Electric field distortions in structures of the twist bend nematic (NTB) phase of a bent-core liquid crystal

Dielectric spectroscopy of a twist bend nematic phase of an achiral bent core liquid crystalline compound under DC bias is used to investigate its response to electric field. Two collective relaxation processes are revealed, these are assigned to distortions of helicoidal structure by the external bias field. Frequency of the mode depends primarily on the helicoidal angle and has anomalous, softening- like behaviour at the nematic to the twist bend nematic transition. A coupling of dielectric anisotropy with electric field gives rise to a new equilibrium periodic structure in the time scale involved. The modulus of the wave vector gradually vanishes on increasing the bias field (except for the initial behaviour, which is just the opposite). Transition from the twist bend to the splay bend structure is clearly observed by a sudden drop in the frequency of this mode, which decreases almost linearly with increasing field. Results agree with predictions from current models for the periodically distorted a twist bend nematic phase.Comment: 14 PAGES, 7 FIGURES, submitted to Physical Review Letter

The effect of confinement on the stability of field induced states and on supercooling in antiferro-ferroelectric phase transitions in chiral smectic liquid crystals

We investigate both the supercooling and the hysteresis phenomena of the phase transitions between the smectic C* and the smectic C * Aphases driven by temperature and electric field, respectively. These two phenomena show similar characteristics for the dependence of transmittance on both the cell thickness and the applied field. The mechanisms for large supercooling and large hysteresis in thin cells are shown to correspond to the suppression of the propagation of solitary wave by the surfaces. Furthermore, these two phenomena are shown to be controlled by a moderate ac field applied across the cell. We present a clear evidence for the existence of at least two field induced subphases (called states here) between the antiferroelectric and the ferroelectric phases. These are found to correspond to the field induced three-layered and four-layered structures through a comparison of experimental results on the tilt angle and its simulation as well as by discrete changes in the texture by increasing the electric field. The correspondence between the thermotropic phases and the field induced states is demonstrated through measurements of the supercooling/ superheating and of the hysteresis as a function of the cell thickness. The instability in the field induced states depends strongly on the cell thickness, and the various states are not observed in a cell of 1.6 ??m thickness.open3

Dielectric Study of Nematic LC Built with Bent-Core Molecules

Here we report results of the study of bent-core (BC) cybotactic nematic LC by dielectric spectroscopy and electro-optic measurements. In the absence of electric field, this material is uniaxial and exhibits only one relaxation process corresponding to the fluctuations of the polar clusters. The application of the external electric field induces biaxialityandthesecondrelaxationprocessappearsinthedielectricspectra.Thisisassigned to the collective mode. Possible models for cluster transformation under electric field are discussed

Observation and Investigation of the Ferrielectric Subphase with qT\u3e1/2

The high-temperature femelectric phase with qT \u3e 1/2 has been found to exist in a antiferroelectric liquid crystal. This phase has been identified using dielectric spectroscopy and conoscopy

Dielectric Study of Liquid Crystals with Large Electroclinic Effect

Here we report results of the study of tilt angle and birefringence of some “high-electroclinic” materials having similar molecular structure with two and three siloxane spacers. The structure of these LC materials were studied by dielectric spectroscopy, birefringence and tilt angle measurements. Three of the four studied materials with a siloxane fragment are the deVries materials, but the siloxane-free sample is conventional SmA phase and is thus different. Possible models for the molecular structures of deVries SmA∗ phases are discussed

Experimental Study of High-Temperature Smectic-C*F12 Phase in Chiral Smectic Liquid Crystals That Exhibit Phase-Sequence Reversal

We report the results of an experimental study of a recently observed phase sequence reversal of smectic-C*FI2 [SmC*(qT=1/2); a four layer antiferroelectric] phase appearing in the temperature range above the smectic-C* (SmC*) phase from the results of optical birefringence, spontaneous polarization, selective reflection, conoscopy, and dielectric spectroscopy. The SmC*FI2 phase is observed in an antiferroelectric liquid crystalline compound, 10OHF, in a temperature range above that of SmC*A phase and is found to be thermodynamically monotropic, i.e., it appears only upon cooling from SmC*A phase. This is also unstable as if it is once transformed to SmC* by the application of the bias, it does not return to its original state unless the sample is heated and cooled again in the absence of the bias. Nevertheless this phase is stabilized by the addition of a chiral smectic compound 9OTBBB1M7 (abbreviated as C9), having a wide temperature range of the SmC*FI2 phase. The temperature range of the low temperature SmC* decreases with increase in the concentration of C9 and for a concentration of 55 wt. %, SmC* disappears and the transition takes place directly from SmC*FI2 to the crystalline phase on cooling. The existence of such a high-temperature SmC*FI2 phase is also supported by a phenomenological model

Development of Ferroelectricity in the Smectic Phases of 4-Cyanoresorcinol Derived Achiral Bent-Core Liquid Crystals with Long Terminal Alkyl Chains

Bent-core molecules can form numerous polar and symmetry broken liquid crystalline phases with fascinating properties. Here we report the characteristics of a previously unknown polar synclinic smectic phase, Sm C S P X , found in the phase sequence of an achiral bent-core material with a 4-cyanoresorcinol bisbenzoate core, terminated by long linear alkyl chains ( n = 18 ) on both ends. This phase exists over a narrow range of temperatures and is sandwiched in between the random polar synclinic smectic phase ( Sm C S P R ) and polar synclinic ferroelectric ( Sm C S P F ) phase with P S ∼ 250 nC / c m 2 . In a planar-aligned cell it exhibits only chirality flipping on application of a conventional ac field but it also exhibits optical switching by rotation of the molecular directors on the tilt cone subjected to a modified sequence of bipolar pulses. This changeover is discussed in terms of the model given by Nakata et al. [Phys. Rev. Lett. 96, 067802 (2006)], involving a competition between the two forms of switching: the rotation around the long molecular axis and the switching through rotation of the molecular directors on the tilt cone. The model is modified to take account of the azimuthal pretilt and the molecular tilt angles. In addition to it, characteristics of Sm C S P R are also discussed

Nematic twist-bend phase with nanoscale modulation of molecular orientation

A state of matter in which molecules show a long-range orientational order and no positional order is called a nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic. Here using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt)

Liquid Crystal Display Modes in a Nontilted Bent-Core Biaxial Smectic Liquid Crystal

Liquid crystal display (LCD) modes associated with the rotation of the secondary director in nontilted, biaxial smectic phase of an achiral bent-core compound are demonstrated. For LCDs, we find that at least four display modes are possible using SmAPA phase of the studied material, in which the minor directors in adjacent layers are aligned antiferroelectrically. The advantages of these modes include low driving field (1–2 V/ µm), high contrast ratio 1000:1, relatively fast switching time of 0.5 ms and continuous gray scale. The molecular short axis or the polar axis in a negative dielectric, biaxial material is oriented by the in-plane electric field by a combination dielectric biaxiality and polarity at low electric fields and polarity at higher fields

Nematic Twist-Bend Phase with Nanoscale Modulation of Molecular Orientation

Peer reviewedPublisher PD