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2013-12-05 | What we can learn about quantum physics from a single qubit | We present an approach for teaching quantum physics at high school level
based on the simplest quantum system - the single quantum bit (qubit). We show
that many central concepts of quantum mechanics, including the superposition
principle, the stochastic behavior and state change under measurements as well
as the Heisenberg uncertainty principle can be understood using simple
mathematics, and can be illustrated using catchy visualizations. We discuss
abstract features of a qubit in general, and consider possible physical
realizations as well as various applications, e.g. in quantum cryptography. | 1312.1463v1 |
2015-09-18 | Electron-phonon-averaged approximation for first-principles computations of electron relaxation times and transport properties in semiconductor materials | We present a simple and efficient approximation to the electron-phonon
scattering rate suitable for high-throughput screening of candidate materials
for thermoelectric devices, based on electronic transport. The method is
applied to calculate the electronic transport coefficients of half-Heusler
compounds, showing agreement with experimental data. By directly computing
electrical and the electronic part of the thermal conductivities, we find
deviations from the Wiedemann-Franz law in these compounds at high temperatures
and low carrier concentrations. | 1509.05800v2 |
2016-07-13 | Unraveling the dominant phonon scattering mechanism in thermoelectric compound ZrNiSn | Determining defect types and concentrations remains a big challenge of
semiconductor materials science. By using ab-initio thermal conductivity
calculations we reveal that Ni/vacancy antisites, and not the previously
claimed Sn/Zr antisites, are the dominant defects affecting thermal transport
in half-Heusler compound ZrNiSn. Our calculations correctly predict the thermal
conductivity dependence with temperature and concentration, in quantitative
agreement with published experimental results. Furthermore, we find a
characteristic proportionality between phonon-antisite scattering rates and the
sixth power of phonon frequency, for which we provide an analytic derivation.
These results suggest that thermal conductivity measurements in combination
with ab-initio calculations can be used to quantitatively assess defect types
and concentrations in semiconductors. | 1607.03624v1 |
2023-04-27 | Effective Tight-Binding Model of Compensated Ferrimagnetic Weyl Semimetal with Spontaneous Orbital Magnetization | The effective tight-binding model with compensated ferrimagnetic
inverse-Heusler lattice Ti$_{2}$MnAl, candidate material of magnetic Weyl
semimetal, is proposed. The energy spectrum near the Fermi level, the
configurations of the Weyl points, and the anomalous Hall conductivity are
calculated. We found that the orbital magnetization is finite, while the total
spin magnetization vanishes, at the energy of the Weyl points. The magnetic
moments at each site are correlated with the orbital magnetization, and can be
controlled by the external magnetic field. | 2304.14009v1 |
2005-06-28 | Geometric, electronic, and magnetic structure of Co$_2$FeSi: Curie temperature and magnetic moment measurements and calculations | In this work a simple concept was used for a systematic search for new
materials with high spin polarization. It is based on two semi-empirical
models. Firstly, the Slater-Pauling rule was used for estimation of the
magnetic moment. This model is well supported by electronic structure
calculations. The second model was found particularly for Co$_2$ based Heusler
compounds when comparing their magnetic properties. It turned out that these
compounds exhibit seemingly a linear dependence of the Curie temperature as
function of the magnetic moment. Stimulated by these models, Co$_2$FeSi was
revisited. The compound was investigated in detail concerning its geometrical
and magnetic structure by means of X-ray diffraction, X-ray absorption and
M\"o\ss bauer spectroscopies as well as high and low temperature magnetometry.
The measurements revealed that it is, currently, the material with the highest
magnetic moment ($6\mu_B$) and Curie-temperature (1100K) in the classes of
Heusler compounds as well as half-metallic ferromagnets. The experimental
findings are supported by detailed electronic structure calculations. | 0506729v4 |
2006-09-22 | Electronic structure, magnetism, and disorder in the Heusler compound Co$_2$TiSn | Polycrystalline samples of the half-metallic ferromagnet Heusler compound
Co$_2$TiSn have been prepared and studied using bulk techniques (X-ray
diffraction and magnetization) as well as local probes ($^{119}$Sn M\"ossbauer
spectroscopy and $^{59}$Co nuclear magnetic resonance spectroscopy) in order to
determine how disorder affects half-metallic behavior and also, to establish
the joint use of M\"ossbauer and NMR spectroscopies as a quantitative probe of
local ion ordering in these compounds. Additionally, density functional
electronic structure calculations on ordered and partially disordered
Co$_2$TiSn compounds have been carried out at a number of different levels of
theory in order to simultaneously understand how the particular choice of DFT
scheme as well as disorder affect the computed magnetization. Our studies
suggest that a sample which seems well-ordered by X-ray diffraction and
magnetization measurements can possess up to 10% of antisite (Co/Ti)
disordering. Computations similarly suggest that even 12.5% antisite Co/Ti
disorder does not destroy the half-metallic character of this material.
However, the use of an appropriate level of non-local DFT is crucial. | 0609578v2 |
2006-11-17 | Electronic structure and magnetism in doped semiconducting half-Heusler compounds | We have studied in details the electronic structure and magnetism in M (Mn
and Cr) doped semiconducting half-Heusler compounds FeVSb, CoTiSb and NiTiSn
(XM$_{x}$Y$_{1-x}$Z) in a wide concentration range using local-spin density
functional method in the framework of tight-binding linearized muffin tin
orbital method(TB-LMTO) and supercell approach. Our calculations indicate that
some of these compounds are not only ferromagnetic but also half-metallic and
may be useful for spintronics applications. The electronic structure of the
doped systems is analyzed with the aid of a simple model where we have
considered the interaction between the dopant transition metal (M) and the
valence band X-Z hybrid. We have shown that the strong X-d - M-d interaction
places the M-d states close to the Fermi level with the M-t$_{2g}$ states lying
higher in energy in comparison to the M-e$_{g}$ states. Depending on the number
of available d-electrons, ferromagnetism is realized provided the d-manifold is
partially occupied. The tendencies toward ferromagnetic(FM) or
antiferromagnetic(AFM) behavior are discussed within Anderson-Hasegawa models
of super-exchange and double-exchange. In our calculations for Mn doped NiTiSn,
the strong preference for FM over AFM ordering suggests a possible high Curie
temperature for these systems. | 0611478v1 |
2013-07-24 | μSR and NMR study of the superconducting Heusler compound YPd2Sn | We report on muon spin rotation/relaxation and $^{119}$Sn nuclear magnetic
resonance (NMR) measurements to study the microscopic superconducting and
magnetic properties of the Heusler compound with the highest superconducting
transition temperature, \ypd\ ($T_c=5.4$ K). Measurements in the vortex state
provide the temperature dependence of the effective magnetic penetration depth
$\lambda(T)$ and the field dependence of the superconducting gap $\Delta(0)$.
The results are consistent with a very dirty s-wave BCS superconductor with a
gap $\Delta(0)=0.85(3)$ meV, $\lambda(0)= 212(1)$ nm, and a Ginzburg-Landau
coherence length $\xi_{\mathrm{GL}}(0)\cong 23$ nm. In spite of its very dirty
character, the effective density of condensed charge carriers is high compared
to the normal state. The \mSR data in a broad range of applied fields are well
reproduced by taking into account a field-related reduction of the effective
superconducting gap. Zero-field \mSR measurements, sensitive to the possible
presence of very small magnetic moments, do not show any indications of
magnetism in this compound. | 1307.6386v2 |
2014-01-10 | Finding unprecedentedly low-thermal-conductivity half-Heusler semiconductors via high-throughput materials modeling | The lattice thermal conductivity ({\kappa}{\omega}) is a key property for
many potential applications of compounds. Discovery of materials with very low
or high {\kappa}{\omega} remains an experimental challenge due to high costs
and time-consuming synthesis procedures. High-throughput computational
pre-screening is a valuable approach for significantly reducing the set of
candidate compounds. In this article, we introduce efficient methods for
reliably estimating the bulk {\kappa}{\omega} for a large number of compounds.
The algorithms are based on a combination of machine-learning algorithms,
physical insights, and automatic ab-initio calculations. We scanned
approximately 79,000 half-Heusler entries in the AFLOWLIB.org database. Among
the 450 mechanically stable ordered semiconductors identified, we find that
{\kappa}{\omega} spans more than two orders of magnitude- a much larger range
than that previously thought. {\kappa}{\omega} is lowest for compounds whose
elements in equivalent positions have large atomic radii. We then perform a
thorough screening of thermodynamical stability that allows to reduce the list
to 77 systems. We can then provide a quantitative estimate of {\kappa}{\omega}
for this selected range of systems. Three semiconductors having
{\kappa}{\omega} < 5 W /(m K) are proposed for further experimental study. | 1401.2439v2 |
2015-01-16 | Topological $R$PdBi half-Heusler semimetals: a new family of non-centrosymmetric magnetic superconductors | We report superconductivity and magnetism in a new family of topological
semimetals, the ternary half Heusler compounds $R$PdBi ($R$ : rare earth). In
this series, tuning of the rare earth $f$-electron component allows for
simultaneous control of both lattice density via lanthanide contraction, as
well as the strength of magnetic interaction via de Gennes scaling, allowing
for a unique tuning of both the normal state band inversion strength,
superconducting pairing and magnetically ordered ground states.
Antiferromagnetism with ordering vector (0.5,0.5,0.5) occurs below a Ne\'eel
temperature that scales with de Gennes factor $dG$, while a superconducting
transition is simultaneously linearly suppressed. With superconductivity
appearing in a system with non-centrosymmetric crystallographic symmetry, the
possibility of spin-triplet Cooper pairing with non-trivial topology analogous
to that predicted for the normal state electronic structure provides a unique
and rich opportunity to realize both predicted and new exotic excitations in
topological materials. | 1501.04096v3 |
2015-02-12 | High Electron Mobility and Large Magnetoresistance in the Half-Heusler Semimetal LuPtBi | Materials with high carrier mobility showing large magnetoresistance (MR)
have recently received much attention because of potential applications in
future high-performance magneto-electric devices. Here, we report on the
discovery of an electron-hole-compensated half-Heusler semimetal LuPtBi that
exhibits an extremely high electron mobility of up to 79000 cm2/Vs with a
non-saturating positive MR as large as 3200% at 2 K. Remarkably, the mobility
at 300 K is found to exceed 10500 cm2/Vs, which is among the highest values
reported in three-dimensional bulk materials thus far. The clean Shubnikov-de
Haas quantum oscillation observed at low temperatures and the first-principles
calculations together indicate that the high electron mobility is due to a
rather small effective carrier mass caused by the distinctive band structure of
the crystal. Our finding provide a new approach for finding large,
high-mobility MR materials by designing an appropriate Fermi surface topology
starting from simple electron-hole-compensated semimetals. | 1502.03523v5 |
2016-02-03 | Robust tunability of magnetorestance in Half-Heusler RPtBi (R = Gd, Dy, Tm, and Lu) compounds | We present the magnetic field dependencies of transport properties for
$R$PtBi ($R$ = Gd, Dy, Tm, and Lu) half-Heusler compounds. Temperature and
field dependent resistivity measurements of high quality $R$PtBi single
crystals reveal an unusually large, non-saturating magnetoresistance (MR) up to
300 K under a moderate magnetic field of $H$ = 140 kOe. At 300 K, the large MR
effect decreases as the rare-earth is traversed from Gd to Lu and the magnetic
field dependence of MR shows a deviation from the conventional $H^{2}$
behavior. The Hall coefficient ($R_{H}$) for $R$ = Gd indicates a sign change
around 120 K, whereas $R_{H}$ curves for $R$ = Dy, Tm, and Lu remain positive
for all measured temperatures. At 300 K, the Hall resistivity reveals a
deviation from the linear field dependence for all compounds. Thermoelectric
power measurements on this family show strong temperature and magnetic field
dependencies which are consistent with resistivity measurements. A highly
enhanced thermoelectric power under applied magnetic field is observed as high
as $\sim$100 $\mu$V/K at 140 kOe. Analysis of the transport data in this series
reveals that the rare-earth-based Half-Husler compounds provide opportunities
to tune MR effect through lanthanide contraction and to elucidate the mechanism
of non-trivial MR. | 1602.01194v1 |
2016-02-23 | The chiral anomaly and thermopower of Weyl fermions in the half-Heusler GdPtBi | The Dirac and Weyl semimetals are unusual materials in which the nodes of the
bulk states are protected against gap formation by crystalline symmetry. The
chiral anomaly~\cite{Adler,Bell}, predicted to occur in both systems, was
recently observed as a negative longitudinal magnetoresistance (LMR) in
Na$_3$Bi and in TaAs. An important issue is whether Weyl physics appears in a
broader class of materials. We report evidence for the chiral anomaly in the
half-Heusler GdPtBi. In zero field, GdPtBi is a zero-gap semiconductor with
quadratic bands. In a magnetic field, the Zeeman energy leads to Weyl nodes. We
have observed a large negative LMR with the field-steering properties specific
to the chiral anomaly. The chiral anomaly also induces strong suppression of
the thermopower. We report a detailed study of the thermoelectric response
function $\alpha_{xx}$ of Weyl fermions. The scheme of creating Weyl nodes from
quadratic bands suggests that the chiral anomaly may be observable in a broad
class of semimetals. | 1602.07219v2 |
2017-01-09 | Spin-Wave versus Joule Heating in Spin-Hall-Effect/Spin-Transfer-Torque Driven Cr/Heusler/Pt Waveguides | We present a time-resolved study of the DC-current driven magnetization
dynamics in a microstructured Cr/Heusler/Pt waveguide by means of Brillouin
light scattering. A reduction of the effective spin-wave damping via the
spin-transfer-torque effect leads to a strong increase in the magnon density.
