PHONON-DISPERSION CURVES OF YBA2CU3O7 AND (LA1-XSRX)2CUO4

Phonon dispersion curves of high T//c superconductors, YBa//2Cu//3O//7 (YBCO) and (La//1// minus //xSr//x)//2CuO//4 (LSCO) have been calculated by making use of the force constants deduced from acoustic measurements. Important characteristics of the results are as follows: The intra-layer and inter-layer optic couplings between Cu and O atoms in the quasi-octahedron CuO//6 or in the incomplete octahedron in YBCO give rise to energies of 20-27 mev at the Brillouin zone center. These optic phonons with small dispersions and with desirable symmetry properties for causing strong electron-phonon interactions seem to be favorable to contribute to the high T//c superconductivity. In YBCO a large contraction of Cu-O side bond along the c-axis which is sticking out from the linear Cu-O chain may be advantageous leading to higher T//c than in LSCO.