Critical behavior of the de Gennes elastic constants near the nematic-smectic-A transition of TBBA

We have studied the behavior of elastic constants A, B, and C deduced from ultrasound velocity anisotropies in the vicinity of the nematic-smectic-A t ransition of terephthal-bis-p-p'-butylaniline. A is associated with compres sibility, B with layer compression, and C with the coupling between compres sibility and layer compression. We show that the exponent of A is of the pr easymptotic 3D-XY type, whereas those of B and C are in between the SD-XY v alues and those associated with the anisotropic fixed point. This behavior is consistent with the extended crossover regime predicted by Patton and An dereck [Phys. Rev. Lett. 69, 1556 (1922)].