This is accompanied by a decrease of the spin-wave frequencies. By evaluating
the time scales of these effects, the origin of this frequency shift can be
identified. However, recently, we found that the experimental setup partially
influences the decay of the spin-wave intensity after the current pulse is
switched off. Thus, further investigations on the presented effect are needed
to allow for a more detailed analysis. For this reason, we need to withdraw the
manuscript at this point and might publish an updated version later. | 1701.02094v2 |
2017-12-29 | From colossal to zero: Controlling the Anomalous Hall Effect in Magnetic Heusler Compounds via Berry Curvature Design | Since the discovery of the anomalous Hall effect (AHE), the anomalous Hall
conductivity (AHC) has been thought to be zero when there is no net
magnetization. However, the recently found relation between the intrinsic AHE
and the Berry curvature predicts other possibilities, such as a large AHC in
non-colinear antiferromagnets with no net magnetization but net Berry
curvature. Vice versa, the AHE in principle could be tuned to zero,
irrespective of a finite magnetization. Here, we experimentally investigate
this possibility and demonstrate that, the symmetry elements of Heusler magnets
can be changed such that the Berry curvature and all the associated properties
are switched while leaving the magnetization unaffected. This enables us to
tune the AHC from 0 {\Omega}-1cm-1 up to 1600 {\Omega}-1cm-1 with an
exceptionally high anomalous Hall angle up to 12 %, while keeping the
magnetization same. Our study shows that the AHC can be controlled by
selectively changing the Berry curvature distribution, independent of the
magnetization. | 1712.10174v3 |
2018-06-18 | Anomalous Nernst effect beyond the magnetization scaling relation in the ferromagnetic Heusler compound Co$_2$MnGa | Applying a temperature gradient in a magnetic material generates a voltage
that is perpendicular to both the heat flow and the magnetization. This is the
anomalous Nernst effect (ANE) which was thought to be proportional to the value
of the magnetization for a long time. However, more generally, the ANE has been
predicted to originate from a net Berry curvature of all bands near the Fermi
level. Subsequently, a large anomalous Nernst thermopower has recently been
observed in topological materials with no net magnetization but large net Berry
curvature around E$_F$. These experiments clearly fall outside the scope of the
conventional magnetization-model of the ANE, but a significant question
remains: Can the value of the ANE in topological ferromagnets exceed the
highest values observed in conventional ferromagnets? Here, we report a
remarkably high anomalous Nernst thermopower value of ~6.0 \mu V/K at 1 T in
the ferromagnetic topological Heusler compound Co$_2$MnGa at room temperature,
which is around 7-times larger than any anomalous Nernst thermopower value ever
reported for a conventional ferromagnet. Combined electrical, thermoelectric
and first-principles calculations reveal that this high value of the ANE arises
from a large net Berry curvature near the Fermi level associated with nodal
lines and Weyl points. | 1806.06753v1 |
2018-09-13 | High-Throughput Calculations of Thermal Conductivity in Nanoporous Materials: The Case of Half-Heusler Compounds | Achieving low thermal conductivity and good electrical properties is a
crucial condition for thermal energy harvesting materials. Nanostructuring
offers a very powerful tool to address both requirements: in nanostructured
materials, boundaries preferentially scatter phonons compared to electrons. The
search for low-thermal-conductivity nanostructures is typically limited to
materials with simple crystal structures, such as silicon, because of the
complexity arising from modeling branch- and wave vector- dependent nanoscale
heat transport. Using the phonon mean-free-path (MFP) dependent Boltzmann
transport equation, a model that overcomes this limitation, we compute thermal
transport in 75 nanoporous half-Heusler compounds for different pore sizes. We
demonstrate that the optimization of thermal transport in nanostructures should
take into account both bulk thermal properties and geometry-dependent phonon
suppression, two aspects that are typically engineered separately. In fact, our
work predicts that, given a set of bulk materials and a system geometry, the
ordering of the thermal conductivity of the nanostructure does not necessarily
align with that of the bulk: We show that what dictates thermal transport is
the interplay between the bulk MFP distribution and the nanostructuring length
scale of the material. Finally, we derive a thermal transport model that
enables fast systems screening within large bulk material repositories and a
given geometry. Our study motivates the need for a holistic approach to
engineering thermal transport and provides a method for high-throughput
materials discovery. | 1809.04751v1 |
2018-11-12 | Evidence for unconventional superconductivity in Half-Heusler YPdBi and TbPdBi compounds revealed by London penetration depth measurements | The half-Heusler compounds YPdBi and TbPdBi, while having similar band
structure, exhibit different magnetic properties. YPdBi is a diamagnet, while
TbPdBi shows antiferromagnetic order below 5.5 K. Both are superconductors with
T${_c}\approx$1 K for YPdBi and T${_c}\approx$1.75 K for TbPdBi. Such a
contrast in properties between these two compounds opens a question about the
effects of band structure or magnetic correlations on superconductivity. Using
the combination of a tunnel diode oscillator (TDO) and a commercial dilution
refrigerator, we measured temperature dependent magnetic penetration depth
$\Delta\lambda(T)$ in single crystals of YPdBi and TbPdBi, down to temperatures
as low as 0.1K. We found that penetration depths of both compounds do not show
exponential temperature dependence and saturation at low temperatures, as
expected for conventional BCS superconductors. Instead, in both compounds, the
penetration depth can be described by a power law $\Delta\lambda(T) = A\times
T^{n}$. The coefficient A was found to be about 50$\%$ smaller in TbPdBi, but
the exponents are very similar, $n = 2.76\pm 0.04$ in YPdBi and $n = 2.6\pm
0.3$ in TbPdBi, respectively. Our results suggest unconventional
superconductivity in both YPdBi and TbPdBi. | 1811.05045v1 |
2021-07-08 | Lattice thermal conductivity of half-Heuslers with density functional theory and machine learning: Enhancing predictivity by active sampling with principal component analysis | Low lattice thermal conductivity is essential for high thermoelectric
performance of a material. Lattice thermal conductivity is often computed using
density functional theory (DFT), typically at a high computational cost.
Training machine learning models to predict lattice thermal conductivity could
offer an effective procedure to identify low lattice thermal conductivity
compounds. However, in doing so, we must face the fact that such compounds can
be quite rare and distinct from those in a typical training set. This
distinctness can be problematic as standard machine learning methods are
inaccurate when predicting properties of compounds with features differing
significantly from those in the training set. By computing the lattice thermal
conductivity of 122 half-Heusler compounds, using the temperature-dependent
effective potential method, we generate a data set to explore this issue. We
first show how random forest regression can fail to identify low lattice
thermal conductivity compounds with random selection of training data. Next, we
show how active selection of training data using feature and principal
component analysis can be used to improve model performance and the ability to
identify low lattice thermal conductivity compounds. Lastly, we find that
active learning without the use of DFT-based features can be viable as a
quicker way of selecting samples. | 2107.03735v2 |
2019-10-17 | High electrical conductivity in the epitaxial polar metals LaAuGe and LaPtSb | Polar metals are an intriguing class of materials that simultaneously host
free carriers and polar structural distortions. Despite the name "polar metal,"
however, most well-studied polar metals are poor electrical conductors. Here,
we demonstrate the molecular beam epitaxial (MBE) growth of LaPtSb and LaAuGe,
two polar metal compounds whose electrical resistivity is an order of magnitude
lower than the well studied oxide polar metals. These materials belong to a
broad family of $ABC$ intermetallics adopting the stuffed wurtzite structure,
also known as hexagonal Heusler compounds. Scanning transmission electron
microscopy (STEM) reveals a polar structure with unidirectionally buckled $BC$
(PtSb, AuGe) planes. Magnetotransport measurements demonstrate good metallic
behavior with low residual resistivity ($\rho_{LaAuGe}=59.05$
$\mu\Omega\cdot$cm and $\rho_{LaAPtSb}=27.81$ $\mu\Omega\cdot$cm at 2K) and
high carrier density ($n_h\sim 10^{21}$ cm$^{-3}$). Photoemission spectroscopy
measurements confirm the band metallicity and are in quantitative agreement
with density functional theory (DFT) calculations. Through DFT-Chemical
Pressure and Crystal Orbital Hamilton Population analyses, the atomic packing
factor is found to support the polar buckling of the structure, though the
degree of direct interlayer $B-C$ bonding is limited by repulsion at the $A-C$
contacts. When combined with insulating hexagonal Heuslers, these materials
provide a new platform for fully epitaxial, multiferroic heterostructures. | 1910.07685v1 |
2018-12-13 | Noncentrosymmetric compensated half-metal hosting pure spin Weyl nodes, triple nodal points, nodal loops, and nexus fermions | Materials containing multiple topological characteristics become more exotic
when combined with noncentrosymmetric crystal structures and unusual magnetic
phases such as the compensated half-metal state, which is gapped in one spin
direction and conducting in the other. First principles calculations reveal
these multiple topological features in the compensated half-metal Cr$_2$CoAl
having neither time-reversal nor inversion symmetries. In the absence of
(minor) spin-orbit coupling (SOC), there are (1) a total of twelve pairs of
magnetic Weyl points, (2) three distinct sets of triple nodal points near the
Fermi level that are (3) interconnected with six symmetry related nodal lines.
This combination gives rise to fully spin polarized nexus fermions, in a system
with broken time-reversal symmetry but negligible macroscopic magnetic field.
The observed high Curie temperature of 750 K and calculated SOC hybridization
mixing of several meV should make these nexus fermions readily measurable.
Unlike topological features discussed for other Heuslers which emphasize their
strong ferromagnetism, this compensated half-metal is impervious to typical
magnetic fields, thus providing a complementary set of experimental phenomena.
