Flux-Flow Noise in a Type-I Superconductor

The noise voltage across a current-carrying type-I superconductor is attributed to flux flow. Measurements of power spectra of this flux-flow noise on In-2 at.% Pb foils show that the noise spectrum has a $\frac{1}{f^{\alpha }}$ frequency dependence with $0.5<\mathrm{}\alpha <1\mathrm{}$. This can be accounted for by a model in which the flux moves in a jerky manner because of interaction with immobile normal regions. This causes a distribution of voltage-pulse duration times. It is concluded from the power spectra that the dc voltage $V$ is caused by a flux-flow component and an ohmic-loss voltage in immobile normal regions. The flux-flow fraction of the dc voltage is found to be a rapidly decreasing universal function of $\frac{V}{V_n}$, where $V_n$ is the normal-state voltage. The size of the moving normal domains is calculated from the noise voltage and the dc voltage, and is shown to increase with field in a range from ${10}^3{\phi }_0$ to about ${10}^5{\phi }_0$. At low fields, flux presumably moves as bundles of small flux tubes, as in type-II superconductors. Close to $T_c$ a structure is found in the power spectra which may be associated with the motion of vortex rings.