Subgap conductivity in SIN-junctions of high barrier transparency

Abstract

We investigate the current-voltage characteristics of high-transparency superconductor-insulator-normal metal (SIN) junctions with the specific tunnel resistance below 30 kOhm per square micron. The junctions were fabricated from different superconducting and normal conducting materials, including Nb, Al, AuPd and Cu. The subgap leakage currents were found to be appreciably larger than those given by the standard tunnelling model. We explain our results using the model of two-electron tunnelling in the coherent diffusive transport regime. We demonstrate that even in the high-transparency SIN-junctions, a noticeable reduction of the subgap current can be achieved by splitting a junction into several submicron sub-junctions. These structures can be used as nonlinear low-noise shunts in Rapid-Single-Flux-Quantum (RSFQ) circuitry for controlling Josephson qubits.Comment: 6 pages, 5 figures, 1 tabl

Are “Pinholes” the Cause of Excess Current in Superconducting Tunnel Junctions? A Study of Andreev Current in Highly Resistive Junctions

“…In SIN junctions, one similarly expects the ratio G sg /G N to be of order 10 −5 −10 −6 . However, a similar saturation is typically observed [17][18][19][20] (though a smaller G sg was recently reported in [21]). This discrepancy has often been explained [7] by assuming microscopic defects in the tunnel barrier, commonly known as "pinholes," which cause a greatly enhanced local transparency (Γ > 10 −3 ).…”

“…This discrepancy has often been explained [7] by assuming microscopic defects in the tunnel barrier, commonly known as "pinholes," which cause a greatly enhanced local transparency (Γ > 10 −3 ). In fact, a large subgap conductance has been considered an indicator of a poor quality tunnel barrier [3,9,15,19].…”

Universality of transport properties of ultrathin oxide films

“…Earlier investigations of subgap processes have been mainly concentrated on symmetric S-I-S [3] and asymmetric S-I-N [22] junctions. At the same time, it was stressed [23] that experimental studies of asymmetric S 1 -I-S 2 devices can provide more information since the structure of I-V curves at voltages below (∆ 1 + ∆ 2 ) /e is more rich and pronounced [24,25].…”

Tunneling characteristics of superconducting junctions with inhomogeneous tunnel barriers

“…The extra leakage currents across nanometer-thick barriers are often attributed to microscopic high-conducting defects in the oxides which shunt the current [6,7]. In this work, using theoretical arguments and experimental data for current-and conductance-versus-voltage characteristics of , MoRe-AlO x -Al-MoRe junctions we show that the nature of the excess currents as well as that of anomalous features in conductance-voltage characteristics is more fundamental (see also related discussion in [8]) and is related to the presence of a universal bimodal distribution of transparencies D across sub-nanometer-thick amorphous oxides in superconducting SINS junctions. The paper presents an overview of our previous results in this field as well as new ones, in particular, calculations of the transparency of dirty metallic films and estimates of current-voltage characteristics of junctions with magnesium diboride, a two-band superconductor with a critical temperature of 39 K. The last section of the paper summarizes our main results.…”

“…Let us note that our assumption about a constant probability density &ðZÞ is similar to that in ref. [8] where the authors assumed a uniform distribution of the barrier thickness around a mean value. With the parametrization D ¼ ð1 þ Z 2 Þ À1 we can transfer from a distribution &ðZÞ to a function of a local transparency D…”