Current-Induced Intermediate State in Thin-Film Type-I Superconductors: Electrical Resistance and Noise

We have investigated the current-induced resistive state in superconducting strips of lead and indium carrying electrical transport currents with an average density up to 10⁶ A/cm². Following previous high-resolution magneto-optical experiments, we report here measurements of the electrical resistance and the electrical-noise-power spectra. The experiments were performed in zero external magnetic field, using micro-strips 60-300 {μ}m wide, 0.3-10 {μ}m thick, and 3-6 mm long. In lead, resistive voltage steps were usually observed during variation of the current resulting in pronounced peaks of the derivative ∂V∂I. These voltage steps are caused by the abrupt creation of channels of normal phase at distinct levels of the transport current. The abrupt growth of these channels of normal phase apparently results from a magnetic instability similar to the kink instability in magnetohydrodynamics. In indium, a step structure in the resistive voltage has been observed only close to Tc, the voltage steps being usually smeared out by fluctuations in time of the number of current-generated channels of normal phase. From the magnitude of the resistive voltage steps and from the observation of dynamic magnetic coupling between two films of a sandwich structure, we conclude that, in the strips of both Pb and In, the current-induced resistive state can best be described by a dynamic model. In this model, the channels of normal phase consist of arrays of flux tubes moving rapidly from the edges to the center of the strip, where they are annihilated by flux tube arrays of opposite sign. The noise-power spectra in the current-induced resistive state usually show a frequency dependence ranging from ω^-1 to ω^-2 behavior. The electrical-noise data are discussed in terms of a model assuming fluctuations in time of the number of channels of normal phase.