On the physical origin of the superconducting transition in transition-edge sensors

Transition-Edge Sensors (TESs) constitute highly sensitive particle and radiation detectors, widely used in many applications. Each of these requires optimization of TES performances and designs, including sizes and geometries. These may have implications on the superconducting transition mechanisms and, therefore, on TESs performances and stability, through the specific shape of the resistance vs temperature and current R(T,I) and the nature of noise. In this study, we investigate the dependence of the superconducting transition, characterized by R(T,I), on TES size and bias current density. Through analyses of R(T,I) in bare Mo/Au TESs with Tc tuned for this study, we observe how the weak link behavior induced by the superconducting leads weakens and disappears as TES length or driving current increase, being substituted by another dominant transition mechanism, which might be related to a Berezinskii–Kosterlitz–Thouless transition. We also observe a significant broadening of the transition's upper part, attributed to the longitudinal proximity effect induced by the pads; for the shorter devices, this effect is observed for R > 70% Rn and results in TES resistances considerably lower than Rn up to temperatures well above the TES transition: R < Rn up to 3 K for a 8 μm-long device.