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Discovery of ferromagnetism with large magnetic anisotropy in ZrMnP and
HfMnP: ZrMnP and HfMnP single crystals are grown by a self-flux growth technique and
structural as well as temperature dependent magnetic and transport properties
are studied. Both compounds have an orthorhombic crystal structure. ZrMnP and
HfMnP are ferromagnetic with Curie temperatures around $370$~K and $320$~K
respectively. The spontaneous magnetizations of ZrMnP and HfMnP are determined
to be $1.9$~$\mu_\textrm{B}$/f.u. and $2.1$~$\mu_\textrm{B}$/f.u. respectively
at $50$~K. The magnetocaloric effect of ZrMnP in term of entropy change
($\Delta S$) is estimated to be $-6.7$ kJm$^{-3}$K$^{-1}$ around $369$~K. The
easy axis of magnetization is [100] for both compounds, with a small anisotropy
relative to the [010] axis. At $50$~K, the anisotropy field along the [001]
axis is $\sim4.6$~T for ZrMnP and $\sim10$~T for HfMnP. Such large magnetic
anisotropy is remarkable considering the absence of rare-earth elements in
these compounds. The first principle calculation correctly predicts the
magnetization and hard axis orientation for both compounds, and predicts the
experimental HfMnP anisotropy field within 25 percent. More importantly, our
calculations suggest that the large magnetic anisotropy comes primarily from
the Mn atoms suggesting that similarly large anisotropies may be found in other
3d transition metal compounds. | cond-mat_mtrl-sci |
Folding Energetics in Thin-Film Diaphragms: We perform experiments on thin-film diaphragms to show that the folding
patterns of anisotropically compressed diaphragms are strikingly different from
those of isotropically compressed ones. We then use a simple von Karman model
to relate the overall features of these folding patterns to the underlying
energetics. We show that the differences between the isotropic and anisotropic
cases can be traced back to fundamental changes in the energy structure of the
diaphragms. Finally, we point out that the energy structure of thin-film
diaphragms is similar to that of many other systems in physics and engineering,
into which our study may provide interesting insights. | cond-mat_mtrl-sci |
Phase Stability in 3d-5d (NiPt and CuAu) and 3d-4d (NiPd and CuAg)
Systems: We show the differences in the stability of 3d-5d (NiPt and CuAu) and 3d-4d
(NiPd and CuAg) alloys arise mainly due to relativistic corrections. The
magnetic properties of disordered NiPd and NiPt alloys also differ due to these
corrections which lead to increase in the separation between s-d bands of 5d
elements in these alloys. For the magnetic case we analyze the results in terms
of splitting of majority and minority spin d-band centers of the 3d elements.
We further examine the effect of relativistic corrections to the pair energies
and order disorder transition temperatures in these alloys. The magnetic
moments and Curie temperatures have also been studied along with the short
range ordering/segregation effects in NiPt/NiPd alloys. | cond-mat_mtrl-sci |
Thermoelectric power factor of Bi-Sb-Te and Bi-Te-Se alloys and doping
strategy: First-principles study: By performing first principles calculations combined with Boltzmann transport
equations, we calculate the thermoelectric power factor (PF) of Bi-Sb-Te and
Bi-Te-Se ternary alloys as a function of alloy composition ratio, carrier
concentration, and temperature. The point defect formation energy calculations
also perform to search potential n-type dopant candidates in ternaries. | cond-mat_mtrl-sci |
Strain engineering of two-dimensional piezo-photocatalytic materials for
hydrogen production: Low-dimensional transition metal dichalcogenides (TMDC) exhibit great
photocatalytic performance and tunability. In this work, using first-principles
simulations based on density functional theory (DFT), we demonstrate that
external electric bias can be employed to further improve the photocatalytic
hydrogen production efficiency of the six AB$_{2}$ (A=Mo, W and B=S, Se, Te)
TMDC monolayers by exploiting their piezoelectric response. In particular, when
subjected to a proper amount of electrically induced tensile biaxial strain,
most TMDC monolayers turn into potentially ideal photocatalyst towards the
hydrogen evolution reaction (HER). The beneficial effects of introducing
tensile biaxial strain on the TMDC monolayers are not limited to the reduction
of the band gap and proper adjustment of the band edge positions, but also to
the modification of the H adsorption free energy in such a way that the HER
reaction is noticeably favored. | cond-mat_mtrl-sci |
Room Temperature Magnetocaloric Effect in Ni-Mn-In: We have studied the effect of magnetic field on a non-stoichiometric Heusler
alloy Ni$_{50}$Mn$_{35}$In$_{15}$ that undergoes a martensitic as well as a
magnetic transition near room temperature. Temperature dependent magnetization
measurements demonstrate the influence of magnetic field on the structural
phase transition temperature. From the study of magnetization as a function of
applied field, we show the occurrence of inverse-magnetocaloric effect
associated with this magneto-structural transition. The magnetic entropy change
attains a value as high as 25 J/kg-K (at 5 T field) at room temperature as the
alloy transforms from the austenitic to martensitic phase with a concomitant
magnetic ordering. | cond-mat_mtrl-sci |
Strain-induced dynamic control over the population of quantum emitters
in two-dimensional materials: The discovery of quantum emitters in two-dimensional materials has triggered
a surge of research to assess their suitability for quantum photonics. While
their microscopic origin is still the subject of intense studies, ordered
arrays of quantum emitters are routinely fabricated using static
strain-gradients, which are used to drive excitons toward localized regions of
the 2D crystals where quantum-light-emission takes place. However, the
possibility of using strain in a dynamic fashion to control the appearance of
individual quantum emitters has never been explored so far. In this work, we
tackle this challenge by introducing a novel hybrid semiconductor-piezoelectric
device in which WSe2 monolayers are integrated onto piezoelectric pillars
delivering both static and dynamic strains. Static strains are first used to
induce the formation of quantum emitters, whose emission shows photon
anti-bunching. Their excitonic population and emission energy are then
reversibly controlled via the application of a voltage to the piezoelectric
pillar. Numerical simulations combined with drift-diffusion equations show that
these effects are due to a strain-induced modification of the
confining-potential landscape, which in turn leads to a net redistribution of
excitons among the different quantum emitters. Our work provides relevant
insights into the role of strain in the formation of quantum emitters in 2D
materials and suggests a method to switch them on and off on demand. | cond-mat_mtrl-sci |
Coercivity and random interfacial exchange coupling in CoPt/Co films: Hard-soft bilayers are analogous to prototype exchange-biased ferromagnetic
-antiferromagnetic systems as the minor loop of the soft layer is biased by the
hard and furthermore they offer bias layer tunability. In sputtered CoPt/Co
hard-soft bilayers we demonstrate that the exchange bias field shows a linear
dependence on the hard layer magnetization, while the coercivity shows a
quadratic dependence. Analysis of the minor hysteresis loop features supported
by Monte-Carlo simulations provide clear evidence that the coercivity of the
soft layer is mainly determined by the tunable randomness of the domain state
of the hard layer. | cond-mat_mtrl-sci |
Thermodynamics of point defects and diffusion mechanisms in B2-ordered
compounds: The point defect thermodynamics in a general family of binary compounds,
including B2 compounds as a specific representative, are classified by way of
two non-trivial energy parameters. The scheme is applied to published ab initio
defect formation energies, and the variety of resulting phenomena is
demonstrated. Further, by introducing model assumptions the consequences for
the active diffusion mechanisms are deduced. It is shown that particularly for
the off-stoichiometric case, the assumed prevalence of either the six-jump
cycle or the triple-defect mechanism has to be reconsidered, as a number of
qualitatively different mechanisms emerge as likely candidates for the dominant
effect. Two of those, the 4+2-jump cycles and the waltzing-step mechanism, are
introduced here. | cond-mat_mtrl-sci |
Tunable Optoelectronic Properties of Triply-Bonded Carbon Molecules with
Linear and Graphyne Substructures: In this paper we present a detailed computational study of the electronic
structure and optical properties of triply-bonded hydrocarbons with linear, and
graphyne substructures, with the aim of identifying their potential in
opto-electronic device applications. For the purpose, we employed a correlated
electron methodology based upon the Pariser-Parr-Pople model Hamiltonian,
coupled with the configuration interaction (CI) approach, and studied
structures containing up to 42 carbon atoms. Our calculations, based upon
large-scale CI expansions, reveal that the linear structures have intense
optical absorption at the HOMO-LUMO gap, while the graphyne ones have those at
higher energies. Thus, the opto-electronic properties depend on the topology of
the {graphyne substructures, suggesting that they can be tuned by means of
structural modifications. Our results are in very good agreement with the
available experimental data. | cond-mat_mtrl-sci |
Nano-scale oxygen octahedral tilting in
0.90(Bi1/2Na1/2)TiO3-0.05(Bi1/2K1/2)TiO3-0.05BaTiO3 lead-free perovskite
piezoelectric ceramics: The oxygen octahedral tilted domains in
0.90(Bi1/2Na1/2)TiO3-0.5(Bi1/2K1/2)TiO3-0.5BaTiO3 lead-free perovskite
piezoelectric ceramic have been studied by transmission electron microscopy
(TEM). Selected-area electron diffraction patterns shows the 1/2ooo and 1/2ooe
reflections, indicating the presence of antiphase (a-a-a-) and in-phase
(aoaoc+) octahedral tilting, respectively. The morphology and distributions of
these tilted domains are shown in the centered dark-field images. Further, the
Bragg-filtered high-resolution TEM image reveals that the size of the in-phase
tilted domains varies from 1 to 8 nm across. The ceramic contains the mixture
of non-tilted and variants of the antiphase and in-phase tilted domains. | cond-mat_mtrl-sci |
A Graphene-Carbon Nanotube Hybrid Material for Photovoltaic Applications: Large area graphene sheets grown by chemical vapor deposition can potentially
be employed as a transparent electrode in photovoltaics if their sheet
resistance can be significantly lowered, without any loss in transparency.
