454 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
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
Dissipative vs dispersive coupling in quantum opto-mechanics: squeezing ability and stability
Generation of squeezed light and optomechanical instability for dissipative
type of opto- mechanical coupling is theoretically addressed for a cavity with
the input mirror, serving as a mechanical oscillator, or an equivalent system.
The problem is treated analytically for the case of resonance excitation or
small detunings, mainly focusing on the bad cavity limit. A qualitative
difference between the dissipative and purely dispersive coupling is reported.
In particular, it is shown that, for the purely dissipative coupling in the bad
cavity regime, the backaction is strongly reduced and the squeezing ability of
the system is strongly suppressed, in contrast to the case of purely dispersive
coupling. It is also shown that, for small detunings, stability diagrams for
the cases of the purely dispersive and dissipative couplings are qualitatively
identical to within the change of the sign of detuning. The results obtained
are compared with those from the recent theoretical publications
Evidence for dielectric aging due to progressive 180 domain wall pinning in polydomain Pb(Zr0.45Ti0.55)O3 thin films
An evidence that the dielectric ageing in the polydomain Pb(Zr0.45Ti0.55)O3
thin films is controlled by progressive pinning of 180 domain walls is
presented. To provide such a conclusion, we use a general method, which is
based on the study of the time evolution of the nonlinear, but anhysteretic,
dielectric response of the ferroelectric to a weak electric field. A
thermodynamic model of the ferroelectric system where the dielectric response
is controlled by bending movements of pinned 180 domain walls is developed.
Within this model, the nonlinear permittivity of the ferroelectric is expressed
as a function of the microstructural parameters of the domain pattern. It is
shown that using the analysis of the time evolution of the nonlinear
permittivity, it is possible to estimate changes in the concentration of the
pinning centers that block the movements of the 180 domain walls during aging
in polydomain perovskite ferroelectrics.Comment: This version is modifed and corrected according to recently published
Erratum: Phys. Rev. B 79, 219903(E) (2009). 21 pages, 3 figure
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