Ultrasonic Studies of Incommensurate Phase Transitions

Abstract
The temperature dependencies of the ultrasonic velocities and attenuation coefficients are studied in the vicinity of the incommensurate phase transitions of BCCD and AHSe crystals. The largest contribution to the longitudinal ultrasonic anomalies is due to the coupling to the amplitude mode, whose relaxation time τ diverges as the transition temperature T_i is approached with the temperature dependence given by : τ=τoTi/T-Ti). The amplitudon relaxation time for AHSe crystal (τo=5.1.10^-12s) was found to be considerably longer than for BCCD (τo=1.3.10^-13s). From the measurements of shear ultrasonic wave velocity in incommensurate phase below T_i = 164 K the critical exponent that describes the order parameter temperature behaviour has been calculated for BCCD. The value β is consistent with three dimensional X - Y model. The substitution of Cl by Br in BCC_1-xBr_xD crystals with x<0.24 manifests itself in smearing of the elastic anomalies and shifting T_i to lower temperatures with increase of Br concentration.