Here, we report the fabrication of a graphene-conducting-carbon-nanotube (CCNT)
hybrid material with a sheet resistance considerably lower than neat graphene,
and with the requisite small reduction in transparency. Graphene is deposited
on top of a a self-assembled CCNT monolayer which creates parallel conducting
paths on the graphene surface. The hybrid thereby circumvents electron
scattering due to defects in the graphene sheet, and reduces the sheet
resistance by a factor of two. The resistance can be further reduced by
chemically doping the hybrid. Moreover, the chemically doped hybrid is more
stable than a standalone chemically doped graphene sheet, as the CCNT network
enhances the dopant binding. In order to understand the results, we develop a
2D resistance network model in which we couple the CCNT layer to the graphene
sheet and demonstrate the model accounts quantitatively for the resistance
decrease. Our results show that a graphene-CCNT hybrid system has high
potential for use as a transparent electrode with high transparency and low
sheet resistance. | cond-mat_mtrl-sci |
Nonvolatile Electric-Field Control of Inversion Symmetry: In condensed-matter systems, competition between ground states at phase
boundaries can lead to significant changes in material properties under
external stimuli, particularly when these ground states have different crystal
symmetries. A key scientific and technological challenge is to stabilize and
control coexistence of symmetry-distinct phases with external stimuli. Using
BiFeO3 (BFO) layers confined between layers of the dielectric TbScO3 as a model
system, we stabilize the mixed-phase coexistence of centrosymmetric and
non-centrosymmetric BFO phases with antipolar, insulating and polar,
semiconducting behavior, respectively at room temperature. Application of
in-plane electric (polar) fields can both remove and introduce centrosymmetry
from the system resulting in reversible, nonvolatile interconversion between
the two phases. This interconversion between the centrosymmetric insulating and
non-centrosymmetric semiconducting phases coincides with simultaneous changes
in the non-linear optical response of over three orders of magnitude, a change
in resistivity of over five orders of magnitude, and a change in the polar
order. Our work establishes a materials platform allowing for novel
cross-functional devices which take advantage of changes in optical,
electrical, and ferroic responses. | cond-mat_mtrl-sci |
Bulk-sensitive photoemission spectroscopy of A_2FeMoO_6 double
perovskites (A=Sr, Ba): Electronic structures of Sr_2FeMoO_6 (SFMO) and Ba_2FeMoO_6 (BFMO) double
perovskites have been investigated using the Fe 2p->3d resonant photoemission
spectroscopy (PES) and the Cooper minimum in the Mo 4d photoionization cross
section. The states close to the Fermi level are found to have strongly mixed
Mo-Fe t_{2g} character, suggesting that the Fe valence is far from pure 3+. The
Fe 2p_{3/2} XAS spectra indicate the mixed-valent Fe^{3+}-Fe^{2+}
configurations, and the larger Fe^{2+} component for BFMO than for SFMO,
suggesting a kind of double exchange interaction. The valence-band PES spectra
reveal good agreement with the LSDA+U calculation. | cond-mat_mtrl-sci |
Thermal Conductivity and Phonon Transport in Suspended Few-Layer
Hexagonal Boron Nitride: The thermal conductivity of suspended few-layer hexagonal boron nitride
(h-BN) was measured using a micro-bridge device with built-in resistance
thermometers. Based on the measured thermal resistance values of 11-12 atomic
layer h-BN samples with suspended length ranging between 3 and 7.5 um, the
room-temperature thermal conductivity of a 11-layer sample was found to be
about 360 Wm-1K-1, approaching the basal plane value reported for bulk h-BN.
The presence of a polymer residue layer on the sample surface was found to
decrease the thermal conductivity of a 5-layer h-BN sample to be about 250
Wm-1K-1 at 300 K. Thermal conductivities for both the 5 layer and the 11 layer
samples are suppressed at low temperatures, suggesting increasing scattering of
low frequency phonons in thin h-BN samples by polymer residue. | cond-mat_mtrl-sci |
Comparison of exact-exchange calculations for solids in
current-spin-density- and spin-density-functional theory: The relative merits of current-spin-density- and spin-density-functional
theory are investigated for solids treated within the exact-exchange-only
approximation. Spin-orbit splittings and orbital magnetic moments are
determined at zero external magnetic field. We find that for magnetic (Fe, Co
and Ni) and non-magnetic (Si and Ge) solids, the exact-exchange
current-spin-density functional approach does not significantly improve the
accuracy of the corresponding spin-density functional results. | cond-mat_mtrl-sci |
The roles of adhesion, internal heat generation and elevated
temperatures in normally loaded, sliding rough surfaces: The thermal effects of plastic and frictional heat generation and elevated
temperature were examined along with the role of adhesion in the context of
galling wear, using a representative crystal plasticity, normally loaded,
sliding surface model. Galling frequency behaviour was predicted for 316L
steel. Deformation of the surfaces was dominated by the surface geometry, with
no significant effect due to variations in frictional models. Plastic and
frictional heating were found to have a minimal effect on the deformation of
the surface, with the rapid conduction of heat preventing any highly localised
heating. There was no corresponding effect on the predicted galling frequency
response.
Isothermal, elevated temperature conditions caused a decrease in galling
resistance, driven by the temperature sensitivity of the critical resolved
shear stress. The extent of deformation, as quantified by the area of
plastically deformed material and plastic reach, increased with temperature.
Comparisons were made with literature results for several surface amplitude and
wavelength conditions. Model results compared favourably with those in the
literature. However, the reduction in predicted galling resistance with
elevated temperature for a fixed surface was not as severe as observations in
the literature, suggesting other mechanisms (e.g. phase transformations,
surface coatings and oxides) are likely important. | cond-mat_mtrl-sci |
Multiscale Kinetic Monte-Carlo for Simulating Epitaxial Growth: We present a fast Monte-Carlo algorithm for simulating epitaxial surface
growth, based on the continuous-time Monte-Carlo algorithm of Bortz, Kalos and
Lebowitz. When simulating realistic growth regimes, much computational time is
consumed by the relatively fast dynamics of the adatoms. Continuum and
continuum-discrete hybrid methods have been developed to approach this issue;
however in many situations, the density of adatoms is too low to efficiently
and accurately simulate as a continuum. To solve the problem of fast adatom
dynamics, we allow adatoms to take larger steps, effectively reducing the
number of transitions required. We achieve nearly a factor of ten speed up, for
growth at moderate temperatures and large D/F. | cond-mat_mtrl-sci |
Undercooling growth and magnetic characterization of ferromagnetic shape
memory alloy Ni2FeGa single crystals: Ni2FeGa single crystals have been grown in undercooling conditions provided
by a glass-purification method. It has been found that trace amounts of gamma
phase embededin the single crystalline matrix preferentially orients in the
<100> orientation along the growth direction. This gamma phase generates
directional residual stress and results in an anisotropic two-way shape memory
effect. Large strains of -2.5% in the [001] and 1.5% in the [010] directions
have been observed. This trace gamma phase also improves the ductility of the
material and thus the crystals could be plastically deformed at room
temperature in the parent phase. The <110> and <111> orientations in Ni2FeGa
alloy were identified as the easy and hard magnetization directions,
respectively, in the parent phase by using low field M-T measurements. | cond-mat_mtrl-sci |
Effect of edge vacancies on localized states in semi-infinite zigzag
graphene sheet: The effect of vacancies on the robustness of zero-energy edge electronic
states in zigzag-type graphene layer is studied at different concentrations and
distributions of defects. All calculations are performed by using the Green's
function method and the tight-binding approximation. It is found that the
arrangement of defects plays a crucial role in the destruction of the edge
states. We have specified a critical distance between edge vacancies when their
mutual influence becomes significant and affects markedly the density of
electronic states at graphene edge. | cond-mat_mtrl-sci |
Ba(Zn,Co)2As2: a II-II-V Diluted Ferromagnetic Semiconductor with N-type
Carriers: Diluted ferromagnetic semiconductors (DMSs) that combine the properties of
semiconductors with ferromagnetism have potential application in spin-sensitive
electronics (spintronics) devices. The search for DMS materials exploded after
the observation of ferromagnetic ordering in III-V (Ga,Mn)As films. Recently, a
series of DMS compounds isostructural to iron-based superconductors have been
reported. Among them, the highest Curie temperature $T_C$ of 230 K has been
achieved in (Ba,K)(Zn,Mn)$_2$As$_2$. However, most DMSs, including (Ga,Mn)As,
are p-type, i.e., the carriers that mediate ferromagnetism are holes. For
practical applications, DMS with n-type carriers are also advantageous. Here we
report the successful synthesis of a II-II-V diluted ferromagnetic
semiconductor with n-type carriers, Ba(Zn,Co)$_2$As$_2$. Magnetization
measurements show that the ferromagnetic transition occurs up to $T_{C} \sim$
45 K. Hall effect and Seebeck effect measurements jointly confirm that the
dominant carriers are electrons. Through muon spin relaxation ($\mu$SR), a
volume sensitive magnetic probe, we have also confirmed that the ferromagnetism
in Ba(Zn,Co)$_2$As$_2$ is intrinsic and the internal field is static. | cond-mat_mtrl-sci |
High-throughput search for triplet point defects with narrow emission
lines in 2D materials: We employ a first-principles computational workflow to screen for optically
accessible, high-spin point defects in wide band gap two-dimensional (2D)
crystals. Starting from an initial set of 5388 point defects, comprising both
intrinsic and extrinsic, single and double defects in ten previously
synthesised 2D host materials, we identify 596 defects with a triplet ground
state. For these defects, we calculate the defect formation energy, the
hyperfine (HF) coupling, and the zero-field splitting (ZFS) tensors. For 39
triplet transitions exhibiting particularly low Huang-Rhys factors, we
calculate the full photo-luminescence (PL) spectrum. Our approach reveals many
new spin defects with narrow PL line shapes and emission frequencies covering a
broad spectral range. Most of the defects are hosted in hexagonal BN, which we
ascribe to its high stiffness, but some are also found in MgI2, MoS2, MgBr2 and
CaI2. As specific examples, we propose the defects vSMoS0 and NiSMoS0 in MoS2
as interesting candidates with potential applications to magnetic field sensors
and quantum information technology. All the data will be made available in the
open access database QPOD. | cond-mat_mtrl-sci |
Ab initio vibrational free energies including anharmonicity for
multicomponent alloys: A density-functional-theory based approach to efficiently compute numerically
exact vibrational free energies - including anharmonicity - for chemically
complex multicomponent alloys is developed. It is based on a combination of
thermodynamic integration and a machine-learning potential. We demonstrate the
performance of the approach by computing the anharmonic free energy of the
prototypical five-component VNbMoTaW refractory high entropy alloy. | cond-mat_mtrl-sci |
Imaging Antiferromagnetic Domains in Nickel-oxide Thin Films by
Magneto-optical Voigt Effect: Recent demonstrations of electrical detection and manipulation of
antiferromagnets (AFMs) have opened new opportunities towards robust and
ultrafast spintronics devices. However, it is difficult to establish the
connection between the spin-transport behavior and the microscopic AFM domain
states due to the lack of the real-time AFM domain imaging technique under the
electric field. Here we report a significant Voigt rotation up to 60 mdeg in
thin NiO(001) films at room temperature. Such large Voigt rotation allows us to
directly observe AFM domains in thin-film NiO by utilizing a wide-field optical
microscope. Further complementary XMLD-PEEM measurement confirms that the Voigt
contrast originates from the NiO AFM order. We examine the domain pattern
evolution at a wide range of temperature and with the application of external
magnetic field. Comparing to large-scale-facility techniques such as the X-ray
photoemission electron microscopy, the use with a wide-field, tabletop optical
imaging method enables straightforward access to domain configurations of
single-layer AFMs. | cond-mat_mtrl-sci |
The role of the catalytic particle temperature gradient for SWNT growth
from small particles: The Vapour-Liquid-Solid (VLS) model, which often includes a temperature
gradient (TG) across the catalytic metal particle, is often used to describe
the nucleation and growth of carbon nanostructures. Although the TG may be
important for the growth of carbon species from large metal particles,
molecular dynamics simulations show that it is not required for single-walled
carbon nanotube growth from small catalytic particles. | cond-mat_mtrl-sci |
Giant optical anisotropy in cylindrical self-assembled InAs/GaAs quantum
rings: Using a single-particle atomistic pseudopotential method followed by a
many-particle configuration interaction method, we investigate the geometry,
electronic structure and optical transitions of a self-assembled InAs/GaAs
quantum ring (QR), changing its shape continously from a lens-shaped quantum
dot (QD) to a nearly one dimensional ring. We find that the biaxial strain in
the ring is strongly asymmetric in the plane perpendicular to the QR growth
direction, leading to giant optical anisotropy. | cond-mat_mtrl-sci |
Quantum Phase Transitions in Ba(1-x)CaxFe12O19: The ground state of BaFe12O19 (BFO) is controversial as three different
quantum states, namely quantum paraelectric, frustrated antiferroelectric and
quantum electric dipole liquid (QEDL), have been proposed. We have investigated
the quantum critical behavior of BFO as a function of chemical pressure (a
non-thermal variable) generated by smaller isovalent ion Ca2+ at the Ba2+ site.
