MAGNETIC-FLUX NOISE IN NB THIN-FILM

We measured the flux flow noise in a type-II superconductor by a direc t flux detection method. The fluctuating component of flux changes in the central part of niobium thin film sample is monitored by a Nb rf-S QUID as a function of the transport current and magnetic field. The fl ux noise power spectrum showed either 1/f or a Lorentzian behavior dep ending on the size of transport current. The overall magnitude of the flux noise showed a peak near the critical current, whereas the voltag e increases monotonically. Based on a modified shot-noise model, we si mulated both Lorentzian and 1/f noise spectra with appropriate assumpt ions and found that the characteristic frequency of noise spectrum is related to the vortex transit time.