Electrically Induced Isotropic-Nematic-Smectic-APhase Transitions in Thermotropic Liquid Crystals

We measure optically the order induced by a stabilizing electric field $E$ in the isotropic phase of a liquid crystal mixture of $4^{\prime }-n-\mathrm{octyl}-\mathrm{cyanobiphenyl}$ and $4^{\prime }-n-\mathrm{decyl}-\mathrm{cyanobiphenyl}$ which shows spontaneously an isotropic-smectic-$A$ transition. Close to the transition, a paranematic to a nonspontaneous nematic (NSN) first-order transition is induced by the field with a critical point. At higher field another first-order transition is induced from the NSN to a likely smectic-$A$ phase with a tricritical point. A phenomenological Landau-de Gennes model is developed to describe the transitions.