Noise Spectroscopy Investigation of Aging-Induced Degradation in Iron-Chalcogenide Superconductors

Iron-based superconductors are subjected to aging when exposed to unprotected environmental conditions, with a consequent degradation of their superconductive properties. The understanding of aging effects is important in view of practical applications. In this paper, electric transport and voltage-noise measurements are reported on FeSe and FeSe _0.5Te_0.5 thin films, just after fabrication and after long aging in protected environment. The pristine films show a superconducting transition at temperatures ranging from 9 to 12 K for FeSe and at 17 K for FeSe_0.5Te_0.5, and an overall metallic behavior above T _c. Preserving the samples for several months in low-humidity and low-pressure conditions, the FeSe_0.5Te_0.5 films show no clear modification of the electrical conduction mechanisms. Conversely, for the FeSe films, the superconductive transition is no longer present, at least for temperatures above 8 K, and an overall larger resistivity is observed, with a characteristic upturn at low temperatures (T > T_min ≈ 50K). An interpretation in terms of weak-localization processes is proposed.