Vortex motion noise in micrometer-sized thin films of the amorphous Nb_0.7Ge_0.3 weak-pinning superconductor

We report high-resolution measurements of voltage (V) noise in the mixed state of micrometer-sized thin films of amorphous Nb_0.7Ge_0.3, which is a good representative of weak-pinning superconductors. There is a remarkable difference between the noise below and above the irreversibility field B_irr. Below B_irr, in the presence of measurable pinning, the noise at small applied currents resembles shot noise, and in the regime of flux flow at larger currents decreases with increasing voltage due to a progressive ordering of the vortex motion. At magnetic fields B between B_irr and the upper critical field B_c2 flux flow is present already at vanishingly small currents. In this regime the noise scales with (1-B/B_c2)²V² and has a frequency (f) spectrum of 1/f type. We interpret this noise in terms of the properties of strongly driven depinned vortex systems at high vortex density.