Flux-noise and ac susceptibility measurements have been performed on epitaxial YBa2Cu3O7 films. The validity of the fluctuation-dissipation-theorem is verified by the proportionality between flux noise and ac susceptibility results. The nominal zero-field flux-noise spectrum (Sφ(f)) can be characterized as follows: Below a temperature dependent frequency f0(T), the flux-noise spectrum is frequency independent. For frequencies f > f0(T), the flux-noise spectrum follows f-x, with x ≈ 1.7-1.8. f0 decreases by 4 orders of magnitude in a temperature interval of ▵T ≈ 0.5 K. Moreover, the flux-noise spectrum scales as fSφ = g(f/f0(T)), where g(·) is a scaling
function. The influence of weak perturbing magnetic fields was investigated. While the general characteristics of the flux-noise spectrum remain, the characteristic frequency f0 increases by 3-4 orders of magnitude increasing the field from zero to 1 Oe. Flux-noise measurements performed using a compensating field to reduce the residual field in the experimental setup reveal new features of the measured noise spectra. The flux-noise is
dominated by thermally generated vortex-antivortex pair fluctuations at temperatures below the mean-field transition temperature, while with decreasing temperature there is a transition to vortex fluctuations being dominated by field generated vortices.
We have performed flux noise and AC-susceptibility measurements on two 400 nm thick MgB2 films. Both measurement techniques give information about the vortex dynamics in the sample, and hence the superconducting transition, and can be linked to each other through the fluctuation-dissipation-theorem. The transition widths for the two films are 0.3 and 0.8 K, respectively, and the transitions show a multi step-like behavior in the AC-susceptibility measurements. The same phenomenon is observed in the flux noise measurements through a change in the frequency dependence of the spectral density at each step in the transition. The results are discussed and interpreted in terms of vortices carrying an arbitrary fraction of a flux quantum as well as in terms of different macroscopic regions in the films having slightly different compositions, and hence, different critical temperatures.
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