Analysis of synchrotron x-ray diffraction data confirms that Ca2+ substitution
generates positive chemical pressure. Our dielectric measurements reveal that
Ca2+ substitution drives BFO away from its quantum critical point (QCP) and
stabilizes a quantum electric dipolar glass state whose dielectric peak
temperature (Tc) increases with increasing Ca2+ content as Tc ~ (x-xc)1/2, a
canonical signature of quantum phase transitions. Our dielectric measurements
reveal that pure BFO is slightly away from its QCP with a Tc of 2.91 K.
Specific heat measurements reveal excess specific heat of non-Debye and
non-magnetic origin with linear temperature dependence below Tc which could be
due to QEDL state of BFO. | cond-mat_mtrl-sci |
Toy nanoindentation model and incipient plasticity: A toy model of two dimensional nanoindentation in finite crystals is
proposed. The crystal is described by periodized discrete elasticity whereas
the indenter is a rigid strain field of triangular shape representing a hard
knife-like indenter. Analysis of the model shows that there are a number of
discontinuities in the load vs penetration depth plot which correspond to the
creation of dislocation loops. The stress vs depth bifurcation diagram of the
model reveals multistable stationary solutions that appear as the
dislocation-free branch of solutions develops turning points for increasing
stress. Dynamical simulations show that an increment of the applied load leads
to nucleation of dislocation loops below the nanoindenter tip. Such
dislocations travel inside the bulk of the crystal and accommodate at a certain
depth in the sample. In agreement with experiments, hysteresis is observed if
the stress is decreased after the first dislocation loop is created. Critical
stress values for loop creation and their final location at equilibrium are
calculated. | cond-mat_mtrl-sci |
NMR in magnetic molecular rings and clusters (I): In the last years there has been a great interest in magnetic systems formed
by a cluster of transition metal ions covalently bonded via superexchange
bridges, embedded in a large organic molecule. Following the synthesis and the
structural and magnetic characterization of these magnetic molecules by
chemists, the physicists realized the great interest of these systems as a
practical realization of zero-dimensional model magnetic systems. In fact the
magnetic molecules can be synthesized in crystalline form whereby each molecule
is magnetically independent since the intramolecular exchange interaction among
the transition metal ions is dominant over the weak intermolecular, usually
dipolar, magnetic interaction. We have undertaken a systematic NMR
investigation of molecular nanomagnets since back in 1996. The present review
tries to give an account of the main results obtained so far and of the many
exciting projects that still lie ahead. The work was done through a continuous
very fruitful collaboration among three NMR laboratories: at the University of
Pavia, Italy, at Iowa State University and Ames Laboratory, Ames, IA, USA and
at Hokkaido University, Sapporo, Japan with occasional very useful
collaborations with the high field NMR lab. in Grenoble, France. None of the
work could be done without the precious collaboration and help of our
colleagues in chemistry at the University of Florence and of Modena, Italy and
at Ames Laboratory in USA who synthesized and characterized the samples used in
the NMR work. | cond-mat_mtrl-sci |
Morphology of supported polymer electrolyte ultra-thin films: a
numerical study: Morphology of polymer electrolytes membranes (PEM), e.g., Nafion, inside PEM
fuel cell catalyst layers has significant impact on the electrochemical
activity and transport phenomena that determine cell performance. In those
regions, Nafion can be found as an ultra-thin film, coating the catalyst and
the catalyst support surfaces. The impact of the hydrophilic/hydrophobic
character of these surfaces on the structural formation of the films has not
been sufficiently explored yet. Here, we report about Molecular Dynamics
simulation investigation of the substrate effects on the ionomer ultra-thin
film morphology at different hydration levels. We use a mean-field-like model
we introduced in previous publications for the interaction of the hydrated
Nafion ionomer with a substrate, characterized by a tunable degree of
hydrophilicity. We show that the affinity of the substrate with water plays a
crucial role in the molecular rearrangement of the ionomer film, resulting in
completely different morphologies. Detailed structural description in different
regions of the film shows evidences of strongly heterogeneous behavior. A
qualitative discussion of the implications of our observations on the PEMFC
catalyst layer performance is finally proposed. | cond-mat_mtrl-sci |
Third order perturbed modified Heisenberg Hamiltonian of fcc structured
ferromagnetic films with seventy spin layers: Magnetic properties of fcc structured ferromagnetic films with the number of
spin layers up to seventy was described using third order perturbed Heisenberg
Hamiltonian. The variation of magnetic easy direction, magnetic energies in
easy and hard directions, magnetic anisotropy energy and the angle between easy
and hard directions was investigated by varying the number of spin layers. Spin
exchange interaction, magnetic dipole interaction, second and fourth order
magnetic anisotropies, in and out of plane applied magnetic fields,
demagnetization factor and stress induced anisotropy were considered in the
model. Because magnetic dipole interaction and demagnetization factor represent
microscopic and macroscopic properties of the sample, respectively, both these
terms were incorporated in our theoretical model. Although our model is a
semi-classical model, some discrete variations of angle of easy axis were
observed. Our theoretical data qualitatively agree with experimental data of Fe
and Ni ferromagnetic films. | cond-mat_mtrl-sci |
Local structural ordering determines the mechanical damage tolerance of
amorphous grain boundary complexions: Amorphous grain boundary complexions act as toughening features within a
microstructure because they can absorb dislocations more efficiently than
traditional grain boundaries. This toughening effect should be a strong
function of the local internal structure of the complexion, which has recently
been shown to be determined by grain boundary crystallography. To test this
hypothesis, molecular dynamics are used here to simulate dislocation absorption
and damage nucleation for complexions with different distributions of
structural short-range order. The complexion with a more disordered structure
away from the dislocation absorption site is actually found to better resist
crack nucleation, as damage tolerance requires delocalized deformation and the
operation of shear-transformation zones through the complexion thickness. The
more damage tolerant complexion accommodates plastic strain efficiently within
the entire complexion, providing the key mechanistic insight that local
patterning and asymmetry of structural short-range order controls the
toughening effect of amorphous complexions. | cond-mat_mtrl-sci |
Segregation-induced phase transformations in grain boundaries: Phase transformations in metallic grain boundaries (GBs) present significant
fundamental interest in the context of thermodynamics of low-dimensional
physical systems. We report on atomistic computer simulations of the Cu-Ag
system that provide direct evidence that GB phase transformations in a
single-component GB can continue to exist in a binary alloy. This gives rise to
segregation-induced phase transformations with varying chemical composition at
a fixed temperature. Furthermore, for such transformations we propose an
approach to calculations of free energy differences between different GB phases
by thermodynamic integration along a segregation isotherm. This opens the
possibility of developing quantitative thermodynamics of GB phases, their
transformations to each other, and critical phenomena in the future. | cond-mat_mtrl-sci |
The growth of ZnO crystals from the melt: The peculiar properties of zinc oxide (ZnO) make this material interesting
for very different applications like light emitting diodes, lasers, and
piezoelectric transducers. Most of these applications are based on epitaxial
ZnO layers grown on suitable substrates, preferably bulk ZnO. Unfortunately the
thermochemical properties of ZnO make the growth of single crystals difficult:
the triple point 1975 deg C., 1.06 bar and the high oxygen fugacity at the
melting point p_O2 = 0.35 bar lead to the prevailing opinion that ZnO crystals
for technical applications can only be grown either by a hydrothermal method or
from "cold crucibles" of solid ZnO. Both methods are known to have significant
drawbacks. Our thermodynamic calculations and crystal growth experiments show,
that in contrast to widely accepted assumptions, ZnO can be molten in metallic
crucibles, if an atmosphere with "self adjusting" p_O2 is used. This new result
is believed to offer new perspectives for ZnO crystal growth by established
standard techniques like the Bridgman method. | cond-mat_mtrl-sci |
Complete mapping of magnetic anisotropy for prototype Ising van der
Waals FePS$_3$: Several Ising-type magnetic van der Waals (vdW) materials exhibit stable
magnetic ground states. Despite these clear experimental demonstrations, a
complete theoretical and microscopic understanding of their magnetic anisotropy
is still lacking. In particular, the validity limit of identifying their
one-dimensional (1-D) Ising nature has remained uninvestigated in a
quantitative way. Here we performed the complete mapping of magnetic anisotropy
for a prototypical Ising vdW magnet FePS$_3$ for the first time. Combining
torque magnetometry measurements with their magnetostatic model analysis and
the relativistic density functional total energy calculations, we successfully
constructed the three-dimensional (3-D) mappings of the magnetic anisotropy in
terms of magnetic torque and energy. The results not only quantitatively
confirm that the easy axis is perpendicular to the $ab$ plane, but also reveal
the anisotropies within the $ab$, $ac$, and $bc$ planes. Our approach can be
applied to the detailed quantitative study of magnetism in vdW materials. | cond-mat_mtrl-sci |
Spin Angular Momentum Transfer and Plasmogalvanic Phenomena: We introduce the continuity equation for the electromagnetic spin angular
momentum (SAM) in matter and discuss the torque associated with the SAM
transfer in terms of effective spin forces acting in a material. In plasmonic
metal, these spin forces result in plasmogalvanic phenomenon which is pinning
the plasmon-induced electromotive force to atomically-thin layer at the metal
interface. | cond-mat_mtrl-sci |
Anomalous Hall effect in a two-dimensional electron gas with spin-orbit
interaction: We discuss the mechanism of anomalous Hall effect related to the contribution
of electron states below the Fermi surface (induced by the Berry phase in
momentum space). Our main calculations are made within a model of
two-dimensional electron gas with spin-orbit interaction of the Rashba type,
taking into account the scattering from impurities. We demonstrate that such an
"intrinsic" mechanism can dominate but there is a competition with the
impurity-scattering mechanism, related to the contribution of states in the
vicinity of Fermi surface. We also show that the contribution to the Hall
conductivity from electron states close to the Fermi surface has the intrinsic
properties as well. | cond-mat_mtrl-sci |
Local Structural Evidence for Strong Electronic Correlations in
LiRh$_2$O$_4$ Spinel: The local structure of the spinel LiRh$_2$O$_4$ has been studied using atomic
pair distribution function (PDF) analysis of powder x-ray diffraction data.
This measurement is sensitive to the presence of short Rh-Rh bonds that form
due to dimerization of Rh$^{4+}$ ions on the pyrochlore sublattice, independent
of the existence of long range order. We show that structural dimers exist in
the low-temperature phase, as previously supposed, with a bond shortening of
$\Delta r \sim 0.15$ \AA . The dimers persist up to 350 K, well above the
insulator-metal transition, with $\Delta r$ decreasing in magnitude on warming.
