31 research outputs found
Ferroelectric Phase Transitions in Films with Depletion Charge
We consider ferroelectric phase transitions in both short-circuited and
biased ferroelectric-semiconductor films with a space (depletion) charge which
leads to some unusual behavior. It is shown that in the presence of the charge
the polarization separates into `switchable' and `non-switchable' parts. The
electric field, appearing due to the space charge, does not wash out the phase
transition, which remains second order but takes place at somewhat reduced
temperature. At the same time, it leads to a suppression of the
ferroelectricity in a near-electrode layer. This conclusion is valid for
materials with both second and first order phase transitions in pure bulk
samples. Influence of the depletion charge on thermodynamic coercive field
reduces mainly to the lowering of the phase transition temperature, and its
effect is negligible. The depletion charge can, however, facilitate an
appearance of the domain structure which would be detrimental for device
performance (fatigue). We discuss some issues of conceptual character, which
are generally known but were overlooked in previous works. The present results
have general implications for small systems with depletion charge.Comment: 11 pages, REVTeX 3.1, five eps-figures included in the text. Minor
clarifications in the text. To appear in Phys. Rev. B 61, Apr 1 (2000
Very large dielectric response of thin ferroelectric films with the dead layers
We study the dielectric response of ferroelectric (FE) thin films with "dead"
dielectric layer at the interface with electrodes. The domain structure
inevitably forms in the FE film in presence of the dead layer. As a result, the
effective dielectric constant of the capacitor increases
abruptly when the dead layer is thin and, consequently, the pattern of
180-degree domains becomes "soft". We compare the exact results for this
problem with the description in terms of a popular "capacitor" model, which is
shown to give qualitatively incorrect results. We relate the present results to
fatigue observed in thin ferroelectric films.Comment: 5 pages, REVTeX 3.1 with one eps-figure. A note added that the linear
response is not changed by electromechanical effect. To appear in Phys. Rev.
The broad Brillouin doublets and central peak of KTaO_3
The incipient ferroelectric KTaO3 presents low-T Brillouin spectra
anomalies,e.g. a broad central peak (CP), and some additional Brillouin
doublets (BD), whose origin is interpreted in terms of phonon-density
fluctuation processes. A parameterisation from new extensive high-resolution
neutron-scattering measurements is used to show that hydrodynamic second sound
from high damping (compared to BD frequency) TA phonons may exist in the
crystal. Furthermore, low damping thermal phonons may scatter light through
two-phonon difference processes and appear on the Brillouin spectra either as a
sharp or a broader BD, depending on the phonon damping and group velocity . The
comparison between computed anisotropies and experimental measurements favours
the second process.Comment: 3 pages, 1 figure, ECNS99 Proceedings. See http://www.ill.fr
A first-principles study of oxygen vacancy pinning of domain walls in PbTiO3
We have investigated the interaction of oxygen vacancies and 180-degree
domain walls in tetragonal PbTiO3 using density-functional theory. Our
calculations indicate that the vacancies do have a lower formation energy in
the domain wall than in the bulk, thereby confirming the tendency of these
defects to migrate to, and pin, the domain walls. The pinning energies are
reported for each of the three possible orientations of the original Ti-O-Ti
bonds, and attempts to model the results with simple continuum models are
discussed.Comment: 8 pages, with 3 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/lh_dw/index.htm
Nature of Sonoluminescence: Noble Gas Radiation Excited by Hot Electrons in "Cold" Water
We show that strong electric fields occurring in water near the surface of
collapsing gas bubbles because of the flexoelectric effect can provoke dynamic
electric breakdown in a micron-size region near the bubble and consider the
scenario of the SBSL. The scenario is: (i) at the last stage of incomplete
collapse of the bubble the gradient of pressure in water near the bubble
surface has such a value and sign that the electric field arising from the
flexoelectric effect exceeds the threshold field of the dynamic electrical
breakdown of water and is directed to the bubble center; (ii) mobile electrons
are generated because of thermal ionization of water molecules near the bubble
surface; (iii) these electrons are accelerated in ''cold'' water by the strong
electric fields; (iv) these hot electrons transfer noble gas atoms dissolved in
water to high-energy excited states and optical transitions between these
states produce SBSL UV flashes in the trasparency window of water; (v) the
breakdown can be repeated several times and the power and duration of the UV
flash are determined by the multiplicity of the breakdowns. The SBSL spectrum
is found to resemble a black-body spectrum where temperature is given by the
effective temperature of the hot electrons. The pulse energy and some other
characteristics of the SBSL are found to be in agreement with the experimental
data when realistic estimations are made.Comment: 11 pages (RevTex), 1 figure (.ps
Pseudoferroelectricity: A possible scenario for doped ZnO
A phenomenological Landau-theory model, where a crystal on cooling from a state with polar symmetry exhibits a maximum of dielectric permittivity and polarization-field hysteresis loops, is offered. The model is equivalent to that of a true ferroelectric in a weak external dc bias. Permittivity maximum occurs as a result of crossing the Widom line. In the model, the role of the external bias plays an intrinsic internal bias originating from a weak polarity of the high temperature state of the material. The results obtained show that the ferroelectriclike phenomena reported for doped ZnO are compatible with a Landau-theory description
Landau Expansion for Ferroelectrics: Which Variable to Use?
