560 research outputs found
Spontaneous symmetry-breaking in optomechanical cavity
A theoretical consideration of the so-called "membrane-in-the-middle"
optomechanical cavity revealed that it undergoes a spontaneous symmetry
breaking as a function of the transparency of the membrane. Such typical
features of this phenomenon as a square-root development of the order parameter
and divergency of the critical susceptibility were identified. In the contract
to classical spontaneous-symmetry-breaking systems of ferromagnets and
ferroelectrics, in the considered system, this divergency remains, due to
interference effects, an "internal" property of the system, which does not
reveal itself in any divergency of its observables. A spontaneous symmetry
breaking in an optomechanical cavity might pave a new way to enhanced
optomechanical interactions
Impact of surface phenomena on direct bulk flexoelectric effect in finite samples
In the framework of a continuum theory, it is shown that the direct
flexoelectric response of a finite sample essentially depends on the surface
polarization energy, even in the thermodynamic limit where the body size tends
to infinity. It is found that a modification of the surface energy can lead to
a change of the polarization response by a factor of two. The origin of the
effect is an electric field produced by surface dipoles induced by the strain
gradient. The unexpected sensitivity of the polarization response to the
surface energy in the thermodynamic limit is conditioned by the fact that the
moments of the surface dipoles may scale as the body size
Bichiral structure of feroelectric domain wall driven by flexoelectricity
The influence of flexoelectric coupling on the internal structure of neutral
domain walls in tetragonal phase of perovskite ferroelectrics is studied. The
effect is shown to lower the symmetry of 180-degree walls which are oblique
with respect to the cubic crystallographic axes, while {100} and {110} walls
stay "untouched". Being of the Ising type in the absence of the flexoelectric
interaction, the oblique domain walls acquire a new polarization component with
a structure qualitatively different from the classical Bloch-wall structure. In
contrast to the Bloch-type walls, where the polarization vector draws a helix
on passing from one domain to the other, in the flexoeffect-affected wall, the
polarization rotates in opposite directions on the two sides of the wall and
passes through zero in its center. Since the resulting polarization profile is
invariant upon inversion with respect to the wall center it does not brake the
wall symmetry in contrast to the classical Bloch-type walls. The flexoelectric
coupling lower the domain wall energy and gives rise to its additional
anisotropy that is comparable to that conditioned by the elastic anisotropy.
The atomic orderof- magnitude estimates shows that the new polarization
component P2 may be comparable with spontaneous polarization Ps, thus
suggesting that, in general, the flexoelectric coupling should be mandatory
included in domain wall simulations in ferroelectrics. Calculations performed
for barium titanate yields the maximal value of the P2, which is much smaller
than that of the spontaneous polarization. This smallness is attributed to an
anomalously small value of a component of the "strain-polarization"
elecrostictive tensor in this material
Optical mode crossings and the low temperature anomalies of SrTiO3
Optical mode crossing is not a plausible explanation for the new broad
Brillouin doublet nor for the strong acoustic anomalies observed at low
temperatures in SrTiO3. Data presented to support that explanation are also
inconclusive.Comment: This is a comment to a paper from J.F. Scott (same ZFP volume
Quantum limited measurements with lossy optical cavity enabled by dissipative optomechanical coupling
We analyze a cavity optomechanical setup, in which position of an oscillator
modulates optical loss. We show that in such setup quantum limited position
measurements can be performed if the external cavity coupling rate matches the
optical loss rate, a condition known as "critical coupling". Additionally,
under this condition the setup exhibits a number of potential benefits for
practical operation including the complete absence of dynamical backaction, and
hence optomechanical instability, and rejection of classical laser noise and
thermal fluctuations of cavity frequency from the measurement record. We
propose two implementations of this scheme: one based on signal-recycled
Michelson-type interferometer and the other on a tilted membrane inside
Fabry-Perot cavity
Effect of mechanical loading on the tuning of acoustic resonances in Ba x Sr1− x TiO3 thin films