Such behavior is inconsistent with the Fermi surface nesting-driven Peierls
transition model. Instead, we argue that LiRh$_2$O$_4$ should properly be
described as a strongly correlated system. | cond-mat_mtrl-sci |
Hybrid-functional and quasi-particle calculations of band structures of
Mg2Si, Mg2Ge, and Mg2Sn: We perform hybrid functional and quasi-particle band structure calculations
with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge,
and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the
characteristics of band edge states, i.e., band and valley degeneracies, and
orbital characters, are found to be conserved, independent of the computational
schemes such as density functional generalized gradient approximation, hybrid
functionals, or quasi-particle calculations. However, the magnitude of the
calculated band gap varies significantly with the computational schemes. Within
density-functional calculations, the one-particle band gaps of Mg2Si, Mg2Ge,
and Mg2Sn are 0.191, 0.090, and -0.346 eV, respectively, and thus severely
underestimated compared to the experimental gaps, due to the band gap error in
the density functional theory and the significant relativistic effect on the
low-energy band structures. By employing hybrid-functional calculations with a
35% fraction of the exact Hartree-Fock exchange energy (HSE-35%), we overcame
the negative band gap issue in Mg2Sn. Finally, in quasi-particle calculations
on top of the HSE-35% Hamiltonians, we obtained band gaps of 0.835, 0.759, and
0.244 eV for Mg2Si, Mg2Ge, and Mg2Sn, respectively, consistent with the
experimental band gaps of 0.77, 0.74, and 0.36 eV, respectively. | cond-mat_mtrl-sci |
Scaling of alloy interfacial properties under compositional strain: Complex morphologies and microstructures that emerge during materials growth
and solidification are often determined by both equilibrium and kinetic
properties of the interface and their crystalline anisotropies. However limited
knowledge is available for the alloying and particularly the compositionally
generated elastic effects on these interface characteristics. Here we
systematically investigate such compositional effects on the interfacial
properties of an alloy model system based on the phase-field-crystal analysis,
including the solid-liquid interfacial free energy, kinetic coefficient, and
lattice pinning strength. Scaling relations for these interfacial quantities
over various ranges of material parameters are identified and predicted. Our
results indicate the important effects of couplings among mesoscopic and
microscopic length scales of alloy structure and concentration, and also the
influence of compressive and tensile interface stresses induced by composition
variations. The approach developed here provides an efficient way to
systematically identify these key material properties beyond the traditional
atomistic and continuum methods. | cond-mat_mtrl-sci |
Computer simulations of defects in perovskite KNbO3 crystals: An ab initio LMTO approach and semi-empirical quantum chemical INDO method
have been used for supercell calculations of basic point defects - F-type
centers and hole polarons bound to cation vacancy - in partly covalent
perovskite KNbO3. We predict the existence of both one-site and two-site
(molecular) polarons with close absorption energies (~ 1 eV). The relevant
experimental data are discussed and interpreted. | cond-mat_mtrl-sci |
Disclinations in the geometric theory of defects: In the geometric theory of defects, media with a spin structure, for example,
ferromagnet, is considered as a manifold with given Riemann--Cartan geometry.
We consider the case with the Euclidean metric corresponding to the absence of
elastic deformations but with nontrivial ${\mathbb S}{\mathbb O}(3)$-connection
which produces nontrivial curvature and torsion tensors. We show that the 't
Hooft--Polyakov monopole has physical interpretation in solid state physics
describing media with continuous distribution of dislocations and
disclinations. The Chern--Simons action is used for the description of single
disclinations. Two examples of point disclinations are considered: spherically
symmetric point "hedgehog" disclination and the point disclination for which
the $n$-field has a fixed value at infinity and essential singularity at the
origin. The example of linear disclinations with the Franc vector divisible by
$2\pi$ is considered. | cond-mat_mtrl-sci |
Nanostructured Immunosensors. Application to the detection of
Progesterone: A novel nanostructured electrochemical immunsensor for the determination of
progesterone is reported. The approach combines the properties of gold
nanoparticles with the use of a graphite-Teflon composite electrode matrix,
into which gold nanoparticles are incorporated by simple physical inclusion.
The antibody anti-progesterone was directly attached to the electrode surface.
The immunosensor functioning is based on competitive assay between progesterone
and alkaline phosphatase-labelled progesterone. Monitoring of the affinity
reaction was accomplished by the electrochemical oxidation of 1-naphtol.
Modification of the graphite -Teflon electrode matrix with gold nanoparticles
improves substantially the electrooxidation response of 1-naphtol. Using a
detection potential of +0.3V, a detection limit for progesterone of 0.84 ng
ml-1 was obtained. Analysis of seven milk samples spiked at a 3.5 ng ml-1
progesterone concentration level yielded a mean recovery of 101+6%. Detection
of the antigen-antibody reaction with a graphite - Teflon - colloidal - gold -
Tyrosinase electrode, using phenylphosphate as alkaline phosphatase substrate
to generate phenol, which is subsequently reduced at -0.1 V at the composite
electrode, produced a high improvement in the sensitivity for progesterone
detection | cond-mat_mtrl-sci |
Fabrication of diamond diffraction gratings for experiments with intense
hard x-rays: The demands on optical components to tolerate high radiation dose and
manipulate hard x-ray beams that can fit the experiment requirements, are
constantly increasing due to the advancements in the available x-ray sources.
Here we have successfully fabricated the transmission type gratings using
diamond, with structure sizes ranging from few tens of nanometres up to
micrometres, and aspect ratio of up to 20. The efficiencies of the gratings
were measured over a wide range of photon energies and their radiation
tolerance was confirmed using the most intense x-ray source in the world. The
fidelity of these grating structures was confirmed by the quality of the
measured experimental results. | cond-mat_mtrl-sci |
Room temperature Epitaxial Stabilization of a Tetragonal Phase in ARuO3
(A=Ca,Sr) Thin Films: We demonstrate that SrRuO3 and CaRuO3 thin films undergo a room temperature
structural phase transition driven by the substrate imposed epitaxial biaxial
strain. As tensile strain increases, ARuO3 (A=Ca, Sr) films transform from the
orthorhombic phase which is usually observed in bulk SrRuO3 and CaRuO3 at room
temperature, into a tetragonal phase which in bulk samples is only stable at
higher temperatures. More importantly, we show that the observed phenomenon
strongly affects the electronic and magnetic properties of ARuO3 thin films
that are grown on different single crystal substrates which in turn offers the
possibility to tune these properties. | cond-mat_mtrl-sci |
Graphene oxyhydride catalysts in view of spin radical chemistry: The article discusses carbocatalysis provided with amorphous carbons. The
discussion is conducted from the standpoint of the spin chemistry of graphene
molecules, in the framework of which the amorphous carbocatalysts are a
conglomerate of graphene-oxynitrothiohydride stable radicals presenting the
basic structural units (BSUs) of the species. The chemical activity of the BSUs
atoms is reliably determined computationally, which allows mapping the
distribution of active sites in these molecular catalysts. The presented maps
reliably evidence the BSUs radicalization provided with carbon atoms only, the
non-terminated edge part of which presents a set of active cites. Spin mapping
of carbocatalysts active cites is suggested as the first step towards the spin
carbocatalysis of the species. | cond-mat_mtrl-sci |
Observation of Magnetopiezoelectric Effect in Antiferromagnetic Metal
EuMnBi2: We have experimentally studied a magnetopiezoelectric effect predicted
recently for magnetic metals with low crystal symmetries. In EuMnBi2 with
antiferromagnetic Mn moments at 77 K, dynamic displacements emerge along the
$a$ direction upon application of ac electric fields in the $c$ direction, and
increase in proportion to the applied electric fields. Such displacements are
not observed along the $c$ direction of EuMnBi2 or EuZnBi2 with nonmagnetic Zn
ions. As temperature increases from 77 K, the displacement signals decrease and
disappear at about 200 K, above which electric conduction changes from coherent
to incoherent. These results demonstrate the emergence of the
magnetopiezoelectric effect in a magnetic metal lacking inversion and
time-reversal symmetries. | cond-mat_mtrl-sci |
Structural defects induced by Fe-ion implantation in TiO2: X-ray photoelectron spectroscopy (XPS) and resonant x-ray emission
spectroscopy (RXES) measurements of pellet and thin film forms of TiO$_2$ with
implanted Fe ions are presented and discussed. The findings indicate that
Fe-implantation in a TiO$_2$ pellet sample induces heterovalent cation
substitution (Fe$^{2+}\rightarrow$ Ti$^{4+}$) beneath the surface region. But
in thin film samples, the clustering of Fe atoms is primarily detected. In
addition to this, significant amounts of secondary phases of Fe$^{3+}$ are
detected on the surface of all doped samples due to oxygen exposure. These
experimental findings are compared with density functional theory (DFT)
calculations of formation energies for different configurations of structural
defects in the implanted TiO$_2$:Fe system. According to our calculations, the
clustering of Fe-atoms in TiO$_2$:Fe thin films can be attributed to the
formation of combined substitutional and interstitial defects. Further, the
differences due to Fe doping in pellet and thin film samples can ultimately be
attributed to different surface to volume ratios. | cond-mat_mtrl-sci |
Dynamic friction force in a carbon peapod oscillator: We investigate a new generation of fullerene nano-oscillators: a
single-walled carbon nanotube with one buckyball inside with an operating
frequency in the tens-of-gigahertz range. A quantitative characterization of
energy dissipation channels in the peapod pair has been performed via molecular
dynamics simulation. Edge effects are found to the dominant cause of dynamic
friction in the carbon-peapod oscillators. A comparative study on energy
dissipation also reveals significant impact of temperature and impulse velocity
on the frictional force. | cond-mat_mtrl-sci |
The structure of amorphous two-dimensional materials: Elemental
monolayer amorphous carbon versus binary monolayer amorphous boron nitride: The structure of amorphous materials has been debated since the 1930's as a
binary question: amorphous materials are either Zachariasen continuous random
networks (Z-CRNs) or Z-CRNs containing crystallites. It was recently
demonstrated, however, that amorphous diamond can be synthesized in either
form. Here we address the question of the structure of single-atom-thick
amorphous monolayers. We reanalyze the results of prior simulations for
amorphous graphene and report kinetic Monte Carlo simulations based on
alternative algorithms. We find that crystallite-containing Z-CRN is the
favored structure of elemental amorphous graphene, as recently fabricated,
whereas the most likely structure of binary monolayer amorphous BN is
altogether different than either of the two long-debated options: it is a
compositionally disordered "pseudo-CRN" comprising a mix of B-N and
noncanonical B-B and N-N bonds and containing "pseudocrystallites", namely
honeycomb regions made of noncanonical hexagons. Implications for other
non-elemental 2D and bulk amorphous materials are discussed. | cond-mat_mtrl-sci |
The decoupled DFT-$\frac{1}{2}$ method for defect excitation energies: The DFT-$\frac{1}{2}$ method is a band gap correction with GW precision at a
DFT computational cost. The method was also extended to correct the gap between
defect levels, allowing for the calculation of optical transitions. However,
this method fails when the atomic character of the occupied and unoccupied
defect levels are similar as we illustrate by two examples, the tetrahedral
hydrogen interstitial and the negatively charged vacancy in diamond. We solve
this problem by decoupling the effect of the occupied and unoccupied defect
levels and call this the decoupled DFT-$\frac{1}{2}$ method for defects. | cond-mat_mtrl-sci |
Direct-written polymer field-effect transistors operating at 20 MHz: Printed polymer electronics has held for long the promise of revolutionizing
technology by delivering distributed, flexible, lightweight and cost-effective
applications for wearables, healthcare, diagnostic, automation and portable
devices. While impressive progresses have been registered in terms of organic
semiconductors mobility, field-effect transistors (FET), the basic building
block of any circuit, are still showing limited speed of operation, thus
limiting their real applicability. So far, attempts with organic FET to achieve
the tens of MHz regime, a threshold for many applications comprising the
driving of high resolution displays, have relied on the adoption of
sophisticated lithographic techniques and/or complex architectures, undermining
the whole concept. In this work we demonstrate polymer FETs which can operate
up to 20 MHz and are fabricated by means only of scalable printing techniques
and direct-writing methods with a completely mask-less procedure. This is
achieved by combining a fs-laser process for the sintering of high resolution
metal electrodes, thus easily achieving micron-scale channels with reduced
parasitism down to 0.19 pF mm-1, and a large area coating technique of a high
mobility polymer semiconductor, according to a simple and scalable process
flow. | cond-mat_mtrl-sci |
Magnetic spin excitations in Mn doped GaAs : A model study: We provide a quantitative theoretical model study of the dynamical magnetic
properties of optimally annealed Ga$_{1-x}$Mn$_x$As. This model has already
been shown to reproduce accurately the Curie temperatures for
Ga$_{1-x}$Mn$_x$As. Here we show that the calculated spin stiffness are in
excellent agreement with those which were obtained from ab-initio based
studies. In addition, an overall good agreement is also found with available
experimental data. We have also evaluated the magnon density of states and the
typical density of states from which the "mobility edge", separating the
extended from localized magnon states, was determined. The power of the model
lies in its ability to be generalized for a broad class of diluted magnetic
semiconductor materials, thus it bridges the gap between first principle
calculations and model based studies. | cond-mat_mtrl-sci |
Superintermolecular orbitals in the C$_{60}$/pentacene complex: We report a group of unusually big molecular orbitals in the C60/pentacene
complex. Our first-principles density functional calculation shows that these
orbitals are very delocalized and cover both C60 and pentacene, which we call
superintermolecular orbitals or SIMOs. Their spatial extension can reach 1 nm
or larger. Optically, SIMOs are dark. Different from ordinary unoccupied
molecular orbitals, SIMOs have a very weak Coulomb and exchange interaction.
