Stabilized hotspots and vortex patterns in superconducting NbN and Nb nanostructured strips and loops

(eng) The superconducting properties are noticeably modified when the samples dimensions approach the nanoscale. This impacts the working regime of many current-biased thin-film devices. The low-temperature electrical transport behavior of niobium and niobium nitride nanostructures has been investigated in this context. Their current-carrying properties turn out to be mainly dictated by the Joule heating. This is evidenced by the numerous normal hotspot domains that are stabilized when the dissipation balances the heat removal capabilities. Importantly, the associated stepwise current-voltage characteristics are quantitatively related to the nanoscale modulations of the cross-section. This dissertation further describes the electrical behavior of the nanostrips under a continuous microwave irradiation. Incorporating the microwave dissipation in the heat balance powerfully corroborates the prevailing role played by the heat exchanges. The present work succeeds in determining the tiny fraction of the total input power which is effectively absorbed by the nanostructures in the normal state, i.e. below the microwatt per cubic micrometer. Furthermore, the critical induction below which the magnetic vortices are completely expelled is determined for structures whose width amounts to a few tens of nanometers. Above this superheating field, the rearrangements of the vortices patterns interestingly go hand in hand with regularly spaced minima of the critical current. In conclusion, this study provides complementary tools to characterize narrow thin-film superconducting devices. The cross-section roughness, the power handling capabilities, and the critical field for vortex entrance are indeed accessible from their electrical transport properties.