Ultrasonic studies of the roles of the electron-phonon interaction in high-Tc superconductors

Abstract

The role of the electron-phonon interaction in high-Tc superconductors MBa2Cu3O7 (M = Y, Nd, and others), (La1−xSrx)2CuO4, and BaPb1−xBixO3 were studied by acoustic means. At scores of MHz, only BaPb1−xBixO3 showed an appreciable electronic contribution to the sound attenuation, suggesting the largeness of the electron-phonon interaction. For higher Tc samples, no evidence of the electronic contribution was found, as can be predicted by the absence of the isotope effect in YBa2Cu3O7. Structural instability was, however, investigated for some samples, and the energies of optical modes were estimated. Based on these experimental results and others, the phonon dispersion curves and the density of state curves D(ε) were calculated in agreement with inelastic neutron scattering data on D(ε). The calculated results show that the optical modes due to Cu-O and O–O bonds in YBa2Cu3O7 provide large contributions to the electron-phonon interaction in view of the energy scale and symmetry properties of wavefunctions. There will be some discussion on what factors counterbalance these advantages. Using single crystals of YBa2Cu3O7 and NdBa2Cu3O7, measurements are being carried out to derive more accurate acoustic information.