Dependence of the low-temperature acoustic spectrum of YBa2Cu3O<i>x</i> ceramic on the oxygen index

Temperature dependences of the oscillation decrement δ(T) and longitudinal ultrasound velocity υ1(T) at frequencies ∼ 105 Hz are studied in HTS ceramic samples of YBa2Cu3Ox with quite different oxygen contents (x = 6.95 and 6.03). Measurements were carried out in the temperature interval 6–340 K under multiple temperature cycling. Upon a transition from orthorhombic to tetragonal phase, hysteresis in the υ1(T) dependence practically disappears and two low-temperature absorption peaks at 60 and 135 K are transformed into a single peak at 83 K. It is shown that the acoustic anomalies in samples with x = 6.95 and 6.03 are due to different relaxation processes. Activation parameters are determined and possible mechanisms for such processes are discussed.

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