Their energy levels are very similar to the native superatomic molecular
orbitals in C60, and can be approximately characterized by orbital angular
momentum quantum numbers. They have a distinctive spatial preference. These
features fit the key characters of charge-generation states that channel
initially-bound electrons and holes into free charge carriers. Thus, our
finding is important for C60/pentacene photovoltaics. | cond-mat_mtrl-sci |
Elastic and magnetic effects on the infrared phonon spectra of MnF2: We measured the temperature dependent infrared reflectivity spectra of MnF2
between 4 K and room temperature. We show that the phonon spectrum undergoes a
strong renormalization at TN. The ab-initio calculation we performed on this
compound accurately predict the magnitude and the direction of the phonon
parameters changes across the antiferromagnetic transition, showing that they
are mainly induced by the magnetic order. In this material, we found that the
dielectric constant is mostly from phonon origin. The large change in the
lattice parameters with temperature seen by X-ray diffraction as well as the
A2u phonon softening below TN indicate that magnetic order induced distortions
in MnF2 are compatible with the ferroelectric instabilities observed in TiO2,
FeF2 and other rutile-type fluorides. This study also shows the anomalous
temperature evolution of the lower energy Eu mode in the paramagnetic phase,
which can be compared to that of the B1g one seen by Raman spectroscopy in many
isostructural materials. This was interpreted as being a precursor of a phase
transition from rutile to CaCl2 structure which was observed under pressure in
ZnF2. | cond-mat_mtrl-sci |
An Efficient DFT Solver for Nanoscale Simulations and Beyond: We present the One-orbital Ensemble Self-Consistent Field (OE-SCF) method, an
{alternative} orbital-free DFT solver that extends the applicability of DFT to
system sizes beyond the nanoscale while retaining the accuracy required to be
predictive. OE-SCF is an iterative solver where the (typically computationally
expensive) Pauli potential is treated as an external potential and updated
after each iteration. Because only up to a dozen iterations are needed to reach
convergence, OE-SCF dramatically outperforms current orbital-free DFT solvers.
Employing merely a single CPU, we carried out the largest ab initio simulation
for silicon-based materials to date. OE-SCF is able to converge the energy of
bulk-cut Si nanoparticles as a function of their diameter up to 16 nm, for the
first time reproducing known empirical results. We model polarization and
interface charge transfer when a Si slab is sandwiched between two metal slabs
where lattice matching mandates a very large slab size. Additionally, OE-SCF
opens the door to adopt even more accurate functionals in orbital-free DFT
simulations while still tackling systems sizes beyond the nanoscale. | cond-mat_mtrl-sci |
Topological surface states of Bi2Se3 with the coexistence of Se
vacancies: Although topological surface states are known to be robust against
non-magnetic surface perturbations, their band dispersions and spatial
distributions are still sensitive to the surface defects. Take Bi2Se3 as an
example, we demonstrated that Se vacancies modifies the surface band structures
considerably. When large numbers of Se vacancies exist on the surface,
topological surface states may sink down from the first to second quintuple
layer and get separated from the vacancies. We simulated STM images to
distinguish the surfaces with Se- and Bi-terminations. | cond-mat_mtrl-sci |
Langevin spin dynamics based on ab initio calculations: numerical
schemes and applications: A method is proposed to study the finite-temperature behaviour of small
magnetic clusters based on solving the stochastic Landau-Lifshitz-Gilbert
equations, where the effective magnetic field is calculated directly during the
solution of the dynamical equations from first principles instead of relying on
an effective spin Hamiltonian. Different numerical solvers are discussed in the
case of a one-dimensional Heisenberg chain with nearest-neighbour interactions.
We performed detailed investigations for a monatomic chain of ten Co atoms on
top of Au(001) surface. We found a spiral-like ground state of the spins due to
Dzyaloshinsky-Moriya interactions, while the finite-temperature magnetic
behaviour of the system was well described by a nearest-neighbour Heisenberg
model including easy-axis anisotropy. | cond-mat_mtrl-sci |
Electric-field control of magnetic domain wall motion and local
magnetization reversal: Spintronic devices currently rely on magnetic switching or controlled motion
of domain walls by an external magnetic field or spin-polarized current.
Achieving the same degree of magnetic controllability using an electric field
has potential advantages including enhanced functionality and low power
consumption. Here, we report on an approach to electrically control local
magnetic properties, including the writing and erasure of regular ferromagnetic
domain patterns and the motion of magnetic domain walls, in multiferroic
CoFe-BaTiO3 heterostructures. Our method is based on recurrent strain transfer
from ferroelastic domains in ferroelectric media to continuous magnetostrictive
films with negligible magnetocrystalline anisotropy. Optical polarization
microscopy of both ferromagnetic and ferroelectric domain structures reveals
that domain correlations and strong inter-ferroic domain wall pinning persist
in an applied electric field. This leads to an unprecedented electric
controllability over the ferromagnetic microstructure, an accomplishment that
produces giant magnetoelectric coupling effects and opens the way to
multiferroic spintronic devices. | cond-mat_mtrl-sci |
The Robustness of Cluster Expansion: Assessing the Roles of Relaxation
and Numerical Error: Cluster expansion (CE) is effective in modeling the stability of metallic
alloys, but sometimes cluster expansions fail. Failures are often attributed to
atomic relaxation in the DFT-calculated data, but there is no metric for
quantifying the degree of relaxation. Additionally, numerical errors can also
be responsible for slow CE convergence. We studied over one hundred different
Hamiltonians and identified a heuristic, based on a normalized mean-squared
displacement of atomic positions in a crystal, to determine if the effects of
relaxation in CE data are too severe to build a reliable CE model. Using this
heuristic, CE practitioners can determine a priori whether or not an alloy
system can be reliably expanded in the cluster basis. We also examined the
error distributions of the fitting data. We find no clear relationship between
the type of error distribution and CE prediction ability, but there are clear
correlations between CE formalism reliability, model complexity, and the number
of significant terms in the model. Our results show that the \emph{size} of the
errors is much more important than their distribution. | cond-mat_mtrl-sci |
2D-MoS2 with Narrowest Excitonic Linewidths Grown by Flow-Less Direct
Heating of Bulk Powders: Developing techniques for high-quality synthesis of mono and few-layered 2D
materials with lowered complexity and cost continues to remain an important
goal, both for accelerating fundamental research and for applications
development. We present the simplest conceivable technique to synthesize
micrometer-scale single-crystal triangular monolayers of MoS2, i.e. by direct
heating of bulk MoS2 powder onto proximally-placed substrates. Room-temperature
excitonic linewidth values of our samples are narrower and more uniform than
those of 2D-MoS2 obtained by most other techniques reported in literature, and
comparable to those of ultraflat h-BN-capped mechanically exfoliated samples,
indicative of their high quality. Feature-rich Raman spectra absent in samples
grown or obtained by most other techniques, also stand out as a testament of
the high quality of our samples. A contact-growth mode facilitates direct
growth of crystallographically-strained circular samples, which allows us to
directly compare the optoelectronic properties of flat vs. strained growth from
the same growth runs. Our method allows, for the first time, to quantitatively
compare the impact of strain on excitonic and Raman peak positions on
identically-synthesized 2D-MoS2. Strain leads to average Red-shifts of ~ 30 meV
in the A-exciton position, and ~ 2 cm-1 in the E12g Raman peak in these
samples. Our findings open-up several new possibilities that expand 2D material
research. By eliminating the need for carrier gas flow, mechanical motion or
chemical reactions, our method can be either miniaturized for substantially
low-cost, high-quality scientific research or potentially scaled-up for
mass-production of 2D crystals for commercial purposes. Moreover, we believe
this technique can also be extended to other transition metal dichalcogenides
and other layered materials. | cond-mat_mtrl-sci |
Topological semimetal phases in a family of monolayer X3YZ6 (X=Nb,Ta,
Y=Si,Ge,Sn, Z=S,Se,Te) with abundant nodal lines and nodes: The electronic and topological properties of single-layer X3YZ6 (X=Nb,Ta,
Y=Si,Ge,Sn, Z=S,Se,Te) materials have been studied with the aid of first
principles calculations. This kind of materials belong to topological
semimetals (TMs) with abundant nodal lines and nodes. Considering their similar
properties, we focus on the analysis of Ta3SnTe6 and Ta3SiSe6. The present of
spin-orbit coupling (SOC) leads to the transition from type-I nodal lines to
Dirac points as well as the disappear of type-II Dirac points. The
three-dimensional (3D) band diagrams reproduce vividly the characteristics of
nodes and nodal lines. The appearance of the flat bands in (110) edge states
further confirm their nontrivial topological properties. We also explore the
relationship among different nodal lines (nodes), crystal symmetry and SOC. The
type-I nodal lines are protected by Mz and My symmetry in the absent of SOC.
Symmetry breaking leads to band splitting even in the presence of SOC. The
single-layer X3YZ6 can be used as candidates for two-dimensional (2D) TMs and
provide a platform for further study of interesting physical phenomena. | cond-mat_mtrl-sci |
Electronic structure of above-room-temperature van der Waals ferromagnet
Fe$_3$GaTe$_2$: Fe$_3$GaTe$_2$, a recently discovered van der Waals ferromagnet, demonstrates
intrinsic ferromagnetism above room temperature, necessitating a comprehensive
investigation of the microscopic origins of its high Curie temperature
($\textit{T}$$_C$). In this study, we reveal the electronic structure of
Fe$_3$GaTe$_2$ in its ferromagnetic ground state using angle-resolved
photoemission spectroscopy and density functional theory calculations. Our
results establish a consistent correspondence between the measured band
structure and theoretical calculations, underscoring the significant
contributions of the Heisenberg exchange interaction ($\textit{J}$$_{ex}$) and
magnetic anisotropy energy to the development of the high-$\textit{T}$$_C$
ferromagnetic ordering in Fe$_3$GaTe$_2$. Intriguingly, we observe substantial
modifications to these crucial driving factors through doping, which we
attribute to alterations in multiple spin-splitting bands near the Fermi level.
