Lifshitz point in the phase diagram of a ferroelectric liquid crystal in an external magnetic held

The Lifshitz point in the (H,T) phase diagram of a ferroelectric liquid cry stal in a transverse magnetic field has been determined using photon correl ation spectroscopy and linear electrooptic response measurements. The extra polated Lifshitz magnetic field is 25(1+/-0.1)T for a smectic-C* material w ith an unperturbed helical period of approximate to 6 mu m, and the Lifshit z point is located approximate to 100 mK above the zero-field transition te mperature. We have observed the reentrant helical modulated smectic-C* phas e just below the lambda line. The width of this phase is 100 mK at the low temperature critical field H-c and decreases continuously to zero as we app roach the Lifshitz point. The phase boundary between the reentrant phase an d the smectic-il phase is of second order, whereas the phase boundary with the unwound smectic-(C) over bar* phase is of first order far away from the Lifshitz point and becomes of second order close to the Lifshitz point. Th e order parameter dynamics is discussed within the Landau theory.