Anomalous ultrasonic absorption in LiNbO₃ crystals

Abstract

We have observed a strongly fluctuating transmission of ultrasonic waves through a lithium niobate crystal as soon as the sample was either optically illuminated or subjected to a temperature change. The origin of these fluctuations is the charging up of the crystalline surface under optical illumination or under changing temperatures, leading to internal electric fields of about 10⁷ V/cm and to external discharges. These electric fields can be completely eliminated by metallizing the surface of the sample.—Using such a metallized sample we were able to measure the ultrasonic absorption in LiNbO₃ in a reproducible way between room temperature and 4.2 K. There is a sharp absorption peak at 246 K and, at lower temperatures, the absorption varies as T^3/2. These results are discussed.