Behavior of the helix in some chiral smectic-C* liquid crystals.

In spite of numerous papers dedicated to the subject, the behavior of the helical pitch in the smectic C* phase near the smectic A phase is still controversial. In particular, it is not clear to what extent the measured pitch values are influenced by surface effects. In order to shed light on this problem, careful pitch measurements have been performed using different methods and sample preparations. We have found that the diffraction of light on planar-oriented samples may give incorrect pitch results because of strong influence of the surfaces and possibly of the sample texture on the helical structure. We have demonstrated that using appropriate preparation of samples the bulk properties of the helix can be determined. It has been found that a maximum of pitch can indeed occur but then only when very close to the transition point smectic C*-->smectic A (about 0.1 K). The results of helical pitch measurements have been compared with predictions of theories.