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Enhancing magnetocrystalline anisotropy of the Fe70Pd30 magnetic shape
memory alloy by adding Cu: Strained epitaxial growth provides the opportunity to understand the
dependence of intrinsic and extrinsic properties of functional materials at
frozen intermediate stages of a phase transformation. In this study, a
combination of thin film experiments and first-principles calculations yields
the binding energy and magnetic properties of tetragonal Fe70Pd30-xCux
ferromagnetic shape memory thin films with x = 0, 3, 7 and structures ranging
from bcc to beyond fcc (1.07<c/a_bct<1.57). We find that Cu enhances the
quality of epitaxial growth, while spontaneous polarisation and Curie
temperature are only moderately lowered as expected from our calculations.
Beyond c/a_bct>1.41 the samples undergo structural relaxations through adaptive
nanotwinning. For all tetragonal structures, we observe a significant increase
of the magnetocrystalline anisotropy constant K1, which reaches a maximum of
K1=-2.4*10^5 Jm^-3 at room temperature around c/a_bct=1.33 and is thus even
larger than for binary Fe70Pd30 and the prototype Ni-Mn-Ga magnetic shape
memory system. Since K1 represents the driving force for variant reorientation
in magnetic shape memory systems, we conclude that Fe-Pd-Cu alloys offer a
promising route towards microactuators applications with significantly improved
work output. | cond-mat_mtrl-sci |
Perpendicular magnetic anisotropy and spin glass-like behavior in
molecular beam epitaxy grown chromium telluride thin films: Reflection high energy electron diffraction (RHEED), scanning tunneling
microscopy (STM), vibrating sample magnetometry and other physical property
measurements are used to investigate the structure, morphology, magnetic and
magneto-transport properties of (001)-oriented Cr$_2$Te$_3$ thin films grown on
Al$_2$O$_3$(0001) and Si(111)-(7$\times$7) surfaces by molecular beam epitaxy
(MBE). Streaky RHEED patterns indicate flat smooth film growth on both
substrates. STM studies show the hexagonal arrangements of surface atoms.
Determination of the lattice parameter from atomically resolved STM image is
consistent with the bulk crystal structures. Magnetic measurements show the
film is ferromagnetic having the Curie temperature of about 180 K, and a spin
glass-like behavior was observed below 35 K. Magneto-transport measurements
show the metallic nature of the film with a perpendicular magnetic anisotropy
along the $c$-axis. | cond-mat_mtrl-sci |
Comment on "Hysteretic transition between states of a filled hexagonal
magnetic dipole cluster": In the paper "Andrew D.P. Smith, Peter T. Haugen, Boyd F. Edwards: Hysteretic
transition between states of a filled hexagonal magnetic dipole cluster,
Journal of Magnetism and Magnetic Materials 549 (2022): 168991" a hysteretic
transition between two stable arrangements of a cluster of seven dipoles is
presented. The relative strength of the center dipole in a hexagonal
arrangement serves as the bifurcation parameter. The authors clearly
demonstrate the existence of two instabilities accompanied by discontinuous
jumps of the dipole arrangement, but leave the question about the nature of
these instabilities unanswered. This comment clarifies the nature of the two
instabilities: the first one is a symmetry-breaking sub-critical bifurcation
with parabolic scaling of the magnetic potential energy difference between the
two branches, and the second one is a fold with its characteristic scaling in
the form of a semi-cubic parabola. | cond-mat_mtrl-sci |
Bulk and Lattice Properties for Rigid Carbon Nanotubes Materials: We use an atom-atom potential between carbon atoms to obtain an interaction
potential between nanotubes (assumed rigid), thereby calculating the cohesive
energy of a bunch of nanotubes in hexagonal two dimensional packing. The model
proposed is quite similar to our earlier work on fullerenes and organic
molecular crystals. The results for inter-nanotube distances, energy per unit
length, bulk modulus and phonons for inter-nanotube vibrations are obtained and
compared with available data from measurements and other available
calculations. We also model formation of multi-wall nanotubes. We find the
results for various calculated quantities agreeing very well with measured
structural parameters and other calculations. The reversible energy stored on
compression of the bunch of nanotubes on application of pressure up to 30 Kbar
calculated in this rigid molecule model is overestimated by about 30% when
compared with measured results, signifying the appreciable flexibility of tubes
at high pressures. The model is considered very suitable for incorporating
flexible nanotubes in bunches of single and multi-wall nanotube materials of
various types. | cond-mat_mtrl-sci |
Self-Assembled Triply Periodic Minimal Surfaces as moulds for Photonic
Band Gap Materials: We propose systems with structures defined by self-assembled triply periodic
minimal surfaces (STPMS) as candidates for photonic bandgap materials. To
support our proposal we have calculated the photonic bands for different STPMS
and we have found that, at least, the double diamond and gyroid structures
present full photonic bandgaps. Given the great variety of systems which
crystalize in these structures, the diversity of possible materials that form
them and the range of lattice constants they present, the construction of
photonic bandgap materials with gaps in the visible range may be presently
within reach. | cond-mat_mtrl-sci |
Imaging of Spin Dynamics in Closure Domain and Vortex Structures: Time-resolved Kerr microscopy is used to study the excitations of individual
micron- scale ferromagnetic thin film elements in their remnant state. Thin (18
nm) square elements with edge dimensions between 1 and 10 $\mu$m form closure
domain structures with 90 degree Neel walls between domains. We identify two
classes of excitations in these systems. The first corresponds to precession of
the magnetization about the local demagnetizing field in each quadrant, while
the second excitation is localized in the domain walls. Two modes are also
identified in ferromagnetic disks with thicknesses of 60 nm and diameters from
2 $\mu$m down to 500 nm. The equilibrium state of each disk is a vortex with a
singularity at the center. As in the squares, the higher frequency mode is due
to precession about the internal field, but in this case the lower frequency
mode corresponds to gyrotropic motion of the entire vortex. These results
demonstrate clearly the existence of well-defined excitations in
inhomogeneously magnetized microstructures. | cond-mat_mtrl-sci |
Limits to crystallization pressure: Crystallization pressure drives deformation and damage in monuments,
buildings and the Earth's crust. Even though the phenomenon has been known for
170 years there is no agreement between theoretical calculations of the maximum
attainable pressure and that found experimentally. We have therefore developed
a novel experimental technique to image the nano-confined crystallization
process while controlling the pressure and applied it to calcite. The results
show that displacement by crystallization pressure is arrested at pressures
well below the thermodynamic limit. We use existing molecular dynamics
simulations and atomic force microscopy data to construct a robust model of the
disjoining pressure in this system and thereby calculate the absolute distance
between the surfaces. Based on the high resolution experiments and modelling we
formulate a novel mechanism for the transition between damage and adhesion by
crystallization that may find application in Earth and materials sciences and
in conservation of cultural heritage. | cond-mat_mtrl-sci |
Giant magnetocaloric effect in exchange-frustrated GdCrTiO5
antiferromagnet: We report the effect of exchange frustration on the magnetocaloric properties
of GdCrTiO$_5$ compound. Due to the highly exchange-frustrated nature of
magnetic interaction, in GdCrTiO$_5$, the long-range antiferromagnetic ordering
occurs at much lower temperature $T_N$=0.9 K and the magnetic cooling power
enhances dramatically relative to that observed in several geometrically
frustrated systems. Below 5 K, isothermal magnetic entropy change (-$\Delta
S_{\rm m}$) is found to be 36 J kg$^{-1}$ K$^{-1}$, for a field change ($\Delta
H$) of 7 T. Further, -$\Delta S_{\rm m}$ does not decrease from its maximum
value with decreasing in $T$ down to very low temperatures and is reversible in
nature. The adiabatic temperature change, $\Delta T_{\rm ad}$, is 15 K for
$\Delta H$=7 T. These magnetocaloric parameters are significantly larger than
that reported for several potential magnetic refrigerants, even for small and
moderate field changes. The present study not only suggests that GdCrTiO$_5$
could be considered as a potential magnetic refrigerant at cryogenic
temperatures but also promotes further studies on the role of exchange
frustration on magnetocaloric effect. In contrast, only the role of geometrical
frustration on magnetocaloric effect has been previously reported theoretically
and experimentally investigated on very few systems. | cond-mat_mtrl-sci |
Structures and velocities of noisy ferroelectric domain walls: Ferroelectric domain wall motion is fundamental to the switching properties
of ferroelectric devices and is influenced by a wide range of factors including
spatial disorder within the material and thermal noise. We build a
Landau-Ginzburg-Devonshire (LGD) model of 180${}^{\circ}$ ferroelectric domain
wall motion that explicitly takes into account the presence of both spatial and
temporal disorder. We demonstrate both creep flow and linear flow regimes of
the domain wall dynamics by solving the LGD equations in a Galilean frame
moving with the wall velocity $v$. Thermal noise plays a key role in the wall
depinning process at small fields $E$. We study the scaling of the velocity $v$
with the applied DC electric field $E$ and show that noise strongly affects
domain wall velocities. We also show that the domain wall widens significantly
in the presence of thermal noise, especially as the material temperature $T$
approaches the critical temperature $T_c$. These calculations therefore point
to the potential of noise and disorder to become control factors for the
switching properties of ferroelectric materials, for example for advancement of
microelectronic applications. | cond-mat_mtrl-sci |
Fascinating interplay between Charge Density Wave Order and magnetic
field in Non-magnetic Rare-Earth Tritelluride LaTe$_{3}$: Charge density wave (CDW) states in solids bear an intimate connection to
underlying fermiology. Modification of the latter by a suitable perturbation
provides an attractive handle to unearth novel CDW states. Here, we combine
extensive magnetotransport experiments and first-principles electronic
structure calculations on a non-magnetic tritelluride LaTe$_{3}$ single crystal
to uncover phenomena rare in CDW systems: $(i)$ hump-like feature in the
temperature dependence of resistivity at low temperature under application of
magnetic field, which moves to higher temperature with increasing field
strength, $(ii)$ highly anisotropic large transverse magnetoresistance (MR)
upon rotation of magnetic field about current parallel to crystallographic
c-axis, (iii) anomalously large positive MR with spike-like peaks at
characteristic angles when the angle between current and field is varied in the
bc-plane, (iv) extreme sensitivity of the angular variation of MR on field and
temperature. Moreover, our Hall measurement reveals remarkably high carrier
mobility $\sim$ 33000 cm$^{2}$/Vs, which is comparable to that observed in some
topological semimetals. These novel observations find a comprehensive
explication in our density functional theory (DFT) and dynamical mean field
theory (DMFT) calculations that capture field-induced electronic structure
modification in LaTe$_{3}$. The band structure theory together with transport
calculations suggest the possibility of a second field-induced CDW transition
from the field-reconstructed Fermi surface, which qualitatively explains the
hump in temperature dependence of resistivity at low temperature. Thus, our
study exposes the novel manifestations of the interplay between CDW order and
field-induced electronic structure modifications in LaTe$_{3}$, and establishes
a new route to tune CDW states by perturbations like magnetic field. | cond-mat_mtrl-sci |
Spatial decomposition of magnetic anisotropy in magnets: application for
doped Fe16N2: We propose a scheme of decomposition of the total relativistic energy in
solids to intra- and interatomic contributions. The method is based on a
variation of the speed of light from its value in relativistic theory to
infinity (a non-relativistic limit). As an illustration of the method, we
tested such decomposition in the case of a spin-orbit interaction variation for
decomposition of the magnetic anisotropy energy (MAE) in CoPt. We further
studied the {\alpha}''-Fe16N2 magnet doped by Bi, Sb, Co and Pt atoms. It has
been found that the addition of Pt atoms can enhance the MAE by as large as
five times while Bi and Sb substitutions double the total MAE. Using the
proposed technique we demonstrate the spatial distribution of these
enhancements. Our studies also suggest that Sb, Pt and Co substitutions could
be synthesized by experiments. | cond-mat_mtrl-sci |
The electronic transport properties and microstructure of carbon
nanofiber/epoxy composites: Carbon nanofibres (CNF) were dispersed into an epoxy resin using a
combination of ultrasonication and mechanical mixing. The electronic transport
properties of the resulting composites were investigated by means of impedance
spectroscopy. It was found that a very low critical weight fraction (pc = 0.064
wt %) which may be taken to correspond to the formation of a tunneling
conductive network inside the matrix. The insulator-to-conductor transition
region spanned about one order of magnitude from 0.1 to 1 wt %. Far from the
transition, the conductivity increased by two orders of magnitude. This
increase and the low value of the conductivity were explained in terms of the
presence of an epoxy film at the contact between CNF. A simple model based on
the CNF-CNF contact network inside the matrix was proposed in order to evaluate
the thickness of that film. | cond-mat_mtrl-sci |
Quest for Dr. Yia-Chung Chang's Calculations about the superlattice
phonon band structures: Quest for Dr. Yia-Chung Chang's Calculations about the superlattice phonon
band structures | cond-mat_mtrl-sci |
Crystal growth and metallic ferromagnetism induced by electron doping in
FeSb$_2$: In order to study the metallic ferromagnetism induced by electron doping in
the narrow-gab semiconductor FeSb$_2$, single crystals of FeSb$_2$,
Fe$_{1-x}$Co$_x$Sb$_2$ ($0 \le x \le 0.5$) and FeSb$_{2-y}$Te$_y$ ($0 \le y \le
0.4$), were grown by a simplified self-flux method. From powder x-ray
diffraction (XRD) patterns, wavelength-dispersive x-ray spectroscopy (WDX) and
x-ray Laue diffraction, pure and doped high-quality single crystals, within the
selected solubility range, show only the orthorhombic $Pnnm$ structure of
FeSb$_2$ with a monotonic change in lattice parameters with increasing the
doping level. In consistence with the model of nearly ferromagnetic small-gap
semiconductor, the energy gap of FeSb$_2$ Pauli paramagnet gradually collapses
by electron doping before it closes at about $x$ or $y$ = 0.15 and subsequent
itinerant electron anisotropic ferromagnetic states are observed with higher
doping levels. A magnetic phase diagram is established and discussed in view of
proposed theoretical scenarios. | cond-mat_mtrl-sci |
In Situ X-Ray Radiography and Tomography Observations of the
Solidification of Alumina Particles Suspensions. Part II: Steady State: This paper investigates the behaviour of colloidal suspensions of alumina
particles during directional solidification, by in situ high-resolution
observations using X-ray radiography and tomography. This second part is
focussed on the evolution of ice crystals during steady state growth (in terms
of interface velocity) and on the particles redistribution taking place in this
regime. In particular, it is shown that diffusion cannot determine the
concentration profile and the particles redistribution in this regime of
interface velocities (20-40 microns/s); constitutional supercooling arguments
cannot be invoked to interpret particles redistribution. Particles are
redistributed by a direct interaction with the moving solidification interface.
