Manifestation of coherent effects in the conductivity of superconductor/insulator/normal-metal/insulator/superconductor junctions

Abstract

An experimental and theoretical study is presented of coherent effects in electron transport in the doublebarrier SINIS junctions ͑where S, I, and N denote a superconductor, insulator, and normal metal, respectively͒. The appearance of a steplike subgap structure in the current-voltage characteristics of the Nb/Al/AlO x /Al/AlO x /Al/Nb superconducting junctions at a voltage Vϳ⌬ Nb /e ͑where ⌬ Nb is the superconducting energy gap of Nb͒ is interpreted as a manifestation of a nonequilibrium supercurrent at finite dc bias voltage ͑Finite-Bias Josephson Effect͒. The origin of this effect lies in the energy-band structure associated with a set of macroscopic quantum states characteristic of a SINIS junction. Specifically, the junction can have an energy level near energy ⌬ Nb , which provides an additional channel for dc Josephson current at V ϳ⌬ Nb /e. In addition, sharp features in the conductivity at a voltage near the gap-sum voltage were observed in both SINIS and SINININIS junctions, implying correlated quasiparticle tunneling in multiple-barrier junctions. Our theoretical model provides a good qualitative description of the quasiparticle conductivity, including narrow peaks at finite voltage and a zero-voltage anomaly observed on some samples, and suggests an alternative explanation of a feature interpreted earlier as gap-difference feature associated with the tunneling extraction of quasiparticles from the middle Al layer.

Josephson (001) tilt grain boundary junctions of high-temperature superconductors

“…This is especially true in the case of the hole-doped cuprates. We note that not only are π-junctions readily obtained with Nb, Al and their oxides, 51 but the associated zero-bias conductance peaks and higher harmonic order Josephson phase relations in such conventional junctions have also been observed, 51,52,53,54,55,56 as in YBCO. 57,58 V. SUMMARY…”

“…Nevertheless, from the results we obtained with these two FS's, it is evident that the results obtained from such devices, the grain boundaries of which have not been analyzed with atomic scale precision as was done with the Bi2212 c-axis twist experiments, 4 might also have a more conventional explanation. 51,52,53,54,55,56 It has been suggested that the experimental I c (θ) observed in (001) tilt grain boundary junctions can only be understood in terms of interface defects, regardless of the OP. 42 Of course, in Bi2212, it has long been established that a significant amount of static defects exist not only on the top cleaved surface, 43 but throughout even the best single crystals.…”

“…44,45,46 Such defects combined with significant oxygen loss at the grain boundaries may be the most important source of the observed π-junctions in the tricrystal and tetracrystal experiments, accounting for the apparent inconsistency between those and the c-axis bicrystal and cross-whisker experiments. 4,18,19,21 Finally, we recall that π-junctions are now routinely produced using junctions of the SNS and SINIS varieties constructed from conventional superconductors, 51,52,53,54,55,56 so that a close reexamination of the tri-and tetracrystal experiments is warranted. This is especially true in the case of the hole-doped cuprates.…”

Resonant states of a double-barrier junction

“…17 Multiple Andreev reflection ͑MAR͒ features [22][23][24] have been observed in the current-voltage characteristics of nanojunctions with superconducting leads. 25 Recently an experimental observation of ABS levels was reported 10,[18][19][20][21] in double barrier SISЈIS junctions.…”

“…10,[16][17][18][19][20][21] Up to now the quantized states ͑re-ferred also as to Andreev bound states or ABSs͒ were experimentally observed in only a few systems. Scanning tunneling spectroscopy measurements provide evidence for ABS levels in ͑i͒ the normal core of an Abrikosov vortex 16 and ͑ii͒ normal wires embedded in a superconducting matrix.…”