Out-of-plane optical conductivity in d-wave superconductors

We study theoretically the out-of-plane optical conductivity of d-wave superconductors in the presence of impurities at T=0K. Unlike the usual approach, we assume that the interlayer quasi-particle transport is due to coherent tunneling. The present model describes the T-2-dependence of the out-of-plane superfluid density observed in YBCO and Tl2201 for example. In the optical conductivity there is no Drude peak in agreement with experiment, and the interlayer Josephson tunneling is also assured in this model. In the unitary limit we predict a step-like behaviour around omega similar to Delta in both the real and imaginary part of the optical conductivity.