98 research outputs found
Theory of hypothetical ferroelectric superlattices incorporating head-to-head and tail-to-tail 180 domain walls
While electrical compatibility constraints normally prevent head-to-head (HH)
and tail-to-tail (TT) domain walls from forming in ferroelectric materials, we
propose that such domain walls could be stabilized by intentional growth of
atomic layers in which the cations are substituted from a neighboring column of
the periodic table. In particular, we carry out predictive first-principles
calculations of superlattices in which Sc, Nb, or other substitutional layers
are inserted periodically into PbTiO. We confirm that this gives rise to a
domain structure with the longitudinal component of the polarization
alternating from domain to domain, and with the substitutional layers serving
as HH and TT domain walls. We also find that a substantial transverse component
of the polarization can also be present.Comment: 5 pages, 4 figure
Role of quantum nuclei and local fields in the x-ray absorption spectra of water and ice
We calculate the x-ray absorption spectra of liquid water at ambient
conditions and of hexagonal ice close to melting, using a static GW approach
that includes approximately local field effects. Quantum dynamics of the nuclei
is taken into account by averaging the absorption cross section over molecular
configurations generated by path integral simulations. We find that inclusion
of quantum disorder is essential to bring the calculated spectra in close
agreement with experiment. In particular, the intensity of the pre-edge
feature, a spectral signature of broken and distorted hydrogen bonds, is
accurately reproduced, in water and ice, only when quantum nuclei are
considered. The effect of the local fields is less important but non
negligible, particularly in ice
Interface enhancement of ferroelectricity in CaTiO/BaTiO superlattices
We carry out first-principles calculations for CaTiO/BaTiO
superlattices with epitaxial strain corresponding to growth on a SrTiO
substrate, and consider octahedral rotations as well as ferroelectric
distortions. The calculations are done as a function of electric displacement
field, and both a macroscopic and a local electrostatic analysis are carried
out. We find that strong octahedral rotations occur for TiO octahedra
sandwiched between CaO layers on both sides, but are strongly suppressed if
either neighboring layer is a BaO layer. Due to the resulting enhancement of
the ferroelectric instability in the BaO-neighboring octahedra, we find that
overall the ferroelectric instability of the superlattice is enhanced by the
interface. Thus, short-period superlattices in this system have a larger
ferroelectric polarization than longer-period ones of the same average
composition, contrary to the expected trend.Comment: 2 figures, 3 table
Modeling functional piezoelectricity in perovskite superlattices with competing instabilities
Based on the locality principle of insulating superlattices, we apply the
method of Wu {\it et al} [Phys. Rev. Letter {\bf 101}, 087610 (2008)] to the
piezoelectric strains of individual layers under fixed displacement field. For
a superlattice of arbitrary stacking sequence an accurate model is acquired for
predicting piezoelectricity. By applying the model in the superlattices where
ferroelectric and antiferrodistortive modes are in competition, functional
piezoelectricity can be achieved. A strong nonlinear effect is observed and can
be further engineered in the PbTiO/SrTiO superlattice and an interface
enhancement of piezoelectricity is found in the BaTiO/CaTiO
superlattice.Comment: 5 pages, 4 figure
Electronic origin of spin-phonon coupling effect in transition-metal perovskites
By applying Wannier-based extended Kugel-Khomskii model, we carry out
first-principles calculations and electronic structure analysis to understand
the spin-phonon coupling effect in transition-metal perovskites. We demonstrate
the successful application of our approach to SrMnO and BiFeO. We show
that both the electron orbitals under crystal field splitting and the
electronic configuration should be taken into account in order to understand
the large variances of spin-phonon coupling effects among various phonon modes
as well as in different materials.Comment: 5 pages, 1 figur
Wannier-based definition of layer polarizations in perovskite superlattices
In insulators, the method of Marzari and Vanderbilt [Phys. Rev. B {\bf 56},
12847 (1997)] can be used to generate maximally localized Wannier functions
whose centers are related to the electronic polarization. In the case of
layered insulators, this approach can be adapted to provide a natural
definition of the local polarization associated with each layer, based on the
locations of the nuclear charges and one-dimensional Wannier centers comprising
each layer. Here, we use this approach to compute and analyze layer
polarizations of ferroelectric perovskite superlattices, including changes in
layer polarizations induced by sublattice displacements (i.e., layer-decomposed
Born effective charges) and local symmetry breaking at the interfaces. The
method provides a powerful tool for analyzing the polarization-related
properties of complex layered oxide systems
Predicting polarization and nonlinear dielectric response of arbitrary perovskite superlattice sequences
We carry out first-principles calculations of the nonlinear dielectric
response of short-period ferroelectric superlattices. We compute and store not
only the total polarization, but also the Wannier-based polarizations of
individual atomic layers, as a function of electric displacement field, and use
this information to construct a model capable of predicting the nonlinear
dielectric response of an arbitrary superlattice sequence. We demonstrate the
successful application of our approach to superlattices composed of SrTiO,
CaTiO, and BaTiO layers.Comment: 5 pages, 4 figures, 2 table
Exploring the Impact of Ions on Oxygen K-Edge X-ray Absorption Spectroscopy in NaCl Solution using the GW-Bethe-Salpeter-Equation Approach
X-ray absorption spectroscopy (XAS) is a powerful experimental tool to probe
the local structure in materials with the core hole excitations. Here, the
oxygen K-edge XAS spectra of the NaCl solution and pure water are computed by
using a recently developed GW-BSE approach, based on configurations modeled by
path-integral molecular dynamics with the deep-learning technique. The neural
network is trained on ab initio data obtained with SCAN density functional
theory. The observed changes in the XAS features of the NaCl solution, compared
to those of pure water, are in good agreement between experimental and
theoretical results. We provided detailed explanations for these spectral
changes that occur when NaCl is solvated in pure water. Specifically, the
presence of solvating ion pairs leads to localization of electron-hole
excitons. Our theoretical XAS results support the theory that the effects of
the solvating ions on the H-bond network are mainly confined within the first
hydration shell of ions, however beyond the shell the arrangement of water
molecules remains to be comparable to that observed in pure water.Comment: 18 pages, 4 figure
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