OBSERVATION OF AN OPTIC-LIKE PHASE MODE IN AN ANTIFERROELECTRIC LIQUID-CRYSTAL

We have observed an acousticlike and an opticlike branch of phase exci tations in a quasielastic light-scattering experiment performed in the antiferroelectric smectic-C-A phase of -ethylheptyloxycarbonyl)pheny l-4'-alkylcarbonyloxy biphenyl-4-carboxylate. In contrast to the nearl y gapless acoustic phason branch, the opticlike phase excitations alwa ys have a finite relaxation rate. The phenomenon is a direct consequen ce of a doubling of a smectic unit cell at the transition from the sme ctic-A phase and is in remarkable agreement with a Landau-type model.