ELECTROCONVECTIVE INSTABILITIES IN SMECTIC-C-ASTERISK LIQUID-CRYSTAL FILMS

We investigate the onset of electroconvection in freely suspended smec tic-C (Sm-C*) liquid crystal films. Compared to nematic liquid crysta ls the Sm-C phase shows in addition a macroscopic electric polarizati on within the smectic planes. Describing the Sm-C film by a two-dimen sional theory we find for low and high frequencies of the applied elec tric ac held, respectively, a conductive and a dielectric instability regime, similar to the case of electroconvection in nematic liquid cry stals. Because of the polarization in the Sm-C phase an additional, ' 'subharmonic regime'' appears at intermediate frequencies, where all t he hydrodynamic and electric variables are moving with half the freque ncy of the applied ac voltage at the onset of convection. For some spe cial but experimentally accessible values of material parameters, geom etric dimensions, and ac voltage frequency also a codimension-three po int is found, where the threshold voltages of all three possible regim es coincide. For de voltages the generalized Frederiks transition is p resented and discussed as far as it restricts the observation of patte rn forming convective instabilities. Both types of instabilities are i nvestigated for de voltages to get a fundamental insight into the mech anisms involved as well as for ac voltages to stimulate experiments.