Masahiro Johno

Temporal and Spatial Behavior of the Field-Induced Transition between Antiferroelectric and Ferroelectric Phases in Chiral Smectics

Optical transmittance change and stroboscopic micrographs have been obtained in homogeneously aligned thin cells of S-MHPOBC, C8H17O–--COO–COO-*CH(CH3)C6H13, and R-TFMNPOBC, C8H17O–--COO–-COO-*CH(CF3)C8H17, by varying the initial applied DC voltage stepwise into the final one across the threshold. There exist two components, fast and slow, in the transmittance change due to the phase transition from antiferroelectric SmCA* to ferroelectric SmC*. The fast component is due to the pretransitional effect. The movement of the domain boundaries is responsible for the slow component. In the transmittance change due to the transition from SmC* to SmCA*, only the slow component is observed. The movement speed is mainly determined by the difference between the final and threshold voltages.

A novel property caused by frustration between ferroelectricity and antiferroelectricity and its application to liquid crystal displays-frustoelectricity and V-shaped switching

We have studied the frustration between ferro- and antiferro-electricity in chiral smectic C like liquid crystalline phases, which is not only fundamentally interesting but also very attractive from an application point of view. It causes temperature induced successive phase transitions as characterized by a devils staircase and the thresholdless, hysteresis-free, V-shaped switching induced by an applied electric field. The devils staircase indicates some type of interlayer ordering, while the V-shaped switching suggests considerably diminished tilting correlation. These two are apparently contradictory to each other, but result from the same cause, i.e. the frustration. We have first summarized experimental facts regarding subphases and successive phase transitions observed in many compounds and mixtures, which we believe are related to one another and result from the frustration. We have introduced several different theoretical explanations for these observed facts, and shown that only the axial next nearest neighbor Ising (ANNNI) model can explain almost all of the facts, provided that it is unified with the XY model appropriately. The unified model can make a comprehensive explanation in the most natural way based on the most probable molecular interactions. We have then emphasised that there are several modes regarding the V-shaped switching, because the system becomes so soft with respect to the tilting direction and sense that any additional external or internal force modifies the in-plane local director alignments. For the practically usable ones, we have emphasised the need for some type of randomization in the molecular alignment at the tip of the V and/or the switching process. In particular, the two dimensional (ideally, cylindrically symmetric) azimuthal angle distribution of local in-plane directors around the smectic layer normal is most attractive. Such a randomized state at the tip of the V is thermodynamically unique under a given condition imposed by interfaces. It stays stable even when the smectic layer structure, such as a chevron, changes with temperature. Finally, we have summarized the so-far reported compounds and mixtures for the V-shaped switching and introduced some prototypes of LCDs using them.

Smectic Layer Switching by an Electric Field in Ferroelectric Liquid Crystal Cells

Switching between two different smectic layer structures, chevron and bookshelf geometries, was observed for the first time in ferroelectric liquid crystal cells. In the absence of an electric field, a chevron layer structure is formed, as in a usual smectic C* phase. An electric field induces layer switching to a bookshelf geometry with a sharp DC threshold. Accompanying the layer switching, the projection of the director onto glass plates changes from the smectic layer normal to one of the uniform states when an electric field is applied. This optical change is equivalent to what is recognized as tristable switching.

Correspondence between Smectic Layer Switching and DC Hysteresis of Apparent Tilt Angle in an Antiferroelectric Liquid Crystal Mixture

We have shown that field-induced antiferro-ferroelectric phase transition is accompanied by reversible smectic layer switching. A layer structure without any electric field applications forms a chevron structure of the same kind as in ordinary SmC*. After the application of a field, the field-induced bookshelf structure relaxes to an obtuse chevron structure. The texture change with focal conics or zigzag defects is interpreted by taking account of the field dependence of the layer tilt angle of the chevron structure. The origin of the layer switching is also discussed on the basis of the texture change.

