Vortex fluctuations in weak magnetic fields

Flux-noise and ac susceptibility measurements have been performed on epitaxial films of YBa2Cu3O7. 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 (S-phi (f)) can be characterized as follows: Below a temperature dependent frequency f(0)(T), the flux-noise spectrum is frequency independent. For frequencies f > f(0)(T), the flux-noise spectrum follows f(-x), with x approximate to 1.7 - 1.8. f(0) decreases by 4 orders of magnitude in a temperature interval of Delta T approximate to 0.5 K. Moreover, the flux-noise spectrum scales as fS(phi) = g(f/f(0)(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 f(0) 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.