Pis'ma v ZhETF, vol. 101, iss. 11, pp. 851-856 © 2015 June 10

Surface 27r-walls in polar free-standing smectic films

P. V. DolganovV. K. Dolganov, A. Fukuda+

Institute of Solid State Physics of the RAS, 142432 Chernogolovka, Russia + Department of Electronic and Electrical Engineering, Trinity College, University of Dublin, Dublin 2, Ireland

Submitted 29 April 2015

It was found that electric field in free-standing smectic films at high temperature leads to formation of unusual 27r-walls, with quite different structure from the structure of classical 27r-walls. The walls are formed by the change of molecular orientation not only along the film plane (as in usual walls) but also in smectic layers between two surfaces of the film. These walls are nuclei of surface field-induced synclinic-anticlinic transition between states with transverse and longitudinal electric polarization.

DOI: 10.7868/S0370274X15110077

Orientational ordering of the molecules in liquid crystals is the reason of the formation of various orientation defects. The most common orientational defects in external electric field are linear walls separating regions with the same direction of molecular orientation. In a nematic liquid crystal the orientational order is described by the average orientation of the long axis of the rodlike molecules, so-called n-director [1]. Due to equivalence of the nematic director orientations n and — n, line defects are 7r-walls, in which the molecules turn by an angle of it across the walls. The smectic liquid-crystalline structure is formed by periodically stacked two-dimensional (2D) fluid layers. In ferroelectric Smectic-C* (SmC*) [2] and antiferroelectric Smectic-C^ (SmC^) [3, 4] the long axes of molecules are tilted by an angle 9 with respect to the layer normal. The projection of the long molecular axis on the layer plane defines the 2D c-director field that describes the azimuthal molecular orientation 4>(r). Two opposite directions of the c-director are nonequivalent [1], which leads to appearance of 27r-walls in electric field. In usual 27r-walls in smectic films the c-director in all layers rotates synchronously by an angle of 2tt. In the direction normal to the film the structure of the wall is the same as far from the wall, but only turns by some angle [512]. Note that in bulk samples of S111C4, contrary to thin films, TV-walls are mainly observed [13-15].

In the present work 27r-walls in free-standing films of antiferroelectric liquid crystal are studied. At high temperature in the same film the states with transverse (perpendicular to the tilt plane) and longitudinal (parallel to the tilt plane) electric polarization can be formed. The existence of the state with longitudinal polarization

-'-'e-mail: pauldol@issp.ac.ru

Sl at low field and the state with transverse polarization St in high field leads to formation of unusual 27r-walls in which the film structure and polarization change across the wall. In particular in the high-field state St in the walls there exist a region which possesses the structure and polarization of the low-field state Sl. The structure transition in the films induced by the electric field can occur via the broadening of this wall region.

Measurements were performed on antiferroelectric liquid crystal (S)-TFMHPBC [4], whose chemical structure is given in Fig. f. Bulk samples of TFMHPBC have

_ _ O _ O CF3

C8H17-^ /\ \-CO-CHC6H13

Fig. 1. Molecular structure of compound (S)-TFMHPBC

the following phase sequence: SmC^-74.3°C-SmC*-75 °C-SmA-83 °C-Isotropic [4]. Films were prepared in a rectangular frame 5 x 0.9 mm2 in size when the sample was in the S111C4 phase. Electric voltage was applied to the two longer sides of the frame. Films with thickness from 4 to 20 smectic layers were studied. The number of smectic layers N was determined by the measurements of light reflection intensity from the film [16]. 27r-walls were observed in reflected light with the use of a polarizing microscope. The image was registered by a digital camera. Films were investigated at temperatures above the SmC*-SmA phase transition. In this temperature region the difference in optical path AnL (An is the difference in main indices of refraction in the plane of the film, L - its thickness) even for a 10-layer film is a small value which provides obstacles for observations with crossed polarizers. For visualization of 27r-walls the depolarized reflected light microscopy

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(DRLM) technique was used, which greatly enhances the contrast of the image [17]. In this method the polarization of the incident light makes an angle of 45° with respect to the direction of the electric field. The difference in indices of refraction along the c-director ne and perpendicular to it nQ and correspondingly the difference in reflection coefficients leads to an effective rotation of the plane of the reflected light polarization towards the direction of the c-director (ne > n0). Observation of the film via a slightly decrossed analyzer enables to visualize regions with different orientation of the c-director in spite of the small value of AnL.

