25 research outputs found
Flexoelectricity and piezoelectricity - reason for rich variety of phases in antiferroelectric liquid crystals
The free energy of antiferroelectric liquid crystal which takes into account
polar order explicitly is presented. Steric, van der Waals, piezoelectric and
flexoelectric interactions to the nearest layers and dipolar electrostatic
interactions to the nearest and to the next nearest layers induce indirect tilt
interactions with chiral and achiral properties, which extend to the third and
to the fourth nearest layers. Chiral indirect interactions between tilts can be
large and induce helicoidal modulations even in systems with negligible chiral
van der Waals interactions. If indirect chiral interactions compete with chiral
van der Waals interactions, the helix unwinding is possible. Although strength
of microscopic interactions change monotonically with decreasing temperature,
effective interlayer interactions change nonmonotonically and give rise to
nonmonotouous change of modulation period through various phases. Increased
enatiomeric excess i.e. increased chirality changes the phase sequence.Comment: 4 pages, 1 figur
Dielectric response of a ferroelectric nematic liquid crystalline phase in thin cells
We studied dielectric properties of a polar nematic phase (NF) sandwiched
between two gold or ITO electrodes, serving as a cell surfaces. In bulk, NF is
expected to exhibit a Goldstone mode (phason), because polarization can
uniformly rotate with no energy cost. However, because the coupling between the
direction of nematic director and polarization is finite, and the confinement,
even in the absence of the aligning surface layer, induces some energy cost for
a reorientation of polarization, the phason dielectric relaxation frequency is
measured in a kHz regime. The phason mode is easily quenched by a bias electric
field, which enables fluctuations in the magnitude of polarization to be
followed in both, the ferronematic and nematic phases. This amplitude (soft)
mode is also influenced by boundary conditions. A theory describing the phase
and amplitude fluctuations in the NF phase shows that the free energy of the
system and, consequently, the dielectric response are dominated by
polarization-related terms with the flexoelectricity being relevant only at a
very weak surface anchoring. Contributions due to the nematic elastic terms are
always negligible. The model relates the observed low frequency mode to the
director fluctuations weakly coupled to polarization fluctuations
Re-entrant ferroelectricity in liquid crystals
The ferroelectric (Sm C) -- antiferroelectric (Sm C) -- reentrant
ferroelectric (re Sm C) phase temperature sequence was observed for system
with competing synclinic - anticlinic interactions. The basic properties of
this system are as follows (1) the Sm C phase is metastable in temperature
range of the Sm C stability (2) the double inversions of the helix
handedness at Sm C -- Sm C and Sm C% -- re-Sm C phase
transitions were found (3) the threshold electric field that is necessary to
induce synclinic ordering in the Sm C phase decreases near both Sm
C -- Sm C and Sm C -- re-Sm C phase boundaries, and it has
maximum in the middle of the Sm C stability region. All these properties
are properly described by simple Landau model that accounts for nearest
neighboring layer steric interactions and quadrupolar ordering only.Comment: 10 pages, 5 figures, submitted to PR
Chiroclinic effect
The effect of a chiral dopant or chiral molecular properties on
the polarly ordered smectic system made of achiral bent-core
molecules is studied theoretically. Two phenomena are predicted:
a) A non-tilted antiferroelectrically ordered system becomes
tilted upon chiral doping. b) Magnitudes of the tilt and
polarization differ in layers with opposite chiralities, if the
synclinic antiferroelectric structure is stable. Both phenomena
are induced by the chiral field of dopants or the chiral field of
chiral bent-core molecules themselves. Therefore the name chiroclinic effect is suggested for the phenomena
Temperature ranges of the chiral polar smectic phases
The paper presents the analysis of static and dynamic properties of achiral polar smectic doped with chiral dopant. In this system the two phases with equal symmetries exist, the and the phase, which continuously transform one to the other upon temperature changes. The analysis of dynamical properties allowed for the development of the criterion which determines the temperature ranges of the two phases. Besides, the analysis of collective modes revealed that they are either amplitude or phase. However, in many of them the character changes from polar to tilt or vice versa. The collective modes occur in pairs of opposite chiralities. The chiral character of modes explains also the gaps which occur between the amplitude modes