Are ``pinholes'' the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions
ORAL
Abstract
In highly resistive superconductor---insulator---superconductor (SIS) and superconductor---insulator---normal-metal (SIN) junctions, ``excess'' subgap current is usually observed. We have studied subgap conductance in Al/AlO$_x$/Al and Al/AlO$_x$/Cu tunnel junctions. In the former, we observed a huge (two orders of magnitude) decrease in subgap conductance upon the transition from the SIS to the SIN regime. In the latter, we observed several signatures of coherent diffusive two-particle transport. We use the quasiclassical Keldysh-Green function theory to quantify the contributions of the single- and two-particle processes on subgap conductance. Our observations indicate insignificance of highly transparent microscopic defects (``pinholes'') in the tunneling barrier, and we therefore argue that the common ``pinhole'' scenario is not the explanation for the observed excess subgap current in SIS tunnel junctions.
*This research was partly funded by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), through the Army Research Office.
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