Thermal properties of ‘stacked hexatic phases’ in liquid crystals

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Abstract
Ever since the hexatic phase was proposed more than 14 years ago in the context of two-dimensional melting theory, the concept of hexatic order has offered unique insight into the nature of several liquid-crystal phases, namely the hexatic-B, smectic-I and smectic-F phases. Moreover, these liquid-crystal phases have offered numerous opportunities to explore and investigate the nature of hexatic order and the related phase transitions. In this paper we present an extensive review of the thermal properties associated with the phase transitions related to these liquid-crystal hexatic phases. Although both electron-beam and X-ray diffraction studies have indicated the importance of hexatic ordering in understanding these phases, the existing experimental data from numerous thermal measurements fail to demonstrate that hexatic order, which can be represented by a complex variable and therefore belongs to the XY universality class, is the only symmetry-breaking field associated with the hexatic-B—smectic-A transition in liquid crystals.