Ultrasonic Study of NaNO₂. III. Critical Behavior of Elastic Stiffness Constants at the Normal-to-Incommensurate Phase Transition

抄録

Two coupling terms between the order parameter Q(q) with wave vector q and strain ε_i, (a) γ_i(q)Q(q)Q(−q)ε_i and (b) η_i(q)Q(q)Q(−q)ε_i², give rise to the anomalous behavior of the elastic stiffness at the phase transition. The coupling (b) causes a similar critical stiffness constant in both longitudinal and transverse waves. On the other hand, (a) brings about the softening of the stiffness constant only in longitudinal waves. Using these characteristics the anomaly caused through (a) is isolated under a minimum assumption and then, γ_i(q) for i=1–3 are estimated. Contrary to a simple theoretical prediction, the above anomaly is not proportional to that of the specific heat. It is partly due to the disregard of the q-dependence of γ_i(q). This effect should be taken into account in the study of the elastic anomaly in the other substances.