These findings provide valuable insights into the underlying electronic
structure and its correlation with the emergence of high-$\textit{T}$$_C$
ferromagnetic ordering in Fe$_3$GaTe$_2$. | cond-mat_mtrl-sci |
Harmonic vibrational excitations in graded elastic networks: transition
from phonons to gradons: We have identified a new type of transition from extended to localized
vibrational states in one-dimensional graded elastic chains of coupled harmonic
oscillators, in which the vibrating masses or nearest-coupling force constants
vary linearly along the chain. We found that the delocalization transition
occurs at the maximum frequency of the corresponding homogeneous chain, which
is in a continuous single band. Although each state in the localized phase,
called gradon, can be regarded as an impurity localized mode, the localization
profile is clearly distinct from usual impurity modes or the Anderson localized
modes. We also argue how gradons may affect the macroscopic properties of
graded systems. Our results can provide insights into many analogous systems
with graded characters. | cond-mat_mtrl-sci |
Switching ferromagnetic spins by an ultrafast laser pulse: Emergence of
giant optical spin-orbit torque: Faster magnetic recording technology is indispensable to massive data storage
and big data sciences. {All-optical spin switching offers a possible solution},
but at present it is limited to a handful of expensive and complex rare-earth
ferrimagnets. The spin switching in more abundant ferromagnets may
significantly expand the scope of all-optical spin switching. Here by studying
40,000 ferromagnetic spins, we show that it is the optical spin-orbit torque
that determines the course of spin switching in both ferromagnets and
ferrimagnets. Spin switching occurs only if the effective spin angular momentum
of each constituent in an alloy exceeds a critical value. Because of the strong
exchange coupling, the spin switches much faster in ferromagnets than
weakly-coupled ferrimagnets. This establishes a paradigm for all-optical spin
switching. The resultant magnetic field (65 T) is so big that it will
significantly reduce high current in spintronics, thus representing the
beginning of photospintronics. | cond-mat_mtrl-sci |
Graphene Nanoribbon based T Junctions: Graphene nanoribbons (GNRs) based T junctions were designed and studied in
this paper. These junctions were made up of shoulders (zigzag GNRs) joined with
stems (armchair GNRs). We demonstrated the intrinsic transport properties and
effective boron (or nitrogen) doping of the junctions by using first principles
quantum transport simulation. Several interesting results were found: i) The
I-V characteristics of the pure-carbon T junctions were shown to obey Ohm law
and the electrical conductivity of the junction depends on the height of the
stem sensitively. ii) boron (or nitrogen) doping on the stems doesnt change the
Ohm law of the T junctions, but the result is opposite when doping process
occurs at the shoulders. This feature could make such quasi-2D T junction a
possible candidate for nanoscale junction devices in a 2D network of
nanoelectronic devices in which conducting pathways can be controlled. | cond-mat_mtrl-sci |
Anisotropic Gilbert damping in perovskite La$_{0.7}$Sr$_{0.3}$MnO$_{3}$
thin film: The viscous Gilbert damping parameter governing magnetization dynamics is of
primary importance for various spintronics applications. Although, the damping
constant is believed to be anisotropic by theories. It is commonly treated as a
scalar due to lack of experimental evidence. Here, we present an elaborate
angle dependent broadband ferromagnetic resonance study of high quality
epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ films. Extrinsic effects are suppressed
and we show convincing evidence of anisotropic damping with twofold symmetry at
room temperature. The observed anisotropic relaxation is attributed to the
magnetization orientation dependence of the band structure. In addition, we
demonstrated that such anisotropy can be tailored by manipulating the stain.
This work provides new insights to understand the mechanism of magnetization
relaxation. | cond-mat_mtrl-sci |
Structure and dynamics of the fullerene polymer Li4 C60 studied with
neutron scattering: The two-dimensional polymer structure and lattice dynamics of the superionic
conductor Li4 C60 are investigated by neutron diffraction and spectroscopy. The
peculiar bonding architecture of this compound is definitely confirmed through
the precise localisation of the carbon atoms involved in the intermolecular
bonds. The spectral features of this phase are revealed through ab-initio
lattice dynamics calculations and inelastic neutron scattering experiments. The
neutron observables are found to be in very good agreement with the simulations
which predict a partial charge transfer from the Li atoms to the C60 cage. The
absence of a well defined band associated to one category of the Li atoms in
the experimental spectrum suggests that this species is not ordered even at the
lowest temperatures. The calculations predict an unstable Li sublattice at a
temperature of 200 K, that we relate to the large ionic diffusivity of this
system. This specificity is discussed in terms of coupling between the low
frequency optic modes of the Li ions to the soft structure of the polymer. | cond-mat_mtrl-sci |
Direct dry transfer of chemical vapor deposition graphene to polymeric
substrates: We demonstrate the direct dry transfer of large area Chemical Vapor
Deposition graphene to several polymers (low density polyethylene, high density
polyethylene, polystyrene, polylactide acid and
poly(vinylidenefluoride-co-trifluoroethylene) by means of only moderate heat
and pressure, and the later mechanical peeling of the original graphene
substrate. Simulations of the graphene-polymer interactions, rheological tests
and graphene transfer at various experimental conditions show that controlling
the graphene-polymer interface is the key to controlling graphene transfer.
Raman spectroscopy and Optical Microscopy were used to identify and quantify
graphene transferred to the polymer substrates. The results showed that the
amount of graphene transferred to the polymer, from no-graphene to full
graphene transfers, can be achieved by fine tuning the transfer conditions. As
a result of the direct dry transfer technique, the graphene-polymer adhesion
being stronger than graphene to Si/SiO2 wafer. | cond-mat_mtrl-sci |
Tantalum STJ for Photon Counting Detectors: Superconducting Tunnel Junctions (STJ's) are currently being developed as
photon detectors for a wide range of applications. Interest comes from their
ability to cumulate photon counting with chromaticity (i.e. energy resolution)
from the near infrared (2 $\mu$m) to the X-rays wavelengths and good quantum
efficiency up to 80%. Resolving power can exceed 10 in the visible wavelength
range. Our main goal is to use STJ's for astronomical observations at low light
level in the near infrared. This paper put the emphasis on two main points: the
improvement of the tantalum absorber epitaxy and the development of a new
version of the fabrication process for making Ta/Al-AlOx-Al/Ta photon counting
STJ's. The main features of this process are that pixels have aligned
electrodes and vias patterned through a protecting SiO2 layer. These vias are
then used to contact the top electrode layer. We use a double thin aluminum
trapping layer on top of a 150 nm thick Ta absorber grown epitaxially. Photon
counting experiments with Ta junction array are presented at \lambda = 0.78
$\mu$m. Digital filtering methods are used to compute the photon counting data
in order to minimize the effects of noise. | cond-mat_mtrl-sci |
Discovery of Weyl nodal lines in a single-layer ferromagnet: Two-dimensional (2D) materials have attracted great attention and spurred
rapid development in both fundamental research and device applications. The
search for exotic physical properties, such as magnetic and topological order,
in 2D materials could enable the realization of novel quantum devices and is
therefore at the forefront of materials science. Here, we report the discovery
of two-fold degenerate Weyl nodal lines in a 2D ferromagnetic material, a
single-layer gadolinium-silver compound, based on combined angle-resolved
photoemission spectroscopy measurements and theoretical calculations. These
Weyl nodal lines are symmetry protected and thus robust against external
perturbations. The coexistence of magnetic and topological order in a 2D
material is likely to inform ongoing efforts to devise and realize novel
nanospintronic devices. | cond-mat_mtrl-sci |
Optimizing Floquet engineering for non-equilibrium steady states with
gradient-based methods: Non-equilibrium steady states are created when a periodically driven quantum
system is also incoherently interacting with an environment -- as it is the
case in most realistic situations. The notion of Floquet engineering refers to
the manipulation of the properties of systems under periodic perturbations.
Although it more frequently refers to the coherent states of isolated systems
(or to the transient phase for states that are weakly coupled to the
environment), it may sometimes be of more interest to consider the final steady
states that are reached after decoherence and dissipation take place. In this
work, we propose a computational method to find the multicolor periodic
perturbations that lead to the final steady states that are optimal with
respect to a given predefined metric, such as for example the maximization of
the temporal average value of some observable. We exemplify the concept using a
simple model for the nitrogen-vacancy center in diamond: the goal in this case
is to find the driving periodic magnetic field that maximizes a time-averaged
spin component. We show that, for example, this technique permits to prepare
states whose spin values are forbidden in thermal equilibrium at any
temperature. | cond-mat_mtrl-sci |
Tuning carrier density and phase transitions in oxide semiconductors
using focused ion beams: We demonstrate spatial modification of the optical properties of thin-film
metal oxides, zinc oxide and vanadium dioxide as representatives, using a
commercial focused ion beam (FIB) system. Using a Ga+ FIB and thermal
annealing, we demonstrated variable doping of a band semiconductor, zinc oxide
(ZnO), achieving carrier concentrations from 10^18 cm-3 to 10^20 cm-3. Using
the same FIB without subsequent thermal annealing, we defect-engineered a
correlated semiconductor, vanadium dioxide (VO2), locally modifying its
insulator-to-metal transition (IMT) temperature by range of ~25 degrees C. Such
area-selective modification of metal oxides by direct writing using a FIB
provides a simple, mask-less route to the fabrication of optical structures,
especially when multiple or continuous levels of doping or defect density are
required. | 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 |
Controlling the Electrical Properties of Undoped and Ta-doped TiO2
Polycrystalline Films via Ultra-Fast Annealing Treatments: We present a study on the crystallization process of undoped and Ta doped
TiO2 amorphous thin films. In particular, the effect of ultra-fast annealing
treatments in environments characterized by different oxygen concentrations is
investigated via in-situ resistance measurements. The accurate examination of
the key parameters involved in this process allows us to reduce the time needed
to obtain highly conducting and transparent polycrystalline thin films
(resistivity about $6 \times 10^{-4}$ {\Omega}cm, mean transmittance in the
visible range about $81\%$) to just 5 minutes (with respect to the 180 minutes
required for a standard vacuum annealing treatment) in nitrogen atmosphere (20
ppm oxygen concentration) at ambient pressure. Experimental evidence of
superficial oxygen incorporation in the thin films and its detrimental role for
the conductivity are obtained by employing different concentrations of
traceable 18O isotopes during ultra-fast annealing treatments. The results are
discussed in view of the possible implementation of the ultra-fast annealing
process for TiO2-based transparent conducting oxides as well as electron
selective layers in solar cell devices; taking advantage of the high control of
the ultra-fast crystallization processes which has been achieved, these two
functional layers are shown to be obtainable from the crystallization of a
single homogeneous thin film. | cond-mat_mtrl-sci |
Oxidation tuning of ferroic transitions in Gd$_2$C monolayer: Tuning of ferroic phases provides great opportunities for material
functionalities, especially in two-dimensional materials. Here, a $4f$
rare-earth carbide Gd$_2$C monolayer is predicted to be ferromagnetic metal
with large magnetization, inherited from its bulk property. Based on
first-principles calculations, we propose a strategy that the surface
passivation can effectively tune its ferroicity, namely switching among
ferromagnetic, antiferromagnetic, and ferroelectric phases. Metal-insulator
transition also occurs accompanying these ferroic transitions. Our calculation
also suggests that the magneto-optic Kerr effect and second harmonic generation
are effective methods to monitor these phase transitions. | cond-mat_mtrl-sci |
Nonthermal effects in solids after swift heavy ion impact: This contribution is a brief introduction to nonthermal effects related to
modifications of the interatomic potential upon ultrafast excitation of the
electronic system of solids, primarily focusing on the swift heavy ion track
problem. We clarify the difference between the exchange of the kinetic energy
of electrons (and holes) scattering on the lattice (electron-phonon coupling,
"thermal effects") and the relaxation of the nonequilibrium potential energy of
a solid ("nonthermal effects"). We discuss that at different degrees of
electronic excitation, the modification of the interatomic potential may result
in various phase transitions without an increase of the atomic temperature,
i.e., at room temperature (nonthermal melting, formation of the superionic
state), or in atomic acceleration causing "nonthermal heating" of the target
atoms. Examples of theoretically predicted various effects are given, supported
by known experimental observations. | cond-mat_mtrl-sci |
Ab initio study of proton-exchanged LiNbO3(I): Structural,
thermodynamic, dielectric, and optical properties: Using first principles calculations, we study the ground-state structure of
bulk proton-exchanged lithium niobate, which is also called hydrogen niobate
and is widely used in waveguides. Thermodynamics helps to establish the most
favorable nonpolar surface as well as the water-deficient and water-rich phases
under different ambient conditions, which we refer to as "dehydrated" and
"rehydrated" phases, respectively. We compute the low-frequency dielectric
response and the optical refractive indices of hydrogen niobate in different
phases. The dielectric constant is greatly enhanced compared to lithium
niobate. At shorter wavelengths, the refractive indices vary between each phase
and have a sharp contrast to lithium niobate. Our study characterizes the
structures and thermal instabilities of this compound and reveals its excellent
dielectric and optical properties, which can be important in the future
application in waveguides. | cond-mat_mtrl-sci |
Spin-orbit Hamiltonian for organic crystals from first principles
electronic structure and Wannier functions: Spin-orbit coupling in organic crystals is responsible for many
spin-relaxation phenomena, going from spin diffusion to intersystem crossing.
