Temporal evolution of normal hot spots in current-driven superconducting films

In addition to the critical current I _c (T), which generates Phase-Slip Centers (PSCs), thin superconducting films possess a well-defined second limiting current I _h, or current intensity able to maintain a preestablished hot spot. By pulsing step functions of the current and monitoring the voltage response on the nanosecond scale, we have determined (T ↔ I _c) and (T ↔ 4). From a dynamic study of the two main modes of dissipation in YBCO and Nb films, it is concluded that PSCs are stable structures in current-biased bridges. In contrast, hot spots grow at a constant rate of a few tens of meters per second, determined by the thermal diffusivity of the material and by its bolometric response time. On reducing the current from T _h, the so-called healing length, or minimum normal length, was found, of the order of 0.2 μm in YBCO and 2 μm in Nb. In summary, the experiment provides three independent measurements (PSC nucleation time, velocity of growth, and minimum length) for only two parameters (D and T).