Several parameters controlling the particles redistribution were identified,
namely the interface velocity, the particle size, the shape of the ice crystals
and the orientation relationships between the crystals and the temperature
gradient. | cond-mat_mtrl-sci |
Density dependent local structures in InTe phase-change materials: Chalcogenide phase-change materials (PCMs) based random access memory (PCRAM)
is one of the leading candidates for the development of non-volatile memory and
neuro-inspired computing technologies. Recent work shows Indium to be an
important alloying element for PCRAM, while a thorough understanding of the
parent compound InTe, in particular, its amorphous phase, is still lacking. In
this work, we carry out ab initio simulations and chemical bonding analyses on
amorphous and various crystalline polymorphs of InTe. We reveal that the local
geometries are highly density dependent in amorphous structures, forming
In-centered tetrahedral motifs under ambient conditions but defective
octahedral motifs under pressure, which stems from the bonding characters of
its crystalline polymorphs. In addition, our ab initio molecular dynamics
simulations predict rapid crystallization capability of InTe under pressure. At
last, we make a suggestion for better use of Indium and propose an "active"
device design to utilize both thermal and mechanical effects for phase-change
applications. | cond-mat_mtrl-sci |
Effects of biaxial strain on the improper multiferroicity in h-LuFeO3
films: Elastic strain is potentially an important approach in tuning the properties
of the improperly multiferroic hexagonal ferrites, the details of which have
however been elusive due to the experimental difficulties. Employing the method
of restrained thermal expansion, we have studied the effect of isothermal
biaxial strain in the basal plane of h-LuFeO3 (001) films. The results indicate
that a compressive biaxial strain significantly enhances the ferrodistortion,
and the effect is larger at higher temperatures. The compressive biaxial strain
and the enhanced ferrodistortion together, cause an increase in the electric
polarization and a reduction in the canting of the weak ferromagnetic moments
in h-LuFeO3, according to our first principle calculations. These findings are
important for understanding the strain effect as well as the coupling between
the lattice and the improper multiferroicity in h-LuFeO3. The experimental
elucidation of the strain effect in h-LuFeO3 films also suggests that the
restrained thermal expansion can be a viable method to unravel the strain
effect in many other epitaxial thin film materials. | cond-mat_mtrl-sci |
Electronic and structural properties of vacancies and hydrogen
adsorbates on trilayer graphene: Using ab initio calculations, we study the electronic and structural
properties of vacancies and hydrogen adsorbates on trilayer graphene. Those
defects are found to share similar low-energy electronic features, since they
both remove a pz electron from the honeycomb lattice and induce a defect level
near the Fermi energy. However, a vacancy also leaves unpaired $\sigma $
electrons on the lattice, which lead to important structural differences and
also contribute to magnetism. We explore both ABA and ABC stackings and compare
properties such as formation energies, magnetic moments, spin density and the
local density of states (LDOS) of the defect levels. These properties show a
strong sensitivity to the layer in which the defect is placed and smaller
sensitivities to sublattice placing and stacking type. Finally, for the ABC
trilayer, we also study how these states behave in the presence of an external
field, which opens a tunable gap in the band structure of the non-defective
system. The pz defect states show a strong hybridization with band states as
the field increases, with reduction and eventually loss of magnetization, and a
non-magnetic, midgap-like state is found when the defect is at the middle
layer. | cond-mat_mtrl-sci |
Concentration behavior of liquidus temperatures and undercooling of
Al-Cu-Co at normal pressure: Differential thermal analysis has been conducted for the Al-Cu-Co alloys with
the composition range of 15 at.% Co and 10 to 30 at.% Cu, and 25 at.% Co and
2.5 to 20 at.% Co. The features of the formation of solid phases have been
studied during the crystallization in a crucible in the conditions of slow
cooling (rate of cooling to 1 K s-1) at normal pressure. On the state diagram
of the Al-Cu-Co system with 15 at.% Co and 25 at.% Cu the concentration
sections have been built, which allows to determine the concentration ranges
from which different phases are formed during the first stage of
crystallization. Along the boundaries of different phase regions, extrema are
observed on the liquidus line. The observed extrema on the liquidus lines and
concentration dependences of undercooling are associated with change in the
chemical short-range order at the considered concentrations both in the liquid
and solid states. | cond-mat_mtrl-sci |
Coherent control of photomagnetic back-switching by double-pump laser
pulses: The control of nonthermal, all-optical magnetization switching under the
regime with an independent state of laser polarization opens up new
opportunities for ultrafast magnetic recording. Here, we investigate the
photo-magnetic back-switching capabilities of the write and erase magnetic
domain pattern using double-pump pulse excitations in an iron garnet film with
pure cubic magnetocrystalline symmetry. It is essential to note that forward
and backward magnetization switching is achievable in two distinctive
scenarios: using identical linearly polarized laser pulses and with pulses
having orthogonal polarization planes. By observing the switch of magnetization
at domains independent of the initial state, one can nonthermally toggle the
magnetization, equivalent to XOR logical operation, at frequencies reaching up
to 50 GHz. | cond-mat_mtrl-sci |
An oxide thermal rectifier: We have experimentally demonstrated thermal rectification as bulk effect.
According to a theoretical design of a thermal rectifier, we have prepared an
oxide thermal rectifier made of two cobalt oxides with different thermal
conductivities, and have made an experimental system to detect the thermal
rectification. The rectifying coefficient of the device is found to be 1.43,
which is in good agreement with the numerical calculation. | cond-mat_mtrl-sci |
Connectivity of the Icosahedral Network and a Dramatically Growing
Static Length Scale in Cu-Zr Binary Metallic Glasses: We report on and characterize, via molecular dynamics (MD) studies, the
evolution of the structure of Cu50Zr50 and Cu64Zr36 metallic glasses (MGs) as
temperature is varied. Interestingly, a percolating icosahedral network appears
in the Cu64Zr36 system as it is supercooled. This leads us to introduce a
static length scale, which grows dramatically as this three dimensional system
approaches the glass transition. Amidst interpenetrating connections,
non-interpenetrating connections between icosahedra are shown to become
prevalent upon supercooling and to greatly enhance the connectivity of the MG's
icosahedral network. Additionally, we characterize the chemical compositions of
the icosahedral networks and their components. These findings demonstrate the
importance of non-interpenetrating connections for facilitating extensive
structural networks in Cu-Zr MGs, which in turn drive dynamical slowing in
these materials. | cond-mat_mtrl-sci |
Microscopic mechanism of high-temperature ferromagnetism in Fe, Mn, and
Cr-doped InSb, InAs, and GaSb magnetic semiconductors: In recent experiments, high Curie temperatures Tc above room temperature were
reported in ferromagnetic semiconductors Fe-doped GaSb and InSb, while low Tc
between 20 K to 90 K were observed in some other semiconductors with the same
crystal structure, including Fe-doped InAs and Mn-doped GaSb, InSb, and InAs.
