430 research outputs found
Ferroelectric Dead Layer Driven by a Polar Interface
Based on first-principles and model calculations we investigate the effect of
polar interfaces on the ferroelectric stability of thin-film ferroelectrics. As
a representative model, we consider a TiO2-terminated BaTiO3 film with LaO
monolayers at the two interfaces that serve as doping layers. We find that the
polar interfaces create an intrinsic electric field that is screened by the
electron charge leaking into the BaTiO3 layer. The amount of the leaking charge
is controlled by the boundary conditions which are different for three
heterostructures considered, namely Vacuum/LaO/BaTiO3/LaO, LaO/BaTiO3, and
SrRuO3/LaO/BaTiO3/LaO. The intrinsic electric field forces ionic displacements
in BaTiO3 to produce the electric polarization directed into the interior of
the BaTiO3 layer. This creates a ferroelectric dead layer near the interfaces
that is non-switchable and thus detrimental to ferroelectricity. Our
first-principles and model calculations demonstrate that the effect is stronger
for a larger effective ionic charge at the interface and longer screening
length due to a stronger intrinsic electric field that penetrates deeper into
the ferroelectric. The predicted mechanism for a ferroelectric dead layer at
the interface controls the critical thickness for ferroelectricity in systems
with polar interfaces.Comment: 33 Pages, 5 figure
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
Concentration phase diagram of Ba(x)Sr(1-x)TiO3 solid solutions
Method of derivation of phenomenological thermodynamic potential of solid
solutions is proposed in which the interaction of the order parameters of
constituents is introduced through the account of elastic strain due to misfit
of the lattice parameters of the end-members. The validity of the method is
demonstrated for Ba(x)Sr(1-x)TiO3 system being a typical example of
ferroelectric solid solution. Its phase diagram is determined using
experimental data for the coefficients in the phenomenological potentials of
SrTiO3 and BaTiO3. In the phase diagram of the Ba(x)Sr(1-x)TiO3 system for
small Ba concentration, there are a tricritical point and two multiphase points
one of which is associated with up to 6 possible phases.Comment: 8 pages, 3 figure
Nanoscale piezoelectric response across a single antiparallel ferroelectric domain wall
Surprising asymmetry in the local electromechanical response across a single
antiparallel ferroelectric domain wall is reported. Piezoelectric force
microscopy is used to investigate both the in-plane and out-of- plane
electromechanical signals around domain walls in congruent and
near-stoichiometric lithium niobate. The observed asymmetry is shown to have a
strong correlation to crystal stoichiometry, suggesting defect-domain wall
interactions. A defect-dipole model is proposed. Finite element method is used
to simulate the electromechanical processes at the wall and reconstruct the
images. For the near-stoichiometric composition, good agreement is found in
both form and magnitude. Some discrepancy remains between the experimental and
modeling widths of the imaged effects across a wall. This is analyzed from the
perspective of possible electrostatic contributions to the imaging process, as
well as local changes in the material properties in the vicinity of the wall
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
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.Comment: 19 pages, with 15 postscript figures embedded. Uses REVTEX4 and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/sai_pol/index.htm
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
- …