An official website of the United States government
Surface alignment and edge dislocations are explored in the recently discovered twist‐bend ferroelectric nematic, NTBF, in which the vector of spontaneous polarization follows an oblique helicoidal trajectory around a polar twist‐bend axis. In a planar cell, the polar axis aligns at some angle to the rubbing direction to mitigate the surface electric charge. It is demonstrated that the pseudolayers in planar cells form chevron defects, a hallmark defect of one‐dimensionally positionally ordered phases, such as smectic A and smectic C. The polar character of the twist‐bend axis prevents the cores of NTBFedge dislocations from splitting into semi‐integer disclinations, in stark contrast to dislocations in paraelectric and ferroelectric chiral nematics. The tilt of pseudolayers around the defect core allows estimation of the elastic penetration length as being close to the pitch of NTBF. Compression/dilation stresses around the core modify the heliconical tilt angle of molecules as evidenced by a substantial variation in local birefringence. The climb of dislocations exhibits high mobility, allowing the system to equilibrate the temperature‐dependent pitch. The uncovered properties facilitate the development of NTBFmaterials for electro‐optical applications, such as electrically controlled diffraction lattices and structural colors.
more »
« less
Confinement and magnetic-field effect on chiral ferroelectric nematic liquid crystals in Grandjean-Cano wedge cells
We explore the structure and magnetic field response of edge dislocations in Grandjean-Cano wedge cells filled with chiral mixtures of the ferroelectric nematic mesogen DIO. Upon cooling, the ordering changes from paraelectric in the cholesteric phase 〖 N〗^* to antiferroelectric in the smectic SmZ_A^* and to ferroelectric in the cholesteric N_F^*. Dislocations of the Burgers vector b equal the helicoidal pitch P are stable in all three phases, while dislocations with b=P/2 exist only in the 〖 N〗^* and SmZ_A^*. The b=P/2 dislocations split into pairs of τ^(-1/2) λ^(+1/2) disclinations, while the thick dislocations b=P are pairs of nonsingular λ^(-1/2) λ^(+1/2) disclinations. The polar order makes the τ^(-1/2) disclinations unstable in the N_F^* phase, as they should be connected to singular walls in the polarization field. We propose a model of transformation of the composite τ^(-1/2) line-wall defect into a nonsingular λ^(-1/2) disclination, which is paired up with a λ^(+1/2) line to form a b=P dislocation. The SmZ_A^* behavior in the in-plane magnetic field is different from that of the N_F^* and N^*: the dislocations show no zigzag instability, and the pitch remains unchanged in the magnetic fields up to 1 T. The behavior is associated with the finite compressibility of smectic layers.
more »
« less
- PAR ID:
- 10514950
- Publisher / Repository:
- American Physical Society
- Date Published:
- Journal Name:
- Physical Review E
- Volume:
- 109
- Issue:
- 5
- ISSN:
- 2470-0045
- Subject(s) / Keyword(s):
- Chiral ferroelectric nematic, chiral antiferroelectric smectic-Z, Grandjean-Cano wedge cell, Edge dislocations, Disclinations, Magnetic field realignment of a liquid crystal.
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Khoo, Iam Choon (Ed.)We explore the structures and confinement-induced edge dislocations in Grandjean-Cano wedge cells filled with the recently discovered chiral ferroelectric nematic (N_F^*) and chiral antiferroelectric smectic-Z 〖(SmZ〗_A^*). The chiral mixture is formed by DIO mesogen doped with a chiral additive. Wedge cells with parallel and antiparallel rubbing at the opposite plates show quantitatively different structures which is attributed to the polar in-plane anchoring of the spontaneous polarization at the rubbed substrates. The helical pitch shows a non-monotonous temperature dependence upon cooling, increasing as the temperature is lowered to the N^*-SmZ_A^* phase transition. The SmZ_A^* formed from an untwisted N^* in the thin portion of the wedge shows a bookshelf (BK) geometry, whereas the twisted N^* transforms into a twisted planar (PA) SmZ_A^* structure. In the N_F^* phase, the untwisted N^* becomes twisted in a wedge with antiparallel assembly of plates and monodomain in wedges with parallel assembly. The twisted regions of N_F^* show only one type of Grandjean zones separated by thick edge dislocations with Burgers vector b=P; the neighboring regions differ by 2π- twist.more » « less
-
We report the observation of the smectic A F , a liquid crystal phase of the ferroelectric nematic realm. The smectic A F is a phase of small polar, rod-shaped molecules that form two-dimensional fluid layers spaced by approximately the mean molecular length. The phase is uniaxial, with the molecular director, the local average long-axis orientation, normal to the layer planes, and ferroelectric, with a spontaneous electric polarization parallel to the director. Polarization measurements indicate almost complete polar ordering of the ∼10 Debye longitudinal molecular dipoles, and hysteretic polarization reversal with a coercive field ∼2 × 10 5 V / m is observed. The SmA F phase appears upon cooling in two binary mixtures of partially fluorinated mesogens: 2N/DIO, exhibiting a nematic (N)–smectic Z A (SmZ A )–ferroelectric nematic (N F )–SmA F phase sequence, and 7N/DIO, exhibiting an N–SmZ A –SmA F phase sequence. The latter presents an opportunity to study a transition between two smectic phases having orthogonal systems of layers.more » « less
-
We carry out Monte Carlo simulations on fluid membranes with orientational order and multiple edges in the presence and absence of external forces. The membrane resists bending and has an edge tension, the orientational order couples with the membrane surface normal through a cost for tilting, and there is a chiral liquid crystalline interaction. In the absence of external forces, a membrane initialized as a vesicle will form a disk at low chirality, with the directors forming a smectic-A phase with alignment perpendicular to the membrane surface except near the edge. At large chirality a catenoid-like shape or a trinoid-like shape is formed, depending on the number of edges in the initial vesicle. This shape change is accompanied by cholesteric ordering of the directors and multiple p walls connecting the membrane edges and wrapping around the membrane neck. If the membrane is initialized instead in a cylindrical shape and stretched by an external force, it maintains a nearly cylindrical shape but additional liquid crystalline phases appear. For large tilt coupling and low chirality, a smectic-A phase forms where the directors are normal to the surface of the membrane. For lower values of the tilt coupling, a nematic phase appears at zero chirality with the average director oriented perpendicular to the long axis of the membrane, while for nonzero chirality a cholesteric phase appears. The p walls are tilt walls at low chirality and transition to twist walls as chirality is increased. We construct a continuum model of the director field to explain this behavior.more » « less
-
Abstract Ferro-rotational magnet RbFe(SO4)2has attracted attention for its stable ferro-rotational phase and electric-field-controlled magnetic chirality. This work presents the multiferroic properties andH–Tphase diagram of RbFe(SO4)2, which have been underexplored. Our measurements of magnetic susceptibility, ferroelectric polarization, and dielectric constant under various magnetic fields reveal four distinct phases: (I) a ferroelectric and helical magnetic phase below 4 K and 6 T, (II) a paraelectric and collinear magnetic phase below 4 K and above 6 T, (III) a paraelectric and non-collinear magnetic phase below 4 K and above 9 T, and (IV) a paraelectric and paramagnetic above 4 K. This study clarifies the multiferroic behavior andH–Tphase diagram of RbFe(SO4)2, providing valuable insights into ferro-rotational magnets.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1103/PhysRevE.109.054702