Here we study systematically the origin of high temperature ferromagnetism in
Fe, Mn, Cr-doped GaSb, InSb, and InAs magnetic semiconductors by combining the
methods of density functional theory and quantum Monte Carlo. In the diluted
impurity limit, the calculations show that the impurities Fe, Mn, and Cr have
similar magnetic correlations in the same semiconductors. Our results suggest
that high (low) Tc obtained in these experiments mainly comes from high (low)
impurity concentrations. In addition, our calculations predict the
ferromagnetic semiconductors of Cr-doped InSb, InAs, and GaSb that may have
possibly high Tc. Our results show that the origin of high Tc in (Ga,Fe)Sb and
(In,Fe)Sb is not due to the carrier induced mechanism because Fe3+ does not
introduce carriers. | cond-mat_mtrl-sci |
Visualization of reaction chemistry in W-KClO4-BaCrO4 delay mixtures via
a Sestak-Berggren model based isoconversional method: The combustion delay mixture of tungsten (W), potassium perchlorate (KClO4),
and barium chromate (BaCrO4), also known as the WKB mixture, has long been
considered to be an integral part of military-grade ammunition. Despite its
long history, however, their progressive reaction dynamics remains a question
mark, especially due to the complex nature of their combustion reaction. As
opposed to a one-step oxidation commonly observed in conventional combustions,
the WKB mixture is associated with a multibody reaction between its solid-state
components. To this end, the emergence of three combustion peaks, which we
corresponded with disparate chemical reactions, was observed using
thermogravimetric analysis on two separate WKB mixtures with differing mixture
ratios. We applied the stepwise isoconversional method on each of the peaks to
match the combustion chemistry it represents to the Sestak-Berggren model and
computed the conceptual activation energy. Further plotting the logarithmic
pre-exponential factor as a function of the reaction progress, we demonstrate a
method of using the plot as an intuitive tool to understand the dynamics of
individual reactions that compose multi-step chemical reactions. Our study
provides a systematic approach in visualizing the reaction chemistry, thereby
strengthening the analytical arsenal against reaction dynamics of combustion
compounds in general. | cond-mat_mtrl-sci |
Dipolar-stabilized first and second-order antiskyrmions in ferrimagnetic
multilayers: Skyrmions and antiskyrmions are topologically protected spin structures with
opposite topological charge. Particularly in coexisting phases, these two types
of magnetic quasi-particles may show fascinating physics and potential for
spintronic devices. While skyrmions are observed in a wide range of materials,
until now antiskyrmions were exclusive to materials with D2d symmetry. In this
work, we show first and second-order antiskyrmions stabilized by magnetic
dipole-dipole interaction in Fe/Gd-based multilayers. We modify the magnetic
properties of the multilayers by Ir insertion layers. Using Lorentz
transmission electron microscopy imaging, we observe coexisting antiskyrmions,
Bloch skyrmions, and type-2 bubbles and determine the range of material
properties and magnetic fields where the different spin objects form and
dissipate. We perform micromagnetic simulations to obtain more insight into the
studied system and conclude that the reduction of saturation magnetization and
uniaxial anisotropy leads to the existence of this zoo of different spin
objects and that they are primarily stabilized by dipolar interaction. | cond-mat_mtrl-sci |
Effect of zirconium doping on mechanical properties of $W_{1-x}Zr_xB_2$
on the base of ab initio calculations and magnetron sputtered films: Potentially superhard $W_{1-x}Zr_xB_2$ polymorph hP6-P6$_3$/mmc-$WB_2$ with
zirconium doping in the range of x=0.0-0.25 was thoroughly analyzed within the
framework of first-principles density functional theory from the structural and
mechanical point of view. The obtained results were subsequently compared with
properties of material deposited by magnetron sputtering method. All predicted
structures are mechanically and thermodynamically stable. Due to theoretical
calculations zirconium doping reduces hardness and fracture toughness $K_{IC}$
of $WB_2$. Deposited films are characterized by greater hardness $H_v$ but
lower fracture toughness $K_{IC}$. The results of experiments show that not
only solid solution hardening is responsible for strengthening of predicted new
material but also change of microstructure, Hall-Petch effect and boron
vacancies. | cond-mat_mtrl-sci |
Melting of hexane monolayers adsorbed on graphite: the role of domains
and defect formation: We present the first large-scale molecular dynamics simulations of hexane on
graphite that completely reproduces all experimental features of the melting
transition. The canonical ensemble simulations required and used the most
realistic model of the system: (i) fully atomistic representation of hexane;
(ii) explicit site-by-site interaction with carbon atoms in graphite; (iii)
CHARMM force field with carefully chosen adjustable parameters of non-bonded
interaction; (iv) numerous $\ge$ 100 ns runs, requiring a total computation
time of ca. 10 CPU-years. This has allowed us to determine correctly the
mechanism of the transition: molecular reorientation within lamellae without
perturbation of the overall adsorbed film structure. We observe that the melted
phase has a dynamically reorienting domain-type structure whose orientations
reflect that of graphite. | cond-mat_mtrl-sci |
Understanding electronic excited states in BiFeO$_3$ via ab initio
calculations and symmetry analysis: BiFeO$_3$ is a technologically relevant multiferroic perovskite featuring
ferroelectricity and antiferromagnetism. Its lattice, magnetic, and
ferroelectric degrees of freedoms are coupled to its optically active
excitations and thus hold the potential to be reversible probed and controlled
by light. In this work, we combine ab initio density functional and many-body
perturbation theory methods with an extensive symmetry and atomic-orbital
analysis to describe and understand the electronic excited states spectrum and
its imprint on the optical absorption spectrum with quantitative accuracy and
qualitative insights. We find that the optical absorption spectrum of BiFeO$_3$
contain several strongly bound and spatially localized electronic transitions
in which the spin-degree of freedom is almost fully flipped. With our analysis
we thoroughly characterize these localized spin-flip transitions in terms of
the unusual crystal field splitting of Fe-$3d$ single-electron orbitals. Our
symmetry analysis further allows us to thoroughly explain how the spin content
and the energetic fine structure of these strongly bound excitons are dictated
by the interplay between crystal symmetry, electron-hole attraction, and the
spin-orbit coupling. | cond-mat_mtrl-sci |
First-order Reversal Curve Analysis of Phase Transitions in
Electrochemical Adsorption: A New Experimental Technique Suggested by
Computer Simulations: The first-order reversal curve (FORC) method for analysis of systems
undergoing hysteresis is applied to dynamical models of electrochemical
adsorption. In this setting, the method can not only differentiate between
discontinuous and continuous phase transitions, but can also quite accurately
recover equilibrium behavior from dynamic analysis for systems with a
continuous phase transition. Discontinuous and continuous phase transitions in
a two-dimensional lattice-gas model are compared using the FORC method. The
FORC diagram for a discontinuous phase transition is characterized by a
negative (unstable) region separating two positive (stable) regions, while such
a negative region does not exist for continuous phase transitions. Experimental
data for FORC analysis could easily be obtained by simple reprogramming of a
potentiostat designed for cyclic-voltammetry experiments. | cond-mat_mtrl-sci |
Computational exfoliation of atomically thin 1D materials with
application to Majorana bound states: We introduce a computational database with calculated structural,
thermodynamic, electronic, magnetic, and optical properties of 820
one-dimensional materials. The materials are systematically selected and
exfoliated from experimental databases of crystal structures based on a
dimensionality scoring parameter. The database is furthermore expanded by
chemical element substitution in the materials. The materials are investigated
in both their bulk form and as isolated one-dimensional components. We discuss
the methodology behind the database, give an overview of some of the calculated
properties, and look at patterns and correlations in the data. The database is
furthermore applied in computational screening to identify materials, which
could exhibit Majorana bound states. | cond-mat_mtrl-sci |
Wave impedance matrices for cylindrically anisotropic radially
inhomogeneous elastic solids: Impedance matrices are obtained for radially inhomogeneous structures using
the Stroh-like system of six first order differential equations for the time
harmonic displacement-traction 6-vector. Particular attention is paid to the
newly identified solid-cylinder impedance matrix ${\mathbf Z} (r)$ appropriate
to cylinders with material at $r=0$, and its limiting value at that point, the
solid-cylinder impedance matrix ${\mathbf Z}_0$. We show that ${\mathbf Z}_0$
is a fundamental material property depending only on the elastic moduli and the
azimuthal order $n$, that ${\mathbf Z} (r)$ is Hermitian and ${\mathbf Z}_0$ is
negative semi-definite. Explicit solutions for ${\mathbf Z}_0$ are presented
for monoclinic and higher material symmetry, and the special cases of $n=0$ and
1 are treated in detail. Two methods are proposed for finding ${\mathbf Z}
(r)$, one based on the Frobenius series solution and the other using a
differential Riccati equation with ${\mathbf Z}_0$ as initial value. %in a
consistent manner as the solution of an algebraic Riccati equation. The
radiation impedance matrix is defined and shown to be non-Hermitian. These
impedance matrices enable concise and efficient formulations of dispersion
equations for wave guides, and solutions of scattering and related wave
problems in cylinders. | cond-mat_mtrl-sci |
Micromagnetic simulations of spinel ferrite particles: This paper presents the results of simulations of the magnetization field
{\it ac} response (at $2$ to $12$ GHz) of various submicron ferrite particles
(cylindrical dots). The ferrites in the present simulations have the spinel
structure, expressed here by M$_{1-n}$Zn$_{n}$Fe$_2$O$_4$ (where M stands for a
divalent metal), and the parameters chosen were the following: (a) for $n=0$: M
= \{ Fe, Mn, Co, Ni, Mg, Cu \}; (b) for $n=0.1$: M = \{ Fe, Mg \} (mixed
ferrites). These runs represent full 3D micromagnetic (one-particle) ferrite
simulations. We find evidences of confined spin waves in all simulations, as
well as a complex behavior nearby the main resonance peak in the case of the M
= \{ Mg, Cu \} ferrites. A comparison of the $n=0$ and $n=0.1$ cases for fixed
M reveals a significant change in the spectra in M = Mg ferrites, but only a
minor change in the M = Fe case. An additional larger scale simulation of a $3$
by $3$ particle array was performed using similar conditions of the Fe$_3$O$_4$
(magnetite; $n=0$, M = Fe) one-particle simulation. We find that the main
resonance peak of the Fe$_3$O$_4$ one-particle simulation is disfigured in the
corresponding 3 by 3 particle simulation, indicating the extent to which
dipolar interactions are able to affect the main resonance peak in that
magnetic compound. | cond-mat_mtrl-sci |
Picosecond acoustic excitation driven ultrafast magnetization dynamics
in dielectric Bi-substituted yttrium iron garnet: Using femtosecond optical pulses, we have investigated the ultrafast
magnetization dynamics induced in a dielectric film of bismuth-substituted
yttrium iron garnet (Bi-YIG) buried below a thick Cu/Pt metallic bilayer. We
show that exciting the sample from Pt surface launches an acoustic strain pulse
propagating into the garnet film. We discovered that this strain pulse induces
a coherent magnetization precession in the Bi-YIG at the frequency of the
ferromagnetic resonance. The observed phenomena can be explain by
strain-induced changes of magnetocristalline anisotropy via the inverse
magnetostriction effect. These findings open new perspectives toward the
control of the magnetization in magnetic garnets embedded in complex
heterostructure devices. | cond-mat_mtrl-sci |
Effect of thermal annealing on the heat transfer properties of reduced
graphite oxide flakes: a nanoscale characterization via scanning thermal
microscopy: This paper reports on the thermal properties of reduced graphite oxide (RGO)
flakes, studied by means of scanning thermal microscopy (SThM). This technique
was demonstrated to allow thermal characterization of the flakes with a spatial
resolution of the order of a few tens of nanometers, while recording nanoscale
topography at the same time. Several individual RGO flakes were analyzed by
SThM, both as obtained after conventional thermal reduction and after a
subsequent annealing at 1700{\deg}C. Significant differences in the thermal
maps were observed between pristine and annealed flakes, reflecting higher heat
dissipation on annealed RGO flakes compared with pristine ones. This result was
correlated with the reduction of RGO structure defectiveness. In particular, a
substantial reduction of oxidized groups and sp3 carbons upon annealing was
proven by X-ray photoelectron and Raman spectroscopies, while the increase of
crystalline order was demonstrated by X-ray diffraction, in terms of higher
correlation lengths both along and perpendicular to the graphene planes.
Results presented in this paper provide experimental evidence for the
qualitative correlation between the defectiveness of graphene-related materials
and their thermal conductivity, which is clearly crucial for the exploitation
of these materials into thermally conductive nanocomposites. | cond-mat_mtrl-sci |
Chiral Spin Bobbers in Exchange-Coupled Hard-Soft Magnetic Bilayers: The spin structure of exchange-coupled MnBi:Co-Fe bilayers is investigated by
X-ray magnetic circular dichroism (XMCD), polarized neutron reflectometry
(PNR), and micromagnetic simu-lations. The purpose of the present research is
two-fold. First, the current search for new permanent-magnet materials includes
hard-soft nanocomposites, and the analysis of coercivity mechanisms in these
structures is an important aspect of this quest. Second, topological
micro-magnetic structures such as skyrmions have recently become of intense
fundamental and applied research, for example in the context of spin-based
electronics. We find that the magnetization reversal of the MnBi:Co-Fe bilayer
structure involves a curling-type twisting of the magnetization in the film
plane. This curling in the exchange-coupled hard-soft magnetic bilayers is
reminiscent of chiral spin structures known as bobbers and, in fact,
establishes a new type of skyrmionic spin structure. | cond-mat_mtrl-sci |
A method to computationally screen for tunable properties of crystalline
alloys: Conventionally, high-throughput computational materials searches start from
an input set of bulk compounds extracted from material databases, and this set
is screened for candidate materials for specific applications. In contrast,
many functional materials, and especially semiconductors, are heavily
engineered alloys or solid solutions of multiple compounds rather than a single
bulk compound. To improve our ability to design functional materials, in this
work we propose a framework and open-source code to automatically construct
possible "alloy pairs" and "alloy systems" and detect "alloy members" from a
set of existing, experimental or calculated ordered compounds, without
requiring any additional metadata beyond their crystal structure. We provide
analysis tools to estimate stability across each alloy. As a demonstration, we
apply this framework to all inorganic materials in the Materials Project
database to create a new database of over 600,000 unique alloy pair entries
that can then be used in materials discovery studies to search for materials
with tunable properties. This new database has been incorporated into the
Materials Project website and linked with corresponding material identifiers
for any user to query and explore. Using an example of screening for p-type
transparent conducting materials, we demonstrate how using this methodology
reveals candidate material systems that might otherwise have been excluded by a
traditional screening. This work lays a foundation from which materials
databases can go beyond stoichiometric compounds, and approach a more realistic
description of compositionally tunable materials. | cond-mat_mtrl-sci |
Low-temperature thermal expansion of rock-salt ZnO: Lattice parameter of metastable high-pressure phase of zinc oxide, rock-salt
ZnO was measured in the 10-300 K temperature range using synchrotron X-ray
powder diffraction. No phase transition was observed down to 10 K. The lattice
parameter of rock-salt ZnO was found to increase from 4.266 {\AA} in the 10-80
K range up to 4.2752(3) {\AA} at 298 K, while the volume thermal expansion
coefficient increases from slight negative values below 40 K up to
4.77\times10^-5 K^-1 at 298 K. | cond-mat_mtrl-sci |
Impact of lattice rotation on dislocation motion: We introduce a phenomenological theory of dislocation motion appropriate for
two dimensional lattices. A coarse grained description is proposed that
involves as primitive variables local lattice rotation and Burgers vector
densities along distinguished slip systems of the lattice. We then use symmetry
considerations to propose phenomenological equations for both defect energies
and their dissipative motion. As a consequence, the model includes explicit
dependences on the local state of lattice orientation, and allows for
differential defect mobilities along distinguished directions. Defect densities
and lattice rotation need to determined self consistently and we show specific
results for both square and hexagonal lattices. Within linear response,
dissipative equations of motion for the defect densities are derived which
contain defect mobilities that depend nonlocally on defect distribution. | cond-mat_mtrl-sci |
Vanadium Dioxide: Metal-Insulator Transition, Electrical Switching and
Oscillations. A Review of State of the Art and Recent Progress: Vanadium dioxide is currently considered as one of the most promising
metarials for oxide elcteronics. Both planar and sandwich thin-film MOM devices
based on VO2 exhibit electrical switching with an S-shaped I-V characteristic,
and this switching effect is associated with the metal-insulator transition
(MIT). In an electrical circuit containing such a switching device, relaxation
oscillations are observed if the load line intersects the I-V curve at a unique
point in NDR region. All these effects are potentially prospective for
designing various devices of oxide electronics, particularly, elements of
dynamical neural networks based on coupled oscillators. | cond-mat_mtrl-sci |
Anionic nickel and nitrogen effects in the chiral antiferromagnetic
antiperovskite Mn$_3$NiN: Magnetic antiperovskites, holding chiral noncollinear antiferromagnetic
ordering, have shown remarkable properties that cover from negative thermal
expansion to anomalous Hall effect. Nevertheless, details on the electronic
structure related to the oxidation states and the octahedral center's site
effect are still scarce. Here, we show a theoretical study, based on
first-principles calculations in the framework of the density-functional
theory, DFT, on the electronic details associated with the nitrogen site effect
into the structural, electronic, magnetic, and topological degrees of freedom.
