Evidence for de Vries structure in a smectic-A liquid crystal observed by polarized Raman scattering Naoki Hayashi * and Tatsuhisa Kato Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan Atsuo Fukuda, Jagdish K. Vij, ² and Yuri P. Panarin Department of Electronic and Electrical Engineering, Trinity College, University of Dublin, Dublin 2, Ireland J. Naciri and R. Shashidhar Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Code 6900, 4555 Overlook Avenue South West, Washington, DC 20375, USA Sachiko Kawada and Shinya Kondoh Citizen Watch Co., Ltd., Tokorozawa 359-8511, Japan Received 3 September 2003; revised manuscript received 26 July 2004; published 25 April 2005 The second- and fourth-order apparent orientational order parameters of the core part of the molecule  P 2  app and  P 4  app , have been measured by polarized vibrational Raman spectroscopy for a homogeneously aligned ferroelectric smectic liquid crystal with three dimethyl siloxane groups in the achiral terminal chain, which shows de Vries–type phenomena, i.e., very large electroclinic effect in the smectic-A Sm-A phase and a negligible layer contraction at the phase transition between the Sm-A and Sm-C * phases. The apparent orien- tational order parameters of the rigid core part of the molecule are extremely small both with and without the external electric field in Sm-A. These results provide evidence for the existence of the de Vries Sm-A phase, where the local molecular director is tilted at a large angle. DOI: 10.1103/PhysRevE.71.041705 PACS numbers: 61.30.Cz, 61.30.Gd, 64.70.Md I. INTRODUCTION The smectic-A Sm-A liquid crystal phase is constructed by piling two-dimensional layers in which the in-layer direc- tor n is parallel to the layer normal. When the constituent liquid crystal molecule has chirality, an application of exter- nal electric field induces an average tilt of the molecules with respect to the layer normal. This phenomenon is known as the electroclinic effect 1. Suppose that the smectic layer spacing is simply determined by the rigid molecular length, the tilting due to the electroclinic effect should cause the layer contraction by a factor of cos , where  is the tilt angle between the in-layer director and the layer normal 2. This layer contraction is observed as stripes parallel to the layer normal in homogeneously aligned sandwich cells 3 and these stripes are found to be detrimental to the working of a good display. In some chiral smectic liquid crystals however, the mol- ecules tilt but the layers do not contract 4. These materials have a large potential for useful applications in displays and devices. This behavior can occur as a function of decreasing temperature into the smectic-C * Sm-C *  phase, or under an applied electric field in the smectic-A phase. Adrien de Vries studied this behavior in nonchiral smectics only 5–7, and suggested that it is caused by a disordered molecular orien- tation in the smectic-A phase, which becomes more ordered under decreasing temperature or increasing electric field. Since then, liquid crystals exhibiting this behavior are gen- erally called de Vries materials and a class of materials in- volving chiral smectics have recently gained importance due to their potential for applications in photonic devices. Sel- inger et al. 8 have reported the results of experiments of tilt angle and birefringence of DSiKN65 and TSiKN65 with one and two siloxane spacers and have concluded that these materials have a disordered distribution of molecular tilt di- rections that is aligned by the electric field. This is found to be the basis of the large electroclinic effect. Recently several other groups 9–14 investigated this behavior using x-ray diffraction and electro-optical studies of such chiral smectics. Their results are consistent with the de Vries concept that the molecular orientation is initially disordered but becomes more ordered under an applied electric field. However, the limitation of those experiments is that they provide only in- direct information about the orientational ordering of the molecules. In this paper, we go beyond those earlier experiments by using polarized Raman scattering to study one of these ma- terials. The technique of polarized Raman scattering provides direct measurements of the second  P 2  and the forth order  P 4  orientational order parameters of the liquid-crystal mol- ecules. We denote the experimentally measured values by a subscript “app” which implies that these are apparent order parameters which are generally different from the molecular orientaional order parameters. Our experiments show that *Present address: Advanced Core Laboratory, Fuji Photo Film Co., 210 Nakanuma, Minami-Ashigara, Kanagawa 250–0193, Japan. Electronic address: naoki_a_hayashi@fujifilm.co.jp ² Electronic address: jvij@tcd.ie PHYSICAL REVIEW E 71, 041705 2005 1539-3755/2005/714/0417058/$23.00 ©2005 The American Physical Society 041705-1