c-axis optical conductivity of layered systems in the superconducting state

We discuss the c axis optical conductivity Re[sigma(c)(omega)] in the high-T-c superconductors, in the superconducting state. The basic premise of this work is that electrons travelling along the c axis between adjacent CuO2 layers must pass through several intervening layers. In earlier work we found that, for weak interlayer coupling, it is preferable for electrons to travel along the c axis by making a series of interband transitions rather than to stay within a single (and very narrow) band. Moreover, we found that many of the properties of the normal-state optical conductivity, including the pseudogap could be explained by interband transitions. In this work we examine the effect of superconductivity on the interband conductivity. We find that, while the onset of superconductivity is clearly evident in the spectrum, there is no clear signature of the symmetry of the superconducting order parameter.