Dislocation-loop melting theory of phase diagrams with nematic regions surrounded by smectic regions

We show that a dislocation-loop theory of the smectic-A–nematic transition predicts the existence of both nematic regions surrounded by smectic regions and a smectic–$A_1$-$A_d$ critical point in the phase diagram, in agreement with recent observations. Fluctuation-corrected mean-field theory leads to a remarkable topology for the isolated nematic region.