DOI 10.1140/epje/i2007-10306-1 Eur. Phys. J. E 25, 395–402 (2008) T HE EUROPEAN P HYSICAL JOURNAL E Multistage polar switching in bent-core mesogens S. Findeisen-Tandel 1 , M.W. Schr¨ oder 1 , G. Pelzl 1 , U. Baumeister 1 , W. Weissflog 1 , S. Stern 2 , A. Nemes 2 , R. Stannarius 2 , and A. Eremin 2, a 1 Martin-Luther-Universit¨at Halle-Wittenberg, Institut f¨ ur Physikalische Chemie, M¨ uhlpforte 1, D-06108 Halle (Saale), Germany 2 Otto-von-Guericke-Universit¨at Magdeburg, IEP, Abt. Nichtlineare Ph¨anomene, Universit¨atsplatz 2, D-39016 Magdeburg, Germany Received 23 October 2007 and Received in final form 23 January 2008 Published online: 21 April 2008 – c  EDP Sciences / Societ`a Italiana di Fisica / Springer-Verlag 2008 Abstract. We report a novel type of electro-optical switching in a tilted smectic phase of bent-shaped mesogens. The switching consists of a continuous stage and two bistable transitions. Detailed optical and electro-optical measurements using high-speed imaging are given and possible interpretations of the experimental results are discussed. PACS. 77.84.Nh Liquids, emulsions, and suspensions; liquid crystals – 78.20.-e Optical properties of bulk materials and thin films – 77.80.-e Ferroelectricity and antiferroelectricity 1 Introduction Since their discovery, ferroelectric liquid crystals have at- tracted particular attention of scientists and applied re- searchers [1–5]. First, ferroelectric order in liquid crys- tals has been discovered in a tilted chiral smectic phase (SmC ∗ ). In a single layer, the polar order occurred as a result of a broken mirror symmetry and the tilt of the molecules. However, the spontaneous polarization in the bulk of SmC ∗ phases is averaged out by a helical wind- ing. SmC ∗ phases confined between two boundaries in surface stabilised states show a clear bistable switching with a spontaneous polarization in the range from a few nC/cm 2 to several hundreds of nC/cm 2 [5]. This kind of behaviour has found a wide range of applications in sci- ence and technology. Apart from commensurate ferroelec- tric SmC ∗ phases, antiferroelectric and a number of in- commensurate ferrielectric phases have been reported up to now [6–8]. The discovery of a new class of liquid crystals formed by bent-shaped molecules considerably expanded the variety and complexity of phenomena encountered in thermotropic liquid crystals [9–12]. Bent-core mesogens are able to form polar liquid-crystalline phases which are an exciting and beautiful object of soft condensed mat- ter systems. As first shown by Niori et al. [13] smectic phases formed by bent-core molecules are able to form polar structures and to show polar switching. The po- lar order of smectic phases results from the sterically in- duced packing of the molecules in bend direction. Bent a e-mail: alexey.eremin@physik.uni-magdeburg.de molecules in the polar smectic phases can be parallel to the layer normal (SmAP phases) or tilted (SmCP phases) where P stands for the polar order in the phase struc- ture. Thus, in contrast to calamitic chiral SmC ∗ phases, the spontaneous polarisation in bent-shaped smectics can appear independently of tilt [14]. However, polarisation- tilt coupling is still present [15]. Interestingly, in tilted polar smectic phases chirality of the smectic layers arises even when the constituent molecules are achiral [16–18]. In most cases SmAP phases as well as SmCP phases are found to be antiferroelectric (SmCP A ) [9–12,19,20]. The preferred occurrence of the antiferroelectric ground state is due to energetic effects, i.e. by the escape of the macro- scopic polar order, but also due to entropic effects de- termined by interlayer fluctuations. There are also SmCP phases with a ferroelectric ground state (SmCP F ) which have been observed in bent-core compounds with end- branched chiral alkyl chains, with terminal oligo(siloxane) or oligo(carbosiloxane) units, furthermore in compounds with non-branched alkyloxy chains adjacent to fluorine substituents (see references in [10]) as well as in laterally non-substituted compounds [21]. The ferro- or antiferro- electric nature of the SmCP phase is mostly examined by the triangular-wave voltage method [22]. Ferroelectric states are characterized by one peak per half-period of the applied triangular voltage, whereas in the antiferroelec- tric ground state two current peaks per half-period can be recorded. In this paper, we present homologous com- pounds of a new series of bent-core mesogens which form a special kind of the SmCP phase. This phase shows a current response which is very distinctive from the usual