectric performance. | 0803.1165v1 |
2008-03-09 | Composition Profiles within Al3Li and Al3Sc/Al3Li Nanoscale Precipitates in Aluminum | An Al-11.3Li-0.11Sc (at. %) alloy was double-aged to induce first
alpha'-Al3Sc and then delta'-Al3Li precipitates. Atom-probe tomography revealed
both single-phase delta'-precipitates and core-shell alpha'/delta'-precipitates
(with respective average radii of 16 and 27 nm, and respective volume fractions
of 12 and 9%) conferring a high strength to the alloy. Although the
delta'-shells contain little Sc (~0.027 at. %), the alpha'-cores have a high Li
content, with an average composition of Al0.72(Sc0.17 Li0.11). The Li
concentrations within the delta'-phase and the Li interfacial excess at the
delta'/alpha'-interface both exhibit wide precipitate-to-precipitate
variations. | 0803.1273v1 |
2008-03-22 | Stoichiometric growth of high Curie temperature heavily-alloyed GaMnAs | Heavily-alloyed, 100 nm Ga1-xMnxAs (x>0.1) films are obtained via low
temperature molecular beam epitaxy utilizing a combinatorial technique which
allows systematic control of excess arsenic during growth. Reproducible,
optimized electronic, magnetic and structural properties are found in a narrow
range of stoichiometric growth conditions. The Curie temperature of
stoichiometric material is 150-165 K and independent of x within a large window
of growth conditions while substitutional Mn content increases linearly,
contradicting the prediction of the Zener Model of hole-mediated
ferromagnetism. | 0803.3245v1 |
2008-04-01 | Interband Effects of Magnetic Field on Hall Effects for Dirac Electrons in Bismuth | Hall effects are investigated for three-dimensional Dirac electrons as a
model of bismuth alloys. It is found that there is unconventional contributions
to the Hall conductivity $\sigma_{xy}$ generated by the interband effects of a
magnetic field. This phenomena is remarkable near the band-edge. The Hall
coefficient exhibits two unexpected properties; a sharp peak at around the
band-edges, and a drastic sign change in the band gap region. Implications of
the present results to bismuth alloys are discussed. | 0804.0076v1 |
2008-04-10 | Character of the Dislocation Bands in the (A+B) regime of the Portevin-Le Chatelier effect in Al-2.5%Mg alloy | The Portevin-Le Chatelier(PLC) effect has been investigated by deforming
Al-2.5%Mg alloy in the strain rate regime where simultaneously two types (type
B and type A) of serrations appear in the stress strain curve. Our analysis
reveal that in this strain rate regime the entire PLC dynamics for a particular
strain rate experiment is governed by a single band which changes its character
during the deformation. | 0804.1670v1 |
2008-04-16 | Site preference of transition-metal elements in L10-TiAl: A first-principles study | The site preference of 3d (Ti-Cu), 4d (Zr-Ag) and 5d (Hf-Au) transition-metal
elements in L10 TiAl is studied using a combination of first-principles
supercell calculations and the statistical mechanical Wagner-Schottky model.
Our key finding is that both temperature and alloy stoichiometry can strongly
affect the site occupancy behavior of ternary alloying elements in L10 TiAl. We
further predict that the tendency of transition metals to occupy the Al sites
in TiAl increases with increasing d-electron number along a series. | 0804.2636v1 |
2008-05-07 | Design Methodology and Manufacture of a Microinductor | Potential core materials to supersede ferrite in the 0.5-10 MHz frequency
range are investigated. The performance of electrodeposited nickel-iron,
cobalt-iron-copper alloys and the commercial alloy Vitrovac 6025 have been
assessed through their inclusion within a custom-made solenoid microinductor.
Although the present inductor, at 500 KHz, achieves 77% power efficiency for
24.7W/cm3 power density, an optimized process predicts a power efficiency of
97% for 30.83W/cm3 power density. The principle issues regarding microinductor
design and performance are discussed. | 0805.0859v1 |
2008-06-03 | Wegner estimates for sign-changing single site potentials | We study Anderson and alloy type random Schr\"odinger operators on
$\ell^2(\ZZ^d)$ and $L^2(\RR^d)$. Wegner estimates are bounds on the average
number of eigenvalues in an energy interval of finite box restrictions of these
types of operators. For a certain class of models we prove a Wegner estimate
which is linear in the volume of the box and the length of the considered
energy interval. The single site potential of the Anderson/alloy type model
does not need to have fixed sign, but it needs be of a generalised step
function form. The result implies the Lipschitz continuity of the integrated
density of states. | 0806.0482v1 |
2008-07-31 | Competing jump cycles for vacancy diffusion in binary alloys | The mean-first-passage-times (MFPTs) for a vacancy that diffuses (via one-
and six-jump cycles) in a two dimensional ordered binary alloy are evaluated
using the properties of random walks on networks. We investigate the effect of
temperature and relative barrier height on the ratio between the MFPTs of the
two cycles. We find that the six-jump cycle takes shorter time than the
one-jump cycle for the range of parameters considered. | 0807.5034v2 |
2008-11-01 | Signature of Martensite transformation on conductivity noise in thin films of NiTi shape memory alloys | Slow time-dependent fluctuations, or noise, in the electrical resistance of
dc magnetron sputtered thin films of Nickel Titanium shape memory alloys have
been measured. Even in equilibrium, the noise was several orders of magnitude
larger than that of simple diffusive metallic films, and was found to be
non-monotonic around the martensitic transformation regime. The results are
discussed in terms of dynamics of structural defects, which also lay foundation
to a new noise-based characterization scheme of martensite transformation. | 0811.0101v1 |
2008-12-01 | Study of phase stability of MnCr using the augmented space recursion based orbital peeling technique | In an earlier communication we have developed a recursion based approach to
the study of phase stability and transition of binary alloys. We had combined
the recursion method introduced by Haydock, Heine and Kelly and the our
augmented space approach with the orbital peeling technique proposed by Burke
to determine the small energy differences involved in the discussion of phase
stability. We extend that methodology for the study of MnCr alloys. | 0812.0216v1 |
2009-07-09 | Anderson localization in a two-particle continuous model with an alloy-type external potential | We establish exponential localization for a two-particle Anderson model in a
Euclidean space ${\mathbb R}^{d}$, $d\ge 1$, in presence of a non-trivial
short-range interaction and a random external potential of the alloy type.
Specifically, we prove that all eigenfunctions with eigenvalues near the lower
edge of the spectrum decay exponentially in $L^2$-norm. | 0907.1459v1 |
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