Theory of the c -axis penetration depth in the cuprates

Recent measurements of the London penetration-depth tensor in the cuprates find a weak temperature dependence along the c direction that is seemingly inconsistent with evidence for d-wave pairing deduced from in-plane measurements. We demonstrate in this paper that these disparate results are not in contradiction, but can be explained within a theory based on incoherent quasiparticle hopping between the CuO₂ layers. By relating the calculated temperature dependence of the penetration depth λc(T) to the c-axis resistivity, we show how the measured ratio λc²(0)λc²(T) can provide insight into the behavior of c-axis transport below Tc and the related issue of "confinement."