Molecular-Field Theories of Reentrance in Liquid Crystals

Within both the Landau and microscopic mean-field theories, reentrance is shown to arise only if either the order parameter of interest is coupled to an additional degree of freedom, or the Hamiltonian is explicitly temperature dependent. Reentrant nematic and smectic phases are exhibited by a fully self-consistent treatment of a simplified McMillan's type model involving coupled nematic and smectic order parameters without any explicitly temperature dependent factors. A reentrant isotropic phase is shown to be exhibited by an explicitly temperature-dependent Hamiltonian as well as by a two-state model.