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1.
Marcerou JP Nguyen HT Bitri N Gharbi A Essid S Soltani T 《The European physical journal. E, Soft matter》2007,23(3):319-328
Usual ferroelectric compounds undergo a paraelectric-to-ferroelectric phase transition when the susceptibility of the electric polarization density changes its sign. The temperature is the only thermodynamic field that governs the phase transition. Chiral tilted smectics may also present an improper ferroelectricity when there is a tilt angle between the average long axis direction and the layer normal. The tilt angle is the order parameter of the phase transition which is governed by the temperature. Although the electric susceptibility remains positive, a polarization proportional to the tilt appears due to their linear coupling allowed by the chiral symmetry. Further complications come in when the chirality increases, as new phases are encountered with the same tilt inside the layers but a distribution of the azimuthal direction which is periodic with a unit cell of two (SmC(A)*, three (SmC(Fi1)*, four (SmC(Fi2)* or more (SmC(alpha)* layers. In most of these phases, the layer normal is a symmetry axis so there is no macroscopic polarization except for the SmC(Fi1)* in which the average long axis is tilted so the phase is ferrielectric. By studying a particular compound with only a SmC(Fi2)* and a SmC(alpha)* phase, we show that we recover the uniformly tilted ferroelectric SmC* when applying an electric field. We are thus led to build field-temperature phase diagrams for this class of compounds by combining different experimental techniques described here. 相似文献
2.
Optical reflectivity studies have been conducted on freely suspended films with synclinic and anticlinic structures. For the
first time quantitative data were obtained on orientational ordering in films with anticlinic structure. In the same wide
temperature range of more than 30 K we observed both transverse and longitudinal ferroelectricity in the films of the same
thickness. Equilibrium transition temperatures between structures with transverse and longitudinal polarization were above
the bulk transition temperature to the SmC phase. The molecule tilt was determined in superthin films with synclinic and anticlinic ordering.
Received 29 October 1999 and Received in final form 17 February 2000 相似文献
3.
In this paper we discuss the direct isotropic to chiral smectic-C phase transition on the basis of a phenomenological theory. The model free energy is written in terms of the coupled order parameters including the spontaneous polarization. We present a detailed analysis of the different phases that can occur and analyze the question under which conditions a direct isotropic to chiral smectic-C phase transition is possible when compared to other phase transitions. On the basis of this model the isotropic-smectic-C* transition is always of first order. The theoretical predictions are compared with the available experimental results.-1 相似文献
4.
Cluzeau P Bonnand V Joly G Dolganov V Nguyen HT 《The European physical journal. E, Soft matter》2003,10(3):231-240
The behaviour of freely suspended smectic-C* ( SmC*) films at the bulk SmC*-cholesteric ( N*) phase transition has been investigated using polarized-reflected-light microscopy. Our experimental observations show that
above the bulk SmC*- N* phase transition the N* order appears in different ways according to the film thickness. In thin films, the conventional layer-by-layer thinning
occurs. In films of intermediate thickness N* inclusions nucleate inside the SmC* film. The distortions of the in-plane orientational order of the SmC* host phase induce elastic interactions between the inclusions and lead to their self-organization in chain-like structures.
Both the dynamic of the chaining and the parameters driving the equilibrium distance between the inclusions in the chain are
investigated. In thick films, N* fingers grow inside the film. The influence of the experimental conditions on the various processes is analysed.
Received 1 July 2002 / Published online: 15 April 2003
RID="a"
ID="a"e-mail: Philippe.cluzeau@univ-lille1.fr 相似文献
5.
Several new kinds of smectic-C twist grain boundary phases (TGBC) have been observed during the last few years. These pure
compounds or mixtures exhibit unusual textures with polygonal lattices (square or hexagonal grids) in the plane normal to
the TGB helix. The structure of these new phases seems to be complex and different from reported and predicted TGBC phases.
