PHONON CONTRIBUTION TO HIGH-Tc STATES IN OXIDE SUPERCONDUCTORS AND Nb3Ge.

The temperature dependences of the ultrasonic attenuation coefficient alpha (T) were investigated for (La//xSr//1// minus //x)//2CuO//4 and YBa//2Cu//3O//7. In each oxide two peaks were observed in the alpha (T) versus T plot which are due to optic phonons. The phonon dispersion curves along the DELTA -line (a-axis) and DELTA -line (c-axis) were calculated taking account of these acoustic data. On the basis of these dispersion curves the origin of the very high T//c in YBa//2Cu//3O//7 is ascribed mainly to the existence of the Cu-O linear chain decorated with contracted Cu-O side bonds. Furthermore, an instability of the high-T//c state of the A-15 type Nb//3Ge phase was also investigated.