Ultrasonic experiments in KDP-type crystals

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Abstract
In this review we summarize a wide variety of ultrasonic experiments performed on KDP-type crystals. The first part of the paper covers the phenomenological and microscopic theories needed for analysis of acoustic data. The second part presents the essential experimental results of ultrasonic velocity and attenuation measurements. The third part gives a critical comparison of the experimental data with theory. The phenomenological theory is based on Landau relaxation theory, whereas the microscopic theory is based on the order-disorder model developed by Silsbee, Uehling and Schmidt. The relationship between the polarization relaxation time derived from ultrasonic experiments and the optical overdamped soft mode is discussed. In this context, the possible role of tunneling in different isomorphs is evaluated. The evidence cited indicates that proton tunneling does not provide a satisfactory model for explaining the substantial isotope effects on Tc and the critical relaxation rate. The effects of external parameters such as electric field and pressure on the elastic properties are also reviewed at the end of the article.