Making use of the soft calculated magnetic state, large magnetic fields can be
used to rotate the direction of magnetism, during which certain topological
features will evolve. Our results suggest that these features may be common in
inverse-Heusler systems, particularly the isostructural and isovalent Ga and In
analogs. | 1812.05273v2 |
2016-12-22 | Co-sputtered PtMnSb thin films and PtMnSb/Pt bilayers for spin-orbit torque investigations | The manipulation of the magnetization by spin-orbit torques (SOTs) has
recently been extensively studied due to its potential for efficiently writing
information in magnetic memories. Particular attention is paid to
non-centrosymmetric systems with space inversion asymmetry, where SOTs emerge
even in single-layer materials. The half-metallic half-Heusler PtMnSb is an
interesting candidate for studies of this intrinsic SOT. Here, we report on the
growth and epitaxial properties of PtMnSb thin films and PtMnSb/Pt bilayers
deposited on MgO(001) substrates by dc magnetron co-sputtering at high
temperature in ultra-high vacuum. The film properties were investigated by
x-ray diffraction, x-ray reflectivity, atomic force microscopy, and electron
microscopy. Thin PtMnSb films present a monocrystalline C1b phase with (001)
orientation, coexisting at increasing thickness with a polycrystalline phase
with (111) texture. Films thinner than about 5 nm grow in islands, whereas
thicker films grow layer-by-layer, forming a perfect MgO/PtMnSb interface. The
thin PtMnSb/Pt bilayers also show island growth and a defective transition
zone, while thicker films grow layer-by-layer and Pt grows epitaxially on the
half-Heusler compound without significant interdiffusion. | 1612.07614v1 |
2017-10-12 | Manifestation of the Berry curvature in Co2TiSn Heusler films | Various Co2 based Heusler compounds are predicted to be Weyl materials. These
systems with broken symmetry possess a large Berry curvature, and introduce
exotic transport properties. The present study on epitaxially grown Co2TiSn
films is an initial approach to understand and explore this possibility. The
anomalous Hall effect in the well-ordered Co2TiSn films has been investigated
both experimentally and theoretically. The measured Hall conductivity is in
good agreement to the calculated Berry curvature. Small deviations between them
are due to the influence of skew scattering on the Hall effect. From
theoretical point of view, the main contribution to the anomalous Hall effect
originates from slightly gapped nodal lines, due to a symmetry reduction
induced by the magnetization. It has been found that only part of the nodal
lines contributed near to the anomalous Hall conductivity at a fixed Fermi
energy which can be explained from a magnetic symmetry analysis. Furthermore,
from hard x-ray photoelectron spectroscopy measurements, we establish the
electronic structure in the film that is comparable to the theoretical density
of states calculations. The present results provide deeper insight into the
spintronics from the prospect of topology. | 1710.04393v1 |
2017-10-27 | Near total magnetic moment compensation without reduction in T_C of Mn_2 V_0.5 Co_0.5 Z (Z=Ga,Al) Heusler compounds | Mn_2V_1-xCo_xZ (Z=Ga,Al and x=0, 0.25, 0.5, 0.75, 1) Heusler compounds have
been synthesized to investigate the effect of Co substitution at the V site on
the magnetic moment and Curie temperatures of half-metallic ferrimagnets
Mn_2VGa and Mn_2VAl. The Co substituted compounds show a non linear decrease in
lattice parameter without altering the crystal structure of the parent
compounds. The end members Mn_2VGa and Mn_2CoGa have the saturation
magnetization of 1.80 {\mu}_B/f.u and 2.05 {\mu}_B/f.u respectively whereas for
the Mn_2V_0.5Co_0.5Ga compound, a near total magnetic moment compensation (0.10
{\mu}_B/f.u) was observed due to the ferrimagnetic coupling of Mn with
parallelly aligned V and Co. The Co substituted Mn_2VAl has also shown the
similar trend with compensated magnetic moment value of 0.06 {\mu}_B/f.u for
x=0.5. The Curie temperatures of the compounds including the x=0.5 composition
are well above the room temperature (more than 700 K) which is in sharp
contrast to the earlier reported values of 171 K for the (MnCo)VGa and 105 K
for the (MnCo)VAl compounds (substitution at the Mn site). The magnetic moment
compensation without significant reduction in T_C indicates that the V site
substitution of Co does not weaken the magnetic interaction in Mn_2VZ (Z=Ga,
Al) compounds which is in contrary to the earlier experimental reports on Mn
site substitution. | 1710.09971v1 |
2019-03-26 | Complex exchange mechanism driven ferromagnetism in half-metallic Heusler Co$_{2}$TiGe: Evidence from critical behavior | We have investigated the critical phenomenon associated with the magnetic
phase transition in the half-metallic full-Heusler Co$_2$TiGe. The compound
undergoes a continuous ferromagnetic to paramagnetic phase transition at the
Curie temperature $T_{C}$=371.5 K. The analysis of magnetization isotherms in
the vicinity of $T_{c}$, following modified Arrott plot method, Kouvel-Fisher
technique, and critical isotherm plot, yields the asymptotic critical exponents
$\beta$=0.495, $\gamma$=1.324, and $\delta$=3.67. The self-consistency and
reliability of the obtained exponents are further verified by the Widom scaling
relation and scaling equation of states. The mean-field-like value of the
critical exponent $\beta$ suggests long-range nature of the exchange
interactions, whereas the values of the critical exponents $\gamma$ and
$\delta$, imply sizeable critical spin fluctuations. The half-metallic
itinerant character of Co$_{2}$TiGe in the presence of magnetic inhomogeneity
may result in such a strong deviation from the three-dimensional Heisenberg
values ($\beta$=0.369, $\gamma$=1.38 and $\delta$=4.8) of the critical
exponents towards the mean field values ($\beta$=0.5, $\gamma$=1 and
$\delta$=3). The results suggest complex nature of exchange couplings that
stabilize the long-range ferromagnetic ordering in the system and are
consistent with the earlier theoretical studies on the exchange mechanism in
Co$_2$TiGe. | 1903.10987v1 |
2020-09-24 | Electronic correlations in the semiconducting half-Heusler compound FeVSb | Electronic correlations are crucial to the low energy physics of metallic
systems with localized $d$ and $f$ states; however, their effect on band
insulators and semiconductors is typically negligible. Here, we measure the
electronic structure of the half-Heusler compound FeVSb, a band insulator with
filled shell configuration of 18 valence electrons per formula unit ($s^2 p^6
d^{10}$). Angle-resolved photoemission spectroscopy (ARPES) reveals a mass
renormalization of $m^{*}/m_{bare}= 1.4$, where $m^{*}$ is the measured
effective mass and $m_{bare}$ is the mass from density functional theory (DFT)
calculations with no added on-site Coulomb repulsion. Our measurements are in
quantitative agreement with dynamical mean field theory (DMFT) calculations,
highlighting the many-body origin of the mass renormalization. This mass
renormalization lies in dramatic contrast to other filled shell intermetallics,
including the thermoelectric materials CoTiSb and NiTiSn; and has a similar
origin to that in FeSi, where Hund's coupling induced fluctuations across the
gap can explain a dynamical self-energy and correlations. Our work calls for a
re-thinking of the role of correlations and Hund's coupling in intermetallic
band insulators. | 2009.11489v2 |
2020-11-20 | An ac-susceptibility study of magnetic relaxation phenomena in the antiskyrmion hosting tetragonal Mn-Pt(Pd)-Sn system | Here, we report an exhaustive study of the frequency-dependent ac-magnetic
susceptibility of the $D_{2d}$ symmetric Heusler system Mn-Pt(Pd)-Sn that hosts
antiskyrmions over a wide temperature range. Magnetic relaxation studies using
Cole-Cole formalism reveal a Debye-type relaxation with a nearly negligible
distribution in relaxation times. In contrast to the archetypical skyrmion
hosts, the high Curie temperature ($ T_C $) of the present system ensures
shorter switching times, and, correspondingly, higher frequencies are required
to probe the relaxation dynamics. We find a non-monotonic variation in the
characteristic relaxation time with distinct maxima at the phase boundaries
\textit{via} helical $\longrightarrow$ antiskyrmion $\longrightarrow$
field-polarized states, indicating slower magnetization dynamics over the
region of phase coexistence. The temperature-dependent relaxation time across
different phases is of the order of $ 10^{-5} - 10^{-4} $ s and follows the
well-known Arrhenius law with reasonable values of the energy barriers. The
present study concerning the magnetization dynamics in the antiskyrmion host
tetragonal Heusler system is an important contribution towards the basic
understanding of the dynamical aspects of antiskyrmions for their potential
applications. | 2011.10229v1 |
2020-11-20 | Mechanical, optoelectronic and thermoelectric properties of half-Heusler p-type semiconductor BaAgP: A DFT investigation | We have explored the mechanical, electronic, optical and thermoelectric
properties of p-type half-Heusler compound BaAgP for the first time using
density functional theory based calculations. The mechanical and dynamical
stability of this compound is confirmed by studying the Born stability criteria
and phonon dispersion curve, respectively. It is soft, ductile and elastically
anisotropic. The atomic bonding along a-axis is stronger than that along
c-axis. The calculated electronic structure reveals that the studied compound
is an indirect band gap semiconductor. The analysis of charge density
distribution map and Mulliken population reveals that the bonding in BaAgP is a
mixture of covalent and ionic. The optical features confirm that BaAgP is
optically anisotropic. The high absorption coefficient and low reflectivity in
the visible to ultraviolet region make this compound a possible candidate for
solar cell and optoelectronic device applications. The thermoelectric
properties have been evaluated by solving the Boltzmann semi-classical
transport equations. The calculated power factor at 1000K along a-axis is 35.2
micro-W/cmK2 (with tau=10-14 s) which is ~3.5 times larger than that of SnSe, a
promising layered thermoelectric materials. The thermoelectric figure of merit,
ZT of BaAgP is 0.44 which is small due to high thermal conductivity. So the
reduction of thermal conductivity is essential to enhance thermoelectric
performance of BaAgP in device applications. | 2011.10506v1 |
2021-01-04 | Influence of the martensitic transformation kinetics on the magnetocaloric effect in Ni-Mn-In | The inverse magnetocaloric effect (MCE) in Ni-Mn-based Heusler compounds
occurs during the magnetostructural transition between low-temperature,
low-magnetization martensite and high-temperature, high-magnetization
austenite. In this study, we analyze the metamagnetic transformation of a
$Ni_{49.8}Mn_{35}In_{15.2}$ compound by simultaneous adiabatic temperature
change and strain measurements in pulsed magnetic fields up to 10 T. We observe
an adiabatic temperature change of -10 K and a strain of -0.22 % when the
reverse martensitic transition is fully induced at a starting temperature of
285 K. By a variation of the magnetic field-sweep rates between 316 Ts$^{-1}$,
865 Ts$^{-1}$ and 1850 Ts$^{-1}$, the transitional dynamics of the reverse
martensitic transformation have been investigated. Our experiments reveal an
apparent delay upon the end of the reverse martensitic transformation at field
rates exceeding 865 Ts$^{-1}$ which is related to the annihilation of retained
martensite. As a consequence, the field hysteresis increases and higher fields
are required to saturate the transition. In contrast, no time-dependent effects
on the onset of the reverse martensitic transformation were observed in the
studied field-sweep range. Our results demonstrate that kinetic effects in
Heusler compounds strongly affect the magnetic cooling cycle, especially when
utilising a multicaloric "exploiting-hysteresis cycle" where high magnetic
field-sweep rates are employed. | 2101.00840v1 |
2021-10-29 | Stability and magnetic properties of grain boundaries in the inverse Heusler phase Fe$_2$CoGa and in bcc Fe | We investigate grain boundaries (GBs) in the cubic inverse Heusler phase
Fe$_2$CoGa by means of first principles calculations based on density
functional theory. Besides the energetic stability, the analysis focuses on the
magnetic properties of a set of 16 GB structures in this intermetallic phase.
We determine the integrated excess magnetization across the GB and analyze it
in terms of the projected local magnetic moments of the atoms and their local
Voronoi volumes. The results are systematically compared to those of
corresponding GBs in body-centered cubic (bcc) Fe. The studied GBs in
Fe$_2$CoGa may have a considerably increased magnetization at the GB, up to
more than twice as much as in bcc Fe, depending on the GB type, while
geometrical quantities like GB widening or local GB excess volume distributions
are similar for both phases. We explain this difference by the higher
flexibility of the ternary Fe$_2$CoGa phase in compensating the disturbance of
a crystal defect by structural relaxation. The GB structures therefore have a
lower energy accompanied with increased local magnetic moments of the Co and
half of the Fe atoms within a distance of a few angstrom around the GB plane. | 2110.15896v1 |
2021-10-30 | Self Doping for Synergistically Tuning the Electronic and Thermal Transport Coefficients in n-type Half-Heuslers | Ternary intermetallic half-Heusler (HH) compounds (XYZ) with 18 valence
electron count viz. ZrCoSb, ZrNiSn, and ZrPdSn, have revealed promising
thermoelectric properties. Exemplarily, it has been experimentally observed
that a slight change in the content of Y-site atoms (by ~3-12.5% i.e., m =0.03,
0.125 in ZrY(1+m)Z) leads to drastic lowering in the lattice thermal
conductivity (kL) by more than 65-80% in many of these compounds. The present
work aims at exploring the possibility of maximizing the electronic transport
scenario after achieving the low kL limit in these compounds. By taking into
account the full anharmonicity of the lattice dynamics, Boltzmann transport
calculations are performed under the framework of density functional theory.
Our results show that these excess atoms present in the vacant lattice site
induce scattering by acting either as a rattling mode or by hybridizing with
the acoustic modes of the host depending upon their mass and bonding chemistry,
respectively. Furthermore, the introduction of these scattering centers may
lead to the formation of a defect mid-gap state in the electronic band
structure (detrimental for electronic transport) or lead to light doping of the
host compound. The latter is found to be particularly conducive for attaining
synergy in both thermal as well as electronic transport. | 2111.00186v1 |
2022-07-11 | Hybrid Bloch-Néel spiral states in Mn$_{1.4}$PtSn probed by resonant soft x-ray scattering | Multiple intriguing phenomena have recently been discovered in tetragonal
Heusler compounds, where $D_{2d}$ symmetry sets a unique interplay between
Dzyaloshinskii-Moriya (DM) and magnetic dipolar interactions. In the prototype
$D_{2d}$ compound Mn$_{1.4}$PtSn, this has allowed the stabilization of exotic
spin textures such as first-reported anti-skyrmions or elliptic Bloch-type
skyrmions. While less attention has so far been given to the low-field spiral
state, this remains extremely interesting as a simplest phase scenario on which
to investigate the complex hierarchy of magnetic interactions in this materials
family. Here, via resonant small-angle soft x-ray scattering experiments on
high-quality single crystals of Mn$_{1.4}$PtSn at low temperatures, we evidence
how the underlying $D_{2d}$ symmetry of the DMI in this material is reflected
in its magnetic texture. Our studies reveal the existence of a novel and
complex metastable phase, which possibly has a mixed character of both the
N\'{e}el-type cycloid and the Bloch-type helix, that forms at low temperature
in zero fields upon the in-plane field training. This hybrid spin-spiral
structure has a remarkable tunability, allowing to tilt its orientation beyond
high-symmetry crystallographic directions and control its spiral period. These
results broaden the reachness of Heusler $D_{2d}$ materials exotic magnetic
phase diagram and extend its tunability, thus enhancing a relevant playground
for further fundamental explorations and potential applications in energy
saving technologies. | 2207.04968v1 |
2022-07-20 | Tunable phase transitions in half-Heusler TbPtBi compound | We report various phase transitions in half-Heusler TbPtBi compound using
Density Functional Theory (DFT). Specifically, inclusion of spin-orbit coupling
(SOC) leads to band inversion resulting in transition from the metallic to the
topological semimetallic phase. However, in presence of SOC, there is a phase
transition from the topological semimetal to the trivial semimetal when the
material is subjected to compressive strain ($\lt -7\%$). Subsequently, under
further increase of compressive strain ($\ge -7\%$), we find an opening of a
direct band gap at the $\Gamma$ point, driving the system from the trivial
semimetallic to the semiconducting state with changes in the sequence of bands.
