Collective excitations and field induced anti-ferroelectric modes in chiral liquid crystal dimer

Abstract

Collective excitations and field influence are investigated in tilted or orthogonal layered liquid crystal smectic (Sm) phases in a chiral hydrogen bonded (HB) calamitic dimer 10OBA:M*SA:10OBA by low-frequency dielectric spectroscopy. Phase transition temperatures, hysteresis, and phase thermal ranges are determined. The hysteresis in SmC* reveals the storage device suitability. The magnitude of the tilt angle in SmC* infers moderate viewing angle. The growth of primary order parameter tilt angle θ(T) infers long range interaction in SmC*. The growth of ferroelectric (FE) relevant secondary order parameter polarization Ps(T) infers strong layer-over-layer helical coupling of transverse dipole moment μt. Dispersion (loss vs permittivity) in 1D SmC*, 3D orthogonal SmBcryst, and 3D hexagonal tilted SmG crystal phases infers two distinct collective modes, viz., high frequency soft mode (SM) and low frequency Goldstone mode (GM). The dielectric strength of the SM in the SmC* phase confirms the FE Curie–Weiss behavior. The GM persists down to low temperature 3D LC phases. The AF order in SmC* is smeared by high temperature thermal fluctuations. A bias field resolves the GM in SmC* to reveal antiferroelectric (AF) modes. AF order manifested as GM1 and GM2 is explained by distinct Ps helices with different relaxation frequencies fR1 and fR2. GM1 and GM2 in 3D SmBcryst and SmG phases are explained by quenching by crystal field. HB LC dimer with a just shifted chiral center promoted the occurrence of field induced AF modes. AF P-switching in SmC* infers the gray scale mode. Trends of dielectric parameters, relaxation frequency, loss maximum, activation energy, dielectric strength, distribution parameter, etc., are discussed with respect to utility in appliances.