Abstract
We have investigated subharmonic energy-gap structures (SGSs) and 1/ f noise spectra in Nb/Al+AlO x /Nb tunnel junctions as a function of bias voltage. The clear SGSs were observed in d V/d I vs. V curves at 4.2 K. The quasiparticle characteristic was explained by the Octavio, Tinkham, Blonder and Klapwijk theory based on multiple-Andreev reflection model. To estimate the 1/ f noise parameter α/ N c in the Nb/Al+AlO x /Nb junctions, we used Hooge's empirical theory. The α/ N c of these junctions biased above superconducting energy-gap did not depend on bias voltage, while below the gap voltage it was larger than that above the gap voltage. Moreover, some peak structures related to the SGSs were observed in α/ N c vs. V characteristics biased below the gap voltage. This enhanced 1/ f noise might be caused by MAR current.
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