NEMATIC-TO-SMECTIC-A TRANSITION IN AEROGEL

We study a model for the nematic-to-smectic-A (NA) transition in aerog el, and find that even arbitrarily weak quenched disorder (i.e., low a erogel density) destroys translational (smectic) order. Ignoring elast ic anharmonicities, but keeping anharmonic couplings to disorder, lead s to the prediction that there is no ''Bragg glass'' phase in this sys tem: it is riddled with dislocation loops induced by the quenched diso rder. Orientational (nematic) order is destroyed as well, as is the th ermodynamically sharp NA transition, in agreement with recent experime ntal results.