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

Abstract

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 A/cm 2 at 4.2 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 A/cm 2 exhibited an appreciable amount of excess current that grew rapidly within an approximate voltage range of less than 5 mV. The critical current versus temperature characteristics of these junctions were found to be explained reasonably well by a superconductornormal-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 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.

Quantitative model for theICRproduct ind-wave Josephson junctions

“…This length estimate is reasonable for the ramp-edge junctions described in Ref. 6. interfaces, large magnitudes of …”

“…4 However, almost all experimental data for high-T C Josephson junctions fabricated so far have shown I C R values much smaller than those predicted by the standard theory, irrespective of what kind of junctions they are. 5,6 This strongly suggests that the interfaces in the cuprates are intrinsically pair breaking. Several models were proposed which treat this issue.…”

“…6 and with the theory for clean DID junctions by Tanaka and Kashiwaya ͑TK͒ 13 using Eq. ͑46͒ in Ref.…”

Anomalous temperature dependence of the stationary Josephson tunnel current in junctions between d-wave superconductors

“…An extremely weak nonmonotonic behavior resembling the transition between the clean and rough limits in the ASJ scenario was found in grain-boundary YBa 2 Cu 3 O 7-δ junctions [53,56]. Other studies of symmetric (S-I-S or S-N-S) junctions [51,52,55] I T may be nonmonotonic owing to the well-known tunnel directionality [25,[30][31][32][33][34][35][36][37][38], with the influence of the latter varying as the interlayer thickness changes. Our model predicts that the ( ) J I T dependences can be either monotonic or weakly nonmonotonic, without any rise of ( ) J I T at 0.…”

“…For our purposes, it is worthwhile to point out only typical features. Relevant experiments were made for various types of junctions involving only YBa 2 Cu 3 O 7-δ electrodes [50][51][52][53][54][55][56]. It turned out that a steep increase of ( ) J I T at 0 T → predicted for clean junctions with the ASJ midgap states is not observed.…”