TEMPERATURE BEHAVIOR OF REFRACTIVE INDICES OF DISCOTIC NEMATICS WITH HIGH BIREFRINGENCE

Temperature behavior of refractive indices n_j(t) and values of <n(t)> = (n_|| + 2n_⊥)/3,

(t) =

^1/2,

= (ε_|| + 2ε_⊥)/3, ε_j = n_j² for the light waves polarized along (j = ||) and across (j = ⊥) the director in discotic nematics N_D was studied within the microscopic approach with accounting for the anisotropy of the local-field tensor f_j(t) =1 + L_j(t)[ε_j(t) – 1] and the Lorentz tensor L_j(t). The known negative derivatives <n(t)>' = d<n(t)>/dt, and the point t_e of minimum at the dependence n_||(t) for nematics N_D were shown to correspond to small and medium values of the birefringence Δn = n_|| – n_⊥. For nematics N_D with high values of Δn and components L_⊥ less than the threshold value L_⊥o(n_j), two new effects were predicted: positive derivatives <n>',

' and the presence of the maximum point t_o at the dependence n_⊥(t). The correlation of t_o with the magnitude of Δn(t_o) and the dependence Δn(λ) on the light wavelength λ was established. The features of the transition from objects with point t_e to objects with point t_o under gradual increase of Δn were cleared up. The molecules were pointed for which one could expect the presence of the predicted effects. The role of these effects in the improvement of visual characteristics of liquid crystal displays with nematic N_D in the boundary compensation film was observed.