Penetration depth in layered superconductors: Application to the cuprates and conventional multilayers

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Abstract
The temperature dependence of the penetration depth of superconductor–normal-metal multilayered metals has been calculated from Eliashberg theory. The results reported here focus on YBa2Cu3O7−δ, but the conclusions apply to any system of proximity-coupled bilayers. Our calculations show that depleting the chains of oxygen, and, consequently the appearance of gaplessness, leads to a transition from an exponential dependence to a power law, initially linear, then quadratic. Different analytical dependences have been measured by various groups, and conflicting conclusions have been made about the pairing state in the cuprate superconductors. We emphasize that our calculations exhibit all of the qualitative features of available experimental data within a single formalism.