Induced helical structure of nematics in a magnetic field

Abstract

Nematics MBBA, EBBA, PBBA, BBBA, 5CB, 8OCB and PCH5 doped with chiral 2-(4′-phenylbenzylidene)-p-menthan-3-one are studied. A numerical solution in terms of the Landau-de Gennes theory is obtained to describe the experimental temperature dependence of P/P ₀, where P ₀ corresponds to P in zero magnetic field. The angle of tilt θ from the plane perpendicular to the helical axis is evaluated and the domain wall energy in a constant magnetic field analysed. To take into account the helical structure deformation, the helicoidal order parameter G is defined, so that G = 0 in the N phase and G = 1 with dϕ/dz = const. and θ = 0 in the N* phase. The relaxation of the helical structure to the equilibrium state (N* → N) is shown to be determined by the domain wall energy with the magnetic field H ≠ 0.