356 research outputs found
A First-Principles Study of the Electronic Reconstructions of LaAlO3/SrTiO3 Heterointerfaces and Their Variants
We present a first-principles study of the electronic structures and
properties of ideal (atomically sharp) LaAlO3/SrTiO3 (001) heterointerfaces and
their variants such as a new class of quantum well systems. We demonstrate the
insulating-to-metallic transition as a function of the LaAlO3 film thickness in
these systems. After the phase transition, we find that conduction electrons
are bound to the n-type interface while holes diffuse away from the p-type
interface, and we explain this asymmetry in terms of a large hopping matrix
element that is unique to the n-type interface. We build a tight-binding model
based on these hopping matrix elements to illustrate how the conduction
electron gas is bound to the n-type interface. Based on the `polar catastrophe'
mechanism, we propose a new class of quantum wells at which we can manually
control the spatial extent of the conduction electron gas. In addition, we
develop a continuous model to unify the LaAlO3/SrTiO3 interfaces and quantum
wells and predict the thickness dependence of sheet carrier densities of these
systems. Finally, we study the external field effect on both LaAlO3/SrTiO3
interfaces and quantum well systems. Our systematic study of the electronic
reconstruction of LaAlO3/SrTiO3 interfaces may serve as a guide to engineering
transition metal oxide heterointerfaces.Comment: 50 pages, 18 figures and 4 table
Phenomenological theory of phase transitions in highly piezoelectric perovskites
Recently discovered fine structure of the morphotropic phase boundaries in
highly piezoelectric mixture compounds PZT, PMN-PT, and PZN-PT demonstrates the
importance of highly non-linear interactions in these systems. We show that an
adequate Landau-type description of the ferroelectric phase transitions in
these compounds is achieved by the use of a twelfth-order expansion of the
Landau potential in terms of the phenomenological order parameter.
Group-theoretical and catastrophe-theory methods are used in constructing the
appropriate Landau potential. A complete phase diagram is calculated in
phenomenological parameter space. The theory describes both PZT and PZN-PT
types of phase diagrams, including the newly found monoclinic and orthorhombic
phases. Anomalously large piezoelectric coefficients are predicted in the
vicinity of the phase transition lines.Comment: RevTex4, 8 pages, 2 figures. Dramatically changed after referees'
Comments, to appear in Phys. Rev. B, 1 April 200
Tunability of the dielectric response of epitaxially strained SrTiO3 from first principles
The effect of in-plane strain on the nonlinear dielectric properties of
SrTiO3 epitaxial thin films is calculated using density-functional theory
within the local-density approximation. Motivated by recent experiments, the
structure, zone-center phonons, and dielectric properties with and without an
external electric field are evaluated for several misfit strains within +-3% of
the calculated cubic lattice parameter. In these calculations, the in-plane
lattice parameters are fixed, and all remaining structural parameters are
permitted to relax. The presence of an external bias is treated approximately
by applying a force to each ion proportional to the electric field. After
obtaining zero-field ground state structures for various strains, the
zone-center phonon frequencies and Born effective charges are computed,
yielding the zero-field dielectric response. The dielectric response at finite
electric field bias is obtained by computing the field dependence of the
structure and polarization using an approximate technique. The results are
compared with recent experiments and a previous phenomenological theory. The
tunability is found to be strongly dependent on the in-plane lattice parameter,
showing markedly different behavior for tensile and compressive strains. Our
results are expected to be of use for isolating the role of strain in the
tunability of real ultrathin epitaxial films.Comment: 11 pages, with postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/ant_srti/index.htm
High frequency polarization switching of a thin ferroelectric film
We consider both experimentally and analytically the transient oscillatory
process that arises when a rapid change in voltage is applied to a
ferroelectric thin film deposited on an substrate.
High frequency () polarization oscillations are observed
in the ferroelectric sample. These can be understood using a simple
field-polarization model. In particular we obtain analytic expressions for the
oscillation frequency and the decay time of the polarization fluctuation in
terms of the material parameters. These estimations agree well with the
experimental results
Ab-initio design of perovskite alloys with predetermined properties: The case of Pb(Sc_{0.5} Nb_{0.5})O_{3}
A first-principles derived approach is combined with the inverse Monte Carlo
technique to determine the atomic orderings leading to prefixed properties in
Pb(Sc_{0.5}Nb_{0.5})O_{3} perovskite alloy. We find that some arrangements
between Sc and Nb atoms result in drastic changes with respect to the
disordered material, including ground states of new symmetries, large
enhancement of electromechanical responses, and considerable shift of the Curie
temperature. We discuss the microscopic mechanisms responsible for these
unusual effects.Comment: 5 pages with 2 postscript figures embedde
Monte Carlo Study of Relaxor Systems: A Minimum Model for Pb(InNb)O}
We examine a simple model for Pb(InNb)O (PIN), which
includes both long-range dipole-dipole interaction and random local anisotropy.
A improved algorithm optimized for long-range interaction has been applied for
efficient large-scale Monte Carlo simulation. We demonstrate that the phase
diagram of PIN is qualitatively reproduced by this minimum model. Some
properties characteristic of relaxors such as nano-scale domain formation, slow
dynamics and dispersive dielectric responses are also examined.Comment: 5 pages, 4 figure
Second harmonic generation and X-ray diffraction studies of pretransitional region in relaxor K(1-x) LixTaO3
Optical second-harmonic-generation(SHG) observations and precise X-ray
diffraction experiments have been performed on quantum paraelectrics KTaO3
(KTO) and relaxor K(1-x)LixTaO3 with x=3% (KLT-3) and 7% (KLT-7). It is found
in KLT-3 and KLT-7 that a pretransitional region exists between two
characteristic temperatures Td and Tp (<Td). The average symmetry of the region
is tetragonal with a weak lattice-deformation but non-polar in average. The
temperature interval between Td and Tp is consistent with the interval on which
neutron diffuse scatterings have been previously reported. These facts strongly
suggest that polar micro-regions (PMRs) nucleate around Td and grow toward Tp.
Below Tp, a larger deformation and a field-induced SH intensity start to
develop, while no significant SHG appear in zero-field cooling process. The
temperature dependence of the SH intensity below Tp coincides well with that of
the tetragonality determined from the lattice deformation. The
Landau-Devonshire phenomenological approach suggests that the ferroelectric
phase transition at Tp is of first order and that it approaches the second
order transition with the decrease of Li concentration. A marked increase of
neutron diffraction intensities below Tp indicates that PMRs are transformed to
ferroelectric micro-domains at Tp, and the micro-domains change to macroscopic
ones under the electric field below Tp.Comment: 31 pages, 10 figure
The polarizability model for ferroelectricity in perovskite oxides
This article reviews the polarizability model and its applications to
ferroelectric perovskite oxides. The motivation for the introduction of the
model is discussed and nonlinear oxygen ion polarizability effects and their
lattice dynamical implementation outlined. While a large part of this work is
dedicated to results obtained within the self-consistent-phonon approximation
(SPA), also nonlinear solutions of the model are handled which are of interest
to the physics of relaxor ferroelectrics, domain wall motions, incommensurate
phase transitions. The main emphasis is to compare the results of the model
with experimental data and to predict novel phenomena.Comment: 55 pages, 35 figure
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