Collective modes and their coupling to pair-breaking excitations in layered d-wave superconductors

Collective modes of a layered superconductor are calculated within the ladder-diagram approximation using a d-wave symmetry model for the gap function. The collective mode dispersion is essentially independent of the order-parameter symmetry (s or d wave) at long wavelengths. The coupling of collective modes to pair-breaking single-particle excitations, however, depends crucially on the order-parameter symmetry. While there is a strong mode-coupling effect as the plasmon approaches the superconducting gap for the s-wave system, the mode coupling in the d-wave case is weak and strongly dependent on the direction of the wave vector, vanishing in the nodal direction of the gap function.