In the absence of SOC, only transition from the metallic to the semiconducting
phase is noticed. Under tensile strain, the TbPtBi compound maintains its phase
as in the unstrained condition but with an increase in the hole pocket at the
Fermi level, both in the absence and presence of SOC. These tunable phase
transitions (especially as a fraction of strain) make this compound very
promising for application in various quantum devices such as highly sensitive
strain gauges. | 2207.09704v4 |
2022-07-21 | First-principles insights into all-optical spin switching in the half-metallic Heusler ferrimagnet Mn$_2$RuGa | All-optical spin switching (AOS) represents a new frontier in magnetic
storage technology -- spin manipulation without a magnetic field, -- but its
underlying working principle is not well understood. Many AOS ferrimagnets such
as GdFeCo are amorphous and renders the high-level first-principles study
unfeasible. The crystalline half-metallic Heusler Mn$_2$RuGa presents an
opportunity. Here we carry out hitherto the comprehensive density functional
investigation into the material properties of Mn$_2$RuGa, and introduce two
concepts - the spin anchor site and the optical active site - as two pillars
for AOS in ferrimagnets. In Mn$_2$RuGa, Mn$(4a)$ serves as the spin anchor
site, whose band structure is below the Fermi level and has a strong spin
moment, while Mn$(4c)$ is the optical active site whose band crosses the Fermi
level. Our magneto-optical Kerr spectrum and band structure calculation jointly
reveal that the delicate competition between the Ru-$4d$ and Ga-$4p$ states is
responsible for the creation of these two sites. These two sites found here not
only present a unified picture for both Mn$_2$RuGa and GdFeCo, but also open
the door for the future applications. Specifically, we propose a
Mn$_2$Ru$_x$Ga-based magnetic tunnel junction where a single laser pulse can
control magnetoresistance. | 2207.10443v1 |
2022-12-01 | Extrinsic to intrinsic mechanism crossover of anomalous Hall effect in the Ir-doped MnPtSn Heusler system | Recent findings of large anomalous Hall signal in nonferromagnetic and
nonferrimagnetic materials suggest that the magnetization of the system is not
a critical component for the realization of the anomalous Hall effect (AHE).
Here, we present a combined theoretical and experimental study demonstrating
the evolution of different mechanisms of AHE in a cubic Heusler system
MnPt$_{1-x}$Ir$_x$Sn. With the help of magnetization and neutron diffraction
studies, we show that the substitution of nonmagnetic Ir in place of Pt
significantly reduces the net magnetic moment from 4.17 $ \mu _B$/f.u. in
MnPtSn to 2.78 $ \mu _B$/f.u. for MnPt$_{0.5}$Ir$_{0.5}$Sn. In contrast, the
anomalous Hall resistivity is enhanced by nearly three times from 1.6 $ \mu
\Omega $ cm in MnPtSn to about 5 $ \mu \Omega $ cm for
MnPt$_{0.5}$Ir$_{0.5}$Sn. The power law analysis of the Hall resistivity data
suggests that the extrinsic contribution of AHE that dominates in the case of
the parent MnPtSn almost vanishes for MnPt$_{0.5}$Ir$_{0.5}$Sn, where the
intrinsic mechanism plays the major role. The experimental results are well
supported by our theoretical study, which shows a considerable enhancement of
the spin-orbit coupling when Ir is introduced into the system. Our finding of a
crossover of the anomalous Hall effect with chemical engineering is a major
contribution toward the recent interest in controlling the band topology of
topological materials, both in bulk and thin-film forms. | 2212.00360v1 |
2023-03-06 | Structural phase transitions and superconductivity in the Heusler intermetallics $X$Pd$_2$Sn ($X$ = Ti, Zr, Hf) | We report the discovery of structural phase transitions and superconductivity
in the full Heusler compounds $X$Pd$_2$Sn ($X$ = Ti, Zr, Hf), by means of
electrical transport, magnetic susceptibility, specific heat and x-ray
diffraction measurements. TiPd$_2$Sn, ZrPd$_2$Sn and HfPd$_2$Sn undergo
structural phase transitions from the room-temperature cubic MnCu$_2$Al-type
structure (space group $Fm\bar{3}m$) to a low-temperature tetragonal structure
at around 160 K, 110 K and 90 K, respectively, which are likely related charge
density wave (CDW) instabilities. Low temperature single crystal x-ray
diffraction measurements of ZrPd$_2$Sn demonstrate the emergence of a
superstructure with multiple commensurate modulations below $T_s$. ZrPd$_2$Sn
and HfPd$_2$Sn have bulk superconductivity (SC) with transition temperatures
$T_c$ $\sim$ 1.2 K and 1.3 K, respectively. Density functional theory (DFT)
calculations reveal evidence for structural and electronic instabilities which
can give rise to CDW formation, suggesting that these $X$Pd$_2$Sn systems are
good candidates for examining the interplay between CDW and SC. | 2303.02908v1 |
1997-02-05 | Adlayer core-level shifts of random metal overlayers on transition-metal substrates | We calculate the difference of the ionization energies of a core-electron of
a surface alloy, i.e., a B-atom in a A_(1-x) B_x overlayer on a
fcc-B(001)-substrate, and a core-electron of the clean fcc-B(001) surface using
density-functional-theory. We analyze the initial-state contributions and the
screening effects induced by the core hole, and study the influence of the
alloy composition for a number of noble metal-transition metal systems. Data
are presented for Cu_(1-x)Pd_x/Pd(001), Ag_(1-x) Pd_x/Pd(001), Pd_(1-x)
Cu_x/Cu(001), and Pd_(1-x) Ag_x/Ag(001), changing x from 0 to 100 %. Our
analysis clearly indicates the importance of final-state screening effects for
the interpretation of measured core-level shifts. Calculated deviations from
the initial-state trends are explained in terms of the change of inter- and
intra-atomic screening upon alloying. A possible role of alloying on the
chemical reactivity of metal surfaces is discussed. | 9702044v1 |
1998-08-10 | Monte Carlo Study of Short-Range Order and Displacement Effects in Disordered CuAu | The correlation between local chemical environment and atomic displacements
in disordered CuAu alloy has been studied using Monte Carlo simulations based
on the effective medium theory (EMT) of metallic cohesion. These simulations
correctly reproduce the chemically-specific nearest-neighbor distances in the
random alloy across the entire Cu\$_x\$Au\$_{1-x}\$ concentration range. In the
random equiatomic CuAu alloy, the chemically specific pair distances depend
strongly on the local atomic environment (i.e. fraction of like/unlike nearest
neighbors). In CuAu alloy with short-range order, the relationship between
local environment and displacements remains qualitatively similar. However the
increase in short-range order causes the average Cu-Au distance to decrease
below the average Cu-Cu distance, as it does in the ordered CuAuI phase. Many
of these trends can be understood qualitatively from the different neutral
sphere radii and compressibilities of the Cu and Au atoms. | 9808101v1 |
1998-12-22 | Magnetic precursor effects in Gd alloys | The behaviour of electrical resistivity ($\rho$) and magnetoresistance in the
vicinity of respective magnetic ordering temperatures in a number of Gd alloys
is reported. In some compounds, e.g., GdNi_2Sn_2 and GdPt_2Ge_2, there is an
enhancement of $\rho$ prior to long range magnetic order over a wide
temperature range which can be highlighted by the suppression of $\rho$ caused
by the application of a magnetic field. However, such features are absent in
many other Gd compounds, e.g., GdCu_2Ge_2, GdAg_2Si_2, GdAu_2Si_2, GdPd_2Ge_2
and GdCo_2Si_2. Attempts to relate such features to magnetic precursor effects
in heat capacity are made. On the basis of our studies, we suggest that better
understanding of magnetic precursor effects in Gd alloys will be helpful to
throw light on some of the current trends in magnetism. Various other
interesting findings in the magnetically ordered state in some of these alloys
are also brought out. | 9812365v1 |
1999-05-12 | Vacancy-assisted domain-growth in asymmetric binary alloys: a Monte Carlo study | A Monte Carlo simulation study of the vacancy-assisted domain-growth in
asymmetric binary alloys is presented. The system is modeled using a
three-state ABV Hamiltonian which includes an asymmetry term, not considered in
previous works. Our simulated system is a stoichiometric two-dimensional binary
alloy with a single vacancy which evolves according to the vacancy-atom
exchange mechanism. We obtain that, compared to the symmetric case, the
ordering process slows down dramatically. Concerning the asymptotic behavior it
is algebraic and characterized by the Allen-Cahn growth exponent x=1/2. The
late stages of the evolution are preceded by a transient regime strongly
affected by both the temperature and the degree of asymmetry of the alloy. The
results are discussed and compared to those obtained for the symmetric case. | 9905163v1 |
2000-02-24 | The local magnetic moments and hyperfine magnetic fields in disordered metal-metalloid alloys | The local magnetic moments and hyperfine magnetic fields (HFF) in the ordered
alloys Fe_{15}Sn and Fe_{15}Si are calculated with the first-principles
full-potential linear augmented plane wave (FP LAPW) method. The results are
compared with the experimental data on Fe-M (M=Si, Sn) disordered alloys at
small metalloid concentration. The relaxation of the lattice around the
impurity and its influence on the quantities under consideration are studied.
The mechanism of the local magnetic moment formation is described. It is proved
that the main distinction between these alloys is connected with the different
lattice parameters. Three contributions to the HFF are discussed: the
contributions of the core and valence electron polarization to the
Fermi-contact part, and the contibution from the orbital magnetic moment. | 0002371v1 |
2000-09-22 | Local Atomic Strain in ZnSe(1-x)Te(x) from High Real Space Resolution Neutron Pair Distribution Function Measurements | High real-space resolution atomic pair distribution functions (PDFs) have
been obtained from ZnSe(1-x)Te(x) using neutron powder diffraction. Distinct
Zn-Se and Zn-Te nearest neighbor (nn) bonds, differing in length by delta_r=
0.14Angstroms, are resolved in the measured PDF allowing the evolution with
composition of the individual bond-lengths to be studied. The local
bond-lengths change much more slowly with doping than the average bond-length
obtained crystallographically. The nn bond-length distributions are constant
with doping but higher-neighbor pair distributions broaden significantly
indicating that most of the strain from the alloying is accommodated by
bond-bending forces in the alloy. The PDFs of alloys across the whole doping
range are well fit using a model based on the Kirkwood potential. The resulting
PDFs give excellent agreement with the measured PDFs over the entire alloy
range with no adjustable parameters. | 0009364v3 |
2001-03-27 | Mechanism of formation of a 2D binary alloy | Direct comparison of scanning tunneling microscopy and high resolution core
level photo-emission experiments provides a rationale for the mechanism of
formation of a two dimensional (2D) binary alloy (1/3 mono-layer (ML) Sn(1-
x)Six/Si(111)-\sqrt3X\sqrt3R30). In contrast with recent theoretical
predictions, the pure metal surface (x=0) results partitioned into two classes
(2/9 ML and 1/9 ML) of ad-atoms occupying non-equivalent T4 sites. During the
formation of the alloy, Si ad-atoms preferably occupy the majority type
adsorption site. This peculiar substitution mechanism leads to a mutual
arrangement of ad-atoms which is not random even at room temperature, but shows
the typical short range order universally observed in 2D and quasi 2D binary
alloys | 0103568v1 |
2001-09-19 | Spin filtering by ferromagnetic nanowires | We show that electrical current flowing through nanowires made of
ferromagnetic disordered alloys can become highly spin polarized. | 0109350v1 |
2001-11-19 | Structure evolution of Pd-Ta-H alloy In Edwards' thermodynamics representation | X-rays diffraction pictures time dependence of deformed alloy Pd-Ta being
charged with hydrogen has been shown to be possibly caused by multi-pits
character of energetic relief in the states space. Phenomenological model
representing alloy structural evolution as a occasional roaming on minima of
internal energy of non-ergodic system, has been offered in Lorenz synergetic
scheme frames. Here, order parameters are the part of minima occupied by the
system, conjugated field is considered to be Edwards entropy and control
parameter is taken to be internal energy. Thermodynamics interpretation of
Pd-Ta-H alloy evolution structure as a complex non-ergodic system is offered | 0111338v1 |
2001-11-28 | Acceleration of Phase Separation in Fe-base Ternary Alloys | Mechanism for acceleration of phase separation in Fe-base ternary alloys was
investigated with use of a model based on the Cahn-Hilliard equation. Behavior
of the minor element in an Fe-base ternary alloy along the trajectory of the
peak of the major element is dependent on the sign of the second derivative of
the chemical free energy with respect to the concentrations of the major and
minor elements. However, the concentration of the major element along the
trajectory of its peak top increases with time regardless of the sign of the
second derivative of the chemical free energy. The addition of a substitutional
element to an Fe-base binary alloy with composition within the spinodal region
was found to accelerate phase separation | 0111520v1 |
2002-01-25 | Self-driven lattice-model Monte Carlo simulations of alloy thermodynamic | Monte Carlo (MC) simulations of lattice models are a widely used way to
compute thermodynamic properties of substitutional alloys. A limitation to
their more widespread use is the difficulty of driving a MC simulation in order
to obtain the desired quantities. To address this problem, we have devised a
variety of high-level algorithms that serve as an interface between the user
and a traditional MC code. The user specifies the goals sought in a high-level
form that our algorithms convert into elementary tasks to be performed by a
standard MC code. For instance, our algorithms permit the determination of the
free energy of an alloy phase over its entire region of stability within a
specified accuracy, without requiring any user intervention during the
calculations. Our algorithms also enable the direct determination of
composition-temperature phase boundaries without requiring the calculation of
the whole free energy surface of the alloy system. | 0201473v2 |
2002-02-21 | Screened Coulomb interactions in metallic alloys: II Screening beyond the single-site and atomic sphere approximations | A quantitative description of the configurational part of the total energy of
metallic alloys with substantial atomic size difference cannot be achieved in
the atomic sphere approximation: It needs to be corrected at least for the
multipole moment interactions in the Madelung part of the one-electron
potential and energy. In the case of a random alloy such interactions can be
accounted for only by lifting the atomic sphere and single-site approximations,
in order to include the polarization due to local environment effects.
