Evidence fora-b-plane coupling to longitudinalc-axis phonons in high-Tcsuperconductors

Abstract

We show that the strong absorption structure observed in the a-b-plane optical conductivity of the high-${\mathit{T}}_{\mathit{c}}$ superconductors is correlated with c-axis longitudinal optical (${\mathrm{LO}}_{\mathit{c}}$) phonons. This suggests a resolution to the long-standing controversy over the origin of these features. The interaction with ${\mathrm{LO}}_{\mathit{c}}$ 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.