Achim Marx, II. Physikalisches Institut, Lehrstuhl fuer Angewandte Physik, Universitaet zu Koeln, Zuelpicherstr 77, D-50937 Koeln, Germany, Phone: +49-221-470-3583, Fax: 49-221-470-5178, email: marx@colorix.ph2.uni-koeln.de, WWW: http://www.uni-koeln.de/math-nat-fak/ph2/gross/index.htm

Low frequency noise with a 1/f frequency dependence can be observed in a large variety of physical systems and up to now no common underlying mechanism for these low frequency fluctuations has been discovered. The investigation of 1/f noise in high temperature superconductor (HTS) Josephson junctions has proven to be a valuable tool to get a detailed understanding of the transport mechanism in these junctions. The noise measurements have been performed in a specially designed experimental setup involving a low-Tc DC-SQUID as a very low noise preamplifier and an optical heating system. We have investigated the noise properties of Bi₂Sr₂CaCu₂O_8+x and YBa₂Cu₃O_7-d bicrystal grain boundary Josephson junctions with misorientation angles and of ramp edge junctions. The voltage fluctuations in these junctions are due to the trapping and release of charge carriers in trapping centers in an insulating barrier giving rise to correlated fluctuations of the junction critical current I_c and normal state resistance R_n. In very small area junctions where individual charge traps dominate the power spectra we found that the effective charge trapping time is thermally activated and shows an exponential voltage dependence. For the normalized fluctuations of the critical current S_I and the normal resistance SR, which were found to be independent of temperature, a linear scaling with Rn has been observed which suggests a constant density of trapping centers for all investigated HTS Josephson junctions. Correlation experiments proved that the fluctuations of I_c and R_n are anti-phase related. The ratio SI/SR of the normalized fluctuations is in close agreement with the scaling of the ICRN-product indicating a common underlying physical mechanism. Our analysis strongly supports the Intrinsically Shunted Junction (ISJ) model based on an insulating grain boundary containing a high density of localized defect states with fluctuating electron occupation causing1/f noise.