Phonon Dispersion Curves of High T_c Superconductors. I. (La_1−xSr_x)₂CuO₄

Abstract

Phonon dispersion curves of a high T_c superconductor (La_1−xSr_x)₂CuO₄ have been calculated by making use of the force constants deduced from our experimental results of ultrasonic attenuation and sound velocity for samples (La_1−xSr_x)₂CuO₄ (x=0.07) with T_c∼38 K. Calculations of the dispersion curves of BaPb_1−xBi_xO₃ having a similar crystal structure have also been made to confirm the rightness of the deduction. The results are resumed as follows: the intra-layer optic couplings between Cu and O atoms and between O and O atoms in the coplanar CuO₄ give rise to the energies around 21 meV and 37 meV, respectively, at the Brillouin zone center. These optic phonons with rather small dispersions and with a desirable symmetry property for causing a strong electron-phonon interaction seem to be very favorable to contribute to the high T_c superconductivity.