Electrically Induced Isotropic-Nematic-Smectic- A Phase 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^'-n-octyl-cyanobiphenyl and 4^'-n-decyl-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.