VORTEX FLUCTUATIONS IN WEAK MAGNETIC FIELDS

Abstract

Flux-noise and ac susceptibility measurements have been performed on epitaxial films of YBa 2 Cu 3 O 7. 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 ΔT≈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.