Magnetic Flux Noise in Nb Thin Film

We measured the flux flow noise in a type-II superconductor by a direct flux detection method. The fluctuating component of flux changes in the central part of niobium thin film sample is monitored by a Nb rf-SQUID as a function of the transport current and magnetic field. The flux noise power spectrum showed either l/f or a Lorentzian behavior depending on the size of transport current. The overall magnetude of the flux noise showed a peak near the critical current, whereas the voltage increases monotonically. Based on a modified shot-noise model, we simulated both Lorentzian and l/f noise spectra with appropriate assumptions and found that the characteristic frequency of noise spectrum is related to the vortex transit time.