Current transport and the fluctuation of critical current in high-temperature superconductor interface-engineered Josephson junctions

The mechanisms of current transport in interface-engineered junctions (IEJs) with ramp-edge geometry were investigated to clarify the possible origin of the statistical fluctuation of the Josephson critical current. More than 1000 junctions with a ramp edge aligned either along the [100] or [110] axis of the high-temperature superconductor electrode were fabricated under various process conditions. These junctions exhibited a critical current density ranging from ${10}^{2}$ to ${10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{A}∕\mathrm{cm}}^{2}$ at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ while maintaining a magnetic field modulation of the critical current exceeding 80% without any indication of the peculiar effect of $d$-wave pairing symmetry. The junctions with a critical current density exceeding ${10}^{4}\phantom{\rule{0.3em}{0ex}}{\mathrm{A}∕\mathrm{cm}}^{2}$ exhibited an appreciable amount of excess current that grew rapidly within an approximate voltage range of less than $5\phantom{\rule{0.3em}{0ex}}\mathrm{mV}$. The critical current versus temperature characteristics of these junctions were found to be explained reasonably well by a superconductor-normal-superconductor (SNS) junction model in the diffusive regime. This model is also consistent with our observation of a weak subharmonic gap structure due to multiple Andreev reflections. In addition, we found that the Josephson critical current $({I}_{c})$ exhibited a good correlation with the differential resistance near $0\phantom{\rule{0.3em}{0ex}}\mathrm{V}$, while the normal resistance defined at a current level of two to three times ${I}_{c}$ varied appreciably even for junctions with a similar ${I}_{c}$. This indicates that another conduction channel with little contribution to the Josephson current coexists within the junctions. The $dI∕dV$ measurement for high resistance junctions revealed that resonant tunneling of quasiparticles through localized states in an insulating barrier constitutes this second conduction channel. All these results suggest that IEJs should be regarded as an array of microscopic SNS contacts embedded in an insulating barrier with random orientation. The fluctuation in the number of SNS contacts in a junction restricts the attainable minimum spread of the ${I}_{c}$ value.