With the goal of constructing effective spin-orbit Hamiltonians to be used in
multiscale approaches to the thermodynamical properties of organic crystals, we
present a method that combines density functional theory with the construction
of Wannier functions. In particular we show that the spin-orbit Hamiltonian
constructed over maximally localised Wannier functions can be computed by
direct evaluation of the spin-orbit matrix elements over the Wannier functions
constructed in absence of spin-orbit interaction. This eliminates the prob- lem
of computing the Wannier functions for almost degenerate bands, a problem
always present with the spin-orbit-split bands of organic crystals. Examples of
the method are presented for isolated molecules, for mono-dimensional chains of
Pb and C atoms and for triarylamine-based one-dimansional single crystals. | cond-mat_mtrl-sci |
Moulding flexural waves in elastic plates lying atop a Faqir's bed of
nails: Platonic crystals (PCs) are the elastic plate analogue of the photonic
crystals widely used in optics, and are thin structured elastic plates along
which flexural waves cannot propagate within certain stop band frequency
intervals. The practical importance of PCs is twofold: these can be used either
in the design of microstructured acoustic metamaterials or as an approximate
model for surface elastic waves propagating in meter scale seismic
metamaterials. Here, we make use of the band spectrum of PCs created with very
small clamped holes, the nails of the title, to achieve surface wave reflectors
at very large wavelengths, a flat lens, an endoscope, a directive antenna near
stop band frequencies and cloaking from Dirac cones. The point pinned, Faqir,
plate is particularly appealing as there is an exact dispersion relation
available so the origin of these phenomena can be explained and interpreted
using Fourier series and high frequency homogenization. | cond-mat_mtrl-sci |
Prediction of grain boundary structure and energy by machine learning: Grain boundaries dramatically affect the properties of polycrystalline
materials because of differences in atomic configuration. To fully understand
the relationship between grain boundaries and materials properties, systematic
studies of the grain boundary atomic structure are crucial. However, such
studies are limited by the extensive computation necessary to determine the
structure of a single grain boundary. If the structure could be predicted with
more efficient computation, the understanding of the grain boundary would be
accelerated significantly. Here, we predict grain boundary structures and
energies using a machine-learning technique. Training data for non-linear
regression of four symmetric-tilt grain boundaries of copper were used. The
results of the regression analysis were used to predict 12 other grain boundary
structures. The method accurately predicts both the structures and energies of
grain boundaries. The method presented in this study is very general and can be
utilized in understanding many complex interfaces. | cond-mat_mtrl-sci |
Cooperative gas adsorption without a phase transition in metal-organic
frameworks: Cooperative adsorption of gases by porous frameworks permits more efficient
uptake and removal than does the more usual non-cooperative (Langmuir-type)
adsorption. Cooperativity, signaled by a step-like isotherm, is usually
attributed to a phase transition of the framework. However, the class of
metal-organic frameworks mmen-M$_2$(dobpdc) exhibit cooperative adsorption of
CO2 but show no evidence of a phase transition. Here we show how cooperativity
emerges in these frameworks in the absence of a phase transition. We use a
combination of quantum and statistical mechanics to show that cooperativity
results from a sharp but finite increase, with pressure, of the mean length of
chains of CO2 molecules that polymerize within the framework. Our study
provides microscopic understanding of the emergent features of cooperative
binding, including the position, slope and height of the isotherm step, and
indicates how to optimize gas storage and separation in these materials. | cond-mat_mtrl-sci |
Characterization and control of ZnGeN2 cation lattice ordering: ZnGeN2 and other heterovalent ternary semiconductors have important potential
applications in optoelectronics, but ordering of the cation sublattice, which
can affect the band gap, lattice parameters, and phonons, is not yet well
understood. Here the effects of growth and processing conditions on the
ordering of the ZnGeN2 cation sublattice were investigated using x-ray
diffraction and Raman spectroscopy. Polycrystalline ZnGeN2 was grown by
exposing solid Ge to Zn and NH3 vapors at temperatures between 758 degree C and
914 degree C. Crystallites tended to be rod-shaped, with growth rates higher
along the c-axis. The degree of ordering, from disordered, wurtzite-like x-ray
diffraction spectra to orthorhombic, with space group Pna21, increased with
increasing growth temperature, as evidenced by the appearance of superstructure
peaks and peak splittings in the diffraction patterns. Annealing disordered,
low-temperature-grown ZnGeN2 at 850 degree C resulted in increased cation
ordering. Growth of ZnGeN2 on a liquid Sn-Ge-Zn alloy at 758 degree C showed an
increase in the tendency for cation ordering at a lower growth temperature, and
resulted in hexagonal platelet-shaped crystals. The trends shown here may help
to guide understanding of the synthesis and characterization of other
heterovalent ternary nitride semiconductors as well as ZnGeN2. | cond-mat_mtrl-sci |
Superheating and solid-liquid phase coexistence in nanoparticles with
non-melting surfaces: We present a phenomenological model of melting in nanoparticles with facets
that are only partially wet by their liquid phase. We show that in this model,
as the solid nanoparticle seeks to avoid coexistence with the liquid, the
microcanonical melting temperature can exceed the bulk melting point, and that
the onset of coexistence is a first-order transition. We show that these
results are consistent with molecular dynamics simulations of aluminum
nanoparticles which remain solid above the bulk melting temperature. | cond-mat_mtrl-sci |
Giant optical birefringence of semiconductor nanowire metamaterials: Semiconductor nanowires exhibit large polarization anisotropy for the
absorption and emission of light, making them ideal building blocks for novel
photonic metamaterials. Here, we demonstrate that a high density of aligned
nanowires exhibits giant optical birefringence, a collective phenomenon
observable uniquely for collections of wires. The nanowire material was grown
on gallium phosphide (GaP) (111) in the form of vertically standing GaP
nanowires. We obtain the largest optical birefringence to date, with a
difference between the in-plane and out-of-plane refractive indices of 0.80 and
a relative birefringence of 43%. These values exceed by a factor of 75 the
natural birefringence of quartz and a by more than a factor of two the highest
values reported so far in other artificial materials. By exploiting the
specific crystallographic growth directions of the nanowires on the substrate,
we further demonstrate full control over the orientation of the optical
birefringence effect in the metamaterial. | cond-mat_mtrl-sci |
Imaging the stick-slip peeling of an adhesive tape under a constant load: Using a high speed camera, we study the peeling dynamics of an adhesive tape
under a constant load with a special focus on the so-called stick-slip regime
of the peeling. It is the first time that the very fast motion of the peeling
point is imaged. The speed of the camera, up to 16000 fps, allows us to observe
and quantify the details of the peeling point motion during the stick and slip
phases: stick and slip velocities, durations and amplitudes. First, in contrast
with previous observations, the stick-slip regime appears to be only transient
in the force controlled peeling. Additionally, we discover that the stick and
slip phases have similar durations and that at high mean peeling velocity, the
slip phase actually lasts longer than the stick phase. Depending on the mean
peeling velocity, we also observe that the velocity change between stick and
slip phase ranges from a rather sudden to a smooth transition. These new
observations can help to discriminate between the various assumptions used in
theoretical models for describing the complex peeling of an adhesive tape. The
present imaging technique opens the door for an extensive study of the velocity
controlled stick-slip peeling of an adhesive tape that will allow to understand
the statistical complexity of the stick-slip in a stationary case. | cond-mat_mtrl-sci |
Glancing-Incidence Focussed Ion Beam Milling: A Coherent X-ray
Diffraction Study of 3D Nano-scale Lattice Strains and Crystal Defects: This study presents a detailed examination of the lattice distortions
introduced by glancing incidence Focussed Ion Beam (FIB) milling. Using
non-destructive multi-reflection Bragg coherent X-ray diffraction we probe
damage formation in an initially pristine gold micro-crystal following several
stages of FIB milling. These experiments allow access to the full lattice
strain tensor in the micro-crystal with ~25 nm 3D spatial resolution, enabling
a nano-scale analysis of residual lattice strains and defects formed. Our
results show that 30 keV glancing incidence milling produces fewer large
defects than normal incidence milling at the same energy. However the resulting
residual lattice strains have similar magnitude and extend up to ~50 nm into
the sample. At the edges of the milled surface, where the ion-beam tails impact
the sample at near-normal incidence, large dislocation loops with a range of
burgers vectors are formed. Further glancing incidence FIB polishing with 5 keV
ion energy removes these dislocation loops and reduces the lattice strains
caused by higher energy FIB milling. However, even at the lower ion energy,
damage-induced lattice strains are present within a ~20 nm thick surface layer.