Thus, we show that the nitrogen-vacancy increases the values of the anomalous
Hall conductivity and retains the chiral $\Gamma_{4g}$ antiferromagnetic
ordering. Moreover, we reveal, based on the Bader charges and the electronic
structure analysis, the negative and positive oxidation states in the Ni and Mn
sites, respectively. The latter is in agreement with the expected
$A_3^{\alpha+}B^{\beta-}X^{\delta-}$ oxidation states to satisfy the charge
neutrality in the antiperovskites, but rare for transition metals. Finally, we
extrapolate our findings on the oxidation states to several Mn$_3B$N compounds
showing that the antiperovskite structure is an ideal platform to encounter
negative oxidation states in metals sitting at the corner $B$-site. | cond-mat_mtrl-sci |
Ion Intercalation in Lanthanum Strontium Ferrite for Aqueous
Electrochemical Energy Storage Devices: Ion intercalation of perovskite oxides in liquid electrolytes is a very
promising method for controlling their functional properties while storing
charge, which opens the potential application in different energy and
information technologies. Although the role of defect chemistry in the oxygen
intercalation in a gaseous environment is well established, the mechanism of
ion intercalation in liquid electrolytes at room temperature is poorly
understood. In this study, the defect chemistry during ion intercalation of
La0.5Sr0.5FeO3-{\delta} thin films in alkaline electrolytes is studied. Oxygen
and proton intercalation into the LSF perovskite structure is observed at
moderate electrochemical potentials (0.5 V to -0.4 V), giving rise to a change
in the oxidation state of Fe (as a charge compensation mechanism). The
variation of the concentration of holes as a function of the intercalation
potential was characterized by in-situ ellipsometry and the concentration of
electron holes was indirectly quantified for different electrochemical
potentials. Finally, a dilute defect chemistry model that describes the
variation of defect species during ionic intercalation was developed. | cond-mat_mtrl-sci |
Lattice Dynamics Calculations based on Density-functional Perturbation
Theory in Real Space: A real-space formalism for density-functional perturbation theory (DFPT) is
derived and applied for the computation of harmonic vibrational properties in
molecules and solids. The practical implementation using numeric atom-centered
orbitals as basis functions is demonstrated exemplarily for the all-electron
Fritz Haber Institute ab initio molecular simulations (FHI-aims) package. The
convergence of the calculations with respect to numerical parameters is
carefully investigated and a systematic comparison with finite-difference
approaches is performed both for finite (molecules) and extended (periodic)
systems. Finally, the scaling tests and scalability tests on massively parallel
computer systems demonstrate the computational efficiency. | cond-mat_mtrl-sci |
Two-dimensional Graphene Heterojunctions: the Tunable Mechanical
Properties: We report the mechanical properties of different two-dimensional carbon
heterojunctions (HJs) made from graphene and various stable graphene
allotropes, including {\alpha}-, {\beta}-, {\gamma}- and 6612-graphyne (GY),
and graphdiyne (GDY). It is found that all HJs exhibit a brittle behaviour
except the one with {\alpha}-GY, which however shows a hardening process due to
the formation of triple carbon rings. Such hardening process has greatly
deferred the failure of the structure. The yielding of the HJs is usually
initiated at the interface between graphene and graphene allotropes, and
monoatomic carbon rings are normally formed after yielding. By varying the
locations of graphene (either in the middle or at the two ends of the HJs),
similar mechanical properties have been obtained, suggesting insignificant
impacts from location of graphene allotropes. Whereas, changing the types and
percentages of the graphene allotropes, the HJs exhibit vastly different
mechanical properties. In general, with the increasing graphene percentage, the
yield strain decreases and the effective Young's modulus increases. Meanwhile,
the yield stress appears irrelevant with the graphene percentage. This study
provides a fundamental understanding of the tensile properties of the
heterojunctions that are crucial for the design and engineering of their
mechanical properties, in order to facilitate their emerging future
applications in nanoscale devices, such as flexible/stretchable electronics. | cond-mat_mtrl-sci |
Phase-Modulated Elastic Properties of Two-Dimensional Magnetic FeTe:
Hexagonal and Tetragonal Polymorphs: Two-dimensional (2D) layered magnets, such as iron chalcogenides, have
emerged these years as a new family of unconventional superconductor and
provided the key insights to understand the phonon-electron interaction and
pairing mechanism. Their mechanical properties are of strategic importance for
the potential applications in spintronics and optoelectronics. However, there
is still lack of efficient approach to tune the elastic modulus despite the
extensive studies. Herein, we report the modulated elastic modulus of 2D
magnetic FeTe and its thickness-dependence via phase engineering. The grown 2D
FeTe by chemical vapor deposition can present various polymorphs, i.e.
tetragonal FeTe (t-FeTe, antiferromagnetic) and hexagonal FeTe (h-FeTe,
ferromagnetic). The measured Young's modulus of t-FeTe by nanoindentation
method showed an obvious thickness-dependence, from 290.9+-9.2 to 113.0+-8.7
GPa when the thicknesses increased from 13.2 to 42.5 nm, respectively. In
comparison, the elastic modulus of h-FeTe remains unchanged. Our results could
shed light on the efficient modulation of mechanical properties of 2D magnetic
materials and pave the avenues for their practical applications in nanodevices. | cond-mat_mtrl-sci |
Thermoelectric probe of defect state induced by ionic liquid gating in
vanadium dioxide: Thermoelectric measurements detect the asymmetry between the density of
states above and below the chemical potential in a material. It provides
insights into small variations in the density of states near the chemical
potential, complementing electron transport measurements. Here, combined
resistance and thermoelectric power measurements are performed on vanadium
dioxide (VO2), a prototypical correlated electron material, under ionic-liquid
(IL) gating. With IL gating, charge transport below the
metal-to-insulator-transition (MIT) temperature remains in the thermally
activated regime, while the Seebeck coefficient exhibits an apparent transition
from semiconducting to metallic behavior. The contrasting behavior indicates
changes in electronic structure upon IL gating, due to the formation of oxygen
defect states. The experimental results are corroborated by numerical
simulations based on a model density of states incorporating a gating induced
defect band. This study reveals thermoelectric measurements to be a convenient
and sensitive probe for the role of defect states induced by IL gating in
suppressing the MIT in VO2, which remains benign in charge transport
measurements, and possibly for studying defect sates in other materials. | cond-mat_mtrl-sci |
Intermediate anomalous Hall states induced by noncollinear spin
structure in magnetic topological insulator MnBi2Te4: The combination of topology and magnetism is attractive to produce exotic
quantum matters, such as the quantum anomalous Hall state, axion insulators and
the magnetic Weyl semimetals. MnBi2Te4, as an intrinsic magnetic topological
insulator, provides a platform for the realization of various topological
phases. Here we report the intermediate Hall steps in the magnetic hysteresis
of MnBi2Te4, where four distinguishable magnetic memory states at zero magnetic
field are revealed. The gate and temperature dependence of the magnetic
intermediate states indicates the noncollinear spin structure in MnBi2Te4,
which can be attributed to the Dzyaloshinskii-Moriya interaction as the
coexistence of strong spin-orbit coupling and local inversion symmetry breaking
on the surface. Moreover, these multiple magnetic memory states can be
programmatically switched among each other through applying designed pulses of
magnetic field. Our results provide new insights of the influence of bulk
topology on the magnetic states, and the multiple memory states should be
promising for spintronic devices. | cond-mat_mtrl-sci |
Ferroelectric field effect of the bulk heterojunction in polymer solar
cells: A ferroelectric field effect in the bulk heterojunction was found when an
external electric field (EEF) was applied on the active layer of polymer solar
cells (PSCs) during the annealing process of the active layer spin-coated with
poly (3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM).
For one direction field, the short circuit current density of PSCs was improved
from 7.2 to 8.0 mA/cm2, the power conversion efficiency increased from 2.4 to
2.8%, and the incident photon-to-current conversion efficiency increased from
42 to 49% corresponding to the different EEF magnitude. For an opposite
direction field, the applied EEF brought a minus effect on the performance
mentioned above. EEF treatment can orientate molecular ordering of the polymer,
and change the morphology of the active layer. The authors suggest a
explanation that the ferroelectric field has been built in the active layer,
and therefore it plays a key role in PSCs system. A needle-like surface
morphology of the active film was also discussed. | cond-mat_mtrl-sci |
Giant enhanced optical nonlinearity of colloidal nanocrystals with a
graded-index host: The effective linear and third-order nonlinear optical properties of metallic
colloidal crystal immersed in a graded-index host fluid are investigated
theoretically. The local electric fields are extracted self-consistently based
on the layer-to-layer interactions, which are readily given by the Lekner
summation method. The resultant optical absorption and nonlinearity enhancement
show a series of sharp peaks, which merge in a broadened resonant band. The
sharp peaks become a continuous band for increasing packing density and number
of layers. We believe that the sharp peaks arise from the in-plane dipolar
interactions and the surface plasmon resonance, whereas the continuous band is
due to the presence of the gradient in the host refractive index. These results
have not been observed in homogeneous and randomly-dispersed colloids, and thus
would be of great interest in optical nanomaterial engineering. | cond-mat_mtrl-sci |
Left handed materials: We review recent progress in the studies of left handed materials. | cond-mat_mtrl-sci |
On the preparation and NMR spectroscopic characterization of potassium
aluminium tetrahydride KAlH4: Potassium aluminium tetrahydride KAlH4 of high phase purity (space group Pnma
(62)) was synthesized via a mechanochemical route. The thus obtained material
was studied by 27Al and 39K MAS NMR spectroscopy. For both nuclei precise data
for the isotropic chemical shift and the quadrupole coupling at T=295 K were
derived (27Al: delta_iso=(107.6+-0.2) ppm, C_Q = (1.29+-0.02) MHz and eta =
0.64+-0.02; 39K: delta_iso=(6.1+-0.2) ppm, C_Q = (0.562+-0.005) MHz and eta =
0.74+-0.02). The straightforward NMR spectroscopic approach applied here should
also work for other complex aluminium hydrides and for many other materials
containing half-integer nuclei experiencing small to medium-sized quadrupole
couplings. | cond-mat_mtrl-sci |
Acoustic phonon scattering in a low density, high mobility AlGaN/GaN
field effect transistor: We report on the temperature dependence of the mobility, $\mu$, of the
two-dimensional electron gas in a variable density AlGaN/GaN field effect
transistor, with carrier densities ranging from 0.4$\times10^{12}$ cm$^{-2}$ to
3.0$\times10^{12}$ cm$^{-2}$ and a peak mobility of 80,000 cm$^{2}$/Vs. Between
20 K and 50 K we observe a linear dependence $\mu_{ac}^{-1} = \alpha$T
indicating that acoustic phonon scattering dominates the temperature dependence
of the mobility, with $\alpha$ being a monotonically increasing function of
decreasing 2D electron density. This behavior is contrary to predictions of
scattering in a degenerate electron gas, but consistent with calculations which
account for thermal broadening and the temperature dependence of the electron
screening. Our data imply a deformation potential D = 12-15 eV. | cond-mat_mtrl-sci |
Energetics of the oxidation and opening of a carbon nanotube: We apply first principles calculations to study the opening of single-wall
carbon nanotubes (SWNT's) by oxidation. We show that an oxygen rim can
stabilize the edge of the open tube. The sublimation of CO$_2$ molecules from
the rim with the subsequent closing of the tube changes from endothermic to
exothermic as the tube radius increases, within the range of experimental
feasible radii. We also obtain the energies for opening the tube at the cap and
at the wall, the latter being significantly less favorable. | cond-mat_mtrl-sci |
Stress-Induced Phase Transitions in Nanoscale CuInP$_2$S$_6$: Using Landau-Ginsburg-Devonshire approach and available experimental results
we reconstruct the thermodynamic potential of the layered ferroelectric
CuInP$_2$S$_6$ (CIPS), which is expected to be applicable a wide range of
temperatures and applied pressures. The analysis of temperature dependences of
the dielectric permittivity and lattice constants for different applied
pressures unexpectedly reveals the critically important role of the nonlinear
electrostriction in this material. With the nonlinear electrostriction included
we calculated temperature and pressure phase diagrams and spontaneous
polarization of bulk CIPS. Using the coefficients of the reconstructed
four-well thermodynamic potential, we study the strain-induced phase
transitions in thin epitaxial CIPS films, as well as the stress-induced phase
transitions in CIPS nanoparticles, which shape varies from prolate needles to
oblate disks. We reveal the strong influence of the mismatch strain, elastic
stress and shape anisotropy on the polar properties and phase diagrams of
nanoscale CIPS. Also, we derived analytical expressions, which allow the
elastic control of the nanoscale CIPS polar properties. Hence obtained results
can be of particular interest for the strain-engineering of nanoscale layered
nanoferroelectrics. | cond-mat_mtrl-sci |
A multimodal operando neutron study of the phase evolution in a graphite
electrode: Obtaining a complete picture of local processes still poses a significant
challenge in battery research. Here we demonstrate an in-situ combination of
multimodal neutron imaging with neutron diffraction for spatially resolved
operando observations of the lithiation-delithiation of a graphite electrode in
a Li-ion battery cell. Throughout the lithiation-delithiation process we image
the Li distribution based on the local beam attenuation. Simultaneously, we
observe the development of the lithiated graphite phases as a function of
cycling time and electrode thickness and integral throughout its volume by
diffraction contrast imaging and diffraction, respectively. While the
conventional imaging data allows to observe the Li uptake in graphite already
during the formation of the solid electrolyte interphase, diffraction indicates
the onset and development of the Li insertion/extraction globally, which
supports the local structural transformation observations by diffraction
contrast imaging. | cond-mat_mtrl-sci |
Pressure-induced Lifshitz transition in NbP: Raman, x-ray diffraction,
electrical transport and density functional theory: We report high pressure Raman, synchrotron x-ray diffraction and electrical
transport studies on Weyl semimetals NbP and TaP along with first-principles
density functional theoretical (DFT) analysis. The frequencies of first-order
Raman modes of NbP harden with increasing pressure and exhibit a slope change
at P$_c$ $\sim$ 9 GPa, and its resistivity exhibits a minimum at P$_c$. The
pressure-dependent volume of NbP exhibits a change in its bulk modulus from 207
GPa to 243 GPa at P$_c$. Using DFT calculations, we show that these anomalies
are associated with pressure induced Lifshitz transition which involves
appearance of electron and hole pockets in its electronic structure. In
contrast, results of Raman and synchrotron x-ray diffraction experiments on TaP
and DFT calculations show that TaP is quite robust under pressure and does not
undergo any phase transition. | cond-mat_mtrl-sci |
Moiré pattern formation in epitaxial growth on a covalent substrate:
Sb on InSb(111)A: Structural moir\'e superstructures arising from two competing lattices may
lead to unexpected electronic behavior, such as superconductivity or Mottness.
