  The order parameter of the smectic liquid crystal phase is the same as that
of a superfluid or superconductor, namely a complex scalar field. We show that
the essential difference in boundary conditions between these systems leads to
a markedly different topological structure of the defects. Screw and edge
defects can be distinguished topologically. This implies an invariant on an
edge dislocation loop so that smectic defects can be topologically linked not
unlike defects in ordered systems with non-Abelian fundamental groups.
