1∕f flux flow noise due to a coexistence of qualitatively different vortex states

We investigate the vortex-motion voltage noise in a hybrid structure consisting of a weak-pinning amorphous Nb0.7Ge0.3 microbridge on top of which a strong-pinning, longitudinal Nb line with a narrow interruption in the middle is added. The Nb part enforces a branching of the applied current, causing a modulation of the current density within the Nb0.7Ge0.3, where vortex motion induces a voltage. When the Nb0.7Ge0.3 is sufficiently dissipative, the modulation is strong and the vortex dynamics is spatially dependent. Under these circumstances, the distribution function for normal excitations in vortex cores varies considerably over the sample, which results in a coexistence of distinct vortex states spreading from nearly equilibrium to strongly nonequilibrium ones. This leads to a range of characteristic times for the voltage fluctuations and, consequently, to the frequency (f) dependence of the noise being of 1∕f type. The noise originates in the fluctuations of the vortex-core size around the average set by the nonequilibrium effects in vortex motion.