Phonon Dispersion Curves of High T_c Superconductors. II. YBa₂Cu₃O₇

抄録

Phonon dispersion curves of YBa₂Cu₃O₇ have been calculated by making use of force constants deduced from those of (La_1−xSr_x)₂CuO₄ in a preceeding paper as well as our experimental results of ultrasonic attenuation and sound velocity for YBa₂Cu₃O₇. Results are resumed as follows. The intra-layer and inter-layer optic couplings between Cu and O atoms in the incomplete octahedron CuO₆ in YBa₂Cu₃O₇ give rise to the energies of 20∼27 meV at the B. Z. center. These optic phonons with desirable symmetry properties for causing strong electron-phonon interactions seem to be favorable to contribute to the high T_c superconductivity. In particular, a large contraction of Cu–O side bond is sticking out from the linear Cu–O chain may be a very advantageous stituation to lead to a stronger electron-phonon interaction in YBa₂Cu₃O₇ than in (La_1−xSr_x)₂CuO₄.