Experimental observation of subharmonic gap structures in long Josephson junctions

The subharmonic gap structure (SGS) in long-overlap Nb-AlOx-Nb Josephson tu nnel junctions has been investigated. The experimental results show peaks i n the differential conductance at both odd and even integer fractions of th e gap voltage, VG Furthermore, the conductance peaks at V-G/2 has been obse rved to split into two peaks with different characteristics. At high magnet ic fields, the I-V characteristics approach a single curve, while retaining the SGS conductance peaks. The gap structure and the SGS show the same tem perature dependence. The SGS can be explained by a Josephson self-coupling effect or multiple Andreev reflections. The results are important for the d evelopment of the flux flow oscillator used a local oscillator in integrate d all-superconducting 200-800 GHz receivers.