Enhancement of the noise spectral densities near the resistive transition region of YBCO micro-bridge

Abstract

We have systematically measured the noise power spectral density ( S υ( f)) of the YBCO micro-bridge as a function of frequency, temperature and magnetic field. In all the cases above, S υ( f) shows V 2/ f α behaviour at the measured frequency range of 0.2 < f < 30 Hz, with V, the d.c. voltage across the sample and, α the fitting parameter to be determined. In the normal state, the normalized noise power spectral density ( S n ( f) = S υ ( f)/ V 2) is four or five orders of magnitude larger than that of the normal metals evaluated by Hooge's formula or thermal fluctuation model. Near the zero resistance temperature ( T c 0, S n( f) was found to further increase by eight orders of magnitude and α was shown to increase from 1 to 2 depending on the frequency range. Furthermore, once a magnetic field is applied, S n( f) showed a marked increase near T cO . This anomaly is attributed to magnetoic vortices coupled with the enhanced instability of space-time coherence associated with Cooper pair as a result of applied magnetic field.