Nevertheless a simple parameterization of the screened Coulomb interactions for
the ordinary single-site methods, including the generalized perturbation
method, is still possible. We obtained such a parameterization for bulk and
surface NiPt alloys, which allows one to obtain quantitatively accurate
effective interactions in this system. | 0202371v1 |
2002-03-21 | Possible complete miscibility of $(BN)_x(C_2)_{1-x}$ alloys | The stabilities of $(BN)_x(C_2)_{1-x}$ alloys and related superlattices are
investigated by ab initio pseudopotential calculations. We find that the
$(BN)_1/(C_2)_1$ superlattices in (111) orientations have the lowest formation
energy among many short-range ordered $BNC_2$ structures due to the smallest
number of B-C and C-N bonds. Based on the calculated formation energies at
several compositions and for various ordered structures and assuming
thermodynamic equilibrium, the solid solution phase diagram of
$(BN)_x(C_2)_{1-x}$ alloys is constructed. We find that the complete
miscibility of $(BN)_x(C_2)_{1-x}$ alloys is possible, which is in contrast
with previous theoretical predictions but in agreement with experimental
reports. | 0203423v2 |
2002-06-06 | Local Charge Excesses in Metallic Alloys: a Local Field Coherent Potential Approximation Theory | Electronic structure calculations performed on very large supercells have
shown that the local charge excesses in metallic alloys are related through
simple linear relations to the local electrostatic field resulting from
distribution of charges in the whole crystal.
By including local external fields in the single site Coherent Potential
Approximation theory, we develop a novel theoretical scheme in which the local
charge excesses for random alloys can be obtained as the responses to local
external fields. Our model maintains all the computational advantages of a
single site theory but allows for full charge relaxation at the impurity sites.
Through applications to CuPd and CuZn alloys, we find that, as a general rule,
non linear charge rearrangements occur at the impurity site as a consequence of
the complex phenomena related with the electronic screening of the external
potential. This nothwithstanding, we observe that linear relations hold between
charge excesses and external potentials, in quantitative agreement with the
mentioned supercell calculations, and well beyond the limits of linearity for
any other site property. | 0206088v3 |
2002-08-26 | Phonons in random alloys: the itinerant coherent-potential approximation | We present the itinerant coherent-potential approximation(ICPA), an analytic,
translationally invariant and tractable form of augmented-space-based,
multiple-scattering theory in a single-site approximation for harmonic phonons
in realistic random binary alloys with mass and force-constant disorder.
We provide expressions for quantities needed for comparison with experimental
structure factors such as partial and average spectral functions and derive the
sum rules associated with them. Numerical results are presented for Ni_{55}
Pd_{45} and Ni_{50} Pt_{50} alloys which serve as test cases, the former for
weak force-constant disorder and the latter for strong. We present results on
dispersion curves and disorder-induced widths. Direct comparisons with the
single-site coherent potential approximation(CPA) and experiment are made which
provide insight into the physics of force-constant changes in random alloys.
The CPA accounts well for the weak force-constant disorder case but fails for
strong force-constant disorder where the ICPA succeeds. | 0208494v1 |
2002-12-06 | The Alloy Theoretic Automated Toolkit: A User Guide | Although the formalism that allows the calculation of alloy thermodynamic
properties from first-principles has been known for decades, its practical
implementation has so far remained a tedious process. The Alloy Theoretic
Automated Toolkit (ATAT) drastically simplifies this procedure by implementing
decision rules based on formal statistical analysis that frees the researchers
from a constant monitoring during the calculation process and automatically
"glues" together the input and the output of various codes, in order to provide
a high-level interface to the calculation of alloy thermodynamic properties
from first-principles. ATAT implements the Structure Inversion Method (SIM),
also known as the Connolly-Williams method, in combination with
semi-grand-canonical Monte Carlo simulations. In order to make this powerful
toolkit available to the wide community of researchers who could benefit from
it, this article present a concise user guide outlining the steps required to
obtain thermodynamic information from ab initio calculations. | 0212159v1 |
2002-12-13 | Aging of a nanostructured Zn50Se50 alloy produced by mechanical alloying | The aging of a nanocrystalline equiatomic ZnSe alloy produced by mechanical
alloying was investigated using X-ray diffraction (XRD) and differential
scanning calorimetry (DSC) techniques. The measured XRD patterns showed that Se
atoms located at interfacial component migrated with aging giving raise to a
crystalline selenium (c-Se) phase. DSC spectra of heat-treated samples at
temperatures above 221oC followed by quenching showed that the c-Se particles
changed to the amorphous state. It was also observed that the as-milled and
aged samples are highly hydrophilic. The lattice parameters and the average
crystallite sizes were calculated as a function of time of aging and
temperature of heat treatment. | 0212322v2 |
2003-02-26 | Anomalous magnetotransport in (Y$_{1-x}$Gd$_{x}$)Co$_{2}$ alloys: interplay of disorder and itinerant metamagnetism | New mechanism of magnetoresistivity in itinerant metamagnets with a
structural disorder is introduced basing on analysis of experimental results on
magnetoresistivity, susceptibility, and magnetization of structurally
disordered alloys (Y$_{1-x}$Gd$_{x}$)Co$_{2}$. In this series, YCo$_{2}$ is an
enhanced Pauli paramagnet, whereas GdCo$_{2}$ is a ferrimagnet (T$_{\rm c}$=400
K) with Gd sublattice coupled antiferromagnetically to the itinerant Co-3d
electrons. The alloys are paramagnetic for $x < 0.12$. Large positive
magnetoresistivity has been observed in the alloys with magnetic ground state
at temperatures T$<$T$_{\rm c}$. We show that this unusual feature is linked to
a combination of structural disorder and metamagnetic instability of itinerant
Co-3d electrons. This new mechanism of the magnetoresistivity is common for a
broad class of materials featuring a static magnetic disorder and itinerant
metamagnetism. | 0302539v2 |
2003-04-18 | Carbon Doping in MgB_2 : Role of Boron and Carbon p_x(y) Bands | We have studied the changes in the electronic structure and the
superconducting transition temperature T_c of Mg(B_{1-x}C_{x})_{2} alloys as a
function of x with 0\leq x\leq 0.3. Our density-functional-based approach uses
coherent-potential approximation to describe the effects of disorder,
Gaspari-Gyorffy formalism to estimate the electron-phonon matrix elements and
Allen-Dynes equation to calculate T_c in these alloys. We find that the changes
in the electronic structure of Mg(B_{1-x}C_{x})_{2} alloys, especially near the
Fermi energy E_F, come mainly from the outward movement of E_F with increasing
x, and the effects of disorder in the B plane are small. In particular, our
results show a sharp decline in both B and C p_{x(y)} states for 0.2\leq x\leq
0.3. Our calculated variation in T_{c} of Mg(B_{1-x}C_{x})_{2} alloys is in
qualitative agreement with the experiments. | 0304436v1 |
2003-07-16 | EXAFS and XRD studies of an amorphous Co$_{57}$Ti$_{43}$ alloy produced by mechanical alloying | We have investigated the local atomic structure of an amorphous
Co$_{57}$Ti$_{43}$ alloy produced by Mechanical Alloying by means of x-ray
diffraction and EXAFS analyses on Co and Ti K-edges. Coordination numbers and
interatomic distances where found and compared with those determined using an
additive hard sphere (AHS) model associated with a RDF$(r)$ deconvolution, and
also with data from bcc-Co$_2$Ti compound. The EXAFS results obtained indicated
a shortening in the Co-Ti and Ti-Ti distances when compared to those found by
the AHS-RDF method and an increase in the Co-Co and Ti-Ti distances and a large
shortening in the Co-Ti one when compared to the distances found in the
bcc-Co$_2$Ti compound. In spite of these differences, coordination numbers
obtained from EXAFS and AHS-RDF are similar to each other and also to those
found in bcc-Co$_2$Ti. | 0307375v2 |
2003-08-25 | Icosahedral quasicrystals in Zn-T-Sc (T=Mn, Fe, Co, Ni) alloys | Starting from the Zn17Sc3 cubic approximant, new quasicrystal alloys were
searched by replacement of Zn with transition elements, T. In the cases of
T=Mn, Fe, Co and Ni, new icosahedral quasicrystals are formed in as-cast alloys
as major phases at the alloy compositions of Zn75T10Sc15. All these
quasicrystals belong to a primitive type, and have 6-dimensional lattice
parameters, a6D, ranging from 7.044 to 7.107 A. They have the valence electron
concentrations, e/a, ranging from 2.01 to 2.14, and almost the same ratios
between the edge-length of the Penrose tile, aR, and the averaged atomic
diameter d: aR/d~1.75. Moreover other Zn- and Cd-based quasicrystals including
the same type of atomic cluster, Tsai-type cluster, also have the same values
of e/a~2.1 and aR/d~1.75. The equality in e/a indicates that the Hume-Rothery
mechanism plays an important role for the formation of these quasicrystals. | 0308480v1 |
2003-12-09 | Magnetic properties of X-Pt (X=Fe,Co,Ni) alloy systems | We have studied the electronic and magnetic properties of Fe-Pt, Co-Pt and
Ni-Pt alloy systems in ordered and disordered phases. The influence of various
exchange-correlation functionals on values of equilibrium lattice parameters
and magnetic moments in ordered Fe-Pt, Co-Pt and Ni-Pt alloys have been studied
using linearized muffin-tin orbital method. The electronic structure
calculations for the disordered alloys have been carried out using augmented
space recursion technique in the framework of tight binding linearized
muffin-tin orbital method. The effect of short range order has also been
studied in the disordered phase of these systems. The results show good
agreements with available experimental values. | 0312215v2 |
2004-01-31 | Structural study of Cu$_{2-x}$Se alloys produced by mechanical alloying | The crystalline structures of superionic high temperature copper selenides
Cu$_{2-x}$Se ($0 \le x \le 0.25$) produced by Mechanical Alloying were
investigated using X-ray diffraction (XRD) technique. The measured XRD patterns
showed the presence of the peaks corresponding to the crystalline superionic
high temperature $\alpha$-Cu$_2$Se phase in the as-milled sample, and its
structural data were determined by means of a Rietveld refinement procedure.
After a heat treatment in argon at 200$^\circ$C for 90 h, this phase transforms
to the superionic high temperature $\alpha$-Cu$_{1.8}$Se phase, whose
structural data where also determined through the Rietveld refinement. In this
phase, a very low occupation of the trigonal 32(f) sites ($\sim 3$%) by Cu ions
is found. In order to explain the evolution of the phases in the samples, two
possible mechanisms are suggested: the high mobility of Cu ions in superionic
phases and the intense diffusive processes in the interfacial component of
samples produced by Mechanical Alloying. | 0402012v1 |
2004-03-11 | Towards a first principles description of phonons in Ni$_{50}$Pt$_{50}$ disordered alloys: the role of relaxation | Using a combination of density-functional perturbation theory and the
itinerant coherent potential approximation, we study the effects of atomic
relaxation on the inelastic incoherent neutron scattering cross sections of
disordered Ni$_{50}$Pt$_{50}$ alloys. We build on previous work, where
empirical force constants were adjusted {\it ad hoc} to agree with experiment.