These results highlight the need for careful consideration and management of
FIB damage. They also show that low-energy FIB-milling is an effective tool for
removing FIB-milling induced lattice strains. This is important for the
preparation of micro-mechanical test specimens and strain microscopy samples. | cond-mat_mtrl-sci |
Hybrid exchange-correlation functional for accurate prediction of the
electronic and structural properties of ferroelectric oxides: Using a linear combination of atomic orbitals approach, we report a
systematic comparison of various Density Functional Theory (DFT) and hybrid
exchange-correlation functionals for the prediction of the electronic and
structural properties of prototypical ferroelectric oxides. It is found that
none of the available functionals is able to provide, at the same time,
accurate electronic and structural properties of the cubic and tetragonal
phases of BaTiO$_3$ and PbTiO$_3$. Some, although not all, usual DFT
functionals predict the structure with acceptable accuracy, but always
underestimate the electronic band gaps. Conversely, common hybrid functionals
yield an improved description of the band gaps, but overestimate the volume and
atomic distortions associated to ferroelectricity, giving rise to an
unacceptably large $c/a$ ratio for the tetragonal phases of both compounds.
This super-tetragonality is found to be induced mainly by the exchange energy
corresponding to the Generalized Gradient Approximation (GGA) and, to a lesser
extent, by the exact exchange term of the hybrid functional. We thus propose an
alternative functional that mixes exact exchange with the recently proposed GGA
of Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] which, for solids, improves
over the treatment of exchange of the most usual GGA's. The new functional
renders an accurate description of both the structural and electronic
properties of typical ferroelectric oxides. | cond-mat_mtrl-sci |
Modelling the Nonlinear Response of Fibre-reinforced Bending Fluidic
Actuators: Soft actuators are receiving increasing attention from the engineering
community, not only in research but even for industrial applications. Among
soft actuators, fibre-reinforced Bending Fluidic Actuators (BFAs) became very
popular thanks to features such as robustness and easy design and fabrication.
However, an accurate modelling of these smart structures, taking into account
all the nonlinearities involved, is a challenging task. In this effort, we
propose an analytical mechanical model to capture the quasi-static response of
fibre-reinforced BFAs. The model is fully 3D and for the first time includes
the effect of the pressure on the lateral surface of the chamber as well as the
non-constant torque produced by the pressure at the tip. The presented model
can be used for design and control, while providing information about the
mechanics of these complex actuators. | cond-mat_mtrl-sci |
Spatially Heterogeneous Dynamics in a Metallic Glass Forming Liquid
Imaged by Electron Correlation Microscopy: Supercooled liquids exhibit spatial heterogeneity in the dynamics of their
fluctuating atomic arrangements. The length and time scales of the
heterogeneous dynamics are central to the glass transition and influence
nucleation and growth of crystals from the liquid. We report direct
experimental visualization of the spatially heterogeneous dynamics as a
function of temperature in the supercooled liquid state of a Pt-based metallic
glass, using electron correlation microscopy with sub-nanometer resolution. An
experimental four point space-time intensity correlation function demonstrates
a growing dynamic correlation length, $\xi$, upon cooling of the liquid toward
the glass transition temperature. $\xi$ as a function of the relaxation time
$\tau$ data are in the good agreement with the Adam-Gibbs, inhomogeneous mode
coupling theory and random first order transition theory of the glass
transition. The same experiments demonstrate the existence of a nanometer
thickness near surface layer with order of magnitude shorter relaxation time
than inside the bulk. | cond-mat_mtrl-sci |
Gigahertz Dielectric Polarization of Single-atom Niobium Substituted in
Graphitic layers: We have synthesized two Nb@C composites with an order of magnitude difference
in the density of single-atom niobium substituted into graphitic layers. The
concentration and sites of single-atom Nb are identified using
aberration-corrected scanning transmission electron microscopy and density
functional theory. Comparing the complex permittivity spectra show that the
representative dielectric resonance at ~16 GHz originates from the intrinsic
polarization of single-atom Nb sites, confirmed by theoretical simulations. The
single-atom dielectric resonance represents the physical limit of the
electromagnetic response of condensed matter, and thus might open up a new
avenue for designing electromagnetic wave absorption materials. Single-atom
resonance also has important implications in understanding the correlation
between the macroscopic dielectric behaviors and the atomic-scale structural
origin. | cond-mat_mtrl-sci |
Failure time in the fiber-bundle model with thermal noise and disorder: The average time for the onset of macroscopic fractures is analytically and
numerically investigated in the fiber-bundle model with quenched disorder and
thermal noise under a constant load. We find an implicit exact expression for
the failure time in the low-temperature limit that is accurately confirmed by
direct simulations. The effect of the disorder is to lower the energy barrier. | cond-mat_mtrl-sci |
Electron-hole versus exciton delocalization in conjugated polymers: the
role of topology: There is currently a great need for solid state lasers that emit in the
infrared. Whether or not conjugated polymers that emit in the IR can be
synthesized is an interesting theoretical challenge. We show that the
requirement for such a material is that the exciton delocalization in the
system be large, such that the optical gap is small. We develop a theory of
exciton delocalization in conjugated polymers, and show that the extent of this
can be predicted from the topology of the conjugated polymer in question. We
determine the precise structural characteristics that would be necessary for
light emission in the IR. | cond-mat_mtrl-sci |
Structural Transitions at Ionic Liquids Interfaces: Recent advances in experimental and computational techniques have allowed for
an accurate description of the adsorption of ionic liquids on metallic
electrodes. It is now well established that they adopt a multi-layered
structure, and that the composition of the layers changes with the potential of
the electrode. In some cases, potential-driven ordering transitions in the
first adsorbed layer have been observed in experiments probing the interface on
the molecular scale or by molecular simulations. This perspective gives an
overview of the current understanding of such transitions and of their
potential impact on the physical and (electro)chemical processes at the
interface. In particular, peaks in the differential capacitance, slow dynamics
at the interface and changes in the reactivity have been reported in
electrochemical studies. Interfaces between ionic liquids and metallic
electrodes are also highly relevant for their friction properties, the
voltage-dependence of which opens the way to exciting applications. | cond-mat_mtrl-sci |
First principles calculations of the interface properties of
amorphous-Al2O3/MoS2 under non-strain and biaxial strain conditions: Al2O3 is a potential dielectric material for metal-oxide-semiconductor (MOS)
devices. Al2O3 films deposited on semiconductors usually exhibit amorphous due
to lattice mismatch. Compared to two-dimensional graphene, MoS2 is a typical
semiconductor, therefore, it has more extensive application. The
amorphous-Al2O3/MoS2 (a-Al2O3/MoS2) interface has attracted people's attention
because of its unique properties. In this paper, the interface behaviors of
a-Al2O3/MoS2 under non-strain and biaxial strain are investigated by first
principles calculations based on density functional theory (DFT). First of all,
the generation process of a-Al2O3 sample is described, which is calculated by
molecular dynamics and geometric optimization. Then, we introduce the band
alignment method, and calculate band offset of a-Al2O3/MoS2 interface. It is
found that the valence band offset (VBO) and conduction band offset (CBO)
change with the number of MoS2 layers. The dependence of leakage current on the
band offset is also illustrated. At last, the band structure of monolayer MoS2
under biaxial strain is discussed. The biaxial strain is set in the range from
-6% to 6% with the interval of 2%. Impact of the biaxial strain on the band
alignment is investigated. | cond-mat_mtrl-sci |
Magneto-transport and magneto-optical properties of ferromagnetic
(III,Mn)V semicondcutors: a review: Rapid developments in material research of metallic ferromagnetic (III,Mn)V
semiconductors over the past few years have brought a much better understanding
of these complex materials. We review here some of the main developments and
current understanding of the bulk properties of these systems within the
metallic regime, focusing principally on the magneto-transport and
magneto-optical properties. Although several theoretical approaches are
reviewed, the bulk of the review uses the effective Hamiltonian approach, which
has proven useful in describing many of these properties namely in (Ga,Mn)As
and (In,Mn)As. The model assumes a ferromagnetic coupling between Mn d-shell
local moments mediated by holes in the semiconductor valence band. | cond-mat_mtrl-sci |
Theory of structural response to macroscopic electric fields in
ferroelectric systems: We have developed and implemented a formalism for computing the structural
response of a periodic insulating system to a homogeneous static electric field
within density-functional perturbation theory (DFPT). We consider the
thermodynamic potentials E(R,eta,e) and F(R,eta,e) whose minimization with
respect to the internal structural parameters R and unit cell strain eta yields
the equilibrium structure at fixed electric field e and polarization P,
respectively. First-order expansion of E(R,eta,e) in e leads to a useful
approximation in which R(P) and eta(P) can be obtained by simply minimizing the
zero-field internal energy with respect to structural coordinates subject to
the constraint of a fixed spontaneous polarization P. To facilitate this
minimization, we formulate a modified DFPT scheme such that the computed
derivatives of the polarization are consistent with the discretized form of the
Berry-phase expression. We then describe the application of this approach to
several problems associated with bulk and short-period superlattice structures
of ferroelectric materials such as BaTiO3 and PbTiO3. These include the effects
of compositionally broken inversion symmetry, the equilibrium structure for
high values of polarization, field-induced structural phase transitions, and
the lattice contributions to the linear and the non-linear dielectric
constants. | cond-mat_mtrl-sci |
Anomalies in non-stoichiometric uranium dioxide induced by pseudo-phase
transition of point defects: A uniform distribution of point defects in an otherwise perfect
crystallographic structure usually describes a unique pseudo phase of that
state of a non-stoichiometric material. With off-stoichiometric uranium dioxide
as a prototype, we show that analogous to a conventional phase transition,
these pseudo phases also will transform from one state into another via
changing the predominant defect species when external conditions of pressure,
temperature, or chemical composition are varied. This exotic transition is
numerically observed along shock Hugoniots and isothermal compression curves in
UO2 with first-principles calculations. At low temperatures, it leads to
anomalies (or quasi-discontinuities) in thermodynamic properties and electronic
structures. In particular, the anomaly is pronounced in both shock temperature
and the specific heat at constant pressure. With increasing of the temperature,
however, it transforms gradually to a smooth cross-over, and becomes less
discernible. The underlying physical mechanism and characteristics of this type
of transition are encoded in the Gibbs free energy, and are elucidated clearly
by analyzing the correlation with the variation of defect populations as a
function of pressure and temperature. The opportunities and challenges for a
possible experimental observation of this phase change are also discussed. | cond-mat_mtrl-sci |
Theory of self-diffusion in GaAs: Ab initio molecular dynamics simulations are employed to investigate the
dominant migration mechanism of the gallium vacancy in gaas as well as to
assess its free energy of formation and the rate constant of gallium
self-diffusion. our analysis suggests that the vacancy migrates by second
nearest neighbour hops. the calculated self-diffusion constant is in good
agreement with the experimental value obtained in ^69 GaAs/ ^71 GaAs isotope
heterostructures and at significant variance with that obtained earlier from
interdiffusion experiments in GaAlAs/GaAs-heterostructures. | cond-mat_mtrl-sci |
Superconductivity in intercalated buckled two-dimensional materials:
KGe$_2$: Germanene has emerged as a novel two-dimensional material with various
interesting properties and applications. Here we report the possibility of
superconductivity in a stable potassium intercalated germanene compound,
KGe$_2$, with a transition temperature $T_c \sim 11$ K, and an electron-phonon
coupling of 1.9. Applying a 5\% tensile strain, which reduces the buckling
height by 4.5\%, leads to the reduction of the electron-phonon coupling by 11\%
and a slight increase in $T_c \sim 12$ K. That is, strong electron-phonon
coupling results from the buckled structure of the germanene layers. Despite
being an intercalated van der Waals material similar to intercalated graphite
superconductors, it does not possess an occupied interlayer state. | cond-mat_mtrl-sci |
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