Most investigated moir\'e heterostructures are based on van der Waals (vdW)
materials, as strong interface interactions typically lead to the formation of
strained films or regular surface reconstructions. Here we successfully
synthesize ultrathin Sb films, that are predicted to show thickness-dependent
topological properties, on semi-insulating InSb(111)A. Despite the covalent
nature of the substrate surface, we prove by scanning transmission electron
microscopy (STEM) that already the first layer of Sb atoms grows completely
unstrained, while azimuthally aligned. Rather than compensating the lattice
mismatch of -6.4% by structural modifications, the Sb films form a pronounced
moir\'e pattern as we evidence by scanning tunneling microscopy (STM)
topography up to film thicknesses of several bilayers. Our model calculations
based on density functional theory (DFT) assign the moir\'e pattern to a
periodic surface corrugation. In agreement with DFT predictions, irrespective
of the moir\'e modulation, the topological surface state known on thick Sb film
is experimentally confirmed to persist down to low film thicknesses, and the
Dirac point shifts towards lower binding energies with decreasing Sb thickness. | cond-mat_mtrl-sci |
An estimate for thermal diffusivity in highly irradiated tungsten using
Molecular Dynamics simulation: The changing thermal conductivity of an irradiated material is among the
principal design considerations for any nuclear reactor, but at present few
models are capable of predicting these changes starting from an arbitrary
atomistic model. Here we present a simple model for computing the thermal
diffusivity of tungsten, based on the conductivity of the perfect crystal and
resistivity per Frenkel pair, and dividing a simulation into perfect and
athermal regions statistically. This is applied to highly irradiated
microstructures simulated with Molecular Dynamics. A comparison to experiment
shows that simulations closely track observed thermal diffusivity over a range
of doses from the dilute limit of a few Frenkel pairs to the high dose
saturation limit at 3 displacements per atom (dpa). | cond-mat_mtrl-sci |
Effect of spin-orbit interaction on the excitonic effects in
single-layer, double-layer, and bulk MoS2: We present converged ab-initio calculations of the optical absorption spectra
of single-layer, bi-layer, and bulk MoS$_2$. Both the quasiparticle-energy
calculations (on the level of the GW approximation) and the calculation of the
absorption spectra (on the level of the Bethe-Salpeter equation) explicitly
include spin-orbit coupling, using the full spinorial Kohn-Sham wave-functions
as input. Without excitonic effects, the absorption spectra would have the form
of a step-function, corresponding to the joint-density of states of a parabolic
band-dispersion in 2D. This profile is deformed by a pronounced bound excitonic
peak below the continuum onset. The peak is split by spin-orbit interaction in
the case of single-layer and (mostly) by inter-layer interaction in the case of
double-layer and bulk MoS$_2$. The resulting absorption spectra are thus very
similar in the three cases but the interpretation of the spectra is different.
Differences in the spectra can be seen around 3 eV where the spectra of single
and double-layer are dominated by a strongly bound exciton. | cond-mat_mtrl-sci |
Spatio-Temporal Electron Propagation Dynamics in Au/Fe/MgO(001) in
nonequilibrium: Revealing Single Scattering Events and the Ballistic Limit: Understanding the microscopic spatio-temporal dynamics of nonequilibrium
charge carriers in heterosystems promises optimization of process and device
design towards desired energy transfer. Hot electron transport is governed by
scattering with other electrons, defects, and bosonic excitations. Analysis of
the energy dependence of scattering pathways and identification of diffusive,
super-diffusive, and ballistic transport regimes are current challenges. We
determine in femtosecond time-resolved two-photon photoelectron emission
spectroscopy the energy-dependent change of the electron propagation time
through epitaxial Au/Fe(001) heteostructures as a function of Au layer
thickness for energies of 0.5 to \unit[2.0]{eV} above the Fermi energy. We
describe the laser-induced nonequilibrium electron excitation and injection
across the Fe/Au interface using real-time time-dependent density functional
theory and analyze the electron propagation through the Au layer by microscopic
electron transport simulations. We identify ballistic transport of minority
electrons at energies with a nascent, optically excited electron population
which is determined by the combination of photon energy and the specific
electronic structure of the material. At lower energy, super-diffusive
transport with 1 to 4 scattering events dominates. The effective electron
velocity accelerates from 0.3 to \unit[1]{nm/fs} with an increase in the Au
layer thickness from 10 to 100~nm. This phenomenon is explained by electron
transport that becomes preferentially aligned with the interface normal for
thicker Au layers, which facilitates electron momentum / energy selection by
choice of the propagation layer thickness. | cond-mat_mtrl-sci |
Properties of heavy rare-gases adlayers on graphene substrates: We investigated properties of heavy rare-gases, Ne, Ar, Kr, Xe and Rn,
adsorbed on graphene substrates using molecular dynamics. We gathered evidences
of commensurate solids for Ne and Kr adlayers, one of them is given by a
typical behavior of the nearest neighbor distance of the adatoms. The specific
heat and the melting temperature were calculated and both indicate continuous
melting for all heavy noble-gases studied. We also determined the distance
between the adlayer and the substrate. | cond-mat_mtrl-sci |
Intrinsic interfacial van der Waals monolayers and their effect on the
high-temperature superconductor FeSe/SrTiO$_3$: The sensitive dependence of monolayer materials on their environment often
gives rise to unexpected properties. It was recently demonstrated that
monolayer FeSe on a SrTiO$_3$ substrate exhibits a much higher superconducting
critical temperature T$_C$ than the bulk material. Here, we examine the
interfacial structure of FeSe / SrTiO$_3$ and the effect of an interfacial
Ti$_{1+x}$O$_2$ layer on the increased T$_C$ using a combination of scanning
transmission electron microscopy and density functional theory. We find
Ti$_{1+x}$O$_2$ forms its own quasi-two-dimensional layer, bonding to both the
substrate and the FeSe film by van der Waals interactions. The excess Ti in
this layer electron-dopes the FeSe monolayer in agreement with experimental
observations. Moreover, the interfacial layer introduces symmetry-breaking
distortions in the FeSe film that favor a T$_C$ increase. These results suggest
that this common substrate may be functionalized to modify the electronic
structure of a variety of thin films and monolayers. | cond-mat_mtrl-sci |
A meta-analysis of the mechanical properties of ice-templated ceramics
and metals: Ice templating, also known as freeze casting, is a popular shaping route for
macroporous materials. Over the past 15 years, it has been widely applied to
various classes of materials, and in particular ceramics. Many formulation and
process parameters, often interdependent, affect the outcome. It is thus
difficult to understand the various relationships between these parameters from
isolated studies where only a few of these parameters have been investigated.
We report here the results of a meta analysis of the structural and mechanical
properties of ice templated materials from an exhaustive collection of records.
