Ground-state phases, orientational ordering

Another most interesting phenomenon has been observed by Mikami et al.31> The ethanol molecules exhibit orientational disorder over three different sites in the high-spin phase, but orient themselves more and more in one of the three sites with decreasing temperature. The variation with temperature of the orientational ground state population seems to be strongly correlated with the temperature dependent spin transition. The authors therefore suggest that the disorder-order transition of the ethanol molecule triggers the spin state transition. 

For large negative crystal field at low temperatures the stable stmcture is the ideal two-sublattice in-plane herringbone phase—that is, the 2-in phase in Fig. 4a. The two sublattices which can be oriented in three different ways relative to the triangular lattice lead to six equivalent ground states. The excitation spectrum of this phase in general has a gap. In this phase the molecular wave functions are localized in the substrate plane, and classically the molecular axes are parallel to the surface see Appendix A of Ref. 141 for an interpretation of the order parameters. Thus, the orientational degeneracy of the pararotational phase is broken by the quadrupolar interactions. A closely related structure was already proposed based on atomistic Lennard-... 

In nematic phase, the liquid crystal director it is uniform in space in the ground state. In reality, the liquid crystal director it may vary spatially because of confinements or external fields. This spatial variation of the director, called the deformation of the direetor, eosts energy. When the variation occurs over a distance much larger than the moleeular size, the orientational order parameter does not change, and the deformation ean be deseribed by a continuum theory in analogue to the classic elastic theory of a solid. The elastie energy is proportional to the square of the spatial variation rate. 

---