External-field-induced tricritical point in a fluctuation-driven nematic-smectic-A transition

We study theoretically the effect of an external field on the nematic-smect ic-A transition close to the tricritical point, where fluctuation effects g overn the qualitative behavior of the transition. An external field suppres ses nematic-director fluctuations by making them massive. For a fluctuation -driven first-order transition, we show that an external field can drive th e transition second order. In an appropriate liquid-crystal system, we pred ict the required magnetic field to be of order 10 T. The equivalent electri c held is of order 1 V/mu m.