We use these results to identify which parameters are the most critical to
control the structure and properties, and to derive guidelines to optimize the
mechanical response of ice templated materials. We hope these results will be a
helpful guide to anyone interested in such materials. | cond-mat_mtrl-sci |
Tunable surface configuration of skyrmion lattices in cubic helimagnets: In bulk helimagnets, the presence of magnetic skyrmion lattices is always
accompanied by a periodic stress field due to the intrinsic magnetoelastic
coupling. The release of this nontrivial stress field at the surface causes a
periodic displacement field, which characterizes a novel particle-like property
of skyrmion: its surface configuration. Here, we derive the analytical solution
of this displacement field for semi-infinite cubic helimagnets when skyrmions
are present. For MnSi, we show that the skyrmion lattices have a bumpy surface
configuration characterized by periodically arranged peaks with a
characteristic height of about 10$^{-13}$ m. The pattern of the peaks can be
controlled by varying the strength of the applied magnetic field. Moreover, we
prove that the surface configuration varies together with the motion and
deformation of the skyrmion lattices. As a result, the surface configuration
can be tuned by application of electric current, mechanical loads, as well as
any other form of external field which has an effect on the skyrmions. | cond-mat_mtrl-sci |
Spin splitting and strain in epitaxial monolayer WSe$_2$ on graphene: We present the electronic and structural properties of monolayer WSe$_{2}$
grown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin
splitting in the WSe$_{2}$ valence band at $\overline{\mathrm{K}}$ was
$\Delta_\mathrm{SO}=0.469\pm0.008$ eV by angle-resolved photoemission
spectroscopy (ARPES). Synchrotron-based grazing-incidence in-plane X-ray
diffraction (XRD) revealed the in-plane lattice constant of monolayer WSe$_{2}$
to be $a_\mathrm{WSe_2}=3.2757\pm0.0008 \mathrm{\r{A}}$. This indicates a
lattice compression of -0.19 % from bulk WSe$_{2}$. By using experimentally
determined graphene lattice constant ($a_\mathrm{MLG}=2.4575\pm0.0007
\mathrm{\r{A}}$), we found that a 3$\times$3 unit cell of the slightly
compressed WSe$_{2}$ is perfectly commensurate with a 4$\times$4 graphene
lattice with a mismatch below 0.03 %, which could explain why the monolayer
WSe$_{2}$ is compressed on MLG. From XRD and first-principles calculations,
however, we conclude that the observed size of strain is negligibly small to
account for a discrepancy in $\Delta_\mathrm{SO}$ found between exfoliated and
epitaxial monolayers in earlier ARPES. In addition, angle-resolved, ultraviolet
and X-ray photoelectron spectroscopy shed light on the band alignment between
WSe$_{2}$ and MLG/SiC and indicate electron transfer from graphene to the
WSe$_{2}$ monolayer. As further revealed by atomic force microscopy, the
WSe$_{2}$ island size depends on the number of carbon layers on top of the SiC
substrate. This suggests that the epitaxy of WSe$_{2}$ favors the weak van der
Waals interactions with graphene while it is perturbed by the influence of the
SiC substrate and its carbon buffer layer. | cond-mat_mtrl-sci |
Emergence of Type-I and Type-II Dirac line nodes in penta-octa-graphene: Carbon allotropes have a large family of materials with varieties of crystal
structures and properties and can realize different topological phases. Using
first principles calculations, we predict a new two-dimensional (2D) carbon
allotrope, namely penta-octa-graphene, which consists of pentagonal and
octagonal carbon rings. We find that penta-octa-graphene can host both type-I
and type-II Dirac line nodes (DLNs). The band inversion between conduction and
valence bands forms the type-I DLNs and the two highest valence bands form the
type-II DLNs. We find that the type-I DLNs are robust to the biaxial strain and
the type-II DLNs can be driven to type-I when applying over 3 $\%$ biaxial
stretching strain. A lattice model based on the $\pi$ orbitals of carbons is
derived to understand the coexistence mechanism of type-I and type-II DLNs in
penta-octa-graphene. Possible realizations and characterizations of this
penta-octa-graphene in the experiment are also discussed. Our findings shed new
light on the study of the coexistence of multiple topological states in the 2D
carbon allotropes. | cond-mat_mtrl-sci |
Thickness of the air-water interface from first-principles
simulation-based hydrogen bond dynamics: The thickness of the air-water interface is determined by interface hydrogen
bond (HB) dynamics. By density functional theory-based molecular dynamics
(DFTMD) simulations, two extreme cases of the interface HB dynamics are
obtained: one underestimates the HB breaking rate constant and the other
overestimates it. The interface HB dynamics in these two cases tends to be the
same as the thickness of the air-water interface increases to 4 Angstroms. The
interface thickness is determined when the interface HB dynamics under the two
cases is converged. | cond-mat_mtrl-sci |
Chemical trends of substitutional transition metal dopants in diamond:
an ab initio study: The electronic and magnetic properties of neutral substitutional
transition-metal dopants in dia- mond are calculated within density functional
theory using the generalized gradient approximation to the exchange-correlation
potential. Ti and Fe are nonmagnetic, whereas the ground state of V, Cr and Mn
are magnetic with a spin entirely localized on the magnetic ion. For Co, Ni,
and Cu, the ground state is magnetic with the spin distributed over the
transition-metal ion and the nearest-neighbor carbon atoms; furthermore a bound
state is found in the gap that originates from the hybridization of the
3d-derived level of the dopant and the 2p-derived dangling bonds of the
nearest-neighbor carbons. A p{d hybridization model is developed in order to
describe the origin of the magnetic interaction. This model predicts high-spin
to low-spin transitions for Ni and Cu under compressive strain. | cond-mat_mtrl-sci |
Brillouin zone spin filtering mechanism of enhanced TMR and correlation
effects in Co(0001)/h-BN/Co(0001) magnetic tunnel junction: The 'Brillouin zone spin filtering' mechanism of enhanced tunneling
magnetoresistance (TMR) is described for magnetic tunnel junctions (MTJ) and
studied on an example of the MTJ with hcp Co electrodes and hexagonal BN (h-BN)
spacer. Our calculations based on local density approximation of density
functional theory (LDA-DFT) for Co(0001)/h-BN/Co(0001) MTJ predict high TMR in
this device due to Brillouin zone filtering mechanism. Owning to the specific
complex band structure of the h-BN the spin-dependent tunneling conductance of
the system is ultra-sensitive to small variations of the Fermi energy position
inside the BN band gap. Doping of the BN and, consequentially, changing the
Fermi energy position could lead to variation of the TMR by several orders of
magnitude. We show also that taking into account correlation effects on beyond
DFT level is required to accurately describe position of the Fermi level and
thus transport propertied of the system. Our study suggests that new MTJ based
on hcp Co-Pt or Co-Pd disordered alloy electrodes and p-doped hexagonal BN
spacer is a promising candidate for the spin-transfer torque magnetoresistive
random-access memory (STT-MRAM). | cond-mat_mtrl-sci |
Effect of Layer-Stacking on the Electronic Structure of Graphene
Nanoribbons: The evolution of electronic structure of graphene nanoribbons (GNRs) as a
function of the number of layers stacked together is investigated using
\textit{ab initio} density functional theory (DFT) including interlayer van der
Waals interactions. Multilayer armchair GNRs (AGNRs), similar to single-layer
AGNRs, exhibit three classes of band gaps depending on their width. In zigzag
GNRs (ZGNRs), the geometry relaxation resulting from interlayer interactions
plays a crucial role in determining the magnetic polarization and the band
structure. The antiferromagnetic (AF) interlayer coupling is more stable
compared to the ferromagnetic (FM) interlayer coupling. ZGNRs with the AF
in-layer and AF interlayer coupling have a finite band gap while ZGNRs with the
FM in-layer and AF interlayer coupling do not have a band gap. The ground state
of the bi-layer ZGNR is non-magnetic with a small but finite band gap. The
magnetic ordering is less stable in multilayer ZGNRs compared to single-layer
ZGNRs. The quasipartcle GW corrections are smaller for bilayer GNRs compared to
single-layer GNRs because of the reduced Coulomb effects in bilayer GNRs
compared to single-layer GNRs. | cond-mat_mtrl-sci |
Raman thresholds and rigid to floppy transitions in calcium silicate
glasses: Alkaline earth silicate glasses $xCaO-(1-x)SiO_2$ exhibit a well marked
threshold in Raman lineshapes which can be related to the onset of network
rigidity as the concentration of calcium oxide $x$ is decreased. The present
results are analyzed by constraint counting algorithms and more deeply
characterized by a size increasing cluster approximation that allows to perform
Maxwell mechanical constraint counting beyond the usual mean-field treatment.
This permits to discuss under which structural conditions an elastic
intermediate phase can be obtained. | cond-mat_mtrl-sci |
Theoretical assessment on the possibility of constraining point defect
energetics by pseudo-phase transition pressures: Making use of the energetics and equations of state of defective uranium
dioxide that calculated with first-principles method, we demonstrate a
possibility of constraining the formation energy of point defects by measuring
the transition pressures of the corresponding pseudo-phase of defects. The
mechanically stable range of fluorite structure of UO2, which dictates the
maximum possible pressure of relevant pseudo-phase transitions, gives rise to
defect formation energies that span a wide band and overlap with the existing
experimental estimates. We reveal that the knowledge about pseudo-phase
boundaries can not only provide important information of energetics that is
helpful for reducing the scattering in current estimates, but also be valuable
for guiding theoretical assessments, even to validate or disprove a theory. In
order to take defect interactions into account and to extrapolate the physical
quantities at finite stoichiometry deviations to that near the stoichiometry,
we develop a general formalism to describe the thermodynamics of a defective
system. We also show that it is possible to include interactions among defects
in a simple expression of point defect model (PDM) by introducing an auxiliary
constant mean-field. This generalization of the simple PDM leads to great
versatility that allows one to study nonlinear effects of stoichiometry
deviation on materials' behavior. It is a powerful tool to extract the defect
energetics from finite defect concentrations to the dilute limit. Besides
these, the full content of the theoretical formalism and some relevant and
interesting issues, including reentrant pseudo-transition, multi-defect
coexistence, charged defects, and possible consequence of instantaneous
defective response in a quantum crystal, are explored and discussed. | cond-mat_mtrl-sci |
One million percent tunnel magnetoresistance in a magnetic van der Waals
heterostructure: We report the observation of a very large negative magnetoresistance effect
in a van der Waals tunnel junction incorporating a thin magnetic semiconductor,
CrI3, as the active layer. At constant voltage bias, current increases by
nearly one million percent upon application of a 2 Tesla field. The effect
arises from a change between antiparallel to parallel alignment of spins across
the different CrI3 layers. Our results elucidate the nature of the magnetic
state in ultrathin CrI3 and present new opportunities for spintronics based on
two-dimensional materials. | cond-mat_mtrl-sci |
Van der Waals Stacking Induced Topological Phase Transition in Layered
Ternary Transition Metal Chalcogenides: Novel materials with nontrivial electronic and photonic band topology are
crucial for realizing novel devices with low power consumption and heat
dissipation, and quantum computing free of decoherence. Here using
first-principles approach, we predict a class of ternary transition metal
chalcogenides (TTMC) MM'Te$_4$ exhibits dual topological characteristics:
quantum spin Hall (QSH) insulators in their 2D monolayers and topological Weyl
semimetals in their 3D noncentrosymmetric crystals upon van der Waals (vdW)
stacking. Remarkably, we find that one can create and annihilate Weyl fermions,
and realize the transition between Type-I and Type-II Weyl fermions by tuning
vdW interlayer spacing. Our calculations show that they possess excellent
thermodynamic stability and weak interlayer binding, implying their great
potentials for experimental synthesis, direct exfoliation and vdW
heterostacking. Moreover, their ternary nature will offer more tunability for
electronic structure by controlling different stoichiometry and valence
charges. Our findings provide an ideal materials platform for realizing QSH
effect and exploring topological phase transition, and will open up a variety
of new opportunities for two-dimensional materials and topological materials
research. | cond-mat_mtrl-sci |
Unconventional Fermi surface spin textures in the Bi_xPb_{1-x}/Ag(111)
surface alloy: The Fermi and Rashba energies of surface states in the Bi_xPb_{1-x}/Ag(111)
alloy can be tuned simultaneously by changing the composition parameter x. We
report on unconventional Fermi surface spin textures observed by spin and
angle-resolved photoemission spectroscopy {that are correlated with a
topological transition of the Fermi surface occurring at x=0.5. We show that
the surface states remain fully spin polarized upon alloying and that the spin
polarization vectors are approximately tangential to the constant energy
contours. We discuss the implications of the topological transition for the
transport of spin. | cond-mat_mtrl-sci |
Low In solubility and band offsets in the small-$x$
$β$-Ga$_2$O$_3$/(Ga$_{1-x}$In$_x$)$_2$O$_3$ system: Based on first-principles calculations, we show that the maximum reachable
concentration $x$ in the (Ga$_{1-x}$In$_x$)$_2$O$_3$ alloy in the low-$x$
regime (i.e. In solubility in $\beta$-Ga$_2$O$_3$) is around 10%. We then
calculate the band alignment at the (100) interface between $\beta$-Ga$_2$O$_3$
and (Ga$_{1-x}$In$_x$)$_2$O$_3$ at 12%, the nearest computationally treatable
concentration. The alignment is strongly strain-dependent: it is of type-B
staggered when the alloy is epitaxial on Ga$_2$O$_3$, and type-A straddling in
a free-standing superlattice. Our results suggest a limited range of
applicability of low-In-content GaInO alloys. | cond-mat_mtrl-sci |
Spin-wave stiffness and micromagnetic exchange interactions expressed by
means of the KKR Green function approach: We represent an approach to calculate micromagnetic model parameters such as
the tensor of exchange stiffness, Dzyaloshinskii-Moriya interaction (DMI) as
well as spin-wave stiffness. The scheme is based on the fully relativistic
Korringa-Kohn-Rostoker Green function (KKR-GF) technique and can be seen as a
relativistic extension of the work of Lichtenstein {\em et al.} The expression
for $D^{z\alpha}$ elements of DMI differ from the expressions for $D^{x\alpha}$
and $D^{y\alpha}$ elements as the former are derived via second-order
perturbation term of the energy caused by spin-spiral while the latter are
associated with the first-order term. Corresponding numerical results are
compared with those obtained using other schemes reported in the literature. | cond-mat_mtrl-sci |
DC Resistance Degradation of SrTiO$_3$: The Role of Virtual-Cathode
Needles and Oxygen Bubbles: This study of highly accelerated lifetime tests of SrTiO$_3$, a model
semiconducting oxide, is motivated by the interest in reliable multilayer
ceramic capacitors and resistance-switching thin-film devices. Our analytical
solution to oxygen-vacancy migration under a DC voltage -- the cause of
resistance degradation in SrTiO$_3$ -- agrees with previous numerical
solutions. However, all solutions fail to explain why degradation kinetics
feature a very strong voltage dependence, which we attribute to the nucleation
and growth of cathode-initiated fast-conducting needles. While they have no
color contrast in SrTiO$_3$ single crystals and are nominally invisible,
needles presence in DC-degraded samples -- in silicone oil and in air -- was
unambiguously revealed by in-situ hot-stage photography. Observations in
silicone oil and thermodynamic considerations of voltage boundary conditions
further revealed a cooccurrence of copious oxygen bubbling and the onset of
final accelerating degradation, suggesting sudden oxygen loss is a precursor of
final failure. Remarkably, both undoped and Fe-doped SrTiO$_3$ can emit
electroluminescence at higher current densities, thus providing a vivid
indicator of resistance degradation and a metal-to-insulator resistance
transition during cooling. The implications of these findings to thin ceramic
and thin film SrTiO$_3$ devices are discussed, along with connections to
similar findings in likewise degraded fast-ion yttria-stabilized zirconia. | cond-mat_mtrl-sci |
Insights into the structural symmetry of single-crystal YCrO$_3$ from
synchrotron X-ray diffraction: We report on the crystallographic information such as lattice parameters,
atomic positions, bond lengths and angles, and local crystalline distortion
size and mode of single-crystal YCrO$_3$ compound by a high-resolution
synchrotron X-ray diffraction study. The data was collected at 120 K (below
$T_\textrm{N} \sim$ 141.5 K), 300 K (within [$T_\textrm{N}$, $T_\textrm{C}$]),
and 500 K (above $T_\textrm{C} \sim$ 473 K). Taking advantages of high
intensity and brilliance of synchrotron X-rays, we are able to refine collected
patterns with the noncentrosymmetric monoclinic structural model ($P12_11$, No.