In this article, we review the main results obtained on these different new phases, and we propose new TGBC structures based
on the well-known splayed polarization --twisted director structures adopted by chiral smectic-C's in planar aligned (bookshelf)
cells. The observed square or hexagonal lattices are made of superimposed pairs of unwinding lines due to the suppression
of the helix within smectic-C blocks by the grain boundaries (unwinding walls). A lattice-free TGBC occurs if the helix within
smectic-C blocks is suppressed completely.
Received 15 March 2001 相似文献
6.
We investigated SmC* films sandwiched between silane coated glass plates and observed formation of textures exhibiting a uniform tilt of the smectic
layers with respect to the boundary plates. The layer tilt angle increases from zero to as the sample is cooled from the smectic A phase to room temperature. These films show linear electro-optical effects because
the permanent polarization can be aligned so that it has a component parallel to the applied field without changing the layer
structure. Our analysis indicates that mainly two effects determine the layer tilt. On the one hand, the surface tension tends
to minimise the layer tilt. On the other hand, the surface energy promotes the director to be normal to the boundary plates.
Received 17 July 1998 相似文献
7.
Smectic-C elastomers can be prepared by cross-linking, e.g., liquid crystal polymers, in the smectic-A phase followed by a cooling through the smectic-A to smectic-C phase transition. This transition from D(infinityh) to C(2h) symmetry spontaneously breaks rotational symmetry in the smectic plane as does the transition from a smectic-A to a biaxial smectic phase with D(2h) symmetry. We study these transitions and the emergent elasticity of the smectic-C and biaxial phases in three related models and show that these phases exhibit soft elasticity analogous to that of nematic elastomers. 相似文献
8.
We describe a chiral axial next nearest neighbour xy-model to account for the various subphases exhibited by antiferroelectric liquid crystals made of chiral rod-like molecules.
The assumed form of the interlayer interactions is based on physical processes which are discussed. Using a discrete model,
the predicted sequence of transitions is SmA-SmC-SmC-FIH-FII-FIL-SmC
*
A, where FI stands for a ferriphase, as seen in many compounds. The ferri and SmC phases are characterized by relatively large angles between the c-vectors of successive layers and occur only when the compounds have high optical purity. The calculated field induced structures
exhibit a plateau of the apparent tilt angle at , where is the tilt angle of the molecules in the ferriphase. The conoscopic figures in the presence of a field and ellipsometric
parameters in the absence of a field have also been generated, which agree extremely well with the experimental results. Recent
anomalous X-ray scattering studies prove the xy-character of the configurations, though the commensurate structures that are found in the ferriphase require an extension
of the model to include lock-in terms.
Received 23 August 1999 相似文献
9.
An anomalous orientation of ferroelectric liquid crystals in an electric field is observed. The tilt planes of the molecules
are oriented in a direction parallel to the electric field. The anomalous orientation is observed in freely suspended films
above the smectic-C*-smectic-A bulk phase transition temperature. The effect is explained by the appearance of giant flexoelectric
polarization, 104 times greater than that in bulk liquid-crystal samples, in the film.
Pis’ma Zh. éksp. Teor. Fiz. 66, No. 6, 414–418 (25 September 1997) 相似文献
10.
Spector MS Heiney PA Naciri J Weslowski BT Holt DB Shashidhar R 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》2000,61(2):1579-1584
Optical and x-ray scattering studies of a chiral, organosiloxane smectic-A liquid crystal indicate a large field induced optical tilt of up to 31 degrees accompanied by a very small contraction of the smectic layers. This result suggests that the molecules have a nonzero tilt even with no applied field, and that the primary effect of the field is to induce long range order in the direction of the molecular tilt. 相似文献
11.
