A Study of Ultra-Thin Superconducting Films at High Bias Currents in Different Cooling Environments

The influence of the cooling method on thermal stability is a non-trivial aspect for superconducting material to be used in several applications, for example, in photon detection. Indeed, a wasteful cooling can induce quenches in the device that can lead to false counts. So far, the efficient cooling has been realized by immersion of the superconducting device in a liquid He bath. However, cheaper cryogen-free (CF) cooling techniques are now commercially available, as well as cryocoolers are becoming the only way to obtain a cooling environment in the liquid He shortage. Here we consider three different cooling methods: one is by liquid He in a standard cryostat and the other two are a dynamic or a static He gas cooling in a CF cryostat. Then, we are able to evaluate the performance of the cooling method by the impact on current-voltage curves at very high bias currents. In particular, we acquire current-voltage characteristics on ultra-thin microbridges made by two different superconducting materials commonly used in detectors fabrication, which are NbN and NbTiN. Here, the flux-flow instability (FFI) phenomenon is used as a tool to determine the influence of the three different cooling techniques on the voltage stability of the devices under current biasing. It results that the CF cooling method has performance comparable to liquid He bath. This finding supports the spread of the CF technique for applications, and it validates the FFI as a tool to test superconducting materials.