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(upsilon)(f)) of the YBCO micro-bridge as a function of frequency, temperature and magnetic field. In all the cases above, S(upsilon)(f) shows V2/f(alpha) behaviour at the measured frequency range of 0.2 < f < 30 Hz, with V, the d.c. voltage across the sample and, alpha the fitting parameter to be determined. In the normal state, the normalized noise power spectral density (S(n)(f) = S(upsilon)(f)/V2) is four or five orders of magnitude larger than that of the normal metals evaluated by Hooge&apos;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 alpha 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(c,0). This anomaly is attributed to magnetic vortices coupled with the enhanced instability of space-time coherence associated with Cooper pair as a result of applied magnetic field.