Collective modes and implications for c-axis optical experiments in layered cuprates.

Abstract

We address recent experiments in the cuprates which demonstrate a coupling of the [ital c]-axis plasma excitation to the superconducting order parameter within a matrix Green's-function formulation. An essential motivation of this work is the premise that a complete unraveling of the order-parameter symmetry cannot be limited to studies of the [ital ab]-plane electromagnetic properties; it must also involve the behavior along the [ital c] axis. Our gauge-invariant formalism for the electromagnetic response of a superconductor is reviewed and the coupling between collective modes and impurity scattering emphasized. The effects on the behavior of the real component of the dielectric function are found to be relatively more sensitive to the presence of inelastic scattering than to the details of the order-parameter symmetry. Future experiments are suggested which probe the ([ital c]-axis) order-parameter collective-mode structure through measurements of the finite wave-vector longitudinal dielectric function.