Anisotropy of the Critical Magnetic Field in a Ferroelectric Liquid Crystal

Abstract

The angular dependence of the critical magnetic field for the unwinding of the Sm-C ‫ء‬ helix of a ferroelectric liquid crystal has been measured by a deuteron NMR sample tilting technique. A minimum at intermediate angles, resulting in a reentrant Sm-C ‫ء‬ Sm-C Sm-C ‫ء‬ behavior, has been observed. The solitonlike deformation of the helix has a 2p symmetry at a general orientation, whereas it has a p symmetry for the magnetic field parallel to the smectic layers. The results can be interpreted in terms of the Landau theory with the inclusion of a nonparallel magnetic field energy term.[S0031-9007(98)06068-2]

Effect of the Magnetic Field on the Supramolecular Structure of Chiral Smectic C Phases: 2H NMR Studies

“…From this pioneering experiment on the chiral smectogen DOBAMBC-D 21 , the authors could evaluate the anisotropy of the critical magnetic field H C . [19] They also found that solitonlike deformations of the helix have p and 2p symmetry for b = 908 and b ¼ 6 908, respectively. Under these experimental conditions, they observed a phase-transition sequence, wound-unwound-wound SmC * (SmC…”

“…This kind of study is very informative and provides an understanding of the phase structure and the stability, and also further supports the Landau-de Gennes theory for the smectic phase. [19] Recently, this type of study was applied to the smectogen 1-methylheptyl 4'-(4''-n-decyloxybenzoyloxy) biphenyl-4-carboxylate (10B1M7) showing an interesting sequence of smectic subphases, which include the antiferroelectric and ferrielectric ones. [21] This work confirms the real potential of 2 H NMR as a sensitive method to understand the structure and the magnetic properties of the ferrielectric mesophases.…”

“…[13] Among the most significant works on this topic, that performed by Zalar and co-workers, which is based on angular-dependent 2 H NMR measurements, must be mentioned. [19,20] The 2 H NMR spectra were recorded as a function of b, the angle between the external magnetic field H and the helicoidal axish. From this pioneering experiment on the chiral smectogen DOBAMBC-D 21 , the authors could evaluate the anisotropy of the critical magnetic field H C .…”

Renormalization-group picture of the Lifshitz critical behavior

“…Some examples include high-T c superconductors [3,4,5], ferroelectric liquid crystals [6,7,8], uniaxial ferroelectrics [9], some types of polymers [10,11,12,13] and magnetic materials [14,15,16,17]. In particular, the confluence of a disordered, a uniformly ordered and a modulated ordered phase characterizes the special critical point associated to this critical behavior, known as the Lifshitz point.…”

Nuclear magnetic resonance: a powerful tool to study liquid crystals

“…In case of fluorinated compounds, 19 F is also a useful nucleus to investigate. The analysis of the spectral shape of 2 H NMR spectra of liquid crystals provided an excellent and powerful method to investigate the supramolecular structure of liquid crystals, such as in frustrated phases (TGBA, TGBC* and so on) [24] and ferroelectric (SmC*) phases [25]. Dynamic properties, such as molecular reorientational diffusion, translational self-diffusion and collective motions are mainly investigated by specific NMR approaches, such as NMR diffusometry [26] and NMR relaxometry [27] (which requires specific equipment), or by analysing the spin-spin (T 2 ) and spin-lattice (T 1 ) relaxation times (mainly 1 H and 2 H) through specific models [28][29][30].…”