Ultrasonic investigation of lattice instability and superconductivity in high-Tc systems

Ultrasonic measurements have indicated that there are possible structural changes near 250 K, 160 K and just above Tc in the high-Tc Y-Ba-Cu-O and Y-Ba-Cu-Nb-O systems, which were confirmed to be first-order-like phase transitions by thermal analysis (DSC) results. It is suggested that the heavily suppressed pre-superconducting transition, which provides the lattice instability and the soft phonon modes, gives rise to the high Tc.