Sound propagation and damping in the vicinity of the smectic-A-hexatic-B phase transition of 4-propionyl-4'-n-heptanoyloxyazobenzene
Abstract
Sound velocity and damping were measured in 4-propionyl-4'-n-heptanoyloxyazobenzene above its Sm-A-Hex-B phase transition. The measurements were taken at 1 MHz (velocity) and at 3, 9, 15, and 21 MHz (damping) as a function of the angle θ between the sound propagation direction and the normal to the smectic layers. The velocity presents a marked anomaly for θ=90°, whereas a much smaller anomaly is observed for θ=0°, indicating that the phase transition occurs essentially within the smectic layers. Analysis of these measurements allows the de Gennes elastic constants A, B, and C to be determined. Like the velocity, the damping presents significant pretransitional effects for θ=90°, reminiscent of those generally observed in the vicinity of second-order or weakly first-order phase transitions. The damping also increases for θ=0° when T-->TA-Hex, but the behavior observed does not resemble the usual critical behavior. It is shown that this pseudocritical behavior stems essentially from a contribution of the anharmonic effects. The anisotropy of the critical effects on velocity and damping can be explained by the theory elaborated by Andereck and Swift for the Sm-A-Sm-C transition and transposed to the Sm-A-Hex-B transition. Analysis of the velocity measurements indicates that the specific-heat exponent is of the order of 0.6. This value, which is far from that associated with the three-dimensional XY universality class -0.007 to which this transition should in principle belong, confirms the results obtained by calorimetry. The critical relaxation time is characterized by the dynamic exponent zν~=1, which corresponds to conventional critical slowing down.
- Publication:
-
Physical Review E
- Pub Date:
- July 1998
- DOI:
- 10.1103/PhysRevE.58.630
- Bibcode:
- 1998PhRvE..58..630C
- Keywords:
-
- 64.70.Md;
- 61.30.-v;
- 62.80.+f;
- Transitions in liquid crystals;
- Liquid crystals;
- Ultrasonic relaxation