Devil's staircase between antiferroelectric SCA and ferroelectric SC phases in liquid crystals observed in free-standing films under temperature gradients

By studying the electro-optical properties and the textures of the subphases successively emerging between antiferroelectric SCA* and ferroelectric SC* (the Devil's staircase), we have revealed several interface effects in both homogeneous and homeotropic cells; free-standing films are most suitable for making observations almost free from the effects. By applying appropriate temperature gradients to the free-standing films, we can directly see any part of the subphase sequence in the visual field of an optical microscope. The two ferrielectric subphases on the low- and high-temperature sides of ferrielectric SCy* together with another ferrielectric subphase between the antiferroelectric subphase (designated as AF in ref. 9) and SC* were thus confirmed to exist definitely. We have discussed the origin of these successive subphases in terms of the several theoretical models reported so far, concluding that the ANNNI model with the third-nearest-neighbour interaction well describes their Devil's staircase character.