The problem of the choice of the order parameter of the Landau expansion of proper ferroelectrics is discussed. The expansion with respect to the (i) electric displacement, (ii) polarization, (iii) ferroelectric contribution to the polarization, (iv) "order" parameter within the pseudo-proper approach, and (v) soft-mode displacements within the weak-ferroelectric approach are compared. The following conclusions concerning applicability of these expansions are made. The description in terms of the electric displacement is only of historical interest. This approach may lead to essentially wrong conclusions while it has no advantage over approach (ii) in terms of the simplicity of the framework. Approaches (ii) to (v) provide progressively more reliable descriptions of properties of ferroelectrics, although they are progressively more complex as well. Depending on the situation addressed, approaches ( ii) to ( iv) may lead to erroneous results or may be unable to describe a certain effect. In case the depolarizing energy comes into play approach (ii) may fail while (iii)-(v) still work well. In materials with anomalously small Curie Weiss constants ("weak ferroelectrics" with anomalously small effective charge of the soft mode) the full description is provided by approach (v), while a partial description is possible within (ii) to (iv)
Novel Electromechanical Phenomena at the Nanoscale: Phenomenological Theory and Atomistic Modeling
In the past two decades, the fact that âsmall is differentâ has been established for a wide variety of phenomena, including electrical, optical, magnetic, and mechanical behavior of materials. However, one largely untapped but potentially very important area of nanoscience involves the interplay of electricity and mechanics at the nanoscale. In this article, predicated on both phenomenological approaches and atomistic calculations, we summarize the state-of-the-art in understanding electromechanical coupling at the nanoscale. First, we address flexoelectricityâthe coupling of strain gradient to polarization. Flexoelectricity exists in both piezoelectric and nonpiezoelectric dielectrics. As a high-order spatial-dispersion effect, the flexoelectricity becomes more and more important with the reduction of the spatial scale of the problem. Exploitation of this phenomenon and the associated nanoscale size effects can lead to tantalizing applications, such as âpiezoelectric nanocomposites without using piezoelectric materials.â The second issue concerns electromechanical effects at the dielectric/metal interface. An interface in solids typically exhibits a lower symmetry compared to that of the associated adhering materials. This symmetry reduction can drastically affect the electromechanical and dielectric behavior of the material at the nanoscale
Electrical tuning of dc bias induced acoustic resonances in paraelectric thin films
A theoretical model for the dc bias dependence of induced acoustic resonances in paraelectric materials is presented. The field dependent piezoelectric constitutive equations were derived from the Landau free energy expansion with respect to the ferroelectric part of the polarization P. To derive the field dependent coefficients correctly, we demonstrate that it is important to take both linear and nonlinear electrostriction as well as the background permittivity into account. Two different resonator geometries, corresponding to the thickness excitation (TE) and the lateral field excitation (LFE) modes, are discussed and compared. In the TE mode the resonance frequency is expected to be much stronger dc bias dependent than the antiresonance frequency. In the LFE mode, both resonance and antiresonance frequencies may exhibit comparable dc bias dependences. In this case the antiresonance frequency shows a stronger tuning with increasing dc bias than the resonance frequency. We model the behavior of the field dependent acoustic resonances in BaxSr1âxTiO3 thin films addressing different compositions and orientations of the films. Our theoretical model corroborates the experimental results available in the literature