Elastic anomalies and phase transitions in high-$T_c$ superconductors

Ultrasound propagation in both known classes of high-$T_c$ ceramic superconductors reveals the presence of an anomaly above $T_c$, at $T_s$≃95 K in ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ and ≃120 K in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, suggesting that the state between $T_s$ and $T_c$ is the ‘‘normal’’ state preceding superconductivity. The anomalous elastic behavior at and below $T_c$ is dominated by the shear modulus and not by the bulk modulus, implying an unusually strong coupling of the order parameter to shear distortions. Implications of these results for superconductivity are discussed.