New interpretation of second-sound measurements at the nematic-smectic-A transition: Nonhydrodynamic behavior of the layer-compression elastic constant.

We show that second-sound measurements taken in the vicinity of the nematic-smectic-$A$ transition are not in the hydrodynamic regime as is presently thought. A dynamic effect associated with relaxation of critical fluctuations explains why the layer-compression modulus $B$ deduced from these experiments deviates from a simple power law and levels off as $T\ensuremath{\rightarrow}{T}_{\mathrm{AN}}$.