M. A. Osipov M. V. Gorkunov H. F. Gleeson S. Jaradat 《The European physical journal. E, Soft matter》2008,26(4):395-404
A molecular theory of the ferroelectric smectic C* phase has been developed using the simple model of a chiral molecule composed
of a uniaxial core and a pair of off-center nonparallel dipoles which determine molecular chirality and polarity. The interaction
between uniaxial cores is modeled by a rather general effective potential which can be used to describe smectic materials
with both conventional and anomalously weak layer contraction in the smectic C* phase. Spontaneous polarization, tilt, and
layer spacing are calculated numerically as functions of temperature, and it is shown that the variation of the polarization
generally deviates from that of the tilt angle. It is shown that this deviation is more pronounced in smectic materials tilting
with low layer contraction which corresponds to existing experimental data. The model has been used to reproduce qualitatively
the experimental data for polarization, tilt and layer spacing for two similar mixtures exhibiting conventional and anomalously
weak layer contraction. The polarization and the tilt are also calculated in the case when the smectic A-smectic C* transition
is characterized by the biaxial primary order parameter. 相似文献
12.
M. Petit M. Nobili P. Barois 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(3):341-345
We report the first observation of an electroclinical effect at the TGB-TGB transition induced by an external DC electric field applied perpendicular to the pitch direction. Upon increasing the field,
the smectic layers rather than the director field tilt over relative to the helical axis, allowing to detect the effect by
X-ray scattering from well aligned samples. The observations are qualitatively interpreted in the frame of a mean field phenomenological
model of a helically modulated electroclinical effect.
Received: 24 April 1998 / Revised: 22 June 1998 / Accepted: 31 July 1998 相似文献
13.
The collapse dynamics of smectic-A bubbles are analyzed experimentally and theoretically. Each bubble is expanded from a flat
film stretched at the end of a hollow cylinder and deflated through a pressure release by means of a capillary tube. Its total
collapse time can be varied between 0.1s and 20s by suitably choosing the length and the internal diameter of the capillary.
This experiment allowed us to show that the collapse takes place in two steps: an initial one, which lasts a fraction of a
second, where the meniscus destabilizes and fills up with focal conics, followed by a much longer period during which the
bubble collapses and exchanges material with the meniscus. By measuring simultaneously the Laplace pressure and the internal
pressure inside the bubble, we were able to fully characterize the shear-thinning behavior of the smectic phase within the
meniscus. We emphasize that this method is generic and could be applied as well to other systems such as soap bubbles, on
condition that inertial effects are negligible. 相似文献
14.
Fukuda J 《The European physical journal. E, Soft matter》2007,24(1):91-98
It was shown experimentally (P.V. Dolganov et al., Europhys. Lett. 76, 250 (2006)) and by numerical calculations (C. Bohley, R. Stannarius, Eur. Phys. J. E 23, 25 (2007)) that the c -director profile of a two-dimensional chiral smectic-C (SmC) film around a circular inclusion adopts dipolar rather than
quadrupolar configuration observed in achiral SmC films. We give an analytical argument on how spontaneous bend inherent in
chiral SmC liquid crystals influences the configuration of a SmC liquid crystal film around a circular inclusion imposing
tangential anchoring. We find how the angle α between two surface defects seen from the center of the inclusion depends on
the radius of the inclusion R and the strength of the spontaneous bend q . We show, however, that the contribution of the spontaneous bend to the free energy suffers from mathematical ambiguity;
it depends on the mathematical treatment of the outer boundary even when it is at infinity. This might indicate that the shape
as well as the treatment of the outer boundary of the film can significantly influence the equilibrium configuration of the
c -director and the position of the surface defects. 相似文献
15.
The field-induced realignment of a smectic-A phase is in principle a complicated process involving the director rotation via
the interaction with the field and the layer rotation via the molecular interactions. Time-resolved X-ray scattering experiments
have revealed major phenomena concerning the maintenance of the integrity of the smectic-A layer structure during the alignment
process. In order to obtain a deeper insight into this process, we have carried out a dissipative particle dynamics study
of the realignment kinetics of a nanodroplet of a smectic-A liquid crystal suspended in an isotropic fluid following a switch
in the direction of an applied magnetic field. The strength of the mesogen-field interaction is small compared to the inter-molecular
interactions. The reaction of the smectic configuration to the field switch was found to depend on the balance between the
inter-molecular interactions stabilising the formation of the smectic layering and the interaction of the mesogens with the
external field. It is found that the rotational behaviour of the smectic layers under the influence of an external magnetic
field arises from a combination of stochastic translational displacements and rotational motions of the centres of mass of
the mesogens in the nanodroplets. The simulations indicate that X-ray scattering and NMR experiments monitoring the orientational
order are sensitive to different aspects of the realignment process. 相似文献
16.
