c-Axis Infrared Conductivity of a d_{x^2-y^2}-Wave Superconductor with   Impurity and Spin-Fluctuation Scattering
  Results of a calculation of the c-axis infrared conductivity sigma_c for a d_{x^2-y^2}-wave superconductor which include 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_0 in sigma_{ab} for T < T_c, it has little effect on the corresponding sigma_c, which exhibits structure only at omega = 2*Delta_0 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. 
P. J. Hirschfeld, S. M. Quinlan, and D. J. Scalapino