Frequency-dependent ultrasonic attenuation of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

At temperatures near $T_c$, experimental measurements of temperature-dependent ultrasonic attenuation at three frequencies exhibit maxima, and velocity measurements display softening of the lattice. These attenuation maxima result from a relaxation process which occurs around the superconducting transition, and the softening of velocity around $T_c$ may evidence a structural instability of Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ at these temperatures.