The magnetic properties of a multi-domain smectic-C phase liquid crystal are calculated as a function of tilt angle and magnetic field orientation. Numerical results are presented for the overall diamagnetic susceptibility and magnetic torque density. It is predicted that a first-order reorientation phase transition can occur for the case of a free smectic-C whose tilt angle exceeds 35° at some temperature. These calculations can be used for critically evaluating the multi-domain model and for studies of the temperature dependence of the smectic-C tilt angle. 相似文献
17.
Achard MF Bedel JP Marcerou JP Nguyen HT Rouillon JC 《The European physical journal. E, Soft matter》2003,10(2):129-134
Taking advantage of the great number of bent-core or “banana" compounds synthesized and studied in the laboratory, we describe
their behaviour under the application of an external electric field. If the field were a static one, we would work within
the frame of an equilibrium phase diagram in a (field E, temperature T) space where some phases would be simple dielectrics and others ferroelectric ones with a macroscopic polarization, either
spontaneous or induced by the field. In this paper, we deal with the basic responses of “banana” liquid crystals under the
application of a low frequency (1 to 100 Hz) AC field. Firstly square-wave voltages allow us to locate the phase boundary
between dielectric (at lower field) and ferroelectric phases (higher field) at a given temperature and field threshold. Then
we apply slowly varying AC voltages with shapes like triangle or “triple-plateau” to check out the stability of the induced
ferroelectric phase versus field removal. Three behaviours are encountered, the unstable one (short lifetime of the high-field ferroelectric phase)
where the macroscopic polarization is destroyed and then rebuilt in the opposite direction during each half period and usually
called “antiferroelectric”; the stable one (long lifetime) with a polarization that rotates at constant modulus which is labeled
as “ferroelectric” and a new one where the macroscopic polarization is proportional to the applied fied, we named this behaviour
as “superparaelectric”. Let us stress that these observations apply to the ferroelectric phases of the (E, T) phase diagram not to the zero field (0,T) phases observed in the usual phase characterization experiments except for an
eventual spontaneous ferroelectric phase.
Received 18 April 2002 and Received in final form 17 January 2003 Published online: 16 April 2003
RID="a"
ID="a"e-mail: marcerou@crpp.u-bordeaux.fr
RID="b"
ID="b"URL: http://www.crpp-bordeaux.cnrs.fr 相似文献
18.
Leonid V. Mirantsev 《Physics letters. A》2008,372(21):3904-3908
A homeotropically oriented smectic-A film on a solid substrate with periodical microrelief is considered. Periodical distortions of the free surface of the film induced by this microrelief are theoretically investigated. The dependence of these distortions on the film thickness, the temperature, and external magnetic (electric) field is obtained. It is shown that, for a certain choice of the shape of the substrate surface microrelief, one can realize a temperature and external magnetic (electric) field control on the microrelief at the free surface of the smectic-A film. 相似文献
19.
20.
Conradi M Ziherl P Sarlah A Musevic I 《The European physical journal. E, Soft matter》2006,20(2):231-236
We study the structure of a free-standing smectic-A film around a micron-size polystyrene colloid adsorbed onto the film.
We find that a colloid or a cluster of colloids is surrounded by an optically distinct and radially decorated meniscus ending
with a sharp edge. The observed strong and finite-range attraction between the adsorbed colloids is driven by the fusion of
menisci. We interpret the structure of the smectic meniscus in terms of a model dominated by the surface free energy and we
argue that the characteristic appearance of the meniscus is due to layer undulations. 相似文献