LOW-TEMPERATURE CROSS-OVER EFFECT IN ULTRASONIC DAMPING IN YBCO CERAMICS

Non-linear acoustic effects in YBCO ceramics with different grain size s are investigated by means of the composite oscillator technique at a frequency of about 100 kHz. Irreversible changes in decrement and res onance frequency introduced during measurements of their amplitude dep endence at 6.5 K are found to recover as a result of annealing at temp eratures up to 65 K. An unusual cross-over effect in the amplitude dep endent damping curves is observed at low temperatures. It is dependent on grain size and seems to decrease at higher temperatures (it is not observed at room temperature). Essential elements of possible models for explaining these observations are briefly discussed.