Antiferroelectric SmC\ structure becomes polar in confined geometry of thin free-standing film [18-20]. Films with an odd number of smectic layers possess transverse polarization Pt. Nonzero Pt appears since layer polarizations directed in opposite directions in neighboring layers are noncompensated in iV-odd films. Films with an even number of layers possess longitudinal polarization Pl that is induced by free surfaces. This universal behavior is known as odd-even layering effect [19] and is observed in the temperature interval of the bulk SmC\ phase. Above the bulk transition temperature to Sm A phase thin films exist in tilted state due to surface ordering effect and the high temperature shift of the surface transitions [21-23]. On heating, after a series of structure transformations a new universal behavior is observed [19, 20, 24, 25]. At high temperature film behavior in electric field does not depend on the parity of its layer number, but is determined by the value of the field. Fig. 2 shows photographs of the film in the high-field state (a) and in the low-field state (b). A difference in brightness of the film (Figs. 2a and b) is connected with different orientation of the c-director with respect to the electric field (vertical direction). In the DRLM geometry used for imaging the low-field structure (b) appears brighter which indicates that the orientation of the molecular tilt planes is parallel to the electric field (low-field state Sl with longitudinal polarization Pl). For the value of E above a critical field Ec, the film appears dark (a) which means that the c-director is perpendicular to the electric field (high-field state St with transverse polarization Pt).

In Fig. 3 the molecular orientation in smectic layers is shown schematically in Sl and Sr-states. At high temperature the molecules tilt mainly near the surfaces and film polarization is determined mainly by surface layers. Two types of polarization exist in the surface layers: 1) ferroelectric polarization orients perpendicular to the tilt plane (transverse polarization), 2) surface polarization is parallel to the tilt plane (longitudinal polarization). Longitudinal polarization in surface lay-

Bottom / T T\

Fig. 2. Photographs of a smectic film in the high-field state (St) with transverse polarization (a) and in the low-field state (Sl) with longitudinal polarization (b). The transition from Sl- to St-state is induced by the electric field. Double dark and bright stripes are classical 2tt-walls. The orientation of the c-director in 2tt-walls in top and bottom surface layers of the film is shown schematically in St- and ^-states. The orientation of surface smectic layers is syn-clinic in St (a) and anticlinic in Sl (b). The horizontal dimension of each photograph is 260 ¿¿m, number of layers N = 11, T=77.4°C, E = 5.1 V/cm (a), E = 3.7V/cm (b)

ers appears due to breaking of up-down symmetry at the air-smectic interface [18]. In Sr-structure (Fig. 3) in the lower and the upper surface layers of the film the molecules tilt in the same direction (synclinic structure, c-director orientation is the same). Directions of the ferroelectric transverse polarizations coincide on two surfaces. Net polarization of the film is perpendicular to the tilt plane (transverse polarization Pt). In S^-structure (Fig. 3, the ground state without the field) in the lower and the upper surface layers the molecules tilt in the opposite directions (anticlinic structure, c-director orientation is the opposite). Net transverse polarization of the film is zero. Net polarization of the film is parallel to the tilt plane (longitudinal polarization Pl). Small difference of the energy between St and S^-states is connected with small tilt angle 6 in the middle of the film at high temperature. The transition from Sl to St~state on increasing field leads to the change of c-director orientation in surface layers and change of the type of polarization. The latter means that Pt > Pl- In antiferroelectric liquid crystals in high electric field the induced polarization [26, 27] can be important and can lead to a number of effects. In particular the induced po-

Surface 2tt- walls in polar free-standing smectic ñlms



® PT


c^/ Top

Fig. 3. Schematic representation of the film structure in St (high field) and Sl (low field) states. In the surface layers the directions of polarization are shown by arrows. In St-state the orientation of molecules in the top and bottom surface layers is the same. The surfaces are syn-clinic and the film possesses transverse polarization Pt - In *Sz,-state the orientation of molecules in top and bottom surface layers is opposite. The surfaces are anticlinic and film possesses longitudinal polarization Pl- Field-induced transition between state with

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