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

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