Evidence for a-b plane coupling to longitudinal c-axis phonons in high-Tc superconductors

We show that the strong absorption structure observed in the a-b-plane optical conductivity of the high-T c superconductors is correlated with c-axis longitudinal optical (LO c ) phonons. This suggests a resolution to the long-standing controversy over the origin of these features. The interaction with LO c phonons is forbidden when the incident wave vector q is normal to the c axis which leads to the surprising result, confirmed with experiments, that the optical properties along a or b are different when measured on the a-b-plane face and a face containing the c axis