The effect of mechanical loading on the tuning performance of a tunable Thin Film Bulk Acoustic Wave Resonator (TFBAR) based on a Ba0.3Sr0.7TiO3 (BST) thin film has been investigated experimentally and theoretically. A membrane-type TFBAR was fabricated by means of micromachining. The mechanical load on the device was increased stepwise by evaporating SiO2 on the backside of the membrane. The device was electrically characterized after each evaporation step and the results were compared to those obtained from modeling. The device with the smallest mechanical load exhibited a tuning of − 2.4% and − 0.6% for the resonance and antiresonance frequencies at a dc electric field of 615kV/cm, respectively. With increasing mechanical load a decrease in the tuning performance was observed. This decrease was rather weak if the thickness of the mechanical load was smaller or comparable to the thickness of the active BST film. If the thickness of the mechanical load was larger than the thickness of the active BST layer, a significant reduction in the tuning performance was observed. The weaker tuning of the antiresonance frequency was due to a reduced tuning of the sound velocity of the BST layer with increasing dc bias. The resonance frequency showed a reduced tuning due to a decrease in the effective electromechanical coupling factor of the device with increasing mechanical load. With the help of the modeling we could de-embed the intrinsic tuning performance of a single, non-loaded BST thin film. We show that the tuning performance of the device with the smallest mechanical load we fabricated is close to the intrinsic tuning characteristics of the BST laye
Landau thermodynamic potential for BaTiO_3
In the paper, the description of the dielectric and ferroelectric properties
of BaTiO_3 single crystals using Landau thermodynamic potential is addressed.
Our results suggest that when using the sixth-power free energy expansion of
the thermodynamic potential, remarkably different values of the fourth-power
coefficient, \beta (the coefficient of P^4_i terms), are required to adequately
reproduce the nonlinear dielectric behavior of the paraelectric phase and the
electric field induced ferroelectric phase, respectively. In contrast, the
eighth-power expansion with a common set of coefficients enables a good
description for both phases at the same time. These features, together with the
data available in literature, strongly attest to the necessity of the
eighth-power terms in Landau thermodynamic potential of BaTiO_3. In addition,
the fourth-power coefficients, \beta and \xi (the coefficient of P^2_i P^2_j
terms), were evaluated from the nonlinear dielectric responses along [001],
[011], and [111] orientations in the paraelectric phase. Appreciable
temperature dependence was evidenced for both coefficients above T_C. Further
analysis on the linear dielectric response of the single domain crystal in the
tetragonal phase demonstrated that temperature dependent anharmonic
coefficients are also necessary for an adequate description of the dielectric
behavior in the ferroelectric phase. As a consequence, an eighth-power
thermodynamic potential, with some of the anharmonic coefficients being
temperature dependent, was proposed and compared with the existing potentials.
In general, the potential proposed in this work exhibits a higher quality in
reproducing the dielectric and ferroelectric properties of this prototypic
ferroelectric substance.Comment: 7 figures, 5 table
Pressure on charged domain walls and additional imprint mechanism in ferroelectrics
The impact of free charges on the local pressure on a charged ferroelectric
domain wall produced by an electric field has been analyzed. A general formula
for the local pressure on a charged domain wall is derived considering full or
partial compensation of bound polarization charges by free charges. It is shown
that the compensation can lead to a very strong reduction of the pressure
imposed on the wall from the electric field. In some cases this pressure can be
governed by small nonlinear effects. It is concluded that the free charge
compensation of bound polarization charges can lead to substantial reduction of
the domain wall mobility even in the case when the mobility of free charge
carriers is high. This mobility reduction gives rise to an additional imprint
mechanism which may play essential role in switching properties of
ferroelectric materials. The effect of the pressure reduction on the
compensated charged domain walls is illustrated for the case of 180-degree
ferroelectric domain walls and of 90-degree ferroelectric domain walls with the
head-to-head configuration of the spontaneous polarization vectors.Comment: subm. to PRB. This verion is extended by appendi
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
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