ANISOTROPY OF ELASTIC AND RELAXATION PROPERTIES OF THE SUPERCONDUCTING 123-YBCO SINGLE-CRYSTAL

Acoustic properties of the YBa2Cu3O6.7 single rystal are studied in th e temperature range 5-300 K. The wave vector of the longitudinal and t orsional standing waves in the frequency range 50 - 90 kHz was directe d along and normal to the: c-axis. It is shown that the acoustic anoma lies observed in the single crystal qualitatively are the same that th ose found earlier in the 123-YBCO ceramics but depend strongly on the mutual orientation of the wave vector and the c-axis. It is establishe d that the significant temperature hysteresis and ''softening'' of the dynamic elastic moduli at thermocycling are observed only in the case when the sound waves have a displacement component along the c-axis. Moreover, in this case the sound absorption peak near 230 K is more pr onounced. The locations of sound absorption peaks are not sensitive to the oscillation mode and sample orientation. The results obtained are qualitatively consistent with the model of bistable behaviour of the sublattice formed by apical oxygen atoms.