Fluctuations in the flux nucleation process of the current-induced resistive state in type-I superconductors

Abstract

The fluctuations of the flux-tube nucleation frequencyν ^* in the current-induced resistive state have been studied using a strictly passive micro-fieldprobe as a flux-tube counter. The measurements were performed with constricted indium films near 2.0K. They included both the bandwidth of the rf signal from the field probe and the temporal variations ofν ^*. From a comparison of the power spectrum of the noise voltage and of the functionν ^*(t) the relative importance of the fluctuations in the size and in the nucleation frequency of the flux tubes can be evaluated. In addition to fluctuations ofv ^* around an average value, switching between two frequenciesν ^*₁ andν ^*₂ can be observed. As a function of sample voltage the bandwidth shows oscillations which appear to be associated with the change in the time-averaged number of flux tubes traveling simultaneously through the constricted film. Narrow-band flux nucleation withΔν ^*/ν ^*<10⁻¹ is observed only in rather restricted regimes of the sample voltage.