Giant proximity effect in single-crystalline MgB₂ bilayers

Although giant proximity effect (GPE) can shed important information on understanding superconducting pairing mechanisms and superconducting electronics, reports on the GPE are few because the fabrication of the junctions with GPE is technologically difficult. Here, we report a GPE in the single-crystalline MgB₂ bilayers (S'/S), where the S' is the damaged MgB₂ layer by cobalt (Co)-ion irradiation and the S is the undamaged MgB₂ layer. Superconducting properties of the S' is remarkably degraded by the irradiation, whereas those of the S is uninfluenced by the irradiation. The degraded superconductivity in the S' is fully recovered by increasing the thickness of undamaged MgB₂ layer S despite almost ten times larger thickness ~ 95 nm of S' than the superconducting coherence length ξ_ab(0) ~ 8.5 nm of the S, indicating a presence of GPE in the S'/S MgB₂ bilayers. A diffusion of electrons in the S' into the S can reduce a pair breaking scattering in the S', and the similar electronic structures of S' and S layers and a finite attractive electron-electron interaction in the S' are thought to be origins of unpredicted GPE between the same superconducting materials. Both upper critical field (μ₀H_c2) and in-field critical current density (J_c) of S'/S bilayers show a significant enhancement, representing a strong correlation between S' and S. These discoveries provide the blue print to the design of the superconducting multilayers for fundamental researches on the mechanism of the GPE as well as their technological applications.