New Interpretation of Second-Sound Measurements at the Nematic-Smectic-ATransition: 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 $T\to T_{\mathrm{AN}}$.