172 research outputs found
Unusual polarization patterns in flat epitaxial ferroelectric nanoparticles
Interest in epitaxial ferroelectric nanoislands was growing rapidly in recent
years driven by their potential for devices, especially ultradense memories.
Recent advances in the "bottom- up" (self-assembly) nanometer scale techniques
have opened up the opportunities of fabricating high-quality epitaxial
ferroelectric nanoislands with extremely small thickness and lateral size on
the order of 1 nm and 20 nm, respectively. On the other hand, recent emergence
of powerful probes, such as piezoresponse force microscopy (PFM), has enabled
imaging of a local domain structure with sub-10 nm resolution. In spite of
those developments, a clear understanding of the polarization patterns in
epitaxial ferroelectric nanoislands is lacking, and some important
characteristics, like a critical lateral size for ferroelectricity, are not yet
established. Here, we perform ab-initio studies of non-electroded epitaxial
Pb(Zr0.5Ti0.5)O3 and BaTiO3 nanoislands and show the existence of novel
polarization patterns driven by the misfit strains and/or anisotropy energy.
The results allow interpretation of the data and design of the ferroelectric
nanostructures with tailored response to external field.Comment: 7 pages, 7 figure
Vortex-to-Polarization Phase Transformation Path in Pb(ZrTi)O Nanoparticles
Phase transformation in finite-size ferroelectrics is of fundamental
relevance for understanding collective behaviors and balance of competing
interactions in low-dimensional systems. We report a first-principles effective
Hamiltonian study of vortex-to-polarization transformation in
Pb(ZrTi)O nanoparticles, caused by homogeneous electric
fields normal to the vortex plane. The transformation is shown to (1) follow an
unusual {\it macroscopic} path that is symmetry non-conforming and
characterized by the occurrence of a previously unknown structure as the
bridging phase; (2) lead to the discovery of a striking collective phenomenon,
revealing how ferroelectric vortex is annihilated {\it microscopically}.
Interactions underlying these behaviors are discussed
Strict baselines for Covid-19 forecasting and ML perspective for USA and Russia
Currently, the evolution of Covid-19 allows researchers to gather the
datasets accumulated over 2 years and to use them in predictive analysis. In
turn, this makes it possible to assess the efficiency potential of more complex
predictive models, including neural networks with different forecast horizons.
In this paper, we present the results of a consistent comparative study of
different types of methods for predicting the dynamics of the spread of
Covid-19 based on regional data for two countries: the United States and
Russia. We used well-known statistical methods (e.g., Exponential Smoothing), a
"tomorrow-as-today" approach, as well as a set of classic machine learning
models trained on data from individual regions. Along with them, a neural
network model based on Long short-term memory (LSTM) layers was considered, the
training samples of which aggregate data from all regions of two countries: the
United States and Russia. Efficiency evaluation was carried out using
cross-validation according to the MAPE metric. It is shown that for complicated
periods characterized by a large increase in the number of confirmed daily
cases, the best results are shown by the LSTM model trained on all regions of
both countries, showing an average Mean Absolute Percentage Error (MAPE) of
18%, 30%, 37% for Russia and 31%, 41%, 50% for US for predictions at forecast
horizons of 14, 28, and 42 days, respectively
Plasmonic resonances and hot spots in Ag octopods
New type of plasmonic nanoparticles - silver octopods that can be synthesized
with a variety of shapes - have been demonstrated to show versatile optical
response using the discrete dipole approximation. The octopods show a complex
behavior at optical (visible, IR) wavelengths, with three major resonances that
can be tuned up to a desired response that makes them especially attractive to
use in e.g. high-performance surface enhanced Raman (SERS) detectors. The
excited resonant modes strongly depend on the geometrical parameters of the
stars, while dependence on their orientation with respect to an incident
radiation is moderate, owing to cubic symmetry. The field "hot spots" are
formed with the local field enhancement up to 50 times compared to an incident
field. They are usually localized at the surface between the arms and may be
both "electric" and "magnetic". While the former are of primary importance for
SERS, the latter may be identified by trapping magnetic nanoparticles in their
vicinity. The results are in very good agreement with the data where available
and may be used as a type of a "shape spectroscopy" for the nanoparticles.Comment: 12 pages. 6 figure
Ferroelectric nanostructure having switchable multi-stable vortex states
A ferroelectric nanostructure formed as a low dimensional nanoscale ferroelectric material having at least one vortex ring of polarization generating an ordered toroid moment switchable between multi-stable states. A stress-free ferroelectric nanodot under open-circuit-like electrical boundary conditions maintains such a vortex structure for their local dipoles when subject to a transverse inhomogeneous static electric field controlling the direction of the macroscopic toroidal moment. Stress is also capable of controlling the vortex\u27s chirality because of the electromechanical coupling that exists in ferroelectric nanodots
Spintronic Quantum Phase Transition in a Heterostructure with Giant Rashba Spin-Orbit Coupling
Mechanical stacking of two dissimilar materials often has surprising
consequences for heterostructure behavior. In particular, a two-dimensional
electron gas (2DEG) is formed in the heterostructure of the topological
crystalline insulator Pb0.24Sn0.76Te and graphene due to contact of a polar
with a nonpolar surface and the resulting changes in electronic structure
needed to avoid polar catastrophe. We study the spintronic properties of this
heterostructure with non-local spin valve devices. We observe spin-momentum
locking at lower temperatures that transitions to regular spin channel
transport only at ~40 K. Hanle spin precession measurements show a spin
relaxation time as high as 2.18 ns. Density functional theory calculations
confirm that the spin-momentum locking is due to a giant Rashba effect in the
material and that the phase transition is a Lifshitz transition. The
theoretically predicted Lifshitz transition is further evident in the phase
transition-like behavior in the Land\'e g-factor and spin relaxation time.Comment: 33 pages, 17 figures, supplemental information include
Charge separation relative to the reaction plane in Pb-Pb collisions at TeV
Measurements of charge dependent azimuthal correlations with the ALICE
detector at the LHC are reported for Pb-Pb collisions at TeV. Two- and three-particle charge-dependent azimuthal correlations in
the pseudo-rapidity range are presented as a function of the
collision centrality, particle separation in pseudo-rapidity, and transverse
momentum. A clear signal compatible with a charge-dependent separation relative
to the reaction plane is observed, which shows little or no collision energy
dependence when compared to measurements at RHIC energies. This provides a new
insight for understanding the nature of the charge dependent azimuthal
correlations observed at RHIC and LHC energies.Comment: 12 pages, 3 captioned figures, authors from page 2 to 6, published
version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/286
A note on comonotonicity and positivity of the control components of decoupled quadratic FBSDE
In this small note we are concerned with the solution of Forward-Backward
Stochastic Differential Equations (FBSDE) with drivers that grow quadratically
in the control component (quadratic growth FBSDE or qgFBSDE). The main theorem
is a comparison result that allows comparing componentwise the signs of the
control processes of two different qgFBSDE. As a byproduct one obtains
conditions that allow establishing the positivity of the control process.Comment: accepted for publicatio
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