After first relaxing all structural parameters within the local-density
approximation for ordered NiPt compounds, density-functional perturbation
theory is then used to compute phonon spectra, densities of states, and the
force constants. The resulting nearest-neighbor force constants are first
compared to those of other ordered structures of different stoichiometry, and
then used to generate the inelastic scattering cross sections within the
itinerant coherent potential approximation. We find that structural relaxation
substantially affects the computed force constants and resulting inelastic
cross sections, and that the effect is much more pronounced in random alloys
than in ordered alloys. | 0403301v1 |
2005-01-09 | Atomic-scale surface demixing in a eutectic liquid BiSn alloy | Resonant x-ray reflectivity of the surface of the liquid phase of the
Bi$_{43}$Sn$_{57}$ eutectic alloy reveals atomic-scale demixing extending over
three near-surface atomic layers. Due to the absence of underlying atomic
lattice which typically defines adsorption in crystalline alloys, studies of
adsorption in liquid alloys provide unique insight on interatomic interactions
at the surface. The observed composition modulation could be accounted for
quantitatively by the Defay-Prigogine and Strohl-King multilayer extensions of
the single-layer Gibbs model, revealing a near-surface domination of the
attractive Bi-Sn interaction over the entropy. | 0501173v3 |
2005-04-13 | Growth and surface alloying of Fe on Pt(997) | The growth of ultra-thin layers of Fe on the vicinal Pt(997) surface is
studied by thermal energy He atom scattering (TEAS) and Auger electron
spectroscopy (AES) in the temperature range between 175K and 800K. We find
three distinct regimes of qualitatively different growth type: Below 450K the
formation of a smooth first monolayer, at and above 600K the onset of bulk
alloy formation, and at intermediate temperature 500K - 550K the formation of a
surface alloy. Monatomic Fe rows are observed to decorate the substrate steps
between 175K and 500K. The importance of the high step density is discussed
with respect to the promotion of smooth layer growth and with respect to the
alloying process and its kinetics. | 0504337v1 |
2005-11-02 | Lattice thermal conductivity of disordered binary alloys : a formulation | We present here a formulation for the calculation of the configuration
averaged lattice thermal conductivity in random alloys. Our formulation is
based on the augmented-space theorem, introduced by one of us, combined with a
generalized diagrammatic technique. The diagrammatic approach simplifies the
problem of including effects of disorder corrections to a great extent. The
approach allows us to obtain an expression for the effective heat current in
case of disordered alloys, which in turn is used in a Kubo-Greenwood type
formula for the thermal conductivity. We show that disorder scattering
renormalizes the phonon propagators as well as the heat currents. The
corrections to the current terms have been shown to be related to the
self-energy of the propagators. We also study the effect of vertex corrections
in a simplified ladder diagram approximation. A mode dependent diffusivity
$D_{\gamma}$ and then a total thermal diffusivity averaged over different modes
are defined. Schemes for implementing the said formalism are discussed. A few
initial numerical results on the frequency and temperature dependence of
lattice thermal conductivity are presented for NiPd alloy and are also compared
with experiment. We also display numerical results on the frequency dependence
of thermal diffusivity averaged over modes. | 0511047v1 |
2005-12-26 | Lattice thermal conductivity of disordered NiPd and NiPt alloys | Numerical calculations of lattice thermal conductivity are reported for the
binary alloys NiPd and NiPt. The present work is a continuation of an earlier
paper by us [PRB, 72, 214207 (2005)]which had developed a theoretical framework
for the calculation of configuration-averaged lattice thermal conductivity and
thermal diffusivity in disordered alloys. The formulation was based on the
augmented space theorem combined with a scattering diagram technique. In this
paper we shall show dependence of the lattice thermal conductivity on a series
of variables like phonon frequency, temperature and alloy composition. The
temperature dependence of $\kappa(T)$ and its realtion to the measured thermal
conductivity is discussed. The concentration dependence of $\kappa$ appears to
justify the notion of a minimum thermal conductivity as discussed by Kittel,
Slack and others. We also study the frequency and composition dependence of the
thermal diffusivity averaged over modes. A numerical estimate of this quantity
gives an idea about the location of mobility edge and the fraction of states in
the frequency spectrum which is delocalized. | 0512654v2 |
2006-02-07 | A first-principles comparison of the electronic properties of MgC_{y}Ni_{3} and ZnC_{y}Ni_{3} alloys | First-principles, density-functional-based electronic structure calculations
are employed to study the changes in the electronic properties of ZnC_{y}Ni_{3}
and MgC_{y}Ni_{3} using the Korringa-Kohn-Rostoker coherent-potential
approximation method in the atomic sphere approximation (KKR-ASA CPA). As a
function of decreasing C at%, we find a steady decrease in the lattice constant
and bulk modulus in either alloys. However, the pressure derivative of the bulk
modulus displays an opposite trend. Following the Debye model, which relates
the pressure derivative of the bulk modulus with the average phonon frequency
of the crystal, it can thus be argued that ZnCNi_{3} and its disordered alloys
posses a different phonon spectra in comparison to its MgCNi_{3} counterparts.
This is further justified by the marked similarity we find in the electronic
structure properties such as the variation in the density of states and the
Hopfield parameters calculated for these alloys. The effects on the equation of
state parameters and the density of states at the Fermi energy, for partial
replacement of Mg by Zn are also discussed. | 0602181v1 |
2006-03-20 | Electronic structure and non-magnetic character of $δ$-Pu-Am alloys | The {\em around-mean-field} LSDA+U correlated band theory is applied to
investigate the electronic and magnetic structure of $fcc$-Pu-Am alloys.
Despite a lattice expansion caused by the Am atoms, neither tendency to 5$f$
localization nor formation of local magnetic moments on Pu atoms in Pu-Am
alloys are found. The $5f$-manifolds in the alloys are calculated being very
similar to a simple weighted superposition of elemental Pu and Am $5f$-states. | 0603502v1 |
2006-06-14 | On the role of confinement on solidification in pure materials and binary alloys | We use a phase-field model to study the effect of confinement on dendritic
growth, in a pure material solidifying in an undercooled melt, and in the
directional solidification of a dilute binary alloy. Specifically, we observe
the effect of varying the vertical domain extent ($\delta$) on tip selection,
by quantifying the dendrite tip velocity and curvature as a function of
$\delta$, and other process parameters. As $\delta$ decreases, we find that the
operating state of the dendrite tips becomes significantly affected by the
presence of finite boundaries. For particular boundary conditions, we observe a
switching of the growth state from 3-D to 2-D at very small $\delta$, in both
the pure material and alloy. We demonstrate that results from the alloy model
compare favorably with those from an experimental study investigating this
effect. | 0606388v1 |
2006-07-03 | First-principles calculation of the thermodynamics of In$_x$Ga$_{1-x}$N alloys: Effect of lattice vibrations | The thermodynamics properties of the wurtzite and zinc-blende \InGaN alloys
are calculated using first-principles density-functional calculations. Special
quasi-random structures are used to describe the disordered alloys, for $x=
1/4, 1/2$, and 3/4. The effect of lattice vibrations on the phase diagram,
commonly omitted from semiconductor alloy phase diagram calculations, are
included through first-principles calculations of phonon spectra. Inclusion of
lattice vibrations leads to a large reduction in the order-disorder critical
temperature ($\sim 29$% and $\sim 26$% for the wurtzite and zinc-blende
structures, respectively) and changes the shape of the solubility and spinodal
curve through changes in the entropies of the competing phases. Neglect of such
effect produces significant errors in the phase diagrams of complex ordered
semiconductor compounds. The critical temperature for phase separation is 1654
K (1771 K) for the wurtzite (zinc-blende) structures. The predicted phase
diagrams are in agreement with experimental measurements on MOCVD \InGaN\
films. | 0607051v1 |
2006-07-07 | Surface Crystallization in a Liquid AuSi Alloy | X-ray measurements reveal a crystalline monolayer at the surface of the
eutectic liquid Au_{82}Si_{18}, at temperatures above the alloy's melting
point. Surface-induced atomic layering, the hallmark of liquid metals, is also
found below the crystalline monolayer. The layering depth, however, is
threefold greater than that of all liquid metals studied to date. The
crystallinity of the surface monolayer is notable, considering that AuSi does
not form stable bulk crystalline phases at any concentration and temperature
and that no crystalline surface phase has been detected thus far in any pure
liquid metal or nondilute alloy. These results are discussed in relation to
recently suggested models of amorphous alloys. | 0607178v1 |
2006-07-11 | Electronic Instabilities in Shape-Memory Alloys | Using a variety of thermodynamic measurements made in magnetic fields, we
show evidence that the diffusionless transition (DT) in many shape-memory
alloys is related to significant changes in the electronic structure. We
investigate three alloys that show the shape-memory effect (In-24 at.% Tl,
AuZn, and U-26 at.% Nb). We observe that the DT is significantly altered in
these alloys by the application of a magnetic field. Specifically, the DT in
InTl-24 at.% shows a decrease in the DT temperature with increasing magnetic
field. Further investigations of AuZn were performed using an ultrasonic
pulse-echo technique in magnetic fields up to 45 T. Quantum oscillations in the
speed of the longitudinal sound waves propagating in the [110] direction
indicated a strong acoustic de Haas-van Alphen-type effect and give information
about part of the Fermi surface. | 0607275v3 |
2006-07-11 | Epitaxial growth of binary alloy nanostructures | Stochastic growth of binary alloys on a weakly interacting substrate is
studied by kinetic Monte Carlo simulation. The underlying lattice model relates
to fcc alloys, and the kinetics are based on deposition, atomic migration with
bond-breaking processes and exchange processes mediated by nearest neighbor
hopping steps. We investigate the interrelation between surface processes and
the emerging nonequilibrium structure at and below the growing surface under
conditions where atoms in the bulk can be regarded as immobile. The parameters
of the model are adapted to CoPt_3 alloys. Growing nanoclusters exhibit an
anisotropic short range order, primarily caused by Pt segregation at the
surface. The overall structural anisotropy depends on both Pt surface
segregation and cluster shape, and can explain the perpendicular magnetic
anisotropy (PMA) recently measured in CoPt_3 nanoclusters on a van der Waals
substrate. The onset of L1_2 ordering in the cluster is induced by surface
processes. The same kinetic model is applied also to continuous thin films,
which in addition can exhibit a small bulk contribution to PMA. | 0607284v1 |
2006-08-28 | First-principles study of polarization in ZnMgO | Wurtzite ZnO can be substituted with up to ~30% MgO to form a metastable
ZnMgO alloy while still retaining the wurtzite structure. Because this alloy
has a larger band gap than pure ZnO, ZnMgO/ZnO quantum wells and superlattices
are of interest as candidates for applications in optoelectronic and electronic
devices. Here, we report the results of an ab-initio study of the spontaneous
polarization of ZnMgO alloys as a function of their composition. We perform
calculations of the crystal structure based on density-functional theory in the
local-density approximation, and the polarization is calculated using the
Berry-phase approach. We decompose the changes in polarization into purely
electronic, lattice-displacement mediated, and strain mediated components, and
quantify the relative importance of these contributions. We consider both
free-stress and epitaxialstrain elastic boundary conditions, and show that our
results can be fairly well reproduced by a simple model in which the
piezoelectric response of pure ZnO is used to estimate the polarization change
of the ZnMgO alloy induced by epitaxial strain. | 0608614v1 |
2006-10-14 | An augmented space approach to the study of phonons in disordered alloys : comparison between the itinerant coherent-potential approximation and the augmented space recursion | A first principles density functional based linear response theory (the so
called Density Functional Perturbation theory \cite{dfpt}) has been combined
separately with two recently developed formalism for a systematic study of the
lattice dynamics in disordered binary alloys. The two formalisms are the
Augmented space recursion (ASR) and the Itinerant coherent potential
approximation (ICPA). The two different theories (DFPT-ASR and DFPT-ICPA)
systematically provides a hierarchy of improvements upon the earlier single
site based theories (like CPA etc.) and includes non-local correlations in the
disorder configurations. The formalisms explicitly take into account
fluctuations in masses, force constants and scattering lengths. The combination
of DFPT with these formulation helps in understanding the actual interplay of
force constants in alloys. We illustrate the methods by applying to a fcc
Fe$_{50}$Pd$_{50}$ alloy. | 0610391v1 |
2007-02-15 | MBE growth of cubic AlxIn1-xN and AlxGayIn1-x-yN lattice matched to GaN | Ternary and quaternary cubic c-AlxIn1-xN/GaN and c-AlxGayIn1-x-y/GaN
heterostructures lattice-matched to c-GaN on freestanding 3C-SiC substrates
were grown by plasma-assisted molecular beam epitaxy. The c-AlxGayIn1-x-y alloy
permits the independent control of band gap and lattice parameter. The ternary
and quaternary films were grown at 620 C. Different alloy compositions were
obtained by varying the Al and Ga fluxes. The alloy composition was measured by
Energy Dispersive X-ray Spectroscopy (EDX) and Rutherford Backscattering
Spectrometry (RBS). X-ray reciprocal space map of asymmetric (-1-13) reflex
were used to measure the lattice parameters and to verify the lattice match
between the alloy and the c-GaN buffer. | 0702363v1 |
2007-02-28 | Finite Volume Analysis of Nonlinear Thermo-mechanical Dynamics of Shape Memory Alloys | In this paper, the finite volume method is developed to analyze coupled
dynamic problems of nonlinear thermoelasticity. The major focus is given to the
description of martensitic phase transformations essential in the modelling of
shape memory alloys. Computational experiments are carried out to study the
thermo-mechanical wave interactions in a shape memory alloy rod, and a patch.
