Friedel transition in layered superconductors

Weakly coupled superconducting layers are described by the anisotropic 3D XY model. A low-temperature layer decoupling due to a proliferation of fluxons between planes, as proposed by Friedel, does not occur. The same is true for a periodic superlattice of high and low T-c layers, although the interplane coherence can become extremely weak. On the other hand a true layer decoupling is found for a random stack.