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 FeSe0.5Te0.5 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 FeSe0.5Te0.5, and an overall metallic behavior above Tc. Preserving the samples for several months in low-humidity and low-pressure conditions, the FeSe0.5Te0.5 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 < Tmin = 50 K). An interpretation in terms of weak-localization processes is proposed