Local lattice Orientational Order in Two-Dimensional Liquid Crystal Films: Its Orientation, Characteristic Defects, Thermal Signature

Ten years ago it was proposed that two dimensional crystals melt in two stages through an intermediate phase which exhibits quasi-long range orientational order of the local lattice but only short range positional order. The observation of this intermediate phase in free-standing liquid crystal films will be described. Both x-ray and electron diffraction were used to probe the detailed nature of its structural order. Furthermore, an ac calorimeter was developed which revealed the distinctive temperature dependence of its heat capacity as well as optical techniques which revealed characteristic defect structures and the orientational fluctuation modes of the local lattice. These measurements will also be presented.