4) that was proposed previously but detailed structural parameters have not
determined yet. Meanwhile, we calculated patterns with the centrosymmetric
orthorhombic space group (\emph{Pmnb}, No. 62) for a controlled study. Lattice
constants \emph{a}, \emph{b}, and \emph{c} as well as unit-cell volume almost
increase linearly upon warming. We observed more dispersive distributions of
bond length and angle and local distortion strength with the $P12_11$ space
group. This indicates that (i) The local distortion mode of Cr2O$_6$ at 120 K
correlates the formation of the canted antiferromagnetic order by Cr1-Cr2 spin
interactions mainly through intermediate of O3 and O4 ions. (ii) The
strain-balanced Cr1-O3(O4) and Cr2-O5(O5) bonds as well as the local distortion
modes of Cr1O$_6$ and Cr2O$_6$ octohedra at 300 K may be a microscopic origin
of the previously-reported dielectric anomaly. Our study demonstrates that
local crystalline distortion is a key factor for the formation of ferroelectric
order and provides a complete set of crystallography for a full understanding
of the interesting magnetic and quasi-ferroelectric properties of YCrO$_3$
compound. | cond-mat_mtrl-sci |
Prediction of Chlorine and Fluorine Crystal Structures at High Pressure
Using Symmetry Driven Structure Search with Geometric Constraints: The high-pressure properties of fluorine and chlorine are not yet well
understood because both are highly reactive and volatile elements, which has
made conducting diamond anvil cell and x-ray diffraction experiments a
challenge. Here we use ab initio methods to search for stable crystal
structures of both elements at megabar pressures. We demonstrate how symmetry
and geometric constraints can be combined to efficiently generate crystal
structures that are composed of diatomic molecules. Our algorithm extends the
symmetry driven structure search method [Phys. Rev. B 98 (2018) 174107] by
adding constraints for the bond length and the number of atoms in a molecule,
while still maintaining generality. As a method of validation, we have tested
our approach for dense hydrogen and reproduced the known molecular structures
of Cmca-12 and Cmca-4. We apply our algorithm to study chlorine and fluorine in
the pressure range from 10--4000 GPa while considering crystal structures with
up to 40 atoms per unit cell. We predict chlorine to follow the same series of
phase transformations as elemental iodine from Cmca to Immm to Fm$\bar{3}$m,
but at substantially higher pressures. We predict fluorine to transition from a
C2/c to an Cmca structure at 70 GPa, to a novel orthorhombic and metallic
structure with P$4_2$/mmc symmetry at 2500 GPa, and finally into its cubic
analogue form with Pm$\bar{3}$n symmetry at 3000 GPa. | cond-mat_mtrl-sci |
Combining experiments on luminescent centres in hexagonal boron nitride
with the polaron model and ab initio methods towards the identification of
their microscopic origin: The two-dimensional material hexagonal boron nitride (hBN) hosts luminescent
centres with emission energies of 2 eV which exhibit pronounced phonon
sidebands. We investigate the microscopic origin of these luminescent centres
by combining ab initio calculations with non-perturbative open quantum system
theory to study the emission and absorption properties of 26 defect
transitions. Comparing the calculated line shapes with experiments we narrow
down the microscopic origin to three carbon-based defects: $\mathrm{C_2C_B}$,
$\mathrm{C_2C_N}$, and $\mathrm{V_NC_B}$. The theoretical method developed
enables us to calculate so-called photoluminescence excitation (PLE) maps,
which show excellent agreement with our experiments. The latter resolves
higher-order phonon transitions, thereby confirming both the vibronic structure
of the optical transition and the phonon-assisted excitation mechanism with a
phonon energy 170 meV. We believe that the presented experiments and
polaron-based method accurately describe luminescent centres in hBN and will
help to identify their microscopic origin. | cond-mat_mtrl-sci |
High-order harmonic generation in solid $\rm \bf C_{60}$: High harmonic generation (HHG) has unleashed the power of strong laser
physics in solids. Here we investigate HHG from a large system, solid C$_{60}$,
with 240 valence electrons engaging harmonic generation at each crystal
momentum, the first of this kind. We employ the density functional theory and
the time-dependent Liouville equation of the density matrix to compute HHG
signals. We find that under a moderately strong laser pulse, HHG signals reach
15th order, consistent with the experimental results from C$_{60}$ plasma. The
helicity dependence in solid C$_{60}$ is weak, due to the high symmetry. In
contrast to the general belief, HHG is unsuitable for band structure mapping in
C$_{60}$. However, we find a window of opportunity using a long wavelength,
where harmonics are generated through multiple-photon excitation. In
particular, the 5th order harmonic energies closely follow the transition
energy dispersion between the valence and conduction bands. This finding is
expected to motivate future experimental investigations. | cond-mat_mtrl-sci |
Excitonic Photoluminescence properties of nanocrystalline GaSb and
Ga0.62In0.38Sb embedded in silica films: The GaSb and Ga0.62In0.38Sb nanocrystals were embedded in the SiO2 films by
radio-frequency magnetron co-sputtering and were grown on GaSb and Si
substrates at different temperatures. We present results on the 10K excitonic
photoluminescence (PL) properties of nanocrystalline GaSb and Ga0.62In0.38Sb as
a function of their size. The measurements show that the PL of the GaSb and
Ga0.62In0.38Sb nanocrystallites follows the quantum confinement model very
closely. By using deconvolution of PL spectra, origins of structures in
photoluminescence were identified. | cond-mat_mtrl-sci |
Self-assembly of Nanometer-scale Magnetic Dots with Narrow Size
Distributions on an Insulating Substrate: The self-assembly of iron dots on the insulating surface of NaCl(001) is
investigated experimentally and theoretically. Under proper growth conditions,
nanometer-scale magnetic iron dots with remarkably narrow size distributions
can be achieved in the absence of a wetting layer Furthermore, both the
vertical and lateral sizes of the dots can be tuned with the iron dosage
without introducing apparent size broadening, even though the clustering is
clearly in the strong coarsening regime. These observations are interpreted
using a phenomenological mean-field theory, in which a coverage-dependent
optimal dot size is selected by strain-mediated dot-dot interactions. | cond-mat_mtrl-sci |
Electronic structure and optical properties of lightweight metal
hydrides: We study the electronic structures and dielectric functions of the simple
hydrides LiH, NaH, MgH2 and AlH3, and the complex hydrides Li3AlH6, Na3AlH6,
LiAlH4, NaAlH4 and Mg(AlH4)2, using first principles density functional theory
and GW calculations. All these compounds are large gap insulators with GW
single particle band gaps varying from 3.5 eV in AlH3 to 6.5 eV in the MAlH4
compounds. The valence bands are dominated by the hydrogen atoms, whereas the
conduction bands have mixed contributions from the hydrogens and the metal
cations. The electronic structure of the aluminium compounds is determined
mainly by aluminium hydride complexes and their mutual interactions. Despite
considerable differences between the band structures and the band gaps of the
various compounds, their optical responses are qualitatively similar. In most
of the spectra the optical absorption rises sharply above 6 eV and has a strong
peak around 8 eV. The quantitative differences in the optical spectra are
interpreted in terms of the structure and the electronic structure of the
compounds. | cond-mat_mtrl-sci |
3D versus 2D domain wall interaction in ideal and rough nanowires: The interaction between transverse magnetic domain walls (TDWs) in planar
(2D) and cylindrical (3D) nanowires is examined using micromagnetic
simulations. We show that in perfect and surface deformed wires the free TDWs
behave differently, as the 3D TDWs combine into metastable states with average
lifetimes of 300ns depending on roughness, while the 2D TDWs do not due to 2D
shape anisotropy. When the 2D and 3D TDWs are pinned at artificial
constrictions, they behave similarly as they interact mainly through the
dipolar field. This magnetostatic interaction is well described by the point
charge model with multipole expansion. In surface deformed wires with
artificial constrictions, the interaction becomes more complex as the depinning
field decreases and dynamical pinning can lead to local resonances. This can
strongly influence the control of TDWs in DW-based devices. | cond-mat_mtrl-sci |
Effective density of states map of undoped microcrystalline Si films: a
combined experimental and numerical simulation approach: The phototransport properties of plasma deposited highly crystalline undoped
hydrogenated microcrystalline silicon films were studied by measuring the
steady state photoconductivity (SSPC) as a function of temperature and light
intensity. The films possessing different thicknesses and microstructures had
been well characterized by various microstructural probes. Microcrystalline Si
films possessing dissimilar microstructural attributes were found to exhibit
different phototransport behaviors. We have employed numerical modeling of SSPC
to corroborate and further elucidate the experimental results. Our study
indicates that the different phototransport behaviors are linked to different
features of the proposed density of states maps of the material which are
different for microcrystalline Si films having different types of
microstructure. | cond-mat_mtrl-sci |
Domains and ferroelectric switching pathways in Ca$_3$Ti$_2$O$_7$ from
first principles: Hybrid improper ferroelectricity, where an electrical polarization can be
induced via a trilinear coupling to two non-polar structural distortions of
different symmetry, has recently been experimentally demonstrated for the first
time in the $n$=2 Ruddlesden-Popper compound Ca$_3$Ti$_2$O$_7$. In this paper
we use group theoretic methods and first-principles calculations to identify
possible ferroelectric switching pathways in Ca$_3$Ti$_2$O$_7$. We identify
low-energy paths that reverse the polarization direction by switching via an
orthorhombic twin domain, or via an antipolar structure. We also introduce a
chemically intuitive set of local order parameters to give insight into how
these paths are relevant to switching nucleated at domain walls. Our findings
suggest that switching may proceed via more than one mechanism in this
material. | cond-mat_mtrl-sci |
Can CF(3)-functionalized La@C(60) be isolated experimentally and become
superconducting?: Superconducting behavior even under harsh ambient conditions is expected to
occur in La@C(60) if it could be isolated from the primary metallofullerene
soot when functionalized by CF(3) radicals. We use ab initio density functional
theory calculations to compare the stability and electronic structure of C(60)
and the La@C(60) endohedral metallofullerene to their counterparts
functionalized by CF(3). We found that CF(3) radicals favor binding to C(60)
and La@C(60), and have identified the most stable isomers. Structures with an
even number m of radicals are energetically preferred for C(60) and structures
with odd m for La@C(60) due to the extra charge on the fullerene. This is
consistent with a wide HOMO-LUMO gap in La@C(60)(CF(3))(m) with odd m, causing
extra stabilization in the closed-shell electronic configuration. CF(3)
radicals are both stabilizing agents and molecular separators in a metallic
crystal, which could increase the critical temperature for superconductivity. | cond-mat_mtrl-sci |
Temperature Dependence of the Energy Levels of Methylammonium Lead
Iodide Perovskite from First Principles: Environmental effects and intrinsic energy-loss processes lead to
fluctuations in the operational temperature of solar cells, which can
profoundly influence their power conversion efficiency. Here we determine from
first principles the effects of temperature on the band gap and band edges of
the hybrid pervoskite CH$_3$NH$_3$PbI$_3$ by accounting for electron-phonon
coupling and thermal expansion. From $290$ to $380$ K, the computed band gap
change of $40$ meV coincides with the experimental change of $30$-$40$ meV. The
calculation of electron-phonon coupling in CH$_3$NH$_3$PbI$_3$ is particularly
intricate, as the commonly used Allen-Heine-Cardona theory overestimates the
band gap change with temperature, and excellent agreement with experiment is
only obtained when including high-order terms in the electron-phonon
interaction. We also find that spin-orbit coupling enhances the electron-phonon
coupling strength, but that the inclusion of nonlocal correlations using hybrid
functionals has little effect. We reach similar conclusions in the metal-halide
perovskite CsPbI$_3$. Our results unambiguously confirm for the first time the
importance of high-order terms in the electron-phonon coupling by direct
comparison with experiment. | cond-mat_mtrl-sci |
Electronic structure and enhanced visible light absorption of N,
B-codoped TiO2: We present the GGA+U calculations to investigate the electronic structure and
visible light absorption of the N, B-codoped anatase TiO2. The NsBi
(substitutional N, interstitial B) codoped TiO2 produces significant Ti 3d and
N 2p mid-gap states when the distance of N and B atoms is far, and the NiBi
(interstitial N and B) and NsBs (substitutional N and B) codoped TiO2 prefer to
form localized p states at 0.3-1.2 eV above the valence band maximum. Further,
the optical band edges of the three codoped systems shift slightly to the
visible region, but only the far distance NsBi codoped TiO2 shows an obvious
visible optical transition. These results indicate that the NsBi codoped TiO2
has a dominant contribution to the visible absorption of the N, B-codoped TiO2. | cond-mat_mtrl-sci |
Human-in-the-loop: The future of Machine Learning in Automated Electron
Microscopy: Machine learning methods are progressively gaining acceptance in the electron
microscopy community for de-noising, semantic segmentation, and dimensionality
reduction of data post-acquisition. The introduction of the APIs by major
instrument manufacturers now allows the deployment of ML workflows in
microscopes, not only for data analytics but also for real-time decision-making
and feedback for microscope operation. However, the number of use cases for
real-time ML remains remarkably small. Here, we discuss some considerations in
designing ML-based active experiments and pose that the likely strategy for the
next several years will be human-in-the-loop automated experiments (hAE). In
this paradigm, the ML learning agent directly controls beam position and image
and spectroscopy acquisition functions, and human operator monitors experiment
progression in real- and feature space of the system and tunes the policies of
the ML agent to steer the experiment towards specific objectives. | cond-mat_mtrl-sci |
Is Cement a Glassy Material?: The nature of Calcium--Silicate--Hydrate (C--S--H), the binding phase of
cement, remains a controversial question. In particular, contrary to the former
crystalline model, it was recently proposed that its nanoscale structure was
actually amorphous. To elucidate this issue, we analyzed the structure of a
realistic simulation of C--S--H, and compared the latter to crystalline
tobermorite, a natural analogue to cement, and to an artificial ideal glass.
