Interband effects in the c-axis optical conductivity in YBa2Cu3O7-delta

The normal state optical conductivity is calculated for a layered metal with two layers per unit cell coupled through a transverse hopping matrix element t(perpendicular to). The optical response involves an interband term id addition to the more familiar intraband term which leads to the usual Drude form. The interband term is only weakly temperature dependent, even for an inelastic scattering rate which is linear in T. It gives a c-axis response which extends in frequency over the entire bandwidth, although there can be structure on this energy scale which reflects details of the electronic structure. In particular, at low energy, the c-axis response can develop a gap or pseudogap as the temperature is lowered. At high temperature, a Drude response will be seen only if the intraband transitions, which are of order t(perpendicular to)(4), become important compared with the interband transitions which are of order t(perpendicular to)(2).