Both mechanically and thermally induced phase transformations, as well as
hysteresis effects, in a one-dimensional structure are successfully simulated
with the developed methodology. In the two-dimensional case, the main focus is
given to square-to-rectangular transformations and examples of martensitic
combinations under different mechanical loadings are provided. | 0702167v1 |
2007-09-06 | Unification of the phonon mode behaviour in semiconductor alloys: Theory and ab initio calculations | We demonstrate how to overcome serious problems in understanding and
classification of vibration spectra in semiconductor alloys, following from
traditional use of the virtual crystal approximation (VCA). We show that such
different systems as InGaAs (1-bond->1-mode behavior), InGaP (modified 2-mode)
and ZnTeSe (2-bond->1-mode) obey in fact the same phonon mode behavior - hence
probably a universal one - of a percolation-type (1-bond->2-mode). The change
of paradigm from the `VCA insight' (an averaged microscopic one) to the
`percolation insight' (a mesoscopic one) offers a promising link towards the
understanding of alloy disorder. The discussion is supported by ab initio
simulation of the phonon density of states at the zone-center of representative
supercells at intermediary composition (ZnTeSe) and at the impurity-dilute
limits (all systems). In particular, we propose a simple ab initio `protocol'
to estimate the basic input parameters of our semi-empirical `percolation'
model for the calculation of the 1-bond->2-mode vibration spectra of zincblende
alloys. With this, the model turns self-sufficient. | 0709.0930v2 |
2008-01-08 | Alloy surface segregation in reactive environments: A first-principles atomistic thermodynamics study of Ag3Pd(111) in oxygen atmospheres | We present a first-principles atomistic thermodynamics framework to describe
the structure, composition and segregation profile of an alloy surface in
contact with a (reactive) environment. The method is illustrated with the
application to a Ag3Pd(111) surface in an oxygen atmosphere, and we analyze
trends in segregation, adsorption and surface free energies. We observe a wide
range of oxygen adsorption energies on the various alloy surface
configurations, including binding that is stronger than on a Pd(111) surface
and weaker than that on a Ag(111) surface. This and the consideration of even
small amounts of non-stoichiometries in the ordered bulk alloy are found to be
crucial to accurately model the Pd surface segregation occurring in
increasingly O-rich gas phases. | 0801.1144v1 |
2008-03-05 | The behavior of magnetic ordering and the KOndo effect in the alloys, Ce2Rh(1-x)Co(x)Si3: Evidence from bulk studies for Fermi-surface change during magnetic ordering - QCP transformation and applicability of SDW pictur | The results of magnetic susceptibility, electrical resistivity (rho), and
heat capacity measurements as a function of temperature are reported for the
alloys, Ce2Rh(1-x)Co(x)Si3, crystallizing in an AlB2-derived hexagonal
strcture. Ce2RhSi3 exhibits antiferromagnetic ordering at 7 K. The Neel
temperature decreases gradually with the increase in Co concentration. For x
greater than 0.6, no magnetic ordering is observed down to 0.5 K.
Interestingly, the x= 0.6 alloy exhibits signatutes of non-Fermi liquid
behavior, while the Co end member is a Fermi liquid. Thus, a transformation of
magnetic ordering state to non-magnetism via non-Fermiliquid state by
isoelectronic chemical doping is evident in this solid solution. The electrical
resistivity data for x= 0.2 and 0.3 alloys show an upturn at respective Neel
temperatures, establishing the formation of a magnetism-induced pseudo-gap for
these intermediate compositions alone as though there is a gradual Fermi
surface transformation as the quantum critical point is approached. | 0803.0652v1 |
2008-06-02 | Screening of magnetic moments in PuAm alloy : LDA+DMFT study | The puzzling absence of Pu magnetic moments in a PuAm environment is explored
using the self-consistent Dynamical Mean Field Theory (DMFT) calculations in
combination with the Local Density Approximation. We argue that delta-Pu -Am
alloys provide an ideal test bed for investigating the screening of moments
from the single impurity limit to the dense limit. Several important effects
can be studied: volume expansion, shift of the bare Pu on-site f energy level,
and the reduction of the hybridization cloud resulting from the collective
character of the Kondo effect in the Anderson lattice. These effects compensate
each other and result in a coherence scale, which is independent of alloy
composition, and is around 800K. We emphasize the role of the DMFT
self-consistency condition, and multiplet splittings in Pu and Am atoms, in
order to capture the correct value of the coherence scale in the alloy. | 0806.0313v1 |
2008-08-27 | Precipitate stability and recrystallisation in the weld nuggets of friction stir welded Al-Mg-Si and Al-Mg-Sc alloys | Two different precipitate hardening aluminium alloys processed by friction
stir welding were investigated. The microstructure and the hardness of the as
delivered materials were compared to that of the weld nugget. Transmission
electron microscopy observations combined with three-dimensional atom probe
analyses clearly show that \beta;" precipitates dissolved in the nugget of the
Al-Mg-Si giving rise to some supersaturated solid solution. It is shown that
the dramatic softening of the weld could be partly recovered by post-welding
ageing treatments. In the Al-Mg-Sc alloy, Al3Sc precipitate size and density
are unchanged in the nugget comparing to the base metal. These precipitates
strongly reduce the boundary mobility of recrystallised grains, leading to a
grain size in the nugget much smaller than in the Al-Mg-Si alloy. Both coherent
and incoherent precipitates were detected. This feature may indicate that a
combination of continuous and discontinuous recrystallisation occurred in the
weld nugget. | 0808.3716v1 |
2008-10-01 | Anomalous behavior of the Debye temperature in Fe-rich Fe-Cr alloys | Debye temperature, $\Theta_D$, of Fe-rich Fe$_{100-x}$Cr$_x$ disordered
alloys with $0\le x \le 22.3$ was determined from the temperature dependence of
the central shift of M\"ossbauer spectra recorded in the temperature range of
60 -- 300 K. Its compositional dependence shows a maximum at $x \approx 5$ with
a relative increase of $\sim 30$% compared to a pure iron. The composition at
which the effect occurs correlates well with that at which several other
quantities, e. g. the Curie temperature and the spin-wave stiffness
coefficient, $D_0$, show their maxima, but the enhancement of $\Theta_D$ is
significantly greater and comparable with the enhancement of the hyperfine
field (spin-density of itinerant $s$-like electrons) in the studied system. The
results suggest that the electron-phonon interaction is important in this alloy
system. | 0810.0123v1 |
2008-12-05 | Reentrant spin glass state in Mn doped Ni2MnSn shape memory alloy | The ground state properties of the ferromagnetic shape memory alloy of
nominal composition Ni2Mn1.36Sn0.64 have been studied by dc magnetization and
ac susceptibility measurements. Like few other Ni-Mn based alloys, this sample
exhibits exchange bias phenomenon. The observed exchange bias pinning was found
to originate right from the temperature where a step-like anomaly is present in
the zero-field-cooled magnetization data. The ac susceptibility study indicates
the onset of spin glass freezing near this step-like anomaly with clear
frequency shift. The sample can be identified as a reentrant spin glass with
both ferromagnetic and glassy phases coexisting together at low temperature at
least in the field-cooled state. The result provides us an comprehensive view
to identify the magnetic character of various Ni-Mn-based shape memory alloys
with competing magnetic interactions. | 0812.1115v1 |
2009-02-25 | Anomalous Lattice Parameter of Magnetic Semiconductor Alloys | The addition of transition metals (TM) to III-V semiconductors radically
changes their electronic, magnetic and structural properties. In contrast to
the conventional semiconductor alloys, the lattice parameter in magnetic
semiconductor alloys, including the ones with diluted concentration (the
diluted magnetic semiconductors - DMS), cannot be determined uniquely from the
composition. By using first-principles calculations, we find a direct
correlation between the magnetic moment and the anion-TM bond lengths. We
derive a simple formula that determines the lattice parameter of a particular
magnetic semiconductor by considering both the composition and magnetic moment.
The formula makes accurate predictions of the lattice parameter behavior of
AlMnN, AlCrN, GaMnN, GaCrN, GaCrAs and GaMnAs alloys. This new dependence can
explain some of the hitherto puzzling experimentally observed anomalies, as
well as, stimulate other kind of theoretical and experimental investigations. | 0902.4368v1 |
2009-04-30 | Structural phase transitions and fundamental band gaps of Mg(x)Zn(1-x)O alloys from first principles | The structural phase transitions and the fundamental band gaps of
Mg(x)Zn(1-x)O alloys are investigated by detailed first-principles calculations
in the entire range of Mg concentrations x, applying a multiple-scattering
theoretical approach (Korringa-Kohn-Rostoker method). Disordered alloys are
treated within the coherent potential approximation (CPA). The calculations for
various crystal phases have given rise to a phase diagram in good agreement
with experiments and other theoretical approaches. The phase transition from
the wurtzite to the rock-salt structure is predicted at the Mg concentration of
x = 0.33, which is close to the experimental value of 0.33 - 0.40. The size of
the fundamental band gap, typically underestimated by the local density
approximation, is considerably improved by the self-interaction correction. The
increase of the gap upon alloying ZnO with Mg corroborates experimental trends.
Our findings are relevant for applications in optical, electrical, and in
particular in magnetoelectric devices. | 0904.4791v1 |
2010-02-12 | Bandgaps and band bowing in semiconductor alloys | The bandgap and band bowing parameter of semiconductor alloys are calculated
with a fast and realistic approach. The method is a dielectric scaling
approximation that is based on a scissor approximation. It adds an energy shift
to the bandgap provided by the local density approximation (LDA) of the density
functional theory (DFT). The energy shift consists of a material-independent
constant weighted by the inverse of the high-frequency dielectric constant. The
salient feature of the approach is the fast calculation of the dielectric
constant of alloys via the Green function (GF) of the TB-LMTO (tight-binding
linear muffin-tin orbitals) in the atomic sphere approximation (ASA). When it
is applied to highly mismatched semiconductor alloys (HMAs) like Zn Te$_x$
Se$_{1-x}$, this method provides a band bowing parameter that is different from
the band bowing parameter calculated with the LDA due to the bowing exhibited
also by the high-frequency dielectric constant. | 1002.2579v1 |
2010-03-31 | Giant magneto-caloric effect near room temperature in Ni-Mn-Sn-Ga alloys | We report the observation of giant magneto-caloric effect (MCE) in
ferromagnetic shape memory alloys (FSMAs) of nominal compositions
Ni$_2$Mn$_{1.36}$Sn$_{0.64-x}$Ga$_{x}$ ($x$ =0.24, 0.28 and 0.32). Irrespective
of the Ga doping, all the samples undergo long range ferromagnetic ordering
below around 330 K. However, the martensitic transition temperature ($T_{MS}$)
of the samples varies strongly with Ga concentration. Clear signature of field
induced transition around $T_{MS}$ is visible for all the samples. The observed
MCE ($\Delta S$ = 13.6 J/kg K for field changing from 0 to 50 kOe) is found to
be highest for $x$ = 0.28 with peak near 249 K, while $x$ = 0.32 shows $\Delta
S$ = 12.8 J/kg K at 274 K. This series of Ga-doped alloys are found to be
interesting materials with high value of $\Delta S$ over a varied range of
temperature | 1003.6116v1 |
2010-06-07 | Phase field crystal dynamics for binary systems: Derivation from dynamical density functional theory, amplitude equation formalism, and applications to alloy heterostructures | The dynamics of phase field crystal (PFC) modeling is derived from dynamical
density functional theory (DDFT), for both single-component and binary systems.
