Nematic-Smectic-A-Smectic-CMulticritical Point

Abstract

When the nematic-smectic-A-smectic C (NAC) multicritical point was first suggested, the two proposed explanations of it were very different. Chu and McMillan 1 (CM) proposed a model in which smectic-C has the dipolar order parameter of McMillan's theory, 2 while the smectic-A order parameter was the one-dimensional density wave of Kobayashi, 3 McMillan, 4 and deGennes. 5 Tilt of the director away from the layer normal entered the model somewhat incidentally through a gradient term coupling the two order parameters. Chen and Lubensky 6 (CL) found an NAC point in a model where the only order parameter was the one-dimensional density wave. Tilt of the director relative to the layer normal is a central feature of the model and occurs when the coefficient of the transverse gradient term becomes negative. From the experimental point of view the differences between the two models are primarily twofold, First, in the CL model the NAC point is a type of Lifshitz point, 7 which leads to the predic-1033 (1979). 9 G.We did not, however, compensate for the intensity reduction by scattering. 14 G. Northrop, to be published. 15 J. Callaway, Phys. Rev. LL3, 1046LL3, (1959.tion that x-ray scattering in the nematic phase near the NAC point falls off in the transverse direction as k ± '* 9 rather than the usual k ± ' 2 predicted by the CM model. Secondly, the nematic-smectic-C (NC) transition entropy is zero in the CM model but finite in the CL model because of the Brazovskii 8 " 10 effect. The NC entropy was found to be finite when the NAC point was discovered experimentally. 11 In this Letter evidence is presented, based on the topology of the phase diagram and the nematicsmectic-C transition entropy, that for the system studied, which is the same as the one studied previously, 11 the NAC point is likely not either of the above kinds of multicritical points. Instead it conforms well with a phenomenological Landau expansion suggested recently by Benguigui, 12 for which is offered a physical interpretation.In earlier work on mixtures of octyl-and heptyloxy-£'-pentylphenyl thiolbenzoate (7S5-8S5), 11 gross features of the phase diagram were deter-A study of the phase diagram near the nematic-smectic-v4. -smectic-C multicritical point and of nematic-smectic-C transition entropies leads to the conclusion that this is not a Lifshitz point. It is shown that a biaxial second-rank tensor order parameter may be needed. 664

Liquid crystalline properties of 4‐hexyloxybenzylidene‐4′‐alkyloxyanilines

“…Especially important is the triple point of the NAC type, for which the critical properties have been fully LC properties of 4-hexyloxybenzylidene-49-alkyloxyanilines 915 discussed in the literature [24,25]. The presented phase diagrams show similar properties.…”

New types of ordering in liquid crystals

“…На линии перехода N-С расходятся все три модуля Франка -8K t ~ | 2 , а в точке Лифшица 8K t 2 ~ In £, 8К 3 ~ |. Появившиеся в самые последние годы экспериментальные данные 27 о топологии фазовой диаграммы в окрестности N-А-С-точки и критических индексах не согласуются ни с одной из описанных выше моделей. В любом случае можно для исследованных в 27 веществ считать твердо установленным, что тройная N-А-С-точка не является точкой Лифшица.…”

Multicritical phenomena in liquid crystals

“…Although the NAC point is relatively common among liquid crystal mixtures, it has not yet been found in a one-component system as a function of pressure. The most thoroughtly studied system is 7S5/8S5 which has been extensively characterized thermodynamically (67,70,71) and by X-ray scattering (72,73). X-ray scattering is particularly effective because the shape of the scattering ring above the NC transition is quite different in the CL and CM theories.…”

“…In addition to the X-ray data, there are now extensive heat capacity data in the literature on the 7S5/8S5 system (70,71). These data, like some aspects of the X-ray data, do not seem to be easily explainable by a Lifshitz point model.…”