Measurement of the hotspot current in superconducting NbTi wires

 

We investigated the formation of the meta-stable states created by a step current pulse. Non-equilibrium superconducting states are induced for a current exceeding the depairing current. Depending on the sample parameters where the dissipation can be governed by different modes, such vortex motion, phase slip lines or hotspot. We focussed our study on the one governed by the hot-spot mechanism. For current value larger than the critical value, the superconductivity collapses. A measurable voltage arises after a delay time t_d, the value of which decreases with increasing current pulse amplitude. The time-dependent Ginzburg-Landau (TDGL) theory is then used to account for the gap relaxation time from the measured delay times for different sample thicknesses. In addition, we present a double pulse technique that allows the determination the hotspot current. It consists of sending two superposed pulses, the first one with a short duration that triggers the hotspot and the second one trays to maintain it. The hotspot has the tendency of expanding or regressing, hence the role of the second pulse is to decuce the hotspot current, that is mostly measured by other technique using I-V characteristics.