Giant Mesoscopic Fluctuations and Long Range Superconducting Correlations in Superconductor--Ferromagnet structures

The fluctuating superconducting correlations emerging in dirty hybrid structures in conditions of the strong proximity effect are demonstrated to affect the validity range of the widely used formalism of Usadel equations at mesoscopic scales. In superconductor -- ferromagnet (SF) structures these giant mesoscopic fluctuations originating from the interference effects for the Cooper pair wave function in the presence of the exchange field can be responsible for an anomalously slow decay of superconducting correlations in a ferromagnet even when the non-collinear and spin-orbit effects are negligible. The resulting sample-to-sample fluctuations of the Josephson current in SFS junctions and local density of states in SF hybrid structures can provide an explanation of the long range proximity phenomena observed in mesoscopic samples with collinear magnetization.