C-axis Transport in Bilayer Cuprates and Relation to Pseudogap
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We consider the effect of interband transitions on the c-axis conductivity,
DC resistivity, and thermal conductivity in a plane-chain bilayer model of a
cuprate. The relation between the c-axis resistivity and thermal conductivity
is governed by the Wiedemann-Franz law. When the perpendicular hopping matrix
element between chain and plane ($t_\perp$) is small, the c-axis DC resistivity
shows a characteristic upturn as the temperature is lowered and the infrared
conductivity develops a pseudogap. As $t_\perp$ is increased, intraband
transitions start to dominate and a more conventional response is obtained.
Analytical results for a simple plane-plane bilayer are also given, including
the frequency sum rule of the optical conductivity.