Current-Induced Metastable States Close to T _c in NbTi Superconducting Bridges

We have studied the characteristics of different metastable states in NbTi thin film deposited on sapphire substrate in a region very close to the transition temperature T _c , which was estimated to be about 7.6 K in our sample. Localized dissipative zones are induced (phase-slip centers (PSC) and hot spots (HS)) when a current pulse larger than the depairing critical ( I_c ) current is sent through the filament. These resistive zones appear after a delay time at zero voltage (transient superconductivity) that depends on the thermal cooling time of the material. A time-dependent Ginzburg–Landau (TDGL) theory developed by M. Tinkham allows to extract the gap relaxation time from the measured time delays at a temperature very close to T _c . Furthermore, it appears that, well below T _c , this relaxation time is dominated by the thermal equilibration time of the film on its substrate. In addition, the niobium-based material showed a clear evidence that PSC can be considered as precursors for hot spots.