Non-monotonic high frequency flux-flow noise spectra in high-T_C superconductors

Nonmonotonic voltage noise spectra have been observed in granular high-T_c YBaCuO thin films. The maximum at low frequencies and oscillations at high frequencies are explained in the framework of the interrupted vortex flow model. Separation between the oscillations corresponds to the time of flight of vortices across the strip. The maximum at low frequencies results from viscous relaxation of pinned vortices. The data indicate that high frequency voltage noise is due to Josephson vortices flowing with a very low viscosity between grains. The pinning frequency determined from the position of low frequency peak is consistent with surface pinning of Abrikosov vortices inside the grains.