Electron-diffraction study of surface-induced phase transitions in hexatic liquid-crystal films

Defect-mediated phase transitions have been generally considered as one of the most exciting topics in condensed matter physics in recent years. The two-dimensional (2D) defect-mediated melting theory predicts that an intermediate hexatic phase exists when a 2D crystal melts into the liquid.; We have performed electron-diffraction studies on various liquid-crystal (LC) thin films and reported the following discoveries:; 1. We have resolved the nature of surfacing-freezing transitions in 40.8 by showing the existence of a surface-induced hexatic-B (HexB) phase which is not present in bulk samples. The surface anomaly observed by heat-capacity and optical-reflectivity measurements is therefore a smectic-A (SmA)-HexB transition rather than a SmA-crystal-B (CryB) transition. We have also discovered a novel two-step layer-by-layer freezing process mediated by the hexatic phase in these 40.8 films.; 2. We reported the first unambiguous identification of a hexatic smectic-L (SmL) phase in FTE1 samples. This discovery represents the first experimental realization in a thermotropic LC system of the theoretical expectation that a 2D tilted smectic can undergo transitions from the smectic-I (SmI) to smectic-F (SmF) phase via an intermediate hexatic SmL phase. We also observed unusual surface-freezing behavior in FTE1 films at lower temperatures, with multiple novel phase transitions made possible by the molecular tilt. These include the orthorhombic crystal-H (CryH) and crystal-N (CryN) phases, observed here for the first time as surface phases with tilted hexatic interiors. By providing unambiguous structural confirmation of surface SmL ordering, our data directly support the proposed explanation of the unusual stripe phase observed optically in FTE1 and other materials.; 3. We reported the results of structural measurements on 70.7 films of up to 25 layers thick that reveal the unusual coexistence of three phases, with a middle phase a few layers thick sandwiched between the outmost layer and the interior. The middle phase appears to be a novel tilted liquid with hexaticlike positional correlations but no long-range bond-orientational order. The result that this novel middle phase appears to be the tilted analog of a possible new phase previously reported between the SmA and the HexB in orthogonal smectics provides important additional indication for the existence of this unusual highly-correlated liquid, which could significantly modify our understanding of two-dimensional melting.