Self-Recovery from Alignment Damage under AC Fields in Antiferroelectric and Ferroelectric Liquid Crystal Cells

Alignment recovery from damage has been confirmed to occur under an AC electric field, e.g., 30 Vppµm-1, in antiferroelectric chiral smectic liquid crystal cells of MHPOBC and a related fluorinated compound. The recovery partly results from internal shearing produced by reversible layer switching between bookshelf and quasi-bookshelf. Contrary to the widely accepted notion, even in a ferroelectric liquid crystal cell of a compound similar to MHPOBC, reversible layer deformation due to an electric field has been observed from X-ray diffraction and a similar alignment recovery from damage has also been confirmed to occur.

Tristable Switching in SmO* of 1-Methylheptyl-Terephthalidene-Bis-Aminocinnamate (MHTAC) and Its Miscibility with SmCA* of Antiferroelectric Chiral Smectic Liquid Crystal

We investigated the electrooptic property of the SmO* phase in 1-methylheptyl-terephthalidene-bis-aminocinnamate (MHTAC), and confirmed its antiferroelectric characteristics. We also studied the miscibility of the SmO* phase with the antiferroelectric SmCA* phase, and found that the two phases are identical.

Layer structure deformation and electro-optic response in electroclinic effect

Abstract Switching in layer structure and molecular orientation due to the electroclinic effect was studied by means of X-ray and light transmittance measurements. The followings were confirmed; (1) A bookshelf layer structure in an unperturbed state deforms in part to bend or tilt. The layer tilt angle linearly increases with increasing field strength. (2) Normalized transmittance change between crossed polarizers is independent of an applied field strength, and is described by an exponential function composed of at least two decay times. The second component may be attributed to the layer structure change.

Tristable Switching in AFS FLCs (Anti-Ferroelectricity Stabilized Ferroelectric Liquid Crystals)

強誘電性液晶 (FLC) の表面安定化 (SS) に対して反強誘電性安定化 (AFS) という概念を提案し, AFSFLCの3安定状態間スイッチングが反強誘電性第3安定状態 (3) と電界により螺旋構造の解かれた強誘電性一様状態 (Ul, Ur) との間で起こることを示す.配向ベクトルがUl, Urではそれぞれ±θ傾き, 3ではスメクティック層法線と平行なので, 直交ニコルで3を暗にとればU1, Urは明となる.本スイッチングの特徴は, (1) AFS状態 (3) は安定で, 配向性が良好である. (2) 電圧無印加時には自発分極が消失しており, SSFLCで問題となるゴースト効果が生じない. (3) 見かけの傾き角一印加電圧特性に急峻な直流閾値とヒステリシスが存在し, メモリー性の付与が可能である. (4) スメクティック層が変形しやすく, 「く」の字層構造を回避できるので, コントラストが高い. (5) 応答特性は印加電圧に敏感に依存し, 高速化が期待できる. (6) スイッチングに伴うドメインの発生が規則的であり, 面積階調性の付与も期待できる.などである.AFSを示すFLC材料が次々に見出されており, ディスプレイなど電気光学デバイスへ応用されつつある.強誘電性液晶 (FLC) の表面安定化 (SS) に対して反強誘電性安定化 (AFS) という概念を提案し, AFSFLCの3安定状態間スイッチングが反強誘電性第3安定状態 (3) と電界により螺旋構造の解かれた強誘電性一様状態 (Ul, Ur) との間で起こることを示す.配向ベクトルがUl, Urではそれぞれ±θ傾き, 3ではスメクティック層法線と平行なので, 直交ニコルで3を暗にとればU1, Urは明となる.本スイッチングの特徴は, (1) AFS状態 (3) は安定で, 配向性が良好である. (2) 電圧無印加時には自発分極が消失しており, SSFLCで問題となるゴースト効果が生じない. (3) 見かけの傾き角一印加電圧特性に急峻な直流閾値とヒステリシスが存在し, メモリー性の付与が可能である. (4) スメクティック層が変形しやすく, 「く」の字層構造を回避できるので, コントラストが高い. (5) 応答特性は印加電圧に敏感に依存し, 高速化が期待できる. (6) スイッチングに伴うドメインの発生が規則的であり, 面積階調性の付与も期待できる.などである.AFSを示すFLC材料が次々に見出されており, ディスプレイなど電気光学デバイスへ応用されつつある.