Ultrasonic study of lattice instability in high-Tc superconductors

We made the experiment of ultrasonic propagation in the sintered Bi2Sr2CaCu2O8 (Bi-2212) and YBa2Cu3O7-x (x=0.35) (YBCO) superconductors in order to study the lattice instability associated with the superconducting transition. The attenuation increase was observed just above the critical temperatures, 0.9 dB/cm for Bi-2212 (Tc=84 K), and 2 dB/cm for YBCO (Tc=60 K), in the longitudinal ultrasonic mode at 9.7 MHz. The attenuation decreased rapidly just below Tc, which showed a condensation mechanism. We could ascribe the attenuation change to the ultrasonic absorption due to the lattice instability caused by the strong correlation between carriers and the local lattice distortion, which led us to the idea of polaron condensation for superconductivity mechanism. (orig.)