Results clearly support that C--S--H is amorphous. However, its structure shows
an intermediate degree of order, retaining some characteristics of the crystal
while acquiring an overall glass-like disorder. Thanks to a detailed
quantification of order and disorder, we show that its amorphous state mainly
arises from its hydration. | cond-mat_mtrl-sci |
Computationally-driven, high throughput identification of CaTe and
Li$_\textrm{3}$Sb as promising candidates for high mobility $p$-type
transparent conducting materials: High-performance $p$-type transparent conducting materials (TCMs) must
exhibit a rare combination of properties including high mobility, transparency
and $p$-type dopability. The development of high-mobility/conductivity $p$-type
TCMs is necessary for many applications such as solar cells, or transparent
electronic devices. Oxides have been traditionally considered as the most
promising chemical space to dig out novel $p$-type TCMs. However, non-oxides
might perform better than traditional $p$-type TCMs (oxides) in terms of
mobility. We report on a high-throughput (HT) computational search for
non-oxide $p$-type TCMs from a large dataset of more than 30,000 compounds
which identified CaTe and Li$_\textrm{3}$Sb as very good candidates for
high-mobility $p$-type TCMs. From our calculations, both compounds are expected
to be $p$-type dopable: intrinsically for Li$_\textrm{3}$Sb while CaTe would
require extrinsic doping. Using electron-phonon computations, we estimate hole
mobilities at room-temperature to be about 20 and 70 cm$^2$/Vs for CaTe and
Li$_\textrm{3}$Sb, respectively. The computed hole mobility for
Li$_\textrm{3}$Sb is quite exceptional and comparable with the electron
mobility in the best $n$-type TCMs. | cond-mat_mtrl-sci |
Preparation of poly(sodium acrylate-co-acrylamide) superabsorbent
copolymer via alkaline hydrolysis of acrylamide using microwave irradiation: In this paper we present a new one-pot synthesis method of
poly(acrylate-co-acrylamide) superabsorbent polymer via partial alkaline
hydrolysis of acrylamide using microwave irradiation. This method allows to
hydrolysis, polymerization and gelation to take place in one pot during a very
short reaction time (90 s), and with no need to operate under inert atmosphere.
The degree of hydrolysis of the gel was determined by a back titration method.
The gel is compact and has a water absorbency of 1031 g/g while the
corresponding copolymer prepared from polymerization of sodium acrylate and
acrylamide, using microwave irradiation and under the same experimental
conditions, has a water absorbency of only 658g/g. This difference in water
absorbency is discussed. FTIR spectroscopy was used to verify the hydrolysis
and the formation of sodium acrylate. Scanning electron microscopy (SEM) showed
that the synthesized hydrogel has a macroporous structure. The influence of the
environmental parameters on water absorbency such as the pH and the ionic force
was also investigated. | cond-mat_mtrl-sci |
Electronic and optical properties of the fully and partially inverse
CoFe$_{2}$O$_{4}$ spinel from first principles calculations including
many-body effects: Using density functional theory (DFT) calculations and state-of-the-art
many-body perturbation theory, we investigate the electronic and optical
properties of the inverse spinel CoFe$_{2}$O$_{4}$, a common anode material for
photocatalytic water splitting. Starting with different exchange-correlation
functionals, at the independent particle level we obtain a direct band gap of
1.38~eV (PBE+$U$) and 1.69 eV (SCAN+$U$), whereas HSE06 renders an indirect
band gap of 2.02~eV. Including quasiparticle effects within $G_{0}W_{0}$, a
larger and indirect band gap is obtained for all functionals: 1.78~eV
(PBE+$U$), 1.95~eV (SCAN+$U$) and 2.17~eV (HSE06), higher than the independent
particle (IP) band gap. Excitonic effects, taken into account by solving the
Bethe-Salpeter equation (BSE) lead to a redshift of the optical band gap to
1.50 (SCAN+$U$) and 1.61~eV (HSE06), in good agreement with the reported
experimental values. The lowest optical transitions in the visible range,
identified by means of oscillator strength, are at 2.0, 3.5, and 5.0~eV,
consistent with experimental observations. We also explored the effect of the
degree of inversion: the band gap is found to decrease from 1.69 ($x=1$) to
1.45 ($x=0.5$), and 1.19~eV ($x=0)$ within the IP approximation with SCAN+$U$.
This trend is reversed after the inclusion of excitonic effects, resulting in a
band gap of 1.50, 1.57, and 1.64~eV for $x$ = 1.0, 0.5, and 0.0, respectively.
The oscillator strength analysis of the BSE calculations indicates that both
$x$ = 0.0 and $x$ = 0.5 exhibit transitions below 1~eV with extremely small
oscillator strengths that are absent in the inverse spinel. This corroborates
previous suggestions that these transitions are due to the presence of
Co$^{2+}$ cations at the tetrahedral sites. | cond-mat_mtrl-sci |
Statistical Analysis of Contacts to Synthetic Monolayer MoS2: Two-dimensional (2D) semiconductors are promising candidates for scaled
transistors because they are immune to mobility degradation at the monolayer
limit. However, sub-10 nm scaling of 2D semiconductors, such as MoS2, is
limited by the contact resistance. In this work, we show for the first time a
statistical study of Au contacts to chemical vapor deposited monolayer MoS2
using transmission line model (TLM) structures, before and after dielectric
encapsulation. We report contact resistance values as low as 330 ohm-um, which
is the lowest value reported to date. We further study the effect of Al2O3
encapsulation on variability in contact resistance and other device metrics.
Finally, we note some deviations in the TLM model for short-channel devices in
the back-gated configuration and discuss possible modifications to improve the
model accuracy. | cond-mat_mtrl-sci |
Thermodynamic driving force in the formation of hexagonal-diamond Si and
Ge nanowires: The metastable hexagonal-diamond phase of Si and Ge (and of SiGe alloys)
displays superior optical properties with respect to the cubic-diamond one. The
latter is the most stable and popular one: growing hexagonal-diamond Si or Ge
without working at extreme conditions proved not to be trivial. Recently,
however, the possibility of growing hexagonal-diamond group-IV nanowires has
been demonstrated, attracting attention on such systems. Based on
first-principle calculations we show that the surface energy of the typical
facets exposed in Si and Ge nanowires is lower in the hexagonal-diamond phase
than in cubic ones. By exploiting a synergic approach based also on a recent
state-of-the-art interatomic potential and on a simple geometrical model, we
investigate the relative stability of nanowires in the two phases up to few
tens of nm in radius, highlighting the surface-related driving force and
discussing its relevance in recent experiments. We also explore the stability
of Si and Ge core-shell nanowires with hexagonal cores (made of GaP for Si
nanowires, of GaAs for Ge nanowires). In this case, the stability of the
hexagonal shell over the cubic one is also favored by the energy cost
associated with the interface linking the two phases. Interestingly, our
calculations indicate a critical radius of the hexagonal shell much lower than
the one reported in recent experiments, indicating the presence of a large
kinetic barrier allowing for the enlargement of the wire in a metastable phase. | cond-mat_mtrl-sci |
Extended Lagrangian formulation of time-reversible Born-Oppenheimer
molecular dynamics for higher-order symplectic integration: A Lagrangian generalization of time-reversible Born-Oppenheimer molecular
dynamics [Niklasson et al., Phys. Rev. Lett. vol. 97, 123001 (2006)] is
proposed. The Lagrangian includes extended electronic degrees of freedom as
auxiliary dynamical variables in addition to the nuclear coordinates and
momenta. While the nuclear degrees of freedom propagate on the Born-Oppenheimer
potential energy surface, the extended auxiliary electronic degrees of freedom
evolve as a harmonic oscillator centered around the adiabatic propagation of
the self-consistent ground state. The formulation enables the application of
higher-order symplectic or geometric integration schemes that are stable and
energy conserving even under incomplete self-consistency convergence. It is
demonstrated how the extended Born-Oppenheimer molecular dynamics improves the
accuracy by over an order of magnitude compared to previous formulations at the
same level of computational cost. | cond-mat_mtrl-sci |
Interfacial and Surface Magnetism in Epitaxial NiCo2O4(001)/MgAl2O4
Films: NiCo2O4 (NCO) films grown on MgAl2O4 (001) substrates have been studied using
magnetometry, x-ray magnetic circular dichroism (XMCD) based on x-ray
absorption spectroscopy, and spin-polarized inverse photoemission spectroscopy
(SPIPES) with various thickness down to 1.6 nm. The magnetic behavior can be
understood in terms of a layer of optimal NCO and an interfacial layer (1.2+/-
0.1 nm), with a small canting of magnetization at the surface. The thickness
dependence of the optimal layer can be described by the finite-scaling theory
with a critical exponent consistent with the high perpendicular magnetic
anisotropy. The interfacial layer couples antiferromagnetically to the optimal
layer, generating exchange-spring styled magnetic hysteresis in the thinnest
films. The non-optimal and measurement-speed-dependent magnetic properties of
the interfacial layer suggest substantial interfacial diffusion. | cond-mat_mtrl-sci |
The strain-induced transitions of the piezoelectric, pyroelectric and
electrocaloric properties of the CuInP$_2$S$_6$ films: The low-dimensional ferroelectrics, ferrielectrics and antiferroelectrics are
of urgent scientific interest due to their unusual polar, piezoelectric,
electrocaloric and pyroelectric properties. The strain engineering and strain
control of the ferroelectric properties of layered 2D Van der Waals materials,
such as CuInP$_2$(S,Se)$_6$ monolayers, thin films and nanoflakes, are of
fundamental interest and especially promising for their advanced applications
in nanoscale nonvolatile memories, energy conversion and storage, nano-coolers
and sensors. Here, we study the polar, piezoelectric, electrocaloric and
pyroelectric properties of thin strained films of a ferrielectric
CuInP$_2$S$_6$ covered by semiconducting electrodes and reveal an unusually
strong effect of a mismatch strain on these properties. In particular, the sign
of the mismatch strain and its magnitude determine the complicated behavior of
piezoelectric, electrocaloric and pyroelectric responses. The strain effect on
these properties is opposite, i.e., "anomalous", in comparison with many other
ferroelectric films, for which the out-of-plane remanent polarization,
piezoelectric, electrocaloric and pyroelectric responses increase strongly for
tensile strains and decrease or vanish for compressive strains. | cond-mat_mtrl-sci |
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