c-Axis transport in a normal-state bilayer cuprate and relation to pseudogap

We study the effect of interband transitions on the normal-state optical conductivity, dc resistivity, and thermal conductivity along the c-axis, for a plane-chain bilayer cuprate coupled by a perpendicular hopping matrix element (t_⊥). When t_⊥ is small, the c-axis dc resistivity shows a characteristic upturn as the temperature is lowered, and the c-axis optical conductivity develops a pseudogap at low frequencies. As t_⊥ is increased, intraband transitions start to dominate and a more conventional response is obtained. Similar pseudogap behavior is predicted in the thermal conductivity for which strong depression at low temperature is found. Analytical results for a simple plane-plane bilayer are also given, including the frequency sum rule of the optical conductivity.