DESTRUCTION OF A 1ST-ORDER SMECTIC-A -] SMECTIC-C-ASTERISK PHASE-TRANSITION BY DIMENSIONAL CROSSOVER IN FREESTANDING FILMS

Shifting and destruction of phase transitions due to the finite thickness of free-standing smectic films are examined. The behavior of smectic films is studied as a function of the temperature and the thickness in a material showing a first-order smectic-A (Sm-A)-->smectic-C* (Sm-C*) bulk phase transition. Measurements of the optical reflectivity and direct observations in an inverted polarizing microscope reveal that the first-order phase transition between Sm-A- and Sm-C*-like structures takes place in films that are thick enough (N > N(c) = 90). In thin films (N < N(c)) this transition disappears. For N almost-equal-to N(c), a critical behavior is detected. A theoretical scheme for interpretation of phase transitions in smectic films is proposed and on its basis a conjecture on tension-induced phase transitions is formulated.