PHASE-EXCITATION SPECTRUM OF FERROELECTRIC LIQUID-CRYSTALS IN AN EXTERNAL STATIC ELECTRIC-FIELD

The dynamics of the phase of the molecular tilt in the helical ferroel ectric liquid crystalline Sm-C phase is studied within the Landau mod el. We show that a static electric held applied parallel to smectic la yers induces a gap in the excitation spectrum of the phase of the tilt at the edge of the Brillouin zone. The gap is shown to grow linearly with the strength of the field up to a critical field beyond which the helical structure finally unwinds. The influence of the phase-excitat ion dynamics on the dielectric response is investigated in order to re solve the contradiction between experimental findings and previous the oretical results. We find the dielectric strength of an infinitely lar ge system to decrease with the bias field and go to zero at the critic al field in accordance with measurements. In order to aquire a better understanding, the properties of a system of a finite length along the helical axis are investigated. It is shown that the dielectric streng th consists of two parts: one related to the local variations of the p hase of the tilt and the other related to the unwinding of the whole h elical structure. The former part of the dielectric strength vanishes at the critical field, whereas the latter part diverges. In addition, the relaxation time for the unwinding of the helical structure is foun d to increase with the length of the system and has a range in real pl anar samples from minutes to hours. In conclusion, the dynamics of the tilt in a finite-size system is found to be crucial in interpreting e xperimental as well as theoretical results.