c-axis infrared conductivity of a d(x2-y2)-wave superconductor with impurity and spin-fluctuation scattering

Results of a calculation of the c-axis infrared conductivity sigma(c) for a d(x2-y2)-wave superconductor, including both elastic impurity and inelastic spin-fluctuation scattering are presented and compared with the ab-plane conductivity sigma(ab) in the same model. In this model, the interlayer c-axis coupling is taken to be weak and diffusive. While in clean systems, inelastic scattering leads to a peak at omega=4 Delta(o) in sigma(ab) for T<T-c, it has little effect on the corresponding sigma(c), which exhibits structure only at omega similar or equal to 2 Delta(o) and is directly related to the single-particle density of states N(omega). The c-axis penetration depth lambda(c) in the same model is predicted to vary as T-3 at low temperatures in clean samples. We discuss recent optical experiments on the cuprates and compare with these predictions.