Phonons in the high temperature Bi₂Ca_n-1Sr₂Cu_nO_4+2n superconductors

Abstract

We consider phonons in two members of the Bi-high temperature superconductor family, Bi2Sr2CuO6 and Bi2CaSr2Cu2O8. For these compounds, we sort the zone center phonon modes according to the irreducible representations of the appropriate point group. When there are two (or more) CuO planes in the unit cell, there is a B1g type of mode that only involves the motion (along the c-axis) of the oxygen atoms in these planes. The Raman spectrum of Bi2CaSr2Cu2O8 is measured by back scattering from ab-plate-like crystals and some mode assignments are given. The unique B1g phonon is assigned to the 464 cm-1 mode. This frequency is higher than that found for the B1g mode in YBa2Cu3O7, but comparable to that in Sr3Ti2O7. © 1988.