Nematic twist-bend phase in an external field
Nematic twist-bend phase in an external field
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DOI：
10.1073/pnas.1721786115
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发表时间：
2018-10-30
影响因子：
11.1
通讯作者：
Chrzanowska, Agnieszka
中科院分区：
文献类型：
--
作者：
Pajak, Grzegorz;Longa, Lech;Chrzanowska, Agnieszka
扭转-弯曲（N-TB）相对外加场的响应可以提供对这种液体结构的重要见解，并可能使我们更接近于理解在非手性分子流体中产生镜像对称性破缺的机制。在这里，我们从理论上研究了外部均匀场如何影响N-TB的结构特性和稳定性。假设负责形成这一阶段的驱动力是包装熵，我们表明，在朗道-德Gennes理论，N-TB可以进行丰富的序列的结构变化与字段。对于具有正各向异性的介电常数的系统，我们首先观察到N-TB的倾斜角减小，直到它通过场诱导相变转变为普通的长轴单轴π相位（N）。然后，在非常高的磁场下，该双相产生垂直于场（Np+）的极化。对于介电常数为负各向异性的系统，结果揭示了新的调制结构。即使是一个无限小的场也会将N-TB变换为它的椭圆对应物（N-TBe），其中主指向矢的圆锥的圆底变成椭圆形。在较强的磁场下，椭圆退化为一条直线，产生一种非手征周期性结构，即展曲弯曲（N-SB），其中两个展曲指向矢被限制在一个平面上。具有调制极性顺序的三种结构-N-TB、N-TBe和N-SB-是全局非极性的。但进一步增加的领域诱导相变到全局极性结构与非零极化沿着场的方向。我们发现了两个这样的结构，其中之一是一个极性和手性修改的N-SB，其中splay和弯曲变形伴随着弱扭曲变形（N*（SBp））。场的进一步增加将该结构展开为平行于场的极化的极线（Np-）。
The response of the nematic twist-bend (N-TB) phase to an applied field can provide important insight into the structure of this liquid and may bring us closer to understanding mechanisms generating mirror symmetry breaking in a fluid of achiral molecules. Here we investigate theoretically how an external uniform field can affect structural properties and the stability of N-TB. Assuming that the driving force responsible for the formation of this phase is packing entropy, we show, within Landau-de Gennes theory, that N-TB can undergo a rich sequence of structural changes with the field. For the systems with positive anisotropy of permittivity, we first observe a decrease of the tilt angle of N-TB until it transforms through a field-induced phase transition to the ordinary prolate uniaxial nematic phase (N). Then, at very high fields, this nematic phase develops polarization perpendicular to the field (Np+). For systems with negative anisotropy of permittivity, the results reveal new modulated structures. Even an infinitesimally small field transforms N-TB to its elliptical counterpart (N-TBe), where the circular base of the cone of the main director becomes elliptic. With stronger fields, the ellipse degenerates to a line, giving rise to a nonchiral periodic structure, the nematic splay-bend (N-SB), where the two nematic directors are restricted to a plane. The three structures-N-TB, N-TBe, and N-SB-with a modulated polar order are globally nonpolar. But further increase of the field induces phase transitions into globally polar structures with nonvanishing polarization along the field's direction. We found two such structures, one of which is a polar and chiral modification of N-SB, where splay and bend deformations are accompanied by weak twist deformations (N*(SBp)). Further increase of the field unwinds this structure into a polar nematic (Np-) of polarization parallel to the field.