The derivation is based on a truncation up to the three-point direct
correlation functions in DDFT, and the lowest order approximation using scale
analysis. The complete amplitude equation formalism for binary PFC is developed
to describe the coupled dynamics of slowly varying complex amplitudes of
structural profile, zeroth-mode average atomic density, and system
concentration field. Effects of noise (corresponding to stochastic amplitude
equations) and species-dependent atomic mobilities are also incorporated in
this formalism. Results of a sample application to the study of surface
segregation and interface intermixing in alloy heterostructures and strained
layer growth are presented, showing the effects of different atomic sizes and
mobilities of alloy components. A phenomenon of composition overshooting at the
interface is found, which can be connected to the surface segregation and
enrichment of one of the atomic components observed in recent experiments of
alloying heterostructures. | 1006.1332v1 |
2010-08-16 | Coherent description of the intrinsic and extrinsic anomalous Hall effect in disordered alloys on an $ab$ $initio$ level | A coherent description of the anomalous Hall effect (AHE) is presented that
is applicable to pure as well as disordered alloy systems by treating all
sources of the AHE on equal footing. This is achieved by an implementation of
the Kubo-St\v{r}eda equation using the fully relativistic
Korringa-Kohn-Rostoker (KKR) Green's function method in combination with the
Coherent Potential Approximation (CPA) alloy theory. Applications to the pure
elemental ferromagnets bcc-Fe and fcc-Ni led to results in full accordance with
previous work. For the alloy systems fcc-Fe$_x$Pd$_{1-x}$ and
fcc-Ni$_x$Pd$_{1-x}$ very satisfying agreement with experiment could be
achieved for the anomalous Hall conductivity (AHC) over the whole range of
concentration. To interpret these results an extension of the definition for
the intrinsic AHC is suggested. Plotting the corresponding extrinsic AHC versus
the longitudinal conductivity a linear relation is found in the dilute regimes,
that allows a detailed discussion of the role of the skew and side-jump
scattering processes. | 1008.2640v2 |
2010-08-23 | Finite-temperature magnetism of Fe$_x$Pd$_{1-x}$ and Co$_x$Pt$_{1-x}$ alloys | The finite-temperature magnetic properties of Fe$_x$Pd$_{1-x}$ and
Co$_x$Pt$_{1-x}$ alloys have been investigated. It is shown that the
temperature-dependent magnetic behaviour of alloys, composed of originally
magnetic and non-magnetic elements, cannot be described properly unless the
coupling between magnetic moments at magnetic atoms (Fe,Co) mediated through
the interactions with induced magnetic moments of non-magnetic atoms (Pd,Pt) is
included. A scheme for the calculation of the Curie temperature ($T_C$) for
this type of systems is presented which is based on the extended Heisenberg
Hamiltonian with the appropriate exchange parameters $J_{ij}$ obtained from
{\em ab-initio} electronic structure calculations. Within the present study the
KKR Green's function method has been used to calculate the $J_{ij}$ parameters.
A comparison of the obtained Curie temperatures for Fe$_x$Pd$_{1-x}$ and
Co$_x$Pt$_{1-x}$ alloys with experimental data shows rather good agreement. | 1008.3784v1 |
2010-10-22 | Investigation on Anharmonicity, Vibrational Anisotropy and Thermal Expansion of an Amorphous Ni$_{46}$Ti$_{54}$ Alloy Produced by Mechanical Alloying using Extended X-ray Absorption Fine Structure | A method to investigate anharmonicity, vibrational anisotropy and thermal
expansion using correlated mean-square relative displacements (MSRD) parallel
and perpendicular to the interatomic bonds obtained only from Extended X-ray
Absorption Fine Structure (EXAFS) analysis based on cumulant expansion is
suggested and applied to an amorphous Ni$_{46}$Ti$_{54}$ alloy produced by
mechanical alloying. From EXAFS measurements taken on Ni and Ti K edges at
several temperatures, the thermal behavior of $\text{MSRD}_\parallel$,
$\text{MSRD}_\perp$ and of the cumulants $C_1^*$, $C_2^*$ and $C_3^*$ of the
real distribution functions $\varrho_{ij}(r,T)$, and also the Einstein
temperatures and frequencies associated with parallel and perpendicular motion
were obtained, furnishing information about the anharmonicity of the
interatomic potential, vibrational anisotropy and the contribution of the
perpendicular motion to the total disorder and thermal expansion. | 1010.4703v1 |
2011-01-11 | Magnetism of fine particles of Kondo lattices, obtained by high-energy ball-milling | Despite intense research in the field of strongly correlated electron
behavior for the past few decades, there has been very little effort to
understand this phenomenon in nano particles of the Kondo lattices. In this
article, we review the results of our investigation on the fine particles (less
than 1 micron) of some of the alloys obtained by high-energy ball-milling to
bring out that this synthetic method paves a way to study strong electron
correlations in nanocrystals of such alloys. We primarily focus on the alloys
of the series, CeRu(2-x)Rh(x)Si2, lying at different positions in Doniach's
magnetic phase diagram. While CeRu2Si2, a bulk paramagnet, appears to become
magnetic (of a glassy type) below about 8 K in fine particle form, in CeRh2Si2,
an antiferromagnet (T_N= 36 K) in bulk form, magnetism is destroyed (at least
down to 0.5 K) in fine particles. In the alloy, CeRu(0.8)Rh(1.2)Si2, at the
quantum critical point, no long range magnetic ordering is found | 1101.2010v1 |
2011-01-18 | Characterizing Solute Segregation and Grain Boundary Energy in a Binary Alloy Phase Field Crystal Model | This paper studies how solute segregation and its relationship to grain
boundary energy in binary alloys is captured in the phase field crystal (PFC)
formalism, a continuum method that incorporates atomic scale elasto-plastic
effects on diffusional time scales. Grain boundaries are simulated using two
binary alloy PFC models --- the original binary model by Elder et al (2007) and
the XPFC model by Greenwood et al (2011). In both cases, grain boundary energy
versus misorientation data is shown to be well described by Read-Shockley
theory. The Gibbs Adsorption Theorem is then used to derive a semi-analytic
function describing solute segregation to grain boundaries. This is used to
characterize grain boundary energy versus average alloy concentration and
undercooling below the solidus. We also investigate how size mismatch between
different species and their interaction strength affects segregation to the
grain boundary. Finally, we interpret the implications of our simulations on
material properties related to interface segregation. | 1101.3464v3 |
2011-02-02 | Interstitial Fe-Cr alloys: Tuning of magnetism by nanoscale structural control and by implantation of nonmagnetic atoms | Using the density functional theory, we perform a full atomic relaxation of
the bulk ferrite with 12.5%-concentration of monoatomic interstitial Cr
periodically located at the edges of the bcc Fe$_\alpha$ cell. We show that
structural relaxation in such artificially engineered alloys leads to
significant atomic displacements and results in the formation of novel highly
stable configurations with parallel chains of octahedrically arranged Fe. The
enhanced magnetic polarization in the low-symmetry metallic state of this type
of alloys can be externally controlled by additional inclusion of nonmagnetic
impurities like nitrogen. We discuss possible applications of generated
interstitial alloys in spintronic devices and propose to consider them as a
basis of novel durable types of stainless steels. | 1102.0432v2 |
2011-02-04 | Depth concentrations of deuterium ions implanted into some pure metals and alloys | Pure metals (Cu, Ti, Zr, V, Pd) and diluted Pd-alloys (Pd-Ag, Pd-Pt, Pd-Ru,
Pd-Rh) were implanted by 25 keV deuterium ions at fluences in the range
(1.2{\div}2.3)x1022 D+/m2. The post-treatment depth distributions of deuterium
ions were measured 10 days and three months after the implantation using
Elastic Recoil Detection Analysis (ERDA) and Rutherford Backscattering (RBS).
Comparison of the obtained results allowed to make conclusions about relative
stability of deuterium and hydrogen gases in pure metals and diluted Pd alloys.
Very high diffusion rates of implanted deuterium ions from V and Pd pure metals
and Pd alloys were observed. Small-angle X-ray scattering revealed formation of
nanosized defects in implanted corundum and titanium. | 1102.0881v1 |
2011-02-06 | Microscopic model for the semiconductor-to-ferromagnetic-metal transition in FeSi$_{1-x}$Ge$_{x}$ Alloys | The simplified bandstructure introduced by Mazurenko et al to model FeSi is
used to analyze the singlet semiconductor to ferromagnetic metal transition in
the isoelectronic isostructural alloys, FeSi$_{1-x}$Ge$_x$. The complex
bandstructure of the alloy is replaced by an alternating chain of doubly and
singly degenerate atoms to represent Fe and Si/Ge respectively. The
former(latter) form narrow(broad) bands with a substantial hybridization
between them. A substantial onsite repulsion including a Hund's rule coupling
is introduced on the Fe sites. The mean field phase diagram contains a first
order phase transition from the singlet semiconductor to a ferromagnetic metal
with increasing temperature and interaction strength similar to the alloys. The
analysis also reproduces the rapid rise of the spin susceptibility in the
semiconductor with a crossover to a Curie-Weiss form at higher temperatures.
Good agreement is found at zero temperature between the mean field and accurate
DMRG calculations. | 1102.1190v1 |
2011-02-22 | Ab-initio calculation of the Gilbert damping parameter via linear response formalism | A Kubo-Greenwood-like equation for the Gilbert damping parameter $\alpha$ is
presented that is based on the linear response formalism. Its implementation
using the fully relativistic Korringa-Kohn-Rostoker (KKR) band structure method
in combination with Coherent Potential Approximation (CPA) alloy theory allows
it to be applied to a wide range of situations. This is demonstrated with
results obtained for the bcc alloy system Fe$_x$Co$_{1-x}$ as well as for a
series of alloys of permalloy with 5d transition metals.
To account for the thermal displacements of atoms as a scattering mechanism,
an alloy-analogy model is introduced. The corresponding calculations for Ni
correctly describe the rapid change of $\alpha$ when small amounts of
substitutional Cu are introduced. | 1102.4551v1 |
2011-03-09 | Magnetostructural Transformation and Magnetoresponsive Properties of MnNiGe1-xSnx Alloys | The martensitic and magnetic phase transformations in MnNiGe1-xSnx (0 \leq x
\leq 0.200) alloys were investigated using X-ray diffraction (XRD),
differential thermal analysis (DTA) and magnetization measurements. Results
indicate that the increasing Sn substitution in MnNiGe1-xSnx results in (i)
decrease of martensitic transformation temperature from 460 to 100 K and (ii)
conversion of AFM spiral to antiparallel AFM strcuture in martensite. Based on
these, the remarkable magnetic-field-induced PM/spiral-AFM and FM/AFM
magnetostructural transformations and, large positive and negative
magnetocaloric effects are obtained. The magnetoresponsive effects of
MnNiGe1-xSnx alloys are enhanced by Sn substitution. A structural and magnetic
phase diagram of MnNiGe1-xSnx alloys has been proposed. | 1103.1736v1 |
2011-05-19 | Waste-recycling Monte Carlo with optimal estimates: application to free energy calculations in alloys | The estimator proposed recently by Delmas and Jourdain for waste-recycling
Monte Carlo achieves variance reduction optimally with respect to a control
variate that is evaluated directly using the simulation data. Here, the
performance of this estimator is assessed numerically for free energy
calculations in generic binary alloys and compared to those of other estimators
taken from the literature. A systematic investigation with varying simulation
parameters of a simplified system, the anti-ferromagnetic Ising model, is first
carried out in the transmutation ensemble using path-sampling. We observe
numerically that (i) the variance of the Delmas-Jourdain estimator is indeed
reduced compared to that of other estimators; and that (ii) the resulting
reduction is close to the maximal possible one, despite the inaccuracy in the
estimated control variate. More extensive path-sampling simulations involving a
FeCr alloy system described by a many-body potential additionally show that
(iii) gradual transmutations accommodate the atomic frustrations, thus
alleviating the numerical ergodicity issue present in numerous alloy systems
and eventually enabling the determination of phase coexistence conditions. | 1105.3874v1 |
2011-05-30 | Predicting the Structure of Alloys using Genetic Algorithms | We discuss a novel genetic algorithm that can be used to find global minima
on the potential energy surface of disordered ceramics and alloys using a
real-space symmetry adapted crossover. Due to a high number of symmetrically
equivalent solutions of many alloys a conventional genetic algorithms using
reasonable population sizes are unable to locate the global minima for even the
smallest systems. We demonstrate the superior performance of the use of
symmetry adapted crossover by the comparison of that of a conventional GA for
finding global minima of two binary Ising-type alloys that either order or
phase separate at low temperature. Comparison of different representations and
crossover operations show that the use of real-space crossover outperforms
crossover operators working on binary representations by several orders of
magnitude. | 1105.5941v1 |
2011-07-21 | Approaching Quantum Criticality in ferromagnetic Ce_2 (Pd_1-x Rh_x)_2 In alloys | Low temperature magnetic and thermal (C_m) properties of the ferromagnetic
(FM) alloys Ce_2.15 (Pd_1-x Rh_x)_1.95 In_0.9 were investigated in order to
explore the possibility for tuning a quantum critical point (QCP) by doping Pd
with Rh. As expected, the magnetic transition observed at T = 4.1K in the
parent alloy decreases with increasing Rh concentration. Nevertheless it splits
into two transitions, the upper being antiferromagnetic (AF) whereas the lower
FM. The AF phase boundary extrapolates to T_N = 0 for x_cr ~ 0.65 whereas the
first order FM transition vanishes at x ~ 0.3. The QC character of the T_N => 0
point arises from the divergent T dependence of the tail of C_m/T observed in
the x = 0.5 and 0.55 alloys, and the tendency to saturation of the maximum of
C_m(T_N)/T as observed in exemplary Ce compounds for T_N => 0. Beyond the
critical concentration the unit cell volume deviates from the Vegard's law in
coincidence with a strong increase of the Kondo temperature. | 1107.4308v1 |
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