Measurement of Current-Phase Relationship in Josephson Junctions with Nb_xSi_1-x Barriers

The current-phase relationship of Josephson junctions is often assumed to be a sinusoid. However, this is not always the case. In order to directly measure the current-phase relationship of junctions with a range of characteristics, including both SIS and SNS junctions, we fabricated Josephson junctions having Nb_xSi_1-x barriers with varied Nb concentration. We constructed a superconducting SQUID-sensor-based circuit to extract their current-phase relationships. SIS-like junctions had the expected near-sinusoidal current-phase relationship, but the current-phase relationship gained additional harmonics as the barriers became less insulating. Thus, SNS-like junctions with normal barriers and high Nb content had sawtooth-like current-phase relationships. This deviation is likely due to additional Andreev reflections. The effects of this non-sinusoidal current phase relationship were observed in current-voltage characteristics through the appearance of non-integer Shapiro steps. Additionally, the same techniques were applied to Si-Mn nanocluster junctions to measure the changes in the current-phase relationship caused by the dynamic tuning of the critical currents of these junctions.