Effect of interband transitions on c-axis penetration depth of layered superconductors

The electromagnetic response of a system with two planes per unit cell involves, in addition to the usual intraband contribution, an added interband term. These transitions affect the temperature dependence and the magnitude of the zero-temperature c-axis penetration depth (lambda(c)). When the interplane hopping t(perpendicular to) is sufficiently small, the interband transitions dominate the low-temperature behavior of lambda(T) which then does not reflect the linear temperature dependence of the intraband term and in comparison becomes quite flat even for a d-wave gap. It is in this regime that the pseudogap was found in our previous normal-state calculations of the c-axis conductivity, and the effects are connected.