The Sound Velocity near the Pressure Induced Tricritical Point of TGSe

The temperature dependence of ultrasonic velocities of TGSe single crystal was studied at several hydrostatic pressures. Particularly, the sound velocities near tricritical point ( T tri =40.6°C, p tri =5 kbar) were measured minutely. At atmospheric pressure, there is a difference between the transition temperature and the temperature at which the anomalous part of the sound velocity takes maximum. On approaching tricritical point, this temperature difference becomes small and disappears ultimately in the first order phase transition region. The pressure dependence of the magnitude of anomalous part of the sound velocity along the a * axis is examined. It is found that these pressure dependences are due to tricritical nature if the polarization relaxation is taken into account.

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