Ultrasonic evidence for a structural phase transition at 220 K inYBa2Cu3O7δ

Abstract
Ultrasonic study of the superconductor YBa2Cu3O7δ reveals the existence of a shear instability centered at ∼220 K. This instability is manifested by a large step in both the longitudinal and transverse velocities. This step is also accompanied by a peak in the attenuation. The analysis of the results suggests that the instability corresponds to a structural order-disorder transition.