Intrinsic fluctuation effects and certain other aspects of superconducting weak links

Abstract

Recent theoretical developments have led to evermore refined theories for the Josephson effects in a variety of superconducting weak link'' structures. One of the major refinements has been the inclusion of the effects of intrinsic thermal fluctuations upon Josephson-like behavior. Some other developments include the investigation of mechanisms for the origin of the Josephson effects in non-tunneling structures, and equivalent circuit representations. Experimental investigations were made which encompass both of these areas; with special emphasis placed upon fluctuation effects; in particular, the effect of fluctuations upon the driven dc Josephson effect has been studied in considerable detail. The weak links studied included niobium point contacts, tin whisker'' crystals, and tin thin film bridges, the latter fabricated via an optical photoresist technique. The experiments were conducted in a temperature regime which extended to within a few mK of T/sub c/, the regime in which fluctuation effects are enhanced (and thus more experimentally accessible). The data consisted of relatively high resolution (ln V) low noise 1- -V characteristics obtained with an apparatus which incorporated ( plus or minus 10 mu K) temperature control with wide-band rf transmission, in a highly shielded environment. Experimental rf-induced step profiles (the driven ac Josephsorin effect) and zero voltage step profiles (the dc Josephson effect) were compared in detail with the relevant intrinsic fluctuation theories (due to Ambegaokar, Halperin, and Stephen) via a one parameter fit, using the respective no-fluctuation step amplitude as she (natural) fitting parameter. We were able to distinguish between external noise effects and intrinsic effects and effectively excluded external noise; thus we maintain a high degree of confidence that the effects reported are indeed intrinsic to the weak links themselves. The agreement between theory an experiment was found to be very good in both the driven and the dc effects over a wide range of experimertal parameters and the single-parameter fits yielded step heights which corresponded reasonably well to the dc critical currents. These results indicate that the conceptually simple theory in which thermal fluctuations are introduced as a rapidly fluctuating (Langevin) force and a tilted periodic potential is introduced by the coupling energy (dc effect) or by phase locking to incident radiation (driven ac effect) provides a rather good description of the intrinsic fluctuation phenomena in superconducting weak links of negligible capacitance. The broad applicability of the theoretical concepts is indicated by the consistency between the dc and driven effect results, and by-the wide range of parameters over which convincing agreement between theory and experiment was obtained. By making some observations in regimes where fluctuation effects were diminished. evidence was found that slight deviations that were observed in the fluctuation experiments could be attributed to deviations from the no-fluctuation model used as the starting point in the fluctuation theories. Supplemental considerations related to junction modeling were also pursued to a certain degree and in some cases, led to new results or confirmations of recently discovered results of other researchers. These observations included 1--V comparisons, rf step amplitude vs voltage dependencies, rf step threshold frequencies, and the Dayem --Wyatt effect.