Bogomol'nyi bound and screw dislocations in a mesoscopic smectic-A
  The de Gennes free energy functional of an infinite smectic-A liquid crystal at the dual point is shown to be topological and to depend only on the number of screw dislocations and the anisotropy. This result generalizes the existence of a Bogomol'nyi bound to an anisotropic system. The role of the boundary of a finite mesoscopic smectic is to provide a mechanism for the existence of thermodynamically stable screw dislocations. We obtain a closed expression for the corresponding free energy and a relation between the applied twist and the number of screw dislocations. 
Eric Akkermans, Sankalpa Ghosh and Amos Schtalheim (Technion I.I.T,   Israel)